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Mutation, by Hugo DeVries
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Title: Species and Varieties, Their Origin by Mutation
Author: Hugo DeVries
Posting Date: September 22, 2014 [EBook #7234]
Release Date: January, 2005
First Posted: March 30, 2003
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK SPECIES, VARIETIES, ORIGIN BY MUTATION ***
Produced by Dave Gowan
Species and Varieties
Their Origin by Mutation
Lectures delivered at the University of California
By
Hugo DeVries
Professor of Botany in the University of Amsterdam
Edited by
Daniel Trembly MacDougal
Director Department of Botanical Research
Carnegie Institution of Washington
Second Edition
Corrected and Revised
CHICAGO
The Open Court Publishing Company
LONDON
Kegan Paul, Trench, Trubner and Co., Ltd.
1906
- - - - -
COPYRIGHT 1904
BY
The Open Court Pub. Co.
CHICAGO
- - - - -
THE ORIGIN OF SPECIES
The origin of species is a natural phenomenon.
LAMARCK
The origin of species is an object of inquiry.
DARWIN
The origin of species is an object of experimental investigation.
DeVRIES.
- - - - -
PREFACE BY THE AUTHOR
THE purpose of these lectures is to point out the means and methods by
which the origin of species and varieties may become an object for
experimental inquiry, in the interest of agricultural and horticultural
practice as well as in that of general biologic science. Comparative
studies have contributed all the evidence hitherto adduced for the
support of the Darwinian theory of descent and given us some general
ideas about the main lines of the pedigree of the vegetable kingdom, but
the way in which one species originates from another has not been
adequately explained. The current belief assumes that species are slowly
changed into new types. In contradiction to this conception the theory
of mutation assumes that new species and varieties are produced from
existing forms by sudden leaps. The parent-type itself remains unchanged
throughout this process, and may repeatedly give birth to new forms.
These may arise simultaneously and in groups or separately at more or
less widely distant periods.
The principal features of the theory of mutation have been dealt with at
length in my book "Die Mutationstheorie" (Vol. I., 1901, Vol. II., 1903.
Leipsic, Veit & Co.), in which I have endeavored to present as
completely as possible the detailed evidence obtained from trustworthy
historical records, and from my own experimental researches, upon which
the theory is based.
The University of California invited me to deliver a series of lectures
on this subject, at Berkeley, during the [vii] summer of 1904, and these
lectures are offered in this form to a public now thoroughly interested
in the progress of modern ideas on evolution. Some of my experiments and
pedigree-cultures are described here in a manner similar to that used in
the "Mutationstheorie," but partly abridged and partly elaborated, in
order to give a clear conception of their extent and scope. New
experiments and observations have been added, and a wider choice of the
material afforded by the more recent current literature has been made in
the interest of a clear representation of the leading ideas, leaving the
exact and detailed proofs thereof to the students of the larger book.
Scientific demonstration is often long and encumbered with difficult
points of minor importance. In these lectures I have tried to devote
attention to the more important phases of the subject and have avoided
the details of lesser interest to the general reader.
Considerable care has been bestowed upon the indication of the lacunae
in our knowledge of the subject and the methods by which they may be
filled. Many interesting observations bearing upon the little known
parts of the subject may be made with limited facilities, either in the
garden or upon the wild flora. Accuracy and perseverance, and a warm
love for Nature's children are here the chief requirements in such
investigations.
In his admirable treatise on Evolution and Adaptation (New York,
Macmillan & Co., 1903), Thomas Hunt Morgan has dealt in a critical
manner with many of the speculations upon problems subsidiary to the
theory of descent, in so convincing and complete a manner, that I think
myself justified in neglecting these questions here. His book gives an
accurate survey of them all, and is easily understood by the general
reader.
In concluding I have to offer my thanks to Dr. D.T. MacDougal and Miss
A.M. Vail of the New York Botanical Garden for their painstaking work in
the preparation of the manuscript for the press. Dr. MacDougal, by
[viii] his publications, has introduced my results to his American
colleagues, and moreover by his cultures of the mutative species of the
great evening-primrose has contributed additional proof of the validity
of my views, which will go far to obviate the difficulties, which are
still in the way of a more universal acceptation of the theory of
mutation. My work claims to be in full accord with the principles laid
down by Darwin, and to give a thorough and sharp analysis of some of the
ideas of variability, inheritance, selection, and mutation, which were
necessarily vague at his time. It is only just to state, that Darwin
established so broad a basis for scientific research upon these
subjects, that after half a century many problems of major interest
remain to be taken up. The work now demanding our attention is
manifestly that of the experimental observation and control of the
origin of species. The principal object of these lectures is to secure a
more general appreciation of this kind of work.
HUGO DE VRIES.
Amsterdam, October, 1904.
[ix]
PREFACE BY THE EDITOR
PROFESSOR DE VRIES has rendered an additional service to all naturalists
by the preparation of the lectures on mutation published in the present
volume. A perusal of the lectures will show that the subject matter of
"Die Mutationstheorie" has been presented in a somewhat condensed form,
and that the time which has elapsed since the original was prepared has
given opportunity for the acquisition of additional facts, and a
re-examination of some of the more important conclusions with the result
that a notable gain has been made in the treatment of some complicated
problems.
It is hoped that the appearance of this English version of the theory of
mutation will do much to stimulate investigation of the various phases
of the subject. This volume, however, is by no means intended to
replace, as a work of reference, the larger book with its detailed
recital of facts and its comprehensive records, but it may prove a
substitute for the use of the general reader.
The revision of the lectures has been a task attended with no little
pleasure, especially since it has given the editor the opportunity for
an advance consideration of some of the more recent results, thus
materially facilitating investigations which have been in progress at
the New York Botanical Garden for some time. So far as the ground has
been covered the researches in question corroborate the conclusions of
de Vries in all important particulars. The preparation of the manuscript
for the printer has consisted chiefly in the adaptation of oral [xii]
discussions and demonstrations to a form suitable for permanent record,
together with certain other alterations which have been duly submitted
to the author. The original phraseology has been preserved as far as
possible. The editor wishes to acknowledge material assistance in this
work from Miss A.M. Vail, Librarian of the New York Botanical Garden.
D.T. MacDougal.
New York Botanical Garden, October, 1904.
PREFACE TO THE SECOND EDITION.
THE constantly increasing interest in all phases of evolution has made
necessary the preparation of a second edition of this book within a few
months after the first appeared. The opportunity has been used to
eliminate typographical errors, and to make alterations in the form of a
few sentences for the sake of clearness and smoothness. The subject
matter remains practically unchanged. An explanatory note has been added
on page 575 in order to avoid confusion as to the identity of some of
the plants which figure prominently in the experimental investigations
in Amsterdam and New York.
The portrait which forms the frontispiece is a reproduction of a
photograph taken by Professor F.E. Lloyd and Dr. W.A. Cannon during the
visit of Professor de Vries at the Desert Botanical Laboratory of the
Carnegie Institution, at Tucson, Arizona, in June, 1904.
D. T. MACDOUGAL.
December 15, 1905.
CONTENTS
A. INTRODUCTION.
LECTURE PAGE
I. Descent: theories of evolution and methods of investigation. 1
The theory of descent and of natural selection. Evolution and
adaptation. Elementary species and varieties. Methods of scientific
pedigree-culture.
B. ELEMENTARY SPECIES.
II. Elementary species in nature. 32
_Viola tricolor_, _Draba verna_, _Primula acaulis_, and other
examples. _Euphorbia pecacuanha_. _Prunus maritima_. _Taraxacum_ and
_Hieracium_.
III. Elementary species of cultivated plants. 63
Beets, apples, pears, clover, flax and coconut.
IV. Selection of elementary species. 92
Cereals. Le Couteur. Running out of varieties. Rimpau and
Risler, _Avena fatua_. Meadows. Old Egyptian cereals. Selection by the
Romans. Shirreff. Hays.
C. RETROGRADE VARIETIES.
V. Characters of retrograde varieties. 121
Seed varieties of pure, not hybrid origin. Differences from
elementary species. Latent characters. Ray-florets of composites.
[xiii] Progressive red varieties. Apparent losses. _Xanthium
canadense_. Correlative variability. Laciniate leaves and petals.
Compound characters.
VI. Stability and real atavism. 154
Constancy of retrograde varieties. Atavism in _Ribes sanguineum
Albidum_, in conifers, in _Iris pallida_. Seedlings of _Acacia_.
Reversion by buds.
VII. Ordinary or false atavism. 185
Vicinism or variation under the influence of pollination by
neighboring individuals. Vicinism in nurseries. Purifying new and
old varieties. A case of running out of corn in Germany.
VIII. Latent characters. 216
Leaves of seedlings, adventitious buds, systematic latency and
retrogressive evolution. Degressive evolution. Latency of specific
and varietal characters in wheat-ear carnation, in the green dahlias,
in white campanulas and others. Systematic latency of flower colors.
IX. Crossing of species and varieties. 247
Balanced and unbalanced, or species and variety crosses.
Constant hybrids of _Oenothera muricata_ and _O. biennis_. _Aegilops_,
_Medicago_, brambles and other instances.
X. Mendel's law of balanced crosses. 276
Pairs of antagonistic characters, one active and one latent.
_Papaver somniferum_. [xiv] _Mephisto Danebrog_. Mendel's laws.
Unit-characters.
D. EVERSPORTING VARIETIES.
XI. Striped flowers. 309
_Antirrhinum majus luteum rubro-striatum_ with pedigree. Striped
flowers, fruits and radishes. Double stocks.
XII. "Five leaved" clover. 340
Origin of this variety. Periodicity of the anomaly.
Pedigree-cultures. Ascidia.
XIII. Polycephalic poppies. 369
Permanency and high variability. Sensitive period of the
anomaly. Dependency on external conditions.
XIV. Monstrosities. 400
Inheritance of monstrosities. Half races and middle races.
Hereditary value of atavists. Twisted stems and fasciations. Middle
races of tricotyls and syncotyls. Selection by the hereditary
percentage among the offspring.
XV. Double adaptations. 430
Analogy between double adaptations and anomalous middle races.
_Polygonum amphibium_. Alpine plants. _Othonna crassifolia_. Leaves
in sunshine and shadow. Giants and dwarfs. Figs and ivy. Leaves of
seedlings.
E. MUTATIONS.
XVI. Origin of the peloric toad-flax. 459
Sudden and frequent origin in the wild state. Origin in the
experiment-garden. Law of repeated mutations. Probable origin of
other pelories.
[xv]
XVII. The production of double flowers. 488
Sudden appearance of double flowers in horticulture. Historical
evidence. Experimental origin of _Chrysanthemum segetum plenum_.
Dependency upon nourishment. Petalody of stamens.
XVIII New species of _Oenothera_. 516
Mutations of _Oenothera lamarckiana_ in the wild state near
Hilversum. New varieties of _O. laevifolia_, _O. brevistylis_, and
_O. nanella_. New elementary species, _O. gigas_, _O. rubrinervis_,
_albida_, and _oblonga_. _O. lata_, a pistillate form.
Inconstancy of _O. scintillans_.
XIX. Experimental pedigree-cultures. 547
Pedigree of the mutative products of _Oenothera lamarckiana_ in
the Botanical Garden at Amsterdam. Laws of mutability. Sudden and
repeated leaps from an unchanging main strain. Constancy of the new
forms. Mutations in all directions.
XX. Origin of wild species and varieties. 576
Problems to solve. _Capsella heegeri_. _Oenothera biennis cruciata_.
_Epilobium hirsutum cruciatum_. _Hibiscus Moscheutos_. Purple beech.
Monophyllous strawberries. Chances of success with new mutations.
XXI. Mutations in horticulture. 604
_Chelidonium majus lacinatum_. Dwarf and spineless varieties.
Laciniate leaves. Monophyllous and broom-like varieties. [xvi] Purple
leaves. _Celosia_. Italian poplar. Cactus dahlia. Mutative origin of
_Dahlia fistulosa_, and _Geranium praetense_ in the experiment-garden.
XXII. Systematic atavism. 630
Reappearance of ancestral characters. _Primula acaulis umbellata_.
Bracts of crucifers. _Zea Mays cryptosperma_. Equisetum, _Dipsacus
sylvestris torsus_. Tomatoes.
XXIII. Taxonomic anomalies. 658
Specific characters occurring in other cases as casual
anomalies. _Papaver bracteatum monopetalum_. _Desmodium gyrans_ and
monophyllous varieties. Peltate leaves and ascidia. Flowers on
leaves. Leaves. _Hordeum trifurcatum_.
XXIV. Hypothesis of periodical mutations. 686
Discovering mutable strains. Periods of mutability and constancy.
Periods of mutations. Genealogical trees. Limited life-time of the
organic kingdom.
F. FLUCTUATIONS.
XXV. General laws of fluctuations. 715
Fluctuating variability. Quetelet's law. Individual and partial
fluctuations. Linear variability. Influence of nutrition.
Periodicity curves.
XXVI. Asexual multiplication of extremes. 742
Selection between species and intra-specific selection.
Excluding individual [xvii] embryonic variability. Sugar-canes.
Flowering cannas. Double lilacs. Other instances. Burbank's method
of selection.
XXVII. Inconstancy of improved races 770
Larger variability in the case of propagation by seed,
progression and regression after a single selection, and after
repeated selections. Selection experiments with corn. Advantages
and effect of repeated selection.
XXVIII. Artificial and natural selection. 798
Conclusions. Specific and intra-specific selection. Natural
selection in the field. Acclimatization. Improvement-selection of
sugar-beets by various methods. Rye. Hereditary percentage and
centgener power as marks by which intraspecific selection may be
guided.
Index 827
[1]
A. INTRODUCTION
LECTURE I
DESCENT: THEORIES OF EVOLUTION
AND METHODS OF INVESTIGATION
Newton convinced his contemporaries that natural laws rule the whole
universe. Lyell showed, by his principle of slow and gradual evolution,
that natural laws have reigned since the beginning of time. To Darwin we
owe the almost universal acceptance of the theory of descent.
This doctrine is one of the most noted landmarks in the advance of
science. It teaches the validity of natural laws of life in its broadest
sense, and crowns the philosophy founded by Newton and Lyell.
Lamarck proposed the hypothesis of a common origin of all living beings
and this ingenious and thoroughly philosophical conception was warmly
welcomed by his partisans, but was not widely accepted owing to lack of
supporting evidence. To Darwin was reserved the task of [2] bringing the
theory of common descent to its present high rank in scientific and
social philosophy.
Two main features in his work have contributed to this early and
unexpected victory. One of them is the almost unlimited amount of
comparative evidence, the other is his demonstration of the possibility
of a physiological explanation of the process of descent itself.
The universal belief in the independent creation of living organisms was
revised by Linnaeus and was put upon a new foundation. Before him the
genera were supposed to be created, the species and minor forms having
arisen from them through the agency of external conditions. In his first
book Linnaeus adhered to this belief, but later changed his mind and
maintained the principle of the separate creation of species. The weight
of his authority soon brought this conception to universal acceptance,
and up to the present time the prevailing conception of a species has
been chiefly based on the definition given by Linnaeus. His species
comprised subspecies and varieties, which were in their turn, supposed
to have evolved from species by the common method.
Darwin tried to show that the links which bind species to genera are of
the same nature as those which determine the relationship of [3]
subspecies and varieties. If an origin by natural laws is conceded for
the latter, it must on this ground be granted for the first also. In
this discussion he simply returned to the pre-Linnean attitude. But his
material was such as to allow him to go one step further, and this step
was an important and decisive one. He showed that the relation between
the various genera of a family does not exhibit any features of a nature
other than that between the species of a genus. What has been conceded
for the one must needs be accepted for the other. The same holds good
for the large groups.
The conviction of the common origin of closely allied forms necessarily
leads to the conception of a similar descent even in remote
relationships.
The origin of subspecies and varieties as found in nature was not
proved, but only generally recognized as evident. A broader knowledge
has brought about the same state of opinion for greater groups of
relationships. Systematic affinities find their one possible explanation
by the aid of this principle; without it, all similarity is only
apparent and accidental. Geographic and paleontologic facts, brought
together by Darwin and others on a previously unequalled scale, point
clearly in the same direction. The vast amount of evidence of all [4]
comparative sciences compels us to accept the idea. To deny it, is to
give up all opportunity of conceiving Nature in her true form.
The general features of the theory of descent are now accepted as the
basis of all biological science. Half a century of discussion and
investigation has cleared up the minor points and brought out an
abundance of facts; but they have not changed the principle. Descent
with modification is now universally accepted as the chief law of nature
in the organic world. In honor of him, who with unsurpassed genius, and
by unlimited labor has made it the basis of modern thought, this law is
called the "Darwinian theory of descent."
Darwin's second contribution to this attainment was his proof of the
possibility of a physiological explanation of the process of descent
itself. Of this possibility he fully convinced his contemporaries, but
in indicating the particular means by which the change of species has
been brought about, he has not succeeded in securing universal
acceptation. Quite on the contrary, objections have been raised from the
very outset, and with such force as to compel Darwin himself to change
his views in his later writings. This however, was of no avail, and
objections and criticisms have since steadily accumulated. Physiologic
facts concerning the origin of [5] species in nature were unknown in the
time of Darwin. It was a happy idea to choose the experience of the
breeders in the production of new varieties, as a basis on which to
build an explanation of the processes of nature. In my opinion Darwin
was quite right, and he has succeeded in giving the desired proof. But
the basis was a frail one, and would not stand too close an examination.
Of this Darwin was always well aware. He has been prudent to the utmost,
leaving many points undecided, and among them especially the range of
validity of his several arguments. Unfortunately this prudence has not
been adopted by his followers. Without sufficient warrant they have laid
stress on one phase of the problem, quite overlooking the others.
Wallace has even gone so far in his zeal and ardent veneration for
Darwin, as to describe as Darwinism some things, which in my opinion,
had never been a part of Darwin's conceptions.
The experience of the breeders was quite inadequate to the use which
Darwin made of it. It was neither scientific, nor critically accurate.
Laws of variation were barely conjectured; the different types of
variability were only imperfectly distinguished. The breeders'
conception was fairly sufficient for practical purposes, but science
needed a clear understanding of the [6] factors in the general process
of variation. Repeatedly Darwin tried to formulate these causes, but the
evidence available did not meet his requirements.
Quetelet's law of variation had not yet been published. Mendel's claim
of hereditary units for the explanation of certain laws of hybrids
discovered by him, was not yet made. The clear distinction between
spontaneous and sudden changes, as compared with the ever-present
fluctuating variations, is only of late coming into recognition by
agriculturists. Innumerable minor points which go to elucidate the
breeders' experience, and with which we are now quite familiar, were
unknown in Darwin's time. No wonder that he made mistakes, and laid
stress on modes of descent, which have since been proved to be of minor
importance or even of doubtful validity.
Notwithstanding all these apparently unsurmountable difficulties, Darwin
discovered the great principle which rules the evolution of organisms.
It is the principle of natural selection. It is the sifting out of all
organisms of minor worth through the struggle for life. It is only a
sieve, and not a force of nature, not a direct cause of improvement, as
many of Darwin's adversaries, and unfortunately many of his followers
also, have so often asserted.
It is [7] only a sieve, which decides what is to live, and what is to
die. But evolutionary lines are of great length, and the evolution of a
flower, or of an insectivorous plant is a way with many sidepaths. It is
the sieve that keeps evolution on the main line, killing all, or nearly
all that try to go in other directions. By this means natural selection
is the one directing cause of the broad lines of evolution.
Of course, with the single steps of evolution it has nothing to do. Only
after the step has been taken, the sieve acts, eliminating the unfit.
The problem, as to the manner in which the individual steps are brought
about, is quite another side of the question.
On this point Darwin has recognized two possibilities. One means of
change lies in the sudden and spontaneous production of new forms from
the old stock. The other method is the gradual accumulation of those
always present and ever fluctuating variations which are indicated by
the common assertion that no two individuals of a given race are exactly
alike. The first changes are what we now call "mutations," the second
are designated as "individual variations," or as this term is often used
in another sense, as "fluctuations." Darwin recognized both lines of
evolution; Wallace disregarded the sudden changes and proposed
fluctuations [8] as the exclusive factor. Of late, however, this point
of view has been abandoned by many investigators, especially in America.
The actual occurrence of mutations is recognized, and the battle rages
about the question, as to whether they are be regarded as the principal
means of evolution, or whether slow and gradual changes have not also
played a large and important part.
The defenders of the theory of evolution by slow accumulation of slight
fluctuations are divided into two camps. One group is called the
Neo-Lamarckians; they assume a direct modifying agency of the
environment, producing a corresponding and useful change in the
organization. The other group call themselves Darwinians or
selectionists, but to my mind with no other right beyond the arbitrary
restriction of the Darwinian principles by Wallace. They assume
fluctuating variations in all directions and leave the choice between
them to the sieve of natural selection.
Of course we are far from a decision between these views, on the sole
ground of the facts as known at present. Mutations under observation are
as yet very rare; enough to indicate the possible and most probable
ways, but no more. On the other hand the accumulation of fluctuations
does not transgress relatively narrow [9] limits as far as the present
methods of selection go. But the question remains to be solved, whether
our methods are truly the right ones, and whether by the use of new
principles, new results might not cause the balance of opinion to favor
the opposite side.
Of late, a thorough and detailed discussion of the opposing views has
been given by Morgan in his valuable book on evolution and adaptation.
He has subjected all the proposed theories to a severe criticism both on
the ground of facts and on that of their innate possibility and logical
value. He decides in favor of the mutation theory. His arguments are
incisive and complete and wholly adapted to the comprehension of all
intelligent readers, so that his book relieves me entirely of the
necessity of discussing these general questions, as it could not be done
in a better or in a clearer way.
I intend to give a review of the facts obtained from plants which go to
prove the assertion, that species and varieties have originated by
mutation, and are, at present, not known to originate in any other way.
This review consists of two parts. One is a critical survey of the facts
of agricultural and horticultural breeding, as they have accumulated
since the time of Darwin. This body of evidence is to be combined with
some corresponding experiments [10] concerning the real nature of
species in the wild state. The other part rests on my own observations
and experiments, made in the botanical garden of the University of
Amsterdam.
For many years past I have tried to elucidate the hereditary conditions
of species and varieties, and the occasional occurrence of mutations,
that suddenly produce new forms.
The present discussion has a double purpose. On one side it will give
the justification of the theory of mutations, as derived from the facts
now at hand. On the other hand it will point out the deficiencies of
available evidence, and indicate the ways by which the lacunae may
gradually be filled. Experimental work on heredity does not require vast
installments or costly laboratory equipment. It demands chiefly
assiduity and exactitude. Any one who has these two qualities, and who
has a small garden at his disposal is requested to take part in this
line of investigation.
In order to observe directly the birth of new forms it is necessary, in
the first place, to be fully clear concerning the question as to what
forms are to be expected to arise from others, and before proceeding to
a demonstration of the origin of species, it is pertinent to raise the
question as to what constitutes a species.
Species is a word, which always has had a [11] double meaning. One is
the systematic species, which is the unit of our system. But these units
are by no means indivisible. Long ago Linnaeus knew them to be compound
in a great number of instances, and increasing knowledge has shown that
the same rule prevails in other instances. Today the vast majority of
the old systematic species are known to consist of minor units. These
minor entities are called varieties in systematic works. However, there
are many objections to this usage. First, the term variety is applied in
horticulture and agriculture to things so widely divergent as to convey
no clear idea at all. Secondly, the subdivisions of species are by no
means all of the same nature, and the systematic varieties include units
the real value of which is widely different in different cases. Some of
these varieties are in reality as good as species, and have been
"elevated," as it is called by some writers, to this rank. This
conception of the elementary species would be quite justifiable, and
would at once get rid of all difficulties, were it not for one practical
obstacle. The number of the species in all genera would be doubled and
tripled, and as these numbers are already cumbersome in many cases, the
distinction of the native species of any given country would lose most
of its charm and interest.
[12] In order to meet this difficulty we must recognize two sorts of
species. The systematic species are the practical units of the
systematists and florists, and all friends of wild nature should do
their utmost to preserve them as Linnaeus has proposed them. These units
however, are not really existing entities; they have as little claim to
be regarded as such as genera and families. The real units are the
elementary species; their limits often apparently overlap and can only
in rare cases be determined on the sole ground of field observations.
Pedigree-culture is the method required and any form which remains
constant and distinct from its allies in the garden is to be considered
as an elementary species.
In the following lectures we shall consider this point at length, to
show the compound nature of systematic species in wild and in cultivated
plants. In both cases, the principle is becoming of great importance,
and many papers published recently indicate its almost universal
acceptation.
Among the systematic subdivisions of species, not all have the same
claim to the title of elementary species. In the first place the cases
in which the differences may occur between parts of the same individual
are to be excluded. Dividing an alpine plant into two halves and [13]
planting one in a garden, varietal differences at once arise and are
often designated in systematic works under different varietal names.
Secondly all individual differences which are of a fluctuating nature
are to be combined into a group. But with these we shall deal later.
Apart from these minor points the subdivisions of the systematic species
exhibit two widely different features. I will now try to make this clear
in a few words, but will return in another lecture to a fuller
discussion of this most interesting contrast.
Linnaeus himself knew that in some cases all subdivisions of a species
are of equal rank, together constituting the group called species. No
one of them outranks the others; it is not a species with varieties, but
a group, consisting only of varieties. A closer inquiry into the cases
treated in this manner by the great master of systematic science, shows
that here his varieties were exactly what we now call elementary
species.
In other cases the varieties are of a derivative nature. The species
constitutes a type that is pure in a race which ordinarily is still
growing somewhere, though in some cases it may have died out. From this
type the varieties are derived, and the way of this derivation is
usually quite manifest to the botanist. It is ordinarily [14] by the
disappearance of some superficial character that a variety is
distinguished from its species, as by the lack of color in the flowers,
of hairs on stems and foliage, of the spines and thorns, &c. Such
varieties are, strictly speaking, not to be treated in the same way as
elementary species, though they often are. We shall designate them by
the term of "retrograde varieties," which clearly indicates the nature
of their relationship to the species from which they are assumed to have
sprung. In order to lay more stress on the contrast between elementary
species and retrograde varieties, it should be stated at once, that the
first are considered to have originated from their parent-form in a
progressive way. They have succeeded in attaining something quite new
for themselves, while retrograde varieties have only thrown off some
peculiarity, previously acquired by their ancestors.
The whole vegetable kingdom exhibits a constant struggle between
progression and retrogression. Of course, the great lines of the general
pedigree are due to progression, many single steps in this direction
leading together to the great superiority of the flowering plants over
their cryptogamous ancestors. But progression is nearly always
accompanied by retrogression in the principal lines of evolution, [15]
as well as in the collateral branches of the genealogical tree.
Sometimes it prevails, and the monocotyledons are obviously a reduced
branch of the primitive dicotyledons. In orchids and aroids, in grasses
and sedges, reduction plays a most important part, leaving its traces on
the flowers as well as on the embryo of the seed. Many instances could
be given to prove that progression and retrogression are the two main
principles of evolution at large. Hence the conclusion, that our
analysis must dissect the complicated phenomena of evolution so far as
to show the separate functions of these two contrasting principles.
Hundreds of steps were needed to evolve the family of the orchids, but
the experimenter must take the single steps for the object of his
inquiry. He finds that some are progressive and others retrogressive and
so his investigation falls under two heads, the origin of progressive
characters, and the subsequent loss of the same. Progressive steps are
the marks of elementary species, while retrograde varieties are
distinguished by apparent losses. They have equal claim to our interest
and our study.
As already stated I propose to deal first with the elementary species
and afterwards with the retrograde varieties. I shall try to depict them
to you in the first place as they are seen in [16] nature and in
culture, leaving the question of their origin to a subsequent
experimental treatment.
The question of the experimental origin of new species and varieties has
to be taken up from two widely separated starting points. This may be
inferred from what we have already seen concerning the two opposing
theories, derived and isolated from Darwin's original broad conception.
One of them considers mutations as the origin of new forms, while the
other assumes fluctuations to be the source of all evolution.
As mentioned above, my own experience has led me to accept the first
view. Therefore I shall have to show that mutations do yield new and
constant forms, while fluctuations are not adequate to do so. Retrograde
varieties and elementary species may both be seen to be produced by
sudden mutations. Varieties have often been observed to appear at once
and quite unexpectedly in horticulture and agriculture, and a survey of
these historical facts will be the subject of one of my lectures. In
some instances I have succeeded in repeating these observations in my
garden under the strict conditions of a scientific experiment, and these
instances teach us the real nature of the process of mutation in all its
visible features. New elementary [17] species are far more rare, but I
have discovered in the great evening-primrose, or _Oenothera
lamarckiana_ a strain which is producing them yearly in the wild state
as well as in my garden. These observations and pedigree-experiments
will be dealt with at due length in subsequent lectures.
Having proved the existence and importance of mutations, it remains to
inquire how far the improvements may go which are due only to
fluctuating variability. As the term indicates, this variability is
fluctuating to and fro, oscillating around an average type. It never
fails nor does it, under ordinary circumstances, depart far from the
fixed average.
But the deviation may be enlarged by a choice of extremes. In sowing
their seed, the average of the strain is seen to be changed, and in
repeating the experiment the change may be considerable. It is not
clear, whether theoretically by such an accumulation, deviations might
be reached which could not be attained at once in a single sowing. This
question is hardly susceptible of an experimental answer, as it would
require such an enormous amount of seed from a few mother plants as can
scarcely ever be produced.
The whole character of the fluctuations shows them to be of an opposite
nature, contrasting [18] manifestly with specific and varietal
characters. By this method they may be proved to be inadequate ever to
make a single step along the great lines of evolution, in regard to
progressive as well as to retrograde development.
First of all fluctuations are linear, amplifying or lessening the
existing qualities, but not really changing their nature. They are not
observed to produce anything quite new, and evolution of course, is not
restricted to the increase of the already existing peculiarities, but
depends chiefly on the continuous addition of new characters to the
stock. Fluctuations always oscillate around an average, and if removed
from this for some time, they show a tendency to return to it. This
tendency, called retrogression, has never been observed to fail, as it
should, in order to free the new strain from the links with the average,
while new species and new varieties are seen to be quite free from their
ancestors and not linked to them by intermediates.
The last few lectures will be devoted to questions concerning the great
problem of the analogy between natural and artificial selection. As
already stated, Darwin made this analogy the foundation stone of his
theory of descent, and he met with the severest objections and
criticisms precisely on this point. But I hope to [19] show that he was
quite right, and that the cause of the divergence of opinions is due
simply to the very incomplete state of knowledge concerning both
processes. If both are critically analyzed they may be seen to comprise
the same factors, and further discussion may be limited to the
appreciation of the part which each of them has played in nature and
among cultivated plants.
Both natural and artificial selection are partly specific, and partly
intra-specific or individual. Nature of course, and intelligent men
first chose the best elementary species from among the swarms. In
cultivation this is the process of variety-testing. In nature it is the
survival of the fittest species, or, as Morgan designates it, the
survival of species in the struggle for existence. The species are not
changed by this struggle, they are only weighed against each other, the
weak being thrown aside.
Within the chosen elementary species there is also a struggle. It is
obvious, that the fluctuating variability adapts some to the given
circumstances, while it lessens the chances of others. A choice results,
and this choice is what is often exclusively called selection, either
natural or artificial. In cultivation it produces the improved and the
local races; in nature little is known about improvement in this way,
but [19] local adaptations with slight changes of the average character
in separate localities, seem to be of quite normal occurrence.
A new method of individual selection has been used in recent years in
America, especially by W.M. Hays. It consists in judging the hereditary
worth of a plant by the average condition of its offspring, instead of
by its own visible characters. If this determination of the "centgener
power," as Hays calls it, should prove to be the true principle of
selection, then indeed the analogy between natural and artificial
selection would lose a large part of its importance. We will reserve
this question for the last lecture, as it pertains more to the future,
than to our present stock of knowledge.
Something should be said here concerning hybrids and hybridism. This
problem has of late reached such large proportions that it cannot be
dealt with adequately in a short survey of the phenomena of heredity in
general. It requires a separate treatment. For this reason I shall limit
myself to a single phase of the problem, which seems to be indispensable
for a true and at the same time easy distinction between elementary
species and retrograde varieties. According to accepted terminology,
some crosses are to be considered as unsymmetrical, while others are
symmetrical. The first are one-sided, [21] some peculiarity being found
in one of the parents and lacking in the other. The second are balanced,
as all the characters are present in both parents, but are found in a
different condition. Active in one of them, they are concealed or
inactive in the other. Hence pairs of contrasting units result, while in
unbalanced crosses no pairing of the particular character under
consideration is possible. This leads to the principal difference
between species and varieties, and to an experimental method of deciding
between them in difficult and doubtful cases.
Having thus indicated the general outlines of the subjects I shall deal
with, something now may be said as to methods of investigation.
There are two points in which scientific investigation differs from
ordinary pedigree-culture in practice. First the isolation of the
individuals and the study of individual inheritance, instead of
averages. Next comes the task of keeping records. Every individual must
be entered, its ancestry must be known as completely as possible, and
all its relations must be noted in such a form, that the most complete
reference is always possible. Mutations may come unexpectedly, and when
once arisen, their parents and grand-parents should be known. Records
must be available which will allow of a most complete knowledge of the
whole ancestral [22] line. This, and approximately this only, is the
essential difference between experimental and accidental observation.
Mutations are occurring from time to time in the wild state as well as
in horticulture and agriculture. A selection of the most interesting
instances will be given later. But in all such cases the experimental
proof is wanting. The observations as a rule, only began when the
mutation had made its appearance. A more or less vague remembrance about
the previous state of the plants in question might be available, though
even this is generally absent. But on doubtful points, concerning
possible crosses or possible introduction of foreign strains, mere
recollection is insufficient. The fact of the mutation may be very
probable, but the full proof is, of course, wanting. Such is the case
with the mutative origin of _Xanthium commune_ Wootoni from New Mexico
and of _Oenothera biennis cruciata_ from Holland. The same doubt exists
as to the origin of the _Capsella heegeri_ of Solms-Laubach, and of the
oldest recorded mutation, that of _Chelidonium laciniatum_ in Heidelberg
about 1600.
First, we have doubts about the fact itself. These, however, gradually
lose their importance in the increasing accumulation of evidence.
Secondly, the impossibility of a closer [23] inquiry into the real
nature of the change. For experimental purposes a single mutation does
not suffice; it must be studied repeatedly, and be produced more or less
arbitrarily, according to the nature of the problems to be solved. And
in order to do this, it is evidently not enough to have in hand the
mutated individual, but it is indispensable to have also the mutable
parents, or the mutable strain from which it sprang.
All conditions previous to the mutation are to be considered as of far
higher importance than all those subsequent to it.
Now mutations come unexpectedly, and if the ancestry of an accidental
mutation is to be known, it is of course necessary to keep accounts of
all the strains cultivated. It is evident that the required knowledge
concerning the ancestry of a supposed mutation, must necessarily nearly
all be acquired from the plants in the experimental garden.
Obviously this rule is as simple in theory, as it is difficult to carry
out in practice. First of all comes the book-keeping. The parents,
grandparents and previous ancestors must be known individually. Accounts
of them must be kept under two headings. A full description of their
individual character and peculiarities must always be available on the
one hand, and on the other, all facts concerning their hereditary [24]
qualities. These are to be deduced from the composition of the progeny,
and in order to obtain complete evidence on this point, two successive
generations are often required. The investigation must ascertain the
average condition of this offspring and the occurrence of any deviating
specimens, and for both purposes it is necessary to cultivate them in
relatively large numbers. It is obvious that, properly speaking, the
whole family of a mutated individual, including all its nearer and more
remote relatives, should be known and recorded.
Hence pedigree-book-keeping must become the general rule. Subordinate to
this are two further points, which should likewise be stated here. One
pertains to the pure or hybrid nature of the original strain, and the
other to the life-conditions and all other external influences. It is
manifest that a complete understanding of a mutation depends upon full
information upon these points.
All experiments must have a beginning. The starting-point may be a
single individual, or a small group of plants, or a lot of seeds. In
many cases the whole previous history is obscure, but sometimes a little
historical evidence is at hand. Often it is evident that the initial
material belongs to a pure species, but with respect to the question of
elementary species it is [25] not rarely open to doubt. Large numbers of
hybrid plants and hybrid races are in existence, concerning the origin
of which it is impossible to decide. It is impossible in many instances
to ascertain whether they are of hybrid or of pure origin. Often there
is only one way of determining the matter; it is to guess at the
probable parents in case of a cross and to repeat the cross. This is a
point which always requires great care in the interpretation of unusual
facts.
Three cases are to be distinguished as to heredity. Many plants are so
constituted as to be fertilized with their own pollen. In this case the
visits of insects have simply to be excluded, which may be done by
covering plants with iron gauze or with bags of prepared paper.
Sometimes they fertilize themselves without any aid, as for instance,
the common evening-primrose; in other cases the pollen has to be placed
on the stigma artificially, as with Lamarck's evening-primrose and its
derivatives. Other plants need cross-fertilization in order to produce a
normal yield of seeds. Here two individuals have always to be combined,
and the pedigree becomes a more complicated one. Such is the case with
the toad-flax, which is nearly sterile with its own pollen. But even in
these cases the visits of insects bringing pollen [26] from other
plants, must be carefully excluded. A special lecture will be devoted to
this very interesting source of impurity and of uncertainty in ordinary
cultures.
Of course, crosses may lie in the proposed line of work, and this is the
third point to be alluded to. They must be surrounded with the same
careful isolation and protection against bees, as any other
fertilizations. And not only the seed-parent, but also the pollen must
be kept pure from all possible foreign admixtures.
A pure and accurately recorded ancestry is thus to be considered as the
most important condition of success in experimental plant breeding. Next
to this comes the gathering of the seeds of each individual separately.
Fifty or sixty, and often more, bags of seeds are by no means uncommon
for a single experiment, and in ordinary years the harvest of my garden
is preserved in over a thousand separate lots.
Complying with these conditions, the origin of species may be seen as
easily as any other phenomenon. It is only necessary to have a plant in
a mutable condition. Not all species are in such a state at present, and
therefore I have begun by ascertaining which were stable and which were
not. These attempts, of course, had to be made in the experimental
garden, and large quantities of seed had to be procured and [27] sown.
Cultivated plants of course, had only a small chance to exhibit new
qualities, as they have been so strictly controlled during so many
years. Moreover their purity of origin is in many cases doubtful. Among
wild plants only those could be expected to reward the investigator
which were of easy cultivation. For this reason I have limited myself to
the trial of wild plants of Holland, and have had the good fortune to
find among them at least one species in a state of mutability. It was
not really a native plant, but one that had been introduced from America
and belongs to an American genus. I refer to the great evening-primrose
or the evening-primrose of Lamarck. A strain of this beautiful species
is growing in an abandoned field in the vicinity of Hilversum, at a
short distance from Amsterdam. Here it has escaped from a park and
multiplied. In doing so it has produced and is still producing quite a
number of new types, some of which may be considered as retrograde
varieties, while others evidently are of the nature of progressive
elementary species.
This interesting plant has afforded me the means of observing directly
how new species originate, and of studying the laws of these changes. My
researches have followed a double line of inquiry. On one side, I have
limited [28] myself to direct field observations, and to tests of seed,
collected from the wild plants in their native locality. Obviously the
mutations are decided within the seed, and the culture of young plants
from them had no other aim than that of ascertaining what had occurred
in the field. And then the many chances of destruction that threaten
young plants in a wild state, could be avoided in the garden, where
environmental factors can be controlled.
My second line of inquiry was an experimental repetition of the
phenomena which were only partly discerned at the native locality. It
was not my aim to intrude into the process, nor to try to bring out new
features. My only object was to submit to the precepts just given
concerning pure treatment, individual seed gathering, exclusion of
crosses and accurate recording of all the facts. The result has been a
pedigree which now permits of stating the relation between all the
descendants of my original introduced plant. This pedigree at once
exhibits the laws followed by the mutating species. The main fact is,
that it does not change itself gradually, but remains unaffected during
all succeeding generations. It only throws off new forms, which are
sharply contrasted with the parent, and which are from the very
beginning as perfect and as constant, as narrowly [29] defined and as
pure of type as might be expected of any species.
These new species are not produced once or in single individuals, but
yearly and in large numbers. The whole phenomenon conveys the idea of a
close group of mutations, all belonging to one single condition of
mutability. Of course this mutable state must have had a beginning, as
it must sometime come to an end. It is to be considered as a period
within the life-time of the species and probably it is only a small part
of it.
The detailed description of this experiment, however, I must delay to a
subsequent lecture, but I may be allowed to state, that the discovery of
this period of mutability is of a definite theoretical importance. One
of the greatest objections to the Darwinian theory of descent arose from
the length of time it would require, if all evolution was to be
explained on the theory of slow and nearly invisible changes. This
difficulty is at once met and fully surmounted by the hypothesis of
periodical but sudden and quite noticeable steps. This assumption
requires only a limited number of mutative periods, which might well
occur within the time allowed by physicists and geologists for the
existence of animal and vegetable life on the earth.
[30] Summing up the main points of these introductory remarks, I propose
to deal with the subjects mentioned above at some length, devoting to
each of them, if possible at least an entire lecture. The decisive facts
and discussions upon which the conclusions are based will be given in
every case. Likewise I hope to point out the weak places and the lacunae
in our present knowledge, and to show the way in which each of you may
try to contribute his part towards the advancement of science in this
subject. Lastly I shall try to prove that sudden mutation is the normal
way in which nature produces new species and new varieties. These
mutations are more readily accessible to observation and experiment than
the slow and gradual changes surmised by Wallace and his followers,
which are entirely beyond our present and future experience.
The theory of mutations is a starting-point for direct investigation,
while the general belief in slow changes has held back science from such
investigations during half a century.
Coming now to the subdivisions and headings under which my material is
to be presented, I propose describing first the real nature of the
elementary species and retrograde varieties, both in normal form and in
hybridizations. A discussion of other types of varieties, including [31]
monstrosities will complete the general plan. The second subdivision
will deal with the origin of species and varieties as taught by
experiment and observation, treating separately the sudden variations
which to my mind do produce new forms, and subsequently the fluctuations
which I hold to be not adequate to this purpose.
[32]
B. ELEMENTARY SPECIES
LECTURE II
ELEMENTARY SPECIES IN NATURE
What are species? Species are considered as the true units of nature by
the vast majority of biologists. They have gained this high rank in our
estimation principally through the influence of Linnaeus. They have
supplanted the genera which were the accepted units before Linnaeus.
They are now to be replaced in their turn, by smaller types, for reasons
which do not rest upon comparative studies but upon direct experimental
evidence.
Biological studies and practical interests alike make new demands upon
systematic botany. Species are not only the subject-material of herbaria
and collections, but they are living entities, and their life-history
and life-conditions command a gradually increasing interest. One phase
of the question is to determine the easiest manner to deal with the
collected forms of a country, and another feature is the problem [33] as
to what groups are real units and will remain constant and unchanged
through all the years of our observations.
Before Linnaeus, the genera were the real units of the system. De
Candolle pointed out that the old common names of plants, such as roses
and clover, poplars and oaks, nearly all refer to genera. The type of
the clovers is rich in color, and the shape of the flower-heads and the
single flowers escape ordinary observation; but notwithstanding this,
clovers are easily recognized, even if new types come to hand. White and
red clovers and many other species are distinguished simply by
adjectives, the generic name remaining the same for all.
Tournefort, who lived in the second half of the 17th century
(1656-1708), is generally considered as the author of genera in
systematic botany. He adopted, what was at that time the general
conception and applied it throughout the vegetable kingdom. He grouped
the new and the rare and the previously overlooked forms in the same
manner in which the more conspicuous plants were already arranged by
universal consent. Species were distinguished by minor marks and often
indicated by short descriptions, but they were considered of secondary
importance.
Based on the idea of a direct creation of all [34] living beings, the
genera were then accepted as the created forms. They were therefore
regarded as the real existing types, and it was generally surmised that
species and varieties owed their origin to subsequent changes under the
influence of external conditions. Even Linnaeus agreed with this view in
his first treatises and in his "Philosophical Botany" he still kept to
the idea that all genera had been created at once with the beginning of
life.
Afterwards Linnaeus changed his opinion on this important point, and
adopted species as the units of the system. He declared them to be the
created forms, and by this decree, at once reduced the genera to the
rank of artificial groups. Linnaeus was well aware that this conception
was wholly arbitrary, and that even the species are not real indivisible
entities. But he simply forbade the study of lesser subdivisions. At his
time he was quite justified in doing so, because the first task of the
systematic botanists was the clearing up of the chaos of forms and the
bringing of them into connection with their real allies.
Linnaeus himself designated the subdivisions of the species as
varieties, but in doing so he followed two clearly distinct principles.
In some cases his species were real plants, and the varieties seemed to
be derived from them by [35] some simple changes. They were subordinated
to the parent-species. In other cases his species were groups of lesser
forms of equal value, and it was not possible to discern which was the
primary and which were the derivatives.
These two methods of subdivision seem in the main, and notwithstanding
their relatively imperfect application in many single examples, to
correspond with two really distinct cases. The derivative varieties are
distinguished from the parent-species by some single, but striking mark,
and often this attribute manifests itself as the loss of some apparent
quality. The loss of spines and of hairs and the loss of blue and red
flower-colors are the most notorious, but in rarer cases many single
peculiarities may disappear, thereby constituting a variety. This
relation of varieties to the parent-species is gradually increasing in
importance in the estimation of botanists, sharply contrasting with
those cases, in which such dependency is not to be met with.
If among the subdivisions of a species, no single one can be pointed out
as playing a primary part, and the others can not be traced back to it,
the relation between these lesser units is of course of another
character. They are to be considered of equal importance. They are
distinguished from each other by more than [36] one character, often by
slight differences in nearly all their organs and qualities. Such forms
have come to be designated as "elementary species." They are only
varieties in a broad and vague systematic significance of the word, not
in the sense accorded to this term in horticultural usage, nor in a
sharper and more scientific conception.
Genera and species are, at the present time, for a large part
artificial, or stated more correctly, conventional groups. Every
systematist is free to delimit them in a wider or in a narrower sense,
according to his judgment. The greater authorities have as a rule
preferred larger genera, others of late have elevated innumerable
subgenera to the rank of genera. This would work no real harm, if
unfortunately, the names of the plants had not to be changed each time,
according to current ideas concerning genera. Quite the same inconstancy
is observed with species. In the Handbook of the British Flora, Bentham
and Hooker describe the forms of brambles under 5 species, while
Babington in his Manual of British Botany makes 45 species out of the
same material. So also in other cases. For instance, the willows which
have 13 species in one and 31 species in the other of these manuals, and
the hawkweeds for which the figures are 7 and 32 [37] respectively.
Other authors have made still greater numbers of species in the same
groups.
It is very difficult to estimate systematic differences on the ground of
comparative studies alone. All sorts of variability occur, and no
individual or small group of specimens can really be considered as a
reliable representative of the supposed type. Many original diagnoses of
new species have been founded on divergent specimens and of course, the
type can afterwards neither be derived from this individual, nor from
the diagnosis given.
This chaotic state of things has brought some botanists to the
conviction that even in systematic studies only direct experimental
evidence can be relied upon. This conception has induced them to test
the constancy of species and varieties, and to admit as real units only
such groups of individuals as prove to be uniform and constant
throughout succeeding generations. The late Alexis Jordan, of Lyons in
France, made extensive cultures in this direction. In doing so, he
discovered that systematic species, as a rule, comprise some lesser
forms, which often cannot easily be distinguished when grown in
different regions, or by comparing dried material. This fact was, of
course, most distasteful to the systematists of his time and even for a
long period afterwards [38] they attempted to discredit it. Milde and
many others have opposed these new ideas with some temporary success.
Only of late has the school of Jordan received due recognition, after
Thuret, de Bary, Rosen and others tested its practices and openly
pronounced for them. Of late Wittrock of Sweden has joined them, making
extensive experimental studies concerning the real units of some of the
larger species of his country.
From the evidence given by these eminent authorities, we may conclude
that systematic species, as they are accepted nowadays, are as a rule
compound groups. Sometimes they consist of two or three, or a few
elementary types, but in other cases they comprise twenty, or fifty, or
even hundreds of constant and well differentiated forms.
The inner constitution of these groups is however, not at all the same
in all cases. This will be seen by the description of some of the more
interesting of them. The European heartsease, from which our
garden-pansies have been chiefly derived, will serve as an example. The
garden-pansies are a hybrid race, won by crossing the _Viola tricolor_
with the large flowered and bright yellow _V. lutea_. They combine, as
everyone knows, in their wide range of [39] varieties, the attributes of
the latter with the peculiarities of the former species.
Besides the _lutea_, there are some other species, nearly allied to
tricolor, as for instance, _cornuta_, _calcarata_, and _altaica_, which
are combined with it under the head of _Melanium_ as a subgenus, and
which together constitute a systematic unity of undoubted value, but
ranging between the common conceptions of genus and species. These forms
are so nearly allied to the heartsease that they have of late been made
use of in crosses, in order to widen the range of variability of
garden-pansies.
_Viola tricolor_ is a common European weed. It is widely dispersed and
very abundant, growing in many localities in large numbers. It is an
annual and ripens its seeds freely, and if opportunity is afforded, it
multiplies rapidly.
_Viola tricolor_ has three subspecies, which have been elevated to the
rank of species by some authors, and which may here be called, for
brevity's sake, by their binary names. One is the typical _V. tricolor_,
with broad flowers, variously colored and veined with yellow, purple and
white. It occurs in waste places on sandy soil. The second is called _V.
arvensis_ or the field-pansy; it has small inconspicuous flowers, with
pale-yellowish petals which are shorter than the sepals. It pollinates
itself without the [40] aid of insects, and is widely dispersed in
cultivated fields. The third form, _V. alpestris_, grows in the Alps,
but is of lesser importance for our present discussion.
Anywhere throughout the central part of Europe _V. tricolor_ and _V.
arvensis_ may be seen, each occupying its own locality. They may be
considered as ranging among the most common native plants of the
particular regions they inhabit. They vary in the color of the flowers,
branching of the stems, in the foliage and other parts, but not to such
an extent as to constitute distinct strains. They have been brought into
cultivation by Jordan, Wittrock and others, but throughout Europe each
of them constitutes a single type.
These types must be very old and constant, fluctuating always within the
same distinct and narrow limits. No slow, gradual changes can have taken
place. In different countries their various habitats are as old as the
historical records, and probably many centuries older. They are quite
independent of one another, the distance being in numerous cases far too
great for the exchange of pollen or of seeds. If slow and gradual
changes were the rule, the types could not have remained so uniform
throughout the whole range of these two species. They would necessarily
have split up into thousands [41] and thousands of minor races, which
would show their peculiar characteristics if tested by cultures in
adjacent beds. This however, is not what happens. As a matter of fact
_V. tricolor_ and _V. arvensis_ are widely distributed but wholly
constant types.
Besides these, there occur distinct types in numerous localities. Some
of them evidently have had time and opportunity to spread more or less
widely and now occupy larger regions or even whole countries. Others are
narrowly limited, being restricted to a single locality. Wittrock
collected seeds or plants from as many localities as possible in
different parts of Sweden and neighboring states and sowed them in his
garden near Stockholm. He secured seeds from his plants, and grew from
them a second, and in many cases a third generation in order to estimate
the amount of variability. As a rule the forms introduced into his
garden proved constant, notwithstanding the new and abnormal conditions
under which they were propagated.
First of all we may mention three perennial forms called by him _Viola
tricolor ammotropha_, _V. tricolor coniophila_ and _V. stenochila_. The
typical _V. tricolor_ is an annual plant; sowing itself in summer and
germinating soon afterwards. The young plants thrive throughout [42] the
latter part of the summer and during the fall, reaching an advanced
stage of development of the branched stems before winter. Early in the
spring the flowers begin to open, but after the ripening of the seeds
the whole plant dies.
The three perennial species just mentioned develop in the same manner in
the first year. During their flowering period, however, and afterwards,
they produce new shoots from the lower parts of the stem. They prefer
dry and sandy soils, often becoming covered with the sand that is blown
on them by the winds. They are prepared for such seemingly adverse
circumstances by the accumulation of food in the older stems and by the
capacity of the new shoots to thrive on this food till they have become
long enough to reach the light. _V. tricolor ammotropha_ is native near
Ystad in Sweden, and the other two forms on Gotland. All three have
narrowly limited habitats.
The typical tricolored heartsease has remained annual in all its other
subspecies. It may be divided into two types in the first place, _V.
tricolor genuina_ and _V. tricolor versicolor_. Both of them have a wide
distribution and seem to be the prototypes from which the rarer forms
must have been derived. Among these latter Wittrock describes seven
local types, which [43] proved to be constant in his pedigree-cultures.
Some of them have produced other forms, related to them in the way of
varieties. They all have nearly the same general habit and do not
exhibit any marked differences in their growth, in the structure and
branching of the stems, or in the character of their foliage.
Differentiating points are to be found mainly in the colors and patterns
of the flowers. The veins, which radiate from the centre of the corolla
are branched in some and undivided in others; in one elementary species
they are wholly lacking. The purple color may be absent, leaving the
flowers of a pale or a deep yellow. Or the purple may be reddish or
bluish. Of the petals all five may have the purple hue on their tips, or
this attribute may be limited to the two upper ones. Contrasting with
this wide variability is the stability of the yellow spot in the centre,
which is always present and becomes inconspicuous only, when the whole
petals are of the same hue. It is a general conception that colors and
color-markings are liable to great variability and do not constitute
reliable standards. But the cultures of Wittrock have proved the
contrary, at least in the case of the violets. No pattern, however
quaint, appears changeable, if one elementary species only is
considered. Hundreds of plants from seeds [44] from one locality may be
grown, and all will exhibit exactly the same markings. Most of these
forms are of very local occurrence. The most beautiful of all, the
_ornatissima_, is found only in Jemtland, the _aurobadia_ only in
Sodermanland, the anopetala_ in other localities in the same country,
the _roseola_ near Stockholm, and the yellow _lutescens_ in Finmarken.
The researches of Wittrock included only a small number of elementary
species, but every one who has observed the violets in the central parts
of Europe must be convinced that many dozens of constant forms of the
typical _Viola tricolor_ might easily be found and isolated.
We now come to the field pansy, the _Viola arvensis_, a very common weed
in the grain-fields of central Europe. I have already mentioned its
small corolla, surpassed by the lobes of the calyx and its capacity of
self-fertilization. It has still other curious differentiating
characters; the pollen grains, which are square in _V. tricolor_, are
five-sided in _V. arvensis_. Some transgressive fluctuating variability
may occur in both cases through the admixture of pollen-grains. Even
three-angled pollen grains are seen sometimes. Other marks are observed
in the form of the anthers and the spur.
There seem to be very many local subspecies [45] of the field-pansy.
Jordan has described some from the vicinity of Lyons, and Wittrock
others from the northern parts of Europe. They diverge from their common
prototype in nearly all attributes, the flowers not showing the
essential differentiating characters as in the _V. tricolor_. Some have
their flower-stalks erect, and in others the flowers are held nearly at
right angles to the stem. _V. pallescens_ is a small, almost unbranched
species with small pale flowers. _V. segetalis_ is a stouter species
with two dark blue spots on the tips of the upper petals. _V. agrestis_
is a tall and branched, hairy form. _V. nemausensis_ attains a height of
only 10 cm., has rounded leaves and long flower-stalks. Even the seeds
afford characters which may be made use of in isolating the various
species.
The above-mentioned elementary forms belong to the flora of southern
France, and Wittrock has isolated and cultivated a number of others from
the fields of Sweden. A species from Stockholm is called _Viola patens_;
_V. arvensis curtisepala_ occurs in Gotland, and _V. arvensis striolata_
is a distinct form, which has appeared in his cultures without its true
origin being ascertained.
The alpine violets comprise a more widespread type with some local
elementary species [46] derived exactly in the same way as the
tricolored field pansies.
Summarizing the general result of this description we see that the
original species _Viola tricolor_ may be split up into larger and lesser
groups of separate forms. These last prove to be constant in
pedigree-cultures, and therefore are to be considered as really existent
units. They are very numerous, comprising many dozens in each of the two
larger subdivisions.
All systematic grouping of these forms, and their combination into
subspecies and species rests on the comparative study of their
characters. The result of such studies must necessarily depend on
principles which underlie them. According to the choice of these
principles, the construction of the groups will be found to be
different. Wittrock trusts in the first place to morphologic characters,
and considers the development as passing from the more simple to the
more complex types. On the other hand the geographic distribution may be
considered as an indication of the direction of evolution, the
wide-spread forms being regarded as the common parents of the minor
local species.
However, such considerations are only of secondary importance. It must
be borne in mind that an ordinary systematic species may include [47]
many dozens of elementary forms, each of which remains constant and
unchanged in successive generations, even if cultivated in the same
garden and under similar external conditions.
Leaving the violets, we may take the vernal whitlow-grass or _Draba
verna_ for a second illustration. This little annual cruciferous plant
is common in the fields of many parts of the United States, though
originally introduced from Europe. It has small basal rosettes which
develop during summer and winter, and produce numerous leafless
flowering stems early in the spring. It is a native of central Europe
and western Asia, and may be considered as one of the most common
plants, occurring anywhere in immense numbers on sandy soils. Jordan was
the first to point out that it is not the same throughout its entire
range. Although a hasty survey does not reveal differences, they show
themselves on closer inspection. De Bary, Thuret, Rosen and many others
confirmed this result, and repeated the pedigree-cultures of Jordan.
Every type is constant and remains unchanged in successive generations.
The anthers open in the flower-buds and pollinate the stigmas before the
expansion of the flowers, thus assuring self-fertilization. Moreover,
these inconspicuous little flowers are only sparingly visited by
insects. Dozens of subspecies [48] may be cultivated in the same garden
without any real danger of their intercrossing. They remain as pure as
under perfect isolation.
It is very interesting to observe the aspect of such types, when growing
near each other. Hundreds of rosettes exhibit one type, and are
undoubtedly similar. The alternative group is distinguishable at first
sight, though the differentiating marks are often so slight as to be
traceable with difficulty. Two elementary species occur in Holland, one
with narrow leaves in the western provinces and one with broader foliage
in the northern parts. I have cultivated them side by side, and was as
much struck with the uniformity within each group, as with the contrast
between the two sets.
Nearly all organs show differences. The most marked are those of the
leaves, which may be small or large, linear or elliptic or oblong and
even rhomboidal in shape, more or less hairy with simple or with
stellate branched hairs, and finally of a pure green or of a glaucous
color. The petals are as a rule obcordate, but this type may be combined
with others having more or less broad emarginations at the summit, and
with differences in breadth which vary from almost linear types to
others which touch along their margins. The pods are short and broad, or
long and narrow, or varying in sundry other [49] ways. All in all there
are constant differences which are so great that it has been possible to
distinguish and to describe large numbers of types.
Many of them have been tested as to their constancy from seed. Jordan
made numerous cultures, some of which lasted ten or twelve years; Thuret
has verified the assertion concerning their constancy by cultures
extending over seven years in some instances; Villars and de Bary made
numerous trials of shorter duration. All agree as to the main points.
The local races are uniform and come true from seed; the variability of
the species is not of a fluctuating, but of a polymorphous nature. A
given elementary species keeps within its limits and cannot vary beyond
them, but the whole group gives the impression of variability by its
wide range of distinct, but nearly allied forms.
The geographic distribution of these elementary species of the
whitlow-grass is quite distinct from that of the violets. Here
predominant species are limited to restricted localities. Most of them
occupy one or more departments of France, and in Holland two of them are
spread over several provinces. An important number are native in the
centre of Europe, and from the vicinity of Lyons, Jordan succeeded in
establishing about fifty elementary [50] species in his garden. In this
region they are crowded together and not rarely two or even more quite
distinct forms are observed to grow side by side on the same spot.
Farther away from this center they are more widely dispersed, each
holding its own in its habitat. In all, Jordan has distinguished about
two hundred species of _Draba verna_ from Europe and western Asia.
Subsequent authors have added new types to the already existing number
from time to time.
The constancy of these elementary species is directly proven by the
experiments quoted above, and moreover it may be deduced from the
uniformity of each type within its own domain. These are so large that
most of the localities are practically isolated from one another, and
must have been so for centuries. If the types were slowly changing such
localities would often, though of course not always, exhibit slighter
differences, and on the geographic limits of neighboring species
intermediates would be found. Such however, are not on record. Hence the
elementary species must be regarded as old and constant types.
The question naturally arises how these groups of nearly allied forms
may originally have been produced. Granting a common origin for all of
them, the changes may have been [51] simultaneous or successive.
According to the geographic distribution, the place of common origin
must probably be sought in the southern part of central Europe, perhaps
even in the vicinity of Lyons. Here we may assume that the old _Draba
verna_ has produced a host or a swarm of new types. Thence they must
have spread over Europe, but whether in doing so they have remained
constant, or whether some or many of them have repeatedly undergone
specific mutations, is of course unknown.
The main fact is, that such a small species as _Draba verna_ is not at
all a uniform type, but comprises over two hundred well distinguished
and constant forms.
It is readily granted that violets and whitlowgrasses are extreme
instances of systematic variability. Such great numbers of elementary
species are not often included in single species of the system. But the
numbers are of secondary importance, and the fact that systematic
species consist, as a rule, of more than one independent and constant
subspecies, retains its almost universal validity.
In some cases the systematic species are manifest groups, sharply
differentiated from one another. In other instances the groups of
elementary forms as they are shown by direct observation, have been
adjudged by many authors [52] to be too large to constitute species.
Hence the polymorphous genera, concerning the systematic subdivisions of
which hardly two authors agree. Brambles and roses are widely known
instances, but oaks, elms, apples, and pears, _Mentha_, _Prunu_s,
_Vitis_, _Lactuca_, _Cucumis_, _Cucurbita_ and numerous others are in
the same condition.
In some instances the existence of elementary species is so obvious,
that they have been described by taxonomists as systematic varieties or
even as good species. The primroses afford a widely known example.
Linnaeus called them _Primula veris_, and recognized three types as
pertaining to this species, but Jacquin and others have elevated these
subspecies to the full rank of species. They now bear the names of
_Primula elatior_ with larger, _P. officinalis_ with smaller flowers,
and _P. acaulis_. In the last named the common flower-stalk is lacking
and the flowers of the umbel seem to be borne in the arils of the basal
leaves.
In other genera such nearly allied species are more or less universally
recognized. _Galium Mollugo_ has been divided into _G. elatum_ with a
long and weak stem, and _G. erectum_ with shorter and erect stems;
_Cochlearia danica_, _anglica_ and _officinalis_ are so nearly allied as
to be hardly distinguishable. _Sagina apetala_ and _patula_, [53]
_Spergula media_ and _salina_ and many other pairs of allied species
have differentiating characters of the same value as those of the
elementary species of _Draba verna_. _Filago_, _Plantago_, _Carex_,
_Ficaria_ and a long series of other genera afford proofs of the same
close relation between smaller and larger groups of species. The
European frost-weeds or _Helianthemum_ include a group of species which
are so closely allied, that ordinary botanical descriptions are not
adequate to give any idea of their differentiating features. It is
almost impossible to determine them by means of the common analytical
keys. They have to be gathered from their various native localities and
cultivated side by side in the garden to bring out their differences.
Among the species of France, according to Jordan, _Helianthemum
polifolium_, _H. apenninum_, _H. pilosum_ and _H. pulverulentum_ are of
this character.
A species of cinquefoil, _Potentilla Tormentilla_, which is
distinguished by its quaternate flowers, occurs in Holland in two
distinct types, which have proved constant in my cultural experiments.
One of them has, broad petals, meeting together at the edges, and
constituting rounded saucer without breaks. The other has narrow petals,
which are strikingly separated from one another and show the sepals
between them. [54] In the same manner bluebells vary in the size and
shape of the corolla, which may be wide or narrow, bell-shaped or
conical, with the tips turned downwards, sidewards or backwards.
As a rule all of the more striking elementary types have been described
by local botanists under distinct specific names, while they are thrown
together into the larger systematic species by other authors, who study
the distribution of plants over larger portions of the world. Everything
depends on the point of view taken. Large floras require large species.
But the study of local floras yields the best results if the many forms
of the region are distinguished and described as completely as possible.
And the easiest way is to give to each of them a specific name. If two
or more elementary species are united in the same district, they are
often treated in this way, but if each region had its own type of some
given species, commonly the part is taken for the whole, and the sundry
forms are described under the same name, without further distinctions.
Of course these questions are all of a practical and conventional
nature, but involve the different methods in which different authors
deal with the same general fact. The fact is that systematic species are
compound groups, exactly like the genera and that their real units [55]
can only be recognized by comparative experimental studies.
Though the evidence already given might be esteemed to be sufficient for
our purpose, I should like to introduce a few more examples; two of them
pertain to American plants.
The Ipecac spurge or _Euphorbia Ipecacuanha_ occurs from Connecticut to
Florida, mainly near the coast, preferring dry and sandy soil. It is
often found by the roadsides. According to Britton and Brown's
"Illustrated Flora" it is glabrous or pubescent, with several or many
stems, ascending or nearly erect; with green or red leaves, which are
wonderfully variable in outline, from linear to orbicular, mostly
opposite, the upper sometimes whorled, the lower often alternate. The
glands of the involucres are elliptic or oblong, and even the seeds vary
in shape.
Such a wide range of variability evidently points to the existence of
some minor types. Dr. John Harshberger has made a study of those which
occur in the vicinity of Whitings in New Jersey. His types agree with
the description given above. Others were gathered by him at Brown's
Mills in the pinelands, New Jersey, where they grew in almost pure sand
in the bright sunlight. He observed still other differentiating
characters. The amount of seed [56] produced and the time of flowering
were variable to a remarkable degree.
Dr. Harshberger had the kindness to send me some dried specimens of the
most interesting of these types. They show that the peculiarities are
individual, and that each specimen has its own characters. It is very
probable that a comparative experimental study will prove the existence
of a large number of elementary species, differing in many points; they
will probably also show differences in the amount of the active chemical
substances, especially of emetine, which is usually recorded as present
in about 1%, but which will undoubtedly be found in larger quantities in
some, and in smaller quantities in other elementary species. In this way
the close and careful distinction of the really existing units might
perhaps prove of practical importance.
MacFarlane has studied the beach-plum or _Prunus maritima_, which is
abundant along the coast regions of the Eastern States from Virginia to
New Brunswick. It often covers areas from two to two hundred acres in
extent, sometimes to the exclusion of other plants. It is most prolific
on soft drifting sand near the sea or along the shore, where it may at
times be washed with ocean-spray. The fruit usually become ripe about
the middle of August, and show extreme [57] variations in size, shape,
color, taste, consistency and maturation period, indicating the
existence of separate races or elementary species, with widely differing
qualities. The earlier varieties begin to ripen from August 10 to 20,
and a continuous supply can be had till September 10, while a few good
varieties continue to ripen till September 20. But even late in October
some other types are still found maturing their fruits.
Exact studies were made of fruit and stone variations, and their
characteristics as to color, weight, size, shape and consistency were
fully described. Similar variations have been observed, as is well
known, in the cultivated plums. Fine blue-black fruits were seen on some
shrubs and purplish or yellow fruits on others. Some exhibit a firmer
texture and others a more watery pulp. Even the stones show differences
which are suggestive of distinct races.
Recently Mr. Luther Burbank of Santa Rosa, California, has made use of
the beach-plum to produce useful new varieties. He observed that it is a
very hardy species, and never fails to bear, growing under the most
trying conditions of dry and sandy, or of rocky and even of heavy soil.
The fruits of the wild shrubs are utterly worthless for anything but
preserving. [58] But by means of crossing with other species and
especially with the Japanese plums, the hardy qualities of the
beach-plum have been united with the size, flavor and other valuable
qualities of the fruit, and a group of new plums have been produced with
bright colors, ovoid and globular forms which are never flattened and
have no suture. The experiments were not finished, when I visited Mr.
Burbank in July, 1904, and still more startling improvements were said
to have been secured.
I may perhaps be allowed to avail myself of this opportunity to point
out a practical side of the study of elementary species. This always
appears whenever wild plants are subjected to cultivation, either in
order to reproduce them as pure strains, or to cross them with other
already cultivated species. The latter practice is as a rule made use of
whenever a wild species is found to be in possession of some quality
which is considered as desirable for the cultivated forms. In the case
of the beach-plum it is the hardiness and the great abundance of fruits
of the wild species which might profitably be combined with the
recognized qualities of the ordinary plums. Now it is manifest, that in
order to make crosses, distinct individual plants are to be chosen, and
that the variability of the wild species may be of very great
importance. [59] Among the range of elementary species those should be
used which not only possess the desired advantages in the highest
degree, but which promise the best results in other respects or their
earliest attainment. The fuller our knowledge of the elementary species
constituting the systematic groups, the easier and the more reliable
will be the choice for the breeder. Many Californian wild flowers with
bright colors seem to consist of large numbers of constant elementary
forms, as for instance, the lilies, godetias, eschscholtias and others.
They have been brought into cultivation many times, but the minutest
distinction of their elementary forms is required to attain the highest
success.
In concluding, I will point out a very interesting difficulty, which in
some cases impedes the clear understanding of elementary species. It is
the lack of self-fertilization. It occurs in widely distant families,
but has a special interest for us in two genera, which are generally
known as very polymorphous groups.
One of them is the hawkweed or _Hieracium_, and the other is the
dandelion or _Taraxacum officinale_. Hawkweeds are known as a genus in
which the delimitation of the species is almost impossible, Thousands of
forms may be cultivated side by side in botanical gardens, exhibiting
[60] slight but undoubted differentiating features, and reproduce
themselves truly by seed. Descriptions were formerly difficult and so
complicated that the ablest writers on this genus, Fries and Nageli are
said not to have been able to recognize the separate species by the
descriptions given by each other. Are these types to be considered as
elementary species, or only as individual differences? The decision of
course, would depend upon their behavior in cultures. Such tests have
been made by various experimenters. In the dandelion the bracts of the
involucre give the best characters. The inner ones may be linear or
linear-lanceolate, with or without appendages below the tip; the outer
ones may be similar and only shorter, or noticeably larger, erect,
spreading or even reflexed, and the color of the involucre may be a pure
green or glaucous; the leaves may be nearly entire or pinnatifid, or
sinuate-dentate, or very deeply runcinate-pinnatifid, or even pinnately
divided, the whole plant being more or less glabrous.
Raunkiaer, who has studied experimentally a dozen types from Denmark,
found them constant, but observed that some of them have no pollen at
all, while in others the pollen, though present, is impotent. It does
not germinate on the stigma, cannot produce the ordinary tube, [61] and
hence has no fertilizing power. But the young ovaries do not need such
fertilization. They are sufficient unto themselves. One may cut off all
the flowers of a head before the opening of the anthers, and leave the
ovaries untouched, and the head will ripen its seeds quite as well. The
same thing occurs in the hawkweeds. Here, therefore, we have no
fertilization and the extensive widening of the variability, which
generally accompanies this process is, of course, wanting. Only partial
or vegetative variability is present. Unfertilized eggs when developing
into embryos are equivalent to buds, separated from the parent-plant and
planted for themselves. They repeat both the specific and the individual
characters of the parent. In the case of the hawkweed and the dandelion
there is at present no means of distinguishing between these two
contrasting causes of variability. But like the garden varieties which
are always propagated in the vegetative way, their constancy and
uniformity are only apparent and afford no real indication of hereditary
qualities.
In addition to these and other exceptional cases, seed-cultures are
henceforth to be considered as the sole means of recognizing the really
existing systematic units of nature. All other groups, including
systematic species and [62] genera, are equally artificial or
conventional. In other words we may state "that current misconceptions
as to the extreme range of fluctuating variability of many native
species have generally arisen from a failure to recognize the composite
nature of the forms in question," as has been demonstrated by MacDougal
in the case of the common evening-primrose, _Oenothera biennis_. "It is
evident that to study the behavior of the characters of plants we must
have them in their simplest combinations; to investigate the origin and
movements of species we must deal with them singly and uncomplicated."
[63]
LECTURE III
ELEMENTARY SPECIES OF CULTIVATED PLANTS
Recalling the results of the last lecture, we see that the species of
the systematists are not in reality units, though in the ordinary course
of floristic studies they may, as a rule, seem to be so. In some cases
representatives of the same species from different countries or regions,
when compared with one another do not exactly agree. Many species of
ferns afford instances of this rule, and Lindley and other great
systematists have frequently been puzzled by the wide range of
differences between the individuals of a single species.
In other cases the differing forms are observed to grow near each other,
sometimes in neighboring provinces, sometimes in the same locality,
growing and flowering in mixtures of two or three or even more
elementary types. The violets exhibit widespread ancient types, from
which the local species may be taken to have arisen. The common
ancestors of the Whitlow-grasses are probably not to be found [64] among
existing forms, but numerous types are crowded together in the southern
part of central Europe and more thinly scattered elsewhere, even as far
as western Asia. There can be little doubt that their common origin is
to be sought in the center of their geographic distribution.
Numerous other cases exhibit smaller numbers of elementary units within
a systematic species; in fact purely uniform species seem to be
relatively rare. But with small numbers there are of course no
indications to be expected concerning their common origin or the
starting point of their distribution.
It is manifest that these experiences with wild species must find a
parallel among cultivated plants. Of course cultivated plants were
originally wild and must have come under the general law. Hence we may
conclude that when first observed and taken up by man, they must already
have consisted of sundry elementary subspecies. And we may confidently
assert that some must have been rich and others poor in such types.
Granting this state of things as the only probable one, we can easily
imagine what must have been the consequences. If a wild species had been
taken into cultivation only once, the cultivated form would have been a
single elementary [65] type. But it is not very likely that such
partiality would occur often. The conception that different tribes at
different times and in distant countries would have used the wild plants
of their native regions seems far more natural than that all should have
obtained plants for cultivation from the same source or locality. If
this theory may be relied upon, the origin of many of the more widely
cultivated agricultural plants must have been multiple, and the number
of the original elementary species of the cultivated types must have
been so much the larger, the more widely distributed and variable the
plants under consideration were before the first period of cultivation.
Further it would seem only natural to explain the wide variability of
many of our larger agricultural and horticultural stocks by such an
incipient multiformity of the species themselves. Through commercial
intercourse the various types might have become mixed so as to make it
quite impossible to point out the native localities for each of them.
Unfortunately historical evidence on this point is almost wholly
lacking. The differences in question could not have been appreciated at
that remote period, and interest the common observer but little even
today. The history of most of the cultivated plants is very obscure,
[66] and even the most skillful historians, by sifting the evidence
afforded by the older writers, and that obtained by comparative
linguistic investigations have been able to do little more than frame
the most general outline of the cultural history of the most common and
most widely used plants.
Some authors assume that cultivation itself might have been the
principal cause of variability, but it is not proved, nor even probable,
that cultivated plants are intrinsically more variable than their wild
prototypes. Appearances in this case are very deceptive. Of course
widely distributed plants are as a rule richer in subspecies than forms
with limited distribution, and the former must have had a better chance
to be taken into cultivation than the latter. In many cases, especially
with the more recent cultivated species, man has deliberately chosen
variable forms, because of their greater promise. Thirdly, wide
variability is the most efficient means of acclimatization, and only
species with many elementary units would have offered the adequate
material for introduction into new countries.
From this discussion it would seem that it is more reasonable to assert
that variability is one of the causes of the success of cultivation,
than to assume that cultivation is a cause of variability [67] at large.
And this assumption would be equally sufficient to explain the existing
conditions among cultivated plants.
Of course I do not pretend to say that cultivated plants should be
expected to be less variable than in the wild state, or that swarms of
elementary species might not be produced during cultivation quite as
well as before. However the chance of such an event, as is easily seen,
cannot be very great, and we shall have to be content with a few
examples of which the coconut is a notable one.
Leaving this general discussion of the subject, we may take up the
example of the beets. The sugar-beet is only one type from among a horde
of others, and though the origin of all the single types is not
historically known, the plant is frequently found in the wild state even
at the present time, and the native types may be compared with the
corresponding cultivated varieties.
The cultivation of beets for sugar is not of very ancient date. The
Romans knew the beets and used them as vegetables, both the roots and
the leaves. They distinguished a variety with white and one with red
flesh, but whether they cultivated them, or only collected them from
where they grew spontaneously, appears to be unknown.
[68] Beets are even now found in large quantities along the shores of
Italy. They prefer the vicinity of the sea, as do so many other members
of the beet family, and are not limited to Italy, but are found growing
elsewhere on the littoral of the Mediterranean, in the Canary Islands
and through Persia and Babylonia to India. In most of their native
localities they occur in great abundance.
The color of the foliage and the size of the roots are extremely
variable. Some have red leafstalks and veins, others a uniform red or
green foliage, some have red or white or yellow roots, or exhibit
alternating rings of a red and of a white tinge on cut surfaces. It
seems only natural to consider the white and the red, and even the
variegated types as distinct varieties, which in nature do not
transgress their limits nor change into one another. In a subsequent
lecture I will show that this at least is the rule with the
corresponding color-varieties in other genera.
The fleshiness or pulpiness of the roots is still more variable. Some
are as thick as the arm and edible, others are not thicker than a finger
and of a woody composition, and the structure of this woody variety is
very interesting. The sugar-beet consists, as is generally known, of
concentric layers of sugar-tissue and of vascular [69] strands; the
larger the first and the smaller the latter, the greater is, as a rule,
the average amount of sugar of the race. Through the kindness of the
late Mr. Rimpau, a well known German breeder of sugar-beet varieties, I
obtained specimens from seed of a native wild locality near Bukharest.
The plants produced quite woody roots, showing almost no sugar tissue at
all. Woody layers of strongly developed fibrovascular strands were seen
to be separated one from another only by very thin layers of
parenchymatous cells. Even the number of layers is variable; it was
observed to be five in my plants; but in larger roots double this number
and even more may easily be met with.
Some authors have distinguished specific types among these wild forms.
While the cultivated beets are collected under the head of _Beta
vulgaris_, separate types with more or less woody roots have been
described as _Beta maritima_ and _Beta patula_. These show differences
in the habit of the stems and the foliage. Some have a strong tendency
to become annual, others to become biennial. The first of course do not
store a large quantity of food in their roots, and remain thin, even at
the time of flowering. The biennial types occur in all sizes of roots.
In the annuals the stems may vary from [70] erect to ascending, and the
name _patula_ indicates stems which are densely branching from the base
with widely spreading branches throughout. Mr. Em. von Proskowetz of
Kwassitz, Austria, kindly sent me seeds of this _Beta patula_, the
variability of which was so great in my cultures as to range from nearly
typical sugar-beets to the thin woody type of Bukharest.
Broad and narrow leaves are considered to be differentiating marks
between _Beta vulgaris_ and _Beta patula_, but even here a wide range of
forms seem to occur.
Rimpau, Proskowetz, Schindler and others have made cultures of beets
from wild localities in order to discover a hypothetical common ancestor
of all the present cultivated types. These researches point to the _B.
patula_ as the probable ancestor, but of course they were not made to
decide the question as to whether the origination of the several now
existing types had taken place before or during culture. From a general
point of view the variability of the wild species is parallel to that of
the cultivated forms to such a degree as to suggest the multiple origin
of the former. But a close investigation of this highly important
problem has still to be made.
The varieties of the cultivated beets are commonly [71] included in four
subspecies. The two smallest are the salad-beets and the ornamental
forms, the first being used as food, and ordinarily cultivated in red
varieties, the second being used as ornamental plants during the fall,
when they fill the beds left empty by summer flowers, with a bright
foliage that is exceedingly rich in form and color. Of the remaining
subspecies, one comprises the numerous sorts cultivated as forage-crops
and the other the true sugar-beets. Both of them vary widely as to the
shape and the size of the roots, the quality of the tissue, the foliage
and other characteristics.
Some of these forms, no doubt, have originated during culture. Most of
them have been improved by selection, and no beet found in the wild
state ever rivals any cultivated variety. But the improvement chiefly
affects the size, the amount of sugar and nutrient substances and some
other qualities which recur in most of the varieties. The varietal
attributes themselves however, are more or less of a specific nature,
and have no relation to the real industrial value of the race. The
short-rooted and the horn-shaped varieties might best be cited as
examples.
The assertion that the sundry varieties of forage-beets are not the
result of artificial selection, [72] is supported in a large measure by
the historic fact that the most of them are far older than the method of
conscious selection of plants itself. This method is due to Louis
Vilmorin and dates from the middle of the last century. But in the
sixteenth century most of our present varieties of beets were already in
cultivation. Caspar Bauhin gives a list of the beets of his time and it
is not difficult to recognize in it a large series of subspecies and
varieties and even of special forms, which are still cultivated. A more
complete list was published towards the close of the same century by
Olivier de Serres in his world-renowned "Theatre d'Agriculture" (Paris,
1600).
The red forage-beets which are now cultivated on so large a scale, had
been introduced from Italy into France only a short time before.
From this historic evidence, the period during which the beets were
cultivated from the time of the Romans or perhaps much later, up to the
time of Bauhin and De Serres, would seem far too short for the
production by the unguided selection of man of all the now existing
types. On the other hand, the parallelism between the characters of some
wild and some cultivated varieties goes to make it very probable that
other varieties have been found in the same way, some in this country
and others in that, [73] and have been taken into cultivation
separately. Afterwards of course all must have been improved in the
direction required by the needs of man.
Quite the same conclusion is afforded by apples. The facts are to some
extent of another character, and the rule of the derivation of the
present cultivated varieties from original wild forms can be illustrated
in this case in a more direct way. Of course we must limit ourselves to
the varieties of pure ancestry and leave aside all those which are of
hybrid or presumably hybrid origin.
Before considering their present state of culture, something must be,
said about the earlier history and the wild state of the apples.
The apple-tree is a common shrub in woods throughout all parts of
Europe, with the only exception of the extreme north. Its distribution
extends to Anatolia, the Caucasus and Ghilan in Persia. It is found in
nearly all forests of any extent and often in relatively large numbers
of individuals. It exhibits varietal characters, which have led to the
recognition of several spontaneous forms, especially in France and in
Germany.
The differentiating qualities relate to the shape and indumentum of the
leaves. Nothing is known botanically as to differences between [74] the
fruits of these varieties, but as a matter of fact the wild apples of
different countries are not at all the same.
Alphonse De Candolle, who made a profound study of the probable origin
of most of our cultivated plants, comes to the conclusion that the apple
tree must have had this wide distribution in prehistoric times, and that
its cultivation began in ancient times everywhere.
This very important conclusion by so high an authority throws
considerable light on the relation between cultivated and wild varieties
at large. If the historic facts go to prove a multiple origin for the
cultivation of some of the more important useful plants, the probability
that different varieties or elementary species have been the starting
points for different lines of culture, evidently becomes stronger.
Unfortunately, this historic evidence is scanty. The most interesting
facts are those concerning the use of apples by the Romans and by their
contemporaries of the Swiss and middle European lake-dwellings. Oswald
Heer has collected large numbers of the relics of this prehistoric
period. Apples were found in large quantities, ordinarily cut into
halves and with the signs of having been dried. Heer distinguished two
varieties, one with large and one with small fruits. The first about 3
and [75] the other about 1.5-2 cm. in diameter. Both are therefore very
small compared with our present ordinary varieties, but of the same
general size as the wild forms of the present day. Like these, they must
have been of a more woody and less fleshy tissue. They would scarcely
have been tasteful to us, but in ancient times no better varieties were
known and therefore no comparison was possible.
There is no evidence concerning the question, as to whether during the
periods mentioned apples were cultivated or only collected in the wild
state. The very large numbers which are found, have induced some writers
to believe in their culture, but then there is no reason why they should
not have been collected in quantity from wild shrubs. The main fact is
that the apple was not a uniform species in prehistoric times but showed
even then at least some amount of variability.
At the present day the wild apples are very rich in elementary species.
Those of Versailles are not the same as those of Belgium, and still
others are growing in England and in Germany. The botanical differences
derived from the blossoms and the leaves are slight, but the flavor,
size and shape of the fruits diverge widely. Two opinions have been
advanced to explain this high degree of variability, but [76] neither of
them conveys a real explanation; their aim is chiefly to support
different views as to the causes of variability, and the origin of
elementary species at large.
One opinion, advocated by De Candolle, Darwin and others, claims that
the varieties owe their origin to the direct influence of cultivation,
and that the corresponding forms found in the wild state, are not at all
original, but have escaped from cultivation and apparently become wild.
Of course this possibility cannot be denied, at least in any single
instance, but it seems too sweeping an assertion to make for the whole
range of observed forms.
The alternative theory is that of van Mons, the Belgian originator of
commercial varieties of apples, who has published his experiments in a
large work called "Arbres fruitiers ou Pomonomie belge." Most of the
more remarkable apples of the first half of the last century were
produced by van Mons, but his greatest merit is not the direct
production of a number of good varieties, but the foundation of the
method, by which new varieties may be obtained and improved.
According to van Mons, the production of a new variety consists chiefly
of two parts. The first is the discovery of a subspecies with new
desirable qualities. The second is the transformation [77] of the
original small and woody apple into a large, fleshy and palatable
variety. Subspecies, or what we now call elementary species were not
produced by man; nature alone creates new forms, as van Mons has it. He
examined with great care the wild apples of his country, and especially
those of the Ardennes, and found among them a number of species with
different flavors. For the flavor is the one great point, which must be
found ready in nature and which may be improved, but can never be
created by artificial selection. The numerous differences in flavor are
quite original; all of them may be found in the wild state and most of
them even in so limited a region as the Ardennes Mountains. Of course
van Mons preferred not to start from the wild types themselves, when the
same flavor could be met with in some cultivated variety. His general
method was, to search for a new flavor and to try to bring the bearer of
it up to the desired standard of size and edibility.
The latter improvement, though it always makes the impression of an
achievement, is only the last stone to be added to the building up of
the commercial value of the variety. Without it, the best flavored apple
remains a crab; with it, it becomes a conquest. According to the method
of van Mons it may be reached within [78] two or three generations, and
a man's life is wholly sufficient to produce in this way many new types
of the very best sorts, as van Mons himself has done. It is done in the
usual way, sowing on a large scale and selecting the best, which are in
their turn brought to an early maturation of their fruit by grafting,
because thereby the life from seed to seed may be reduced to a few
years.
Form, taste, color, flavor and other valuable marks of new varieties are
the products of nature, says van Mons, only texture, fleshiness and size
are added by man. And this is done in each new variety by the same
method and according to the same laws. The richness of the cultivated
apples of the present day was already present in the large range of
original wild elementary species, though unobserved and requiring
improvement.
An interesting proof of this principle is afforded by the experience of
Mr. Peter M. Gideon, as related by Bailey. Gideon sowed large quantities
of apple-seeds, and one seed produced a new and valuable variety called
by him the "Wealthy" apple. He first planted a bushel of apple-seeds,
and then every year, for nine years, planted enough seeds to produce a
thousand trees. At the end of ten years all seedlings had perished
except one hardy seedling [79] crab. This experiment was made in
Minnesota, and failed wholly. Then he bought a small lot of seeds of
apples and crab-apples in Maine and from these the "Wealthy" came. There
were only about fifty seeds in the lot of crab-apple seed which produced
the "Wealthy," but before this variety was obtained, more than a bushel
of seed had been sown. Chance afforded a species with an unknown taste;
but the growing of many thousands of seedlings of known varieties was
not the best means to get something really new.
Pears are more difficult to improve than apples. They often require six
or more generations to be brought from the wild woody state to the
ordinary edible condition. But the varieties each seem to have a
separate origin, as with apples, and the wide range of form and of taste
must have been present in the wild state, long before cultivation. Only
recently has the improvement of cherries, plums, currants and
gooseberries been undertaken with success by Mr. Burbank, and the
difference between the wild and cultivated forms has hitherto been very
small. All indications point to the existence, before the era of
cultivation, of larger or smaller numbers of elementary species.
The same holds good with many of the larger forage crops and other
plants of great industrial [80] value. Clover exhibits many varieties,
which have been cultivated indiscriminately, and often in motley
mixtures. The flower heads may be red or white, large or small,
cylindric or rounded, the leaves are broader or narrower, with or
without white spots of a curious pattern. They may be more or less hairy
and so forth. Even the seeds exhibit differences in size, shape or
color, and of late Martinet has shown, that by the simple means of
picking out seeds of the same pattern, pure strains of clover may be
obtained, which are of varying cultural value. In this way the best
subspecies or varieties may be sought out for separate cultivation. Even
the white spots on the leaflets have proved to be constant characters
corresponding with noticeable differences in yield.
Flax is another instance. It was already cultivated, or at least made
use of during the period of the lake-dwellers, but at that time it was a
species referred to as _Linum angustifolium_, and not the _Linum
usitatissimum_, which is our present day flax. There are now many
subspecies, elementary species, and varieties under cultivation. The
oldest of them is known as the "springing flax," in opposition to the
ordinary "threshing flax." It has capsules which open of themselves, in
order to disseminate the seeds, while the ordinary heads of the [81]
flax remain closed until the seeds are liberated by threshing. It seems
probable that the first form or _Linum crepitans_ might thrive in the
wild state as well as any other plant, while in the common species those
qualities are lacking which are required for a normal dissemination of
the seeds. White or blue flowers, high or dwarf stems, more or less
branching at the base and sundry other qualities distinguish the
varieties, aside from the special industrial difference of the fibres.
Even the life-history varies from annual and biennial, to perennial.
It would take us too long to consider other instances. It is well known
that corn, though considered as a single botanical species, is
represented by different subspecies and varieties in nearly every region
in which it is grown. Of course its history is unknown and it is
impossible to decide whether all the tall and dwarf forms, or starchy
and sweet varieties, dented or rounded kernels, and hundreds of others
are older than culture or have come into existence during historic
times, or as some assume, through the agency of man. But our main point
now is not the origin, but only the existence of constant and sharply
differentiated forms within botanical species. Nearly every cultivated
plant affords instances of such diversity. Some include a few types
only, while [82] others show, a large number of forms clearly separated
to a greater or lesser degree.
In some few instances it is obvious that this variability is of later
date than culture. The most conspicuous case is that of the coconut.
This valuable palm is found on nearly all tropical coasts, in America,
as well as in Asia, but in Africa and Australia there are many hundreds
of miles of shore line, where it is not found. Its importance is not at
all the same everywhere. On the shores and islands of the Indian Ocean
and the Malay Archipelago, man is chiefly dependent upon it, but in
America it is only of subordinate usefulness.
In connection with these facts, it abounds in subspecies and varieties
in the East Indian regions, but on the continent of America little
attention has as yet been given to its diverging qualities. In the
Malayan region it affords nearly all that is required by the
inhabitants. The value of its fruit as food, and the delicious beverage
which it yields, are well known. The fibrous rind is not less useful; it
is manufactured into a kind of cordage, mats and floor-cloths. An
excellent oil is obtained from the kernel by compression. The hard
covering of the stem is converted into drums and used in the
construction of huts; the lower part is so hard as to take on a
beautiful polish [83] when it resembles agate. Finally the unexpanded
terminal bud is a delicate article of food. Many other uses could be
mentioned, but these may suffice to indicate how closely the life of the
inhabitants is bound up with the culture of this palm, and how sharply,
in consequence, its qualities must have been watched by early man. Any
divergence from the ordinary type must have been noted; those which were
injurious must have been rejected, but the useful ones must have been
appreciated and propagated. In a word any degree of variability afforded
by nature must have been noticed and cultivated.
More than fifty different sorts of the coconut are described from the
Indian shores and islands, with distinct local and botanical names.
Miquel, who was one of the best systematists of tropical plants, of the
last century, described a large number of them, and since, more have
been added. Nearly all useful qualities vary in a higher or lesser
degree in the different varieties. The fibrous strands of the rind of
the nut are developed in some forms to such a length and strength as to
yield the industrial product known as the coir-fibre. Only three of them
are mentioned by Miquel that have this quality, the _Cocos nucifera
rutila_, _cupuliformis_ and _stupposa_. Among them the _rutila_ [84]
yields the best and most supple fibres, while those of the _stupposa_
are stiff and almost unbending.
The varieties also differ greatly in size, color, shape and quality, and
the trees have also peculiar characteristics. One variety exhibits
leaves which are nearly entire, the divisions being only imperfectly
separated, as often occurs in the very first leaves of the seedlings of
other varieties. The flavor of the flesh, oil and milk likewise yield
many good varietal marks.
In short, the coconut-palm comes under the general rule, that botanical
species are built up of a number of sharply distinguishable types, which
prove their constancy and relative independence by their wide
distribution in culture. In systematic works all these forms are called
varieties, and a closer investigation of their real systematic value has
not yet been made. But the question as to the origin of the varieties
and of the coconut itself has engrossed the attention of many botanists,
among whom are De Candolle in the middle of the last century, and Cook
at its close.
Both questions are closely connected. De Candolle claimed an Asiatic
origin for the whole species, while Cook's studies go to prove that its
original habitat is to be sought in the northern countries of South
America. Numerous [85] varieties are growing in Asia and have as yet not
been observed to occur in America, where the coconut is only of
subordinate importance, being one of many useful plants, and not the
only one relied upon by the natives for their subsistence. If therefore,
De Candolle's opinion is the right one, the question as to whether the
varieties are older or younger than the cultivated forms of the species,
must always remain obscure. But if the proofs of an American origin
should be forthcoming, the possibility, and even the probability that
the varieties are of later date than the beginning of their culture, and
have originated while in this condition must at once be granted. An
important point in the controversy is the manner in which the coconuts
were disseminated from shore to shore, from island to island. De
Candolle, Darwin and most of the European writers claim that the
dispersal was by natural agencies, such as ocean-currents. They point
out that the fibrous rind or husk would keep the fruits afloat, and
uninjured, for many days or even many weeks, while being carried from
one country to another in a manner that would explain their geographic
distribution. But the probability of the nuts being thrown upon the
strand, and far enough from the shore to find suitable conditions for
their germination, is a very small one. To insure [86] healthy and
vigorous seedlings the nuts must be fully ripe, after which planting
cannot be safely delayed for more than a few weeks. If kept too moist
the nuts rot. If once on the shore, and allowed to lie in the sun, they
become overheated and are thereby destroyed; if thrown in the shade of
other shrubs and trees, the seedlings do not find the required
conditions for a vigorous growth.
Some authors have taken the fibrous rind to be especially adapted to
transport by sea, but if this were so, this would argue that water is
the normal or at least the very frequent medium of dissemination, which
of course it is not. We may, claim with quite as much right that the
thick husk is necessary to enable the heavy fruit to drop from tall
trees with safety. But even for this purpose the protection is not
sufficient, as the nuts often suffer from falling to such a degree as to
be badly injured as to their germinating qualities. It is well known
that nuts, which are destined for propagation, are as a rule not allowed
to fall off, but are taken from the trees with great care.
Summing up his arguments, Cook concludes that there is little in the way
of known facts to support the poetic theory of the coconut palm dropping
its fruits into the sea to float away to barren islands and prepare them
for [87] human habitation. Shipwrecks might furnish a successful method
of launching viable coconuts, and such have no doubt sometimes
contributed to their distribution. But this assumption implies a
dissemination of the nuts by man, and if this principal fact is granted,
it is far more natural to believe in a conscious intelligent
dissemination.
The coconut is a cultivated tree. It may be met with in some spots
distant from human dwellings, but whenever such cases have been
subjected to a closer scrutiny, it appears that evidently, or at least
probably, huts had formerly existed in their neighborhood, but having
been destroyed by some accident, had left the palm trees uninjured. Even
in South America, where it may be found in forests at great distances
from the sea-shore, it is not at all certain that true native localities
occur, and it seems to be quite lost in its natural condition.
Granting the cultivated state of the palms as the only really important
one, and considering the impossibility or at least great improbability
of its dissemination by natural means, the distribution by man himself,
according to his wants, assumes the rank of an hypothesis fully adequate
to the explanation of all the facts concerning the life-history of the
tree.
We now have to inquire into the main question, [88] whether it is
probable that the coconut is of American or of Asiatic origin, leaving
aside the historic evidence which goes to prove that nothing is known
about the period in which its dissemination from one hemisphere to
another took place, we will now consider only the botanic and geographic
evidence, brought forward by Cook. He states that the whole family of
coconut-palms, consisting of about 20 genera and 200 species, are all
strictly American with the exception of the rather aberrant African
oilpalm, which has, however, an American relative referred to the same
genus. The coconut is the sole representative of this group which is
connected with Asia and the Malayan region, but there is no manifest
reason why other members of the same group could not have established
themselves there, and maintained an existence under conditions, which
are not at all unfavorable to them. The only obvious reason is the
assumption already made, that the distribution was brought about by man,
and thus only affected the species, chosen by him for cultivation. That
the coconut cannot have been imported from Asia into America seems to be
the most obvious conclusion from the arguments given. It should be
briefly noted, that it was known and widely distributed in tropical
America at the time of the discovery of that continent [89] by Columbus,
according to accounts of Oviedo and other contemporary Spanish writers.
Concluding we may state that according to the whole evidence as it has
been discussed by De Candolle and especially by Cook, the coconut-palm
is of American origin and has been distributed as a cultivated tree by
man through the whole of its wide range. This must have happened in a
prehistoric era, thus affording time enough for the subsequent
development of the fifty and more known varieties. But the possibility
that at least some of them have originated before culture and have been
deliberately chosen by man for distribution, of course remains
unsettled.
Coconuts are not very well adapted for natural dispersal on land, and
this would rather induce us to suppose an origin within the period of
cultivation for the whole group. There are a large number of cultivated
varieties of different species which by some peculiarity do not seem
adapted for the conditions of life in the wild state. These last have
often been used to prove the origin of varietal forms during culture.
One of the oldest instances is the variety or rather subspecies of the
opium-poppy, which lacks the ability to burst open its capsules. The
seeds, which are thrown out by the wind, in the common forms, through
the apertures underneath [90] the stigma, remain enclosed. This is
manifestly a very useful adaptation for a cultivated plant, as by this
means no seeds are lost. It would be quite a disadvantage for a wild
species, and is therefore claimed to have been connected from the
beginning with the cultivated form.
The large kernels of corn and grain, of beans and peas, and even of the
lupines were considered by Darwin and others to be unable to cope with
natural conditions of life. Many valuable fruits are quite sterile, or
produce extremely few seeds. This is notoriously the case with some of
the best pears and grapes, with the pine-apples, bananas, bread-fruits,
pomegranate and some members of the orange tribe. It is open to
discussion as to what may be the immediate cause of this sterility, but
it is quite evident, that all such sterile varieties must have
originated in a cultivated condition. Otherwise they would surely have
been lost.
In horticulture and agriculture the fact that new varieties arise from
time to time is beyond all doubt, and it is not this question with which
we are now concerned. Our arguments were only intended to prove that
cultivated species, as a rule, are derived from wild species, which obey
the laws discussed in a previous lecture. The botanic units are compound
entities, and [91] the real systematic units in elementary species play
the same part as in ordinary wild species. The inference that the origin
of the cultivated plants is multiple, in most cases, and that more than
one, often many separate elementary forms of the same species must
originally have been taken into cultivation, throws much light upon many
highly important problems of cultivation and selection. This aspect of
the question will therefore be the subject of the next lecture.
[92]
LECTURE IV
SELECTION OF ELEMENTARY SPECIES
The improvement of cultivated plants must obviously begin with already
existing forms. This is true of old cultivated sorts as well as for
recent introductions. In either case the starting-point is as important
as the improvement, or rather the results depend in a far higher degree
on the adequate choice of the initial material than on the methodical
and careful treatment of the chosen varieties. This however, has not
always been appreciated as it deserves, nor is its importance at present
universally recognized. The method of selecting plants for the
improvement of the race was discovered by Louis Vilmorin about the
middle of the last century. Before his time selection was applied to
domestic animals, but Vilmorin was the first to apply this principle to
plants. As is well known, he used this method to increase the amount of
sugar in beets and thus to raise their value as forage-crops, with such
success, that his plants have since been used for the production [93] of
sugar. He must have made some choice among the numerous available sorts
of beets, or chance must have placed in his hands one of the most
appropriate forms. On this point however, no evidence is at hand.
Since the work of Vilmorin the selection-principle has increased
enormously in importance, for practical purposes as well as for the
theoretical aspect of the subject. It is now being applied on a large
scale to nearly all ornamental plants. It is the one great principle now
in universal practice as well as one of preeminent scientific value. Of
course, the main arguments of the evolution theory rest upon
morphologic, systematic, geographic and paleontologic evidence. But the
question as to how we can coordinate the relation between existing
species and their supposed ancestors is of course one of a physiologic
nature. Direct observation or experiments were not available for Darwin
and so he found himself constrained to make use of the experience of
breeders. This he did on a broad scale, and with such success that it
was precisely this side of his arguments that played the major part in
convincing his contemporaries.
The work of the breeders previous to Darwin's time had not been very
critically performed. Recent analyses of the evidence obtained [94] from
them show that numerous types of variability were usually thrown
together. What type in each case afforded the material, which the
breeder in reality made use of, has only been inquired into in the last
few decades. Among those who have opened the way for thorough and more
scientific treatment are to be mentioned Rimpau and Von Rumker of
Germany and W.M. Hays of America.
Von Rumker is to be considered as the first writer, who sharply
distinguished between two phases of methodical breeding-selection. One
side he calls the production of new forms, the other the improvement of
the breed. He dealt with both methods extensively. New forms are
considered as spontaneous variations occurring or originating without
human aid. They have only to be selected and isolated, and their progeny
at once yields a constant and pure race. This race retains its character
as long as it is protected against the admixture of other minor
varieties, either by cross-pollination, or by accidental seeds.
Improvement, on the other hand, is the work of man. New varieties of
course can only be isolated if chance offers them; the improvement is
not incumbent on chance. It does not create really anything new, but
develops characters, which were already existing. It brings [95] the
race above its average, and must guard constantly against the regression
towards this average which usually takes place.
Hays has repeatedly insisted upon the principle of the choice of the
most favorable varieties as the foundation for all experiments in
improving races. He asserts that half the battle is won by choosing the
variety which is to serve as a foundation stock, while the other half
depends upon the selection of parent-plants within the chosen variety.
Thus the choice of the variety is the first principle to be applied in
every single case; the so-called artificial selection takes only a
secondary place. Calling all minor units within the botanic species by
the common name of varieties, without regard to the distinction between
elementary species and retrograde varieties, the principle is designated
by the term of "variety-testing." This testing of varieties is now, as
is universally known, one of the most important lines of work of the
agricultural experiment stations. Every state and every region, in some
instances even the larger farms, require a separate variety of corn, or
wheat, or other crops. They must be segregated from among the hundreds
of generally cultivated forms, within each single botanic species. Once
found, the type may be ameliorated according to the local conditions
[96] and needs, and this is a question of improvement.
The fact that our cultivated plants are commonly mixtures of different
sorts, has not always been known. The first to recognize it seems to
have been the Spanish professor of botany, Mariano Lagasca, who
published a number of Spanish papers dealing with useful plants and
botanical subjects between 1810 and 1830, among them a catalogue of
plants cultivated in the Madrid Botanical Garden. Once when he was on a
visit to Colonel Le Couteur on his farm in Jersey, one of the Channel
Islands off the coast of France, in discussing the value of the fields
of wheat, he pointed out to his host, that they were not really pure and
uniform, as was thought at that time, and suggested the idea that some
of the constituents might form a larger part in the harvest than others.
In a single field he succeeded in distinguishing no less than 23
varieties, all growing together. Colonel Le Couteur took the hint, and
saved the seeds of a single plant of each supposed variety separately.
These he cultivated and multiplied till he got large lots of each and
could compare their value. From among them he then chose the variety
producing the greatest amount of the finest, whitest and most nutritious
flour. This he eventually placed in the [97] market under the name of
"Talavera de Bellevue." It is a tall, white variety, with long and
slender white heads, almost without awns, and with fine white pointed
kernels. It was introduced into commerce about 1830, and is still one of
the most generally cultivated French wheats. It was highly prized in the
magnificent collection of drawings and descriptions of wheats, published
by Vilmorin under the title "Les meilleurs bles" and is said to have
quite a number of valuable qualities, branching freely and producing an
abundance of good grain and straw. It is however, sensitive to cold
winters in some degree and thereby limited in its distribution. Hallett,
the celebrated English wheat-breeder, tried in vain to improve the
peculiar qualities of this valuable production of Le Couteur's.
Le Couteur worked during many years along this line, long before the
time when Vilmorin conceived the idea of improvement by race selections,
and he used only the simple principle of distinguishing and isolating
the members of his different fields. Later he published his results in a
work on the varieties, peculiarities and classification of wheat (1843),
which though now very rare, has been the basis and origin of the
principle of variety-testing.
The discovery of Lagasca and Le Couteur was [98] of course not
applicable to the wheat of Jersey alone. The common cultivated sorts of
wheat and other grains were mixtures then as they are even now. Improved
varieties are, or at least should be, in most cases pure and uniform,
but ordinary sorts, as a rule, are mixtures. Wheat, barley and oats are
self-fertile and do not mix in the field through cross-pollination.
Every member of the assemblage propagates itself, and is only checked by
its own greater or less adaptation to the given conditions of life.
Rimpau has dealt at large with the phenomenon as it occurs in the
northern and middle parts of Germany. Even Rivett's "Bearded wheat,"
which was introduced from England as a fine improved variety, and has
become widely distributed throughout Germany, cannot keep itself pure.
It is found mingled almost anywhere with the old local varieties, which
it was destined to supplant. Any lot of seed exhibits such impurities,
as I have had the opportunity of observing myself in sowings in the
experimental-garden. But the impurities are only mixtures, and all the
plants of Rivett's "Bearded wheat," which of course constitute the large
majority, are of pure blood. This may be confirmed when the seeds are
collected and sown separately in cultures that can be carefully guarded.
[99] In order to get a closer insight into the causes of this confused
condition of ordinary races, Rimpau made some observations on Rivett's
wheat. He found that it suffers from frost during winter more than the
local German varieties, and that from various causes, alien seeds may
accidentally, and not rarely, become mixed with it. The
threshing-machines are not always as clean as they should be and may be
the cause of an accidental mixture. The manure comes from stables, where
straw and the dust from many varieties are thrown together, and
consequently living kernels may become mixed with the dung. Such stray
grains will easily germinate in the fields, where they find more
congenial conditions than does the improved variety. If winter arrives
and kills quantities of this latter, the accidental local races will
find ample space to develop. Once started, they will be able to multiply
so rapidly, that in one or two following generations they will
constitute a very considerable portion of the whole harvest. In this way
the awnless German wheat often prevails over the introduced English
variety, if the latter is not kept pure by continuous selection.
The Swiss wheat-breeder Risler made an experiment which goes to prove
the certainty of the explanation given by Rimpau. He observed on his
farm at Saleves near the lake of Geneva that after a lapse of time the
"Galland wheat" deteriorated and assumed, as was generally believed, the
characters of the local sorts. In order to ascertain the real cause of
this apparent change, he sowed in alternate rows in a field, the
"Galland" and one of the local varieties. The "Galland" is a race with
obvious characters and was easily distinguished from the other at the
time when the heads were ripe. They are bearded when flowering, but
afterwards throw off the awns. The kernels are very large and yield an
extraordinarily good, white flour.
During the first summer all the heads of the "Galland" rows had the
deciduous awns but the following year these were only seen on half of
the plants, the remainder having smooth heads, and the third year the
"Galland" had nearly disappeared, being supplanted by the competing
local race. The cause of this rapid change was found to be twofold.
First the "Galland," as an improved variety, suffers from the winter in
a far higher degree than the native Swiss sorts, and secondly it ripens
its kernels one or two weeks later. At the time of harvest it may not
have become fully ripe, while the varieties mixed with it had reached
maturity. The wild oat, _Avena fatua_, is very common in [101] Europe
from whence it has been introduced in the United States. In summers
which are unfavorable to the development of the cultivated oats it may
be observed to multiply with an almost incredible rapidity. It does not
contribute to the harvest, and is quite useless. If no selection were
made, or if selection were discontinued, it would readily supplant the
cultivated varieties.
From these several observations and experiments it may be seen, that it
is not at all easy to keep the common varieties of cereals pure and that
even the best are subject to the encroachment of impurities. Hence it is
only natural that races of cereals, when cultivated without the utmost
care, or even when selected without an exact knowledge of their single
constituents, are always observed to be more or less in a mixed
condition. Here, as everywhere with cultivated and wild plants, the
systematic species consist of a number of minor types, which pertain to
different countries and climates, and are growing together in the same
climate and under the same external conditions. They do not mingle, nor
are their differentiating characters destroyed by intercrossing. They
each remain pure, and may be isolated whenever and wherever the
desirability for such a proceeding should arise. The purity of [102] the
races is a condition implanted in them by man, and nature always strives
against this arbitrary and one-sided improvement. Numerous slight
differences in characters and numerous external influences benefit the
minor types and bring them into competition with the better ones.
Sometimes they tend to supplant the latter wholly, but ordinarily sooner
or later a state of equilibrium is reached, in which henceforth the
different sorts may live together. Some are favored by warm and others
by cool summers, some are injured by hard winters while others thrive
then and are therefore relatively at an advantage. The mixed condition
is the rule, purity is the exception.
Different sorts of cereals are not always easily distinguishable by the
layman and therefore I will draw your attention to conditions in
meadows, where a corresponding phenomenon can be observed in a much
simpler way.
Only artificial pasture-grounds are seen to consist of a single species
of grass or clover. The natural condition in meadows is the occurrence
of clumps of grasses and some clovers, mixed up with perhaps twenty or
more species of other genera and families. The numerical proportion of
these constituents is of great interest, and has been studied at
Rothamstead in England and on a number of other farms. It is [103]
always changing. No two successive years show exactly the same
proportions. At one time one species prevails, at another time one or
two or more other species. The weather during the spring and summer
benefits some and hurts others, the winter may be too cold for some, but
again harmless for others, the rainfall may partly drown some species,
while others remain uninjured. Some weeds may be seen flowering
profusely during some years, while in other summers they are scarcely to
be found in the same meadow. The whole population is in a fluctuating
state, some thriving and others deteriorating. It is a continuous
response to the ever changing conditions of the weather. Rarely a
species is wholly annihilated, though it may apparently be so for years;
but either from seeds or from rootstocks, or even from neighboring
lands, it may sooner or later regain its foothold in the general
struggle for life.
This phenomenon is a very curious and interesting one. The struggle for
life, which plays so considerable a part in the modern theories of
evolution, may be seen directly at work. It does not alter the species
themselves, as is commonly supposed, but it is always changing their
numerical proportion. Any lasting change in the external conditions will
of course alter the average oscillation and the influence [104] of such
alterations will manifest itself in most cases simply in new numerical
proportions. Only extremes have extreme effects, and the chance for the
weaker sorts to be completely overthrown is therefore very small.
Any one, who has the opportunity of observing a waste field during a
series of years, should make notes concerning the numerical proportions
of its inhabitants. Exact figures are not at all required; approximate
estimates will ordinarily prove to be sufficient, if only the standard
remains the same during the succeeding years.
The entire mass of historic evidence goes to prove that the same
conditions have always prevailed, from the very beginning of cultivation
up to the present time. The origin of the cultivation of cereals is to
be sought in central Asia. The recent researches of Solms Laubach show
it to be highly probable that the historic origin of the wheat
cultivated in China, is the same as that of the wheat of Egypt and
Europe. Remains of cereals are found in the graves of Egyptian mummies,
in the mounds of waste material of the lake-dwellings of Central Europe,
and figures of cereals are to be seen on old Roman coins. In the
sepulchre of King Ra-n-Woser of the Fifth Dynasty of Egypt, who lived
about 2000 years B.C., two [105] tombs have recently been opened by the
German Oriental Society. In them were found quantities of the tares of
the _Triticum dicoccum_, one of the more primitive forms of wheat. In
other temples and pyramids and among the stones of the walls of Dashur
and El Kab studied by Unger, different species and varieties of cereals
were discovered in large quantities, that showed their identity with the
present prevailing cultivated races of Egypt.
The inhabitants of the lake-dwellings in Switzerland possessed some
varieties of cereals, which have entirely disappeared. They are
distinguished by Heer under special names. The small barley and the
small wheat of the lake-dwellers are among them. All in all there were
ten well distinguished varieties of cereals, the Panicum and the Setaria
or millet being of the number. Oats were evidently introduced only
toward the very last of the lake-dwelling period, and rye is of far
later introduction into western Europe. Similar results are attained by
the examination of the cereals figured by the Romans of the same period.
All these are archaeologic facts, and give but slight indications
concerning the methods of cultivation or the real condition of the
cultivated races of that time. Virgil has left us some knowledge of the
requirements of methodical [106] culture of cereals of his time. In his
poem _Georgics_ (I. 197) the following lines are found:
_Vidi lecta din, et multo spectata labore
Degenerare tamen, ni vis humana quotannis
Maxima quaeque manu legeret_.
(The chosen seed, through years and labor improved,
Was seen to run back, unless yearly
Man selected by hand the largest and fullest of ears.)
Elsewhere Virgil and also some lines of Columella and Varro go to prove
in the same way that selection was applied by the Romans to their
cereals, and that it was absolutely necessary to keep their races pure.
There is little doubt, but that it was the same principle as that which
has led, after many centuries, to the complete isolation and improvement
of the very best races of the mixed varieties. It further proves that
the mixed conditions of the cereals was known to man at that time,
although distinct ideas of specific marks and differences were of course
still wholly lacking. It is proof also that cultivated cereals from the
earliest times must have been built up of numerous elementary forms.
Moreover it is very probable, that in the lapse of centuries a goodly
number of such types must have disappeared. [107] Among the vanished
forms are the special barley and wheat of the lake-dwellings, the
remains of which have been accidentally preserved, but most of the forms
must have disappeared without leaving any trace.
This inference is supported by the researches of Solms-Laubach, who
found that in Abyssinia numerous primitive types of cereals are still in
culture. They are not adequate to compete with our present varieties,
and would no doubt also have disappeared, had they not been preserved by
such quite accidental and almost primitive isolation.
Closing this somewhat long digression into history we will now resume
our discussion concerning the origin of the method of selecting cereals
for isolation and segregate-cultivation. Some decades after Le Couteur,
this method was taken up by the celebrated breeder Patrick Sheriff of
Haddington in Scotland. His belief, which was general at that time, was
"That cultivation has not been found to change well defined kinds, and
that improvement can be best attained by selecting new and superior
varieties, which nature occasionally produces, as if inviting the
husbandman to stretch forth his hand and cultivate them."
Before going into the details of Sheriff's work it is as well to say
something concerning [108] the use of the word "selection." This word
was used by Sheriff as seen in the quotation given, and it was obviously
designed to convey the same idea as the word "lecta" in the quotation
from Virgil. It was a choice of the best plants from among known mixed
fields, but the chosen individuals were considered to be representatives
of pure and constant races, which could only be isolated, but not
ameliorated. Selection therefore, in the primitive sense of the word, is
the choice of elementary species and varieties, with no other purpose
than that of keeping them as pure as possible from the admixture of
minor sorts. The Romans attained this end only imperfectly, simply
because the laws governing the struggle for life and the competition of
numerous sorts in the fields were unsuspected by them.
Le Couteur and Sheriff succeeded in the solution of the problem, because
they had discovered the importance of isolation. The combination of a
careful choice with subsequent isolation was all they knew about it, and
it was one of the great achievements to which modern agriculture owes
its success.
The other great principle was that of Vilmorin. It was the improvement
within the race, or the "amelioration of the race" as it was termed by
him. It was introduced into [109] England by F.F. Hallett of Brighton in
Sussex, who at once called it "pedigree-culture," and produced his first
new variety under the very name of "Pedigree-wheat." This principle,
which yields improved strains, that are not constant but dependent on
the continued and careful choice of the best plants in each succeeding
generation, is now generally called "selection." But it should always be
remembered that according to the historic evolution of the idea, the
word has the double significance of the distinction and isolation of
constant races from mixtures, and that of the choice of the best
representatives of a race during all the years of its existence. Even
sugar-beets, the oldest "selected" agricultural plants, are far from
having freed themselves from the necessity of continuous improvement.
Without this they would not remain constant, but would retrograde with
great rapidity.
The double meaning of the word selection still prevailed when Darwin
published his "Origin of Species." This was in the year 1859, and at
that time Shirreff was the highest authority and the most successful
breeder of cereals. Vilmorin's method had been applied only to beets,
and Hallett had commenced his pedigree-cultures only a few years before
and his first publication of the "Pedigree-wheat" [110] appeared some
years later at the International Exhibition of London in 1862. Hence,
whenever Darwin speaks of selection, Shirreff's use of the word may as
well be meant as that of Vilmorin.
However, before going deeper into such theoretical questions, we will
first consider the facts, as given by Shirreff himself.
During the best part of his life, in fact during the largest part of the
first half of the nineteenth century, Shirreff worked according to a
very simple principle. When quite young he had noticed that sometimes
single plants having better qualities than the average were seen in the
fields. He saved the grains, or sometimes the whole heads of such plants
separately, and tried to multiply them in such manner as to avoid
intermixtures.
His first result was the "Mungoswell's wheat." In the spring of 1819 he
observed quite accidentally in a field of the farm of that name, a
single plant which attracted his attention by a deeper green and by
being more heavily headed out. Without going into further details, he at
once chose this specimen as the starting point of a new race. He
destroyed the surrounding plants so as to give it more space, applied
manure to its roots, and tended it with special care. It yielded 63
heads and nearly [111] 2500 grains. All of these were sown the
following fall, and likewise in the succeeding years the whole harvest
was sown in separate lots. After two years of rapid multiplication it
proved to be a good new variety and was brought into commerce. It has
become one of the prominent varieties of wheat in East Lothian, that
county of Scotland of which Haddington is the principal borough.
The grains of "Mungoswell's wheat" are whiter than those of the allied
"Hunter's wheat," more rounded but otherwise of the same size acid
weight. The straw is taller and stronger, and each plant produces more
culms and more heads.
Shirreff assumed, that the original plant of this variety was a sport
from the race in which he had found it, and that it was the only
instance of this sport. He gives no details about this most interesting
side of the question, omitting even to tell the name of the parent
variety. He only asserts that it was seen to be better, and afterwards
proved so by the appreciation of other breeders and its success in
trade. He observed it to be quite constant from the beginning, no
subsequent selection being needed. This important feature was simply
assumed by him to be true as a matter of course.
[112] Some years afterwards, in the summer of 1824, he observed a large
specimen of oats in one of the fields of the same farm. Being at that
time occupied in making a standard collection of oats for a closer
comparison of the varieties, he saved the seeds of that plant and sowed
them in a row in his experiment-field. It yielded the largest culms of
the whole collection and bore long and heavy kernels with a red streak
on the concave side and it excelled all other sorts by the fine
qualities of its very white meal. In the unequal length of its stalks it
has however a drawback, as the field appears thinner and more meager
than it is in reality. "Hopetown oats," as it is called, has found its
way into culture extensively in Scotland and has even been introduced
with success into England, Denmark and the United States. It has been
one of the best Scottish oats for more than half a century.
The next eight years no single plant judged worthy of selection on his
own farm attracted Shirreff's attention. But in the fall of 1832 he saw
a beautiful plant of wheat on a neighboring farm and he secured a head
of it with about 100 grains. From this he produced the "Hopetown wheat."
After careful separation from the kernels this original ear was
preserved, and was afterwards exhibited at the Stirling Agricultural
[113] Museum. The "Hopetown wheat" has proved to be a constant variety,
excelling the ordinary "Hunter's wheat" by larger grains and longer
heads; it yields likewise a straw of superior quality and has become
quite popular in large districts of England and Scotland, where it is
known by the name of "White Hunter's" from its origin and the brilliant
whiteness of its heads.
In the same way Shirreff's oats were discovered in a single plant in a
field where it was isolated in order to be brought into commerce after
multiplication. It has won the surname of "Make-him-rich." Nothing is on
record about the details of its origin.
Four valuable new varieties of wheat and oats were obtained in this way
in less than forty years. Then Shirreff changed his ideas and his method
of working. Striking specimens appeared to be too rare, and the
expectation of a profitable result too small. Therefore he began work on
a larger scale. He sought and selected during the summer of 1857 seventy
heads of wheat, each from a single plant showing some marked and
presumably favorable peculiarity. These were not gathered on one field,
but were brought together from all the fields to which he had access in
his vicinity. The grains of each of these selected heads were [114] sown
separately, and the lots compared during their whole life-period and
chiefly at harvest time. Three of the lots were judged of high
excellence, and they alone were propagated, and proving to be constant
new varieties from the outset were given to the trade under the names of
"Shirreff's bearded white," "Shirreff's bearded red," and "Pringle's
wheat." They have found wide acceptance, and the first two of them are
still considered by Vilmorin as belonging to the best wheats of France.
This second method of Shirreff evidently is quite analogous to the
principle of Lagasca and Le Couteur. The previous assumption that new
varieties with striking features were being produced by nature from time
to time, was abandoned, and a systematic inquiry into the worth of all
the divergent constituents of the fields was begun. Every single ear at
once proved to belong to a constant and pure race, but most of these
were only of average value. Some few however, excelled to a degree,
which made them worth multiplying, and to be introduced into trade as
separate varieties.
Once started, this new method of comparison, selection and isolated
multiplication was of course capable of many improvements. The culture
in the experiment-field was improved, so as to insure a fuller and more
rapid growth.
[115] The ripe heads had to be measured and counted and compared with
respect to their size and the number of their kernels. Qualities of
grain and of meal had to be considered, and the influence of climate and
soil could not be overlooked.
Concerning the real origin of his new types Shirreff seems never to have
been very inquisitive. He remarks that only the best cultivated
varieties have a chance to yield still better types, and that it is
useless to select and sow the best heads of minor sorts. He further
remarks that it is not probable that he found a new sport every time; on
the contrary he assumes that his selections had been present in the
field before, and during a series of succeeding generations. How many
years old they were, was of course impossible to determine. But there is
no reason to believe that the conditions in the fields of Scotland were
different from those observed on the Isle of Jersey by Le Couteur.
In the year 1862 Shirreff devoted himself to the selection of oats,
searching for the best panicles from the whole country, and comparing
their offspring in his experimental garden. "Early Fellow," "Fine
Fellow," "Longfellow" and "Early Angus" are very notable varieties
introduced into trade in this way.
[116] Some years later Patrick Shirreff described his experiments and
results in a paper entitled, "On the improvement of cereals," but the
descriptions are very short, and give few details of systematic value.
The leading principle, however, is clearly indicated, and anyone who
studies with care his method of working, may confidently attempt to
improve the varieties of his own locality in the same way.
This great principle of "variety-testing," as it has been founded by Le
Couteur and Patrick Shirreff, has increased in importance ever since.
Two main features are to be considered here. One is the production of
local races, the other the choice of the best starting-point for
hybridizing experiments, as is shown in California by the work of Luther
Burbank in crossing different elementary species of _Lilium pardalinum_
and others.
Every region and locality has its own conditions of climate and soil.
Any ordinary mixed race will contain some elementary forms which are
better adapted to a given district, while others are more suitable to
divergent conditions. Hence it can readily be inferred that the choice
cannot be the same for different regions. Every region should select its
own type from among the various forms, and variety testing therefore
becomes a task which every [117] one must undertake under his own
conditions. Some varieties will prove, after isolation, to be profitable
for large districts and perhaps for whole states. Others will be found
to be of more local value, but in such localities to excel all others.
As an example we may take one of the varieties of wheat originated by
the Minnesota Experiment Station. Hays described it as follows. It was
originated from a single plant. From among 400 plants of "Blue stem"
several of the best were chosen, each growing separately, a foot apart
in every direction. Each of the selected plants yielded 500 or more
grains of wheat, weighing 10 or more grams. The seeds from these
selected plants were raised for a few years until sufficient was
obtained to sow a plot. Then for several years the new strains were
grown in a field beside the parent-variety. One of them was so much
superior that all others were discarded. It was the one named "Minnesota
No. 169." For a large area of Minnesota this wheat seems capable of
yielding at least 1 or 2 bushels more grain per acre than its parent
variety, which is the best kind commonly and almost universally found on
the farms in southern and central Minnesota.
It would be quite superfluous for our present purpose to give more
instances. The fact of [118] the compound nature of so-called species of
cultivated plants seems to be beyond all doubt, and its practical
importance is quite obvious.
Acclimatization is another process, which is largely dependent on the
choice of adequate varieties. This is shown on a large scale by the slow
and gradual dispersion of the varieties of corn in this country. The
largest types are limited to temperate and subtropical regions, while
the varieties capable of cultivation in more northern latitudes are
smaller in size and stature and require a smaller number of days to
reach their full development from seed to seed. Northern varieties are
small and short lived, but the "Forty-day-corn" or "Quarantino maize" is
recorded to have existed in tropical America at the time of Columbus. In
preference, or rather to the entire exclusion of taller varieties, it
has thriven on the northern boundaries of the corn-growing states of
Europe since the very beginning of its cultivation.
According to Naudin, the same rule prevails with melons, cucumbers and
gherkins, and other instances could easily be given.
Referring now to the inferences that may be drawn from the experience of
the breeders in order to elucidate the natural processes, we will return
to the whitlow-grasses and pansies.
[119] Nature has constituted them as groups of slightly different
constant forms, quite in the same way as wheat and oats and corn.
Assuming that this happened ages ago somewhere in central Europe, it is
of course probable that the same differences in respect to the influence
of climatic conditions will have prevailed as with cereals. Subsequent
to the period which has produced the numerous elementary species of the
whitlow-grass came a period of widespread distribution. The process must
have been wholly comparable with that of acclimatization. Some species
must have been more adapted to northern climates, others to the soils of
western or eastern regions and so on. These qualities must have decided
the general lines of the distribution, and the species must have been
segregated according to their respective climatic qualities, and their
adaptability to soil and weather. A struggle for life and a natural
selection must have accompanied and guided the distribution, but there
is no reason to assume that the various forms were changed by this
process, and that we see them now endowed with other qualities than they
had at the outset.
Natural selection must have played, in this and in a large number of
other cases, quite the same part as the artificial method of variety
testing.
[120] Indeed it may be surmised that this has been its chief and
prominent function. Taking up again our metaphor of the sieve we can
assert that in such cases climate and soil exercise sifting action and
in this way the application of the metaphor becomes more definite. Of
course, next to the climate and soil in importance, come ecological
conditions, the vegetable and animal enemies of the plants and other
influences of the same nature.
In conclusion it is to be pointed out that this side of the problem of
natural selection and the struggle for life appears to offer the best
prospects for experimental, or for continued statistical inquiry. Direct
observations are possible and any comparison of numerical proportions of
species in succeeding years affords clear proof of the part it plays.
And above all, such observations can be made quite independently of
doubtful theoretical considerations about presumed changes of character.
The fact of natural selection is plain and it should be studied in its
most simple conditions.
[121]
C. RETROGRADE VARIETIES
LECTURE V
CHARACTERS OF RETROGRADE VARIETIES
Every one admires the luxuriance of garden-flowers, and their diversity
of color and form. All parts of the world have contributed to their
number and every taste can find its preference among them. New forms
produced by the skill of the breeder are introduced every year. This has
been done mostly by crossing and intermingling the characters of
introduced species of the same genus. In some of the cases the history
of our flowers is so old that their hybrid origin is forgotten, as in
the case of the pansies. Hybridizations are still going on in other
groups on a large scale, and new forms are openly claimed to be of
hybrid origin.
Breeders and amateurs generally have more interest in the results than
in the way in which they have been brought about. Excellent flowers and
fruit recommend themselves and there seems to be no reason for inquiring
[122] about their origin. In some cases the name of the originator may
be so widely known that it adds weight to the value of the new form, and
therefore may advantageously be coupled with it. The origin and history
of the greater part of our garden-flowers, fruits and vegetables are
obscure; we see them as they are, and do not know from whence they came.
The original habitat for a whole genus or for a species at large, may be
known, but questions as to the origin of the single forms, of which it
is built up, ordinarily remain unanswered.
For these reasons we are restricted in most cases to the comparison of
the forms before us. This comparison has led to the general use of the
term "variety" in opposition to "species." The larger groups of forms,
which are known to have been introduced as such are called species. All
forms which by their characters belong to such a species are designated
as varieties, irrespective of their systematic relation to the form,
considered as the ancestor of the group.
Hence, we distinguish between "hybrid varieties" and "pure varieties"
according to their origin from different parents or from a single line
of ancestors. Moreover, in both groups the forms may be propagated by
seeds, or in the vegetative way by buds, by grafting or [123] by
cutting, and this leads to the distinction of "seed-varieties" and
"vegetative varieties." In the first case the inheritance of the special
characters through the seeds decides the status of the variety, in the
latter case this point is left wholly out of consideration.
Leaving aside all these different types, we are concerned here only with
the "seed-varieties" of pure origin, or at least with those, that are
supposed to be so. Hybridization and vegetative multiplication of the
hybrids no doubt occur in nature, but they are very rare, when compared
with the ordinary method of propagation by seed. "Seed-varieties" may
further be divided into constant and inconstant ones. The difference is
very essential, but the test is not always easy to apply. Constant
varieties are as sharply defined and as narrowly limited as are the best
wild species, while inconstant types are cultivated chiefly on account
of their wide range of form and color. This diversity is repeated
yearly, even from the purest seed. We will now discuss the constant
seed-varieties, leaving the inconstant and eversporting types to a
subsequent lecture.
In this way we may make an exact inquiry into the departures from the
species which are ordinarily considered to constitute the essential
character of such a constant and pure seed-variety [124] and need only
compare these differences with those that distinguish the elementary
species of one and the same group from each other.
Two points are very striking. By far the greatest part of the ordinary
garden-varieties differ from their species by a single sharp character
only. In derivative cases two, three or even more such characters may be
combined in one variety, for instance, a dwarfed variety of the larkspur
may at the same time bear white flowers, or even double white flowers,
but the individuality of the single characters is not in the least
obscured by such combinations.
The second point is the almost general occurrence of the same variety in
extended series of species. White and double flowers, variegated leaves,
dwarfs and many other instances may be cited. It is precisely this
universal repetition of the same character that strikes us as the
essential feature of a variety.
And again these two characteristics may now be considered separately.
Let us begin with the sharpness of the varietal characters. In this
respect varieties differ most obviously from elementary species. These
are distinguished from their nearest allies in almost all organs. There
is no prominent distinctive feature between the single forms of _Draba_
[125] _Verna_, _Helianthemum_ or of _Taraxacum_; all characters are
almost equally concerned. The elementary species of _Draba_ are
characterized, as we have seen, by the forms and the hairiness of the
leaves, the number and height of the flower-stalks, the breadth and
incision of the petals, the forms of the fruits, and so on. Every one of
the two hundred forms included in this collective species has its own
type, which it is impossible to express by a single term. Their names
are chosen arbitrarily. Quite the contrary is the case with most of the
varieties, for which one word ordinarily suffices to express the whole
difference.
White varieties of species with red or blue flowers are the most common
instances. If the species has a compound color and if only one of the
constituents is lost, partially colored types arise as in _Agrostemma
Coronaria bicolor_. Or the spots may disappear and the color become
uniform as in _Gentiana punctata concolor_ and the spotless Arum or
_Arum maculatum immaculatum_. Absence of hairs produces forms as
_Biscutella laevigata glabra_; lack of prickles gives the varieties
known as _inermis, as for instance, _Ranunculus arvensis inermis_.
_Cytisus prostratus_ has a variety _ciliata_, and _Solanum Dulcamara_,
or the bitter-sweet, has a variety called _tomentosum_. The curious
monophyllous [126] variety of the strawberry and many other forms will
be discussed later.
To enlarge this list it would only be necessary to extract from a flora,
or from a catalogue of horticultural plants, the names of the varieties
enumerated therein. In nearly every instance, where true varieties and
not elementary species are concerned, a single term expresses the whole
character.
Such a list would also serve to illustrate the second point since the
same names would recur frequently. Long lists of varieties are called
alba, or inermis, or canescens or lutea, and many genera contain the
same appellations. In some instances the systematists use a diversity of
names to convey exactly the same idea, as if to conceal the monotony of
the character, as for instance in the case of the lack of hairs, which
is expressed by the varietal names of _Papaver dubium glabrum_, _Arabis
ciliata glabrata_, _Arabis hirsuta glaberrima_, _Veronica spicata
nitens_, _Amygdalus persica laevis_, _Paeonia corallina Leiocarpa_, &c.
On the contrary we find elementary species in different genera based on
the greatest possible diversity of features. The forms of _Taraxacum_ or
_Helianthemum_ do not repeat those of _Draba_ or _Viola_. In roses and
brambles the distinguishing features are characteristic of the type, as
[127] they are evidently derived from it and limited to it. And this is
so true that nobody claims the grade of elementary species for white
roses or white brambles, but everyone recognizes that forms diverging
from the nearest species by a single character only, are to be regarded
as varieties.
This general conviction is the basis on which we may build up a more
sharply defined distinction between elementary species and varieties. It
is an old rule in systematic botany, that no form is to be constituted a
species upon the basis of a single character. All authors agree on this
point; specific differences are derived from the totality of the
attributes, not from one organ or one quality. This rule is intimately
connected with the idea that varieties are derived from species. The
species is the typical, really existing form from which the variety has
originated by a definite change. In enumerating the different forms the
species is distinguished by the term of genuine or typical, often only
indicated as _a_ or the first; then follow the varieties sometimes in
order of their degree of difference, sometimes simply in alphabetical
order. In the case of elementary species there is no real type; no one
of them predominates because all are considered to be equal in rank, and
the systematic species to which they [128] are referred is not a really
existing form, but is the abstraction of the common type of all, just as
it is in the case of a genus or of a family.
Summarizing the main points of this discussion, we find that elementary
species are of equal rank and together build up the collective or
systematic ideal species. Varieties on the other hand are derived from a
real and commonly, still existing type.
I hope that I have succeeded in showing that the difference between
elementary species, or, as they are often called, smaller or subspecies,
on the one hand and varieties on the other, is quite a marked one.
However, in order to recognize this principle it is necessary to limit
the term variety, to those propagating themselves by seed and are of
pure and not of hybrid origin.
But the principle as stated here, does not involve an absolute contrast
between two groups of characters. It is more a difference in our
knowledge and appreciation of them than a difference in the things
themselves. The characters of elementary species are, as a rule, new to
us, while those of varieties are old and familiar. It seems to me that
this is the essential point.
And what is it that makes us familiar with them? Obviously the
continuous recurrence of the same changes, because by a constant
repetition they must of course lose their novelty.
[129] Presently we shall look into these characters more in detail and
then we shall find that they are not so simple as might be supposed at
first sight; but precisely because we are so familiar with them, we
readily see that their different features really belong to a single
character; while in elementary species everything is so new that it is
impossible for us to discern the unities of the new attributes.
If we bear in mind all these difficulties we cannot wonder at the
confusion on this question that seems to prevail everywhere. Some
authors following Linnaeus simply call all the subdivisions of species,
varieties; others follow Jordan and avoid the difficulty by designating
all smaller forms directly as species. The ablest systematists prefer to
consider the ordinary species as collective groups, calling their
constituents "The elements of the species," as was done by A.P. De
Candolle, Alph. De Candolle and Lindley.
By this method they clearly point out the difference between the
subdivisions of wild species as they ordinarily occur, and the varieties
in our gardens, which would be very rare, were they not singled out and
preserved.
Our familiarity with a character and our grounds for calling it an old
acquaintance may result from two causes, which in judging a new [130]
variety are essentially different. The character in question may be
present in the given species or it may be lacking, but present in the
other group. In the first case a variety can only be formed by the loss
of the character, in the second case it arises by the addition of a new
one.
The first mode may be called a negative process, while the second is
then to be designated as positive. And as it is more easy to lose what
one has than to obtain something new, negative varieties are much more
common than are positive ones.
Let us now take an instance of a character that is apt to vary in both
ways, for this is obviously the best way of making clear what is meant
by a negative and a positive change.
In the family of the composites we find a group of genera with two forms
of florets on each flower-head. The hermaphrodite ones are tubular with
5, or rarely 4, equal teeth, and occupy the center of the head. These
are often called the flosculous florets or disk-florets. Those of the
circumference are ligulate and ordinarily unisexual, without stamens. In
many cases they are sterile, having only an imperfect ovary. They are
large and brightly colored and are generally designated as ray-florets.
As instances we may cite the camomile (_Anthemis nobilis_), the wild
camomile (_Matricaria Chamomilla_), [131] the yarrow (_Achillea
Millefolium_), the daisies, the Dahlia and many others. Species occur in
this group of plants from time to time that lack the ray-florets, as in
the tansy (_Tanacetum vulgare_) and some _artemisias_. And the genus of
the marigolds or _Bidens_ is noted for containing both of these types.
The smaller and the three-toothed marigold (_B. cernua_ and _B.
tripartita_) are very common plants of wet soil and swamps, ordinarily
lacking the ray-florets, and in some countries they are very abundant
and wholly constant in this respect, never forming radiate flower-heads.
On the other hand the white-flowered and the purple marigold (_B.
leucantha_ and _B. atropurpurea_) are cultivated species of our gardens,
prized for their showy flower-heads with large white or deeply colored,
nearly black-purple florets.
Here we have opportunity to observe positive and negative varieties of
the same character. The smaller, and the three-toothed marigold occur
from time to time, provided with ray florets, showing a positive
variation. And the white marigold has produced in our gardens a variety
without rays. Such varieties are quite constant, never returning to the
old species. Positive and negative varieties of this kind are by no
means rare among the compositae.
[132] In systematic works the positive ones are as a rule called
"radiate," and the negative ones "discoid." Discoid forms of the
ordinary camomile, of the daisy, of some asters (_Aster Tripolium_), and
of some centauries have been described. Radiate forms have been observed
in the tansy (_Tanacetum vulgare_), the common horse-weed or Canada
fleabane (_Erigeron canadensis_) and the common groundsel (_Senecio
vulgaris_). Taken broadly the negative varieties seem to be somewhat
more numerous than the positive ones, but it is very difficult to come
to a definite conclusion on this point.
Quite the contrary is the case with regard to the color-varieties of red
and blue flowers. Here the loss of color is so common that every one
could give long lists of examples of it. Linnaeus himself supposed that
no blue or red-colored wild species would be without a white variety. It
is well known that he founded his often criticized prescript never to
trust to color in recognizing or describing a species, on this belief.
On the other hand there are some red varieties of white-flowered
species. But they are very rare, and little is known about their
characters or constancy. Blue varieties of white species are not found.
The yarrow (_Achillea Millefolium_) has a red-flowered form, which
occurs [133] from time to time in sunny and sandy localities. I have
isolated it and cultivated it during a series of years and during many
generations. It is quite true to its character, but the degree of its
coloring fluctuates between pink and white and is extremely variable.
Perhaps it can be considered as an inconstant variety. A redflowered
form of the common _Begonia semperflorens_ is cultivated under the name
of "Vernon," the white hawthorn (_Crataegus Oxyacantha_) is often seen
with red flowers, and a pink-flowered variety of the "Silverchain" or
"Bastard acacia" (_Robinia Pseud-Acacia_) is not rarely cultivated. The
"Crown" variety of the yellow wall-flower and the black varieties, are
also to be considered as positive color variations, the black being due
in the latter cases to a very great amount of the red pigment.
Among fruits there are also some positive red varieties of greenish or
yellowish species, as for instance the red gooseberry (_Ribes
Grossularia_) and the red oranges. The red hue is far more common in
leaves, as seen among herbs, in cultivated varieties of _Coleus_ and in
the brown leaved form of the ordinary white clover, among trees and
shrubs in the hazelnut (_Corylus_), the beech (_Fagus_), the birch
(_Betula_), the barberry (_Berberis_) and many others. But though most
of these forms are very ornamental and abundant [134] in parks and
gardens, little is as yet known concerning the origin of their varietal
attributes and their constancy, when propagated by seeds. Besides the
ray-florets and the colors, there are of course a great many other
characters in which varieties may differ from their species. In most of
the cases it is easy to discern whether the new character is a positive
or a negative one. And it is not at all necessary to scrutinize very
narrowly the list of forms to become convinced that the negative form is
the one which prevails nearly everywhere, and that positive aberrations
are in a general sense so rare that they might even be taken for
exceptions to the rule.
Many organs and many qualities may be lost in the origination of a
variety. In some instances the petals may disappear, as in _Nigella_, or
the stamens, as in the Guelder-rose (_Viburnum Opulus_) and the
_Hortensia_ and in some bulbs even the whole flowers may be wanting, as
in the beautiful "Plumosa" form of the cultivated grape-hyacinth or
_Muscari comosum_. Fruits of the pineapples and bananas without seeds
are on record as well as some varieties of apples and pears, of raisins
and oranges. And some years ago Mr. Riviere of Algeria described a date
growing in his garden that forms fruit without pits. The stoneless plum
of Mr. [135] Burbank of Santa Rosa, California, is also a very curious
variety, the kernel of which is fully developed but naked, no hard
substance intervening between it and the pulp.
More curious still are the unbranched varieties consisting of a single
stem, as may be seen sometimes in the corn or maize and in the fir.
Fir-trees of some three or four meters in height without a single
branch, wholly naked and bearing leaves only on the shoots of the last
year's growth at the apex of the tree, may be seen. Of course they
cannot bear seed, and so it is with the sterile maize, which never
produces any seed-spikes or staminate flowers. Other seedless varieties
can be propagated by buds; their origin is in most cases unknown, and we
are not sure as to whether they should be classified with the constant
or with the inconstant varieties.
A very curious loss is that of starch in the grains of the sugar-corn
and the sugar-peas. It is replaced by sugar or some allied substance
(dextrine). Equally remarkable is the loss of the runners in the
so-called "Gaillon" strawberries.
Among trees the pendulous or weeping, and the broomlike or fastigiate
forms are very marked varieties, which occur in species belonging to
quite different orders. The ash, the beach, some willows, many other
trees and some [136] finer species of garden-plants, as _Sophora
japonica_, have given rise to weeping varieties, and the yew-tree or
_Taxus_ has a fastigiate form which is much valued because of its
ascending branches and pyramidal habit. So it is with the pyramidal
varieties of oaks, elms, the bastard-acacia and some others.
It is generally acknowledged that these forms are to be considered as
varieties on the ground of their occurrence in so wide a range of
species, and because they always bear the same attributes. The pendulous
forms owe their peculiarity to a lengthening of the branches and a loss
of their habit of growing upwards; they are too weak to retain a
vertical position and the response to gravity, which is ordinarily the
cause of the upright growth, is lacking in them. As far as we know, the
cause of this weeping habit is the same in all instances. The fastigiate
trees and shrubs are a counterpart of the weeping forms. Here the
tendency to grow in a horizontal direction is lacking, and with it the
bilateral and symmetric structure of the branches has disappeared. In
the ordinary yew-tree the upright stem bears its needles equally
distributed around its circumference, but on the branches the needles
are inserted in two rows, one to the left and one to the right. All the
needles turn their upper surfaces upwards, [137] and their lower
surfaces downwards, and all of them are by this means placed in a single
horizontal plane, and branching takes place in the same plane. Evidently
this general arrangement is another response to gravity, and it is the
failure of this reaction which induces the branches to grow upwards and
to behave like stems.
Both weeping and fastigiate characters are therefore to be regarded as
steps in a negative direction, and it is highly important that even such
marked departures occur without transitions or intermediate forms. If
these should occur, though ever so rarely, they would probably have been
brought to notice, on account of the great prospect the numerous
instances would offer. The fact that they are lacking, proves that the
steps, though apparently great, are in reality to be considered as
covering single units, that cannot be divided into smaller parts.
Unfortunately we are still in the dark as to the question of the
inheritance of these forms, since in most cases it is difficult to
obtain pure seed.
We now consider the cases of the loss of superficial organs, of which
the nectarines are example. These are smooth peaches, lacking the soft
hairy down, that is a marked peculiarity of the true peaches. They occur
in different [138] races of the peach. As early as the beginning of the
past century, Gallesio described no less than eight subvarieties of
nectarines, each related to a definite race of peach. Most of them
reproduce themselves truly from seed, as is well known in this country
concerning the clingstones, freestones and some other types. Nectarines
have often varied, giving rise to new sorts, as in the case of the white
nectarine and many others differing greatly in appearance and flavor. On
the other hand it is to be remarked, that the trees do not differ in
other respects and cannot be distinguished while young, the varietal
mark being limited to the loss of the down on the fruit. Peaches have
been known to produce nectarines, and nectarines to yield true peaches.
Here we have another instance of positive and negative steps with
reference to the same character, but I cannot withhold an expression of
some doubt as to the possibility of crossing and subsequently splitting
up of the hybrids as a more probable explanation of at least some of the
cases quoted by various writers.
Smooth or glabrous varieties often occur, and some of them have already
been cited as instances of the multiplication of varietal names.
Positive aberrations are rather rare, and are mostly restricted to a
greater density of the [139] pubescence in some hairy species, as in
_Galeopsis Ladanum canescens_, _Lotus corniculatus hirsutus_ and so on.
But _Veronica scutellata_ is smooth and has a pubescent variety, and
Cytisus prostratus and _C. spinescens_ are each recorded to have a
ciliate form.
Comparable with the occurrence and the lack of hairs, is the existence
or deficiency of the glaucous effect in leaves, as is well known in the
common _Ricinus_. Here the glaucous appearance is due to wax distributed
in fine particles over the surface of the leaves, and in the green
variety this wax is lacking. Other instances could be given as in the
green varieties of _Papaver alpinum_ and _Rumex scutatus_. No positive
instances are recorded in this case.
Spines and prickles may often disappear and give rise to unarmed and
defenceless types. Of the thorn-apples both species, the whiteflowered
_Datura Stramonium_ and the purple _D. Tatula_ have such varieties.
Spinach has a variety called the "Dutch," which lacks the prickles of
the fruit; it is a very old form and absolutely constant, as are also
the thornless thorn-apples. Last year a very curious instance of a
partial loss of prickles was discovered by Mr. Cockerell of East Las
Vegas in New Mexico. It is a variety of the American cocklebur, often
called sea-burdock, or the [140] hedgehog-burweed, a stout and common
weed of the western states. Its Latin name is _Xanthium canadense_ or
_X. commune_ and the form referred to is named by Mr. Cockerell, _X.
Wootoni_, in honor of Professor E.o. Wooton who described the first
collected specimens.
The burs of the common species are densely covered with long prickles,
which are slightly hooked at the apex. In the new form, which is similar
in all other respects to the common cocklebur, the burs are more slender
and the prickles much less numerous, about 25 to the bur and mostly
stouter at the base. It occurs abundantly in New Mexico, always growing
with the common species, and seems to be quite constant from seed. Mr.
Cockerell kindly sent me some burs of both forms, and from these I
raised in my garden last year a nice lot of the common, as well as of
the _Wootoni_ plants.
Spineless varieties are recorded for the bastard-acacia, the holly and
the garden gooseberry (_Ribes Grossularia_, or _R. Uva-crispa_). A
spineless sport of the prickly Broom (_Ulex europaeus_) has been seen
from time to time, but it has not been propagated.
Summarizing the foregoing facts, we have excellent evidence of varieties
being produced either by the loss of some marked peculiarity or by the
acquisition of others that are already [141] present in allied species.
There are a great many cases however, in which the morphologic cause of
the dissimilarity is not so easily discerned. But there is no reason to
doubt that most of them will be found to conform to the rule on closer
investigation. Therefore we can consider the following as the principal
difference between elementary species and varieties; that the first
arise by the acquisition of entirely new characters, and the latter by
the loss of existing qualities or by the gain of such peculiarities as
may already be seen in other allied species.
If we suppose elementary species and varieties originated by sudden
leaps or mutations, then the elementary species have mutated in the line
of progression, some varieties have mutated in the line of
retrogression, while others have diverged from their parental types in a
line of depression, or in the way of repetition. This conception agrees
quite well with the current idea that in the building up of the
vegetable kingdom according to the theory of descent, it is species that
form the links of the chain from the lower forms to the more highly
organized later derivatives. Otherwise expressed, the system is built up
of species, and varieties are only local and lateral, but never of real
importance for the whole structure.
[142] Heretofore we have generally assumed, that varieties differ from
the parent-species in a single character only, or at least that only one
need be considered. We now come to the study of those varieties, which
differ in more than one character. Of these there are two types. In the
first the points of dissimilarity are intimately connected with one
another, in the second they are more or less independent.
The mutually related peculiarities may be termed correlative, and we
therefore speak, in such cases, of correlative variability. This
phenomenon is of the highest importance and is of general occurrence.
But before describing some examples, it is as well to note that in the
lecture on fluctuating variability, cases of a totally different nature
will be dealt with, which unfortunately are designated by the same term.
Such merely fluctuating variations are therefore to be left out of the
present discussion.
The purple thorn-apple, which is considered by some writers as a variety
of the white-flowered species or _Datura Stramonium_, and by others as a
separate species, _D. Tatula_, will serve as an illustration. But as its
distinguishing attributes, as far as we are concerned with them here,
are of the nature described above as characteristic of varietal
peculiarities no objection [143] can be made to our using them as a case
of correlative variability.
The essential character of the purple thornapple lies in the color of
the flowers, which are of a very beautiful pale blue. But this color is
not limited to the corolla. It is also to be seen in the stems and in
the stalks and veins of the leaves, which are stained with a deep
purple, the blue color being added to the original green. Even on the
surface of the leaves it may spread into a purplish hue. On the stems it
is to be met with everywhere, and even the young seedlings show it. This
is of some importance, as the young plants when unfolding their
cotyledons and primary leaves, may be distinguished by this means from
the seedlings of the white flowered species.
In crossing experiments it is therefore possible to distinguish the
whites and the blues, even in young seedlings, and experience shows that
the correlation is quite constant. The color can always be relied upon;
if lacking in the seedlings, it will be lacking in the stems and flowers
also; but if the axis of the young plant is ever so slightly tinged, the
color will show itself in its beauty in the later stages of the life of
the plant.
This is what we term correlation. The colors of the different organs are
always in agreement. It is true that they require the concurrence of
[144] light for development, and that in the dark or in a faint light
the seedlings are apt to remain green when they should become purple,
but aside from such consideration all organs always come true to their
color, whether pure green and white, or whether these are combined with
the blue tinge. This constancy is so absolute that the colors of the
different organs convey the suggestion, that they are only separate
marks of a single character.
It is on this suggestion that we must work, as it indicates the cause of
the correlation. Once present, the faculty of producing the anthocyan,
the color in question, will come into activity wherever and whenever
opportunity presents itself. It is the cell-sap of the ordinary cell
tissue or parenchyma, which is colored by the anthocyan, and for this
reason all organs possessing this tissue, may exhibit the color in
question.
Thus the color is not a character belonging to any single organ or cell,
nor is it bound to a morphologic unit; it is a free, physiologic
quality. It is not localized, but belongs to the entire plant. If we
wish to assume for its basis material representative particles, these
particles must be supposed to be diffused throughout the whole body of
the plant.
This conception of a physiologic unit as the [145] cause of colors and
other qualities is evidently opposed to the current idea of the cells
and tissues as the morphologic units of the plants. But I do not doubt,
that in the long run it will recommend itself as much to the scientist
as to the breeder. For the breeder, when desiring to keep his varieties
up to their standard, or when breeding to a definite idea, obviously
keeps his standard and his ideal for the whole plant, even if he breeds
only for flowers or for fruit.
I have chosen the color of the purple thornapple as a first example, but
the colors of other plants show so many diverging aspects, all pointing
so clearly to the same conclusion, that it would be well to take a more
extensive view of this interesting subject.
First we must consider the correlation in the colors of flowers and
fruits. If both are colored in the species, whether red or brown or
purple or nearly black, and a variety lacking this hue is known, it will
be lacking in both organs. If the color is pure, the flowers and berries
will become white, but such cases are rare. Ordinarily a yellowish or
greenish tinge underlies the ornamental color, and if this latter
disappears, the yellowish ground will become manifest. So for instance
in the Belladonna, a beautiful perennial herb with great shiny black,
but very poisonous, fruits. Its flowers are brown, but in [146] some
woods a variety with greenish flowers and bright yellow berries occurs,
which is also frequently seen in botanic gardens. The anthocyan dye is
lacking in both organs, and the same is the case with the stems and the
leaves. The lady's laurel or _Daphne Mezereum_ has red corollas, purple
leaves and red fruits; its white flowered variety may be distinguished
by lack of the red hue in the stems and leaves, and by their beautiful
yellow berries. Many other instances could be given, since the loss of
color in berries is a very common occurrence, so common that for
instance, in the heath-family or Ericaceae, with only a few exceptions,
all berry-bearing species have white-fruited varieties.
The same correlation is observed in the seeds. The white-flowered flax
may be seen to yield yellow and not brown seeds as in the blue species.
Many varieties of flowers may be recognized by the color of their seeds,
as in the poppies, stocks and others. Other white-flowered varieties may
be distinguished when germinating, their young axes being of a pure
instead of a purplish green. It is a test ordinarily used by gardeners,
to purify their flower beds long before the blooming time, when thinning
or weeding them. Even in wild plants, as in _Erodium_, _Calluna_,
_Brunella_ and others, a botanist may recognize the rare white-flowered
[147] variety by the pure green color of the leaves, at times when it is
not in flower. Some sorts of peas bear colored flowers and a red mark on
the stipules of their leaves. Among bulbous plants many varieties may be
recognized even in the dry bulbs by the different tinges of the outer
scales.
Leaving the colors, we come now to another instance of correlation,
which is still more astonishing. For it is as rare, as color-varieties
are common. It is afforded by some plants the leaves of which, instead
of being entire or only divided into large parts, are cleft to a greater
extent by repeated fissures of the marginal lobes. Such foliar
variations are often seen in gardens, where they are cultivated for
their beauty or singularity, as the laciniated alders, fern-leaved,
beeches and limes, oakleaved laburnums, etc. Many of them are described
under the varietal name of _laciniata_. In some cases this fissure
extends to the petals of the flowers, and changes them in a way quite
analogous to the aberrancy of the leaves.
This is known to occur with a variety of brambles, and is often seen in
botanic gardens in one of the oldest and most interesting of all
anomalies, the laciniated variety of the greater celandine or
_Chelidonium majus_. Many other instances could be given. Most of them
belong to the [148] group of negative variations, as we have defined
them. But the same thing occurs also with positive varieties, though of
course, such cases are very rare. The best known instance is that of the
ever-flowering begonia, _Begonia semperflorens_, which has green leaves
and white flowers, but which has produced garden varieties with a brown
foliage and pink flowers. Here also the new quality manifests itself in
different organs.
Enough has now been said on correlative changes, to convince us that
they are as a rule to be considered as the expression of some general
internal or physiologic quality, which is not limited to a single organ,
but affects all parts of the organism, provided they are capable of
undergoing the change. Such characters are therefore to be considered as
units, and should be referred to the group of single characters.
Opposed to these are the true compound characters, which consist of
different units. These may be segregated by the production of varieties,
and thereby betray the separate factors of the complex group.
The most beautiful instances of such complex characters are offered by
the colors of some of the most prized garden-flowers. Rarely these are
of a single hue, often two or three shades contribute to the effect, and
in some cases special [149] spots or lines or tracings are to be seen on
a white or on a colored background. That such spots and lines are
separate units is obvious and is demonstrated by the fact that sometimes
spotless varieties occur, which in all other respects have kept the
colors of the species. The complexity of the color is equally evident,
whenever it is built up of constituents of the anthocyan and of the
yellow group. The anthocyan dye is limited to the sap-cavity of the
cells, while the yellow and pure orange colors are fixed in special
organs of the protoplasm. The observation under the microscope shows at
once the different units, which though lying in the same cell and in
almost immediate vicinity of each other are always wholly separated from
one another by the wall of the vacuole or sapfilled cell-cavity.
The combination of red and yellow gives a brown tinge, as in the
cultivated wall-flower, or those bright hues of a dark orange-red, which
are so much sought in tulips. By putting such flowers for a short time
in boiling water, the cells die and release the red pigment, which
becomes diffused in the surrounding fluids and the petals are left
behind with their yellow tinge. In this way it is easy to separate the
constituents, and demonstrate the compound nature of the original
colors.
[150] But the diversity of the color patterns is far from being
exhausted with these simple instances. Apart from them, or joined to
them, other complications are frequently seen, which it is impossible to
analyze in such an artificial way. Here we have to return to our former
principle, the comparison of different varieties. Assuming that single
units may be lost, irrespective of the others, we may expect to find
them segregated by variation, wherever a sufficiently wide range of
color-varieties is in cultivation. In fact, in most cases a high degree
of dissimilarity may be reached in the simplest way by such a separation
of the components, and by their combination into most diverse smaller
groups. A very nice instance of such an analysis of flower-colors is
afforded by the ordinary snapdragon. The beautiful brown red color of
this common garden-plant is composed on one side of yellow elements, on
the other of red units. Of the yellow there are two, one staining the
whole corolla with a light hue, as is to be seen in the pure yellow
variety called _luteum. This form has been produced by the loss of the
whole group of the red constituents. If the yellow tinge is also lost,
there arises a white variety, but this is not absolutely colorless, but
shows the other yellow constituent. This last stains only some small
parts [151] of the lips of the flower around the throat, brightening, as
it seems, the entrance for the visiting insects. In many of the red or
reddish varieties this one yellow patch remains, while the general
yellow hue fails. In the variety called "Brilliant" the yellow ground
makes the red color more shiny, and if it is absent the pure carmine
tinge predominates.
It is readily seen, that in the ordinary form the lips are of a darker
red than the tube. This evident dissimilarity indicates some complexity.
And in fact we have two varieties which exhibit the two causes of this
attribute separately. One of them is called "Delila," and has the red
color limited to the lips, whilst the tube is pure white. The other is
called "Fleshy," and is of a pale pink throughout the whole corolla.
Adding these two units to one another, we get the original dark red of
the wild type, and it may be briefly stated here, that the way of
effecting such an addition is given us in the crossing of the "Fleshy"
and the "Delila" variety, the hybrid showing the two colors and
returning thereby to the old prototype.
Other cases of compound flower colors or of color patterns might be
given as in the _Mimulus_ and the poppy, and in most of these cases some
varieties are to be seen in our gardens which show only the single
constituents of the group.
[152] Many dark flowers have an intermediate bright hued form besides
the white variety, as in the case of roses, asters, _Nicandra_ and so
on.
Intermediate forms with respect to stature may also be seen. The
opium-poppy, the snapdragon, peas, the _Nicandra_, and many other
garden-plants have not only dwarf varieties, but also some of
intermediate height. These, though they are intermediate between the
tall and dwarf types, cannot be considered as transitions, as between
them and the extremes, intermediates are, as a rule wholly lacking.
Instances of the same occurrence of three types may be seen in the seeds
of maize ("Cuzco," "Horse-dent" and "Gracillima") of beans and some
other plants. The _Xanthium Wootoni_, above referred to, with only part
of the prickles of Xanthium commune is also a very curious instance of
the demonstration of the compound nature of a character.
Summarizing the conclusions that may be drawn from the evidence given in
this lecture, we have seen that varieties differ from elementary species
in that they do not possess anything really new. They originate for the
greater part in a negative way, by the apparent loss of some quality,
and rarely in a positive manner by acquiring a character, already seen
in allied species. These characters are not of the nature of [153]
morphologic entities, but are to be considered as physiologic units,
present in all parts of the organisms, and manifesting themselves where
ever occasion is afforded. They are units in the sense that they may
appear and disappear singly. But very often they are combined to yield
compound characters, which are capable of analysis. Opportunities for
such an analysis are afforded by these groups of cultivated varieties,
of which some members show a single distinguishing quality, or a number
of them.
[154]
LECTURE VI
STABILITY AND REAL ATAVISM
It is generally believed that varieties are principally distinguished
from species by their inconstancy. This conception is derived from some
special cases and transferred to others, and in its common form this
belief must have originated from the confusion which exists as to the
meaning of the term variety. It is true that vegetative varieties as a
rule run back, when propagated by seeds; they are an obvious instance of
inconstancy. In the second place we have considered the group of
inconstant or sporting varieties, which of course we must exclude when
studying the stability of other types. However, even these sporting
varieties are unstable only to a certain degree, and in a broader sense
will prove to be as true to their character as the most constant types.
Having separated these two groups, which include also the wide range of
hybrid forms, we may next consider only those varieties of pure origin,
and ordinarily propagated by seeds, [155] which have been discussed in
former chapters. Their general character lies in their fidelity to type,
and in the fact that this is single, and not double, as in the sporting
varieties.
But the current belief is, that they are only true to their
peculiarities to a certain degree, and that from time to time, and not
rarely, they revert to the type from which they have arisen. Such
reversion is supposed to prove that they are mere varieties, and at the
same time to indicate empirically the species from which they have
sprung.
In the next lecture we shall examine critically the evidence on which
this assumption rests. Before doing so however, it will be necessary to
collate the cases in which there is no reversion at all, or in which the
reversion is absent at least in experimental and pure sowings.
In the present state of our knowledge it is very difficult to decide,
whether or not true reversion occurs in constant varieties. If it does
occur, it surely does so very rarely and only under unusual
circumstances, or in particular individuals. However when such
individuals are multiplied by buds and especially when they are the only
representatives of their type, the reversion, though theoretically rare,
will be shown by nearly every specimen of the variety. Examples of this
will be given below.
[156] They are generally called atavists or reversionists, but even
these terms are sometimes used in a different sense.
Lastly it is to be said that the empirical and experimental evidence as
to the question of constancy is not as extensive as it should be. The
experimental conditions are seldom described, and it is only recently
that an interest in the matter has been awakened. Much remains to be
done. Among other things the innumerable varieties of trees, shrubs and
perennial herbs should be tested as to their constancy when grown from
purely fertilized seeds. Many of them may be included among the number
that sport constantly.
Leaving aside the doubtful or insufficiently studied cases, we may now
turn our attention to the facts that prove the absolute stability of a
large number of varieties, at least as far as such completeness can be
attained by experiment or observation.
The best proof is afforded by the varieties which grow wild in
localities where they are quite isolated from the species, and where for
this reason, no possibility of crossing disturbs the significance of the
proof. As one instance the rayless form of the wild camomile, or the
_Matricaria Chamomilla discoidea_ may be mentioned. Many systematists
have been so strongly [157] impressed with its absolute constancy and
its behavior as an ordinary species, that they have elevated it, as it
is called, to the rank of a species. As such it is described under the
name of _Matricaria discoidea_ DC. It is remarkable for its rapid and
widespread distribution, as of late years it has become naturalized in
different parts of America and of Europe, where it is to be seen
especially in France and in Norway. Experimentally I raised in
succeeding years between 1000 and 2000 seedlings, but observed no trace
of reversion, either in the strongest or in the numerous very small and
weak individuals which appeared in the cultures.
The tansy-ragwort or _Senecio Jacobaea_ may be chosen as a second
instance. It is a perennial herb with short rootstocks and stout stems
bearing numerous short-peduncled heads in large compact corymb; it
multiplies itself abundantly by seeds and is very common on the sand
dunes of Holland. It has two forms, differing only in the occurrence or
the lack of the ray florets. But these two varieties occupy different
localities and are even limited to different provinces. As far as I have
been able to ascertain on numerous excursions during a series of years,
they never sport, and are only intermingled on the outskirts of their
habitats. The rayless form is generally considered as the [158] variety
but it is quite as stable as the radiate species.
The radiate varieties of marigold, quoted in a former lecture, seem to
be equally constant, when growing far away from their prototypes. I
sowed the seeds of a single plant of the radiate form of _Bidens
cernua_, and found all of the seedlings came true, and in the next year
I had from their seed between 2,000 and 3,000 flowering individuals, all
equally radiate. Many species of composites have been tried, and they
are all constant. On the other hand rare sports of this kind have been
observed by Murr and other authors.
Many kinds of vegetables and of fruits give instances of stability.
White strawberries, green grapes, white currants, crisped lettuce,
crisped parsley and some other crisped forms may be cited. The spinage
without prickles is a widely known instance. White-flowered flax never
reverts to the blue prototype, if kept pure. Sugar-peas and sugar-corn
afford further instances. Strawberries without runners have come true
from seed ever since their first appearance, over a hundred years ago.
Many garden-varieties, the stability of which under ordinary
circumstances is doubtful, because of their being sown too close to
other varieties of the same species, have been tested in [159] respect
to their stability by different writers and at different times. In doing
this it is plain that it is very essential to be sure of the purity of
the seed. Specimens must be grown in positions isolated from their
allies, and if possible be pollinated artificially with the exclusion of
the visits of insects. This may be done in different ways. If it is a
rare species, not cultivated in the neighborhood, it is often sufficient
to make sure of this fact. Pollen may be conveyed by bees from distances
of some ten or twenty meters, or in rare cases from some hundred meters
and more, but a greater distance is ordinarily sufficient for isolation.
If the flowers fertilize themselves, as is more often the case than is
generally supposed, or if it is easy to pollinate them artificially,
with their own pollen or in small groups of similar individuals, the
best way is to isolate them by means of close coverings. When flowering,
the plants are as a rule too large to be put under bell-glasses, and
moreover such coverings would keep the air moist, and cause the
flower-buds to be thrown off. The best coverings are of netting, or of
canvas of sufficiently wide mesh, although after a long experience I
greatly prefer cages of fine iron-wire, which are put around and over
the whole plant or group of plants, and fastened securely and tightly to
the ground.
[160] Paper bags also may be made use of. They are slipped over the
flowering branches, and bound together around the twigs, thus enclosing
the flowers. It is necessary to use prepared papers, in order that they
may resist rain and wind. The best sort, and the one that I use almost
exclusively in my fertilization-experiments, is made of parchment-paper.
This is a wood-pulp preparation, freed artificially from the so-called
wood-substance or lignin. Having covered the flowers with care, and
having gathered the seeds free from intermixtures and if possible
separately for each single individual, it only remains to sow them in
quantities that will yield the greatest possible number of individuals.
Reversions are supposed to be rare and small groups of seedlings of
course would not suffice to bring them to light. Only sowings of many
hundreds or thousands of individuals are decisive. Such sowings can be
made in one year, or can be extended over a series of years and of
generations. Hildebrand and Hoffman have preferred the last method, and
so did Hofmeister and many others. Hildebrand sowed the white hyacinth,
and the white varieties of the larkspur, the stock and the sweet pea.
Hoffman cultivated the white flax and many other varieties and
Hofmeister extended his sowings [161] over thirty years with the white
variety of the yellow foxglove (_Digitalis parviflora_). White-flowered
varieties of perennial garden plants were used in my own experiments. I
bought the plants, flowered them under isolation in the way described
above, gathered the seeds from each individual separately and sowed them
in isolated groups, keeping many hundreds and in some cases above a
thousand plants up to the time of flowering. Among them I found only one
inconstant variety, the white form of the yellow columbine, _Aquilegia
chrysantha_. It evidently belonged to the group of sporting varieties
already referred to. All others came absolutely true to type without any
exception. The species experimented with, were _Campanula persicifolia_,
_Hyssopus officinalis_, _Lobelia syphilitica_, _Lychnis chalcedonica_,
_Polemonium dissectum_, _Salvia sylvestris_ and some others. Tested in
the same way I found the white varieties of the following annual plants
also quite true: _Chrysanthemum coronarium_, _Godetia amoena_, _Linum
usitatissimum_, _Phlox drummondi_, and _Silene Armeria_. To these may be
added the white hemlock stork's-bill (_Erodium cicutarium album_) which
grows very abundantly in some parts of my fatherland, and is easily
recognizable by its pure green leaves and stems, even when not
flowering. I cultivated it, in large numbers [162] during five
succeeding generations, but was never able to find even the slightest
indication of a reversion to the red prototype. The scarlet pimpernel or
_Anagallis arvensis_ has a blue variety which is absolutely constant.
Even in Britton and Brown's "Flora," which rarely enumerates varieties,
it is mentioned as being probably a distinct species. Eight hundred
blooming seedlings were obtained from isolated parents, all of the same
blue color. The New Zealand spinage (_Tetragonia expansa_) has a
greenish and a brownish variety, the red color extending over the whole
foliage, including the stems and the branches. I have tried both of them
during several years, and they never sported into each other. I raised
more than 5,000 seedlings, from the different seeds of one lot of the
green variety in succeeding years, but neither those germinating in the
first year, nor the others coming into activity after two, three or four
years of repose gave any sign of the red color of the original species.
It is an old custom to designate intermediate forms as hybrids,
especially when both the types are widely known and the intermediates
rare. Many persons believe that in doing so, they are giving an
explanation of the rarer forms. But since the laws of hybridism are
coming to be known we shall have to break with [163] all such usages. So
for instance there are numerous flowers which are of a dark red or a
dark blue color, and which, besides a white variety, have a pink or a
pale blue form. Such pale varieties are of exactly the same value as
others, and on testing they are found to be equally stable. So for
instance the pink variety of the Sweet William (_Silene Armeria rosea_),
the _Clarkia pulchella carnea_ and the pale variety of the corn-cockle,
called usually _Agrostemma Githago nicaeensis_ or even simply _A.
nicaeensis_. The latter variety I found pure during ten succeeding
generations. Another notable stable intermediate form is the poppy
bearing the Danish flag (_Papaver somniferum Danebrog_). It is an old
variety, and absolutely pure when cultivated separately. A long list of
other instances might easily be given.
Many garden-varieties, that are still universally prized and cultivated
are very old. It is curious to note how often such forms have been
introduced as novelties. The common foxglove is one of the best
examples. It has a monstrous variety, which is very showy because it
bears on the summit of its raceme and branches, large erect cup-shaped
flowers, which have quite a different aspect from the normal
thimbleshaped side-blossoms. These flowers are ordinarily described as
belonging to the anomaly [164] known as "peloria," or regular form of a
normally symmetric type; they are large and irregular on the stems and
the vigorous branches but slender and quinate on the weaker twigs. Their
beauty and highly interesting anomalous character has been the cause of
their being described many times, and nearly always as a novelty; they
have been recently re-introduced into horticulture as such, though they
were already cultivated before the middle of the last century. About
that time very good descriptions with plates were published in the
journal "Flora" by Vrolik, but afterwards they seem to have been
forgotten. The peloric variety of the foxglove always comes true from
seed, though in the strict sense of the word which we have chosen for
our discussion, it does not seem to be a constant and pure variety.
It is very interesting to compare old botanical books, or even old
drawings and engravings containing figures of anomalous plants. The
celebrated Pinacothec of Munich contains an old picture by Holbein
(1495-1543) representing St. Sebastian in a flower-garden. Of the plants
many are clearly recognizable, and among others there is one of the
"one-leaved" variety of the strawberry, which may still be met with in
botanical gardens. In the year 1671 a Dutch botanist, Abraham Munting
published [165] a large volume on garden-plants, containing a great
number of very good engravings. Most of them of course show normal
plants, but intermixed with these are varieties, that are still in
cultivation and therefore must be at least two centuries old. Others,
though not figured, are easily recognized by their names and
descriptions. The cockscomb is the most widely known, but many white or
double flowered varieties were already cultivated at that time. The
striped Jalappa, the crested Sedum, the fasciated crown-imperial, white
strawberries, red gooseberries and many others were known to Munting.
Some varieties are as old as culture itself, and it is generally known
that the Romans cultivated the white form of the opium-poppy and used
the foliage of the red variety of the sugarbeet as a vegetable.
In our time flowers and fruits are changing nearly as rapidly as the
fancies and tastes of men. Every year new forms are introduced and usurp
the place of older ones. Many are soon forgotten. But if we look at old
country gardens, a goodly number of fine and valued old sorts are still
to be found. It would be worth while to make special collections of
living plants of old varieties, which surely would be a good and
interesting work and bring about a conviction [166] of the stability of
pure strains. Coming now to the other side of the question, we may
consider those cases of reversion which have been recorded from time to
time, and which always have been considered as direct proofs of the
varietal character of the reverting form. Reversion means the falling
back or returning to another type, and the word itself expresses the
idea that this latter type is the form from which the variety has
arisen.
Some instances of atavism of this kind are well known, as they are often
repeated by individuals that are multiplied by buds or by grafting.
Before looking attentively into the different features of the many cases
of rare reversions it will be advisable to quote a few examples.
The flowering-currant of the Pacific Coast or North American scarlet
ribes (_Ribes sanguineum_), a very popular ornamental shrub, will serve
as a good example. It is prized because of its brilliant red racemes of
flowers which blossom early in the spring, before the appearance of the
leaves. From this species a white form has arisen, which is an old and
widely cultivated one, but not so highly prized because of its pale
flowers. These are not of a pure white, but have retained a faint
reddish hue. The young twigs and the stalks of the [167] leaves afford
an instance of correlated variability since in the species the red color
shows itself clearly mixed with the green, while in the variety this
tinge is wholly wanting.
Occasionally this white-flowered currant reverts back to the original
red type and the reversion takes place in the bud. One or two buds on a
shrub bearing perhaps a thousand bunches of white flowers produce twigs
and leaves in which the red pigment is noticeable and the flowers of
which become brightly colored. If such a twig is left on the shrub, it
may grow further, ramify and evolve into a larger group of branches. All
of them keep true to the old type. Once reverted, the branches remain
forever atavistic. It is a very curious sight, these small groups of red
branches among the many white ones. And for this reason attention is
often called to it, and more than once I myself have had the opportunity
of noting its peculiarities. It seems quite certain that by planting
such shrubs in a garden, we may rely upon seeing sooner or later some
new buds reverting to the prototype.
Very little attention seems hitherto to have been given to this curious
phenomenon, though in many respects it deserves a closer investigation.
The variety is said to have originated from seed in Scotland, many years
ago, and [168] seems to be propagated only by cuttings or by grafting.
If this is true, all specimens must be considered as constituting
together only one individual, notwithstanding their wide distribution in
the gardens and parks of so many countries. This induces me to suppose,
that the tendency to reversion is not a character of the variety as
such, but rather a peculiarity of this one individual. In other words it
seems probable that when the whitish variety arises a second time from
the red species, it is not at all necessary that it should exhibit this
same tendency to revert. Or to put it still in another way, I think that
we may suppose that a variety, which might be produced repeatedly from
the same original stock, would only in rare individuals have a tendency
to revert, and in most cases would be as absolutely constant as the
species itself.
Such a conception would give us a distinct insight into the cause of the
rarity of these reversions. Many varieties of shrubs and trees have
originated but once or twice. Most of them must therefore, if our
supposition is correct, be expected to be stable and only a few may be
expected to be liable to reversions.
Among the conifers many very good cases of reversions by buds are to be
found in gardens and glasshouses. They behave exactly like the whitish
currant. But as the varietal characters [169] are chiefly found in the
foliage and in the branches, these aberrations are to be seen on the
plants during the whole year. Moreover they are in some cases much more
numerous than in the first instance. The _Cryptomeria_ of Japan has a
variety with twigs resembling ropes. This is not caused by a twisting,
but only by a curvature of the needles in such a way that they seem to
grow in spiral lines around the twigs. This variety often reverts to the
type with widely spread, straight needles. And on many a specimen four,
five, or more reverted branches may be seen on different parts of the
same shrub. Still more widely cultivated is the shrub called
_Cephalotaxus pedunculata fastigiata_, and more commonly known under its
old name of _Podocarpus koraiana_. It is the broomlike variety of a
species, nearly allied to the common American and European species of
yew, (_Taxus minor_ and _T. baccata_). It is a low shrub, with broadly
linear leaves of a clear green. In the species the leaves are arranged
in two rows, one to the left and one to the right of the horizontally
growing and widely spreading branches. In the variety the branches are
erect and the leaves inserted on all sides. When sporting, it returns to
the bilateral prototype and flat wings of fan-shaped twigs are produced
laterally on its dense broom-like tufts.
[170] Wherever this variety is cultivated the same reversion may be
seen; it is produced abundantly, and even under seemingly normal
circumstances. But as in the case of the _Ribes_ all the specimens are
derived by buds from a single original plant. The variety was introduced
from Japan about the year 1860, but is probably much older. Nothing is
known as to its real origin. It never bears flowers or fruits. It is
curious to note that the analogous variety of the European yew, _Taxus
baccata fastigiata_, though much more commonly cultivated than the
_Cephalotaxus_, never reverts, at least as far as I have been able to
ascertain. This clearly corroborates the explanation given above.
After considering these rare instances of more widely known reversions,
we may now examine the question of atavism from a broader point of view.
But in doing so it should once more be remembered, that all cases of
hybridism and also all varieties sporting annually or frequently, are to
be wholly excluded. Only the very rare occurrence of instances of
atavism in varieties that are for the rest known to be absolutely
constant, is to be considered.
Atavism or reversion is the falling back to a prototype. But what is a
prototype? We may take the word in a physiologic or in a systematic
sense. Physiologically the signification is a [171] very narrowly
restricted one; and includes only those ancestors from which a form is
known to have been derived. But such evidence is of course historic. If
a variety has been observed to spring from a definite species, and if
the circumstances have been sufficiently ascertained not to leave the
slightest doubt as to its pure origin, and if moreover all the evidence
has been duly recorded, we may say that the origin of the variety is
historically known. In most cases we must be content with the testimony,
given somewhat later, and recorded after the new variety had the
opportunity of showing its greater merits.
If it now happens that such a variety of recorded origin should
occasionally revert to its parent-species, we have all we can wish for,
in the way of a thoroughly proved case of atavism. But such instances
are very rare, as the birth of most varieties has only been very
imperfectly controlled.
Next to this comes the systematic relation of a variety to its species.
The historic origin of the variety may be obscure, or may simply be
forgotten. But the distinguishing marks are of the order described in
our last lecture, either in the positive or in the negative direction,
and on this ground the rarer form is considered to be a variety of the
more wide-spread one. If [172] now the presumed variety sports and runs
over to the presumed type, the probability of the supposed relation is
evidently enhanced. But it is manifest that the explanation rests upon
the results of comparative studies, and not upon direct observations of
the phenomena themselves.
The nearer the relations between the two types in question, the less
exposed to doubt and criticism are the conclusions. But the domain of
atavism is not restricted to the cases described. Quite on the contrary
the facts that strike us most forcibly as being reversions are those
that are apt to give us an insight into the systematic affinity of a
higher degree. We are disposed to make use of them in our attempts to
perfect the natural system and to remould it in such a way as to become
a pedigree of the related groups. Such cases of atavism no doubt occur,
but the anomalies referred to them must be interpreted merely on the
ground of our assumptions as to the relative places in the system to be
assigned to the different forms.
Though such instances cannot be considered as belonging strictly to the
subject we are dealing with, I think it may be as well to give an
example, especially as it affords an occasion for referring to the
highly important researches of Heinricher on the variability and
atavistic [173] tendencies of the pale blue flag or _Iris pallida_. The
flowers of the blue flags have a perianth of six segments united below
into a tube. The three outer parts are dilated and spreading, or
reflexed, while the three inner usually stand erect, but in most species
are broad and colored like the outer ones. Corresponding to the outer,
perianth-segments are the three stamens and the three, petal-like
divisions of the style, each bearing a transverse stigma immediately
above the anther. They are pollinated by bumble-bees, and in some
instances by flies of the genus _Rhingia_, which search for the honey,
brush the pollen out of the anthers and afterwards deposit it on the
stigma. According to systematic views of the monocotyledons the original
prototype of the genus _Iris_ must have had a whorl of six equal, or
nearly equal perianth-segments and six stamens, such as are now seen in
the more primitive types of the family of the lilies, as for instance in
the lilies themselves, the tulips, hyacinths and others. As to the
perianth this view is supported by the existence of one species, the
_Iris falcifolia_, the perianth of which consists of six equal parts.
But species with six stamens are wholly lacking. Heinricher however, in
cultivating some anomalous forms of _Iris pallida_, succeeded in filling
out this gap and in producing [174] flowers with a uniform perianth and
six stamens, recalling thereby the supposed ancestral type. The way in
which he got these was as follows: he started from some slight
deviations observed in the flowers of the pale species, sowed the seeds
in large numbers and selected from the seedlings only those which
clearly showed anomalies in the expected atavistic direction. By
repeating this during several generations he at last reached his goal
and was able to give reality to the prototype, which formerly was only a
hypothetical one. The _Iris kaempferi_, a large-flowered Japanese
species much cultivated in gardens, is very variable in the number of
the different parts of its flowers, and may in some instances be seen
even with six stamens. If studied in the same way as Heinricher's iris,
it no doubt will yield highly interesting and confirmatory results.
Many other instances of such systematic atavism could be given, and
every botanist can easily add some from memory. Many anomalies,
occurring spontaneously, are evidently due to the same principle, but it
would take too long to describe them.
Reversion may occur either by buds or by seeds. It is highly probable
that it occurs more readily by sexual than by asexual propagation. But
if we restrict the discussion to the limits [175] hitherto observed,
seed-reversions must be said to be extremely rare. Or rather cases which
are sufficiently certain to be relied upon, are very rare, and perhaps
wholly lacking. Most of the instances, recorded by various writers, are
open to question. Doubts exist as to the purity of the seeds and the
possibility of some unobserved cross disturbing the results.
In the next lecture we shall deal in general with the ordinary causes
and results of such crosses. We shall then see that they are so common
and occur so regularly under ordinary circumstances that we can never
rely on the absolute purity of any seeds, if the impossibility of an
occasional cross has not been wholly excluded, either by the
circumstances themselves, or by experimental precautions taken during
the flowering period.
For these reasons cases of atavism given without recording the
circumstances, or the precautions that guarantee the purity of the
fertilization, should always be disregarded. And moreover another proof
should always be demanded. The parent which yielded the seeds might be
itself a hybrid and liable to reversions by the ordinary laws of the
splitting up of hybrids. Such cases should likewise be discarded, since
they bring in confusing elements. If we review the long list of recorded
cases by these [176] strict methods of criticism very few instances will
be found that satisfy legitimate demands. On this ground it is by far
safer in the present state of our knowledge, to accept bud-variations
only as direct proofs of true atavism. And even these may not always be
relied on, as some hybrids are liable to split up in a vegetative way,
and in doing so to give rise to bud-variations that are in many respects
apparently similar to cases of atavism. But fortunately such instances
are as yet very rare.
After this discussion it would be bold indeed to give instances of
seed-atavism, and I believe that it will be better to refrain wholly
from doing so.
Many instances of so-called atavism are of purely morphologic nature.
The most interesting cases are those furnished by the forms which some
plants bear only while young, and which evidently connect them with
allied species, in which the same features may be seen in the adult
state. Some species of the genus _Acacia_ bear bipinnate leaves, while
others have no leaves at all, but bear broadened and flattened petioles
instead. The second type is presumed to be descended from the first by
the loss of the leaflets and the modification of the stalks into flat
and simple phyllodes. But many of them are liable to recall this
primitive form [177] when very young, in the first two or three, or
sometimes in eight or ten primary leaves. These leaves are small because
of the weakness of the young plant and therefore often more or less
reduced in structure. But they are usually strictly bipinnate and
thereby give testimony as to their descent from species which bear such
leaves throughout their life.
Other similar cases could be given, but this will suffice. They once
more show how necessary it is to separate the different cases, thrown
together until now, under this general name of atavism. It would be far
better to give them all special names, and as long as these are not
available we must be cautious not to be misguided by the name, and
especially not to confuse different phenomena with one another, because
at the present time they bear the same names.
Taking into consideration the relatively numerous restrictions resulting
from this discussion, we will now make a hasty survey of some of the
more notable and generally acknowledged cases of atavism by
bud-propagation. But it should be repeated once more that most of the
highly cultivated plants, grown as vegetables, or for their fruit or
flowers, have so many crosses in their ancestry, that it seems better to
exclude them from all considerations, in which purity of [178] descent
is a requisite. By so doing, we exclude most of the facts which were
until now generally relied upon. For the roses, the hyacinths, the
tulips, the chrysanthemums always have furnished the largest
contributions to the demonstrations of bud-variation. But they have been
crossed so often, that doubt as to the purity of the descent of any
single form may recur, and may destroy the usefulness of their many
recorded cases of bud-variation for the demonstration of real atavism.
The same assertion holds good in many other cases, as with _Azalea_ and
_Camellia_. And the striped varieties of these genera belong to the
group of ever-sporting forms, and therefore will be considered later on.
So it is with carnations and pinks, which occasionally vary by layering,
and of which some kinds are so uncertain in character that they are
called by floriculturists "catch-flowers." On the other hand there is a
larger group of cases of reversion by buds, which is probably not of
hybrid nature, nor due to innate inconstancy of the variety, but must be
considered as pure atavism. I refer to the bud-variations of so many of
our cultivated varieties of shrubs and trees. Many of them are
cultivated because of their foliage. They are propagated by grafting,
and in most cases it is probable that all the numerous specimens [179]
of the same variety have been derived in this way from one primitive,
aberrant individual. We may disregard variegated leaves, spotted or
marked with white or yellow, because they are too inconstant types.
We may next turn our attention to the varieties of trees with cut
leaves, as the oakleaved _Laburnum_, the parsley-leaved vine and the
fern-leaved birch. Here the margin of the leaves is deeply cut and
divided by many incisions, which sometimes change only the outer parts
of the blade, but in other cases may go farther and reach, or nearly
reach, the midvein, and change the simple leaf into a seemingly compound
structure. The anomaly may even lead to the almost complete loss of all
the chorophyll-tissue and the greater part of the lateral veins, as in
the case of the cut-leaved beech or _Fagus sylvatica pectinata_.
Such varieties are often apt to revert by buds to the common forms. The
cut-leaved beech sometimes reverts partially only, and the branches
often display the different forms of cut-leaved, fern-like, oak-leaved
and other variously shaped leaves on the same twigs. But this is merely
due to the wide variability of the degree of fissure and is to be
considered only as a fluctuation between somewhat widely distant
extremes, which may even apparently include [180] the form of the common
beech-leaves. It is not a bud-variation at all, and it is to be met with
quite commonly while the true reversions by buds are very rare and are
of the nature of sports appearing suddenly and remaining constant on the
same twig. Analogous phenomena of wide variability with true reversion
may be seen in the variety of the European hornbeam called _Carpinus
Betulus heterophylla_. The leaves of this tree generally show the
greatest diversity in form. Some other cases have been brought together
by Darwin. In the first place a subvariety of the weeping-willow with
leaves rolled up into a spiral coil. A tree of this kind kept true for
twenty-five years and then threw out a single upright shoot bearing flat
leaves. The barberry (_Berberis_) offers another case; it has a well
known variety with seedless fruit, which can be propagated by cuttings
or layers, but its runners are said always to revert to the common form,
and to produce ordinary berries with seeds. Most of the cases referred
to by Darwin, however, seem to be doubtful and cannot be considered as
true proofs of atavism until more is known about the circumstances under
which they were produced.
Red or brown-leaved varieties of trees and shrubs also occasionally
produce green-leaved branches, and in this way revert to the type [181]
from which they must evidently have arisen. Instances are on record of
the hazel, _Corylus Avellana_, of the allied _Corylus tubulosa_, of the
red beech, the brown birch and of some other purple varieties. Even the
red bananas, which bear fruits without seeds and therefore have no other
way of being propagated than by buds, have produced a green variety with
yellow fruits. The _Hortensia_ of our gardens is another instance of a
sterile form which has been observed to throw out a branch with cymes
bearing in their center the usual small staminate and pistillate flowers
instead of the large radiate and neutral corollas of the variety,
thereby returning to the original wild type. Crisped weeping-willows,
crisped parsley and others have reverted in a similar manner.
All such cases are badly in need of a closer investigation. And as they
occur only occasionally, or as it is commonly stated, by accident, the
student of nature should be prepared to examine carefully any case which
might present itself to him. Many phases of this difficult problem could
no doubt be solved in this way. First of all the question arises as to
whether the case is one of real atavism, or is only seemingly so, being
due to hybrid or otherwise impure descent of the varying individual, and
secondly whether it may be only an instance of the regularly [182]
occurring so-called atavism of the sporting varieties with which we
shall deal in a later lecture. If it proves to be real atavism and rare,
the case should be accurately described and figured, or photographed if
possible; and the exact position of the reverting bud should be
ascertained. Very likely the so-called dormant or resting buds are more
liable to reversions than the primary ones in the arils of the leaves of
young twigs. Then the characters of the atavistic branches should be
minutely compared with those of the presumed ancestor; they may be quite
identical with them or slightly divergent, as has been asserted in some
instances. The atavism may be complete in one case, but more or less
incomplete in others. By far the most interesting point is the question,
as to what is to be expected from the seeds of such an atavistic branch.
Will they keep true to the reverted character, or return to the
characters of the plant which bears the retrograde branch? Will all of
them do so, or only part of them, and how large a part? It is very
astonishing that this question should still be unsolved where so many
individual trees bear atavistic branches that remain on them through
long series of years. But then many such branches do not flower at all,
or if they flower and bear seed, no care is taken to prevent [183]
cross-fertilization with the other flowers of the same plant, and the
results have no scientific value. For anyone who cares to work with the
precautions prescribed by science, a wide field is here open for
investigation, because old reverted branches may be met with much less
rarely than new ones.
Finally the possibility is always to be considered that the tendency to
bud-reversions may be a special feature of some individuals, and may not
be met with in others of the same variety. I have spoken of this before.
For the practical student it indicates that a specimen, once observed to
produce atavistic buds, may be expected to do the same thing again. And
then there is a very good chance that by combining this view with the
idea that dormant buds are more apt to revert than young ones, we may
get at a method for further investigation, if we recur to the practice
of pruning. By cutting away the young twigs in the vicinity of dormant
buds, we may incite these to action. Evidently we are not to expect that
in so doing they will all become atavistic. For this result is not at
all assured; on the contrary, all that we might hope to attain would be
the possibility of some of them being induced to sport in the desired
direction.
Many questions in scientific research can only [184] be answered by long
and arduous work in well equipped laboratories; they are not to be
attempted by every one. But there are other problems which the most
complete of institutions are not able to study if opportunity is not
offered them, and such opportunities are apt to occur more often in
fields, gardens, parks, woods and plains, than in the relatively small
experimental gardens of even the largest institution. Therefore,
whosoever has the good fortune to find such sports, should never allow
the occasion to pass without making an investigation that may bring
results of very great importance to science.
[185]
LECTURE VII
FALSE ATAVISM OR VICINISM
About the middle of the last century Louis de Vilmorin showed that it
was possible to subject plants to the methods of amelioration of races
then in use for domestic animals, and since that time atavism has played
a large part in all breeding-processes. It was considered to be the
greatest enemy of the breeder, and was generally spoken of as a definite
force, working against and protracting the endeavors of the
horticulturist.
No clear conception as to its true nature had been formulated, and even
the propriety of designating the observed phenomena by the term atavism
seemed doubtful. Duchesne used this word some decades ago to designate
those cases in which species or varieties revert spontaneously, or from
unknown internal causes, to some long-lost characters of their
ancestors. Duchesne's definition was evidently a sharp and useful one,
since it developed for the first time the idea of latent or dormant
qualities, [186] formerly active, and awaiting probably through
centuries an occasion to awaken, and to display the lost characters.
Cases of apparent reversion were often seen in nurseries, especially in
flower culture, which under ordinary circumstances are rarely wholly
pure, but always sport more or less into the colors and forms of allied
varieties. Such sporting individuals have to be extirpated regularly,
otherwise the whole variety would soon lose its type and its uniformity
and run over to some other form in cultivation in the vicinity. For this
reason atavism in nurseries causes much care and labor, and consequently
is to be dealt with as a very important factor.
From time to time the idea has suggested itself to some of the best
authorities on the amelioration of plants, that this atavism was not due
to an innate tendency, but, in many cases at least, was produced by
crosses between neighboring varieties. It is especially owing to Verlot
that this side of the question was brought forward. But breeders as a
rule have not attached much importance to this supposition, chiefly
because of the great practical difficulties attending any attempt to
guard the species of the larger cultures against intermixture with other
varieties. Bees and humble-bees fly from bud to bud, and carry the
pollen from one [187 ] sort to another, and separation by great
distances would be required to avoid this source of impurity.
Unfortunately the arrangements and necessities of large cultures make it
impossible to isolate the allied varieties from each other.
From a theoretical point of view the origin of these impurities is a
highly important question. If the breeders' atavism is due to crosses,
and only to this cause, it has no bearing at all on the question of the
constancy of varieties. And the general belief, that varieties are
distinguished from true species by their repeated reversion and that
even such reversibility is the real distinction of a variety, would not
hold.
For this reason I have taken much trouble in ascertaining the
circumstances which attend this form of atavism. I have visited a number
of the leading nurseries of Europe, tested their products in various
ways, and made some experiments on the unavoidable conditions of
hybridizing and on their effect on the ensuing generations. These
investigations have led me to the conclusion, that atavism, as it is
generally described, always or nearly always is due to hybridization,
and therefore it is to be considered as untrue or false atavism.
True atavism, or reversion caused by an innate latent tendency, seems
to be very rare, [188] and limited to such cases as we have spoken of
under our last heading. And since the definition, given to this term by
its author, Duchesne, is generally accepted in scientific works, it
seems better not to use it in another sense, but rather to replace it in
such cases by another term. For this purpose I propose the word
vicinism, derived from the Latin vicinus or neighbor, as indicating the
sporting of a variety under the influence of others in its vicinity.
Used in this way, this term has the same bearing as the word atavism of
the breeders, but it has the advantage of indicating the true cause
thereof.
It is well known that the term variability is commonly employed in the
broadest possible sense. No single phenomenon can be designated by this
name, unless some primary restriction be given. Atavism and vicinism are
both cases of variability, but in wholly different sense. For this
reason it may be as well, to insert here a short survey of the general
meanings to be conveyed by the term variation. It implies in the first
place the occurrence of a wide range of forms and types, irrespective of
their origin, and in the second place the process of the change in such
forms. In the first signification it is nearly identical with
polymorphy, or richness of types, especially so when these [189] types
are themselves quite stable, or when it is not at all intended to raise
the question of their stability. In scientific works it is commonly used
to designate the occurrence of subspecies or varieties, and the same is
the case in the ordinary use of the term when dealing with cultivated
plants. A species may consist of larger or smaller groups of such units,
and they may be absolutely constant, never sporting if hybridization is
precluded, and nevertheless it may be called highly variable. The
opium-poppy affords a good instance. It "varies" in height, in color of
foliage and flowers; the last are often double or laciniated; it may
have white or bluish seeds, the capsules may open themselves or remain
closed and so on. But every single variety is absolutely constant, and
never runs into another, when the flowers are artificially pollinated
and the visits of insects excluded. So it is with many other species.
They are at the same time wholly stable and very variable.
The terms variation and variety are used frequently when speaking of
hybrids. By crossing forms, which are already variable in the sense just
mentioned, it is easy to multiply the number of the types, and even in
crossing pure forms the different characters may be combined in
different ways, the resulting combinations [190] yielding new, and very
often, valuable varieties. But it is manifest that this form of
variation is of quite another nature from the variations of pure races.
Many hybrid varieties are quite constant, and remain true to their type
if no further crosses are made; many others are artificially propagated
only in a vegetative way, and for this reason are always found true.
Hybrid varieties as a rule were formerly confused with pure varieties,
and in many instances our knowledge as to their origin is quite
insufficient for sharp distinctions. To every student of nature it is
obvious, that crossing and pure variability are wholly distinct groups
of phenomena, which should never be treated under the same head, or
under the same name.
Leaving aside polymorphy, we may now discuss those cases of variability,
in which the changes themselves, and not only their final results play a
part. Of such changes two types exist. First, the ever-recurring
variability, never absent in any large group of individuals, and
determining the differences which are always to be seen between parents
and their children, or between the children themselves. This type is
commonly called "individual variability" and since this term also has
still other meanings, it has of late become customary to use instead the
term "fluctuating variability." [191] And to avoid the repetition of the
latter word it is called "fluctuation." In contrast to these
fluctuations are the so-called sports or single varieties, not rarely
denominated spontaneous variations, and for which I propose to use the
term "mutations." They are of very rare occurrence and are to be
considered as sudden and definite steps.
Lastly, we have to consider those varieties, which vary in a much wider
range than the ordinary ones, and seem to fluctuate between two opposite
extremes, as for instance variegated leaves, cultivated varieties with
variegated or striped flowers, double flowers and some other anomalies.
They are eversporting and ever-returning from one type to the other. If
however, we take the group of these extremes and their intermediates as
a whole, this group remains constant during the succeeding generations.
Here we find once more an instance of the seemingly contradictory
combination of high variability and absolute constancy. It means that
the range of variability has quite definite limits, which in the common
course of things, are never transgressed.
We may infer therefore that the word variability has such a wide range
of meanings that it ought never be used without explanation. [192]
Nothing indeed, is more variable than the signification of the term
variable itself.
For this reason, we will furthermore designate all variations under the
influence of neighbors with the new and special term "vicinism." It
always indicates the result of crossing.
Leaving this somewhat lengthy terminological discussion, we now come to
the description of the phenomenon itself. In visiting the plantations of
the seedsmen in summer and examining the large fields of garden-flowers
from which seed is to be gathered, it is very rare to find a plot quite
pure. On the contrary, occasional impurities are the rule. Every plot
shows anomalous individuals, red or white flowers among a field of blue,
normal among laciniated, single among double and so on. The most curious
instance is afforded by dwarf varieties, where in the midst of hundreds
and thousands of small individuals of the same height, some specimens
show twice their size. So for instance, among the dwarfs of the
larkspur, _Delphinium Ajacis_.
Everywhere gardeners are occupied in destroying these "atavists," as
they call them. When in full bloom the plants are pulled up and thrown
aside. Sometimes the degree of impurity is so high, that great piles of
discarded plants of the same species lie about the [193] paths, as I
have seen at Erfurt in the ease of numerous varieties of the Indian
cress or _Tropaeolum_.
Each variety is purified at the time when it shows its characters most
clearly. With vegetables, this is done long before flowering, but with
flowers only when in full bloom, and with fruits, usually after
fertilization has been accomplished. It needs no demonstration to show
that this difference in method must result in very diverging degrees of
purity.
We will confine ourselves to a consideration of the flowers, and ask
what degree of purity may be expected as the result of the elimination
of the anomalous plants during the period of blooming.
Now it is evident that the colors and forms of the flowers can only be
clearly distinguished, when they are fully displayed. Furthermore it is
impossible to destroy every single aberrant specimen as soon as it is
seen. On the contrary, the gardener must wait until all or nearly all
the individuals of the same variety have displayed their characters, as
only in this way can all diverging specimens be eliminated by a single
inspection. Unfortunately the insects do not wait for this selection.
They fertilize the flowers from the beginning, and the damage will have
been done [194] long before the day of inspection comes around. Crosses
are unavoidable and hybrid seeds will unavoidably come into the harvest.
Their number may be limited by an early eradication of the vicinists, or
by the elimination of the first ripe seeds before the beginning of the
regular harvest, or by other devices. But some degree of impurity will
remain under ordinary circumstances.
It seems quite superfluous to give more details. In any case in which
the selection is not done before the blooming period, some impurities
must result. Even if it is done before that time, errors may occur, and
among hundreds and thousands of individuals a single anomalous one may
escape observation.
The conclusion is, that flower seeds as they are offered in commerce,
are seldom found absolutely pure. Every gardener knows that he will have
to weed out aberrant plants in order to be sure of the purity of his
beds. I tested a large number of samples of seeds for purity, bought
directly from the best seed growers. Most of them were found to contain
admixtures and wholly pure samples were very rare.
I will now give some illustrative examples. From seeds of a yellow
snapdragon, I got one red-flowered specimen among half a hundred [195]
yellow ones, and from the variety "Delila" of the same species two red
ones, a single white and two belonging to another variety called
"Firefly." _Calliopsis tinctoria_ has three varieties, the ordinary
type, a brown-flowered one and one with tubular rays. Seeds of each of
these three sorts ordinarily contain a few belonging to the others.
_Iberis umbellata rosea_ often gives some white and violet examples. The
"Swan" variety of the opium-poppy, a dwarfish double-flowered form of a
pure white, contained some single-flowered and some red-flowered plants,
when sown from commercial seed are said to be pure. But these were only
occasional admixtures, since after artificial fertilization of the
typical specimens the strain at once became absolutely pure, and
remained so for a series of generations, as long as the experiment was
continued. Seeds of trees often contain large quantities of impurities,
and the laciniated varieties of birch, elder and walnut have often been
observed to come true only in a small number of seedlings.
In the case of new or young varieties, seed merchants often warn their
customers as to the probable degree of purity of the seeds offered, in
order to avoid complaints. For example the snow-white variety of the
double daisy, _Bellis perennis plena_, was offered at the start as
containing [196] as much as 20% of red-flowered specimens.
Many fine varieties are recorded to come true from seed, as in the case
of the holly with yellow fruits, tested by Darwin. Others have been
found untrue to a relatively high degree, as is notorious in the case of
the purple beech. Seeds of the laciniated beech gave only 10% of
laciniated plants in experiments made by Strasburger; seeds of the
monophyllous acacia, _Robinia Pseud-Acacia monophylla_, were found to be
true in only 30% of the seedlings. Weeping ashes often revert to the
upright type, red May-thorns (_Crataegus_) sometimes revert nearly
entirely to the white species and the yellow cornel berry is recorded to
have reverted in the same way to the red berries of the _Cornus Mas_.
Varieties have to be freed by selection from all such impurities, since
isolation is a means which is quite impracticable under ordinary
circumstances. Isolation is a scientific requirement that should never
be neglected in experiments, indeed it may be said to be the first and
most important requisite for all exact research in questions of
variability and inheritance. But in cultivating large fields of allied
varieties for commercial purposes, it is impossible to grow them at such
distances from each other [197] as to prevent cross-pollination by the
visits of bees.
This purification must be done in nearly every generation. The oldest
varieties are to be subjected to it as well as the latest. There is no
regular amelioration, no slow progression in the direction of becoming
free from these admixtures. Continuous selection is indispensable to
maintain the races in the degree of purity which is required in
commerce, but it does not lead to any improvement. Nor does it go so far
as to become unnecessary in the future. This shows that there must be a
continuous source of impurities, which in itself is not neutralized by
selection, but of which selection can only eliminate the deteriorating
elements.
The same selection is usually applied to new varieties, when they
occasionally arise. In this case it is called "fixing," as gardeners
generally believe that through selection the varieties are brought to
the required degree of purity. This belief seems to rest mainly on
observations made in practice, where, as we have seen, isolation is of
very rare application. Most varieties would no doubt be absolutely pure
from the first moment of their existence, if it were only possible to
have them purely fertilized. But in practice this is seldom to be
obtained. Ordinarily the breeder is content with such slow [198]
improvement as may be obtained with a minimum of cost, and this mostly
implies a culture in the same part of the nursery with older varieties
of the same species. Three, four or five years are required to purify
the novelty, and as this same length of time is also required to produce
sufficient quantities of seed for commercial purposes, there is no
strong desire to shorten the period of selection and fixation. I had
occasion to see this process going on with sundry novelties at Erfurt in
Germany. Among them a chamois-colored variety of the common stock, a
bluish _Clarkia elegans_ and a curiously colored opium-poppy may be
mentioned. In some cases the crossfertilization is so overwhelming, that
in the next generation the novelty seems entirely to have disappeared.
The examples given may suffice to convey a general idea of the
phenomenon, ordinarily called atavism by gardeners, and considered
mostly to be the effect of some innate tendency to revert to the
ancestral form. It is on this conception that the almost universal
belief rests, that varieties are distinguished, as such, from species by
their inconstancy. Now I do not deny the phenomenon itself. The impurity
of seeds and cultures is so general and so manifest, and may so easily
be tested by every one [199] that it cannot reasonably be subjected to
any doubt. It must be conceded to be a fact, that varieties as a rule
revert to their species under the ordinary circumstances of commercial
culture. And I cannot see any reason why this fact should not be
considered as stating a principal difference between varieties and
species, since true species never sport into one another.
My objection only refers to the explanation of the observed facts.
According to my view nearly all these ordinary reversions are due to
crosses, and it is for this reason that I proposed to call them by a
separate name, that of "vicinists." Varieties then, by means of such
spontaneous intercrossing sport into one another, while species either
do not cross, or when crossing produce hybrids that are otherwise
constituted and do not give the impression of atavistic reversion.
I must not be content with proposing this new conception, but must give
the facts on which this assumption rests. These facts are the results of
simple experiments, which nevertheless are by no means easy to carry
out, as they require the utmost care to secure the absolute purity of
the seeds that are employed. This can only be guaranteed by previous
cultures of isolated plants or groups of plants, or by artificial
pollination.
[200] Once sure of this preliminary condition, the experiment simply
consists in growing a variety at a given distance from its species and
allowing the insects to transfer the pollen. After harvesting the seed
thus subjected to the presumed cause of the impurities, it must be sown
in quantities, large enough to bring to light any slight anomaly, and to
be examined during the period of blooming.
The wild seashore aster, _Aster Tripolium_, will serve as an example. It
has pale violet or bluish rays, but has given rise to a white variety,
which on testing, I have found pure from seed. Four specimens of this
white variety were cultivated at a distance of nearly 100 meters from a
large lot of plants of the bluish species. I left fertilization to the
bees, harvested the seeds of the four whites separately and had from
them the following year more than a thousand flowering plants. All of
them were of the purest white, with only one exception, which was a
plant with the bluish rays of the species, wholly reverting to its
general type. As the variety does not give such reversions when
cultivated in isolation, this sport was obviously due to some cross in
the former year. In the same way I tried the white Jacob's ladder,
_Polemonium coeruleum_ album in the neighborhood of the blue-flowered
species, the distance [202] in this case being only 40 meters. Of two
hundred seeds one became a blue atavist, or rather vicinist, while all
others remained true to the white type. The same was observed in the
white creeping thyme, or _Thymus Serpyllum album_, and the white
self-heal, _Brunella vulgaris alba_, gave even so much as 28% seedlings
with purple corollas out of some 400 specimens, after being cultivated
in close proximity to its parent-species. I have tried many other
species, but always with the same result. Such atavists only arise by
cultivation in the proximity of allied varieties, never in isolation.
They are not real atavists, but only vicinists.
In order to show this yet more clearly, I made another experiment with
the white selfheal. I had a lot of the pinnate-leaved variety with
purple flowers and somewhat stouter stems, and cultivated single plants
of the whiteflowering sort at distances that varied from 2-16 meters.
The seeds of each plant were collected and sown separately, those of the
nearest gave up to 5 or 6 hybrids from the seeds of one parent, while
those of the farthest gave only one purple-flowered plant for each
parent. Evidently the chance of the pollen being carried by bees is much
greater on short than on longer distances.
True hybrids between species may arise in quite the same way, and since
it is obviously impossible to attribute them to an innate tendency to
reversion, they afford an absolutely irrefutable proof of the assertion
that pollen is often brought by insects from one lot of plants to
another. In this way I obtained a hybrid between the common Jacob's
ladder and the allied species _Polemonium dissectum_. With a distance of
100 meters between them I had two hybrid seeds among a hundred of pure
ones. At a similar distance pollen was carried over from the wild
radish, _Raphanus Raphanistrum_, to the allied _Raphanus caudatus_, and
I observed the following year some very nice hybrids among my seedlings.
A hybrid-bean between _Phaseolus nanus_ and _P. multiflorus_, and some
hybrids between the yellow daisy, _Chrysanthemum segetum_ and the allied
_Chrysanthemum coronarium_ or ox-eye daisy which also arose
spontaneously in my garden between parents cultivated at recorded
distances, might further be noted. Further details of these experiments
need not be given. Suffice to say, that occasional crosses between
species do occur, and not even rarely, that they are easily recognized
as such and cannot be confused with cases of atavism, and that therefore
they give proof to the assumption that in the same way crosses
ordinarily occur also between varieties [203] of the same species, if
cultivated at small distances apart, say 40-50 meters or even more.
Vicinism therefore, may play a part in all such cultures, enough to
account for all the impurities observed in the nurseries or in
commercial seed-samples.
Of course this whole discussion is limited to such species as are not
only as a rule visited by insects, but are dependent on these visits for
their fertilization. Most of our garden-flowers are included in this
category. If not then we may expect to find the cultures and seeds pure,
irrespective of the distances between allied varieties, as for instance
with peas, which are known to be self-fertilizing. Another instance is
given by the barley. One of the most curious anomalous varieties of this
cereal, is the "Nepaul-barley," with its small adventitious flowers on
the palets or inner scales. It is a very old, widely cultivated sort,
which always comes true from seed, and which has been tested in repeated
experiments in my garden. The spikelets of this curious plant are
oneflowered and provided with two linear glumes or outer scales. Of the
inner scales or palets, the outer one is three-lobed at the summit,
hence the varietal name of _Hordeum vulgare trifurcatum_. The central
lobe is oblong and hollow, covering a small supernumerary floret
inserted [204] at its base. The two lateral lobes are narrower,
sometimes linear, and are often prolonged into an awn, which is
generally turned away from the center of the spike. The central lobe
sometimes bears two florets at its base, although but one is usually
present and it may be incomplete.
I might give one more instance from my own experience. A variety of the
evening-primrose with small linear petals was once found by one of my
sons growing wild near Amsterdam. It was represented by only one
individual, flowering among a great many of the ordinary type with broad
petals. But the evening-primroses open their anthers in the morning,
fertilize themselves during the day, and only display their beautiful
flowers in the evening, after the pollination has been accomplished.
They then allure evening moths, such as _Agrotis_ and _Plusia_, by their
bright color, their sweet honeysmell and their nectar. Since the
fertilization is accomplished many hours before opening, crosses are
effected only in rare instances, and the seeds commonly remain true to
the parent type. The seeds of this one plant, when sown separately in my
garden, produced exclusively flowers with the small linear petals of
their parent. Although I had a hundred individuals bearing many
thousands of flowers, there was not an instance of reversion. And such
would [205] immediately have been observed, had it occurred, because the
hybrids between the cruciate and the normal flowers are not
intermediate, but bear the broad petals of the _O. biennis_.
We may now take up another phase of the question, that of the running
out of new varieties, shortly after their introduction into a new
country, or later.
The most widely known instance of this is that of the American corn in
Baden, recorded by Metzger and quoted by Darwin as a remarkable instance
of the direct and prompt action of climate on a plant. It has since been
considered as a reversion to the old type. Such reversions invariably
occur, according to Wallace, in cases of new varieties, which have been
produced quickly. But as we now know, such reversions are due to
spontaneous crosses with the old form, and to the rule, that the hybrids
of such origin are not intermediate, but assume the features of the
older of the two parents. In the light of this experience, Metzger's
observation becomes a typical instance of vicinism. It relates to the
"Tuscarora" corn of St. Louis, a variety with broad and flat white
seeds.
About the year 1840, this corn was introduced into Baden in Germany, and
cultivated by Metzger. In the first year it came true to type, and [206]
attained a height of 12 feet, but the season did not allow its seeds to
ripen normally. Only a few kernels were developed before the winter.
From this seed plants of a wholly different type came the next year, of
smaller stature, and with more brownish and rounded kernels. They also
flowered earlier and ripened a large number of seeds. The depression on
the outer side of the seed had almost disappeared, and the original
white had become darker. Some of the seeds had even become yellow and in
their rounded form they approached the common European maize. Obviously
they were hybrids, assuming the character of their pollen-parent, which
evidently was the ordinary corn, cultivated all around. The observation
of the next year showed this clearly, for in the third generation nearly
all resemblance to the original and very distinct American species was
lost. If we assume that only those seeds ripened which reverted to the
early-ripening European type, and that those that remained true to the
very late American variety could not reach maturity, the case seems to
be wholly comprehensible, without supposing any other factors to have
been at work than those of vicinism, which though unknown at the period
of Metzger's and Darwin's writings, seems now to be fully understood. No
innate tendency to run out and no changing influence of the climate are
required for an adequate explanation of the facts.
In the observation quoted, what astonishes us most, is the great
rapidity of the change, and the short time necessary for the offspring
of the accidental crosses to completely supplant the introduced type. In
the lecture on the selection of elementary species, closely analogous
cases were described. One of them was the wild oat or _Avena fatua_
which rapidly supplants the cultivated oats in bad years in parts of the
fields. Other instances were the experiments of Risler with the
"Galland" wheat and the observation of Rimpau on "Rivett's bearded"
wheat.
Before leaving the question of vicinism and its bearing on the general
belief of the instability of varieties, which when tested with due care,
prove to be quite stable, it may be as well to consider the phenomena
from another point of view. Our present knowledge of the effects of
crosses between varieties enables us to formulate some general rules,
which may be used to calculate, and in some way to predict, the nature
of the impurities which necessarily attend the cultivation of allied
species in close vicinity. And this mode of cultivation being in almost
universal use in the larger nurseries, [208] we may, by this discussion,
arrive at a more scientific estimation of the phenomena of vicinism,
hitherto described.
The simplest case that may be given, is when an ordinary retrograde
variety is cultivated with the species to which it belongs. For
instance, if dwarfs are cultivated next to the taller type, or a white
variety next to the red or blue-flowering species, or thornless forms in
neighboring beds with the armed species. Bees and Bumble-bees,
butterflies and moths are seen flying from flower to flower, collecting
the honey and carrying pollen. I frequently saw them cross the limits of
the neighboring beds. Loaded with the pollen of the variety they visit
the flowers of the different species and impregnate the stigma with it.
And returning to the variety they bring about similar crosses in the
flowers of the latter. Hybrid seeds will develop in both cases and
become mixed with the crop. We now have to ask the question, what sort
of plants will arise from these hybrid seeds. As a general rule we may
state, first, that the hybrids of either form of cross are practically
the same, secondly that they are not intermediate, but that the
character of one parent prevails to the almost absolute exclusion of the
other and in the third place that the older character dominates the
younger.
[209] The hybrid offspring will therefore, in the main, have the
character of the species and be indistinguishable from it, or show only
such differences as escape ordinary observation. When occurring in the
seeds of the variety they betray themselves as soon as the differential
characters are displayed. Between the thousands of flowering plants of a
white variety the hybrids will instantly catch the eye by their red or
blue corollas. Quite the contrary effect results from the admixture of
hybrids with the seeds of the species itself. Here no difference will
show itself, even in the fullest bloom. The effect of the spontaneous
crosses will pass unobserved. The strain, if pure in the first year,
will seem to be still in the same condition. Or in other terms, the
unavoidable spontaneous crosses will disturb the purity of the variety
in the second year, while they do not seem to interfere at all with the
uniformity of the species. The direct effect of the visits of the
insects is evident in the first case, but passes unobserved in the
latter.
From this it would seem, that spontaneous crosses are hurtful to
varieties, but are innocuous to true species. Certainly this would be
so, were there no selection. But it is easily seen, that through this
operation the effect becomes quite the opposite. For when the fields
[210] are inspected at the time of the fullest display of the varietal
characters, the obvious hybrids will be eliminated, but the hidden ones
will of necessity be spared, as they are concealed among the species by
the similarity of their type. Hence, the harvest of the variety may be
rendered pure or nearly so, while the harvest of the species will retain
the seeds of the hybrids. Moreover it will contain seeds originated by
the spontaneous but numerous crosses of the true plants with the
sparsely intermingled hybrids.
This brings us to the question, as to what will be the visible
consequences of the occurrence of such invisible hybrids in the
following generation. In opposition to the direct effects just
described, we may call them indirect. To judge of their influence, we
must know how hybrid seeds of the first generation behave.
In one of our lectures we will deal with the laws that show the
numerical relations known as the laws of Mendel. But for our present
purpose, these numerical relations are only of subordinate importance.
What interests us here is the fact that hybrids of varieties do not
remain constant in the second generation but usually split as it is
said, remaining hybrid only in part of their offspring, the other
portion returning to the parental types. This however, will show itself
only in those individuals [211] which reassume the character of the
varietal parent, all the others apparently remaining true to the type of
the species. Now it is easy to foresee what must happen in the second
generation if the first generation after the cross is supposed to be
kept free from new vicinistic influences, or from crosses with
neighboring varieties.
We may limit ourselves in the first place to the seeds of the unobserved
hybrids. For the greater part they will repeat the character of their
parents and still remain concealed. But a small number will display the
varietal marks, as for example showing white flowers in a field of blue
ones. Hence, the indirect consequence of the spontaneous crosses will be
the same in the species, as was the direct effect in the variety, only
that it appears a year later. It will then be eliminated in the process
of selection.
Obviously, this elimination conduces only to a partial purification. The
conspicuous plants will be destroyed, but a greater number of hybrids
will remain, still concealed by their resemblance to the general type
and will be spared to repeat the same process next year. So while the
variety may be freed every year from the impurities brought into it in
the preceeding summer, the admixtures of the species [212] will continue
during a number of years, and it may not be possible to get rid of them
at all.
It is an often recurring assertion that white varieties of colored
species are the most stable of all horticultural races. They are often
said to be at least as constant as the species itself, and even to
surpass it in this quality. With our present state of knowledge, the
explanation of this general experience is easily given. For selection
removes the effect of spontaneous crosses from the variety in each year,
and renders it practically pure, while it is wholly inadequate to
produce the same effects on the species, because of the concealed
hybrids.
The explanation given in this simple instance may be applied to the case
of different varieties of the same species, when growing together and
crossed naturally by insects.
It would take too long to go into all the details that present
themselves here to the student of nature and of gardens. I will only
state, that since varieties differ principally from their species by the
lack of some sharp character, one variety may be characterized by the
lack of color of the flowers, another by the lack of pubescence, a third
by being dwarfed, and so on. Every character must be studied separately
in its effects on the offspring [213] of the crosses. And it is
therefore easily seen, that the hybrids of two varieties may resemble
neither of them, but revert to the species itself. This is necessarily
and commonly the case, since it is always the older or positive
characters that prevail in the hybrids and the younger or negative that
lie hidden. So for instance, a blue dwarf larkspur, crossed with a tall
white variety, must give a tall blue hybrid, reassuming in both
characters the essentials of the species.
Keeping this rule in view, it will be easy to calculate what may be
expected from spontaneous crosses for a wide range of occurrences, and
thus to find an explanation of innumerable cases of apparent variability
and reversion in the principle of vicinism. Students have only to
recollect that specific characters prevail over varietal ones, and that
every character competes only with its own antagonist. Or to give a
sharper distinction: whiteness of flowers cannot be expected to be
interchanged with pubescence of leaves.
In concluding I will point out another danger which in the principle of
vicinism may be avoided. If you see a plant in a garden with all the
characteristics of its species, how can you be sure that it is truly a
representative of the species, and not a hybrid? The prevailing [214]
characters are in either case the same. Perhaps on close inspection you
may find in some cases a slight difference, some character being not as
fully developed in the hybrid as in the species. But when such is not
the case, or where the opportunity for such a closer examination is
wanting, a hybrid may easily be taken for a specimen of the pure race.
Now take the seeds of your plant and sow them. If you had not supposed
it to be hybrid you will be astonished at finding among its progeny some
of a wholly different type. You will be led to conclude that you are
observing a sudden change in structure such as is usually called a
sport.
Or in other words you may think that you are assisting at the
origination of a new variety. If you are familiar with the principle of
vicinism, you will refrain from such an inference and consider the
supposition of a hybrid origin. But in former times, when this principle
was still unknown and not even guessed at, it is evident that many
mistakes must have been made, and that many an instance, which until now
has been considered reliable proof of a so-called single variation, is
in fact only a case of vicinism. In reading the sparse literature on
sports, numerous cases will be found, which cannot stand this test. In
many instances crossing must be looked to as an explanation, [215] and
in other cases the evidence relied upon does not suffice to exclude this
assumption. Many an old argument has of late lost its force by this
test.
Returning to our starting point we may now state that regular reversions
to a specific type characterize a form as a variety of that species.
These reversions, however, are not due to an innate tendency, but to
unobserved spontaneous crosses.
[217]
LECTURE VIII
LATENT CHARACTERS
No organism exhibits all of its qualities at any one time. Many of them
are generally dormant and await a period of activity. For some of them
this period comes regularly, while in others the awakening depends upon
external influences, and consequently occurs very irregularly. Those of
the first group correspond to the differences in age; the second
constitute the responses of the plant to stimuli including
wound-injuries.
Some illustrative examples may be quoted in order to give a precise idea
of this general conception of dormant or latent characters. Seed leaves
are only developed in the seed and the seedling; afterwards, during the
entire lifetime of the plant, the faculty of producing them is not made
use of. Every new generation of seeds however, bears the same kind of
seed leaves, and hence it is manifest that it is the same quality, which
shows itself from time to time.
The primary leaves, following the seed-leaves, are different in many
species, from the later ones, and the difference is extremely pronounced
in some cases of reduction. Often, when leaves are lacking in the adult
plant, being replaced by flattened stalks as in the case of the acacias,
or by thorns, or green stems and twigs as in the prickly broom or _Ulex
europaeus_, the first leaves of the young plant may be more highly
differentiated, being pinnate in the first case and bearing three
leaflets in the second instance. This curious behavior which is very
common, brings the plants, when young, nearer to their allies than in
the adult state, and manifestly implies that the more perfect state of
the leaves is latent throughout the life of the plant, with the
exception of the early juvenile period.
_Eucalyptus Globulus_, the Australian gum tree, has opposite and broadly
sessile leaves during the first years of its life. Later these disappear
and are replaced by long sickle-shaped foliage organs, which seem to be
scattered irregularly along the branches. The juvenile characters
manifestly lie dormant during the adult period, and that this is so, may
be shown artificially by cutting off the whole crown of the tree, when
the stem responds by producing numerous new branches, which assume the
[218] shape proper to the young trees, bearing sessile and opposite
leaves.
It seems quite unnecessary to give further instances. They are familiar
to every student. It is almost safe to say that every character has its
periods of activity and of inactivity, and numbers of flowers and fruits
can be mentioned as illustrations. One fact may be added to show that
nearly every part of the plant must have the power of producing all or
nearly all the characters of the individual to which it belongs. This
proof is given by the formation of adventitious buds. These, when once
formed, may grow out into twigs, with leaves and flowers and roots. They
may even be separated from the plants and used as cuttings to reproduce
the whole. Hence we may conclude that all tissues, which possess the
power of producing adventitious buds, must conceal in a latent state,
all the numerous characters required for the full development of the
whole individual.
Adventitious buds may proceed from specialized cells, as on the margin
of the leaves of _Bryophyllum calycinum_; or from the cells of special
tissues, as in the epidermis of the begonias; or they may be provoked by
wounds in nearly every part of the plant, provided it be able to heal
the wound by swelling tissues or [219] callus. The best instance is
afforded by elms and by the horse-chestnut. If the whole tree is hewn
down the trunk tries to repair the injury by producing small
granulations of tissue between the wood and the bark, which gradually
coalesce while becoming larger. From this new ring of living matter
innumerable buds arise, that expand into leafy branches, showing clearly
that the old trunk possesses, in a latent state, all the qualities of
the whole crown. Indeed, such injured stumps may be used for the
production of copses and hedges.
All the hitherto recorded cases of latency have this in common, that
they may become active during the life-time of any given individual
once, or oftener. This may be called the ordinary type of latency.
Besides this there is another form of latent characters, in which this
awakening power is extremely limited, or wholly absent. It is the
systematic latency, which may be said to belong to species and varieties
in the same way as the ordinary latency belongs to individuals. As this
individual latency may show itself from time to time during the life of
a given plant, the first may only become active from time to time during
the whole existence of the variety or the species. It has no regular
period of activity, nor may it be incited by artificial stimulation.
[220] It emerges from concealment only very rarely and only on its own
initiative. Such instances of atavism have been described in previous
lectures, and their existence has been proved beyond doubt.
Systematic latency explains the innumerable instances in which species
are seen to lack definite characteristics which ordinarily do not fail,
either in plants at large, or in the group or family to which the plant
belongs. If we take for instance the broom-rape or _Orobanche_, or some
other pale parasite, we explain their occurrence in families of plants
with green leaves, by the loss of the leaves and of the green color. But
evidently this loss is not a true one, but only the latency of those
characters. And even this latency is not a complete one, as little
scales remind us of the leaves, and traces of chlorophyll still exist in
the tissues. Numerous other cases will present themselves to every
practical botanist.
Taking for granted that characters, having once been acquired, may
become latent, and that this process is of universal occurrence
throughout the whole vegetable and animal kingdom, we may now come to a
more precise and clear conception of the existing differences between
species and varieties.
For this purpose we must take a somewhat [221] broader view of the whole
evolution of the vegetable kingdom. It is manifest that highly developed
plants have a larger number of characters than the lower groups. These
must have been acquired in some way, during preceding times. Such
evolution must evidently be called a process of improvement, or a
progressive evolution. Contrasted to this is the loss, or the latency of
characters, and this may be designated retrogressive or retrograde
evolution. But there is still a third possibility. For a latent
character may reassume its activity, return to the active state, and
become once more an important part of the whole organization. This
process may be designated as degressive evolution; it obviously
completes the series of the general types of evolution.
Advancement in general in living nature depends on progressive
evolution. In different parts of the vegetable kingdom, and even in
different families this progression takes place on different lines. By
this means it results in an ever increasing divergency between the
several groups. Every step is an advance, and many a step must have been
taken to produce flowering plants from the simplest unicellular algae.
But related to, and very intimately connected with this advancement is
the retrogressive [222] evolution. It is equally universal, perhaps
never failing. No great changes have been attained, without acquiring
new qualities on one side, and reducing others to latency. Everywhere
such retrogressions may be seen. The polypetalous genera _Pyrola_,
_Ledum_, and _Monotropa_ among the sympetalous heaths, are a remarkable
instance of this. The whole evolution of the monocotyledons from the
lowest orders of dicotyledons implies the seeming loss of cambial growth
and many other qualities. In the order of aroids, from the calamus-root
or sweet flag, with its small but complete flowers, up to the reduced
duckweeds (_Lemna_), almost an unbroken line of intermediate steps may
be traced showing everywhere the concurrence of progressive and
retrogressive evolution.
Degressive evolution is not so common by far, and is not so easy to
recognize, but no doubt it occurs very frequently. It is generally
called atavism, or better, systematic atavism, and the clearest cases
are those in which a quality which is latent in the greater part of a
family or group, becomes manifest in one of its members. Bracts in the
inflorescence of crucifers are ordinarily wanting, but may be seen in
some genera, _Erucastrum pollichii_ being perhaps the [223] most widely
known instance, although other cases might easily be cited.
For our special purpose we may take up only the more simple cases that
may be available for experimental work. The great lines of evolution of
whole families and even of genera and of many larger species obviously
lie outside the limits of experimental observation. They are the outcome
of the history of the ancestors of the present types, and a repetition
of their history is far beyond human powers. We must limit ourselves to
the most recent steps, to the consideration of the smallest differences.
But it is obvious that these may be included under the same heads as the
larger and older ones. For the larger movements are manifestly to be
considered only as groups of smaller steps, going in the same direction.
Hence we conclude, that even the smallest steps in the evolution of
plants which we are able to observe, may be divided into progressive,
retrogressive and degressive ones. The acquisition of a single new
quality is the most simple step in the progressive line, the becoming
latent and the reactivating of this same quality are the prototypes of
the two other classes.
Having taken this theoretical point of view, it remains to inquire, how
it concurs with the [224] various facts, given in former lectures and
how it may be of use in our further discussions.
It is obvious that the differences between elementary species and
varieties on the one hand, and between the positive and negative
varieties as distinguished above, are quite comparable with our
theoretical views. For we have seen that varieties can always be
considered as having originated by an apparent loss of some quality of
the species, or by the resumption of a quality which in allied species
is present and visible. In our exposition of the facts we have of course
limited ourselves to the observable features of the phenomena without
searching for a further explanation. For a more competent inquiry
however, and for an understanding of wider ranges of facts, it is
necessary to penetrate deeper into the true nature of the implied
causes.
Therefore we must try to show that elementary species are distinguished
from each other by the acquisition of new qualities, and that varieties
are derived from their species either by the reduction of one or more
characteristics to the latent state, or by the energizing of dormant
characters.
Here we meet with a great difficulty. Hitherto varieties and subspecies
have never been clearly defined, or when they have been, it was [225]
not by physiological, but only by morphological research. And the claims
of these two great lines of inquiry are obviously very diverging.
Morphological or comparative studies need a material standard, by which
it may be readily decided whether certain groups of animals and plants
are to be described or de-nominated as species, as subspecies or as
varieties. To get at the inner nature of the differences is in most
cases impossible, but a decision must be made. The physiological line of
inquiry has more time at its disposal; it calls for no haste. Its
experiments ordinarily cover years, and a conclusion is only to be
reached after long and often weary trials. There is no making a decision
on any matter until all doubtful points have been cleared up. Of course,
large groups of facts remain uncertain, awaiting a closer inquiry, and
the teacher is constrained to rely on the few known instances of
thoroughly investigated cases. These alone are safe guides, and it seems
far better to trust to them and to make use of them for the construction
of sharp conceptions, which may help us to point out the lines of
inquiry which are still open.
Leaving aside all such divisions and definitions, as were stamped with
the name of provisional species and varieties by the great systematist,
[226] Alphonse De Candolle, we may now try to give the proofs of our
assertion, by using only those instances that have been thoroughly
tested in every way.
We may at once proceed to the retrogressive or negative varieties. The
arguments for the assumption that elementary species owe their origin to
the acquisition of new qualities may well be left for later lectures
when we shall deal with the experimental proofs in this matter.
There are three larger groups of facts, on which the assumption of
latent characters in ordinary varieties rests. These are true atavism,
incomplete loss of characters, and systematic affinity. Before dealing
with each of these separately, it may be as well to recall once more
that in former lectures we have treated the apparent losses only as
modifications in a negative way, without contemplating the underlying
causes.
Let us recall the cases of bud-atavism given by the whitish variety of
the scarlet _Ribes_, by peaches and nectarines, and by conifers,
including _Cephalotaxus_ and _Cryptomeria_. These and many other
analogous facts go to prove the relation of the variety to the species.
Two assumptions are allowable. In one the variety differs from the
species by the total loss of the [227] distinctive character. In the
other this character is simply reduced to an inactive or dormant state.
The fact of its recurrence from time to time, accompanied by secondary
characters previously exhibited, is a manifest proof of the existence of
some relation between the lost and the resumed peculiarity. Evidently
this relation cannot be accounted for on the assumption of an absolute
disappearance; something must remain from which the old features may be
restored.
This lengthy discussion may be closed by the citation of the cases, in
which plants not only show developmental features of a former state, but
also reproduce the special features they formerly had, but seemingly
have lost. Two good illustrative examples may be given. One is afforded
by the wheat-ear carnation, the other by the green dahlias, and both
have occurred of late in my own cultures.
A very curious anomaly may from time to time be observed in large beds
of carnations. It bears no flowers, but instead of them small green
ears, which recall the ears of wheat. Thence the name of "Wheat-ear"
carnation. On closer inspection it is easily seen how they originate.
The normal flowers of the carnations are preceded by a small group of
bracts, [228] which are arranged in opposite pairs and therefore
constitute four rows.
In this variety the flower is suppressed and this loss is attended by a
corresponding increase of the number of the pairs of bracts. This
malformation results in square spikes or somewhat elongated heads
consisting only of the greenish bracts. As there are no flowers, the
variety is quite sterile, and as it is not regarded by horticulturists
as an improvement on the ordinary bright carnations, it is seldom
multiplied by layering. Notwithstanding this, it appears from time to
time and has been seen in different countries and at different periods,
and, what is of great importance for us, in different strains of
carnations. Though sterile, and obviously dying out as often as it
springs into existence, it is nearly two centuries old. It was described
in the beginning of the 18th century by Volckamer, and afterwards by
Jaeger, De Candolle, Weber, Masters, Magnus and many other botanists. I
have had it twice, at different times and from different growers.
So far as I have been able to ascertain reversions of this curious
carnation to normal flowers have not yet been recorded. Such a
modification occurred last summer in my garden on a plant which had not
been divided or layered, but on which the slender branches had [229]
been left on the stem. Some of them remained true to the varietal type
and bore only green spikes. Others reverted wholly or partially to the
production of normal flowers. Some branches bore these only, others had
spikes and flowers on neighboring twigs, and in still other instances
little spikes had been modified in such manner that a more or less well
developed flower was preceded by some part of an ear.
The proof that this retrograde modification was due to the existence of
a character in the latent state was given by the color of the flowers.
If the reverted bud had only lost the power of producing spikes, they
would evidently simply have returned to the characteristics of the
ordinary species, and their color would have been a pale pink. Instead
of this, all flowers displayed corollas of a deep brown. They obviously
reverted to their special progenitor, the chance variety from which they
had sprung, and not to the common prototype of the species. Of course it
was not possible to ascertain from which variety the plant had really
originated, but the reproduction of any one clearly defined varietal
mark is in itself proof enough of their origin, and of the latency of
the dark brown flower-color in this special case.
A still better proof is afforded by a new type of green dahlia. The
ordinary green dahlia [230] has large tufts of green bracts instead of
flowering heads, the scales of the receptacle having assumed the texture
and venation of leaves, and being in some measure as fleshy. But the
green heads retain the form of the ordinary flower-heads, and as they
have no real florets that may fade away, they remain unchanged on the
plants, and increase in number through the whole summer. The new types
of green dahlia however, with which I have now to deal, are
distinguished by the elongation of the axis of the head, which is
thereby changed into a long leafy stalk, attaining a length of several
inches. These stalks continue growing for a very long time, and for the
most part die without producing anything else than green fleshy scales.
This long-headed green dahlia originated at Haarlem some years ago, in
the nursery of Messrs. Zocher & Co. It was seen to arise twice, from
different varieties. Both of these were double-flowered, one a deep
carmine with white tips on the rays, the other of a pale orange tint,
known by the name of "Surprise." As they did not bear any florets or
seeds, they were quite sterile. The strain arising from the carmine
variety was kindly given to me by Messrs. Zocher & Co., and was
propagated in my garden, while the other was kept in the nursery. In the
earlier cultures both remained true to [231] their types, never
producing true florets. No mark of the original difference was to be
seen between them. But last summer (1903) both reverted to their
prototypes, bearing relatively large numbers of ordinary double
flowerheads among the great mass of green stalks. Some intermediate
forms also occurred consisting of green-scaled stalks ending in small
heads with colored florets.
Thus far we have an ordinary case of reversion. But the important side
of the phenomenon was, that each plant exactly "recollected" from which
parent it had sprung. All of those in my garden reverted to the carmine
florets with white tips, and all of those in the nursery to the pale
orange color and the other characteristics of the "Surprise" variety.
It seems absolutely evident, that no simple loss can account for this
difference. Something of the character of the parent-varieties must have
remained in the plant. And whatever conception we may formulate of these
vestigial characters it is clear that the simplest and most obvious idea
is their preservation in a dormant or latent state. Assuming that the
distinguishing marks have only become inactive by virescence, it is
manifest that on returning each will show its own peculiarities, as
recorded above. Our second point was the incomplete loss of [232] the
distinguishing quality in some varieties. It is of general occurrence,
though often overlooked. Many white varieties of colored flowers give
striking instances, among them many of the most stable and most prized
garden-flowers. If you look at them separately or in little bouquets
they seem to be of irreproachable purity. But if you examine large beds
a pale hue will become visible. In many cases this tinge is so slight as
to be only noticeable in a certain illumination, or by looking in an
oblique direction across the bed; in others it is at once evident as
soon as it has been pointed out. It always reminds the observer of the
color of the species to which the variety belongs, being bluish in
violets and harebells, reddish in godetias and phloxes, in _Silene
Armeria_ and many others. It proves that the original color quality of
the species has not wholly, but only partly disappeared. It is dormant,
but not entirely obliterated; latent, but not totally concealed;
inactive, but only partially so. Our terminology is an awkward one; it
practically assumes, as it so often does in other cases, a conventional
understanding, not exactly corresponding to the simple meaning of the
words. But it would be cumbrous to speak always of partial inactivity,
incomplete latency or half awakening qualities. Even such words as
sub-latent, [233] which would about express the real state of things,
would have little chance of coming into general use.
Such sub-latent colors are often seen on special parts in white
varieties of flowers. In many cases it is the outer side of the petals
which recalls the specific color, as in some white roses. In violets it
is often on the spur that the remains of the original pigment are to be
seen. In many instances it is on the tips of the petals or of the
segments of the corolla, and a large number of white or yellow flowers
betray their affinity to colored species by becoming red or bluish at
the edges or on the outer side.
The reality of such very slight hues, and their relation to the original
pigment of the species may in some cases be proved by direct experiment.
If it is granted that latency is not an absolute quality, then it will
be readily accepted, that even latency must be subjected to the laws of
gradual variation or fluctuating variability. We will deal with these
laws in a later lecture but every one knows that greater deviations than
the ordinary may be attained by sowing very large numbers and by
selecting from among them the extreme individuals and sowing anew from
their seed. In this way the slightest tinge of any latent color may be
[234] strengthened, not indeed to the restoration of the tinge of the
species, but at least so far as to leave no doubt as to the identity of
the visible color of the species and the latent or sublatent one of the
variety.
I made such an experiment with the peach leaved harebell or _Campanula
persicifolia_. The white variety of this species, which is often met
with in our gardens, shows a very pale bluish hue when cultivated in
large quantities, which however is subject to individual variations. I
selected some plants with a decided tinge, flowered them separately,
sowed their seeds, and repeated this during two generations. The result
was an increase of the color on the tips of the segments of the corolla
in a few individuals, most of them remaining as purely white as the
original strain. But in those few plants the color was very manifest,
individually variable in degree, but always of the same blue as in the
species itself.
Many other instances could be given. Smooth varieties are seldom
absolutely so, and if scattering hairs are found on the leaves or only
on some more or less concealed parts, they correspond in their character
to those of the species. So it is with prickles, and even the thornless
thorn-apple has fruits with surfaces far from smooth. The thornless
horse-chestnut [235] has in some instances such evident protuberances on
the valves of its fruits, that it may seem doubtful whether it is a pure
and stable variety.
Systematic latency may betray itself in different ways, either by normal
systematic marks, or by atavism. With the latter I shall deal at length
on another occasion, and therefore I will give here only one very clear
and beautiful example. It is afforded by the common red clover.
Obviously the clovers, with their three leaflets in each leaf, stand in
the midst of the great family of papilionaceous plants, the leaves of
which are generally pinnate. Systematic affinity suggests that the
"three leaved" forms must have been derived from pinnate ancestors,
evidently by the reduction of the number of the leaflets. In some
species of clover the middle of the three is more or less stalked, as is
ordinarily the case in pinnate leaves; in others it is as sessile as are
its neighbors. In a subsequent chapter I will describe a very fine
variety, which sometimes occurs in the wild state and may easily be
isolated and cultivated. It is an ordinary red clover with five leaflets
instead of three, and with this number varying between three and seven,
instead of being nearly wholly stable as in the common form. It produces
from time to time pinnate leaves, [236] very few indeed, and only
rarely, but then often two or three or even more on the same individual.
Intermediate stages are not wanting, but are of no consequence here. The
pinnate leaves obviously constitute a reversion to some prototype, to
some ancestor with ordinary papilionaceous leaves. They give proof of
the presence of the common character of the family, concealed here in a
latent state. Any other explanation of this curious anomaly would
evidently be artificial. On the other hand nothing is really known about
the ancestors of clover, and the whole conception rests only on the
prevailing views of the systematic relationships in this family. But, as
I have already said, further proof must be left for a subsequent
occasion.
Many instances, noted in our former lectures, could be quoted here. The
systematic distribution of rayed and rayless species and varieties among
the daisy-group of the composites affords a long series of examples.
Accidental variations in both directions occur. The Canada fleabane or
_Erigeron canadensis_, the tansy or _Tanacetum vulgare_ and some others
may at times be seen with ray-florets, and according to Murr, they may
sometimes be wanting in _Aster Tripolium_, _Bellis perennis_, some
species of _Anthemis_, _Arnica montana_ and in a number [237] of other
well-known rayed species. Another instance may be quoted; it has been
pointed out by Grant Allen, and refers to the dead-nettle or Lamium
album. Systematically placed in a genus with red-flowering species, we
may regard its white color as due to the latency of the general red
pigment.
But if the flower of this plant is carefully examined, it will be found
in most cases not to be purely white, but to have some dusky lines and
markings on its lower lip. Similar devices are observed on the lip of
the allied _Lamium maculatum_, and in a less degree on the somewhat
distant _Lamium purpureum_. With _Lamium maculatum_ or spotted
dead-nettle, the affinity is so close that even Bentham united the two
in a single species, considering the ordinary dead-nettle only as a
variety of the dappled purple type. For the support of this conception
of a specific or varietal retrograde change many other facts are
afforded by the distribution of the characteristic color and of the
several patterns of the lips of other labiates, and our general
understanding of the relationships of the species and genera in this
family may in a broad sense be based on the comparison of these
seemingly subordinate characteristics.
The same holds good in many other cases, and systematists have often
become uncertain [238] as to the true value of some form, by its
relationship to the allied types in the way of retrogressive
modification. Color-differences are so showy, that they easily
overshadow other characters. The white and the blue thorn-apple, the
white and the red campion (_Lychnis vespertina_ and _diurna_) and many
other illustrative cases could be given, in which two forms are
specifically separated by some authors, but combined by others on the
ground of the retrograde nature of some differentiating mark.
Hitherto we have dealt with negative characters and tried to prove that
the conception of latency of the opposite positive characteristics is a
more natural explanation of the phenomenon than the idea of a complete
loss. We have now to consider the positive varieties, and to show that
it is quite improbable that here the species have struck out for
themselves a wholly new character. In some instances such may have been
the case, but then I should prefer to treat these rather as elementary
species. But in the main we will have to assume the latency of the
character in the species and its reassumption by the variety when
originating, as the most probable explanation.
Great stress is laid upon this conception by the fact, that positive
varieties are so excessively rare when compared with the common
occurrence [239] of negative ones. Indeed, if we put aside the radiate
and the color-varieties of flowers and foliage, hardly any cases can be
cited. We have dealt with this question in a former lecture, and may now
limit ourselves to the positive color-varieties.
The latency of the faculty of producing the red pigment in leaves must
obviously be accepted for nearly the whole vegetable kingdom. Oaks and
elms, the beautiful climbing species of Ampelopsis, many conifers, as
for instance _Cryptomeria japonica_, some brambles, the Guelder-rose
(_Viburnum Opulus_) and many other trees and shrubs assume a more or
less bright red color in the fall. During summer this tendency must have
been dormant, and that this is so, is shown by the young leaves of oaks
and others, which, when unfolding in the spring show a similar but paler
hue. Moreover, there is a way of awakening the concealed powers at any
time. We have only to inflict small wounds on the leaves, or to cut
through the nerves or to injure them by a slight bruising, and the
leaves frequently respond with an intense reddening of the living
tissues around and especially above the wounds. _Azolla caroliniana_, a
minute mosslike floating plant allied to the ferns, responds to light
and cold with a reddish tinge, and to shade or warmth with a pure green.
The foliage [240] of many other plants behaves likewise, as also do
apples and peaches on the insolated sides of the fruits. It is quite
impossible to state these groups of facts in a more simple way than by
the statement that the tendency to become red is almost generally
present, though latent in leaves and stems, and that it comes into
activity whenever a stimulus provokes it.
Now it must be granted that the energizing of such a propensity under
ordinary circumstances is quite another thing from the origination of a
positive variety by the evolution of the same character. In the variety
the activity has become independent of outer influences or dependent
upon them in a far lesser degree. The power of producing the red
pigments is shown to be latent by the facts given above, and we see that
in the variety it is no longer latent but is in perfect and lasting
activity throughout the whole life of the plant.
Red varieties of white flowers are much more rare. Here the latency of
the red pigment may be deduced partly from general arguments like those
just given, partly from the special systematic relations in the given
cases. Hildebrand has clearly worked out this mode of proof. He showed
by the critical examination of a large number of instances that the
occurrence of the red-flowered varieties is contingent upon the [241]
existence of red species in the same genus, or in some rare cases, in
nearly allied genera. Colors that are not systematically present in the
group to which a white species belongs are only produced in its
varieties in extremely rare cases.
We may quote some special rules, indicated by Hildebrand. Blue species
are n the main very rare, and so are blue varieties of white species
also. Carnations, Asiatic or cultivated buttercups (_Ranunculus
asiaticus_), _Mirabilis_, poppies, _Gladiolus_, _Dahlia_, and some other
highly cultivated or very old garden-plants have not been able to
produce true blue flowers. But the garden-anemone (_Anemone coronaria_)
has allies with very fine blue flowers. The common stock has bluish
varieties and is allied to _Aubretia_ and _Hesperis_, and gooseberries
have a red form, recalling the ordinary currant. In nearly all other
instances of blue or red varieties every botanist will be able to point
out some allied red or blue species, as an indication of the probable
source of the varietal character.
Dark spots on the lower parts of the petals of some plants afford
another instance, as in poppies and in the allied _Glaucium_, where they
sometimes occur as varietal and in other cases as specific marks.
The yellow fails in many highly developed [242] flowers, which are not
liable to produce yellow variations, as in _Salvia_, _Aster_,
_Centaurea_, _Vinca_, _Polygala_ and many others. Even the rare pale
yellowish species of some of these genera have no tendency in this
direction. The hyacinths are the most remarkable, if not the sole known
instance of a species having red and blue and white and yellow
varieties, but here the yellow is not the bright golden color of the
buttercups.
The existence of varietal colors in allied species obviously points to a
common cause, and this cause can be no other than the latency of the
pigment in the species that do not show it.
The conception of latency of characters as the common source of the
origination of varieties, either in the positive or in the negative way,
leads to some rules on variability, which are known under the names
given to them by Darwin. They are the rules of repeated, homologous,
parallel and analogous variability. Each of them is quite general, and
may be recognized in instances from the most widely distant families.
Each of them is quite evident and easily understood on the principle of
latency.
By the term of repeated variability is meant the well-known phenomenon,
that the same variety has sprung at different times and in different
[243] countries from the same species. The repetition obviously
indicates a common internal cause. The white varieties of blue- and
red-flowered plants occur in the wild state so often, and in most of the
instances in so few individuals that a common pedigree is absolutely
improbable. In horticulture this tendency is widely and vexatiously
known, since the repetition of an old variety does not bring any
advantage to the breeder. The old name of "conquests," given by the
breeders of hyacinths, tulips and other flower-bulbs to any novelty, in
disregard of the common occurrence of repetitions, is an indication of
the same experience in the repeated appearance of certain varieties.
The rule of parallel variations demands that the same character
occasionally makes its appearance in the several varieties or races,
descended from the same species, and even in widely distinct species.
This is a rule, which is very important for the general conception of
the meaning of the term variety as contrasted with elementary species.
For the recurrence of the same deviation always impresses us as a
varietal mark. Laciniated leaves are perhaps the most beautiful
instance, since they occur in so many trees and shrubs, as the walnut
tree, the beech, the birch, the hazelnut, and even in [244] brambles and
some garden-varieties of the turnip (_Brassica_).
In such cases of parallel variations the single instances obviously
follow the same rules and are therefore to be designated as analogous.
Pitchers or ascidia, formed by the union of the margins of a leaf, are
perhaps the best proof. They were classified by Morren under two heads,
according to their formation from one or more leaves. Monophyllous
pitchers obey the same law, viz.: that the upper side of the leaf has
become the inner side of the pitcher. Only one exception to this rule is
known to me. It is afforded by the pitchers of the banyan or holy
fig-tree, _Ficus religiosus_, but it does not seem to belong to the same
class as other pitchers, since as far as it has been possible to
ascertain the facts, these pitchers are not formed by a few leaves as in
all other cases, but by all the leaves of the tree.
In some cases pitchers are only built up of part of the leaf-blade. Such
partial malformations obey a rule, that is common to them and to other
foliar enations, viz.: that the side of the leaf from which they emerge,
is always their outer side. The inner surface of these enations
corresponds to the opposite side of the leaf, both in color and in
anatomical structure. The last of the four rules above mentioned is
[245] that of the homologous variability. It asserts that the same
deviation may occur in different, but homologous parts of the same
plant. We have already dealt with some instances, as the occurrence of
the same pigment in the flowers and foliage, in the fruits and seeds of
the same plant, as also illustrated by the loss of the red or blue tinge
by flowers and berries. Other instances are afforded by the curious fact
that the division of the leaves into numerous and small segments is
repeated by the petals, as in the common celandine and some sorts of
brambles.
It would take too long to make a closer examination of the numerous
cases which afford proof of these statements. Suffice it to say that
everywhere the results of close inspection point to the general rule,
that the failure of definite qualities both in species and in varieties
must, in a great number of cases, be considered as only apparent. Hidden
from view, occasionally reappearing, or only imperfectly concealed, the
same character must be assumed to be present though latent.
In the case of negative or retrogressive varieties it is the transition
from the active into a dormant state to which is due the origin of the
variety. Positive varieties on the contrary owe their origin to the
presence of some character [246] in the species in the latent state, and
to the occasional re-energizing thereof.
Specific or varietal latency is not the same thing as the ordinary
latency of characters that only await their period of activity, or the
external influence which will awake them. They are permanently latent,
and could well be designated by the word perlatent. They spring into
activity only by some sudden leap, and then at once become independent
of ordinary external stimulation.
[247]
LECTURE IX
CROSSES OF SPECIES AND VARIETIES
In the foregoing lectures I have tried to show that there is a real
difference between elementary species and varieties. The first are of
equal rank, and together constitute the collective or systematic
species. The latter are usually derived from real and still existing
types. Elementary species are in a sense independent of each other,
while varieties are of a derivative nature.
Furthermore I have tried to show that the ways in which elementary or
minor species must have originated from their common ancestor must be
quite different from the mode of origin of the varieties. We have
assumed that the first come into existence by the production of
something new, by the acquirement of a character hitherto unnoticed in
the line of their ancestors. On the contrary, varieties, in most cases,
evidently owe their origin to the loss of an already existing character,
or in other less frequent cases, to the re-assumption of a quality [248]
formerly lost. Some may originate in a negative, others in a positive
manner, but in both cases nothing really new is acquired.
This distinction holds good for all cases in which the relationship
between the forms in question is well known. It seems entirely
justifiable therefore to apply it also to cases in which the systematic
affinity is doubtful, as well as to instances in which it is impossible
to arrive at any taxonomic conclusions. The extreme application of the
principle would no doubt disturb the limits between many species and
varieties as now recognized. It is not to be forgotten however that all
taxonomic distinctions, which have not been confirmed by physiologic
tests are only provisional, a view acknowledged by the best
systematists. Of course the description of newly discovered forms can
not await the results of physiologic inquiries; but it is absolutely
impossible to reach definite conclusions on purely morphologic evidence.
This is well illustrated by the numerous discords of opinion of
different authors on the systematic worth of many forms.
Assuming the above mentioned principle as established, and disregarding
doubtful cases as indicated, the term progressive evolution is used to
designate the method in which elementary species must have originated.
It is the [249] manner in which all advance in the animal and vegetable
kingdoms must have taken place, continuously adding new characters to
the already existing number. Contrasted with this method of growing
differentiation, are the retrogressive modifications, which simply
retrace a step, and the degressive changes in which a backward step is
retraced and old characters revived. No doubt both of these methods have
been operative on a large scale, but they are evidently not in the line
of general advancement.
In all of these directions we see that the differentiating marks show
more or less clearly that they are built up of units. Allied forms are
separated from each other without intermediates. Transitions are wholly
wanting, although fallaciously apparent in some instances owing to the
wide range of fluctuating variability of the forms concerned, or to the
occurrence of hybrids and subvarieties.
These physiologic units, which in the end must be the basis for the
distinction of the systematic units, may best be designated by the term
of "unit-characters." Their internal nature is as yet unknown to us, and
we will not now look into the theories, which have been propounded as to
the probable material basis underlying them. For our present purpose the
empirical evidence of the general occurrence of [250] sharp limits
between nearly related characters must suffice. As Bateson has put it,
species are discontinuous, and we must assume that their characters are
discontinuous also.
Moreover there is as yet no reason for trying to make a complete
analysis of all the characters of a plant. No doubt, if attained, such
an analysis would give us a deep insight into the real internal
construction of the intricate properties of organisms in general. But
taxonomic studies in this direction are only in their infancy and do not
give us the material required for such an analysis. Quite on the
contrary, they compel us to confine our study to the most recently
acquired, or youngest characters, which constitute the differentiating
marks between nearly allied forms.
Obviously this is especially the case in the realm of the hybrids, since
only nearly related forms are able to give hybrid offspring. In dealing
with this subject we must leave aside all questions concerning more
remote relationships.
It is not my purpose to treat of the doctrine of hybridization at any
length. Experience is so rapidly increasing both in a practical and in a
purely scientific direction that it would take an entire volume to give
only a brief survey of the facts and of all the proposed theories.
[251] For our present purposes we are to deal with hybrids only in so
far as they afford the means of a still better distinction between
elementary species and varieties. I will try to show that these two
contrasting groups behave in quite a different manner, when subjected to
crossing experiments, and that the hope is justified that some day
crosses may become the means of deciding in any given instance, what is
to be called a species, and what a variety, on physiologic grounds. It
is readily granted that the labor required for such experiments, is
perhaps too great for the results to be attained, but then it may be
possible to deduce rules from a small series of experiments, which may
lead us to a decision in wider ranges of cases.
To reach such a point of view it is necessary to compare the evidence
given by hybrids, with the conclusions already attained by the
comparison of the differentiating characteristics of allied forms.
On this ground we first have to inquire what may be expected respecting
the internal nature and the outcome of the process of crossing in the
various cases cited in our former discussion.
We must always distinguish the qualities, which are the same in both
parents, from those that constitute the differentiating marks in every
single cross. In respect to the first [252] group the cross is not at
all distinguished from a normal fertilization, and ordinarily these
characters are simply left out of consideration. But it should never be
forgotten that they constitute the enormous majority, amounting to
hundreds and thousands, whereas the differentiating marks in each case
are only one or two or a few at most. The whole discussion is to be
limited to these last-named exceptions. We must consider first what
would be the nature of a cross when species are symmetrically combined,
and what must be the case when varieties are subjected to the same
treatment. In so doing, I intend to limit the discussion to the most
typical cases. We may take the crosses between elementary species of the
same or of very narrowly allied systematic species on the one side, and
on the other, limit treatment to the crossing of varieties with the
species, from which they are supposed to have sprung by a retrograde
modification. Crosses of different varieties of the same species with
one another obviously constitute a derivative case, and should only be
discussed secondarily. And crosses of varieties with positive or
depressive characters have as yet so rarely been made that we may well
disregard them.
Elementary species differ from their nearest allies by progressive
changes, that is by the acquirement [253] of some new character. The
derivative species has one unit more than the parent. All other
qualities are the same as in the parent. Whenever such a derivative is
combined with its parent the result for these qualities will be exactly
as in a normal fertilization. In such ordinary cases it is obvious that
each character of the pollen-parent is combined with the same character
of the pistil-parent. There may be slight individual differences, but
each unit character will become opposed to, and united with, the same
unit-character in the other parent. In the offspring the units will thus
be paired, each pair consisting of two equivalent units. As to their
character the units of each single pair are the same, only they may
exhibit slight differences as to the degree of development of this
character.
Now we may apply this conception to the sexual combination of two
different elementary species, assuming one to be the derivative of the
other. The differentiating mark is only present in one of the parents
and wanting in the other. While all other units are paired in the
hybrid, this one is not. It meets with no mate, and must therefore
remain unpaired. The hybrid of two such elementary species is in some
way incomplete and unnatural. In the ordinary course of things all
individuals derive [254] their qualities from both parents; for each
single mark they possess at least two units. Practically but not
absolutely equal, these two opponents always work together and give to
the offspring a likeness to both parents. No unpaired qualities occur in
normal offspring; these constitute the essential features of the hybrids
of species and are at the same time the cause of their wide deviations
from the ordinary rules.
Turning now to the varieties, we likewise need discuss their
differentiating marks only. In the negative types, these consist of the
apparent loss of some quality which was active in the species. But it
was pointed out in our last lecture that such a change is an apparent
loss. On a closer inquiry we are led to the assumption of a latent or
dormant state. The presumably lost characters have not absolutely, or at
least not permanently disappeared. They show their presence by some
slight indication of the quality they represent, or by occasional
reversions. They are not wanting, but only latent.
Basing our discussion concerning the process of crossing on this
conception, and still limiting the discussion to one differentiating
mark, we come to the inference, that this mark is present and active in
the species, and present but dormant in the variety. Thus it is present
in both, and as all other characters not differentiating [255] find
their mates in the cross, so these two will also meet one another. They
will unite just as well as though they were both active or both dormant.
For essentially they are the same, only differing in their degree of
activity. From this we can infer, that in the crossing of varieties, no
unpaired remainder is left, all units combining in pairs exactly as in
ordinary fertilization.
Setting aside the contrast between activity and latency in this single
pair, the procedure in the inter-crossing of varieties is the same as in
ordinary normal fertilization.
Summarizing this discussion we may conclude that in normal fertilization
and in the inter-crossing of varieties all characters are paired, while
in crosses between elementary species the differentiating marks are not
mated.
In order to distinguish these two great types of fertilization we will
use the term bisexual for the one and unisexual for the other. The term
balanced crosses then conveys the idea of complete bisexuality, all
unit-characters combining in pairs. Unbalanced crosses are those in
which one or more units do not find their mates and therefore remain
unpaired. This distinction was proposed by Macfarlane when studying the
minute structure of plant-hybrids in comparison with that of their
parents (1892).
[256] In the first place it shows that a species hybrid may inherit the
distinguishing marks of both parents. In this way it may become
intermediate between them, having some characters in common with the
pollen-parent and others with the pistil-parent. As far as these
characters do not interfere with each other, they may be fully developed
side by side, and in the main this is the way in which hybrid characters
are evolved. But in most cases our existing knowledge of the units is
far too slender to give a complete analysis, even of these
distinguishing marks alone. We recognize the parental marks more or less
clearly, but are not prepared for exact delimitations. Leaving these
theoretical considerations, we will pass to the description of some
illustrative examples.
In the first place I will describe a hybrid between two species of
_Oenothera_, which I made some years ago. The parents were the common
evening-primrose or _Oenothera biennis_ and of its small-flowered
congener, _Oenothera muricata_. These two forms were distinguished by
Linnaeus as different species, but have been considered by subsequent
writers as elementary species or so-called systematic varieties of one
species designated with the name of the presumably older type, the _O.
biennis_. Varietal differences in a physiologic sense they [257] do not
possess, and for this reason afford a pure instance of unbalanced union,
though differing in more than one point.
I have made reciprocal crosses, taking at one time the small-flowered
and at the other the common species as pistillate parent. These crosses
do not lead to the same hybrid as is ordinarily observed in analogous
cases; quite on the contrary, the two types are different in most
features, both resembling the pollen-parent far more than the
pistil-parent. The same curious result was reached in sundry other
reciprocal crosses between species of this genus. But I will limit
myself here to one of the two hybrids.
In the summer of 1895 I castrated some flowers of _O. muricata_, and
pollinated them with _O. biennis_, surrounding the flowers with paper
bags so as to exclude the visits of insects. I sowed the seeds in 1896
and the hybrids were biennial and flowered abundantly the next year and
were artificially fertilized with their own pollen, but gave only a very
small harvest. Many capsules failed, and the remaining contained only
some few ripe seeds.
From these I had in the following year the second hybrid generation, and
in the same way I cultivated also the third and fourth. These were as
imperfectly fertile as the first, and in [258] some years did not give
any seed at all, so that the operation had to be repeated in order to
continue the experiment. Last summer (1903) I had a nice lot of some 25
biennial specimens blooming abundantly. All in all I have grown some 500
hybrids, and of these about 150 specimens flowered.
These plants were all of the same type, resembling in most points the
pollen-parent, and in some others the pistil-parent of the original
cross. The most obvious characteristic marks are afforded by the
flowers, which in _O. muricata_ are not half so large as in _biennis_,
though borne by a calyx-tube of the same length. In this respect the
hybrid is like the _biennis_ bearing the larger flowers. These may at
times seem to deviate a little in the direction of the other parent,
being somewhat smaller and of a slightly paler color. But it is very
difficult to distinguish between them, and if _biennis_ and hybrid
flowers were separated from the plants and thrown together, it is very
doubtful whether one would succeed in separating them.
The next point is offered by the foliage. The leaves of _O. biennis_ are
broad, those of _O. muricata_ narrow. The hybrid has the broad leaves of
_O. biennis_ during most of its life and at the time of flowering. Yet
small deviations in the [259] direction of the other parent are not
wanting, and in winter the leaves of the hybrid rosettes are often much
narrower than those of _O. biennis_, and easily distinguishable from
both parents. A third distinction consists in the density of the spike.
The distance between the insertion of the flowers of _O. biennis_ is
great when compared with that of _O. muricata_. Hence the flowers of the
latter species are more crowded and those of _O. biennis_ more
dispersed, the spikes of the first being densely crowned with flowers
and flower-buds while those of _O. biennis_ are more elongated and
slender. As a further consequence the _O. biennis_ opens on the same
evening only one, two or three flowers on the same spike, whereas _O.
muricata_ bears often eight or ten or more flowers at a time. In this
respect the hybrid is similar to the pistil-parent, and the crowding of
the broad flowers at the top of the spikes causes the hybrids to be much
more showy than either of the parent types.
Other distinguishing marks are not recorded by the systematists, or are
not so sharply separated as to allow of the corresponding qualities of
the hybrids being compared with them.
This hybrid remains true to the description given. In some years I
cultivated two generations [260] so as to be able to compare them with
one another, but did not find any difference. The most interesting point
however, is the likeness between the first generation, which obviously
must combine in its internal structure the units of both parents, and
the second and later generations which are only of a derivative nature.
Next to this stands the fact that in each generation all individuals are
alike. No reversion to the parental forms either in the whole type or in
the single characteristics has ever been observed, though the leaves of
some hundreds, and the spikes and flowers of some 150 individual plants
have been carefully examined. No segregation or splitting up takes
place.
Here we have a clear, undoubted and relatively simple, case of a true
and pure species hybrid. No occurrence of possible varietal
characteristics obscures the result, and in this respect this hybrid
stands out much more clearly than all those between garden-plants, where
varietal marks nearly always play a most important part.
From the breeder's point of view our hybrid _Oenothera_ would be a
distinct gain, were it not for the difficulty of its propagation. But to
enlarge the range of the varieties this simple and stable form would
need to be treated anew, by [261] crossing it with the parent-types.
Such experiments however, have miscarried owing to the too stable nature
of the unit-characters.
This stability and this absence of the splitting shown by varietal marks
in the offspring of hybrids is one of the best proofs of unisexual
unions. It is often obscured by the accompanying varietal marks, or
overlooked for this reason. Only in rare cases it is to be met with in a
pure state and some examples are given of this below.
Before doing so, I must call your attention to another feature of the
unbalanced unions. This is the diminution of the fertility, a phenomenon
universally known as occurring in hybridizations. It has two phases.
First, the diminished chance of the crosses themselves of giving full
crops of seed, as compared with the pure fertilization of either parent.
And, secondly, the fertility of the hybrids themselves. Seemingly, all
grades of diminished fertility occur and the oldest authors on hybrids
have pointed out that a very definite relation exists between the
differences of the parents and the degree of sterility, both of the
cross and of the hybrid offspring. In a broad sense these two factors
are proportionate to each other, the sterility being the greater, the
lesser the affinity between the parents. Many writers have [262] tried
to trace this rule in the single cases, but have met with nearly
unsurmountable difficulties, owing chiefly to our ignorance of the units
which form the differences between the parents in the observed cases.
In the case of _Oenothera muricata x biennis_ the differentiating units
reduce the fertility to a low degree, threatening the offspring with
almost complete infertility and extinction. But then we do not know
whether these characters are really units, or perhaps only seemingly so
and are in reality composed of smaller entities which as yet we are not
able to segregate. And as long as we are devoid of empirical means of
deciding such questions, it seems useless to go farther into the details
of the question of the sterility. It should be stated here however, that
pure varietal crosses, when not accompanied by unbalanced characters,
have never showed any tendency to diminished fertility. Hence there can
be little doubt that the unpaired units are the cause of this decrease
in reproductive power.
The genus _Oenothera_ is to a large degree devoid of varietal
characteristics, especially in the subgenus _Onagra_, to which
_biennis_, _muricata_, _lamarckiana_ and some others belong. On the
other hand it seems to be rich in elementary species, but an adequate
study of [263] them has as yet not been made. Unfortunately many of the
better systematists are in the habit of throwing all these interesting
forms together, and of omitting their descriptive study. I have made a
large number of crosses between such undescribed types and as a rule got
constant hybrid races. Only one or two exceptions could be quoted, as
for instance the _Oenothera brevistylis_, which in its crosses always
behaves as a pure retrogressive variety. Instead of giving an exhaustive
survey of hybrids, I simply cite my crosses between _lamarckiana_ and
_biennis_, as having nearly the aspect of the last named species, and
remaining true to this in the second generation without any sign of
reversion or of splitting. I have crossed another elementary species,
the _Oenothera hirtella_ with some of my new and with some older Linnean
species, and got several constant hybrid races. Among these the
offspring of a cross between _muricata_ and _hirtella_ is still in
cultivation. The cross was made in the summer of 1897 and last year
(1903) I grew the fourth generation of the hybrids. These had the
characters of the _muricata_ in their narrow leaves, but the elongated
spikes and relatively large flowers of the _hirtella_ parent, and
remained true to this type, showing only slight fluctuations and never
reverting or segregating [264] the mixed characters. Both parents bear
large capsules with an abundance of seed, but in the hybrids the
capsules remain narrow and weak, ripening not more than one-tenth the
usual quantity of seed. Both parents are easily cultivated in annual
generations and the same holds good for the hybrid. But whereas the
hybrid of muricata and biennis is a stout plant, this type is weak with
badly developed foliage, and very long strict spikes. Perhaps it was not
able to withstand the bad weather of the last few years.
A goodly number of constant hybrids are described in literature, or
cultivated in fields and gardens. In such cases the essential question
is not whether they are now constant, but whether they have been so from
the beginning, or whether they prove to be constant whenever the
original cross is repeated. For constant hybrids may also be the issue
of incipient splittings, as we shall soon see.
Among other examples we may begin with the hybrid alfalfa or hybrid
lucerne (_Medicago media_). It often originates spontaneously between
the common purple lucerne or alfalfa and its wild ally with yellow
flowers and procumbent stems, the _Medicago falcata_. This hybrid is
cultivated in some parts of Germany on a large scale, as it is more
productive than [265] the ordinary lucerne. It always comes true from
seed and may be seen in a wild state in parks and on lawns. It is one of
the oldest hybrids with a pure and known lineage. The original cross has
been repeated by Urban, who found the hybrid race to be constant from
the beginning.
Another very notorious constant hybrid race is the _Aegilops
speltaeformis_. It has been cultivated in botanic gardens for more than
half a century, mostly in annual or biennial generations. It is
sufficiently fertile and always comes true. Numerous records have been
made of it, since formerly it was believed by Fabre and others to be a
spontaneous transition from some wild species of grass to the ordinary
wheat, not a cross. Godron, however, showed that it can be produced
artificially, and how it has probably sprung into existence wherever it
is found wild. The hybrid between _Aegilops ovata_, a small weed, and
the common wheat is of itself sterile, producing no good pollen. But it
may be fertilized by the pollen of wheat and then gives rise to a
secondary hybrid, which is no other than the _Aegilops speltaeformis_.
This remained constant in Godron's experiments during a number of
generations, and has been constant up to the present time.
[266] Constant hybrids have been raised by Millardet between several
species of strawberries. He combined the old cultivated forms with newly
discovered types from American localities. They ordinarily showed only
the characteristics of one of their parents and did not exhibit any new
combination of qualities, but they came true to this type in the second
and later generations.
In the genus _Anemone_, Janczewski obtained the same results. Some
characters of course may split, but others remain constant, and when
only such are present, hybrid races result with new combinations of
characters, which are as constant as the best species of the same genus.
The hybrids of Janczewski were quite fertile, and he points out that
there is no good reason why they should not be considered as good new
species. If they had not been produced artificially, but found in the
wild state, their origin would have been unknown, and there can be no
doubt that they would have been described by the best systematists as
species of the same value as their parents. Such is especially the case
with a hybrid between _Anemone magellanica_ and the common _Anemone
sylvestris_.
Starting from similar considerations Kerner von Marilaun pointed out the
fact long ago that many so-called species, of rare occurrence, [267]
standing between two allied types, may be considered to have originated
by a cross. Surely a wide field for abuse is opened by such an
assertion, and it is quite a common habit to consider intermediate forms
as hybrids, on the grounds afforded by their external characters alone,
and without any exact knowledge of their real origin and often without
knowing anything as to their constancy from seed. All such apparent
explanations are now slowly becoming antiquated and obsolete, but the
cases adduced by Kerner seem to stand this test.
Kerner designates a willow, _Salix ehrhartiana_ as a constant hybrid
between _Salix alba_ and _S. pentandra_. _Rhododendron intermedium_ is
an intermediate form between the hairy and the rusty species from the
Swiss Alps, _R. hirsutum_ and _R. ferrugineum_, the former growing on
chalky, and the other on silicious soils. Wherever both these types of
soil occur in the same valley and these two species approach one
another, the hybrid _R. intermedium_ is produced, and is often seen to
be propagating itself abundantly. As is indicated by the name, it
combines the essential characters of both parents.
_Linaria italica_ is a hybrid toad-flax between _L. genistifolia_ and
_L. vulgaris_, a cross which I have repeated in my garden. _Drosera
obovata_ [268] is a hybrid sundew between _D. anglica_ and _D.
rotundifolia_. _Primula variabilis_ is a hybrid between the two common
primroses, _P. officinalis_ and _P. grandiflora_. The willow-herb
(_Epilobium_), the self-heal (_Brunella_) and the yellow pond-lilies
(Nuphar) afford other instances of constant wild hybrids.
Macfarlane has discovered a natural hybrid between two species of sundew
in the swamps near Atco, N.J. The parents, _D. intermedia_ and _D.
filiformis_, were growing abundantly all around, but of the hybrid only
a group of eleven plants was found. A detailed comparison of the hybrid
with its parents demonstrated a minute blending of the anatomical
peculiarities of the parental species.
Luther Burbank of Santa Rosa, California, has produced a great many
hybrid brambles, the qualities of which in many respects surpass those
of the wild species. Most of them are only propagated by cuttings and
layers, not being stable from seed. But some crosses between the
blackberry and the raspberry (_R. fruticosus_ and _R. idaeus_) which
bear good fruit and have become quite popular, are so fixed in their
type as to reproduce their composite characters from seed with as much
regularity as the species of _Rubus_ found in nature. Among them are the
"Phenomenal" and the [269] "Primus." The latter is a cross between the
Californian dewberry and the Siberian raspberry and is certainly to be
regarded as a good stable species, artificially produced. Bell Salter
crossed the willow-herbs _Epilobium tetragonum_ and _E. montanum_, and
secured intermediate hybrids which remained true to their type during
four successive generations.
Other instances might be given. Many of them are to be found in
horticultural and botanical journals which describe their systematic and
anatomical details. The question of stability is generally dealt with in
an incidental manner, and in many cases it is difficult to reach
conclusions from the facts given. Especially disturbing is the
circumstance that from a horticultural point of view it is quite
sufficient that a new type should repeat itself in some of its offspring
to be called stable, and that for this reason absolute constancy is
rarely proved.
The range of constant hybrids would be larger by far were it not for two
facts. The first is the absolute sterility of so many beautiful hybrids,
and the second is the common occurrence of retrogressive characters
among cultivated plants. To describe the importance of both these groups
of facts would take too much [270] time, and therefore it seems best to
give some illustrative examples instead.
Among the species of _Ribes_ or currant, which are cultivated in our
gardens, the most beautiful are without doubt the Californian and the
Missouri currant, or _Ribes sanguineum_ and _R. aureum_. A third form,
often met with, is "Gordon's currant," which is considered to be a
hybrid between the two. It has some peculiarities of both parents. The
leaves have the general form of the Californian parent, but are as
smooth as the Missouri species. The racemes or flower-spikes are densely
flowered as in the red species, but the flowers themselves are of a
yellow tinge, with only a flesh-red hue on the outer side of the calyx.
It grows vigorously and is easily multiplied by cuttings, but it never
bears any fruit. Whether it would be constant, if fertile, is therefore
impossible to decide. _Berberis ilicifolia_ is considered as a hybrid
between the European barberry (_B. vulgaris_) and the cultivated shrub
_Mahonia aquifolia_. The latter has pinnate leaves, the former undivided
ones. The hybrid has undivided leaves which are more spiny than those of
the European parent, and which are not deciduous like them, but persist
during the winter, a peculiarity inherited from the _Mahonia_. As far as
I [271] have been able to ascertain, this hybrid never produces seed.
Another instance of an absolutely sterile hybrid is the often quoted
_Cytisus adami_. It is a cross between the common laburnum (_Cytisus
Laburnum_) and another species of the same genus, _C. purpureus_, and
has some traits of both. But since the number of differentiating marks
is very great in this case, most of the organs have become intermediate.
It is absolutely sterile. But it has the curious peculiarity of
splitting in a vegetative way. It has been multiplied on a large scale
by grafting and was widely found in the parks and gardens of Europe
during the last century. Nearly all these specimens reverted from time
to time to the presumable parents. Not rarely a bud of Adam's laburnum
assumed all the qualities of the common laburnum, its larger leaves,
richer flowered racemes, large and brightly yellow flowers and its
complete fertility. Other buds on the same tree reverted to the purple
parent, with its solitary small flowers, its dense shrublike branches
and very small leaves. These too are fertile, though not producing their
seeds as abundantly as the _C. Laburnum_ reversions. Many a botanist has
sown the seeds of the latter and obtained only pure common _C. Laburnum_
plants. I had a lot of nearly a hundred seedlings [272] myself, many of
which have already flowered, bearing the leaves and flowers of the
common species. Seeds of the purple reversions have also been sown, and
also yielded the parental type only.
Why this most curious hybrid sports so regularly and why others always
remain true to their type is as yet an open question.
But recalling our former consideration of this subject the supposition
seems allowable that the tendency to revert is not connected with the
type of the hybrid, but is apt to occur in some rare individuals of
every type. But since most of the sterile hybrids are only known to us
in a single individual and its vegetative offspring, this surmise offers
an explanation of the rare occurrence of sports.
Finally, we must consider some of the so called hybrid races or strains
of garden-plants. _Dahlia_, _Gladiolus_, _Amaryllis_, _Fuchsia_,
_Pelargonium_ and many other common flowers afford the best known
instances. Immeasurable variability seems here to be the result of
crossing. But on a closer inspection the range of characters is not so
very much wider in these hybrid races than in the groups of parent
species which have contributed to the origin of the hybrids. Our
tuberous begonias owe their variability to at least seven original
parent species, [273] and to the almost incredible number of
combinations which are possible between their characters. The first of
these crosses was made in the nursery of Veitch and Sons near London by
Seden, and the first hybrid is accordingly known as _Begonia sedeni_ and
is still to be met with. It has been superseded by subsequent crosses
between the _sedeni_ itself and the _Veitchi_ and _rosiflora_, the
_davisii_, the _clarkii_ and others. Each of them contributed its
advantageous qualities, such as round flowers, rosy color, erect flower
stalks, elevation of the flowers above the foliage and others. New
crosses are being made continuously, partly between the already existing
hybrids and partly with newly introduced wild species. Only rarely is it
possible to get pure seeds, and I have not yet been able to ascertain
whether the hybrids would come true from seed. Specific and varietal
characters may occur together in many of the several forms, but nothing
is as yet accurately known as to their behavior in pure fertilizations.
Constancy and segregation are thrown together in such a manner that
extreme variability results, and numerous beautiful types may be had,
and others may be expected from further crosses. For a scientific
analysis, however, the large range of recorded facts and the written
history, which at first sight [274] seems to have no lacunae, are not
sufficient. Most of the questions remain open and need investigation. It
would be a capital idea to try to repeat the history of the begonias or
any other hybrid race, making all the described crosses and then
recording the results in a manner requisite for complete and careful
scientific investigations.
Many large genera of hybrid garden-flowers owe their origin to species
rich in varieties or in elementary subspecies. Such is the case with the
gladiolus and the tulips. In other cases the original types have not
been obtained from the wild state but from the cultures of other
countries.
The dahlias were cultivated in Mexico when first discovered by
Europeans, and the chrysanthemums have been introduced from the old
gardens of Japan. Both of them consisted of various types, which
afterwards have been increased chiefly by repeated intercrossing.
The history of many hybrid races is obscure, or recorded by different
authorities in a different way. Some have derived their evidence from
one nursery, some from another, and the crosses evidently may have been
different in different places. The early history of the gladiolus is an
instance. The first crosses are recorded to have been made between
_Gladiolus_ [275] _psittacinus_ and _G. cardinalis_, and between their
hybrid, which is still known under the name of gandavensis_ and the
_purpureo-auratus_. But other authors give other lines of descent. So it
is with _Amaryllis_, which is said by De Graaff to owe its stripes to
_A. vittata_, its fine form to _A. brasiliensis_, the large petals to
_A. psittacina_, the giant flowers to _A. leopoldi_, and the piebald
patterns to _A. pardina_. But here, too, other authors give other
derivations.
Summarizing the results of our inquiry we see in the first place how
very much remains to be done. Many old crosses must be repeated and
studied anew, taking care of the purity of the cross as well as of the
harvesting of the seeds. Many supposed facts will be shown to be of
doubtful validity. New facts have to be gathered, and in doing so the
distinction between specific and varietal marks must be taken strictly
into account. The first have originated as progressive mutations; they
give unbalanced crosses with a constant offspring, as far as experience
now goes. The second are chiefly due to retrograde modifications, and
will be the subject of the next lecture.
[276]
LECTURE X
MENDEL'S LAW OF BALANCED CROSSES
In the scientific study of the result of crosses, the most essential
point is the distinction of the several characters of the parents in
their combination in the hybrids and their offspring. From a theoretical
point of view it would be best to choose parents which would differ only
in a single point. The behavior of the differentiating character might
then easily be seen.
Unfortunately, such simple cases do not readily occur. Most species, and
even many elementary species are distinguished by more than one quality.
Varieties deviating only in one unit-character from the species, are
more common. But a closer inspection often reveals some secondary
characters which may be overlooked in comparative or descriptive
studies, but which reassume their importance in experimental crossings.
In a former lecture we have dealt with the qualities which must be
considered as being due to the acquisition of new characters. If we
[277] compare the new form in this case with the type from which it has
originated, it may be seen that the new character does not find its
mate, or its opposite, and it will be unpaired in the hybrid.
In the case of retrogressive changes the visible modification is due, at
least in the best known instances, to the reduction of an active quality
to a state of inactivity or latency. Now if we make a cross between a
species and its variety, the differentiating character will be due to
the same internal unit, with no other difference than that it is active
in the species and latent in the variety. In the hybrid these two
corresponding units will make a pair. But while all other pairs in the
same hybrid individuals consist of like antagonists, only this pair
consists of slightly unlike opponents.
This conception of varietal crosses leads to three assertions, which
seem justifiable by actual experience.
First, there is no reason for a diminution of the fertility, as all
characters are paired in the hybrid, and no disturbance whatever ensues
in its internal structure. Secondly, it is quite indifferent, how the
two types are combined, or which of them is chosen as pistillate and
which as staminate parent. The deviating pair will have the same
constitution in both cases, being [278] built up of one active and one
dormant unit. Thirdly this deviating pair will exhibit the active unit
which it contains, and the hybrid will show the aspect of the parent in
which the character was active and not that of the parent in which it
was dormant. Now the active quality was that of the species, and its
latent state was found in the variety. Hence the inference that hybrids
between a species and its retrograde variety will bear the aspect of the
species. This attribute may be fully developed, and then the hybrid will
not be distinguishable from the pure species in its outer appearance. Or
the character may be incompletely evolved, owing to the failure of
cooperation of the dormant unit. In this case the hybrid will be in some
sense intermediate between its parents, but these instances are more
rare than the alternate ones, though presumably they may play an
important part in the variability of many hybrid garden-flowers.
All of these three rules are supported by a large amount of evidence.
The complete fertility of varietal hybrids is so universally
acknowledged that it is not worth while to give special instances. With
many prominent systematists it has become a test between species and
varieties, and from our present point of view this assumption is
correct. Only the test is of little use in practice, as fertility may be
diminished [279] in unbalanced unions in all possible degrees, according
to the amount of difference between the parents. If this amount is
slight, if for instance, only one unit-character causes the difference,
the injury to fertility may, be so small as to be practically nothing.
Hence we see that this test would not enable us to judge of the doubtful
cases, although it is quite sufficient as a proof in cases of wider
differences.
Our second assertion related to the reciprocal crosses. This is the name
given to two sexual combinations between the same parents, but with
interchanged places as to which furnishes the pollen. In unbalanced
crosses of the genus _Oenothera_ the hybrids of such reciprocal unions
are often different, as we have previously shown. Sometimes both
resemble the pollen parent more, in other instances the pistil-parent.
In varietal crosses no such divergence is as yet known. It would be
quite superfluous to adduce single cases as proofs for this rule, which
was formerly conceived to hold good for hybrids in general. The work of
the older hybridists, such as Koelreuter and Gaertner affords numerous
instances.
Our third rule is of a wholly different nature. Formerly the distinction
between elementary species and varieties was not insisted upon, and the
principle which stamps retrograde changes [280] as the true character of
varieties is a new one. Therefore it is necessary to cite a considerable
amount of evidence in order to prove the assertion that a hybrid bears
the active character of its parent-species and not the inactive
character of the variety chosen for the cross.
We may put this assertion in a briefer form, stating that the active
character prevails in the hybrid over its dormant antagonist. Or as it
is equally often put, the one dominates and the other is recessive. In
this terminology the character of the species is dominant in the hybrid
while that of the variety is recessive. Hence it follows that in the
hybrid the latent or dormant unit is recessive, but it does not follow
that these three terms have the same meaning, as we shall see presently.
The term recessive only applies to the peculiar state into which the
latent character has come in the hybrid by its pairing with the
antagonistic active unit.
In the first place it is of the highest importance to consider crosses
between varieties of recorded origin and the species from which they
have sprung. When dealing with mutations of celandine we shall see that
the laciniated form originated from the common celandine in a garden at
Heidelberg about the year 1590. Among my _Oenotheras_ one of the eldest
of the recent productions is the _O. brevistylis_ or short [281] styled
species which was seen for the first time in the year 1889. The third
example offered is a hairless variety of the evening campion, _Lychnis
vespertina_, found the same year, which hitherto had not been observed.
For these three cases I have made the crosses of the variety with the
parent-species, and in each case the hybrid was like the species, and
not like the variety. Nor was it intermediate. Here it is proved that
the older character dominates the younger one.
In most cases of wild, and of garden-varieties, the relation between
them and the parent-species rests upon comparative evidence. Often the
variety is known to be younger, in other cases it may be only of local
occurrence, but ordinarily the historic facts about its origin have
never been known or have long since been forgotten.
The easiest and most widely known varietal crosses are those between
varieties with white flowers and the red- or blue-flowered species. Here
the color prevails in the hybrid over the lack of pigment, and as a rule
the hybrid is as deeply tinted as the species itself, and cannot be
distinguished from it, without an investigation of its hereditary
qualities. Instances may be cited of the white varieties of the
snapdragon, of the red clover, the long-spurred violet (_Viola_ [282]
_cornuta_) the sea-shore aster (_Aster Tripolium_), corn-rose
(_Agrostemma Githago_), the Sweet William (_Silene Armeria_), and many
garden flowers, as for instance, the _Clarkia pulchella_, the
_Polemonium coeruleum_, the _Veronica longifolia_, the gloxinias and
others. If the red hue is combined with a yellow ground-color in the
species, the variety will be yellow and the hybrid will have the red and
yellow mixture of the species as for instance, in the genus _Geum_. The
toad-flax has an orange-colored palate, and a variety occurs in which
the palate is of the same yellow tinge as the remaining parts of the
corolla. The hybrid between them is in all respects like the
parent-species.
Other instances could be given. In berries the same rule prevails. The
black nightshade has a variety with yellow berries, and the black color
returns in the hybrid. Even the foliage of some garden-plants may afford
instances, as for instance, the purplish amaranth (_Amaranthus
caudatus_). It has a green variety, but the hybrid between the two has
the red foliage of the species.
Special marks in leaves and in flowers follow the same rule. Some
varieties of the opium poppy have large black patches at the basal end
of the petals, while in others this pattern is entirely white. In
crossing two such varieties, [283] for instance, the dark "Mephisto"
with the white-hearted "Danebrog," the hybrid shows the active character
of the dark pattern.
Hairy species crossed with their smooth varieties produce hairy hybrids,
as in some wheats, in the campion (_Lychnis_), in _Biscutella_ and
others. The same holds good for the crosses between spiny species and
their unarmed derivatives, as in the thorn-apple, the corn-crowfoot
(_Ranunculus arvensis_) and others.
Lack of starch in seeds is observed in some varieties of corn and of
peas. When such derivatives are crossed with ordinary starch-producing
types, the starch prevails in the hybrid.
It would take too much time to give further examples. But there is still
one point which should be insisted upon. It is not the systematic
relation of the two parents of a cross, that is decisive, but only the
occurrence of the same quality, in the one in an active, and in the
other in an inactive condition. Hence, whenever this relation occurs
between the parents of a cross, the active quality prevails in the
hybrid, even when the parents differ from each other in other respects
so as to be distinguished as systematic species. The white and red
campions give a red hybrid, the black and pale henbane (_Hyoscyamus
niger_ and _H. pallidus_) give a hybrid [284] with the purple veins and
center in the corolla of the former, the white and blue thornapple
produce a blue hybrid, and so on. Instances of this sort are common in
cultivated plants.
Having given this long list of examples of the rule of the dominancy of
the active character over the opposite dormant unit, the question
naturally arises as to how the antagonistic units are combined in the
hybrid. This question is of paramount importance in the consideration of
the offspring of the hybrids. But before taking it up it is as well to
learn the real signification of recessiveness in the hybrids themselves.
Recessive characters are shown by those rare cases, in which hybrids
revert to the varietal parent in the vegetative way. In other words by
bud-variations or sports, analogous to the splitting of Adam's laburnum
into its parents, by means of bud-variation already described. But here
the wide range of differentiating characters of the parents of this most
curious hybrid fail. The illustrative examples are extremely simple, and
are limited to the active and inactive condition of only one quality.
An instance is given by the long-leaved veronica (_Veronica
longifolia_), which has bluish flowers in long spikes. The hybrid
between [285] this species and its white variety has a blue corolla. But
occasionally it produces some purely white flowers, showing its power of
separating the parental heritages, combined in its internal structures.
This reversion is not common, but in thousands of flowering spikes one
may expect to find at least one of them. Sometimes it is a whole stem
springing from the underground system and bearing only white flowers on
all its spikes. In other instances it is only a side branch which
reverts and forms white flowers on a stem, the other spikes of which
remain bluish. Sometimes a spike even differentiates longitudinally,
bearing on one side blue and on the other white corollas, and the white
stripe running over the spike may be seen to be long and large, or
narrow and short in various degrees. In such cases it is evident that
the heritages of the parents remain uninfluenced by each other during
the whole life of the hybrid, working side by side, but the active
element always prevails over its latent opponent which is ready to break
free whenever an opportunity is offered.
It is now generally assumed that this incomplete mixture of the parental
qualities in a hybrid, this uncertain and limited combination is the
true cause of the many deviations, exhibited by varietal hybrids when
compared with their [286] parents. Partial departures are rare in the
hybrids themselves, but in their offspring the divergence becomes the
rule.
Segregation seems to be a very difficult process in the vegetative way,
but it must be very easy in sexual reproduction, indeed so easy as to
show itself in nearly every single instance.
Leaving this first generation, the original hybrids, we now come to a
discussion of their offspring. Hybrids should be fertilized either by
their own pollen, or by that of other individuals born from the same
cross. Only in this case can the offspring be considered as a means of
arriving at a decision as to the internal nature of the hybrids
themselves. Breeders generally prefer to fertilize hybrids with the
pollen of their parents. But this operation is to be considered as a new
cross, and consequently is wholly excluded from our present discussion.
Hence it follows that a clear insight into the heredity of hybrids may
be expected only from scientific experiments. Furthermore some of the
diversity observed as a result of ordinary crosses, may be due to the
instability of the parents themselves or at least of one of them, since
breeders ordinarily choose for their crosses some already very variable
strain. Combining such a strain with the desirable qualities of some
newly imported species, a new strain may [287] result, having the new
attribute in addition to all the variability of the old types. In
scientific experiments made for the purpose of investigating the general
laws of hybridity, such complex cases are therefore to be wholly
excluded. The hereditary purity of the parents must be considered as one
of the first conditions of success.
Moreover the progeny must be numerous, since neither constancy, nor the
exact proportions in the case of instability, can be determined with a
small lot of plants.
Finally, and in order to come to a definite choice of research material,
we should keep in mind that the chief object is to ascertain the
relation of the offspring to their parents. Now in nearly all cases the
seeds are separated from the fruits and from one another, before it
becomes possible to judge of their qualities. One may open a fruit and
count the seeds, but ordinarily nothing is noted as to their characters.
In this respect no other plant equals the corn or maize, as the kernels
remain together on the spike, and as it has more than one variety
characterized by the color, or constitution, or other qualities of the
grains. A corn-grain, however, is not a seed, but a fruit containing a
seed. Hence the outer parts pertain to the parent plant and only the
innermost ones to the [288] seedling and therefore to the following
generation. Fruit-characters thus do not offer the qualities we need,
only the qualities resulting from fertilizations are characteristic of
the new generation. Such attributes are afforded in some cases by the
color, in others by the chemical constitution.
We will choose the latter, and take the sugarcorn in comparison with the
ordinary or starch producing forms for our starting point. Both sugar-
and starch-corns have smooth fruits when ripening. No difference is to
be seen in the young ripe spikes. Only the taste, or a direct chemical
analysis might reveal the dissimilarity. But as soon as the spikes are
dried, a diversity is apparent. The starchy grains remain smooth, but
the sugary kernels lose so much water that they become wrinkled. The
former becomes opaque, the latter more or less transparent. Every single
kernel may instantly be recognized as belonging to either of the types
in question, even if but a single grain of the opposite quality might be
met with on a spike. Kernels can be counted on the spike, and since
ordinary spikes may bear from 300-500 grains and often more, the
numerical relation of the different types may be deduced with great
accuracy.
Coming now to our experiment, both starchy [289] and sugary varieties
are in this respect wholly constant, when cultivated separately. No
change is to be seen in the spikes. Furthermore it is very easy to make
the crosses. The best way is to cultivate both types in alternate rows
and to cut off the staminate panicles a few days before they open their
first flowers. If this operation is done on all the individuals of one
variety, sparing all the panicles of the other, it is manifest that all
the plants will become fertilized by the latter, and hence that the
castrated plants will only bear hybrid seeds.
The experiment may be made in two ways; by castrating the sugary or the
starchy variety. In both cases the hybrid kernels are the same. As to
their composition they repeat the active character of the starchy
variety. The sugar is only accumulated as a result of an incapacity of
changing it into starch, and the lack of this capacity is to be
considered as a retrogressive varietal mark. The starch-producing unit
character, which is active in the ordinary sorts of corns, is therefore
latent in sugar-corn.
In order to obtain the second generation, the hybrid grains are sown
under ordinary conditions, but sufficiently distant from any other
variety of corn to insure pure fertilization. The several individuals
may be left to pollinate [290] each other, or they may be artificially
pollinated with their own pollen.
The outcome of the experiments is shown by the spikes, as soon as they
dry. Each spike bears two sorts of kernels irregularly dispersed over
its surface. In this point all the spikes are alike. On each of them one
may see on the first inspection that the majority of the kernels are
starch-containing seeds, while a minor part becomes wrinkled and
transparent according to the rule for sugary seeds. This fact shows at
once that the hybrid race is not stable, but has differentiated the
parental characters, bringing those of the varietal parent to perfect
purity and isolation. Whether the same holds good for the starchy
parent, it is impossible to judge from the inspection of the spikes,
since it has been seen in the first generation that the hybrid kernels
are not visibly distinguished from those of the pure starch-producing
grains.
It is very easy to count the number of both sorts of grains in the spike
of such a hybrid. In doing so we find, that the proportion is nearly the
same on all the spikes, and only slight variations would be found in
hundreds of them. One-fourth of the seeds are wrinkled and three-fourths
are always smooth. The number may vary in single instances and be a
little more or a little less than 25%, ranging, for [291] instance, from
20 to 27%, but as a rule, the average is found nearly equal to 25%.
The sugary kernels, when separated from the hybrid spikes and sown
separately, give rise to pure sugary race, in no degree inferior in
purity to the original variety. But the starchy kernels are of different
types, some of them being internally like the hybrids of the first
generation and others like the original parent. To decide between these
two possibilities, it is necessary to examine their progeny.
For the study of this third hybrid generation we will now take another
example, the opium poppies. They usually have a dark center in the
flowers, the inferior parts of the four petals being stained a deep
purple, or often nearly black. Many varieties exhibit this mark as a
large black cross in the center of the flower. In other varieties the
pigment is wanting, the cross being of a pure white. Obviously it is
only reduced to a latent condition, as in so many other cases of loss of
color, since it reappears in a hybrid with the parent-species.
For my crosses I have taken the dark-centered "Mephisto" and the
"Danebrog," or Danish flag, with a white cross on a red field. The
second year the hybrids were all true to the type of "Mephisto." From
the seeds of each artificially self-fertilized capsule, one-fourth
(22.5%) [292] in each instance reverted to the varietal mark of the
white cross, and three-fourths (77.5%) retained the dark heart. Once
more the flowers were self-pollinated and the visits of insects
excluded. The recessives now gave only recessives, and hence we may
conclude that the varietal marks had returned to stability. The dark
hearted or dominants behaved in two different ways. Some of them
remained true to their type, all their offspring being dark-hearted.
Evidently they had returned to the parent with the active mark, and had
reassumed this type as purely as the recessives had reached theirs. But
others kept true to the hybrid character of the former generation,
repeating in their progeny exactly the same mixture as their parents,
the hybrids of the first generation, had given.
This third generation therefore gives evidence, that the second though
apparently showing only two types, really consists of three different
groups. Two of them have reassumed the stability of their original
grandparents, and the third has retained the instability of the hybrid
parents.
The question now arises as to the numerical relation of these groups.
Our experiments gave the following results: [293]
Cross 1. Generation 2. Generation 3. Generation
Mephisto 4- 100% Mephisto
| /
| /
| 77.5 % Dom.
| / \
> --All Mephisto \
| \ 9- all hybrids with 83-68%
| 22.5 % Rec. dominants and 17-32%
| recessives. 100% Danebrog.
Danebrog
Examining these figures we find one-fourth of constant recessives, as
has already been said, further one-fourth of constant dominants, and the
rest or one half as unstable hybrids. Both of the pure groups have
therefore reappeared [293] in the same numbers. Calling A the specimens
with the pure active mark, L those with the latent mark, and H the
hybrids, these proportions may be expressed as follows:
1A+2H+1L.
This simple law for the constitution of the second generation of
varietal hybrids with a single differentiating mark in their parents is
called the law of Mendel. Mendel published it in 1865, but his paper
remained nearly unknown to scientific hybridists. It is only of late
years that it has assumed a high place in scientific literature, and
attained the first rank as an investigation on fundamental questions of
heredity. [294] Read in the light of modern ideas on unit characters it
is now one of the most important works on heredity and has already
widespread and abiding influence on the philosophy of hybridism in
general.
But from its very nature and from the choice of the material made by
Mendel, it is restricted to balanced or varietal crosses. It assumes
pairs of characters and calls the active unit of the pair dominant, and
the latent recessive, without further investigations of the question of
latency. It was worked out by Mendel for a large group of varieties of
peas, but it holds good, with only apparent exceptions, for a wide range
of cases of crosses of varietal characters. Recently many instances have
been tested, and even in many cases third and later generations have
been counted, and whenever the evidence was complete enough to be
trusted, Mendel's prophecy has been found to be right.
According to this law of Mendel's the pairs of antagonistic characters
in the hybrid split up in their progeny, some individuals reverting to
the pure parental types, some crossing with each other anew, and so
giving rise to a new generation of hybrids. Mendel has given a very
suggestive and simple explanation of his formula. Putting this in the
terminology of to-day, and limiting it to the occurrence of only [295]
one differential unit in the parents, we may give it in the following
manner. In fertilization, the characters of both parents are not
uniformly mixed, but remain separated though most intimately combined in
the hybrid throughout life. They are so combined as to work together
nearly always, and to have nearly equal influence on all the processes
of the whole individual evolution. But when the time arrives to produce
progeny, or rather to produce the sexual cells through the combination
of which the offspring arises, the two parental characters leave each
other, and enter separately into the sexual cells. From this it may be
seen that one-half of the pollen-cells will have the quality of one
parent, and the other the quality of the other. And the same holds good
for [296] the egg-cells. Obviously the qualities lie latent in the
pollen and in the egg, but ready to be evolved after fertilization has
taken place.
Granting these premises, we may now ask as to the results of the
fertilization of hybrids, when this is brought about by their own
pollen. We assume that numerous pollen grains fertilize numerous egg
cells. This assumption at once allows of applying the law of
probability, and to infer that of each kind of pollen grains one-half
will reach egg-cells with the same quality [297] and the other half
ovules with the opposite character.
Calling P pollen and O ovules, and representing the active mark by P and
O, the latent qualities by P' and O', they would combine as follows:
P + 0 giving uniform pairs with the active mark,
P + 0' giving unequal pairs,
P' + 0 giving unequal pairs,
P' + 0' giving uniform pairs with the latent mark.
In this combination the four groups are obviously of the same size, each
containing one-fourth of the offspring. Manifestly they correspond
exactly to the direct results of the experiments, P + O representing the
individuals which reverted to the specific mark, P' + O' those who
reassumed the varietal quality and P + O' and P + O' those who
hybridized [298] for the second time. These considerations lead us to
the following form of Mendel's,
P + O = 1/4 Active or 1A,
P + O'
> = 1/2 Hybrid or 2 H,
P' + O
P' + O' = 1/4 Latent or 1 L,
Which is evidently the same as Mendel's empirical law given above.
To give the proof of these assumptions Mendel has devised a very simple
crossing experiment, [299] which he has effected with his varieties of
peas. I have repeated it with the sugar-corn, which gives far better
material for demonstration. It starts from the inference that if
dissimilarity among the pollen grains is excluded, the diversity of the
ovules must at once became manifest and vice versa. In other terms, if a
hybrid of the first generation is not allowed to fertilize itself, but
is pollinated by one of its parents, the result will be in accordance
with the Mendelian formula.
In order to see an effect on the spikes produced in this way, it is of
course necessary to fertilize them with the pollen of the variety, and
not with that of the specific type. The latter would give partly pure
starchy grains and partly hybrid kernels, but these would assume the
same type. But if we pollinate the hybrid with pollen of a pure
sugar-corn, we may predict the result as follows.
If the spike of the hybrid contains dormant paternal marks in one-half
of its flowers and in the other half maternal latent qualities, the
sugar-corn pollen will combine with one-half of the ovules to give
hybrids, and with the other half so as to give pure sugar-grains. Hence
we see that it will be possible to count out directly the two groups of
ovules on inspecting the ripe and dry spikes. Experience teaches us
[298] that both are present, and in nearly equal numbers; one-half of
the grains remaining smooth, and the other half becoming wrinkled.
The corresponding experiment could be made with plants of a pure
sugar-race by pollination with hybrid pollen. The spikes would show
exactly the same mixture as in the above case, but now this may be
considered as conclusive proof that half the pollen-grains represent the
quality of one parent and the other half the quality of the other.
Another corollary of Mendel's law is the following. In each generation
two groups return to purity, and one-half remains hybrid. These last
will repeat the same phenomenon of splitting in their progeny, and it is
easily seen that the same rule will hold good for all succeeding
generations. According to Mendel's principle, in each year there is a
new hybridization, differing in no respect from the first and original
one. If the hybrids only are propagated, each year will show one-fourth
of the offspring returning to the specific character, one-fourth
assuming the type of the variety and one-half remaining hybrid. I have
tested this with a hybrid between the ordinary nightshade with black
berries, and its variety, _Solanum nigrum chlorocarpum_, with pale
yellow fruits. Eight generations of the hybrids were cultivated, [299]
disregarding always the reverting offspring. At the end I counted the
progeny of the sixth and seventh generations and found figures for their
three groups of descendants, which exactly correspond to Mendel's
formula.
Until now we have limited ourselves to the consideration of single
differentiating units. This discussion gives a clear insight into the
fundamental phenomena of hybrid fertilization. It at once shows the
correctness of the assumption of unit-characters, and of their pairing
in the sexual combinations.
But Mendel's law is not at all restricted to these simple cases. Quite
on the contrary, it explains the most intricate questions of
hybridization, providing they do not transgress the limits of
symmetrical unions. But in this realm nearly all results may be
calculated beforehand, on the ground of the principle of probability.
Only one more assumption need be discussed. The several pairs of
antagonistic characters must be independent from, and uninfluenced by,
one another. This premise seems to hold good in the vast majority of
cases, though rare exceptions seem to be not entirely wanting. Hence the
necessity of taking all predictions from Mendel's law only as
probabilities, which will prove true in most, but not necessarily in all
cases. [300] But here we will limit ourselves to normal cases.
The first example to be considered is obviously the assumption that the
parents of a cross differ from each other in respect to two characters.
A good illustrative example is afforded by the thorn-apple. I have
crossed the blue flowered thorny form, usually known as _Datura Tatula_,
with the white thornless type, designated as _D. Stramonium inermis_.
Thorns and blue pigment are obviously active qualities, as they are
dominant in the hybrids. In the second generation both pairs of
characters are resolved into their constituents and paired anew
according to Mendel's law. After isolating my hybrids during the period
of flowering, I counted among their progeny:
128 individuals with blue flowers and thorns
47 individuals with blue flowers and without thorns
54 individuals with white flowers and thorns
21 individuals with white flowers and without thorns
----
250
The significance of these numbers may easily be seen, when we calculate
what was to be expected on the assumption that both characters follow
Mendel's law, and that both are independent from each other. Then we
would have three-fourths blue offspring and one-fourth individuals with
white flowers. Each of these [301] two groups would consist of
thorn-bearing and thornless plants, in the same numerical relation.
Thus, we come to the four groups observed in our experiment, and are
able to calculate their relative size in the following way:
Proportion
Blue with thorns 3/4 X 3/4 = 9/16 = 56.25% 9
Blue, unarmed 3/4 X 1/4 = 3/16 = 18.75% 3
White with thorns 1/4 X 3/4 = 3/16 = 18.75% 3
White, unarmed 1/4 X 1/4 = 1/16 = 6.25% 1
In order to compare this inference from Mendel's law and the assumption
of independency, with the results of our experiments, we must calculate
the figures of the latter in percentages. In this way we find:
Found Calculated
Blue with thorns 128=51% 56.25%
Blue unarmed 47=19% 18.75%
White with thorns 54=22% 18.75%
White unarmed 21= 8% 6.25%
The agreement of the experimental and the theoretical figures is as
close as might be expected.
This experiment is to be considered only as an illustrative example of a
rule of wide application. The rule obviously will hold good in all such
cases as comply with the two conditions already premised, viz.: that
each character agrees with Mendel's law, and that both are wholly
independent of each other. It is clear that our figures show the
numerical composition [302] of the hybrid offspring for any single
instance, irrespective of the morphological nature of the qualities
involved.
Mendel has proved the correctness of these deductions by his experiments
with peas, and by combining their color (yellow or green) with the
chemical composition (starch or sugar) and other pairs of characters. I
will now give two further illustrations afforded by crosses of the
ordinary campion. I used the red-flowered or day-campion, which is a
perennial herb, and a smooth variety of the white evening-campion, which
flowers as a rule in the first summer. The combination of flower-color
and pubescence gave the following composition for the second hybrid
generation:
Number % Calculation
Hairy and red 70 44 56.25%
Hairy and white 23 14 18.75%
Smooth and red 46 23 18.75%
Smooth and white 19 12 6.25%
For the combination of pubescence and the capacity of flowering in the
first year I found:
Number % Calculated
Hairy, flowering 286 52 56.25%
Hairy, without stem 128 23 18.75%
Smooth, flowering 96 17 18.75%
Smooth, without stem 42 8 6.25%
Many other cases have been tested by different writers and the general
result is the [303] applicability of Mendel's formula to all cases
complying with the given conditions.
Intentionally I have chosen for the last example two pairs of
antagonisms, relating to the same pair of plants, and which may be
tested in one experiment and combined in one calculation.
For the latter we need only assume the same conditions as mentioned
before, but now for three different qualities. It is easily seen that
the third quality would split each of our four groups into two smaller
ones in the proportion of 3/4 : 1/4.
We would then get eight groups of the following composition:
9/16 X 3/4 = 27/64 or 42.2%
9/16 X 1/4 = 9/64 " 14.1%
3/16 X 3/4 = 9/64 " 14.1%
3/16 X 1/4 = 3/64 " 4.7%
3/16 X 3/4 = 9/64 " 14.1%
3/16 X 1/4 = 3/64 " 4.7%
1/16 X 3/4 = 3/64 " 4.7%
1/16 X 1/4 = 1/64 " 1.6%
The characters chosen for our experiment include the absence of stem and
flowers in the first year, and therefore would require a second year to
determine the flower-color on the perennial specimens. Instead of doing
so I have taken another character, shown by the teeth of the capsules
when opening. These curve outwards [304] in the red campion, but lack
this capacity in the evening-campion, diverging only until an upright
position is reached. The combination of hairs, colors and teeth gives
eight groups, and the counting of their respective numbers of
individuals gave the following:
Teeth
Hairs Flowers of capsules Number % Calculated
Hairy red curved 91 47 42.2%
Hairy red straight 15 7.5 14.1%
Hairy white curved 23 12 14.1%
Hairy white straight 17 8.5 4.7%
Smooth red curved 23 12 14.1%
Smooth red straight 9 4.5 4.7%
Smooth white curved 5 2.5 4.7%
Smooth white straight 12 6 1.6%
The agreement is as comprehensive as might be expected from an
experiment with about 200 plants, and there can be no doubt that a
repetition on a larger scale would give still closer agreement.
In the same way we might proceed to crosses with four or more
differentiating characters. But each new character will double the
number of the groups. Four characters will combine into 16 groups, five
into 32, six into 64, seven into 128, etc. Hence it is easily seen that
the size of the experiments must be made larger and larger in the same
ratio, if we intend to expect numbers equally trustworthy. For [305]
seven differentiating marks 16,384 individuals are required for a
complete series. And in this set the group with the seven attributes all
in a latent condition would contain only a single individual.
Unfortunately the practical value of these calculations is not very
great. They indicate the size of the cultures required to get all the
possible combinations, and show that in ordinary cases many thousands of
individuals have to be cultivated, in order to exhaust the whole range
of possibilities. They further show that among all these thousands, only
very few are constant in all their characters; in fact, it may easily be
seen that with seven differentiating points among the 16,384 named
above, only one individual will have all the seven qualities in pure
active, and only one will have them all in a purely dormant condition.
Then there will be some with some attributes active and others latent,
but their numbers will also be very small. All others will split up in
the succeeding generation in regard to one or more of their apparently
active marks. And since only in very rare cases the stable hybrids can
be distinguished by external characters from the unstable ones, the
stability of each individual bearing a desired combination of characters
would have to be established by experiment [306] after pure
fertilization. Mendel's law teaches us to predict the difficulties, but
hardly shows any way to avoid them. It lays great stress on the old
prescript of isolation and pure fertilization, but it will have to be
worked out and applied to a large number of practical cases before it
will gain a preeminent influence in horticultural practice.
Or, as Bailey states it, we are only beginning to find a pathway through
the bewildering maze of hybridization.
This pathway is to be laid out with regard to the following
considerations. We are not to cross species or varieties, or even
accidental plants. We must cross unit-characters, and consider the
plants only as the bearers of these units. We may assume that these
units are represented in the hereditary substance of the cell-nucleus by
definite bodies of too small a size to be seen, but constituting
together the chromosomes. We may call these innermost representatives of
the unit-characters pangenes, in accordance with Darwin's hypothesis of
pangenesis, or give them any other name, or we may even wholly abstain
from such theoretical discussion, and limit ourselves to the conception
of the visible character-units. These units then may be present, or
lacking and in the first case active, or latent.
[307] True elementary species differ from each other in a number of
unit-characters, which do not contrast. They have arisen by progressive
mutation. One species has one kind of unit, another species has another
kind. On combining these, there can be no interchange. Mendelism assumes
such an interchange between units of the same character, but in a
different condition. Activity and latency are such conditions, and
therefore Mendel's law obviously applies to them. They require pairs of
antagonistic qualities, and have no connection whatever with those
qualities, which do not find an opponent in the other parent. Now, only
pure varieties afford such pure conditions. When undergoing further
modifications, some of them may be in the progressive line and others in
the retrogressive. Progressive modifications give new units, which are
not in contrast with any other, retrograde changes turn active units
into the latent condition and so give rise to pairs. Ordinary species
generally originate in this way, and hence differ from each other partly
in specific, partly in varietal characters. As to the first, they give
in their hybrids stable peculiarities, while as to the latter, they
split up according to Mendel's law.
Unpaired or unbalanced characters lie side by side with paired or
balanced qualities, and they [308] do so in nearly all the crosses made
for practical purposes, and in very many scientific experiments. Even
Mendel's peas were not pure in this respect, much less do the campions
noted above differ only in Mendelian characters.
Comparative and systematic studies must be made to ascertain the true
nature of every unit in every single plant, and crossing experiments
must be based on these distinctions in order to decide what laws are
applicable in any case.
[309]
D. EVER-SPORTING VARIETIES
LECTURE XI
STRIPED FLOWERS
Terminology is an awkward thing. It is as disagreeable to be compelled
to make new names, as to be constrained to use the old faulty ones.
Different readers may associate different ideas with the same terms, and
unfortunately this is the case with much of the terminology of the
science of heredity and variability. What are species and what are
varieties? How many different conceptions are conveyed by the terms
constancy and variability? We are compelled to use them, but we are not
at all sure that we are rightly understood when we do so.
Gradually new terms arise and make their way. They have a more limited
applicability than the old ones, and are more narrowly circumscribed.
They are not to supplant the older terms, but permit their use in a more
general way.
[310] One of these doubtful terms is the word _sport_. It often means
bud-variation, while in other cases it conveys the same idea as the old
botanical term of mutation. But then all sorts of seemingly sudden
variations are occasionally designated by the same term by one writer or
another, and even accidental anomalies, such as teratological ascidia,
are often said to arise by sports.
If we compare all these different conceptions, we will find that their
most general feature is the suddenness and the rarity of the phenomenon.
They convey the idea of something unexpected, something not always or
not regularly occurring. But even this demarcation is not universal, and
there are processes that are regularly repeated and nevertheless are
called sports. These at least should be designated by another name.
In order to avoid confusion as far as possible, with the least change in
existing terminology, I shall use the term "ever-sporting varieties" for
such forms as are regularly propagated by seed, and of pure and not
hybrid origin, but which sport in nearly every generation. The term is a
new one, but the facts are for the most part new, and require to be
considered in a new light. Its meaning will become clearer at once when
the illustrations afforded by [311] striped flowers are introduced. In
the following discussion it will be found most convenient to give a
summary of what is known concerning them, and follow this by a
consideration of the detailed evidence obtained experimentally, which
supports the usage cited.
The striped variety of the larkspur of our gardens is known to produce
monochromatic flowers, in addition to striped ones. They may be borne by
the same racemes, or on different branches, or some seedlings from the
same parent-plant may bear monochromatic flowers while others may be
striped. Such deviations are usually called sports. But they occur
yearly and regularly and may be observed invariably when the cultures
are large enough. Such a variety I shall call "ever-sporting."
The striped larkspur is one of the oldest garden varieties. It has kept
its capacity of sporting through centuries, and therefore may in some
sense be said to be quite stable. Its changes are limited to a rather
narrow circle, and this circle is as constant as the peculiarities of
any other constant species or variety. But within this circle it is
always changing from small stripes to broad streaks, and from them to
pure colors. Here the variability is a thing of absolute constancy,
while the constancy consists in eternal changes. Such apparent [312]
contradictions are unavoidable, when we apply the old term to such
unusual though not at all new cases. Combining the stability and the
qualities of sports in one word, we may evidently best express it by the
new term of eversporting variety.
We will now discuss the exact nature of such varieties, and of the laws
of heredity which govern them. But before doing so, I might point out,
that this new type is a very common one. It embraces most of the
so-called variable types in horticulture, and besides these a wide range
of anomalies.
Every ever-sporting variety has at least two different types, around and
between which it varies in numerous grades, but to which it is
absolutely limited. Variegated leaves fluctuate between green and white,
or green and yellow, and display these colors in nearly all possible
patterns. But there variability ends, and even the patterns are
ordinarily narrowly prescribed in the single varieties. Double flowers
afford a similar instance. On one side the single type, on the other the
nearly wholly double model are the extreme limits, between which the
variability is confined. So it is also with monstrosities. The race
consists of anomalous and normal individuals, and displays between them
all possible combinations of normal and monstrous [313] parts. But its
variability is restricted to this group. And large as the group may seem
on first inspection, it is in reality very narrow. Many monstrosities,
such as fasciated branches, pitchers, split leaves, peloric flowers, and
others constitute such ever-sporting varieties, repeating their
anomalies year by year and generation after generation, changing as much
as possible, but remaining absolutely true within their limits as long
as the variety exists.
It must be a very curious combination of the unit-characters which
causes such a state of continuous variability. The pure quality of the
species must be combined with the peculiarity of the variety in such a
way, that the one excludes the other, or modifies it to some extent,
although both never fully display themselves in the same part of the
same plant. A corolla cannot be at once monochromatic and striped, nor
can the same part of a stem be twisted and straight. But neighboring
organs may show the opposite attributes side by side.
In order to look closer into the real mechanism of this form of
variability, and of this constant tendency to occasional reversions, it
will be best to limit ourselves first to a single case, and to try to
gather all the evidence, which can be obtained by an examination of the
hereditary relations of its sundry constituents.
[314] This may best be done by determining the degree of inheritance for
the various constituents of the race during a series of years. It is
only necessary to apply the two precautions of excluding all
cross-fertilization, and of gathering the seeds of each individual
separately. We do not need to ascertain whether the variety as such is
permanent; this is already clear from the simple fact of its antiquity
in so many cases. We wish to learn what part each individual, or each
group of individuals with similar characters, play in the common line of
inheritance. In other words, we must build up a genealogical tree,
embracing several generations and a complete set of the single cases
occurring within the variety, in order to allow of its being considered
as a part of the genealogy of the whole. It should convey to us an idea
of the hereditary relations during the life-time of the variety.
It is manifest that the construction of such a genealogical tree
requires a number of separate experiments. These should be extended over
a series of years. Each should include a number of individuals large
enough to allow the determination of the proportion of the different
types among the offspring of a single plant. A species which is easily
fertilized by its own pollen, and which bears capsules with [315] large
quantities of seeds, obviously affords the best opportunities. As such,
I have chosen the common snapdragon of the gardens, _Antirrhinum majus_.
It has many striped varieties, some tall, others of middle height, or of
dwarfed stature. In some the ground-color of the flowers is yellow, in
others it is white, the yellow disappearing, with the exception of a
large mark in the throat. On these ground-colors the red pigment is seen
lying in streaks of pure carmine, with white intervals where the yellow
fails, but combined with yellow to make a fiery red, and with yellow
intervals when that color is present. This yellow color is quite
constant and does not vary in any marked degree, notwithstanding the
fact that it seems to make narrower and broader stripes, according to
the parts of the corolla left free by the red pigment. But it is easily
seen that this appearance is only a fallacious one.
The variety of snapdragon chosen was of medium height and with the
yellow ground-color, and is known by horticulturists as _A. majus luteum
rubro-striatum_. As the yellow tinge showed itself to be invariable; I
may limit my description to the red stripes.
Some flowers of this race are striped, others are not. On a hasty survey
there seem to be three types, pure yellow, pure red, and stripes [316]
with all their intermediate links of narrower and broader, fewer and
more numerous streaks. But on a close inspection one does not succeed in
finding pure yellow racemes. Little lines of red may be found on nearly
every flower. They are the extreme type on this side of the range of
variability. From them an almost endless range of patterns passes over
to the broadest stripes and even to whole sections of a pure red. But
then, between these and the wholly red flowers we observe a gap, which
may be narrower by the choice of numerous broad striped individuals, but
which is never wholly filled up. Hence we see that the red flowers are a
separate type within the striped variety.
This red type springs yearly from the striped form, and yearly reverts
to it. This is what in the usual descriptions of this snapdragon, is
called its sporting. The breadth of the streaks is considered to be an
ordinary case of variability, but the red flowers appear suddenly,
without the expected links. Therefore they are to be considered as
sports. Similarly the red forms may suddenly produce striped ones, and
this too is to be taken as a sport, according to the usual conception of
the word.
Such sports may occur in different ways. Either by seeds, or by buds, or
even within the single spikes. Both opposite reversions, [317] from
striped to red and from red to stripes, occur by seed, even by the
strictest exclusion of cross-fertilization. As far as my experiments go,
they are the rule, and parent-plants that do not give such reversions,
at least in some of their offspring, are very rare, if not wholly
wanting. Bud-variations and variations within the spike I have as yet
only observed on the striped individuals, and never on the red ones,
though I am confident that they might appear in larger series of
experiments. Both cases are more common on individuals with broad
stripes than on plants bearing only the narrower red lines, as might be
expected, but even on the almost purely yellow individuals they may be
seen from time to time. Bud-variations produce branches with spikes of
uniform red flowers. Every bud of the plant seems to have equal chances
to be transformed in this way. Some striped racemes bear a few red
flowers, which ordinarily are inserted on one side of the spike only. As
they often cover a sharply defined section of the raceme, this
circumstance has given rise to the term of sectional variability to
cover such cases. Sometimes the section is demarcated on the axis of the
flower-spike by a brownish or reddish color, sharply contrasting with
the green hue of the remaining parts. Sectional variation may be looked
at as a [318] special type of bud-variation, and from this point of view
we may simplify our inquiry and limit ourselves to the inheritance of
three types, the striped plants, the red plants and the red asexual
variants of the striped individuals. In each case the heredity should be
observed not only for one, but at least for two successive generations.
Leaving these introductory remarks I now come at once to the
genealogical tree, as it may be deduced from my experiments:
Year
1896 95% Striped 84% Red
| |
1895 Striped Individual Red Indiv.
\ /
1895 98% Striped 71% Red
| |
1894 Striped branches. Red branches.
\ /
1894 98% Striped 76% Red
| |
1893 90% Striped Indiv. 10% Red Indiv.
\ /
1892 Striped Individual
This experiment was begun in the year 1892 with one individual out of a
large lot of striped plants grown from seeds which I had purchased from
a firm in Erfurt. The capsules were gathered separately from this
individual and about 40 flowering plants were obtained from the seeds in
the following year. Most of them had neatly striped flowers, some
displayed broader stripes and spare flowers were seen with one [319]
half wholly red. Four individuals were found with only uniform red
flowers. These were isolated and artificially pollinated, and the same
was done with some of the best striped individuals. The seeds from every
parent were sown separately, so as to allow the determination of the
proportion of uniform red individuals in the progeny.
Neither group was constant in its offspring. But as might be expected,
the type of the parent plant prevailed in both groups, and more strongly
so in the instances with the striped, than with the red ones. Or, in
other words seed-reversions were more numerous among the already
reverted reds than among the striped type itself. I counted 2% reversion
in the latter case, but 24% from the red parents.
Among the striped plants from the striped parents, I found some that
produced bud variations. I succeeded in isolating these red flowering
branches in paper bags and in pollinating them with their own pollen,
and subjected the striped spikes of the same individuals to a similar
treatment. Three individuals gave a sufficient harvest from both types,
and these six lots of seeds were sown separately. The striped flowers
repeated their character in 98% of their offspring, the red twigs in
only 71%, the [320] remaining individuals sporting into the opposite
group.
In the following year I continued the experiment with the seeds of the
offspring of the red bud-variations. The striped individuals gave 95%,
but in the red ones only 84% of the progeny remained true to the parent
type.
From these figures it is manifest that the red and striped types differ
from one another not only in their visible attributes, but also in the
degree of their heredity. The striped individuals repeat their
peculiarity in 90-98% of their progeny, 2-10% sporting into the uniform
red color. On the other hand the red individuals are constant in 71-84%
of their offspring, while 16-29% go over to the striped type. Or,
briefly, both types are inherited to a high degree, but the striped type
is more strictly inherited than the red one.
Moreover the figures show that the degree of inheritance is not
contingent upon the question as to how the sport may have arisen.
Bud-sports show the same degree of inheritance as seed-sports. Sexual
and asexual variability therefore seem to be one and the same process in
this instance. But the deeper meaning of this and other special features
of our genealogical tree are still awaiting further investigation. It
seems that much important evidence might [321] come from an extension of
this line of work. Perhaps it might even throw some light on the
intimate nature of the bud-variations of ever-sporting varieties in
general. Sectional variations remain to be tested as to the degree of
inheritance exhibited, and the different occurrences as to the breadth
of the streaks require similar treatment.
In ordinary horticultural practice it is desirable to give some
guarantee as to what may be expected to come from the seeds of brightly
striped flowers. Neither the pure red type, nor the nearly yellow
racemes are the object of the culture, as both of them may be had pure
from their, own separate varieties. In order to insure proper striping,
both extremes are usually rejected and should be rooted out as soon as
the flowering period begins. Similarly the broad-striped ones should be
rejected, as they give a too large amount of uniform red flowers.
Clearly, but not broadly striped individuals always yield the most
reliable seed.
Summing up once more the results of our pedigree-experiment, we may
assert that the striped variety of the snapdragon is wholly permanent,
including the two opposite types of uniform color and of stripes. It
must have been so since it first originated from the invariable uniform
[322] varieties, about the middle of the last century, in the nursery of
Messrs. Vilmorin, and probably it will remain so as long as popular
taste supports its cultivation. It has never been observed to transgress
its limits or to sport into varieties without reversions or sports. It
fluctuates from one extreme to the other yearly, always recurring in the
following year, or even in the same summer by single buds. Highly
variable within its limits, it is absolutely constant or permanent, when
considered as a definite group.
Similar cases occur not rarely among cultivated plants. In the wild
state they seem to be wholly wanting. Neither are they met with as
occasional anomalies nor as distinct varieties. On the contrary, many
garden-flowers that are colored in the species, and besides this have a
white or yellow variety, have also striped sorts. The oldest instance is
probably the marvel of Peru, _Mirabilis Jalappa_, which already had more
than one striped variety at the time of its introduction from Peru into
the European gardens, about the beginning of the seventeenth century.
Stocks, liver-leaf (_Hepatica_), dame's violet (_Hesperis_), Sweet
William (_Dianthus barbatus_), and periwinkles (_Vinca minor_) seem to
be in the same condition, as their striped varieties were already quoted
[323] by the writers of the same century. Tulips, hyacinths, _Cyclamen_,
_Azalea_, _Camellia_, and even such types of garden-plants as the meadow
crane's-bill (_Geranium pratensev) have striped varieties. It is always
the red or blue color which occurs in stripes, the underlying ground
being white or yellow, according to the presence or absence of the
yellow in the original color mixture.
All these varieties are known to be permanent, coming true during long
series of successive generations. But very little is known concerning
the more minute details of their hereditary qualities. They come from
seed, when this is taken from striped individuals, and thence revert
from time to time to the corresponding monochromatic type. But whether
they would do so when self-fertilized, and whether the reversionary
individuals are always bound to return towards the center of the group
or towards the opposite limit, remains to be investigated. Presumably
there is nowhere a real transgression of the limits, and never or only
very rarely and at long intervals of time a true production of another
race with other hereditary qualities.
In order to satisfy myself on these points, I made some
pedigree-cultures with the striped forms of dame's violet (_Hesperis
matronalis_) [324] and of _Clarkia pulchella_. Both of them are
ever-sporting varieties. The experiments were conducted during five
generations with the violet, and during four with the striped Clarkia,
including the progeny of the striped and of the monochromatic red
offspring of a primitive striped plant. I need not give the figures here
for the numerical relations between the different types of each group,
and shall limit myself to the statement that they behaved in exactly the
same manner as the snapdragon.
It is worth while to dwell a moment on the capacity of the individuals
with red flowers to reproduce the striped type among their offspring.
For it is manifest that this latter quality must have lain dormant in
them during their whole life. Darwin has already pointed out that when a
character of a grandparent, which is wanting in the progeny, reappears
in the second generation, this quality must always be assumed to have
been present though latent in the intermediate generation. To the many
instances given by him of such alternative inheritance, the
monochromatic reversionists of the striped varieties are to be added as
a new type. It is moreover, a very suggestive type, since the latency is
manifestly of quite another character than for instance in the case of
Mendelian hybrids, and probably more allied to those instances, [325]
where secondary sexual marks, which are as a rule only evolved by one
sex, are transferred to the offspring through the other.
Stripes are by no means limited to flowers. They may affect the whole
foliage, or the fruits and the seeds, and even the roots. But all such
cases occur much more rarely than the striped flowers. An interesting
instance of striped roots is afforded by radishes. White and red
varieties of different shapes are cultivated. Besides them sometimes a
curious motley sort may be seen in the markets, which is white with red
spots, which are few and narrow in some samples, and more numerous and
broader in others. But what is very peculiar and striking is the
circumstance, that these stripes do not extend in a longitudinal, but in
a transverse direction. Obviously this must be the effect of the very
notable growth in thickness. Assuming that the colored regions were
small in the beginning, they must have been drawn out during the process
of thickening of the root, and changed into transverse lines. Rarely a
streak may have had its greatest extension in a transverse direction
from the beginning, in which case it would only be broadened and not
definitely changed in its direction.
This variety being a very fine one, and more agreeable to the eye than
the uniform colors, is [326] being more largely cultivated in some
countries. It has one great drawback: it never comes wholly true from
seed. It may be grown in full isolation, and carefully selected, all red
or nearly monochromatic samples being rooted out long before blooming,
but nevertheless the seed will always produce some red roots. The most
careful selection, pursued through a number of years, has not been
sufficient to get rid of this regular occurrence of reversionary
individuals. Seed-growers receive many complaints from their clients on
this account, but they are not able to remove the difficulty. This
experience is in full agreement with the experimental evidence given by
the snapdragon, and it would certainly be very interesting to make a
complete pedigree-culture with the radishes to test definitely their
compliance with the rules observed for striped flowers.
Horticulturists in such cases are in the habit of limiting themselves to
the sale of so-called mixed seeds. From these no client expects purity,
and the normal and hereditary diversity of types is here in some sense
concealed under the impurities included in the mixture from lack of
selection. Such cases invite scrutiny, and would, no doubt, with the
methods of isolation, artificial pollination, and the sowing of the
seeds separately from each parent, yield [327] results of great
scientific value. Any one who has a garden, and sufficient perseverance
to make pure cultures during a series of years might make important
contributions to scientific knowledge in this way.
Choice might be made from among a wide range of different types. A
variety of corn called "Harlequin" shows stripes on its kernels, and one
ear may offer nearly white and nearly red seeds and all the possible
intermediate steps between them. From these seeds the next generation
will repeat the motley ears, but some specimens will produce ears of
uniform kernels of a dark purple, showing thus the ordinary way of
reversion. Some varieties of beans have spotted seeds, and among a lot
of them one may be sure to find some purely red ones. It remains to be
investigated what will be their offspring, and whether they are due to
partial or to individual variation.
The cockscomb (_Celosia cristata_) has varieties of nearly all colors
from white and yellow to red and orange, and besides them some striped
varieties occur in our gardens, with the stripes going from the lower
parts of the stem up to the very crest of the comb. They are on sale as
constant varieties, but nothing has as yet been recorded concerning
their peculiar behavior in the inheritance of the stripes. [328] Striped
grapes, apples and other fruits might be mentioned in this connection.
Before leaving the striped varieties, attention is called to an
interesting deduction, which probably gives an explanation of one of the
most widely known instances of ever-sporting garden plants. Striped
races always include two types. Both of them are fertile, and each of
them reproduces in its offspring both its own and the alternate type. It
is like a game of ball, in which the opposing parties always return the
ball. But now suppose that only one of the types were fertile and the
other for some reason wholly sterile, and assume the reversionary, or
primitive monochromatic individuals to be fertile, and the derivative
striped specimens to bloom without seed. If this were the case,
knowledge concerning the hereditary qualities would be greatly limited.
In fact the whole pedigree would be reduced to a monochromatic strain,
which would in each generation sport in some individuals into the
striped variety. But, being sterile, they would not be able to propagate
themselves.
Such seems to be the case with the double flowered stocks. Their double
flowers produce neither stamens nor pistils, and as each individual is
either double or single in all its flowers, the doubles are wholly
destitute of seed. [329] Nevertheless, they are only reproduced by seed
from single flowers, being an annual or biennial species.
Stocks are a large family, and include a wonderful variety of colors,
ranging from white and yellow to purple and red, and with some
variations toward blue. They exhibit also diversity in the habit of
growth. Some are annuals, including the ten-week and pyramidal forms;
others are intermediates and are suitable for pot-culture; and the
biennial sorts include the well-known "Brompton" and "Queen" varieties.
Some are large and others are small or dwarf. For their brightness,
durability and fragrance, they are deservedly popular. There are even
some striped varieties. Horticulturists and amateurs generally know that
seed can be obtained from single stocks only, and that the double
flowers never produce any. It is not difficult to choose single plants
that will produce a large percentage of double blossoms in the following
generation. But only a percentage, for the experiments of the most
skilled growers have never enabled them to save seed, which would result
entirely in double flowering plants. Each generation in its turn is a
motley assembly of singles and doubles.
Before looking closer into the hereditary peculiarities of this old and
interesting ever-sporting [330] variety, it may be as well to give a
short description of the plants with double flowers. Generally speaking
there are two principal types of doubles. One is by the conversion of
stamens into petals, and the other is an anomaly, known under the name
of _petalomany_.
The change of stamens into petals is a gradual modification. All
intermediate steps are easily to be found. In some flowers all stamens
may be enlarged, in others only part of them. Often the broadened
filaments bear one or two fertile anthers. The fertility is no doubt
diminished, but not wholly destroyed. Individual specimens may occur,
which cannot produce any seed, but then others of the same lot may be as
fertile as can be desired. As a whole, such double varieties are
regularly propagated by seed.
Petalomany is the tendency of the axis of some flowers never to make any
stamens or pistils, not even in altered or rudimentary form. Instead of
these, they simply continue producing petals, going on with this
production without any other limit than the supply of available food.
Numerous petals fill the entire space within the outer rays, and in the
heart of the flower innumerable young ones are developed half-way, not
obtaining food enough to attain [331] full size. Absolute sterility is
the natural consequence of this state of things.
Hence it is impossible to have races of petalomanous types. If the
abnormality happens to show itself in a species, which normally
propagates itself in an asexual way, the type may become a vegetative
variety, and be multiplied by bulbs, buds or cuttings, etc. Some
cultivated anemones and crowfoots (_Ranunculus_) are of this character,
and even the marsh-marigold (_Caltha palustris_) has a petalomanous
variety. I once found in a meadow such a form of the meadow-buttercup
(_Ranunculus_ acris_), and succeeded in keeping it in my garden for
several years, but it did not make seeds and finally died. Camellias are
known to have both types of double flowers. The petalomanous type is
highly regular in structure, so much so as to be too uniform in all its
parts to be pleasing, while the conversion of stamens into petals in the
alternative varieties gives to these flowers a more lively diversity of
structure. Lilies have a variety called _Lilium candidum flore pleno_,
in which the flowers seem to be converted into a long spike of bright,
white narrow bracts, crowded on an axis which never seems to cease their
production.
It is manifestly impossible to decide how all such sterile double
flowers have originated. [332] Perhaps each of them originally had a
congruent single-flowered form, from which it was produced by seed in
the same way as the double stocks now are yearly. If this assumption is
right, the corresponding fertile line is now lost; it has perhaps died
out, or been masked. But it is not absolutely impossible that such
strains might one day be discovered for one or another of these now
sterile varieties.
Returning to the stocks we are led to the conception that some varieties
are absolutely single, while others consist of both single-flowered and
double-flowered individuals. The single varieties are in respect to this
character true to the original wild type. They never give seed which
results in doubles, providing all intercrossing is excluded. The other
varieties are ever-sporting, in the sense of this term previously
assumed, but with the restriction that the sports are exclusively
one-sided, and never return, owing to their absolute sterility.
The oldest double varieties of stocks have attained an age of a century
and more. During all this time they have had a continuous pedigree of
fertile and single-flowered individuals, throwing off in each generation
a definite number of doubles. This ratio is not at all dependent on
chance or accident, nor is it even variable to a remarkable degree.
Quite on the contrary [333] it is always the same, or nearly the same,
and it is to be considered as an inherent quality of the race. If left
to themselves, the single individuals always produce singles and doubles
in the same quantity; if cultivated after some special method, the
proportion may be slightly changed, bringing the proportion of doubles
up to 60% or even more.
Ordinarily the single and double members of such a race are quite equal
in the remainder of their attributes, especially in the color of their
flowers. But this is not always the case. The colors of such a race may
repeat for themselves the peculiarities of the ever-sporting characters.
It often happens that one color is more or less strictly allied to the
doubles, and another to the singles. This sometimes makes it difficult
to keep the various colors true. There are certain sorts, which
invariably exhibit a difference in color between the single and the
double flowers. The sulphur-yellow varieties may be adduced as
illustrative examples, because in them the single flowers always come
white. Hence in saving seed, it is impossible so to select the plant,
that an occasional white does not also appear among the double flowers,
agreeing in this deviation with the general rule of the eversporting
varieties.
I commend all the above instances to those [334] who wish to make
pedigree-cultures. The cooperation of many is needed to bring about any
notable advancement, since the best way to secure isolation is to
restrict one's self to the culture of one strain, so as to avoid the
intermixture of others. So many facts remain doubtful and open to
investigation, that almost any lot of purchased seed may become the
starting point for interesting researches. Among these the
sulphur-yellow varieties should be considered in the first place.
In respect to the great questions of heredity, the stocks offer many
points of interest. Some of these features I will now try to describe,
in order to show what still remains to be done, and in what manner the
stocks may clear the way for the study of the ever-sporting varieties.
The first point, is the question, which seeds become double-flowered and
which single-flowered plants? Beyond all doubt, the determination has
taken place before the ripening of the seed. But though the color of the
seed is often indicative of the color of the flowers, as in some red or
purple varieties, and though in balsams and some other instances the
most "highly doubled" flowers are to be obtained from the biggest and
plumpest seeds, no such rule seems to exist respecting the double
stocks. Now if one half of the seeds gives doubles, and [335] the other
half singles, the question arises, where are the singles and the doubles
to be found on the parent-plant?
The answer is partly given by the following experiment. Starting from
the general rule of the great influence of nutrition on variability, it
may be assumed that those seeds will give most doubles, that are best
fed. Now it is manifest that the stem and larger branches are, in a
better condition than the smaller twigs, and that likewise the first
fruits have better chances than the ones formed later. Even in the same
pod the uppermost seeds will be in a comparatively disadvantageous
position. This conception leads to an experiment which is the basis of a
practical method much used in France in order to get a higher percentage
of seeds of double-flowering plants.
This method consists in cutting off, in the first place the upper parts
of all the larger spikes, in the second place, the upper third part of
each pod, and lastly all the small and weak twigs. In doing so the
percentage is claimed to go up to 67-70%, and in some instances even
higher. This operation is to be performed as soon as the required number
of flowers have ceased blossoming. All the nutrient materials, destined
for the seeds, are now forced to flow into these relatively few embryos,
and it is clear that [336] they will be far better nourished than if no
operation were made.
In order to control this experiment some breeders have made the
operation on the fruits when ripe, instead of on the young pods, and
have saved the seeds from the upper parts separately. This seed,
produced in abundance, was found to be very poor in double flowers,
containing only some 20-30%. On the contrary the percentage of doubles
in the seed of the lower parts was somewhat augmented, and the average
of both would have given the normal proportion of 50%.
Opposed to the French method is the German practice of cultivating
stocks, as I have seen it used on a very large scale at Erfurt and at
other places. The stocks are grown in pots on small scaffolds, and not
put on or into the earth. The obvious aim of this practice is to keep
the earth in the pots dry, and accordingly they are only scantily
watered. In consequence they cannot develop as fully as they would have
done when planted directly in the beds, and they produce only small
racemes and no weak twigs, eliminating thereby without further operation
the weaker seeds as by the French method. The effect is increased by
planting from 6-10 separate plants in each pot.
It would be very interesting to make comparative [337] trials of both
methods, in order to discover the true relation between the practice and
the results reached. Bath should also be compared with cultures on open
plots, which are said to give only 50% of doubles. This last method of
culture is practiced wherever it is desired to produce great quantities
of seeds at a low cost. Such trials would no doubt give an insight into
the relations of hereditary characters to the distribution of the food
within the plant.
A second point is the proportional increase of the double-flowering
seeds with age. If seed is kept for two or three years, the greater part
of the grains will gradually die, and among the remainder there is found
on sowing, a higher percentage of double ones. Hence we may infer that
the single-flowered seeds are shorter lived than the doubles, and this
obviously points to a greater weakness of the first. It is quite evident
that there is some common cause for these facts and for the above cited
experience, that the first and best pods give more doubles. Much,
however, remains to be investigated before a satisfactory answer can be
made to these questions.
A third point is the curious practice, called by the French "esimpler,"
and which consists in pulling out the singles when very young. It seems
to be done at an age when the flower-buds [338] are not yet visible, or
at least are not far enough developed to show the real distinctive
marks. Children may be employed to choose and destroy the singles. There
are some slight differences in the fullness and roundness of the buds
and the pubescence of the young leaves. Moreover the buds of the doubles
are said to be sweeter to the taste than those of the singles. But as
yet I have not been able to ascertain, whether any scientific
investigation of this process has ever been made, though according to
some communications made to me by the late Mr. Cornu, the practice seems
to be very general in the environs of Paris. In summer large fields may
be seen, bearing exclusively double flowers, owing to the weeding out of
the singles long before flowering.
Bud-variation is the last point to be taken up. It seems to be very rare
with stocks, but some instances have been recorded in literature. Darwin
mentions a double stock with a branch bearing single flowers, and other
cases are known to have occurred. But in no instance does the seed of
such a bud-variant seem to have been saved. Occasionally other
reversions also occur. From time to time specimens appear with more
luxurious growth and with divergent instead of erect pods. They are
called, in Erfurt, "generals" on account [339] of their stiff and erect
appearance, and they are marked by more divergent horns crowning the
pods. They are said to produce only a relatively small number of doubles
from their seeds, and even this small number might be due to
fertilization with pollen of their neighbors. I saw some of these
reversionary types; when inspecting the nurseries of Erfurt, but as they
are, as a rule, thrown out before ripening their seed, nothing is
exactly known about their real hereditary qualities.
Much remains to be cleared up, but it seems that one of the best means
to find a way through the bewildering maze of the phenomena of
inheritance, is to make groups of related forms and to draw conclusions
from a comparison of the members of such groups. Such comparisons must
obviously give rise to questions, which in their turn will directly lead
to experimental investigation.
[340]
LECTURE XII
FIVE-LEAVED CLOVER
Every one knows the "four-leaved" clover. It is occasionally found on
lawns, in pastures and by the roadsides. Specimens with five leaflets
may be found now and then in the same place, or on the same plant, but
these are rarer. I have often seen isolated plants with quaternate
leaves, but only rarely have I observed individuals with more than one
such leaf.
The two cases are essentially dissimilar. They may appear to differ but
little morphologically, but from the point of view of heredity they are
quite different. Isolated quaternate leaves are of but little interest,
while the occurrence of many on the same individual indicates a distinct
variety. In making experiments upon this point it is necessary to
transplant the divergent individuals to a garden in order to furnish
them proper cultural conditions and to keep them under constant
observation. When a plant bearing a quaternate leaf is thus transplanted
however, it rarely repeats the [341] anomaly. But when plants with two
or more quaternate leaves on the same individual are chosen it indicates
that it belongs to a definite race, which under suitable conditions may
prove to become very rich in the anomalies in question.
Obviously it is not always easy to decide definitely whether a given
individual belongs to such a race or not. Many trials may be necessary
to secure the special race. I had the good fortune to find two plants of
clover, bearing one quinate and several quaternate leaves, on an
excursion in the neighborhood of Loosdrecht in Holland. After
transplanting them into my garden, I cultivated them during three years
and observed a slowly increasing number of anomalous leaves. This number
in one summer amounted to 46 quaternate and 16 quinate leaves, and it
was evident that I had secured an instance of the rare "five-leaved"
race which I am about to describe.
Before doing so it seems desirable to look somewhat closer into the
morphological features of the problem. Pinnate and palmate leaves often
vary in the number of their parts. This variability is generally of the
nature of a common fluctuation, the deviations grouping themselves
around an average type in the ordinary way. Ash leaves bear five pairs,
and [342] the mountain-ash (_Sorbus Aucuparia_) has six pairs of
leaflets in addition to the terminal one. But this number varies
slightly, the weaker leaves having less, the stronger more pairs than
the average. Such however, is not the case, with ternate leaves, which
seem to be quite constant. Four leaflets occur so very rarely that one
seems justified in regarding them rather as an anomaly than, as a
fluctuation. And this is confirmed by the almost universal absence of
two-bladed clover-leaves.
Considering the deviation as an anomaly, we may look into its nature.
Such an inquiry shows that the supernumerary leaflets owe their origin
to a splitting of one or more of the normal ones. This splitting is not
terminal, as is often the case with other species, and as it may be seen
sometimes in the clover. It is for the most part lateral. One of the
lateral nerves grows out becoming a median nerve of the new leaflet.
Intermediate steps are not wanting, though rare, and they show a gradual
separation of some lateral part of a leaflet, until this division
reaches the base and divides the leaflet into two almost equal parts. If
this splitting occurs in one leaflet we get the "four-leaved" Clover, if
it occurs in two there will be five leaflets. And if, besides this, the
terminal leaflet produces a derivative on one or both of its sides,
[343] we obtain a crown of six or seven leaflets on one stalk. Such were
often met with in the race I had under cultivation, but as a rule it did
not exceed this limit.
The same phenomenon of a lateral doubling of leaflets may of course be
met with in other instances. The common laburnum has a variety which
often produces quaternate and quinate leaves, and in strawberries I have
also seen instances of this abnormality. It occurs also in pinnate
leaves, and complete sets of all the intermediate links may often be
found on the false or bastard-acacia (_Robinia Pseud_Acacia_).
Opposed to this increase of the number of leaflets, and still more rare
and more curious is the occurrence of "single-leaved" varieties among
trees and herbs with pinnate or ternate leaves. Only very few instances
have been described, and are cultivated in gardens. The ashes and the
bastard-acacia may be quoted among trees, and the "one-leaved"
strawberry among herbs. Here it seems that several leaflets have been
combined into one, since this one is, as a rule, much larger than the
terminal leaflet of an ordinary leaf of the same species. These
monophyllous varieties are interesting also on account of their
continuous but often incomplete reversion to the normal type.
[344] Pinnate and palmate leaves are no doubt derivative types. They
must have originated from the ordinary simple leaf. The monophylly may
therefore be considered as a reversion to a more primitive state and the
monophyllous varieties may be called atavistic.
On the other hand we have seen that these atavistic varieties may revert
to their nearest progenitors, and this leads to the curious conception
of positive and negative atavism. For if the change of compound leaves
into single ones is a retrograde or negative step, the conversion of
single or ternate leaves into pinnate and palmate ones must evidently be
considered in this case as positive atavism.
This discussion seems to throw some light on the increase of leaflets in
the clover. The pea family, or the group of papilionaceous plants, has
pinnate leaves ordinarily, which, according to our premises, must be
considered as a derivative type. In the clovers and their allies this
type reverts halfway to the single form, producing only three leaflets
on each stalk. If now the clover increases its number of leaflets, this
may be considered as a reversion to its nearest progenitors, the
papilionaceous plants with pinnate leaves. Hence a halfway returning and
therefore positive atavism. And as I have already mentioned in a former
lecture, pinnate [345] leaves are also sometimes produced by my new race
of clover.
Returning to the original plants of this race, it is evidently
impossible to decide whether they were really the beginning of a new
strain, and had originated themselves by some sudden change from the
common type, or whether they belonged to an old variety, which had
propagated itself perhaps during centuries, unobserved by man. But the
same difficulty generally arises when new varieties are discovered. Even
the behavior of the plants themselves or of their progeny does not
afford any means of deciding the question. The simplest way of stating
the matter therefore, is to say that I accidentally found two
individuals of the "five-leaved" race. By transplanting them into my
garden, I have isolated them and kept them free from cross-fertilization
with the ordinary type. Moreover, I have brought them under such
conditions as are necessary for the full development of their
characters. And last but not least, I have tried to improve this
character as far as possible by a very rigid and careful selection.
The result of all this effort has been a rapid improvement of my strain.
I saved the seed of the original plants in 1889 and cultivated the
second generation in the following year. It [346] showed some increase
of the anomaly, but not to a very remarkable degree. In the flowering
period I selected four plants with the largest number of quaternate and
quinate leaves and destroyed all the others. I counted in the average 25
anomalous organs on each of them. From their seed I raised the third
generation of my culture in the year 1891.
This generation included some 300 plants, on which above 8,000 leaves
were counted. More than 1,000 were quaternate or quinate, the ternate
leaves being still in the majority. But the experiment clearly showed
that "four-leaved" clovers may be produced in any desired quantity,
provided that the seed of the variety is available. In the summer only
three, four and five leaflets on one stalk were seen, but towards the
fall, and after the selection of the best individuals, this number
increased and came up to six and seven in some rare instances.
The selection in this year was by no means easy. Nearly all the
individuals produced at least some quaternate leaves, and thereby showed
the variety to be quite pure. I counted the abnormal organs on a large
group of the best plants, and selected 20 excellent specimens from them,
with more than one-third of all their leaves changed in the desired
manner. Having brought my race up to this point, I [347] was able to
introduce a new and far more easy mark, afforded by the seedlings, for
my selections. This mark has since remained constant, and has brought
about a rapid continuance of the improvement, without necessitating such
large cultures.
This seedling in the various species of clover usually begins with a
first leaf above the cotyledons of a different structure from those that
follow. It has only one blade instead of three. But in my variety the
increase of the number of the leaflets may extend to these primary
organs, and make them binate or even ternate. Now it is obvious that an
individual, which begins with a divided primary leaf, will have a
greater tendency to produce a large number of supernumerary leaflets
than a plant which commences in the ordinary way. Or in other words, the
primary leaves afford a sure criterion for the selection, and this
selection may be made in the seed-pans. In consequence, no young
individual with an undivided primary leaf was planted out. Choosing the
20 or 30 best specimens in the seed-pan, no further selection was
required, and the whole lot could be left to cross-fertilization by
insects.
The observation of this distinguishing mark in the young seedlings has
led to the discovery of another quality as a starting-paint for further
[348] selection. According to the general rule of pedigree-culture, the
seeds of each individual plant are always saved and sowed separately.
This is done even with such species as the clover, which are infertile
when self-pollinated, and which are incapable of artificial pollination
on the required scale, since each flower produces only one seed. My
clover was always left free to be pollinated by insects. Obviously this
must have led to a diminution of the differentiating characters of the
individual plants. But this does not go far enough to obliterate the
differences, and the selection made among the seedlings will always
throw out at least a large part of those that have suffered from the
cross.
Leaving this discussion, we may inquire closer into the nature of the
new criterion afforded by the seedlings. Two methods present themselves.
First, the choice of the best seedlings. In the second place it becomes
possible to compare the parent-plants by counting the number of
deviating seedlings. This leads to the establishment of a percentage for
every single parent, and gives data for comparisons. Two or three
hundreds of seeds from a parent may easily be grown in one pan, and in
this way a sufficiently high degree of accuracy may be reached. Only
those parents that give [349] the highest percentage are chosen, and
among their progeny only the seedlings with trifoliolate primary leaves
are planted out. The whole procedure of the selection is by this means
confined to the glasshouse during the spring, and the beds need not be
large, nor do they require any special care during the summer.
By this method I brought my strain within two years up to an average of
nearly 90% of the seedlings with a divided primary leaf. Around this
average the real numbers fluctuated between the maximum of 99% and the
minimum of 70% or thereabouts. This condition was reached by the sixth
generation in the year 1894, and has since proved to be the limit, the
group of figures remaining practically the same during all the
succeeding generations.
Such selected plants are very rich in leaves with four, five and six
blades. Excluding the small leaves at the tops of the branches, and
those on the numerous weaker side-branches, these three groups include
the large majority of all the stronger leaves. In summer the range is
wider, and besides many trifoliolate leaves the curiously shaped
seven-bladed ones are not at all rare. In the fall and in the winter the
range of variability is narrowed, and at first sight the plants often
seem to bear only quinquefoliolate leaves.
[350] I have cultivated a new generation of this race nearly every year
since 1894, using always the strictest selection. This has led to a
uniform type, but has not been adequate to produce any further
improvement. Obviously the extreme limit, under the conditions of
climate and soil, has been reached. This extreme type is always
dependent upon repeated selection. No constant variety, in the older
sense, has been obtained, nor was any indication afforded that such a
type might ever be produced. On the contrary, it is manifest that the
new form belongs to the group of ever-sporting varieties. It is never
quite free from the old atavistic type of the trifoliolate leaves, and
invariably, when external conditions become less favorable, this
atavistic form is apt to gain dominion over the more refined varietal
character. Reversions always occur, both partial and individual.
Some instances of these reversions may now be given. They are not of
such a striking character as those of the snapdragon. Intermediate steps
are always occurring, both in the leaves themselves, and in the
percentages of deviating seedlings of the several parent plants.
On normal plants of my variety the quinquefoliolate leaves usually
compose the majority, when there are no weak lateral branches, or when
they are left out of consideration. Next [351] to these come the fours
and the sixes, while the trifoliolate and seven-bladed types are nearly
equal in number. But out of a lot of plants, grown from seed of the same
parent, it is often possible to choose some in which one extreme
prevails, and others with a preponderating number of leaves with the
other extreme number of leaflets. If seed from these extremes are saved
separately, one strain, that with numerous seven-bladed leaves will
remain true to the type, but the other will diverge more or less,
producing leaves with a varying number of subdivisions.
Very few generations of such opposite selection are required to reduce
the race to an utterly poor one. In three years I was able to nearly
obliterate the type of my variety. I chose the seedlings with an
undivided primary leaf, cultivated them and counted their offspring
separately after the sowing. I found some parents with only 2-3% of
seedlings with divided primary leaves. And by a repeated selection in
this retrograde direction I succeeded in getting a great number of
plants, which during the whole summer made only very few leaves with
more than three blades. But an absolute reversion could no more be
reached in this direction than in the normal one. Any sowing without
selection would be [352] liable to reduce the strain to an average
condition.
The production of varietal and of atavistic leaves is dependent to a
high degree on external conditions. It agrees with the general rule,
that favorable circumstances strengthen the varietal peculiarities,
while unfavorable conditions increase the number of the parts with the
atavistic attribute. These influences may be seen to have their effect
on the single individuals, as well as on the generations growing from
their seed. I cannot cite here all the experimental material, but a
single illustrative example may be given. I divided a strong individual
into two parts, planted one in rich soil and the other in poor sand, and
had both pollinated by bees with the pollen of some normal individuals
of my variety growing between them. The seeds of both were saved and
sown separately, and the two lots of offspring cultivated close to each
other under the same external conditions. In the beginning no difference
was seen, but as soon as the young plants had unfolded three or four
leaves, the progeny of the better nourished half of the parent plant
showed a manifest advance. This difference increased rapidly and was
easily seen in the beds, even before the flowering period.
This experience probably gives an explanation [353] why the
quinquefoliolate variety is so seldom met with in the wild state. For
even if it did occur more often, the plants would hardly find
circumstances favorable enough for the full development of their
varietal character. They must often be so poor in anomalous leaves as to
be overlooked, or to be taken for instances of the commonly occurring
quadrifoliolate leaves and therefore as not indicating the true variety.
In the beginning of my discussion I have asserted the existence of two
different races of "four-leaved" clovers, a poor one and a rich one, and
have insisted on a sharp distinction between them. This distinction
partly depends on experiments with clover, but in great part on tests
with other plants. The previously mentioned circumstance, that clover
cannot be pollinated on a sufficiently large scale otherwise than by
insects, prevents trials in more than one direction at the same time and
in the same garden. For this reason I have chosen another species of
clover to be able to give proof or disproof of the assertion quoted.
This species is the Italian, or crimson clover, which is sometimes also
called scarlet clover (_Trifolium incarnatum_). It is commonly used in
Europe as a crop on less fertile soils than are required by the red
clover. It is annual [354] and erect and more or less hairy, and has
stouter leaves than other kinds of clover. It has oblong or cylindrical
heads with bright crimson flowers, and may be considered as one of the
most showy types. As an annual it has some manifest advantages over the
perennial species, especially in giving its harvest of hay at other
seasons of the year.
I found some stray quaternate leaves of this plant some years ago, and
tried to win from them, through culture and selection, a race that would
be as rich in these anomalies as the red clover. But the utmost care and
the most rigid selection, and all the attention I could afford, failed
to produce any result. It is now ten years since I commenced this
experiment, and more than once I have been willing to give it up. Last
year (1903) I cultivated some hundreds of selected plants, but though
they yielded a few more instances of the desired anomaly than in the
beginning, no trace of a truly rich race could be discovered. The
experimental evidence of this failure shows at least that stray
"four-leaves" may occur, which do not indicate the existence of a true
"four-" or "five-leaved" variety.
This conception seems destined to become of great value in the
appreciation of anomalies, as they are usually found, either in the wild
state [355] or in gardens. And before describing the details of my
unsuccessful pedigree-culture, it may be as well to give some more
instances of what occurs in nature.
Stray anomalies are of course rare, but not so rare that they might not
be found in large numbers when perseveringly sought for. Pitcher-like
leaves may be found on many trees and shrubs and herbs, but ordinarily
one or only two of them are seen in the course of many years on the same
plant, or in the same strain. In some few instances they occur annually
or nearly so, as in some individuals of the European lime-tree (_Tilia
parvifolia_) and of the common magnolia (_Magnolia obovata_). Many of
our older cultivated plants are very rich in anomalies of all kinds, and
_Cyclamen_, _Fuchsia_, _Pelargonium_ and some others are notorious
sources of teratologic phenomena. Deviations in flowers may often be
seen, consisting of changes in the normal number of the several organs,
or alterations in their shape and color. Leaves may have two tips,
instead of one, the mid-vein being split near the apex, and the fissure
extending more or less towards the base. Rays of the umbels of
umbelliferous plants may grow together and become united in groups of
two or more, and in the same way the fruits of [356] the composites may
be united into groups. Many other instances could easily be given.
If we select some of these anomalies for breeding-experiments, our
results will not agree throughout, but will tend to group themselves
under two heads. In some cases the isolation of the deviating
individuals will at once show the existence of a distinct variety, which
is capable of producing the anomaly in any desired number of instances;
only dependent on a favorable treatment and a judicious selection. In
other cases no treatment and no selection are adequate to give a similar
result, and the anomaly remains refractory despite all our endeavors to
breed it. The cockscomb and the peloric fox-glove are widely known
instances of permanent anomalies, and others will be dealt with in
future lectures. On the other hand I have often tried in vain to win an
anomalous race from an accidental deviation, or to isolate a teratologic
variety out of more common aberrations. Two illustrative examples may be
quoted. In our next lecture we shall deal with a curious phenomenon in
poppies, consisting in the change of the stamens into pistils and giving
rise to a bright crown of secondary capsules around the central one.
Similar anomalies may be occasionally met with in other species of the
same genus. But they are rare, and may show [357] the conversion of only
a single stamen in the described manner. I observed this anomaly in a
poppy called _Papaver commutatum_, and subjected it during several years
to a rigid selection of the richest individuals. No amelioration was to
be gained and the culture had to be given up. In the same way I found on
the bulbous buttercup (_Ranunculus bulbosus_) a strain varying largely
in the number of the petals, amounting often to 6-8, and in some flowers
even yet to higher figures. During five succeeding years I cultivated
five generations, often in large numbers, selecting always those which
had the highest number of petals, throwing out the remainder and saving
the seed only from the very best plants. I got a strain of selected
plants with an average number of nine petals in every flower, and found
among 4,000 flowers four having 20 petals or more, coming up even to 31
in one instance. But such rare instances had no influence whatever on
the selection, since they were not indicative of individual qualities,
but occurred quite accidentally on flowers of plants having only the
average number of petals. Now double flowers are widely known to occur
in other species of the buttercups, both in the cultivated varieties and
in some wild forms. For this reason it might be expected that through a
continuous selection of [358] the individuals with the largest numbers a
tendency to become double would be evolved. Such, however, was not the
case. No propensity to vary in any definite direction could be observed.
Quite on the contrary, an average condition was quickly reached, and
then remained constant, strongly counteracting all selection.
Such experiences clearly show that the same anomaly may occur in
different species, and no doubt in strains of the same species from
different localities, according to at least two different standards. The
one is to be called the poor, and the other the rich variety. The first
always produces relatively few instances of the deviation, the last is
apt to give as many of them as desired. The first is only half-way a
variety, and therefore would deserve the name of a half-race; the second
is not yet a full constant variety, but always fluctuates to and fro
between the varietal and the specific mark, ever-sporting in both
directions. It holds a middle position between a half-race and a
variety, and therefore might be called a "middle-race." But the term
ever-sporting variety seems more adequate to convey a right idea of the
nature of this curious type of inheritance.
From this discussion it will be seen that the behavior of the crimson
clover is not to be considered [359] as an exception, but as a widely
occurring type of phenomenon, occurring perhaps in all sorts of
teratologic deviations, and in wide ranges of species and genera. Hence
it may be considered worth while to give some more details of this
extended experiment.
Ten years ago (1894-5) I bought and sowed about a pound of seed of the
crimson clover. Among many thousands of normal seedlings I found two
with three and one with four cotyledons. Trusting to the empirical rules
of correlation, I transplanted these three individuals in order to
isolate them in the flowering period.
One of them produced during the ensuing summer one four-bladed and one
five-bladed leaf. The seeds were saved separately and sown the following
spring and the expected result could soon be seen. Among some 250
individual plants I counted 22 with one or two deviations, and 10 with
from three to nine four- or five-bladed leaves. Proportions nearly
similar have been observed repeatedly. Better nourished individuals have
produced more deviating leaves on one plant, partly owing to the larger
number of stems and branches, and poor or average specimens have mostly
been without any aberration or with only one or two abnormal leaves. No
further improvement could be attained. Quadrifoliolate leaves were
always rare, never [360] attaining a number that would put its stamp on
a whole bed. I have endeavored to get some six- and seven-bladed crimson
clover leaves, but in vain; selection, culture of many hundreds of
individuals, manure, and the best possible treatment has not been
adequate to produce them. Of course I am quite convinced that a
repetition of my experiment on a far larger scale would yield the
desired types, but then only in such rare instances that they would have
no influence whatever on the average, or on the improvement of the race.
The eighth generation in the year 1903 has not been noticeably better
than the second and third generations after the first selection.
In comparing this statement with the results gained in the experiment
with the red clover, the difference is at once striking. In one case a
rich variety was isolated, and, by better treatment and sharp methods of
selection, was brought up in a few years to its highest pitch of
development. In the other case a very weak race was shown to exist, and
no amount of work and perseverance was adequate to improve it to any
noticeable degree.
I wish to point out that the decision of what is to be expected from
deviating specimens may become manifest within one or two generations.
Even the generation grown from the seeds of [361] the first observed
aberrant-individuals, if gathered after sufficient isolation during the
period of blossoming, may show which type of inheritance is present,
whether it is an unpromising half-race, or a richly endowed sporting
variety. I have kept such strains repeatedly after the first isolation,
and a special case, that of cotyledoneous aberrations, will be dealt
with later. The first generation always gave a final decision, provided
that a suitable method of cultivation for the species under observation
was found at the beginning. This however, is a condition, which it is
not at all easy to comply with, when new sorts are introduced into a
garden. Especially so when they had been collected in the wild state.
Often one or two years, sometimes more, are necessary to find the proper
method of sowing, manuring, transplanting and, other cultural methods
satisfactory to the plants. Many wild species require more care and more
manure in gardens than the finest garden flowers. And a large number are
known to be dependent on very particular conditions of soil.
One of the most curious features of anomalies, which has been learned
from accumulated instances, is the fact that they obey definite laws as
to their occurrence on the different parts of the plant. Obviously such
laws are [362] not apparent as long as each plant produces only one or
two, or, at most, a few instances of the same deviation. On the
contrary, any existing regularity must betray itself, as soon as a
larger number of instances is produced. A rule of periodicity becomes
most clearly manifest in such cases.
This rule is shown by no other race in a more undoubted and evident
manner than by the "five-leaved" clover. Evidently the several degrees
of deviation, going from three to seven leaflets, may be regarded as
responses to different degrees of variation, and their distribution over
the stems and branches, or over the whole plant, may be considered as
the manifestation of the ever-changing internal tendency to vary.
Considered from this point of view, my plants always showed a definite
periodicity in this distribution, which is the same for the whole plant.
Each of them, and each of the larger branches, begin with atavistic
leaves or with slight deviations. These are succeeded by greater
deviations, but only the strongest axes show as many as seven leaflets
on a stalk. This ordinarily does not occur before the height of
development is reached, and often only towards its close. Then the
deviation diminishes rapidly, returning often to atavistic leaves at the
summit of the stem or branch. I give the numbers of the [363] leaves of
a branch, in their order from the base to the top. They were as follows:
3. 4. 5. 6. 7. 5. 5. 4.
But this is a selected case, and such regular examples of the expected
periodicity are rarely found. Often one or more of the various steps are
lacking, or even leaves with smaller numbers may be interspersed among
those with larger numbers of leaflets. But while the regularity of the
periodicity is in some degree diminished by such occurrences, yet the
rule always holds good, when taken broadly. It may be expressed by
stating that the bases and apices have on the average fewer leaflets on
each leaf than the middle parts of the stem and branches, and that the
number of leaflets gradually increases from the base toward a maximum,
which is reached in organs on the middle or upper part of the axis, and
then diminishes from this toward the apex.
This periodicity is not limited to the stems and branches, considered
singly, but also holds good in a comparison made between the branches of
a single stem, in regard to their relative places on that stem. So it is
also for the whole plant. The first stems, produced by the subterranean
axis, ordinarily show only a low maximum deviation: the next succeeding
being [364] more divergent and the last ones returning to less
differentiated forms.
It is evident that on a given stem the group of deviating leaves will be
extended upward and downward, with the increase of the number of these
organs. This shows that a stem, or even a plant, promises a higher
degree of differentiation if it commences with its aberration earlier.
Hence it becomes possible to discern the most promising individuals in
early youth, and this conclusion leads to a very easy and reliable
method of selection, which may be expressed simply as follows: the
seedlings which commence earliest with the production of four- and
five-foliolate leaves are the best and should be selected for the
continuance of the race. And it is easily seen that this rule agrees
with that given above, and which was followed in my pedigree-culture.
Furthermore it is seen that there is a complete agreement between the
law of periodicity and the responses of the deviations to nourishment
and other conditions of life. Weak plants only produce low degrees of
deviation, the stronger the individual becomes, the higher it reaches in
the scale of differentiation, and the more often it develops leaves with
five or more blades. Whether weakness or strength are derived from outer
causes, or from the internal [365] succession of the periods of life, is
evidently of no consequence, and in this way the law of periodicity may
be regarded as a special instance of the more general law of response to
external conditions.
The validity of this law of periodicity is of course not limited to our
"five-leaved" clover. Quite on the contrary it is universal in
eversporting varieties. Moreover it may be ascertained and studied in
connection with the most widely different morphologic abnormalities, and
therefore affords easily accessible material for statistical inquiry. I
will now give some further instances, but wish to insist first upon the
necessity of an inquiry on a far larger scale, as the evidence as yet is
very scanty.
The great celandine (_Chelidonium majus_) has a very curious double
variety. Its flowers are simpler and much more variable than in ordinary
garden-varieties. The process of doubling consists mainly in a change of
stamens into petals. This change is dependent on the season. On each
stem the earliest flowers are single. These are succeeded by blossoms
with one or two converted stamens, and towards the summer this number
increases gradually, attaining 10-11 and in some instances even more
altered filaments. Each year the same succession may be seen repeating
itself on the stems of [366] the old roots. Double tuberous begonias are
ordinarily absolutely sterile throughout the summer, but towards autumn
the new flowers become less and less altered, producing some normal
stamens and pistils among the majority of metamorphosed organs. From
these flowers the seeds are saved. Sometimes similar flowers occur at
the beginning of the flowering-period. Double garden-camomiles
(_Chrysanthemum inodorum plenissimum_) and many other double varieties
of garden-plants among the great family of the composites are very
sensitive to external agencies, and their flower-heads are fuller the
more favorable the external conditions. Towards the autumn many of them
produce fewer and fewer converted heads and often only these are fertile
and yield seeds.
Ascidia afford another instance of this periodicity, though ordinarily
they are by far too rare to show any regularity in their distribution.
However, it is easy to observe that on lime-trees they prefer the lower
parts of each twig, while on magnolias the terminal leaves of the
branches are often pitcher-bearing. Ascidia of the white clover have
been found in numbers, in my own experiment-garden, but always in the
springtime. The thickleaved saxifrage (_Saxifraga crassifolia_) is often
very productive of ascidia, especially in [367] the latter part of the
season, and as these organs may be developed to very different degrees,
they afford fine material for the study of the law of periodicity. On a
garden-cytisus (_Cytisus candicans attleyanus_) I once had the good
fortune to observe a branch with ascidia, which ordinarily are very rare
in this species. It had produced seven ascidia in all, each formed by
the conversion of one leaflet on the trifoliolate leaves. The first six
leaves were destitute of this malformation and were quite normal. Then
followed a group of five leaves, constituting the maximum of the period.
The first bore one small pitcher-like blade, the second and third, each
one highly modified organ, the fourth, two ascidia, and the last, one
leaflet with slightly connate margins. The whole upper part of the
branch was normal, with the exception of the seventeenth leaf, which
showed a slight change in the same direction. All in all, the tendency
to produce ascidia increased from the beginning to the tenth leaf, and
decreased from this upward.
The European Venus' looking-glass was observed in my garden to produce
some quaternate and some quinate flowers on the same specimens. The
quinate were placed at the end of the branches, those with four petals
and sepals lower down. The peloric fox-glove shows the [368] highest
degree of metamorphy in the terminal flowers of the stem itself, the
weaker branches having but little tendency towards the formation of the
anomaly. The European pine or _Pinus sylvestris_ ordinarily has two
needles in each sheath, but trifoliolate sheaths occur on the stems and
stronger branches, where they prefer, as a rule, the upper parts of the
single annual shoots. _Camellia japonica_ is often striped in the fall
and during the winter, but when flowering in the spring it returns to
the monochromatic type.
Peloric flowers are terminal in some cases, but occur in the lower parts
of the flower-spikes in others. Some varieties of gladiolus commence on
each spike with more or less double flowers, which, higher up, are
replaced by single ones. A wide range of bulbs and perennial
garden-plants develop their varietal characters only partly when grown
from seed and flowering for the first time. The annual
garden-forget-me-not of the Azores (_Myosotis azorica_) has a variety
with curiously enlarged flowers, often producing 20 or more
corolla-segments in one flower. But this number gradually diminishes as
the season advances. It would be quite superfluous to give further proof
of the general validity of the law of periodicity in ever-sporting
varieties.
[369]
LECTURE XIII
PISTILLODY IN POPPIES
One of the most curious anomalies that may be met with in ornamental
garden-plants is the conversion of stamens into pistils. It is neither
common nor rare, but in most cases the change is so slight comparatively
that it is ordinarily overlooked. In the opium-poppy, on the contrary,
it is very showy, and heightens the ornamental effect of the young
fruits after the fading of the flowers. Here the central capsule is
surrounded by a large crown of metamorphosed stamens.
This peculiarity has attracted the attention both of horticulturists and
of botanists. As a rule not all the stamens are changed in this way but
only those of the innermost rows. The outer stamens remain normal and
fertile, and the flowers, when pollinated with their own pollen, bear as
rich a harvest of seeds as other opium-poppies. The change affects both
the filament and the anther, the former of which is dilated into a
sheath. Within this sheath perfect [370] and more or less numerous
ovules may be produced. The anthers become rudimentary and in their
place broad leafy flaps are developed, which protrude laterally from the
tip and constitute the stigmas. Ordinarily these altered organs are
sterile, but in some instances a very small quantity of seed is
produced, and when testing their viability I succeeded in raising a few
plants from them.
The same anomaly occurs in other plants. The common wall-flower
(_Cheiranthus Cheiri_) and the houseleek (_Sempervivum tectorum_) are
the best known instances. Both have repeatedly been described by various
investigators. In compiling the literature of this subject it is very
interesting to observe the two contrasting views respecting the nature
of this anomaly. Some writers, and among them Masters in his "Vegetable
Teratology" consider the deviations to be merely accidental. According
to them some species are more subject to this anomaly than others, and
the houseleek is said to be very prone to this change. Goeppert,
Hofmeister and others occasionally found the pistilloid poppies in
fields or gardens, and sowed their seeds in order to ascertain whether
the accidental peculiarity was inheritable or not. On the other hand De
Candolle in his "Prodromus" mentions the pistilloid wall-flowers as a
distinct [371] variety, under the name of _Cheiranthus Cheiri
gynantherus_, and the analogous form of the opium-poppy is not at all an
accidental anomaly, but an old true horticultural variety, which can be
bought everywhere under the names of _Papaver somniferum monstruosum_ or
_polycephalum_. Since it is an annual plant, only the seeds are for
sale, and this at once gives a sufficient proof of its heredity. In all
cases, where it was met with accidentally by botanists, it is to be
assumed that stray seeds had been casually mixed with those of other
varieties, or that the habit had been transmitted by a spontaneous
cross.
Wherever opportunity led to experiments on heredity, distinct races were
found to be in possession of this quality, while others were not. It is
of no use to cultivate large numbers of wall-flowers in the hope of one
day seeing the anomaly arise; the only means is to secure the strain
from those who have got it. With poppies the various varieties are so
often intercrossed by bees, that the appearance of an accidental change
may sometimes be produced, and in the houseleek the pistilloid warily
seems to be the ordinary one, the normal strain being very rare or
perhaps wholly wanting.
Our three illustrative examples are good and permanent races, producing
their peculiar qualities [372] regularly and abundantly. In this respect
they are however very variable and dependent on external circumstances.
Such a regularity is not met with in other instances. Often
pedigree-experiments lead to poor races, betraying their tendency to
deviate only from time to time and in rare cases. Such instances
constitute what we have called in a former lecture, "half races," and
their occurrence indicates that the casual observation of an anomaly is
not in itself adequate to give an opinion as to the chance of repetition
in sowing experiments. A large number of species seem to belong to this
case, and their names may be found in the above mentioned work by
Masters and elsewhere. But no effort has yet been made to separate
thoroughly the pistilloid half-races from the corresponding
ever-sporting varieties. Some plants are recorded as being more liable
to this peculiarity than others.
Stamens are sometimes replaced by open carpels with naked ovules arising
from their edges and even from their whole inner surfaces. This may be
seen in distinct strains of the cultivated bulbous Begonia, and more
rarely in primroses. Here the apex of the carpellary leaf is sometimes
drawn out into a long style, terminated by a flattened spatulate stigma.
The pistillody of the stamens is frequently [373] combined with another
deviation in the poppies. This is the growing together of some of the
altered stamens so as to constitute smaller or larger connate groups.
Often two are united, sometimes three, four or more. Flowers with
numerous altered stamens are seldom wholly free from this most
undesirable secondary anomaly. I call it undesirable with respect to
experiments on the variability of the character. For it may easily be
seen that while it is feasible to count the stamens even when converted
into pistils, it is not possible when groups of them are more or less
intimately united into single bodies. This combination makes all
enumeration difficult and inaccurate and often wholly unreliable. In
such cases the observation is limited to a computation of the degree of
the change, rather than to a strict numerical inquiry. Happily the
responses to the experimental influences are so marked and distinct that
even this method of describing them has proved to be wholly sufficient.
In extreme instances I have seen all the changed stamens of a flower of
the opium-poppy united into a single body, so as to form a close sheath
all around the central ovary. Lesser sheaths, surrounding one-half or
one-third of the capsule are of course less rarely met with. Leaving
this description of the outer appearance [374] of our anomaly, we may
now consider it from the double point of view of inheritance and
variability.
The fact of inheritance is shown by the experience of many authors, and
by the circumstance already quoted, that the variety has been propagated
from seed for more than half a century, and may be obtained from various
seed merchants. In respect to the variability, the variety belongs to
the ever-sporting group, constituting a type which is more closely
related to the "five-leaved" clover than to the striped flowers or even
the double stocks.
It fluctuates around an average type with half filled crowns, going as
far as possible in both directions, but never transgressing either
limit. It is even doubtful whether the presumable limits are, under
ordinary circumstances, ever reached. Obviously one extreme would be the
conversion of all the stamens, and the other the absolute deficiency of
any marked tendency to such a change. Both may occur, and will probably
be met with from time to time. But they must be extremely rare, since in
my own extensive experiments, which were strictly controlled, I never
was able to find a single instance of either of them. Some of the outer
stamens have always remained unchanged, yielding enough pollen for the
artificial pollination of [375] the central ovary, and on the other hand
some rudiments of hardened filaments were always left, even if they were
reduced to small protuberances on the thalamus of the flower. Between
these extremes all grades occur. From single, partially or wholly
changed stamens upwards to 150 and over, all steps may be seen. It is a
true fluctuating variability. There is an average of between 50 and 100,
constituting a nearly filled crown around the central capsule. Around
this average the smaller deviations are most numerous and the larger
ones more rare. The inspection of any bed of the variety suffices to
show that, taken broadly, the ordinary laws of fluctuating variability
are applicable. No counting of the single individuals is required to
dispel all doubts on this point.
Moreover all intermediate steps respecting the conversion of the single
stamens may nearly always be seen. Rarely all are changed into normal
secondary ovaries with a stigma and with a cavity filled with ovules.
Often the stigma is incomplete or even almost wanting, in other
instances the ovules are lacking or the cavity itself is only partially
developed. Not rarely some stamens are reduced and converted into thin
hard stalks, without any appearance of an ovary at their tip. But then
the demarcation [376] between them and the thalamus fails, so that they
cannot be thrown off when the flower fades away, but remain as small
stumps around the base of the more fully converted filaments. This fact
would frequently render the enumeration of the altered organs quite
unreliable.
For these reasons I have chosen a group of arbitrary stages in order to
express the degree of deviation for a given lot of plants. The limits
were chosen so as to be sufficiently trustworthy and easy to ascertain.
In each group the members could be counted, and a series of figures was
reached by this means which allowed of a further comparison of the
competing sets of plants.
It should be stated that in such experiments and especially in the case
of such a showy criterion as the pistilloid heads afford after the time
of flowering is over, the inspection of the controlling beds at once
indicates the result of the experiment. Even a hasty survey is in most
cases sufficient to get a definite conclusion. Where this is not the
case, the counting of the individuals of the various groups often does
not add to the evidence, and the result remains uncertain. On the other
hand, the impression made by the groups of plants on the experimenter
and on his casual visitors, cannot well be conveyed to the readers of
his account by [377] other means than by figures. For this reason the
result of the experiments is expressed in this way.
I made six groups. The first includes the cases where the whole circle
is reduced to small rudiments. The second shows 1-10 secondary capsules.
The two following constitute half a crown around the central fruit, the
third going up to this limit, the fourth going from this limit to a
nearly filled circle. Wholly filled circles of secondary capsules
without gaps give the two last degrees, the fifth requiring only
continuity of the circle, the sixth displaying a large and bright crown
all around the central head. The fifth group ordinarily includes from
90-100 altered stamens, while the sixth has from 100-150 of these
deviating parts.
In ordinary cultures the third and fourth group, with their interrupted
crowns, predominate. Large crowns are rare and flowers which at first
sight seem to be wholly normal, occur only under circumstances
definitely known to be unfavorable to growth, and to the development of
the anomaly.
Having reached by this means a very simple and easy method of stating
the facts shown by equal lots under contrasting influences, we will now
make use of it to inquire into the relation [378] of this exceptionally
high degree of variability to the inner and outer conditions of life.
As a rule, all experiments show the existence of such a relation.
Unfavorable conditions reduce the numbers of altered stamens, favorable
circumstances raise it to its highest point. This holds true for lots
including hundreds of specimens, but also for the sundry heads of one
bed, and often for one single plant.
We may compare the terminal flower with those of the lateral branches on
a plant, and when no special influences disturb the experiment, the
terminal head ordinarily bears the richest crown. If the first has more
than 100 metamorphosed parts, the latter have often less than 50 on the
same plant. In poor soil, terminal heads are often reduced to 10-20
monstrous organs, and in such cases I found the lateral flowers of the
same plants ordinarily with less than 10 altered stamens. In some cases
I allowed the branches of the third and fourth degree, in other words,
the side twigs of the first branches of my selected plants to grow out
and produce flowers in the fall. They were ordinarily weak, sometimes
very small, having only 5-9 stigmas on their central fruit. Secondary
capsules were not seen on such flowers, even when the experiment was
repeated on a [379] somewhat larger scale and during a series of years.
Among the same lot of plants individual differences almost always occur.
They are partly due to inequalities already existing in the seeds, and
partly to the diversity of the various parts of the same bed. Some of
the plants become stout and have large terminal heads. Others remain
very weak, with a slender stem, small leaves and undersized flowers. The
height and thickness of the stem, the growth of the foliage and of the
axillary buds are the most obvious measures of the individual strength
of the plant. The development of the terminal flower and the size of its
ovary manifestly depends largely on this individual strength, as may be
seen at once by the inspection of any bed of opium-poppies. Now this
size of the head can easily be measured, either by its height or
circumference, or by its weight. Moreover we can arrange them into a
series according to their size. If we do this with the polycephalous
variety, the relation between individual strength and degree of
metamorphosis at once becomes manifest. The largest heads have the
brightest crowns, and the number of supernumerary carpels diminishes in
nearly exact proportion to the size of the fruits. Fruits with less than
50 altered stamens weighed on an average 5 grams, [380] those with
50-100 such organs 7 grams and those with a bright crown 10 grams, the
appendices being removed before the weighing. Corresponding results have
been reached by the comparison of the height of the capsules with their
abnormal surroundings. The degree of development of the monstrosity is
shown by this observation to be directly dependent on, and in a sense
proportionate to the individual strength of the plant.
The differences between the specimens grown from a single lot of seeds,
for instance from the seeds of one self-fertilized capsule are, as I
have said, partly due to the divergences which are always present in a
bed, even if the utmost care has been taken to make it as uniform as
possible. These local differences are ordinarily underrated and
overlooked, and it is often considered to be sufficient to cultivate
small lots of plants under apparently similar conditions on neighboring
beds, to be justified in imputing all the observed deviations of the
plants to hereditary inequalities. This of course is true for large
lots, whenever the averages only are compared. In smaller experiments
the external conditions of the single individuals should always be
considered carefully. Lots of one or two square meters suffice for such
comparisons, but smaller lots are always subject to chances and [381]
possibilities, which should never be left out of consideration.
Therefore I will now point out some circumstances, which are ordinarily
different on various parts of one and the same bed.
In the first place comes the inequality of the seeds themselves. Some of
them will germinate earlier and others later. Those that display their
cotyledons on a sunny day will be able to begin at once with the
production of organic food. Others appear in bad weather, and will thus
be retarded in their development. These effects are of a cumulative
nature as the young plants must profit by every hour of sunshine,
according to the size of the cotyledons. Any inequality between two
young seedlings is apt to be increased by this cumulative effect.
The same holds good for the soil of the bed. It is simply impossible to
mix the manure so equally that all individuals receive the same amount
of it from the very beginning. I am in the habit of using manures in a
dry and pulverized condition, of giving definite quantities to each
square meter, and of taking the utmost care to get equal distribution
and mixture with the soil, always being present myself during this most
important operation. Nevertheless it is impossible to make the
nourishment exactly equal for all the plants of even a small bed.
[382] Any inequality from this cause will increase the difference in the
size of the young leaves, augment the inequality of their production of
organic matter and for this reason go on in an ever increasing rate.
Rain and spraying, or on the other hand dryness of the soil, have still
greater consequences. The slightest unevenness of the surface will cause
some spots to dry rapidly and others to retain moisture during hours and
even sometimes during days.
Seeds, germinating in such little moist depressions grow regularly and
rapidly, while those on the dryer elevations may be retarded for hours
and days, before fully unfurling their seed-leaves. After heavy rains
these differences may be observed to increase continually, and in some
instances I found that plants were produced only on the wet spots, while
the dry places remained perfectly bare. From this the wet spots seem to
be the most favorable, but on the other hand, seeds may come to
germinate there too numerously and so closely that the young plants will
be crowded together and find neither space nor light enough, for a free
and perfect development. The advantage may change to disadvantage in
this way unless the superfluous individuals are weeded out in due time.
[383] From all these and other reasons some plants will be favored by
the external conditions from the beginning, while others will be
retarded, and the effects will gradually increase until at last they
become sufficient to account for a considerable amount of individual
variability. There is no doubt that the difference in the strength of
the plant and in the size of the capsules, going from 5-10 grams for a
single fruit, are for the most part due to these unavoidable
circumstances. I have tried all conceivable means to find remedies for
these difficulties, but only by sowing my seeds in pans in a glass-house
have I been able to reach more constant and equal conditions. But
unfortunately such a method requires the planting out of the young
seedlings in the beginning of the summer, and this operation is not
without danger for opium-poppies, and especially not without important
influence on the monstrosity of the pistilloid variety. Consequently my
sowings of this plant have nearly always been made in the beds.
In order to show how great the influence of all these little things may
become, we only have to make two sowings on neighboring beds and under
conditions which have carefully been made as equal as possible. If we
use for these controlling experiments seeds from one and the same
capsule, it will soon become evident that [384] no exact similarity
between the two lots may be expected. Such differences as may be seen in
these cases are therefore never to be considered of value when comparing
two lots of seeds of different origin, or under varying conditions. No
amount of accuracy in the estimation of the results of a trial, or in
the counting out of the several degrees of the anomaly, is adequate to
overcome the inaccuracy resulting from these differences.
It is certainly of great importance to have a correct conception in
regard to the influence of the surrounding conditions on the growth of a
plant and on the development of the attribute we are to deal with. No
less important is the question of the sensibility of the plants to these
factors. Obviously this sensibility must not be expected to remain the
same during the entire life-period, and periods of stronger and of
weaker responses may be discerned.
In the first place it is evident that external or inner influences are
able to change the direction of the development of an organ only so long
as this development is not yet fully finished. In the young flower-bud
of the pistilloid poppy there must evidently be some moment in which it
is definitely decided whether the young stamens will grow out normally
or become metamorphosed into secondary pistils. From this [385] moment
no further change of external conditions is able to produce a
corresponding change in the degree of the anomaly. The individual
strength of the whole plant may still be affected in a more or less
manifest degree, but the number of converted stamens of the flower has
been definitely fixed. The sensitive period has terminated.
In order to determine the exact moment of this termination of the period
of sensibility, I have followed the development of the flower buds
during the first weeks of the life of the young plants. The terminal
flower may already be seen in young plants only seven weeks old, with a
stem not exceeding 5-6 cm. in height and a flower-bud with a diameter of
nearly 1 mm., in which the stamens and secondary pistils are already
discernible, but still in the condition of small rounded protuberances
on the thalamus. Though it is not possible at that time to observe any
difference between the future normal and converted stamens, it does not
seem doubtful that the development is so far advanced, that in the inner
tissues the decision has already definitely been taken. In the next few
days this decision rapidly becomes visible, and the different parts of
the normal stamens and the metamorphosed carpels soon become apparent.
From this observation it [386] can be inferred that the sensitive period
of the anomaly is limited for the terminal flower-head, to the first few
weeks of the life of the young plants. The secondary heads manifestly
leave this period at a somewhat later stage.
In order to prove the accuracy of this conclusion I have tried to injure
the anomalies after the expiration of the first six or seven weeks. I
deprived them of their leaves, and damaged them in different ways. I
succeeded in making them very weak and slender, without being able to
diminish the number of the supernumerary carpels. The proportionality of
the size of the central fruit and the development of the surrounding
crown can often be modified or even destroyed by this means, and the
apparent exceptions from this rule, which are often observed, may find
their explanation in this way.
In the second place I have tried to change the development of the
anomaly during the period of sensibility, and even in the last part of
it. This experiment succeeded fully when carried out within the fifth or
sixth week after the beginning of the germination. As means of injury I
transplanted the young plants. To this end I sowed my seeds in pans in
unmanured soil, planted them out in little pots with richly prepared
earth, grew them in these during a few weeks and afterwards transferred
them to the [387] beds, taking care that the pats were removed, but the
balls of earth not broken.
In consequence of this treatment the plants became very large and
strong, with luxuriant foliage and relatively numerous large flowers and
fruits. But almost without exception they were poor in anomalous
stamens, at least so on the terminal heads. On a lot of some 70 plants
more than 50 had less than half a crown of secondary capsules, while
from the same packet of seed the control-plants gave in an equal number
more than half of filled crowns on all plants with the exception of five
weak specimens.
It is curious to compare such artificially injured plants with the
ordinary cultures. Strong stems and heavy fruits, which otherwise are
always indicative of showy crowns, now bear fruits wholly or nearly
destitute of any anomalous change. The commonly prevailing rule seems to
be reversed, showing thereby the possibility of abolishing the
correlation between individual strength and anomaly by an artificial
encroachment upon the normal conditions.
Aside from these considerations the experiments clearly give proof of
the existence of a period of sensibility limited to the first weeks of
the life of the plant for the terminal flower. This knowledge enables us
to explain many apparent [388] parent abnormalities, which may occur in
the experiments.
We now may take a broader view of the period of sensibility. Evidently
the response to external influences will be greater the younger the
organ. Sensibility will gradually diminish, and the phenomena observed
in the last part of this period may be considered as the last remainder
of a reaction which previously must have been much stronger and much
readier, providing that it would be possible to isolate them from, and
contrast them with, the other responses of the same plant.
With the light thus cast upon the question, we may conclude that the
sensitive period commences not only at the beginning of the germination,
but must also be considered to include the life of the seed itself. From
the moment of fertilization and the formation of the young embryo the
development must be subjected to the influence of external agencies
which determine the direction it will take and the degree of development
it will finally be able to acquire. Probably the time of growth of the
embryo and of the ripening of the seed correspond exactly to the period
of highest sensibility. This period is only interrupted during the
resting stage of the seed, to be repeated in germination. Afterwards the
sensibility [389] slowly and gradually decreases, to end with the
definite decision of all further growth sometime before the outer form
of the organ becomes visible under the microscope. The last period of
life includes only an expansion of the tissues, which may still have
some influence on their final size, but not on their form. This has been
definitely arrested before the end of the sensitive period, and
ordinarily before the commencement of that rapid development, which is
usually designated by the name of growth, as contrasted with evolution.
Within the seed the evolution of the young plant manifestly depends upon
the qualities and life-conditions of the parent-plant. The stronger this
is, and the more favorable circumstances it is placed under, the more
food will be available for the seed, and the healthier will be the
development of the embryo. Only well-nourished plants give
well-nourished seeds, and the qualities of each plant are for this
reason at least, partly dependent on the properties of its parents and
even of its grandparents.
From these considerations the inference is forced upon us that the
apparently hereditary differences, which are observed to exist among the
seeds of a species or a variety and even of a single strain or a single
parent-plant, may for a large part, and perhaps wholly, be the result
[390] of the life-conditions of their parents and grandparents. Within
the race all ssvariability would in this way be reduced to the effects
of external circumstances. Among these nourishment is no doubt the most
momentous, and this to such a degree that older writers designated the
external conditions by the term nourishment. According to Knight
nutrition reigns supreme in the whole realm of variability, the kind of
food and the method of nourishment coming into consideration only in a
secondary way. The amount of useful nutrition is the all-important
factor.
If this is so, and if nutrition decides the degree of deviation of any
given character, the widest deviating individuals are the best nourished
ones. The best nourished not only during the period of sensibility of
the attribute under consideration, but also in the broadest sense of the
word.
This discussion casts a curious light upon the whole question of
selection. Not of course upon the choice of elementary species or
varieties out of the original motley assembly which nature and old
cultures offer us, but upon the selection of the best individuals for
isolation and for the improvement of the race. These are, according to
my views, only the best nourished ones. Their external conditions have
been the [391] most favorable, not only from the beginning of their own
life in the field, but also during their embryonic stages, and even
during the preparation of these latter in the life of their parents and
perhaps even their grandparents. Selection then, would only be the
choice of the best nourished individuals.
In connection with the foregoing arguments I have tried to separate the
choicest of the poppies with the largest crown of pistilloid stamens,
from the most vigorous individuals. As we have already seen, these two
attributes are as a rule proportional to one another. Exceptions occur,
but they may be explained by some later changes in the external
circumstances, as I have also pointed out. As a rule, these exceptions
are large fruits with comparatively too few converted stamens; they are
exactly the contrary from what is required for a selection. Or plants,
which from the beginning were robust, may have become crowded together
by further growth, and for these reasons become weaker than their
congeners, though retaining the full development of the staminodal
crown, which was fixed during the sensitive period and before the
crowding. I have searched my beds yearly for several years in vain to
find individuals which might recommend themselves for selection without
having the stamp of permanent, [392] or at least temporarily better,
nourishment. No starting-point for such an independent selection has
ever been met with.
Summing up the consequences of this somewhat extended discussion, we may
state it as a rule that a general proportion between the individual
strength and the degree of development of the anomaly exists. And from
this point of view it is easy to see that all external causes which are
known to affect the one, must be expected to influence the other also.
It will therefore hardly be necessary to give a full description of all
my experiments on the relations of the monstrosity to external
conditions. A hasty survey will suffice.
This survey is not only intended to convey an idea of the relations of
pistilloid poppies to their environment, but may serve as an example of
the principle involved. According to my experience with a large range of
other anomalies, the same rule prevails everywhere. And this rule is so
simple that exact knowledge of one instance may be considered as
sufficient to enable us to calculate from analogy what is to be expected
from a given treatment of any other anomaly. Our appreciation of
observed facts and the conditions to be chosen for intended cultures are
largely dependent on such calculations. What I am now going to describe
[393] is to be considered therefore as an experimental basis for such
expectations.
First of all comes the question how many individuals are to be grown in
a given place. When sowing plants for experimental purposes it is always
best to sow in rows, and to give as few seeds to each row as possible,
so as to insure all necessary space to the young plants. On the other
hand the seeds do not all germinate, and after sowing too thinly, gaps
may appear in the rows. This would cause not only a loss of space, but
an inequality between the plants in later life, as those nearest the
gaps would have more space and more light, and a larger area for their
roots than those growing in the unbroken rows. Hence the necessity of
using large quantities of seed and of weeding out a majority of young
plants on the spots where the greatest numbers germinate.
Crowded cultures as a rule, will give weak plants with thin stems,
mostly unbranched and bearing only small capsules. According to the
rule, these will produce imperfect crowns of secondary pistils. The
result of any culture will thus be dependent to a high degree on the
number of individuals per square meter. I have sown two similar and
neighboring beds with the thoroughly mixed seeds of parent-plants of the
same strain and culture, using as much [394] as 2.5 cu. cm. per square
meter. On one of the beds I left all the germinating plants untouched
and nearly 500 of them flowered, but among them 360 were almost without
pistillody, and only 10 had full crowns. In the other bed I weeded away
more than half of the young plants, leaving only some 150 individuals
and got 32 with a full crown, nearly 100 with half crowns and only 25
apparently without monstrosity.
These figures are very striking. From the same quantity of seed, in
equal spaces, by similar exposure and treatment I got 10 fully developed
instances in one and 32 in the other case. The weeding out of
supernumerary individuals had not only increased the percentage of
bright crowns, but also their absolute number per square meter. So the
greatest number of anomalies upon a given space may be obtained by
taking care that not too many plants are grown upon it: any increase of
the number beyond a certain limit will diminish the probability of
obtaining these structures. The most successful cultures may be made
after the maximum number of individuals per unit of area has been
determined. A control-experiment was made under the same conditions and
with the same seed, but allowing much less for the same space. I sowed
only 1 cu. cm. on my bed of 2 square meters, and thereby avoided [395]
nearly all weeding out. I got 120 plants, and among them 30 with full
crowns of converted stamens, practically the same number as after the
weeding out in the first experiment. This shows that smaller quantities
of seed give an equal chance for a greater number of large crowns, and
should therefore always be preferred, as it saves both seed and labor.
Weeding out is a somewhat dangerous operation in a comparative trial.
Any one who has done it often, knows that there is a strong propensity
to root out the weaker plants and to spare the stronger ones. Obviously
this is the best way for ordinary purposes, but for comparisons
evidently one should not discriminate. This rule is very difficult in
practice, and for this reason one should never sow more than is
absolutely required to meet all requirements.
Our second point is the manuring of the soil. This is always of the
highest importance, both for normal and for anomalous attributes. The
conversion of the stamens into pistils is in a large measure dependent
upon the conditions of the soil. I made a trial with some 800 flowering
plants, using one sample of seed, but sowing one-third on richly manured
soil, one-third on an unprepared bed of my garden, and one-third on
nearly pure sand. In all other respects the three groups were treated in
the same way. Of [396] the manured plants one-half gave full crowns, of
the non-manured only one-fifth, and on the sandy soil a still smaller
proportion. Other trials led to the same results. I have often made use
of steamed and ground horn, which is a manure very rich in nitrogenous
substances. One-eighth of a kilo per square meter is an ample amount.
And its effect was to increase the number of full crowns to an
exceptional degree.
In the controlling trial and under ordinary circumstances this figure
reached some 50%, but with ground horn it came up as high as 90%. We may
state this result by the very striking assertion that the number of
large crowns in a given culture may be nearly doubled by rich manure.
All other external conditions act in a similar manner. The best
treatment is required to attain the best result. A sunny exposure is one
of the most essential requisites, and in some attempts to cultivate my
poppies in the shade, I found the pistillody strongly reduced, not a
single full crown being found in the whole lot. Often the weather may be
hurtful, especially during the earlier stages of the plants. I protected
my beds during several trials by covering them with glass for a few
weeks, until the young plants reached the glass covering. I got a normal
number of full crowns, some 55%, at a time [397] when the weather was so
bad as to reduce the number in the control experiments to 10%.
It would be quite superfluous to give more details or to describe
additional experiments. Suffice to say, that the results all point in
the same direction, and that pistillody of the poppies always clearly
responds to the treatment, especially to external conditions during the
first few weeks, that is, during the period of sensitiveness. The
healthier and the stronger the plants the more fully they will develop
their anomaly.
In conclusion something is to be said about the choice of the seed.
Obviously it is possible to compare seeds of different origin by sowing
and treating them in the same way, giving attention to all the points
above mentioned. In doing so the first question will be, whether there
is a difference between the seeds of strong plants with a bright crown
around the head and those of weaker individuals with lesser development
of the anomaly. It is evident that such a difference must be expected,
since the nutrition of the seed takes place during the period of the
greatest sensitiveness.
But the experiments will show whether this effect holds good against the
influences which tend to change the direction of the development of the
anomaly during the time of germination. [398] The result of my attempt
has shown that the choice of the seeds has a manifest influence upon the
ultimate development of the monstrosity, but that this influence is not
strong enough to overwhelm all other factors.
The choice of the fullest or smallest crowns may be repeated during
succeeding generations, and each time compared with a culture under
average conditions. By this means we come to true selection-experiments,
and these result in a notable and rapid change of the whole strain. By
selecting the brightest crowns I have come up in three years from 40 to
90 and ultimately to 120 converted stamens in the best flower of my
culture, and in selecting the smallest crowns I was able in three years
to exclude nearly all good crowns, and to make cultures in which heads
with less than half-filled crowns constituted the majority. But such
selected strains always remain very sensitive to treatment, and by
changing the conditions the effect may be wholly lost in a single year,
or even turned in the contrary direction. In other words, the anomaly is
more dependent on external conditions during the germinating period than
on the choice of the seeds, providing these belong to the pistilloid
variety and have not deteriorated by some crossing with other sorts.
At the beginning of this lecture I stated that [399] no selection is
adequate to produce either a pure strain of brightly crowned
flower-heads without atavism, or to conduce to an absolute and permanent
loss of the anomaly. During a series of years I have tested my plants in
both directions, but without the least effect. Limits are soon reached
on both sides, and to transgress these seems quite impossible.
Taking these limits as the marks of the variety, and considering all
fluctuations between them as responses to external influences working
during the life of the individual or governing the ripening of the
seeds, we get a clear picture of a permanent ever-sporting type. The
limits are absolutely permanent during the whole existence of this
already old variety. They never change. But they include so wide a range
of variability, that the extremes may be said to sport into one another,
so much the more so as one of the extremes is to be considered
morphologically as the type of the variation, while the other extreme
can hardly be distinguished from the normal form of the species.
[400]
LECTURE XIV
MONSTROSITIES
I have previously dealt with the question of the hereditary tendencies
that cause monstrosities. These tendencies are not always identical for
the same anomaly. Two different types may generally, be distinguished.
One of them constitutes a poor variety, the other a rich one. But this
latter is abundant and the first one is poor in instances of exactly the
same conformation. Therefore the difference only lies in the frequency
of the anomaly, and not in its visible features. In discovering an
instance of any anomaly it is therefore impossible to tell whether it
belongs to a poor or to a rich race. This important question can only be
answered by direct sowing-experiments to determine the degree of
heredity.
Monstrosities are often considered as accidents, and rightfully so, at
least as long as they are considered from a morphological point of view.
Physiology of course excludes all accidentality. And in our present ease
it shows [401] that some internal hereditary quality is present, though
often latent, and that the observed anomalies are to be regarded as
responses of this innate tendency to external conditions. Our two types
differ in the frequency of these responses. Rare in the poor race, they
are numerous in the rich variety. The external conditions being the same
for both, the hereditary factor must be different. The tendency is weak
in the one and strong in the other. In both cases, according to my
experience, it may be weakened or strengthened by selection and by
treatment. Often to a very remarkable degree, but not so far as to
transgress the limits between the two races. Such transgression may
apparently be met with from time to time, but then the next generation
generally shows the fallacy of the conclusion, as it returns more or
less directly to the type from which the strain had been derived.
Monstrosities should always be studied by physiologists from this point
of view. Poor and rich strains of the same anomaly seem at first sight
to be so nearly allied that it might be thought to be very easy to
change the one into the other. Nevertheless such changes are not on
record, and although I have made several attempts in this line, I never
succeeded in passing the limit. I am quite convinced that sometime [402]
a method will be discovered of arbitrarily producing such conversions,
and perhaps the easiest way to attain artificial mutations may lie
concealed here. But as yet not the slightest indication of this
possibility is to be found, save the fallacious conclusions drawn from
too superficial observations.
Unfortunately the poor strains are not very interesting. Their chance of
producing beautiful instances of the anomaly for which they are
cultivated is too small. Exceptions to this rule are only afforded by
those curious and rare anomalies, which command general attention, and
of which, therefore, instances are always welcome. In such cases they
are searched for with perseverance, and the fact of their rarity
impresses itself strongly on our mind.
Twisted stems are selected as a first example. This monstrosity, called
_biastrepsis_, consists of strongly marked torsions as are seen in many
species with decussate leaves, though as a rule it is very rare. Two
instances are the most generally known, those of the wild valerian
(_Valeriana officinalis_) and those of cultivated and wild sorts of
teasels (_Dipsacus fullonum_, _D. sylvestris_, and others). Both of
these I have cultivated during upwards of fifteen years, but with
contradictory results. The valerian is a perennial herb, multiplying
itself yearly by [403] slender rootstocks or runners producing at their
tips new rosettes of leaves and in the center of these the flowering
stem. My original plant has since been propagated in this manner, and
during several years I preserved large beds with hundreds of stems, in
others I was compelled to keep my culture within more restricted limits.
This plant has produced twisted stems of the curious shape, with a
nearly straight flag of leaves on one side, described by De Candolle and
other observers, nearly every year. But only one or two instances of
abnormal stems occurred in each year, and no treatment has been found
that proved adequate to increase this number in any appreciable manner.
I have sown the seeds of this plant repeatedly, either from normal or
from twisted stems, but without better results. It was highly desirable
to be able to offer instances of this rare and interesting peculiarity
to other universities and museums, but no improvement of the race could
be reached and I have been constrained to give it up. My twisted
valerian is a poor race, and hardly anything can be done with it.
Perhaps, in other countries the corresponding rich race may be hidden
somewhere, but I have never had the good fortune of finding it.
This good fortune however, I did have with the wild teasel or _Dipsacus
sylvestris_. [404] Stems of this and of allied species are often met
with and have been described by several writers, but they were always
considered as accidents and nobody had ever tried to cultivate them. In
the summer of 1885 I saw among a lot of normal wild teasels, two nicely
twisted stems in the botanical garden of Amsterdam. I at once proposed
to ascertain whether they would yield a hereditary race and had all the
normal individuals thrown away before the flowering time. My two plants
flowered in this isolated condition and were richly pollinated by
insects. Of course, at that time, I knew nothing of the dependency of
monstrosities on external conditions, and made the mistake of sowing the
seeds and cultivating the next generation in too great numbers on a
small space. But nevertheless the anomaly was repeated, and the aberrant
individuals were once more isolated before flowering. The third
generation repeated the second, but produced sixty twisted stems on some
1,600 individuals. The result was very striking and quite sufficient for
all further researches, but the normal condition of the race was not
reached. This was the case only after I had discovered the bad effects
of growing too many plants in a limited space. In the fourth generation
I restricted my whole culture to about 100 individuals, and by this
simple [405] means at once got up to 34% of twisted stems. This
proportion has since remained practically the same. I have selected and
isolated my plants during five succeeding generations, but without any
further result, the percentage of twisted stems fluctuating between 30
and about 45 according to the size of the cultures and the favorableness
or unfavorableness of the weather.
It is very interesting to note that all depends on the question whether
one has the good fortune of finding a rich race or not, as this
pedigree-culture shows. Afterwards everything depends on treatment and
very little on selection. As soon as the treatment becomes adequate, the
full strength of the race at once displays itself, but afterwards no
selection is able to improve it to any appreciable amount. Of course, in
the long run, the responses will be the same as those of the pistilloid
poppies on the average, and some influence of selection will show itself
on closer scrutiny.
Compared with the polycephalous poppies my race of twisted teasels is
much richer in atavists. They are never absent, and always constitute a
large part of each generation and each bed, comprising somewhat more
than half of the individuals. Intermediate stages between them and the
wholly twisted stems are not wanting, [406] and a whole series of steps
may easily be observed from sufficiently large cultures. But they are
always relatively rare, and any lot of plants conveys the idea of a
dimorphous race, the small twisted stems contrasting strongly with the
tall straight ones.
A sharper contrast between good representatives of a race and their
atavists is perhaps to be seen in no other instance. All the details
contribute to the differentiation in appearance. The whole stature of
the plants is affected by the varietal mark. The atavists are not, as in
the case of the poppies, obviously allied with the type by a full range
of intermediate steps, but quite distant from it by their rarity. There
seems to be a gap in the same way as between the striped flowers of the
snapdragon and their uniform red atavists, while with the poppies the
atavists may be viewed as being only the extremes of a series of
variations fluctuating around some average type.
From this reason it is as interesting to appreciate the hereditary
position of the atavists of twisted varieties as it was for the
red-flowered descendants of the striped flowers. In order to ascertain
this relation it is only necessary to isolate some of them during the
blooming-period. I made this experiment in the summer of 1900 with the
eighth generation of my race, and contrived [407] to isolate three
groups of plants by the use of parchment bags, covering them
alternately, so the flowers of only one group were accessible to
insects, at a time. I made three groups, because the atavists show two
different types. Some specimens have decussate stems, others bear all
their leaves in whorls of three, but in respect to the hereditary
tendency of the twisting character this difference does not seem to be
of any importance.
In this way I got three lots of seeds and sowed enough of them to have
three groups of plants each containing about 150-200 well developed
stems. Among these I counted the twisted individuals, and found nearly
the same numbers for all three. The twisted parents gave as many as 41%
twisted children, but the decussate atavists gave even somewhat more,
viz., 44%, while the ternate specimens gave 37%. Obviously the
divergences between these figures are too slight to be dwelt upon, but
the fact that the atavists are as true or nearly as true inheritors of
the twisted race as the best selected individuals is clearly proved by
this experience.
It is evident that here we have a double race, including two types,
which may be combined in different degrees. These combinations determine
a wide range of changes in the stature of the plants, and it seems
hardly right to use the [408] same term for such changes as for common
variations. It is more a contention of opposite characters than a true
phenomenon of simple variability. Or perhaps we might say that it is the
effect of the cooperation of a very variable mark, the twisting, with a
scarcely varying attribute of the normal structure of the stem. Between
the two types an endless diversity prevails, but outwardly there are
limits which are never transgressed. The double race is as permanent,
and in this sense as constant, as any ordinary simple variety, both in
external form, and in its intimate hereditary qualities.
I have succeeded in discovering some other rich races of twisted plants.
One of them is the Sweet William (_Dianthus barbatus_), which yielded,
after isolation, in the second generation, 25% of individuals with
twisted stems, and as each individual produces often 10 and more stems,
I had a harvest of more than half a thousand of instances of this
curious, and ordinarily very rare anomaly. My other race is a twisted
variety of _Viscaria oculata_, which is still in cultivation, as it has
the very consistent quality of being an annual. It yielded last summer
(1903) as high a percentage as 65 of twisted individuals, many of them
repeating the monstrosity on several branches. After some occasional
observations _Gypsophila paniculata_ [409] seems to promise similar
results. On the other hand I have sowed in vain the seeds of twisted
specimens of the soapwort and the cleavewort (_Saponaria officinalis_
and _Galium Aparine_). These and some others seem to belong to the same
group as the valerian and to constitute only poor or so-called
half-races.
Next to the torsions come the fasciated stems. This is one of the most
common of all malformations, and consists, in its ordinary form, of a
flat ribbon-like expansion of the stems or branches. Below they are
cylindrical, but they gradually lose this form and assume a flattened
condition. Sometimes the rate of growth is unequal on different portions
or on the opposite sides of the ribbon, and curvatures are produced and
these often give to the fasciation a form that might be compared with a
shepherd's crook. It is a common thing for fasciated branches and stems
to divide at the summit into a number of subdivisions, and ordinarily
this splitting occurs in the lower part, sometimes dividing the entire
fasciated portion. In biennial species the rosette of the root-leaves of
the first year may become changed by the monstrosity, the heart
stretching in a transverse direction so as to become linear. In the next
year this line becomes the base from which the stem grows. In such cases
the fasciated stems [410] are broadened and flattened from the very
beginning, and often retain the incipient breadth throughout their
further development. Species of primroses (_Primula japonica_ and
others), of buttercups (_Ranunculus bulbosus_), the rough hawksbeard
(_Crepis biennis_), the Aster _Tripolium_, and many others could be
given as instances.
Some of these are so rare as to be considered as poor races, and in
cultural trials do not produce the anomaly except in a very few
instances. Heads of rye are found in a cleft condition from time to
time, single at their base and double at the top, but this anomaly is
only exceptionally repeated from seed. Flattened stems of _Rubia
tinctorum_ are not unfrequently met with on the fields, but they seem to
have as little hereditary tendency as the split rye (_Secale Cereale_).
Many other instances could be given. Both in the native localities and
in pedigree-cultures such ribboned stems are only seen from time to
time, in successive years, in annual and biennial as well as in
perennial species. The purple pedicularis (_Pedicularis palustris_) in
the wild state, and the sunflower among cultivated plants, may be cited
instead of giving a long list of analogous instances.
On the other hand rich races of flattened stems are not entirely
lacking. They easily betray [411] themselves by the frequency of the
anomaly, and therefore may be found, and tried in the garden. Under
adequate cultivation they are here as rich in aberrant individuals as
the twisted races quoted above, producing in good years from 30-40% and
often more instances. I have cultivated such rich races of the dandelion
(_Taraxacum officinale_), of _Thrincia hirta_, of the dame's violet
(_Hesperis matronalis_), of the hawkweed (_Picris hieracioides_), of the
rough hawksbeard (_Crepis biennis_), and others.
Respecting the hereditary tendencies these rich varieties with flattened
stems may be put in the same category with the twisted races. Two points
however, seem to be of especial interest and to deserve a separate
treatment.
The common cockscomb or _Celosia cristata_, one of the oldest and most
widely cultivated fasciated varieties may be used to illustrate the
first point. In beds it is often to be seen in quite uniform lots of
large and beautiful crests, but this uniformity is only secured by
careful culture and selection of the best individuals. In experimental
trials such selection must be avoided, and in doing so a wide range of
variability at once shows itself. Tall, branched stems with fan-shaped
tops arise, constituting a series of steps towards complete atavism.
This last [412] however, is not to be reached easily. It often requires
several successive generations grown from seed collected from the most
atavistic specimens. And even such selected strains are always reverting
to the crested type. There is no transgression, no springing over into a
purely atavistic form, such as may be supposed to have once been the
ancestor of the present cockscomb. The variety includes crests and
atavists, and may be perpetuated from both. Obviously every gardener
would select the seeds of the brightest crests, but with care the full
crests may be recovered, even from the worst reversionists in two or
three generations. It is a double race of quite the same constitution as
the twisted teasels.
My second point is a direct proof of this assertion, but made with a
fasciated variety of a wild species. I took for my experiment the rough
hawksbeard. In the summer of 1895 I isolated some atavists of the fifth
generation of my race, which, by ordinary selection, gave in the average
from 20-40% of fasciated stems. My isolated atavists bore abundant
fruit, and from these I had the next year a set of some 350 plants, out
of which about 20% had broadened and linear rosettes. This proportion
corresponds with the degree of inheritance which is shown in many years
by the largest and strongest [413] fasciated stems. It strengthens our
conclusion as to the innermost constitution of the double races or
ever-sporting varieties.
Twisted stems and fasciations are very striking monstrosities. But they
are not very good for further investigation. They require too much space
and too much care. The calculation of a single percentage requires the
counting of some hundreds of individuals, taking many square meters for
their cultivation, and this, as my best races are biennial, during two
years. For this reason the countings must always be very limited, and
selection is restrained to the most perfect specimens.
Now the question arises, whether this mark is the best upon which to
found selection. This seems to be quite doubtful. In the experiments on
the heredity of the atavists, we have seen that they are, at least
often, in no manner inferior to even the best inheritors of the race.
This suggests the idea that it is not at all certain that the visible
characters of a given individual are a trustworthy measure of its value
as to the transmission of the same character to the offspring. In other
words, we are confronted with the existence of two widely different
groups of characters in estimating the hereditary tendency. One is the
visible quality of the individuals and the other is the direct
observation [414] of the degree in which the attribute is transmitted.
These are by no means parallel, and seem in some sense to be nearly
independent of each other. The fact that the worst atavists may have the
highest percentage of varietal units seems to leave no room for another
explanation.
Developing this line of thought, we gradually arrive at the conclusion
that the visible attribute of a varying individual is perhaps the most
untrustworthy and the most unreliable character for selection, even if
it seems in many cases practically to be the only available one. The
direct determination of the degree of heredity itself is obviously
preferable by far. This degree is expressed by the proportion of its
inheritors among the offspring, and this figure therefore should be
elevated to the highest rank, as a measure of the hereditary qualities.
Henceforward we will designate it by the name of hereditary percentage.
In scientific experiments this figure must be determined for every plant
of a pedigree-culture singly, and the selection should be founded
exclusively or at least mainly on it. It is easily seen that this method
requires large numbers of individuals to be grown and counted. Some two
or three hundred progeny of one plant are needed to give the decisive
figure for this one [415] individual, and selection requires the
comparison of at least fifty or more individuals. This brings the total
amount of specimens to be counted up to some tens of thousands. In
practice, where important interests depend upon the experiments, such
numbers are usually employed and often exceeded, but for the culture of
monstrosities, other methods are to be sought in order to avoid these
difficulties.
The idea suggests itself here that the younger the plants are, when
showing their distinguishing marks, the more of them may be grown on a
small space. Hence the best way is to choose such attributes, as may
already be seen in the young seedlings, in the very first few weeks of
their lives. Fortunately the seed-leaves themselves afford such
distinctive marks, and by this means the plants may be counted in the
pans, requiring no culture at all in the garden. Only the selected
individuals need be grown to ripen their seeds, and the whole selection
may be made in the spring, in the glasshouse. Instead of being very
troublesome, the determination of the hereditary percentages becomes a
definite reduction of the size of the experiments. Moreover it may
easily be effected by any one who cares for experimental studies, but
has not the means required for cultures on a larger scale. And lastly,
there are [416] a number of questions about heredity, periodicity,
dependency on nourishment and other life conditions, and even about
hybridizing, which may be answered by this new method.
Seed-leaves show many deviations from the ordinary shape, especially in
dicotyledonous plants. A very common aberration is the multiplication of
their number, and three seed-leaves in a whorl are not rarely met with.
The whorl may even consist of four, and in rare cases of five or more
cotyledons. Cleft cotyledons are also to be met with, and the fissure
may extend varying distances from the tips. Often all these deviations
may be seen among the seedlings of one lot, and then it is obvious that
together they constitute a scale of cleavages, the ternate and
quaternate whorls being only cases where the cleaving has reached its
greatest development. All in all it is manifest that here we are met by
one type of monstrosity, but that this type allows of a wide range of
fluctuating variability. For brevity's sake all these cleft and ternate,
double cleft and quaternate cotyledons and even the higher grades are
combined under one common name and indicated as tricotyls.
A second aberration of young seed-plants is exactly opposite to this. It
consists of the union of the two seed-leaves into a single organ. This
ordinarily betrays its origin by [417] having two separate apices, but
not always. Such seedlings are called syncotyledonous or syncotyls.
Other monstrosities have been observed from time to time, but need not
be mentioned here.
It is evident that the determination of the hereditary percentage is
very easy in tricotylous or syncotylous cultures. The parent plants must
be carefully isolated while blooming. Many species pollinate themselves
in the absence of bees; from these the insects are to be excluded.
Others have the stamens and stigmas widely separated and have to be
pollinated artificially. Still others do not lend themselves to such
operations, but have to be left free to the visits of bees and of
humble-bees, this being the only means of securing seed from every
plant. At the time of the harvest the seeds should be gathered
separately from each plant, and this precaution should also be observed
in studies of the hereditary percentage at large, and in all scientific
pedigree-cultures. Every lot of seeds is to be sown in a separate pan,
and care must be taken to sow such quantities the three to four hundred
seedlings will arise from each. As soon as they display their
cotyledons, they are counted, and the number is the criterion of the
parent-plant. Only parent-plants with the highest percentages are
selected, and out of [418] their seedlings some fifty or a hundred of
the best ones are chosen to furnish the seeds for the next generation.
This description of the method shows that the selection is a double one.
The first feature is the hereditary percentage. But then not all the
seedlings of the selected parents can be planted out, and a choice has
to be made. This second selection may favor the finest tricotyls, or the
strongest individuals, or rely on some other character, but is
unavoidable.
We now come to the description of the cultures. Starting points are the
stray tricotyls which are occasionally found in ordinary sowings. In
order to increase the chance of finding them, thousands of seeds of the
same species must be inspected, and the range of species must be widened
as much as possible.
Material for beginning such experiments is easily obtained, and almost
any large sample of seeds will be found suitable. Some tricotyls will be
found among every thousand seedlings in many species, while in others
ten or a hundred times, as many plants must be examined to secure them,
but species with absolutely pure dicotylous seeds are very rare.
The second phase of the experiment, however, is not so promising. Some
species are rich, and others are poor in this anomaly. This difference
[419] often indicates what can be expected from further culture. Stray
tricotyls point to poor species or half-races, while more frequent
deviations suggest rich or double-races. In both cases however, the
trial must be made, and this requires the isolation of the aberrant
individuals and the determination of their hereditary percentage.
In some instances the degree of their inheritance is only a very small
one. The isolated tricotyls yield 1 or 2% of inheritors, in some cases
even less, or upwards up to 3 or 4%. If the experiment is repeated, no
amelioration is observed, and this result remains the same during a
series of successive generations. In the case of _Polygonum
convolvulus_, the Black bindweed, I have tried as many as six
generations without ever obtaining more than 3%. With other species I
have limited myself to four successive years with the same negative
result, as with spinage, the Moldavian dragon-head, (_Dracocephalum
moldavicum_), and two species of corn catch-fly (_Silene conica_ and _S.
conoidea_).
Such poor races hardly afford a desirable material for further
inquiries. Happily the rich races, though rare, may be discovered also
from time to time. They seem to be more common among cultivated plants
and horticultural as well as agricultural species may be used. Hemp
[420] and mercury (_Mercurialis annua_) among the first, snapdragon,
poppies, _Phacelia_, _Helichrysum_, and _Clarkia_ among garden-flowers
may be given as instances of species containing the rich tricotylous
double races.
It is very interesting to note how strong the difference is between such
cases and those which only yield poor races. The rich type at once
betrays itself. No repeated selection is required. The stray tricotyls
themselves, that are sought out from among the original samples, give
hereditary percentages of a much higher type after isolation than those
quoted above. They come up to 10-20% and in some cases even to 40%. As
may be expected, individual differences occur, and it must even be
supposed that some of the original tricotyls may not be pure, but
hybrids between tricotylous and dicotylous parents. These are at once
eliminated by selection, and if only the tricotyls which have the
highest percentages are chosen for the continuance of the new race, the
second generation comes up with equal numbers of dicotyls and tricotyls
among the seedlings. The figures have been observed to range from 51-58%
in the majority of the cases, and average 55%, rarely diverging somewhat
more from this average.
Here we have the true type of an ever-sporting variety. Every year it
produces in the [421] same way heirs and atavists. Every plant, if
fertilized with its own pollen, gives rise to both types. The parent
itself may be tricotylous or dicotylous, or show any amount of
multiplication and cleavage in its seed-leaves, but it always gives the
entire range among its progeny of the variation. One may even select the
atavists, pollinate them purely and repeat this in a succeeding
generation without any chance of changing the result. On an average the
atavists may give lower hereditary figures, but the difference will be
only slight.
Such tricotylous double races offer highly interesting material for
inquiries into questions of heredity, as they have such a wide range of
variability. There is little danger in asserting that they go upwards to
nearly 100%, and downwards to 0%, diverging symmetrically on both sides
of their average (50-55%). These limits they obviously cannot
transgress, and are not even able to reach them. Samples of seed
consisting only of tricotyls are very rare, and when they are met with
the presumption is that they are too few to betray the rare aberrants
they might otherwise contain. Experimental evidence can only be reached
by the culture of a succeeding generation, and this always discloses the
hidden qualities, showing that the double [422] type was only
temporarily lost, but bound to return as soon as new trials are made.
This wide range of variability between definite limits is coupled with a
high degree of sensibility and adequateness to the most diverging
experiments. Our tricotylous double races are perhaps more sensitive to
selection than any other variety, and equally dependent on outer
circumstances. Here, however, I will limit myself to a discussion of the
former point.
In the second generation after the isolation of stray tricotylous
seedlings the average condition of the race is usually reached, but only
by some of the strongest individuals, and if we continue the race,
sowing or planting only from their offspring, the next generation will
show the ordinary type of variability, going upwards in some and
downwards in other instances. With the _Phacelia_ and the mercury and
some others I had the good luck in this one generation to reach as high
as nearly 90% of tricotylous seedlings, a figure indicating that the
normal dicotylous type had already become rare in the race. In other
cases 80% or nearly 80% was easily attained. Any further divergence from
the average would have required very much larger sowings, the effect of
selection between a limited number of parents being only to retain the
high degree once [423] reached; so for instance with the mercury, I had
three succeeding generations of selection after reaching the average of
55%, but their extremes gave no increasing advance, remaining at 86, 92
and 91%.
If we compare these results with the effects of selection in twisted and
fasciated races, we observe a marked contrast. Here they reached their
height at 30-40%, and no number of generations had the power of making
any further improvement. The tricotyls come up in two generations to a
proportion of about 54%, which shows itself to correspond to the average
type. And as soon as this is reached, only one generation is required to
obtain a very considerable improvement, going up to 80 or even 90%.
It is evident that the cause of this difference does not lie in the
nature of the monstrosity, but is due to the criterion upon which the
selection is made. Selection of the apparently best individuals is one
method, and it gives admirable results. Selection on the ground of the
hereditary percentages is another method and gives results which are far
more advantageous than the former.
In the lecture on the pistillody of the poppies we limited ourselves to
the selection of the finest individuals and showed that there is always
a manifest correlation between the individual [424] strength of the
plant and the degree of development of its anomaly. The same holds good
with other monstrosities, and badly nourished specimens of rich races
with twisted or fasciated stems always tend to reversion. This
reversion, however, is not necessarily correlated with the hereditary
percentage and therefore does not always indicate a lessening of the
degree of inheritance. This shows that even in those cases an
improvement may be expected, if only the means can be found to subject
the twisted and the fasciated races to the same sharp test as the
tricotylous varieties.
Much remains to be done, and the principle of the selection of parents
according to the average constitution of their progeny seems to be one
of the most promising in the whole realm of variability.
Besides tricotylous, the syncotylous seedlings may be used in the same
way. They are more rarely met with, and in most instances seem to belong
only to the unpromising half-races. The black bindweed (_Polygonum
Convolvulus_), the jointed charlock (Raphanus Raphanistrum), the
glaucous evening-primrose (_Oenothera glauca_) and many other plants
seem to contain such half-races. On the other hand I found a plant of
_Centranthus macrosiphon_ yielding as much as 55% of syncotylous
children [425] and thereby evidently betraying the nature of a rich or
double race. Likewise the mercury was rich in such deviations. But the
best of all was the Russian sunflower, and this was chosen for closer
experiments.
In the year of 1888 I had the good luck to isolate some syncotylous
seedlings and of finding among them one with 19% of inheritors among its
seeds. The following generation at once surpassed the ordinary average
and came up in three individuals to 76, 81 and even 89%. My race was at
once isolated and ameliorated by selection. I have tried to improve it
further and selected the parents with the highest percentages during
seven more generations; but without any remarkable result. I got figures
of 90% and above, coming even in one instance up to the apparent purity
of 100%. These, however, always remained extremes, the averages
fluctuating yearly between 80-90% or thereabouts, and the other extremes
going nearly every year downwards to 50%, the value which would be
attained, if no selection were made.
Contra-selection is as easily made as normal selection. According to our
present principle it means the choice of the parents with the smallest
hereditary percentage. One might easily imagine that by this means the
dicotylous seedlings could be rendered pure. This, however, [426] is not
at all the case. It is easy to return from so highly selected figures as
for instance 95% to the average about of 50%, as regression to
mediocrity is always an easy matter. But to transgress this average on
the lower side seems to be as difficult as it is on the upper side. I
continued the experiment during four succeeding generations, but was not
able to go lower than about 10%, and could not even exclude the high
figures from my strain. Parents with 65-75% of syncotylous seedlings
returned in each generation, notwithstanding the most careful
contra-selection. The attribute is inherent in the race, and is not to
be eliminated by so simple a means as selection, nor even by a selection
on the ground of hereditary percentages.
We have dealt with torsions and fasciations and with seedling variations
at some length, in order to point out the phases needing investigation
according to recent views. It would be quite superfluous to consider
other anomalies in a similar manner, as they all obey the same laws. A
hasty survey may suffice to show what prospects they offer to the
student of nature.
First of all come the variegated leaves. They are perhaps the most
variable of all variations. They are evidently dependent on external
circumstances, and by adequate nutrition the leaves may even become
absolutely white or [427] yellowish, with only scarcely perceptible
traces of green along the veins. Some are very old cultivated varieties,
as the wintercress, or _Barbarea vulgaris_. They continuously sport into
green, or return from this normal color, both by seeds and by buds.
Sports of this kind are very often seen on shrubs or low trees, and they
may remain there and develop during a long series of years. Bud-sports
of variegated holly, elms, chestnuts, beeches and others might be cited.
One-sided variegation on leaves or twigs with the opposite side wholly
green are by no means rare. It is very curious to note that variegation
is perhaps the most universally known anomaly, while its hereditary
tendencies are least known.
Cristate and plumose ferns are another instance. Half races or rare
accidental cleavages seem to be as common with ferns as cultivated
double races, which are very rich in beautiful crests. But much depends
on cultivation. It seems that the spores of crested leaves are more apt
to reproduce the variety than those of normal leaves, or even of normal
parts of the same leaves. But the experiments on which this assertion is
made are old and should be repeated. Other cases of cleft leaves should
also be tested. Ascidia are far more common than is usually believed.
Rare instances point [428] to poor races, but the magnolias and
lime-trees are often so productive of ascidia as to suggest the idea of
ever-sporting varieties. I have seen many hundred ascidia on one
lime-tree, and far above a hundred on the magnolia. They differ widely
in size and shape, including in some cases two leaves instead of one, or
are composed of only half a leaf or of even still a smaller part of the
summit. Rich ascidia-bearing varieties seem to offer notable
opportunities for scientific pedigree-cultures.
Union of the neighboring fruits and flowers on flower-heads, of the rays
of the umbellifers or of the successive flowers of the racemes of
cabbages and allied genera, seem to be rare. The same holds good for the
adhesion of foliar to axial organs, of branches to stems and other cases
of union. Many of these cases return regularly in each generation, or
may at least be seen from time to time in the same strains.
Proliferation of the inflorescence is very common and changes in the
position of staminate and pistillate flowers are not rare. We find
starting points for new investigations in almost any teratological
structure. Half-races and double-races are to be distinguished and
isolated in all cases, and their hereditary qualities, the periodicity
of the recurrence of the anomaly, the dependency on external
circumstances [429] and many other questions have to be answered.
Here is a wide field for garden experiments easily made, which might
ultimately yield much valuable information on many questions of heredity
of universal interest.
[430]
LECTURE XV
DOUBLE ADAPTATIONS
The chief object of all experimentation is to obtain explanations of
natural phenomena. Experiments are a repetition of things occurring in
nature with the conditions so guarded and so closely followed that it is
possible to make a clear analysis of facts and their causes, it being
rightfully assumed that the laws are the same in both cases.
Experiments on heredity and the experience of the breeder find their
analogy in the succession of generations in the wild state. The
stability of elementary species and of retrograde varieties is quite the
same under both conditions. Progression and retrogression are narrowly
linked everywhere, and the same laws govern the abundance of forms in
cultivated and in wild plants.
Elementary species and retrograde varieties are easily recognizable.
Ever-sporting varieties on the contrary are far less obvious, and in
many cases their hereditary relations have [431] had to be studied anew.
A clear analogy between them and corresponding types of wild plants has
yet to be pointed out. There can be no doubt that such analogy exists;
the conception that they should be limited to cultivated plants is not
probable. Striped flowers and variegated leaves, changes of stamens into
carpels or into petals may be extremely rare in the wild state, but the
"five-leaved" clover and a large number of monstrosities cannot be said
to be typical of the cultivated condition. These, however, are of rare
occurrence, and do not play any important part in the economy of nature.
In order to attain a better solution of the problem we must take a
broader view of the facts. The wide range of variability of
ever-sporting varieties is due to the presence of two antagonistic
characters which cannot be evolved at the same time and in the same
organ, because they exclude one another. Whenever one is active, the
other must be latent. But latency is not absolute inactivity and may
often only operate to encumber the evolution of the antagonistic
character, and to produce large numbers of lesser grades of its
development. The antagonism however, is not such in the exact meaning of
the word; it is rather a mutual exclusion, because one of the opponents
simply takes the place of the other when absent, or supplements [432] it
to the extent that it may be only imperfectly developed. This completion
ordinarily occurs in all possible degrees and thus causes the wide range
of the variability. Nevertheless it may be wanting, and in the case of
the double stocks only the two extremes are present.
It is rather difficult to get a clear conception of the substitution,
and it seems necessary to designate the peculiar relationship between
the two characters forming such a pair by a simple name. They might be
termed alternating, if only it were clearly understood that the
alternation may be complete, or incomplete in all degrees. Complete
alternation would result in the extremes, the incomplete condition in
the intermediate states. In some cases as with the stocks, the first
prevails, while in other cases, as with the poppies, the very extremes
are only rarely met with.
Taking such an alternation as a real character of the ever-sporting
varieties, a wide range of analogous cases is at once revealed among the
normal qualities of wild plants. Alternation is here almost universal.
It is the capacity of young organs to develop in two diverging
directions. The definitive choice must be made in extreme youth, or
often at a relatively late period of development. Once made, this [433]
choice is final, and a further change does not occur in the normal
course of things.
The most curious and most suggestive instance of such an alternation is
the case of the water-persicaria or _Polygonum amphibium_. It is known
to occur in two forms, one aquatic and the other terrestrial. These are
recorded in systematic works as varieties, and are described under the
names of _P. amphibium_ var. _natans_ Moench, and _P. amphibium_ var.
_terrestre_ Leers or _P. amphibium_ var. _terrestris_ Moench. Such
authorities as Koch in his German flora, and Grenier and Godron in their
French flora agree in the conception of the two forms as varieties.
Notwithstanding this, the two varieties may often be observed to sport
into one another. They are only branches of the same plant, grown under
different conditions. The aquatic form has floating or submerged stems
with oblong or elliptic leaves, which are glabrous and have long
petioles. The terrestrial plants are erect, nearly simple, more or less
hispid throughout, with lanceolate leaves and short petioles, often
nearly sessile. The aquatic form flowers regularly, producing its
peduncle at right angles from the floating stems, but the terrestrial
specimens are ordinarily seen without flower-spikes, which are but
rarely met with, at least as far as my own experience goes. Intermediate
[434] forms are very rare, perhaps wholly wanting, though in swamps the
terrestrial plants may often vary widely in the direction of the
floating type.
That both types sport into each other has long been recognized in
field-observations, and has been the ground for the specific name of
_amphibium_, though in this respect herbarium material seems usually to
be scant. The matter has recently been subjected to critical and
experimental studies by the Belgian botanist Massart, who has shown that
by transplanting the forms into the alternate conditions, the change may
always be brought about artificially. If floating plants are established
on the shore they make ascending hairy stems, and if the terrestrial
shoots are submerged, their buds grow into long and slack, aquatic
stems. Even in such experiments, intermediates are rare, both types
agreeing completely with the corresponding models in the wild state.
Among all the previously described cases of horticultural plants and
monstrosities there is no clearer case of an ever-sporting variety than
this one of the water-persicaria. The var. _terrestris_ sports into the
var. _natans_, and as often as the changing life conditions may require
it. It is-true that ordinary sports occur without our discerning the
cause and without [435] any relation to adaptation. This however is
partly due to our lack of knowledge, and partly to the general rule that
in nature only such sports as are useful are spared by natural
selection, and what is useful we ordinarily term adaptive.
Another side of the question remains to be considered. The word variety,
as is now becoming generally recognized, has no special meaning
whatever. But here it is assumed in the clearly defined sense of a
systematic variety, which includes all subdivisions of species. Such
subdivisions may be, from a biological point of view, elementary species
and also be eversporting varieties. They may be retrograde varieties,
and the two alternating types may be described as separate varieties.
It is readily granted that many writers would not willingly accept this
conclusion. But it is simply impossible to avoid it. The two forms of
the water-persicaria must remain varieties, though they are only types
of the different branches of a single plant.
If not, hundreds and perhaps thousands of analogous cases are at once
exposed to doubt, and the whole conception of systematic varieties would
have to be thrown over. Biologists of course would have no objection to
this, but the student of the flora of any given country [436] or region
requires the systematic subdivisions and should always use his utmost
efforts to keep them as they are. There is no intrinsic difficulty in
the statement that different parts of the same plant should constitute
different varieties.
In some cases different branches of the same plant have been described
as species. So for instance with the climbing forms of figs. Under the
name of _Ficus repens_ a fine little plant is quite commonly cultivated
as a climber in flower baskets. It is never seen bearing figs. On the
other hand a shrub of our hothouses called _Ficus stipulata_, is
cultivated in pots and makes a small tree which produces quite large,
though non-edible figs. Now these two species are simply branches of the
same plant. If the _repens_ is allowed to climb up high along the walls
of the hothouses, it will at last produce stipulate branches with the
corresponding fruits. _Ficus radicans_ is another climbing form,
corresponding to the shrub _Ficus ulmifolia_ of our glasshouses. And
quite the same thing occurs with ivy, the climbing stems of which never
flower, but always first produce erect and free branches with rhombic
leaves. These branches have often been used as cuttings and yield little
erect and richly flowering shrubs, which are known in [437] horticulture
under the varietal name of _Hedera Helix arborea_.
Manifestly this classification is as nearly right as that of the two
varieties of the water-persicaria. Going one step further, we meet with
the very interesting case of alpine plants. The vegetation of the higher
regions of mountains is commonly called alpine, and the plants show a
large number of common features, differentiating them from the flora of
lower stations. The mountain plants have small and dense foliage, with
large and brightly-colored flowers. The corresponding forms of the
lowlands have longer and weaker stems, bearing their leaves at greater
distances, the leaves themselves being more numerous. The alpine forms,
if perennial, have thick, strongly developed and densely branched
rootstocks with heavy roots, in which a large amount of food material is
stored up during the short summer, and is available during the long
winter months of the year.
Some species are peculiar to such high altitudes, while many forms from
the lowlands have no corresponding type on the mountains. But a large
number of species are common to both regions, and here the difference of
course is most striking. _Lotus corniculatus_ and _Calamintha Acinos_,
_Calluna vulgaris_ and _Campanula_ [438] _rotundifolia_ may be quoted as
instances, and every botanist who has visited alpine regions may add
other examples. Even the edelweiss of the Swiss Alps, _Gnaphalium
Leontopodium_, loses its alpine characters, if cultivated in lowland
gardens. Between such lowland and alpine forms intermediates regularly
occur. They may be met with whenever the range of the species extends
from the plains upward to the limit of eternal snow.
In this case the systematists formerly enumerated the alpine plants as
_forma alpestris_, but whenever the intermediate is lacking the term
_Varietas alpestris_ was often made use of.
It is simply impossible to decide concerning the real relation between
the alpine and lowland types without experiments. About the middle of
the last century it was quite a common thing to collect plants not only
for herbarium-material, but also for the purpose of planting them in
gardens and thus to observe their behavior under new conditions. This
was done with the acknowledged purpose of investigating the systematic
significance of observed divergencies. Whenever these held good in the
garden they were considered to be reliable, but if they disappeared they
were regarded as the results of climatic conditions, or of the influence
of soil or nourishment. Between [439] these two alternatives, many
writers have tried to decide, by transplanting their specimens after
some time in the garden, into arid or sandy soil, in order to see
whether they would resume their alpine character.
Among the systematists who tested plants in this way, Nageli especially,
directed his attention to the hawkweeds or _Hieracium_. On the Swiss
Alps they are very small and exhibit all the characters of the pure
alpine type. Thousands of single plants were cultivated by him in the
botanical garden of Munich, partly from seed and partly from introduced
rootstocks. Here they at once assumed the tall stature of lowland forms.
The identical individual, which formerly bore small rosettes of basal
leaves, with short and unbranched flower-stalks, became richly leaved
and often produced quite a profusion of flower-heads on branched stems.
If then they were transplanted to arid sand, though remaining in the
same garden and also under the same climatic conditions they resumed
their alpine characters. This proved nutrition to be the cause of the
change and not the climate.
The latest and most exact researches on this subject are due to Bonnier,
who has gone into all the details of the morphologic as well as of the
physiologic side of the problem. [440] His purpose was the study of
partial variability under the influence of climate and soil. In every
experiment he started from a single individual, divided it into two
parts and planted one half on a mountain and the other half on the
plain. The garden cultures were made chiefly at Paris and Fontainebleau,
the alpine cultures partly in the Alps, partly in the Pyrenees. From
time to time the halved plants were compared with each other, and the
cultures lasted, as a rule, during the lifetime of the individual, often
covering many years.
The common European frostweed or _Helianthemum vulgare_ will serve to
illustrate his results. A large plant growing in the Pyrenees in an
altitude of 2,400 meters was divided. One half was replanted on the same
spot, and the other near Cadeac, at the base of the mountain range (740
M.). In order to exclude the effect of a change of soil, a quantity of
the earth from the original locality was brought into the garden and the
plant put therein. Further control experiments were made at Paris. As
soon as the two halved individuals commenced to grow and produced new
shoots, the influence of the different climates made itself felt. On the
mountain, the underground portions remained strong and dense, the leaves
and internodes small and hairy, the flowering stems nearly [441]
procumbent, the flowers being large and of a deep yellow. At Cadeac and
at Paris the whole plant changed at once, the shoots becoming elongated
and loose, with broad and flattened, rather smooth leaves and numerous
pale-hued flowers. The anatomical structure exhibited corresponding
differences, the intercellular spaces being small in the alpine plant
and large in the one grown in the lowlands, the wood-tissues strong in
the first and weak in the second case.
The milfoil (_Achillea Millefolium_) served as a second example, and the
experiments were carried on in the same localities. The long and thick
rootstocks of the alpine plant bearing short stems only with a few dense
corymbs contrasted markedly with the slender stems, loose foliage and
rich groups of flowerheads of the lowland plant. The same differences,
in inner and outer structures were observed in numerous instances,
showing that the alpine type in these cases is dependent on the climate,
and that the capacity for assuming the antagonistic characters is
present in every individual of the species. The external conditions
decide which of them becomes active and which remains inactive, and the
case seems to be exactly parallel to that of the water-persicaria.
In the experiments of Bonnier the influence of the soil was, as a rule,
excluded by transplanting [442] part of the original earth with the
transplanted half of the plant. From this he concluded that the observed
changes were due to the inequality of the climate. This involved three
main factors, light, moisture and temperature. On the mountains the
light is more intense, the air drier and cooler. Control-experiments
were made on the mountains, depriving the plants of part of the light.
In various ways they were more or less shaded, and as a rule responded
to this treatment in the same way as to transplantation to the plain
below. Bonnier concluded that, though more than one factor takes part in
inciting the morphologic changes, light is to be considered as the chief
agency. The response is to be considered as a useful one, as the whole
structure of the alpine varieties is fitted to produce a large amount of
organic material in a short time, which enables the plants to thrive
during the short summers and long winters of their elevated stations.
In connection with these studies on the influences of alpine climates,
Bonnier has investigated the internal structure of arctic plants, and
made a series of experiments on growth in continuous electric light. The
arctic climate is cold, but wet, and the structure of the leaves is
correspondingly loose, though the plants become [443] as small as on the
Alps. Continuous electric light had very curious effects; the plants
became etiolated, as if growing in darkness, with the exception that
they assumed a deep green tinge. They showed more analogy with the
arctic than with the alpine type.
The influence of the soil often produces changes similar to that of
climate. This was shown by the above cited experiments of Nageli with
the hawkweeds, and may easily be controlled in other cases. The
ground-honeysuckle or _Lotus corniculatus_ grows in Holland partly on
the dry and sandy soil of the dunes, and occasionally in meadows. It is
small and dense in the first case, with orange and often very darkly
colored petals, while it is loose and green in the meadows, with
yellower flowers. Numerous analogous cases might be given. On mountain
slopes in South Africa, and especially in Natal, a species of composite
is found, which has been introduced into culture and is used as a
hanging plant. It is called _Othonna crassifolia_ and has fleshy, nearly
cylindrical leaves, and exactly mimics some of the crassulaceous
species. On dry soil the leaves become shorter and thicker and assume a
reddish tinge, the stems remain short and woody and bear their leaves in
dense rosettes. On moist and rich garden-soil this aspect becomes [444]
changed at once, the stems grow longer and of a deeper green.
Intermediates occur, but notwithstanding this the two extremes
constitute clearly antagonistic types.
The flora of the deserts is known to exhibit a similar divergent type.
Or rather two types, one adapted to paucity of water, and the other to a
storage of fluid at one season in order to make use of it at other
times, as is the case with the cactuses. Limiting ourselves to the
alternate group, we observe a rich and dense branching, small and
compact leaves and extraordinarily long roots. Here the analogy with the
alpine varieties is manifest, and the dryness of the soil evidently
affects the plants in a similar way, as do the conditions of life in
alpine regions. The question at once comes up as to whether here too we
have only instances of partial variability, and whether many of the
typical desert-species would lose their peculiar character by
cultivation under ordinary conditions. The varieties of _Monardella
macrantha_, described by Hall, from the San Jacinto Mountain, Cal., are
suggestive of such an intimate analogy with the cases studied by
Bonnier, that it seems probable that they might yield similar results,
if tested by the same method.
Leaving now the description of these special [445] cases, we may resume
our theoretical discussion of the subject, and try to get a clearer
insight into the analogy of ever-sporting varieties and the wild species
quoted. All of them may be characterized by the general term of
dimorphism. Two types are always present, though not in the same
individual or in the same organ. They exclude one another, and during
their juvenile stage a decision is taken in one direction or in the
other. Now, according to the theory of natural selection, wild species
can only retain useful or at least innocuous qualities, since all
mutations in a wrong direction must perish sooner or later. Cultivated
species on the other hand are known to be largely endowed with
qualities, which would be detrimental in the wild condition.
Monstrosities are equally injurious and could not hold their own if left
to themselves.
These same principles may be applied to ever-sporting or antagonistic
pairs of characters. According to the theory of mutations such pairs may
be either useful or useless. But only the useful will stand further
test, and if they find suitable conditions will become specific or
varietal characters. On this conclusion it becomes at once clear, why
natural dimorphism is, as a rule, a very useful quality, while the
cultivated dimorphous varieties [446] strike us as something unnatural.
The relation between cause and effect, is in truth other than it might
seem to be at first view, but nevertheless it exists, and is of the
highest importance.
From this same conclusion we may further deduce some explanation of the
hereditary races characterized by monstrosities. It is quite evident
that the twisted teasels are inadequate for the struggle with their tall
congeners, or with the surrounding plants. Hence the conclusion that a
pure and exclusively twisted race would soon die out. The fact that such
races are not in existence finds its explanation in this circumstance,
and therefore it does not prove the impossibility or even the
improbability that some time a pure twisted race might arise. If chance
should put such an accidental race in the hands of an experimenter, it
could be protected and preserved, and having no straight atavistic
branches, but being twisted in all its organs, might yield the most
curious conceivable monstrosity, surpassing even the celebrated dwarf
twisted shrubs of Japanese horticulturists.
Such varieties however, do not exist at present. The ordinary twisted
races on the other hand, are found in the wild state and have only to be
isolated and cultivated to yield large numbers [447] of twisted
individuals. In nature they are able to maintain themselves during long
centuries, quite as well as normal species and varieties. But they owe
this quality entirely to their dimorphous character. A twisted race of
teasels might consist of successive generations of tall atavistic
individuals, and produce yearly some twisted specimens, which might be
destroyed every time before ripening their seeds. Reasoning from the
evidence available, and from analogous cases, the variety would, even
under such extreme circumstances, be able to last as long as any other
good variety or elementary species. And it seems to me that this
explanation makes clear how it is possible that varieties, which are
potentially rich in their peculiar monstrosity, are discovered from time
to time among plants when tested by experimental methods.
Granting these conclusions, monstrosities on the one side, and
dimorphous wild species on the other, constitute the most striking
examples of the inheritance of latent characters.
The bearing of the phenomena of dimorphism upon the principles of
evolution formulated by Lamarck, and modified by his followers to
constitute Neo-Lamarckianism, remains to be considered. Lamarck assumed
that the external conditions directly affected the organisms in [448]
such a way as to make them better adapted to life, under prevailing
circumstances. Nageli gave to this conception the name "Theory of direct
causation" (Theorie der directen Bewirkung), and it has received the
approval of Von Wettstein, Strasburger and other German investigators.
According to this conception a plant, when migrating from lowlands into
the mountains would slowly be changed and gradually assume alpine
habits. Once acquired this habit would become fixed and attain the rank
of specific characters. In testing this theory by field-observations and
culture-experiments, the defenders of the Nagelian principle could
easily produce evidence upon the first point. The change of
lowland-plants into alpine varieties can be brought about in numerous
cases, and corresponding changes under the influence of soil, or
climate, or life-conditions are on record for the most various
characters and qualities.
The second point, however, is as difficult to prove as the first is of
easy treatment. If after hundreds and thousands of years of exposure to
alpine or other extreme conditions a fixed change is proved to have
taken place, the question remains unanswered, whether the change has
been a gradual or a sudden one. Darwin pointed out that long periods of
life afford a [449] chance for a sudden change in the desired direction,
as well as for the slow accumulation of slight deviations. Any mutations
in a wrong direction would at once be destroyed, but an accidental
change in a useful way would be preserved, and multiply itself. If in
the course of centuries this occurred, they would be nearly sure to
become established, however rare at the outset. Hence the positive
assertion is scarcely capable of direct proof.
On the other hand the negative assertion must be granted full
significance. If the alpine climate has done no more than produce a
transitory change, it is clear that thousands of years do not,
necessarily, cause constant and specific alterations. This requirement
is one of the indispensable supports of the Lamarckian theory. The
matter is capable of disproof however, and such disproof seems to be
afforded by the direct evidence of the present condition of the alpine
varieties at large, and by many other similar cases.
Among these the observations of Holtermann on some desert-plants of
Ceylon are of the highest value. Moreover they touch questions which are
of wide importance for the study of the biology of American deserts. For
this reason I may be allowed to introduce them here at some length.
[450] The desert of Kaits, in Northern Ceylon, nourishes on its dry and
torrid sands some species, represented by a large number of individuals,
together with some rarer plants. The commonest forms are _Erigeron
Asteroides_, _Vernonia cinerea_, _Laurea pinnatifida_, _Vicoa
auriculata_, _Heylandia latebrosa_ and _Chrysopogon montanus_. In direct
contrast with the ordinary desert-types they have a thin epidermis, with
exposed stomata, features that ordinarily were characteristic of species
of moister regions. They are annuals, growing rapidly, blooming and
ripening their seeds before the height of the dry season. Evidently they
are to be considered as the remainder of the flora of a previous period,
when the soil had not yet become arid. They might be called relics. Of
course they are small and dwarf-like, when compared with allied forms.
These curious little desert-plants disprove the Nagelian views in two
important points. First, they show that extreme conditions do not
necessarily change the organisms subjected to them, in a desirable
direction. During the many centuries that these plants must have existed
in the desert in annual generations, no single feature in the anatomical
structure has become changed. Hence the conclusion that small leaves,
abundant rootstocks and short [451] stems, a dense foliage, a strongly
cuticularized epidermis, few and narrow air-cavities in the tissues and
all the long range of characteristics of typical desert-plants are not a
simple result of the influence of climate and soil. There is no direct
influence in this sense.
The second point, in which Nageli's idea is broken down by Holtermann's
observations, results from the behavior of the plants of the Kaits
desert when grown or sown on garden soil. When treated in this way they
at once lose the only peculiarity which might be considered as a
consequence of the desert-life of their ancestors, their dwarf stature.
They behave exactly like the alpine plants in Bonnier's experiments, and
with even more striking differences. In the desert they attain a height
of a few centimeters, but in the garden they attain half a meter and
more in height. Nothing in the way of stability has resulted from the
action of the dry soil, not even in such a minor point as the height of
the stems.
From the facts and discussions we may conclude that double adaptation is
not induced by external influences, at least not in any way in which it
might be of use to the plant. It may arise by some unknown cause, or may
not be incited at all. In the first case the plant becomes capable of
living under the alternating [452] circumstances, and if growing near
the limits of such regions it will overlap and get into the new area.
All other species, which did not acquire the double habit, are of course
excluded, with such curious exceptions as those of Kaits. The typical
vegetation under such extreme conditions however, finds explanation
quite as well by the one as by the other view.
Leaving these obvious cases of double adaptation, there still remains
one point to be considered. It is the dwarf stature of so many desert
and alpine plants. Are these dwarfs only the extremes of the normal
fluctuating variability, or is their stature to be regarded as the
expression of some peculiar adaptive but latent quality? It is as yet
difficult to decide this question, because statistical studies of this
form of variability are still wanting. The capacity of ripening the seed
on individuals of dwarf stature however, is not at all a universal
accompaniment of a variable height. Hence it cannot be considered as a
necessary consequence of it. On the other hand the dwarf varieties of
numerous garden-plants, as for instance: of larkspurs, snapdragon,
opium-poppies and others are quite stable and thence are obviously due
to peculiar characteristics. Such characteristics, if combined with tall
stature into a pair of antagonists, would yield a double [453]
adaptation, and on such a base a hypothetical explanation could no doubt
be rested. Instead of discussing this problem from the theoretical side,
I prefer to compare those species which are capable of assuming a dwarf
stature under less uncommon conditions than those of alpine and
desert-plants. Many weeds of our gardens and many wild species have this
capacity. They become very tall, with large leaves, richly branched
stems and numerous flowers in moist and rich soil. On bad soil, or if
germinating too late, when the season is drier, they remain very small,
producing only a few leaves and often limiting themselves to one
flower-head. This is often seen with thorn-apples and amaranths, and
even with oats and rye, and is notoriously the case with buckwheat.
Gauchery has observed that the extremes differ often as much from one
another as 1:10. In the case of the Canadian horseweed or _Erigeron
canadensis_, which is widely naturalized in Europe, the tallest
specimens are often twenty-five times as tall as the smallest, the
difference increasing to greater extremes, if besides the main stem, the
length of the numerous branches of the tall plants are taken into
consideration. Other instances studied by the French investigator are
_Erythraea pulchella_ and _Calamintha Acinos_.
[454] Dimorphism is of universal occurrence in the whole vegetable
kingdom. In some cases it is typical, and may easily be discerned from
extreme fluctuating variability. In others the contrast is not at all
obvious, and a closer investigation is needed to decide between the two
possibilities. Sometimes the adaptive quality is evident, in other cases
it is not. A large number of plants bear two kinds of leaves linked with
one another by intermediate forms. Often the first leaves of a shoot, or
those of accidentally strong shoots, exhibit deviating shapes, and the
usefulness of such occurrences seems to be quite doubtful. The
elongation of stems and linear leaves, and the reduction of lateral
organs in darkness, is manifestly an adaptation. Many plants have
stolons with double adaptations which enable them to retain their
character of underground stems with bracts or to exchange it for the
characteristics of erect stems with green leaves according to the outer
circumstances. In some shrubs and trees the capacity of a number of buds
to produce either flowers or shoots with leaves seems to be in the same
condition. The capacity of producing spines is also a double adaptation,
active on dry and arid soil and latent in a moist climate or under
cultivation, as with the wild and cultivated apple, and in the
experiments of Lothelier [455] with _Berberis_, _Lycium_ and other
species, which lose their spines in damp air.
In some conifers the evolution of horizontal branches may be modified by
simply turning the buds upside down. Or the lateral branches can be
induced to become erect stems by cutting off the normal summit of a
tree. Numerous organs and functions lie dormant until aroused by
external agencies, and many other cases could be cited, showing the wide
occurrence of double adaptation.
There are, however, two points, which should not be passed over without
some mention. One of them is the influence of sun and shade on leaves,
and the other the atavistic forms, often exhibited during the juvenile
period.
The leaves of many plants, and especially those of some shrubs and
trees, have the capacity of adapting themselves either to intense or to
diffuse light. On the circumference of the crown of a tree the light is
stronger and the leaves a small and thick, with a dense tissue. In the
inner parts of the crown the light is weak and the leaves are broader in
order to get as much of it as possible. They become larger but thinner,
consisting often of a small number of cell layers. The definitive
formation is made in extreme youth, often even during the previous
summer, at the time of the [456] very first evolution of the young
organs within the buds. _Iris_, and _Lactuca Scariola_ or the prickly
lettuce, and many other plants afford similar instances. As the
definitive decision must be made in these cases long before the direct
influence of the conditions which would make the change useful is felt,
it is hardly conceivable how they could be ascribed to this cause.
It is universally known that many plants show deviating features when
very young, and that these often remind us of the characters of their
probable ancestors. Many plants that must have been derived from their
nearest systematic relatives, chiefly by reductions, are constantly
betraying this relation by a repetition of the ancestral marks during
their youth.
There can be hardly a doubt that the general law of natural selection
prevails in such cases as it does in others. Or stated otherwise, it is
very probable, that in most cases the atavistic characters have been
retained during youth because of their temporary usefulness.
Unfortunately, our knowledge of utility of qualities is as yet, very
incomplete. Here we must assume that what is ordinarily spared by
natural selection is to be considered as useful, [457] until direct
experimental investigations have been made.
So it is for instance with the submerged leaves of water-plants. As a
rule they are linear, or if compound, are reduced to densely branching
filiform threads. Hence we may conclude that this structure is of some
use to them. Now two European and some corresponding American species of
water-parsnip, the _Sium latifolium_ and _Berula angustifolia_ with
their allies, are umbellifers, which bear pinnate instead of bi- or
tri-pinnate leaves. But the young plants and even the young shoots when
developing from the rootstocks under water comply with the above rule,
producing very compound, finely and pectinately dissected leaves. From a
systematic point of view these leaves indicate the origin of the
water-parsnips from ordinary umbellifers, which generally have bi- and
tripinnate leaves.
Similar cases of double adaptation, dependent on external conditions at
different periods of the evolution of the plant are very numerous. They
are most marked among leguminous plants, as shown by the trifoliolate
leaves of the thorn-broom and allies, which in the adult state have
green twigs destitute of leaves.
As an additional instance of dimorphism and probable double adaptation
to unrecognized external [458] conditions I might point to the genus
_Acacia_. As we have seen in a previous lecture some of the numerous
species of this genus bear bi-pinnate leaves, while others have only
flattened leaf-stalks. According to the prevailing systematic
conceptions, the last must have been derived from the first by the loss
of the blades and the corresponding increase of size and superficial
extension of the stalk. In proof of this view they exhibit, as we have
described, the ancestral characters in the young plantlets, and this
production of bi-pinnate leaves has probably been retained at the period
of the corresponding negative mutations, because of some distinct,
though still unknown use.
Summarizing the results of this discussion, we may state that useful
dimorphism, or double adaptation, is a substitution of characters quite
analogous to the useless dimorphism of cultivated ever-sporting
varieties and the stray occurrence of hereditary monstrosities. The same
laws and conditions prevail in both cases.
[459]
E. MUTATIONS
LECTURE XVI
THE ORIGIN OF THE PELORIC TOAD-FLAX
I have tried to show previously that species, in the ordinary sense of
the word, consist of distinct groups of units. In systematic works these
groups are all designated by the name of varieties, but it is usually
granted that the units of the system are not always of the same value.
Hence we have distinguished between elementary species and varieties
proper. The first are combined into species whose common original type
is now lost or unknown, and from their characters is derived an
hypothetical image of what the common ancestor is supposed to have been.
The varieties proper are derived in most cases from still existing
types, and therefore are subjoined to them. A closer investigation has
shown that this derivation is ordinarily produced by the loss of some
definite attribute, or by the re-acquisition of an apparently [460] lost
character. The elementary species, on the other hand, must have arisen
by the production of new qualities, each new acquisition constituting
the origin of a new elementary form.
Moreover we have seen, that such improvements and such losses constitute
sharp limits between the single unit-forms. Every type, of course,
varies around an average, and the extremes of one form may sometimes
reach or even overlap those of the nearest allies, but the offspring of
the extremes always return to the type. The transgression is only
temporary and a real transition of one form to another does not come
within ordinary features of fluctuating variability. Even in the cases
of eversporting varieties, where two opposite types are united within
one race, and where the succeeding individuals are continually swinging
from one extreme to the other, passing through a wide range of
intermediate steps, the limits of the variety are as sharply defined and
as free from real transgression as in any other form.
In a complete systematic enumeration of the real units of nature, the
elementary species and varieties are thus observed to be discontinuous
and separated by definite gaps. Every unit may have its youth, may lead
a long life in the adult state and may finally die. But through [461]
the whole period of its existence it remains the same, at the end as
sharply defined from its nearest allies as in the beginning. Should some
of the units die out, the gaps between the neighboring ones will become
wider, as must often have been the case. Such segregations, however
important and useful for systematic distinctions, are evidently only of
secondary value, when considering the real nature of the units
themselves.
We may now take up the other side of the problem. The question arises as
to how species and varieties have originated. According to the Darwinian
theory they have been produced from one another, the more highly
differentiated ones from the simpler, in a graduated series from the
most simple forms to the most complicated and most highly organized
existing types. This evolution of course must have been regular and
continuous, diverging from time to time into new directions, and linking
all organisms together into one common pedigree. All lacunae in our
present system are explained by Darwin as due to the extinction of the
forms, which previously filled them.
Since Lamarck first propounded the conception of a common origin for all
living beings, much has been done to clear up our ideas as to the real
nature of this process. The broader [462] aspect of the subject,
including the general pedigree of the animal and vegetable kingdom, may
be said to have been outlined by Darwin and his followers, but this
phase of the subject lies beyond the limits of our present discussion.
The other phase of the problem is concerned with the manner in which the
single elementary species and varieties have sprung from one another.
There is no reason to suppose that the world is reaching the end of its
development, and so we are to infer that the production of new species
and varieties is still going on. In reality, new forms are observed to
originate from time to time, both wild and in cultivation, and such
facts do not leave any doubt as to their origin from other allied types,
and according to natural and general laws.
In the wild state however, and even with cultivated plants of the field
and garden, the conditions, though allowing of the immediate observation
of the origination of new forms, are by no means favorable for a closer
inquiry into the real nature of the process. Therefore I shall postpone
the discussion of the facts till another lecture, as their bearing will
be more easily understood after having dealt with more complete cases.
These can only be obtained by direct experimentation. Comparative
studies, of course, [463] are valuable for the elucidation of general
problems and broad features of the whole pedigree, but the narrower and
more practical question as to the genetic relation of the single forms
to one another must be studied in another way, by direct experiment. The
exact methods of the laboratory must be used, and in this case the
garden is the laboratory. The cultures must be guarded with the
strictest care and every precaution taken to exclude opportunities for
error. The parents and grandparents and their offspring must be kept
pure and under control, and all facts bearing upon the birth or origin
of the new types should be carefully recorded.
Two great difficulties have of late stood in the way of such
experimental investigation. One of them is of a theoretical, the, other
of a practical nature. One is the general belief in the supposed
slowness of the process, the other is the choice of adequate material
for experimental purposes. Darwin's hypothesis of natural selection as
the means by which new types arise, is now being generally interpreted
as stating the slow transformation of ordinary fluctuating divergencies
from the average type into specific differences. But in doing so it is
overlooked that Quetelet's law of fluctuating variability was not yet
discovered at the time, when Darwin propounded his theory. So there
[464] is no real and intimate connection between these two great
conceptions. Darwin frequently pointed out that a long period of time
might be needed for slow improvements, and was also a condition for the
occurrence of rare sports. In any case those writers have been in error,
according to my opinion, who have refrained from experimental work on
the origin of species, on account of this narrow interpretation of
Darwin's views. The choice of the material is quite another question,
and obviously all depends upon this choice. Promising instances must be
sought for, but as a rule the best way is to test as many plants as
possible. Many of them may show nothing of interest, but some might lead
to the desired end.
For to-day's lecture I have chosen an instance, in which the grounds
upon which the choice was based are very evident. It is the origin of
the peloric toad-flax (_Linaria vulgaris peloria_).
The ground for this choice lies simply in the fact that the peloric
toad-flax is known to have originated from the ordinary type at
different times and in different countries, under more or less divergent
conditions. It had arisen from time to time, and hence I presumed that
there was a chance to see it arise again. If this should happen under
experimental circumstances [465] the desired evidence might easily be
gathered. Or, to put it in other words, we must try to arrange things so
as to be present at the time when nature produces another of these rare
changes.
There was still another reason for choosing this plant for observational
work. The step from the ordinary toad-flax to the peloric form is short,
and it appears as if it might be produced by slow conversion. The
ordinary species produces from time to time stray peloric flowers. These
occur at the base of the raceme, or rarely in the midst of it. In other
species they are often seen at the summit. Terminal pelories are usually
regular, having five equal spurs. Lateral pelories are generally of
zygomorphic structure, though of course in a less degree than the normal
bilabiate flowers, but they have unequal spurs, the middle one being of
the ordinary length, the two neighboring being shorter, and those
standing next to the opposite side of the flower being the shortest of
all. This curious remainder of the original, symmetrical structure of
the flower seems to have been overlooked hitherto by the investigators
of peloric toad-flaxes.
The peloric variety of this plant is characterized by its producing only
peloric flowers. No single bilabiate or one-spurred flower remains.
[466] I once had a lot of nearly a hundred specimens of this fine
variety, and it was a most curious and beautiful sight to observe the
many thousands of nearly regular flowers blooming at the same time. Some
degree of variability was of course present, even in a large measure.
The number of the spurs varied between four and six, transgressing these
limits in some instances, but never so far as to produce really
one-spurred flowers. Comparing this variety with the ordinary type, two
ways of passing over from the one to the other might be imagined. One
would entail a slow increase of the number of the peloric flowers on
each plant, combined with a decrease of the number of the normal ones,
the other a sudden leap from one extreme to the other without any
intermediate steps. The latter might easily be overlooked in field
observations and their failure may not have the value of direct proof.
They could never be overlooked, on the other hand, in experimental
culture.
The first record of the peloric toad-flax is that of Zioberg, a student
of Linnaeus, who found it in the neighborhood of Upsala. This curious
discovery was described by Rudberg in his dissertation in the year 1744.
Soon afterwards other localities were discovered by Link near Gottingen
in Germany about 1791 and afterwards [467] in the vicinity of Berlin, as
stated by Ratzeburg, 1825. Many other localities have since been
indicated for it in Europe, and in my own country some have been noted
of late, as for instance near Zandvoort in 1874 and near Oldenzaal in
1896. In both these last named cases the peloric form arose
spontaneously in places which had often been visited by botanists before
the recorded appearance, and therefore, without any doubt, they must
have been produced directly and independently by the ordinary species
which grows in the locality. The same holds good for other occurrences
of it. In many instances the variety has been recorded to disappear
after a certain lapse of time, the original specimens dying out and no
new ones being produced. _Linaria_ is a perennial herb, multiplying
itself easily by buds growing on the roots, but even with this means of
propagation its duration seems to have definite limits.
There is one other important point arguing strongly for the independent
appearance of the peloric form in its several localities. It is the
difficulty of fertilization and the high degree of sterility, even if
artificially pollinated. Bees and bumble-bees are unable to crawl into
the narrow tubular flowers, and to bring the fertilizing pollen to the
stigma. Ripe capsules with seeds [468] have never been seen in the wild
state. The only writer who succeeded in sowing seeds of the peloric
variety was Wildenow and he got only very few seedlings. But even in
artificial pollination the result is the same, the anthers seeming to be
seriously affected by the change. I tried both self-fertilization and
cross-pollination, and only with utmost care did I succeed in saving
barely a hundred seeds. In order to obtain them I was compelled to
operate on more than a thousand flowers on about a dozen peloric plants.
The variety being wholly barren in nature, the assumption that the
plants in the different recorded localities might have a common origin
is at once excluded. There must have been at least nearly as many
mutations as localities. This strengthens the hope of seeing such a
mutation happen in one's own garden. It should also be remembered that
peloric flowers are known to have originated in quite a number of
different species of _Linaria_, and also with many of the allied species
within the range of the Labiatiflorae.
I will now give the description of my own experiment. Of course this did
not give the expected result in the first year. On the contrary, it was
only after eight years' work that I had the good fortune of observing
the mutation. [469] But as the whole life-history of the preceding
generations had been carefully observed and recorded, the exact
interpretation of the fact was readily made.
My culture commenced in the year 1886. I chose some plants of the normal
type with one or two peloric flowers besides the bilabiate majority
which I found on a locality in the neighborhood of Hilversum in Holland.
I planted the roots in my garden and from them had the first flowering
generation in the following summer. From their seeds I grew the second
generation in three following years. They flowered profusely and
produced in 1889 only one, and in 1890 only two peloric structures. I
saved the seeds in 1889 and had in 1890-1891 the third generation. These
plants likewise flowered only in the second year, and gave among some
thousands of symmetrical blossoms, only one five-spurred flower. I
pollinated this flower myself, and it produced abundant fruit with
enough seeds for the entire culture in 1892, and they only were sown.
Until this year my generations required two years each, owing to the
perennial habit of the plants. In this way the prospects of the culture
began to decrease, and I proposed to try to heighten my chances by
having a new generation yearly. With this intention I sowed the [470]
selected seeds in a pan in the glasshouse of my laboratory and planted
them out as soon as the young stems had reached a length of some few
centimeters. Each seedling was put in a separate pot, in heavily manured
soil. The pots were kept under glass until the beginning of June, and
the young plants produced during this period a number of secondary stems
from the curious hypocotylous buds which are so characteristic of the
species. These stems grew rapidly and as soon as they were strong
enough, the plants were put into the beds. They all, at least nearly
all, some twenty specimens, flowered in the following month.
I observed only one peloric flower among the large number present. I
took the plant bearing this flower and one more for the culture of the
following year, and destroyed all others. These two plants grew on the
same spot, and were allowed to fertilize each other by the agency of the
bees, but were kept isolated from any other congener. They flowered
abundantly, but produced only one-spurred bilabiate flowers during the
whole summer. They matured more than 10 cu. cm. of seeds.
It is from this pair of plants that my peloric race has sprung. And as
they are the ancestors of the first closely observed case of peloric
mutation, [471] it seems worth while to give some details regarding
their fertilization.
Isolated plants of _Linaria vulgaris_ do not produce seed, even if
freely pollinated by bees. Pollen from other plants is required. This
requirement is not at all restricted to the genus _Linaria_, as many
instances are known to occur in different families. It is generally
assumed that the pollen of any other individual of the same species is
capable of producing fertilization, although it is to be said that a
critical examination has been made in but few instances.
This, however, is not the case, at least not in the present instance. I
have pollinated a number of plants, grown from seed of the same strain
and combined them in pairs, and excluded the visits of insects, and
pollen other than that of the plant itself and that of the specimen with
which it was paired. The result was that some pairs were fertile and
others barren. Counting these two groups of pairs, I found them nearly
equal in number, indicating thereby that for any given individual the
pollen of half of the others is potent, but that of the other half
impotent. From these facts we may conclude the presence of a curious
case of dimorphy, analogous to that proposed for the primroses, but
without visible differentiating marks in the flowers. At least such
opposite characters [472] have as yet not been ascertained in the case
of our toad-flax.
In order to save seed from isolated plants it is necessary, for this
reason, to have at least two individuals, and these must belong to the
two physiologically different types. Now in the year 1892, as in other
years, my plants, though separated at the outset by distances of about
20 cm. from each other, threw out roots of far greater length, growing
in such a way as to abolish the strict isolation of the individuals. Any
plot may produce several stems from such roots, and it is manifestly
impossible to decide whether they all belong to one original plant or to
the mixed roots of several individuals. No other strains were grown on
the same bed with my plants however, and so I considered all the stems
of the little group as belonging to one plant. But their perfect
fertility showed, according to the experience described, that there must
have been at least two specimens mingled together.
Returning now to the seeds of this pair of plants, I had, of course, not
the least occasion to ascribe to it any higher value than the harvest of
former years. The consequence was that I had no reason to make large
sowings, and grew only enough young plants to have about 50 in bloom in
the summer of 1894. Among [473] these, stray peloric flowers were
observed in somewhat larger number than in the previous generations, 11
plants bearing one or two, or even three such abnormalities. This
however, could not be considered as a real advance, since such plants
may occur in varying, though ordinarily small numbers in every
generation.
Besides them a single plant was seen to bear only peloric flowers; it
produced racemes on several stems and their branches. All were peloric
without exception. I kept it through the winter, taking care to preserve
a complete isolation of its roots. The other plants were wholly
destroyed. Such annihilation must include both the stems and roots and
the latter of course requires considerable labor. The following year,
however, gave proof of the success of the operation, since my plant
bloomed luxuriously for the second time and remained true to the type of
the first year, producing peloric flowers exclusively.
Here we have the first experimental mutation of a normal into a peloric
race. Two facts were clear and simple. The ancestry was known for over a
period of four generations, living under the ordinary care and
conditions of an experimental garden, isolated from other toad-flaxes,
but freely fertilized by bees or at times by myself. This ancestry was
quite constant as to [474] the peloric peculiarity, remaining true to
the wild type as it occurs everywhere in my country, and showing in no
respect any tendency to the production of a new variety.
The mutation took place at once. It was a sudden leap from the normal
plants with very rare peloric flowers to a type exclusively peloric. No
intermediate steps were observed. The parents themselves had borne
thousands of flowers during two summers, and these were inspected nearly
every day, in the hope of finding some pelories and of saving their seed
separately. Only one such flower was seen. If there had been more, say a
few in every hundred flowers, it might be allowable to consider them as
previous stages, showing a preparation of the impending change. But
nothing of this kind was observed. There was simply no visible
preparation for the sudden leap.
This leap, on the other hand, was full and complete. No reminiscence of
the former condition remained. Not a single flower on the mutated plant
reverted to the previous type. All were thoroughly affected by the new
attribute, and showed the abnormally augmented number of spurs, the
tubular structure of the corolla and the round and narrow entrance of
its throat. The whole plant departed absolutely from the old type of its
progenitors.
[475] Three ways were open to continue my experiment. The first was
indicated by the abundant harvest from the parent-plants of the
mutation. It seemed possible to compare the numerical proportion of the
mutated seeds with those of normal plants. In order to ascertain this
proportion I sowed the greatest part of my 10 cu. cm. of seed and
planted some 2,000 young plants in little pots with well-manured soil. I
got some 1,750 flowering plants and observed among them 16 wholly
peloric individuals. The numerical proportion of the mutation was
therefore in this instance to be calculated equal to about 1% of the
whole crop.
This figure is of some importance. For it shows that the chance of
finding mutations requires the cultivation of large groups of
individuals. One plant in each hundred may mutate, and cultures of less
than a hundred specimens must therefore be entirely dependent on chance
for the appearance of new forms, even if such should accidentally have
been produced and lay dormant in the seed. In other cases mutations may
be more numerous, or on the contrary, more rare. But the chance of
mutative changes in larger numbers is manifestly much reduced by this
experiment, and they may be expected to form a very small proportion of
the culture.
[476] The second question which arose from the above result was this.
Could the mutation be repeated? Was it to be ascribed to some latent
cause which might be operative more than once? Was there some hidden
tendency to mutation, which, ordinarily weak, was strengthened in my
cultures by some unknown influence? Was the observed mutation to be
explained by a common cause with the other cases recorded by
field-observations? To answer this question I had only to continue my
experiment, excluding the mutated individuals from any intercrossing
with their brethren. To this end I saved the seeds from duly isolated
groups in different years and sowed them at different times. For various
causes I was not prepared to have large cultures from these seeds, but
notwithstanding this, the mutation repeated itself. In one instance I
obtained two, in another, one peloric plant with exclusively
many-spurred flowers. As is easily understood, these were related as
"nieces" to the first observed mutants. They originated in quite the
same way, by a sudden leap, without any preparation and without any
intermediate steps.
Mutation is proved by this experience to be of an iterative nature. It
is the expression of some concealed condition, or as it is generally
[477] called, of some hidden tendency. The real nature of this state of
the hereditary qualities is as yet wholly unknown. It would not be safe
to formulate further conclusions before the evidence offered by the
evening-primroses is considered.
Thirdly, the question arises, whether the mutation is complete, not only
as to the morphologic character, but also as to the hereditary
constitution of the mutated individuals. But here unfortunately the high
degree of sterility of the peloric plants, as previously noted, makes
the experimental evidence a thing of great difficulty. During the course
of several years I isolated and planted together the peloric individuals
already mentioned, all in all some twenty plants. Each individual was
nearly absolutely sterile when treated with its own pollen, and the aid
of insects was of no avail. I intercrossed my plants artificially, and
pollinated more than a thousand flowers. Not a single one gave a normal
fruit, but some small and nearly rudimentary capsules were produced,
bearing a few seeds. From these I had 119 flowering plants, out of which
106 were peloric and 13 one-spurred. The great majority, some 90%, were
thus shown to be true to their new type. Whether the 10% reverting ones
were truly atavists, or whether they were [478] only vicinists, caused
by stray pollen grains from another culture, cannot of course be decided
with sufficient certitude.
Here I might refer to the observations concerning the invisible
dimorphous state of the flowers of the normal toad-flax. Individuals of
the same type, when fertilized with each other, are nearly, but not
absolutely, sterile. The yield of seeds of my peloric plants agrees
fairly well with the harvest which I have obtained from some of the
nearly sterile pairs of individuals in my former trial. Hence the
suggestion is forced upon us that perhaps, owing to some unknown cause,
all the peloric individuals of my experiment belonged to one and the
same type, and were sterile for this reason only. If this is true, then
it is to be presumed that all previous investigators have met the same
condition, each having at hand only one of the two required types. And
this discussion has the further advantage of showing the way, in which
perhaps a full and constant race of peloric toad-flaxes may be obtained.
Two individuals of different type are required to start from. They seem
as yet never to have arisen from one group of mutations. But if it were
possible to combine the products of two mutations obtained in different
countries and under different conditions, there would be a chance [479]
that they might belong to the supposed opposite types, and thus be
fertile with one another. My peloric plants are still available, and the
occurrence of this form elsewhere would give material for a successful
experiment. The probability thereof is enhanced by the experience that
my peloric plants bear large capsules and a rich harvest of seeds when
fertilized from plants of the normal one-spurred race, while they remain
nearly wholly barren by artificial fertilization with others. I suppose
that they are infertile with the normal toad-flaxes of their own sexual
disposition, but fertile with those of the opposite constitution. At all
events the fact that they may bear abundant seed when properly
pollinated is an indication of successful experiments on the possibility
of gaining a hereditary race with exclusively peloric flowers. And such
a race would be a distinct gain for sundry physiologic inquiries, and
perhaps not wholly destitute of value from an horticultural point of
view.
Returning now to the often recorded occurrence of peloric toad-flaxes in
the wild state and recalling our discussion about the improbability of a
dispersion from one locality to another by seed, and the probability of
independent origin for most of these cases, we are confronted with the
conception that a latent [480] tendency to mutation must be universally
present in the whole species. Another observation, although it is of a
negative character, gains in importance from this point of view. I refer
to the total lack of intermediate steps between normal and peloric
individuals. If such links had ordinarily been produced previous to the
purely peloric state they would no doubt have been observed from time to
time. This is so much the more probable as _Linaria_ is a perennial
herb, and the ancestors of a mutation might still be in a flowering
condition together with their divergent offspring. But no such
intermediates are on record. The peloric toad-flaxes are, as a rule,
found surrounded by the normal type, but without intergrading forms.
This discontinuity has already been insisted upon by Hofmeister and
others, even at the time when the theory of descent was most under
discussion, and any link would surely have been produced as a proof of a
slow and continuous change. But no such proof has been found, and the
conclusion seems admissible that the mutation of toad-flaxes ordinarily,
if not universally, takes place by a sudden step. Our experiment may
simply be considered as a thoroughly controlled instance of an often
recurring phenomenon. It teaches us how, in the [481] main, the peloric
mutations must be assumed to proceed.
This conception may still be broadened. We may include in it all similar
occurrences, in allied and other species. There is hardly a limit to the
possibilities which are opened up by this experience. But it will be
well to refrain from hazardous theorizing, and consider only those cases
which may be regarded as exact repetitions of the same phenomenon and of
which our culture is one of the most recent instances on record. We will
limit ourselves to the probable origin of peloric variations at large,
of which little is known, but some evidence may be derived from the
recorded facts. Only one case can be said to be directly analogous to
our observations.
This refers to the peloric race of the common snapdragon, or
_Antirrhinum majus_ of our gardens. It is known to produce peloric races
from time to time in the same way as does the toadflax. But the
snapdragon is self-fertile and so is its peloric variety. Some cases are
relatively old, and some of them have been recorded and in part observed
by Darwin. Whence they have sprung and in what manner they were
produced, seems never to have been noted. Others are of later origin,
and among these one or two varieties have been accidentally produced
[482] in the nursery of Mr. Chr. Lorenz in Erfurt, and are now for sale,
the seeds being guaranteed to yield a large proportion of peloric
individuals. The peloric form in this case appeared at once, but was not
isolated, and was left free to visiting insects, which of course crossed
it with the surrounding varieties. Without doubt the existence of two
color-varieties of the peloric type, one of a very dark red, indicating
the "Black prince" variety as the pollen-parent, and the other with a
white tube of the corolla, recalling the form known as "Delila," is due
to these crossings. I had last year (1903) a large lot of plants, partly
normal and partly peloric, but evidently of hybrid origin, from seeds
from this nursery, showing moreover all intermediate steps between
nearly wholly peloric individuals and apparently normal ones. I have
saved the seeds of the isolated types and before seeing the flowers of
their offspring, nothing can be said about the purity and constancy of
the type, when freed from hybrid admixtures. The peloric snapdragon has
five small unequal spurs at the base of its long tube, and in this
respect agrees with the peloric toad-flax.
Other pelories are terminal and quite regular, and occur in some species
of _Linaria_, where I observed them in _Linaria dalmatica_. The [483]
terminal flowers of many branches were large and beautifully peloric,
bearing five long and equal spurs. About their origin and inheritance
nothing is known.
A most curious terminal pelory is that of the common foxglove or
_Digitalis purpurea_. As we have seen in a previous lecture, it is an
old variety. It was described and figured for the first time by Vrolik
of Amsterdam, and the original specimens of his plates are still to be
seen in the collections of the botanic garden of that university. Since
his time it has been propagated by seed as a commercial variety, and may
be easily obtained. The terminal flower of the central stem and those of
the branches only are affected, all other flowers being wholly normal.
Almost always it is accompanied by other deviations, among which a
marked increase of the number of the parts of the corolla and other
whorls is the most striking. Likewise supernumerary petals on the outer
side of the corolla, and a production of a bud in the center of the
capsule may be often met with. This bud as a rule grows out after the
fading away of the flower, bursting through the green carpels of the
unripe fruit, and producing ordinarily a secondary raceme of flowers.
This raceme is a weak but exact repetition of the first, bearing
symmetrical foxgloves all [484] along and terminating in a peloric
structure. On the branches these anomalies are more or less reduced,
according to the strength of the branch, and conforming to the rule of
periodicity, given in our lecture on the "five-leaved" clover. Through
all this diminution the peloric type remains unchanged and therefore
becomes so much the purer, the weaker the branches on which it stands.
I am not sure whether such peloric flowers have ever been purely
pollinated and their seed saved separately, but I have often observed
that the race comes pure from the seed of the zygomorphic flowers. It is
as yet doubtful whether it is a half race or a double race, and whether
it might be purified and strengthened by artificial selection. Perhaps
the determination of the hereditary percentage described when dealing
with the tricotyls might give the clue to the acquisition of a higher
specialized race. The variety is old and widely disseminated, but must
be subjected to quite a number of additional experiments before it can
be said to be sufficiently understood.
The most widespread peloric variety is that of _Gloxinia_. It has erect
instead of drooping flowers; and with the changed position the structure
is also changed. Like other pelories it has five equal stamens instead
of four unequal [485] ones, and a corolla with five equal segments
instead of an upper and a lower lip. It shows the peloric condition in
all of its flowers and is often combined with a small increase of the
number of the parts of the whorls. It is for sale under the name of
_erecta_, and may be had in a wide range of color-types. It seems to be
quite constant from seed.
Many other instances of peloric flowers are on record. Indian cress or
_Tropaeolum majus_ loses the spur in some double varieties and with it
most of its symmetrical structure; it seems to be considered justly as a
peloric malformation. Other species produce such anomalies only from
time to time and nothing is known about their hereditary tendency. One
of the most curious instances is the terminal flower of the raceme of
the common laburnum, which loses its whole papilionaceous character and
becomes as regularly quinate as a common buttercup.
Some families are more liable to pelorism than others. Obviously all the
groups, the flowers of which are not symmetrical, are to be excluded.
But then we find that labiates and their allies among the dicotyledonous
plants, and orchids among the monocotyledonous ones are especially
subjected to this alteration. In both groups many genera and a long list
of species [486] could be quoted as proof. The family of the labiates
seems to be essentially rich in terminal pelories, as for instance in
the wild sage or _Salvia_ and the dead-nettle or _Lamium_. Here the
pelories have long and straight corolla-tubes, which are terminated by a
whorl of four or five segments. Such forms often occur in the wild state
and seem to have a geographic distribution as narrowly circumscribed as
in the case of many small species. Those of the labiates chiefly belong
to southern Europe and are unknown at least in some parts of the other
countries. On the contrary terminal pelories of _Scrophularia nodosa_
are met with from time to time in Holland. Such facts clearly point to a
common origin, and as only the terminal flowers are affected by the
malformation, the fertility of the whole plant is evidently not
seriously infringed upon.
Before leaving the labiates, we may cite a curious instance of pelorism
in the toad-flax, which is quite different from the ordinary peloric
variety. This latter may be considered from a morphologic standpoint to
be owing to a five-fold repetition of the middle part of the underlip.
This conception would at once explain the occurrence of five spurs and
of the orange border all around the corolla-tube. We might readily
imagine that any other of the five [487] parts of the corolla could be
repeated five-fold, in which case there would be no spur, and no orange
hue on the upper corolla-ring. Such forms really occur, though they seem
to be more rare than the five-spurred pelories. Very little is known
about their frequency and hereditary qualities.
Orchids include a large number of peloric monstrosities and moreover a
wild pelory which is systematically described not only as a separate
species but even as a new genus. It bears the name of _Uropedium
lindenii_, and is so closely related to _Cypripedium caudatum_ that many
authors take it for the peloric variety of this plant. It occurs in the
wild state in some parts of Mexico, where the _Cypripedium_ also grows.
Its claims to be a separate genus are lessened by the somewhat monstrous
condition of the sexual organs, which are described as quite abnormal.
But here also, intermediates are lacking, and this fact points to a
sudden origin.
Many cases of pelorism afford promising material for further studies of
experimental mutations. The peloric toad-flax is only the prototype of
what may be expected in other cases. No opportunity should be lost to
increase the as yet too scanty, evidence on this point.
[488]
LECTURE XVII
THE PRODUCTION OF DOUBLE FLOWERS
Mutations occur as often among cultivated plants as among those in the
wild state. Garden flowers are known to vary markedly. Much of their
variability, however, is due to hybridism, and the combination of
characters previously separate has a value for the breeder nearly equal
the production of really new qualities. Nevertheless there is no doubt
that some new characters appear from time to time.
In a previous lecture we have seen that varietal characters have many
features in common. One of them is their frequent recurrence both in the
same and in other, often very distantly related, species. This
recurrence is an important factor in the choice of the material for an
experimental investigation of the nature of mutations.
Some varieties are reputed to occur more often and more readily than
others. White-colored varieties, though so very common, seem for the
most part to be of ancient date, but only few [489] have a known origin,
however. Without any doubt many of them have been found in a wild state
and were introduced into culture. On the other hand double flowers are
exceedingly rare in the wild state, and even a slight indication of a
tendency towards doubling, the stray petaloid stamens, are only rarely
observed growing wild. In cultivation, however, double flowers are of
frequent occurrence; hence the conclusion that they have been produced
in gardens and nurseries more frequently than perhaps any other type of
variety.
In the beginning of my experimental work I cherished the hope of being
able to produce a white variety. My experiments, however, have not been
successful, and so I have given them up temporarily. Much better chances
for a new double variety seemed to exist, and my endeavors in this
direction have finally been crowned with success.
For this reason I propose to deal now with the production of double
flowers, to inquire what is on record about them in horticultural
literature, and to give a full description of the origin thereof in an
instance which it was my good fortune to observe in my garden.
Of course the historical part is only a hasty survey of the question and
will only give such evidence as may enable us to get an idea of the
[490] chances of success for the experimental worker. In the second half
of the seventeenth century (1671), my countryman, Abraham Munting,
published a large book on garden plants with many beautiful figures. It
is called "Waare Oeffeninge der Planters," or "True Exercises With
Plants." The descriptions pertain to ordinary typical species in greater
part, but garden varieties receive special attention. Among these a long
list of double flowers are to be seen. Double varieties of poppies,
liverleaf (Hepatica), wallflowers (_Cheiranthus_), violets, _Caltha_,
_Althaea_, _Colchicum_, and periwinkles (_Vinca_), and a great many
other common flowers were already in cultivation at that time.
Other double forms have been since added. Many have been introduced from
Japan, especially the Japanese marigold, _Chrysanthemum indicum_. Others
have been derived from Mexico, as for instance the double zinnias. The
single dahlias only seem to have been originally known to the
inhabitants of Mexico. They were introduced into Spain at about 1789,
and the first double ones were produced in Louvain, Belgium, in 1814.
The method of their origin has not been described, and probably escaped
the originators themselves. But in historical records we find the
curious statement that it took place after three years' work. This
indicates [491] a distinct plan, and the possibility of carrying it to a
practical conclusion within a few years' time.
Something more is known about other cases. Garden anemones, _Anemone
coronaria_, are said to have become double in the first half of the last
century in an English nursery. The owner, Williamson, observing in his
beds a flower with a single broadened stamen, saved its seeds
separately, and in the next generations procured beautifully filled
flowers. These he afterwards had crossed by bees with a number of
colored varieties, and in this way succeeded in producing many new
double types of anemone.
The first double petunia is known to have suddenly and accidentally
arisen from ordinary seed in a private garden at Lyons about 1855. From
this one plant all double races and-varieties have been derived by
natural and partly by artificial crosses. Carriere, who reported this
fact, added that likewise other species were known at that time to
produce new double varieties rapidly. The double fuchsias originated
about the same time (1854) and ten years later the range of double
varieties of this plant had become so large that Carriere found it
impossible to enumerate all of them.
Double carnations seem to be relatively old, double corn-flowers and
double blue-bells being [492] of a later period. A long list could
easily be made, to show that during the whole history of horticulture
double varieties have arisen from time to time. As far as we can judge,
such appearances have been isolated and sudden. Sometimes they sprang
into existence in the full display of their beauty, but most commonly
they showed themselves for the first time, exhibiting only spare
supernumerary petals. Whenever such sports were worked up, a few years
sufficed to reach the entire development of the new varietal attribute.
From this superficial survey of historical facts, the inference is
forced upon us that the chance of producing a new double variety is good
enough to justify the attempt. It has frequently succeeded for practical
purposes, why should it not succeed as well for purely scientific
investigation? At all events the type recommends itself to the student
of nature, both on account of its frequency, and of the apparent
insignificance of the first step, combined with the possibility of
rapidly working up from this small beginning of one superfluous petal
towards the highest degree of duplication.
Compared with the tedious experimental production of the peloric
toad-flax, the attempt to produce a double flower has a distinct
attraction. The peloric toad-flax is nothing new; the [493] experiment
was only a repetition of what presumably takes place often within the
same species. To attempt to produce a double variety we may choose any
species, and of course should select one which as yet has not been known
to produce double flowers. By doing so we will, if we succeed, produce
something new. Of course, it does not matter whether the new variety has
an horticultural interest or not, and it seems preferable to choose a
wild or little cultivated species, to be quite sure that the variety in
question is not already in existence. Finally the prospect of success
seems to be enhanced if a species is chosen, the nearest allies of which
are known to have produced double flowers.
For these reasons and others I chose for my experiment the
corn-marigold, or _Chrysanthemum segetum_. It is also called the golden
cornflower. In the wheat and rye fields of central Europe it associates
with the blue-bottle or blue corn-flower. It is sometimes cultivated and
the seeds are offered for sale by many nurserymen. It has a cultivated
variety, called _grandiflorum_, which is esteemed for its brilliancy and
long succession of golden bloom. This variety has larger flower-heads,
surrounded with a fuller border of ray-florets. The species belongs to a
genus many species of which have produced [494] double varieties. One of
them is the Japanese marigold, others are the _carinatum_ and the
_imbricatum_ species. Nearly allied are quite a number of garden-plants
with double flower-heads, among which are the double camomiles.
My attention was first drawn to the structure of the heads and
especially to the number of the ray-florets of the corn-marigold. The
species appertains to that group of composites which have a head of
small tubular florets surrounded by a broad border of rays. These rays,
when counted, are observed to occur in definite numbers, which are
connected with each other by a formula, known as "the series" of Braun
and Schimper. In this formula, which commences with 1 and 2, each number
is equal to the sum of the two foregoing figures. Thus 5, 8 and 13 are
very frequent occurrences, and the following number, 21, is a most
general one for apparently full rays, such as in daisies, camomiles,
_Arnica_ and many other wild and cultivated species.
These numbers are not at all constant. They are only the averages,
around which the real numbers fluctuate. There may even be an
overlapping of the extremes, since the fluctuation around 13 may even go
beyond 8 and 21, and so on. But such extremes are only found in stray
flowers, occurring on the same [495] individuals with the lesser degrees
of deviation.
Now the marigold averages 13, and the _grandiflorum_ 21 rays. The wild
species is pure in this respect, but the garden-variety is not. The
seeds which are offered for sale usually contain a mixture of both forms
and their hybrids. So I had to isolate the pure types from this mixture
and to ascertain their constancy and mutual independency. To this end I
isolated from the mixture first the 13-rayed, and afterwards the
21-rayed types. As the marigolds are not sufficiently self-fertile, and
are not easily pollinated artificially, it seemed impossible to carry on
these two experiments at the same time and in the same garden. I devoted
the first three years to the lower form, isolated some individuals with
12-13 rays out of the mixture of 1892 and counted the ray-florets on the
terminal head of every plant of the ensuing generation next year. I
cultivated and counted in this way above 150 individuals and found an
average of exactly 13 with comparatively few individuals displaying 14
or only 12 rays, and with the remainder of the plants grouped
symmetrically around this average. I continued the experiment for still
another year and found the same group of figures. I was then satisfied
as to the purity of the isolated strain. Next year I sowed a new mixture
in [496] order to isolate the reputed pure _grandiflorum_ type. During
the beginning of the flowering period I ruthlessly threw away all plants
displaying less than 21 rays in the first or terminal head. But this
selection was not to be considered as complete, because the 13-rayed
race may eventually transgress its boundary and come over to the 21 and
more. This made a second selection necessary. On the selected plants all
the secondary heads were inspected and their ray-florets counted. Some
individuals showed an average of about 13 and were destroyed. Others
gave doubtful figures and were likewise eliminated, and only 6 out of a
lot of nearly 300 flowering plants reached an average of 21 for all of
the flowers.
Our summer is a short one, compared with the long and beautiful summer
of California, and it was too late to cut off the faded and the open
flowers, and await new ones, which might be purely fertilized after the
destruction of all minor plants. So I had to gather the seed from
flowers, which might have been partially fertilized by the wrong pollen.
This however, is not so great a drawback in selection experiments as
might be supposed at first sight. The selection of the following year is
sure to eliminate the offspring of such impure parentage.
[497] A far more important principle is that of the hereditary
percentage, already discussed in our lecture on the selection of
monstrosities. In our present case it had to be applied only to the six
selected plants of 1895. To this end the seeds of each of them were sown
separately, the ray-florets of the terminal heads of each of the new
generation were counted, and curves and averages were made up for the
six groups. Five of them gave proof of still being mixtures and were
wholly rejected. The children of the sixth parent, however, formed a
group of uniform constitution, all fluctuating around the desired
average of 21. All in all the terminal heads of over 1,500 plants have
been subjected to the somewhat tedious work of counting their
ray-florets. And this not in the laboratory, but in the garden, without
cutting them off. Otherwise it would obviously have been impossible to
recognize the best plants for preservation. I chose only two plants
which in addition recommended themselves by the average number of rays
on their secondary heads, sowed their seeds next year separately and
compared the numerical constitution of their offspring. Both groups
averaged 21 and were distributed very symmetrically around this mean.
This result [498] showed that no further selection could be of any
avail, and that I had succeeded in purifying the 21-rayed _grandiflorum_
variety.
It is from this _grandiflorum_ that I have finally produced my double
variety. In the year 1896 I selected from among the above quoted 1,500
plants, 500 with terminal heads bearing 21 or more rays. On these I
counted the rays of all the secondary heads about the middle of August
(1896) and found that they had, as a rule, retrograded to lower figures.
On many thousands of heads only two were found having 22 rays. All
others had the average number of 21 or even less. I isolated the
individual which bore these two heads, allowed them to be fertilized by
insects with the pollen of some of the best plants of the same group,
but destroyed the remainder.
This single exceptional plant has been the starting point of my double
variety. It was not remarkable for its terminal head, which exhibited
the average number of rays of the 21-rayed race. Nor was it
distinguished by the average figure for all its heads. It was only
selected because it was the one plant which had some secondary heads
with one ray more than all the others. This indication was very slight,
and could not have been detected save by the counting of the rays of
thousands of heads.
[499] But the rarity of the anomaly was exactly the indication wanted,
and the same deviation would have had no signification whatever, had it
occurred in a group fluctuating symmetrically around the average figure.
On the other hand, the observed anomaly was only an indication, and no
guarantee of future developments.
Here it should be remarked that the indication alluded to was not the
appearance of the expected character of doubling in ever so slight a
measure. It was only a guide to be followed in further work. The real
character of double flower-heads among composites lies in the production
of rays on the disk. No increase of the number of the outer rays can
have the same significance. A hasty inspection of double flower heads
may convey the idea that all rays are arranged around a little central
cluster of disk-florets, the remainder of the original disk-florets but
a closer investigation will always reveal the fallacy of this
conclusion. Hidden between the inner rays, and covered by them, lie the
little tubular and fertile florets everywhere on the disk. They may not
be easily seen, but if the supernumerary rays are pulled out, the disk
may be seen to bear numerous small florets at intervals. But these
intervals are not at all numerous, showing thereby that only a
relatively small number of tubes has been [500] converted into rays.
This conversion is obviously the true mark of the doubling, and before
traces of it are found, no assertion whatever can be given as to the
issue of the pedigree experiment.
Three more years were required before this first, but decisive trace was
discovered. During these years I subjected my strain to the same sharp
selection as has already been described. The chosen ancestor of the race
had flowered in 1896, and the next year I sowed its seeds only. From
this generation I chose the one plant with the largest number of rays in
its terminal head, and repeated this in the following year.
The consequence was that the average number of rays increased rapidly,
and with it the absolute maximum of the whole strain. The average came
up from 21 to 34. Brighter and brighter crowns of the yellow rays
improved my race, until it became difficult and very time consuming to
count all the large rays of the borders. The largest numbers determined
in the succeeding generations increased by leaps from 21 to 34 in the
first year, and thence to 48 and 66 in the two succeeding summers. Every
year I was able to save enough seed from the very best plant and to use
it only for the continuance of the race. Before the selected plants were
allowed to open the flowers from which the seed [501] was to be
gathered, nearly the whole remaining culture was exterminated, excepting
only some of the best examples, in order to have the required material
for cross-pollination by insects. Each new generation was thereby as
sharply selected as possible with regard to both parents.
All flower-heads were of course closely inspected. Not the slightest
indication of real doubling was discovered, even in the summer of 1899
in the fourth generation of my selected race. But among the best the new
character suddenly made its appearance. It was at the commencement of
September (1899), too late to admit of the seeds ripening before winter.
An inspection of the younger heads was made, which revealed three heads
with some few rays in the midst of the disk on one plant, the result of
the efforts of four years. Had the germ of the mutation lain hidden
through all this time? Had it been present, though dormant in the
original sample of seed? Or had an entirely new creation taken place
during my continuous endeavors? Perhaps as their more or less immediate
result? It is obviously impossible to answer these questions, before
further and similar experiments shall have been performed, bringing to
light other details that will enable us to reach a more definite
conclusion.
[502] The fact that the origination of such forms is accessible to
direct investigation is proven quite independently of all further
considerations. The new variety came into existence at once. The leap
may have been made by the ancestor of the year 1895, or by the plant of
1899, which showed the first central rays, or the sport may have been
gradually built up during those four years. In each case there was a
leap, contrasting with the view which claims a very long succession of
years for the development of every new character.
Having discovered this first trace of doubling, it was to be expected
that the new variety would be at once as pure and as rich as other
double composites usually are. Some effect of the crossing with the
other seed-bearing individuals might still disturb this uniformity in
the following year, but another year's work would eliminate even this
source of impurity.
These two years have given the expected result. The average number of
the rays, which had already arisen from 13 to 34 now at once came up to
47 and 55, the last figure being the sum of 21 and 34 and therefore the
probable uttermost limit to be reached before absolute doubling. The
maximum numbers came as high as 100 in 1900, and reached even 200 in
1901. Such heads are as completely double as are the [503] brightest
heads of the most beautiful double commercial varieties of composites.
Even the best white camomiles (_Chrysanthemum inodorum_) and the
gold-flowers or garden-marigolds (_Calendula officinalis_) do not come
nearer to purity since they always have scores of little tubular florets
between the rays on their disks.
Real atavists or real reversionists were seen no more after the first
purification of the race. I have continued my culture and secured last
summer (1903) as many and as completely doubled heads as previously. The
race has at once become permanent and constant. It has of course a wide
range of fluctuating variability, but the lower limit has been worked up
to about 34 rays, a figure never reached by the _grandiflorum_ parent,
from which my new variety is thus sharply separated.
Unfortunately the best flowers and even the best individuals of my race
are wholly barren. Selection has reached its practical limit. Seeds must
be saved from less dense heads, and no way has been found of avoiding
it. The ray-florets are sterile, even in the wild species, and when
growing in somewhat large numbers on the disk, they conceal the fertile
flowers from the visiting insects, and cause them also to be sterile.
The same is the case with the best cultivated forms. Their showiest
individuals are [504] barren, and incapable of the reproduction of the
race.
This last is therefore, of necessity, always continued by means of
individuals whose deviation from the mean average is the least. But in
many cases the varieties are so highly differentiated that selection has
become quite superfluous for practical purposes. I have already
discussed the question as to the actual moment, in which the change of
the _grandiflorum_ variety into the new _plenum_ form must be assumed to
have taken place. In this respect some stress is to be laid on the fact
that the improvement through selection has been gradual and continuous,
though very rapid from the first moment. But with the appearance of the
first stray rays within the disk, this continuity suddenly changed. All
the children of this original mutated plant showed the new character,
the rays within the disk, without exception. Not on all the heads, nor
even on the majority of the heads on some individuals, but on some heads
all gave clear proof of the possession of the new attribute. This was
present in all the representatives of the new race, and had never been
seen in any of their parents and grandparents. Here there was evidently
a sudden leap, at least in the external form of the plants. And it seems
to me to be the most simple conception, [505] that this visible leap
directly corresponded to that inner change, which brought about the
complete inheritability of the new peculiarity. It is very interesting
to observe how completely my experience agrees with the results of the
observations of breeders at large. No doubt a comparison is difficult,
and the circumstances are not adequate to a close study.
Isolation and selection have been applied commonly only so far as was
consistent with the requirements of practical horticulture, and of
course a determination of the hereditary percentage was never made. The
disregard of this feature made necessary a greater length of time and a
larger number of generations to bring about the desired changes.
Notwithstanding this, however, it has been seen that double varieties
are produced suddenly. This may have occurred unexpectedly or after a
few years' effort toward the end desired. Whether this sudden appearance
is the consequence of a single internal differentiating step, or of the
rapid succession of lesser changes, cannot yet be made out. The extreme
variability of double flowers and the chance of their appearance with
only slight indications of the previous petaloid alterations of a few
stamens may often result in their origin being overlooked, while
subsequent generations may come in for full notice. [506] In the greater
number of cases recorded it remains doubtful whether the work said to be
done to obtain a new double variety was done before the appearance of
these preliminary indications or afterward.
In the first case, it would correspond with our selection of large
numbers of florets in the outer rays, in the second however, with the
ordinary purification of new races from hybrid mixtures.
In scientific selection-experiments such crosses are of course avoided,
and the process of purification is unnecessary, even as in the
_Chrysanthemum_ culture. The first generation succeeding the original
plant with disk-rays was in this respect wholly uniform and true to the
new type.
In practice the work does not start from such slight indications, and is
done with no other purpose in view than to produce double flowers in
species in which they did not already exist. Therefore it is of the
highest importance to know the methods used and the chances of success.
Unfortunately the evidence is very scanty on both points.
Lindley and other writers, on horticultural theory and practice assert
that a large amount of nourishment tends to produce double flowers,
while a culture under normal conditions, [507] even if the plants are
very strong and healthy, has no such effect. But even here it remains
doubtful whether it applies to the period before or after the internal
mutation. On the other hand success is not at all to be relied upon, nor
is the work to be regarded as easy. The instances of double flowers said
to be obtainable at will, are too rare in comparison with the number of
cases, where the first indication of them was found accidentally.
Leaving all these doubtful points, which will have to be cleared up by
further scientific investigation, the high degree of variability
requires further discussion. It may be considered from three different
points of view according to the limit of the deviation from the average,
to the dependency on external conditions and to periodicity. It seems
best to take up the last two points first.
On a visit to a nursery at Erfurt I once inspected an experiment with a
new double variety of the common blue-bottle or blue corn-flower. The
plants were dependent on the weather to a high degree. Bad weather
increased the number of poorly filled flower-heads, while warm and sunny
days were productive of beautiful double flowers. The heads that are
borne by strong branches have a greater tendency to become double than
those of the weaker ones, [508] and towards the autumn, when all those
of the first group are faded away, and only a weak though large section
of the heads is still flowering, the whole aspect of the variety
gradually retrogrades. The same law of dependency and periodicity is
prevalent everywhere. In my own cultures of the improved field-marigold
I have observed it frequently. The number of the ray-florets may be
considered as a direct response to nourishment, both when this is
determined by external circumstances, and when it depends on the
particular strength of the branch, which bears the head in question. It
is a case exactly similar to that of the supernumerary carpels of the
pistilloid poppy, and the deductions arrived at with that variety may be
applied directly to double flowers.
This dependency upon nourishment is of high practical importance in
combination with the usual effect of the doubling which makes the
flowers sterile. It is a general rule that the most perfect flowers do
not produce seed. At the height of the flowering period the external
circumstances are the most favorable, and the flowering branches still
constitute the stronger axes of the plants. Hence we may infer that
sterility will prevail precisely in this period. Many varieties are
known to yield only seeds from the very last flowers, as for instance
some [509] double begonias. Others bear only seed on their weaker
lateral branches, as the double camomile, or become fertile only towards
the fall, as is often the case with the above quoted Erfurt variety of
the blue-bottle. As far as I have been able to ascertain, such seeds are
quite adequate for the reproduction and perpetuation of the double
varieties, but the question whether there are differences between the
seeds of the more or less double flowers of the same plants still
remains open. It is very probable, from a theoretical point of view,
that such differences exist, but perhaps they are so slight, as to have
practically no bearing on the question.
On the ground of their wide range of variability, the double varieties
must be regarded as pertaining to the group of ever-sporting forms. On
one side they fluctuate in the direction towards such petalomanous
flowers as are borne by the stocks and others, which we have previously
discussed. Here no trace of the fertile organs is left. But this extreme
is never reached by petaloid double flowers. A gap remains which, often
overlooked, always exists, and which sharply separates the two types. On
the other hand the alteration of the stamens gradually relapses to
perfectly single flowers. Here the analogy with the pistillody of the
poppies and with the "five-leaved" clover is obvious.
[510] This conception of the inner nature of double flowers explains the
fact that the varietal mark is seldom seen to be complete throughout
larger groups of individuals, providing these have not been already
selected by this character. _Tagetes africana_ is liable to produce some
poorly filled specimens, and some double varieties of carnations are
offered for sale with the note that the seed yields only 80% of doubles.
With _Chrysanthemum coronarium_ and blue-bottles this figure is often
announced to be only about 50%. No doubt it is partly due to impurities,
caused by vicinism, but it is obviously improbable that the effect of
these impurities should be so large.
Some cases of partial reversion may be interpreted in the same way.
Among the garden anemones, _Anemone coronaria_, there is a variety
called the "Bride," on account of its pure white dowers. It is for sale
with single and with double flowers, and these two forms are known to
sport into one another, although they are multiplied in the vegetative
way. Such cases are known to be of quite ordinary occurrence. Of course
such sports must be considered as partial, and the same stem may bear
both types of flowers. It even happens that some particular flower is
partly double and partly single. Mr. Krelage, of Haarlem, had the
kindness to [511] send me such a curious flower. One half of it was
completely double, while the other half was entirely single, bearing
normal and fertile stamens in the ordinary number.
The same halfway doubling is recorded to occur among composites
sometimes, and from the same source I possess in my collection a head of
_Pyrethrum roseum_, bearing on half of its disk elongated corolla tubes,
and on the other half the small disk-florets of the typical species.
It is a current belief, that varieties are improved by continued
culture. I have never been able to ascertain the grounds on which this
conviction rests. It may be referred either to the purity of the race or
to the complete development of the varietal character. In the first case
it is a question of hybrid mixtures from which many young varieties must
be freed before being placed on the market. But as we have already seen
in a former lecture, this requires only three or four years, and
afterwards the degree of purity is kept up to the point which proves to
be the most suitable for practical purposes. The complete development of
the varietal character is a question restricted to ever-sporting
varieties, since in white flowers and other constant varieties this
degree is variable in a very small and unimportant measure. [512] Hence
the double flowers seem to afford a very good example for this
discussion.
It can be decided by two facts. First by a consideration of the oldest
double varieties, and secondly by that of the very youngest. Are the
older ones now in a better condition than at the outset? Have they
really been gradually improved during the centuries of their existence?
Obviously this can only be answered by a comparison of the figures given
by older writers, with the varieties as they are now in culture.
Munting's drawings and descriptions are now nearly two centuries and a
half old, but I do not find any real difference between his double
varieties and their present representatives. So it is in other cases in
which improvements by crossing or the introduction of new forms does not
vitiate the evidence. Double varieties, as a rule, are exactly the same
now, as they were at the time of their first introduction.
If this were otherwise one would expect that young double varieties
should in the main display only slight grades of the anomaly, and that
they would require centuries to reach their full development. Nothing of
the kind is on record. On the contrary the newest double sorts are said
to be not only equal to their predecessors, but to excel them. As a rule
such claims may be exaggerated, but not to any great extent. [513] This
is proven in the simplest way by the result of our own experiment.
In the double field-marigold we have the very first generation of a
variety of pure and not hybrid origin. It shows the new attribute in its
full development. It has flower-heads nearly as completely filled as the
best double varieties of allied cultivated composites. In the second
generation it reached heads with 200 rays each, and much larger numbers
will seldom be seen in older species on heads of equal size. I have
compared my novelty with the choicest double camomiles and others, but
failed to discover any real difference. Improvement of the variety
developed in the experiments carried on by myself seems to be excluded
by the fact that it comes into conflict with the same difficulty that
confronts the older cultivated species, viz.: the increasing sterility
of the race.
It is perfectly evident that this double marigold is now quite constant.
Continuously varying about a fixed average it may live through
centuries, but the mean and the limits will always remain the same, as
in the case of the ever-sporting varieties.
Throughout this lecture I have spoken of double flowers and double
flower-heads of composites as of one single group. They are as nearly
related from the hereditary point of [514] view, as they are divergent
in other respects. It would be superfluous to dwell any longer upon the
difference between heads and flowers. But it is as well to point out,
that the term double flowers indicates a motley assemblage of different
phenomena. The hen-and-chicken daisy, and the corresponding variety of
the garden cineraria (_Cineraria cruenta_), are extremes on one side.
The hen-and-chicken type occurs even in other families and is known to
produce most curious anomalies, as with _Scabiosa_, the supernumerary
heads of which may be produced on long stalks and become branched
themselves in the same manner.
Petalody of the stamens is well known to be the ordinary type of
doubling. But it is often accompanied by a multiplication of the organs,
both of the altered stamens and of the petals themselves. This
proliferation may consist in median or in lateral cleavages, and in both
cases the process may be repeated one or more times. It would be quite
superfluous to give more details, which may be gathered from any
morphologic treatise on double flowers. But from the physiologic point
of view all these cases are to be considered as one large group,
complying with previously given definitions of the ever-sporting
varieties. They are very variable and wholly permanent. Obviously this
[515] permanency agrees perfectly with the conception of their sudden
origin.
[516]
LECTURE XVIII
NEW SPECIES OF _OENOTHERA_
In our experiments on the origin of peloric varieties and double flowers
we were guided in the choice of our material by a survey of the evidence
already at hand. We chose the types known to be most commonly produced
anew, either in the wild state or under the conditions of cultivation.
In both instances our novelty was a variety in the ordinary sense of the
word. Our pedigree-culture was mainly an experimental demonstration of
the validity of conclusions, which had previously been deduced from such
observations as can be made after the accidental birth of new forms.
From these facts, and even from these pedigree-experiments, it is
scarcely allowable to draw conclusions as to the origin of real species.
If we want to know how species originate, it is obviously necessary to
have recourse to direct observation. The question is of the highest
importance, both for the theory of descent, and for our conception of
the real nature of [517] systematic affinities at large. Many authors
have tried to solve it on the ground of comparative studies and of
speculations upon the biologic relations of plants and animals. But in
vain. Contradiction and doubt still reign supreme. All our hopes now
rest on the result of experiments.
Unfortunately such experiments seemed simply impossible a few years ago.
What is to guide us in the choice of the material? The answer may only
be expected from a consideration of elementary species. For it is
obvious that they only can be observed to originate, and that the
systematic species, because they are only artificial groups of lower
unities, can never become the subject of successful experimental
inquiry.
In previous lectures we tried to clear up the differences existing
between nearly related elementary species. We have seen that they affect
all of the attributes of the plants, each of them changing in some
measure all of the organs. Nevertheless they were due to distinct
unities and of the lowest possible degree. Such unit-steps may therefore
be expected to become visible some time or other by artificial means. On
the other hand, mutations as a rule make their appearance in groups, and
there are many systematic species which on close inspection [518] have
been shown to be in reality composite assemblages. Roses and brambles,
hawkweeds and willows are the best known examples. Violets and _Draba
verna_, dandelions and helianthemums and many other instances were dealt
with in previous lectures. Even wheat and barley and corn afford
instances of large groups of elementary species. Formerly mixed in the
fields, they became separated during the last century, and now
constitute constant races, which, for brevity's sake, are dealt with
under the name of varieties.
In such groups of nearly allied forms the single members must evidently
be of common origin. It is not necessary for them to have originated all
in the same place or at the same time. In some cases, as with _Draba
verna_, the present geographic distribution points to a common
birthplace, from whence the various forms may about the same period have
radiated in all directions. The violets on the other hand seem to
include widely diffused original forms, from which branches have started
at different times and in different localities.
The origin of such groups of allied forms must therefore be the object
of our research. Perhaps we might find a whole group, perhaps only part
of it. In my opinion we have the right to assume that if _Draba_ and
violets and [519] others have formerly mutated in this way, other
species must at present be in the same changeable condition. And if
mutations in groups, or such periodic mutations should be the rule, it
is to be premised that these periods recur from time to time, and that
many species must even now be in mutating condition, while others are
not.
It is readily granted that the constant condition of species is the
normal one, and that mutating periods must be the exception. This fact
does not tend to increase our prospect of discovering a species in a
state of mutability. Many species will have to be tested before finding
an instance. On the other hand, a direct trial seems to be the only way
to reach the goal. No such special guides as those that led us to the
choice of pelories and double flowers are available. The only indication
of value is the presumption that a condition of mutability might be
combined with a general state of variability at large, and that groups
of plants of very uniform features might be supposed to be constant in
this respect too. On the contrary, anomalies and deviations if existent
in the members of one strain, or found together in one native locality
of a species, might be considered as an indication in the desired
direction.
Few plants vary in the wild state in such a [520] measure as to give
distinct indications. All have to be given a trial in the garden under
conditions as similar as possible to their natural environments.
Cultivated plants are of course to be excluded. Practically they have
already undergone the experience in question and can not be expected to
change their habits soon enough. Moreover they are often of hybrid
origin. The best way is to experiment with the native plants of one's
own country.
I have made such experiments with some hundred species that grow wild in
Holland. Some were very variable, as for instance, the jointed charlock
(_Raphanus Raphanistrum_) and the narrow-leaved plantain (_Plantago
lanceolata_). Others seemed more uniform, but many species, collected
without showing any malformation, subsequently produced them in my
garden, either on the introduced plants themselves or among their
offspring. From this initial material I have procured a long series of
hereditary races, each with some peculiar anomaly for its special
character. But this result was only a secondary gain, a meager
consolation for the negative fact that no real mutability could be
discovered.
My plants were mostly annuals or biennials, or such perennials as under
adequate treatment might produce flowers and seeds during their [521]
first summer. It would be of no special use to enumerate them. The
negative result does not apply to the species as such, but only to the
individual strain, which I collected and cultivated. Many species, which
are quite constant with us, may be expected to be mutable in other parts
of their range.
Only one of all my tests met my expectations. This species proved to be
in a state of mutation, producing new elementary forms continually, and
it soon became the chief member of my experimental garden. It was one of
the evening primroses.
Several evening-primroses have at different times been introduced into
European gardens from America. From thence they have spread into the
vicinity, becoming common and exhibiting the behavior of indigenous
types. _Oenothera biennis_ was introduced about 1614 from Virginia, or
nearly three centuries ago. _O. muricata_, with small corollas and
narrow leaves, was introduced in the year 1789 by John Hunneman, and _O.
suaveolens_, or sweet-scented primrose, a form very similar to the
_biennis_, about the same time, in 1778, by John Fothergill. This form
is met with in different parts of France, while the _biennis_ and
_muricata_ are very common in the sandy regions of Holland, where I have
observed them for [522] more than 40 years. They are very constant and
have proven so in my experiments. Besides these three species, the
large-flowered evening-primrose, or _Oenothera lamarckiana_, is found in
some localities in Holland and elsewhere. We know little concerning its
origin. It is supposed to have come from America in the same way as its
congeners, but as yet I have not been able to ascertain on what grounds
this supposition rests. As far as I know, it has not been seen growing
wild in this country, though it may have been overlooked. The fact that
the species of this group are subject to many systematic controversies
and are combined by different writers into systematic species in
different ways, being often considered as varieties of one or two types,
easily accounts for it having been overlooked. However, it would be of
great interest to ascertain whether _O. lamarckiana_ yet grows in
America, and whether it is in the same state of mutability here as it is
in Holland.
The large-flowered evening-primrose was also cultivated about the
beginning of the last century in the gardens of the Museum d'Histoire
Naturelle, at Paris, where it was noticed by Lamarck, who at once
distinguished it as an undescribed species. He wrote a complete
description [523] of it and his type specimens are still preserved in
the herbarium of the Museum, where I have compared them with the plants
of my own culture. Shortly afterwards it was renamed by Seringe, in
honor of its eminent discoverer, whose name it now bears. So Lamarck
unconsciously discovered and described himself the plant, which after a
century, was to become the means of an empirical demonstration of his
far-reaching views on the common origin of all living beings.
_Oenothera lamarckiana_ is considered in Europe as a garden-plant, much
prized for parks and ornamental planting. It is cultivated by
seed-merchants and offered for sale. It has escaped from gardens, and
having abundant means for rapid multiplication, has become wild in many
places. As far as I know its known localities are small, and it is to be
presumed that in each of them the plant has escaped separately from
culture. It was in this state that I first met with this beautiful
species.
Lamarck's evening-primrose is a stately plant, with a stout stem,
attaining often a height of 1.6 meters and more. When not crowded the
main stem is surrounded by a large circle of smaller branches, growing
upwards from its base so as often to form a dense bush. These branches
in their turn have numerous lateral [524] branches. Most of them are
crowned with flowers in summer, which regularly succeed each other,
leaving behind them long spikes of young fruits. The flowers are large
and of a bright yellow color, attracting immediate attention, even from
a distance. They open towards evening, as the name indicates, and are
pollinated by humble-bees and moths. On bright days their duration is
confined to one evening, but during cloudy weather they may still be
found open on the following morning. Contrary to their congeners they
are dependent on visiting insects for pollination. _O. biennis_ and _O.
muricata_ have their stigmas in immediate contact with the anthers
within the flower-buds, and as the anthers open in the morning preceding
the evening of the display of the petals, fecundation is usually
accomplished before the insects are let in. But in _O. lamarckiana_ no
such self-fertilization takes place. The stigmas are above the anthers
in the bud, and as the style increases in length at the time of the
opening of the corolla, they are elevated above the anthers and do not
receive the pollen. Ordinarily the flowers remained sterile if not
visited by insects or pollinated by myself, although rare instances of
self-fertilization were seen.
In falling off, the flowers leave behind them a stout ovary with four
cells and a large number [525] of young seeds. The capsule when ripe,
opens at its summit with four valves, and contains often from two to
three hundred seeds. A hundred capsules on the main stem is an average
estimate, and the lateral branches may ripen even still more fruits, by
which a very rapid dissemination is ensured.
This striking species was found in a locality near Hilvers, in the
vicinity of Amsterdam, where it grew in some thousands of individuals.
Ordinarily biennial, it produces rosettes in the first, and stems in the
second year. Both the stems and the rosettes were at once seen to be
highly variable, and soon distinct varieties could be distinguished
among them.
The first discovery of this locality was made in 1886. Afterwards I
visited it many times, often weekly or even daily during the first few
years, and always at least once a year up to the present time. This
stately plant showed the long-sought peculiarity of producing a number
of new species every year. Some of them were observed directly on the
field, either as stems or as rosettes. The latter could be transplanted
into my garden for further observation, and the stems yielded seeds to
be sown under like control. Others were too weak to live a sufficiently
long time in the field. They were discovered by sowing seed from
indifferent plants [526] of the wild locality in the garden. A third and
last method of getting still more new species from the original strain,
was the repetition of the sowing process, by saving and sowing the seed
which ripened on the introduced plants. These various methods have led
to the discovery of over a dozen new types, never previously observed or
described.
Leaving the physiologic side of the relations of these new forms for the
next lecture, it would be profitable to give a short description of the
several novelties. To this end they may be combined under five different
heads, according to their systematic value. The first head includes
those which are evidently to be considered as varieties, in the narrower
sense of the word, as previously given. The second and third heads
indicate the real progressive elementary species, first those which are
as strong as the parent-species, and secondly a group of weaker types,
apparently not destined to be successful. Under the fourth head I shall
include some inconstant forms, and under the last head those that are
organically incomplete.
Of varieties with a negative attribute, or real retrograde varieties, I
have found three, all of them in a flowering condition in the field. I
have given them the names of _laevifolia_, _brevistylis_ and _nanella_.
[527] The _laevifolia_, or smooth-leaved variety, was one of the very
first deviating types found in the original field. This was in the
summer of 1887, seventeen years ago. It formed a little group of plants
growing at some distance from the main body, in the same field. I found
some rosettes and some flowering stems and sowed some seed in the fall.
The variety has been quite constant in the field, neither increasing in
number of individual plants nor changing its place, though now closely
surrounded by other _Lamarckiana_s. In my garden it has proved to be
constant from seed, never reverting to the original _lamarckiana_,
provided intercrossing was excluded.
It is chiefly distinguished from Lamarck's evening-primrose by its
smooth leaves, as the name indicates. The leaves of the original form
show numerous sinuosities in their blades, not at the edge, but anywhere
between the veins. The blade shows numbers of convexities on either
surface, the whole surface being undulated in this manner; it lacks also
the brightness of the ordinary evening-primrose or _Oenothera biennis_.
These undulations are lacking or at least very rare on the leaves of the
new _laevifolia_. Ordinarily they are wholly wanting, but at times
single leaves with slight manifestations of this [528] character may
make their appearance. They warn us that the capacity for such
sinuosities is not wholly lost, but only lies dormant in the new
variety. It is reduced to a latent state, exactly as are the apparently
lost characters of so many ordinary horticultural varieties.
Lacking the undulations, the _laevifolia_ leaves are smooth and bright.
They are a little narrower and more slender than those of the
_lamarckiana_. The convexities and concavities of leaves are said to be
useful in dry seasons, but during wet summers, such as those of the last
few years, they must be considered as very harmful, as they retain some
of the water which falls on the plants, prolonging the action of the
water on the leaves. This is considered by some writers to be of some
utility after slight showers, but was observed to be a source of
weakness during wet weather in my garden, preventing the leaves from
drying. Whether the _laevifolia_ would do better under such
circumstances, remains to be tested.
The flowers of the _laevifolia_ are also in a slight degree different
from those of _lamarckiana_. The yellow color is paler and the petals
are smoother. Later, in the fall, on the weaker side branches these
differences increase. The _laevifolia_ petals become smaller and are
often not emarginated at the apex, becoming ovate [529] instead of
obcordate. This shape is often the most easily recognized and most
striking mark of the variety. In respect to the reproductive organs, the
fertility and abundance of good seed, the _laevifolia_ is by no means
inferior or superior to the original species.
_O. brevistylis_, or the short-styled evening primrose, is the most
curious of all my new forms. It has very short styles, which bring the
stigmas only up to the throat of the calyx tube, instead of upwards of
the anthers. The stigmas themselves are of a different shape, more
flattened and not cylindrical. The pollen falls from the anthers
abundantly on them, and germinates in the ordinary manner.
The ovary which in _lamarckiana_ and in all other new forms is wholly
underneath the calyx-tube, is here only partially so. This tube is
inserted at some distance under its summit. The insertion divides the
ovary into two parts: an upper and a lower one. The upper part is much
reduced in breadth and somewhat attenuated, simulating a prolongation of
the base of the style. The lower part is also reduced, but in another
manner. At the time of flowering it is like the ovary of _lamarckiana_,
neither smaller nor larger. But it is reached by only a very few
pollen-tubes, and is therefore always incompletely fertilized. It does
[530] not fall off after the fading away of the flower, as unfertilized
ovaries usually do; neither does it grow out, nor assume the upright
position of normal capsules. It is checked in its development, and at
the time of ripening it is nearly of the same length as in the
beginning. Many of them contain no good seeds at all; from others I have
succeeded in saving only a hundred seeds from thousands of capsules.
These seeds, if purely pollinated, and with the exclusion of the visits
of insects, reproduce the variety, entirely and without any reversion to
the _lamarckiana_ type.
Correlated with the detailed structures is the form of the flower-buds.
They lack the high stigma placed above the anthers, which in the
_lamarckiana_, by the vigorous growth of the style, extends the calyx
and renders the flower bud thinner and more slender. Those of the
_brevistylis_ are therefore broader and more swollen. It is quite easy
to distinguish the individuals by this striking character alone,
although it differs from the parent in other particulars.
The leaves of the _O. brevistylis_ are more rounded at the tip, but the
difference is only pronounced at times, slightly in the adult rosettes,
but more clearly on the growing summits of the stems and branches. By
this character, the plants [531] may be discerned among the others, some
weeks before the flowers begin to show themselves. But the character by
which the plants may be most easily recognized from a distance in the
field is the failure of the fruits. They were found there nearly every
year in varying, but always small numbers.
Leaving the short-styled primrose, we come now to the last of our group
of retrograde varieties. This is the _O. nanella_, or the dwarf, and is
a most attractive little plant. It is very short of stature, reaching
often a height of only 20-30 cm., or less than one-fourth of that of the
parent. It commences flowering at a height of 10-15 cm., while the
parent-form often measures nearly a meter at this stage of its
development. Being so very dwarfed the large flowers are all the more
striking. They are hardly inferior to those of the _lamarckiana_, and
agree with them in structure. When they fade away the spike is rapidly
lengthened, and often becomes much longer than the lower or vegetative
part of the stem.
The dwarfs are one of the most common mutations in my garden, and were
observed in the native locality and also grown from seeds saved there.
Once produced they are absolutely constant. I have tried many thousands
of seeds from various dwarf mutants, and never observed [532] any trace
of reversion to the _lamarckiana_ type. I have also cultivated them in
successive generations with the same result. In a former lecture we have
seen that contrary to the general run of horticultural belief, varieties
are as constant as the best species, if kept free from hybrid
admixtures. This is a general rule, and the exceptions, or cases of
atavism are extremely rare. In this respect it is of great interest to
observe that this constancy is not an acquired quality, but is to be
considered as innate, because it is already fully developed at the very
moment when the original mutation takes place.
From its first leaves to the rosette period, and through this to the
lengthening of the stem, the dwarfs are easily distinguished from any
other of their congeners. The most remarkable feature is the shape of
the leaves. They are broader and shorter, and especially at the base
they are broadened in such a way as to become apparently sessile. The
stalk is very brittle, and any rough treatment may cause the leaves to
break off. The young seedlings are recognizable by the shape of the
first two or three leaves, and when more of them are produced, the
rosettes become dense and strikingly different from others. Later leaves
are more nearly like the parent-type, but the petioles remain short. The
bases of the blades are frequently [533] almost cordate, the laminae
themselves varying from oblong-ovate to ovate in outline. The stems are
often quite unbranched, or branched only at the base of the spike.
Strong secondary stems are a striking attribute of the _lamarckiana_
parent, but they are lacking, or almost so in the dwarfs. The stem is
straight and short, and this, combined with the large crown of bright
flowers, makes the dwarfs eminently suitable for bed or border plants.
Unfortunately they are very sensitive, especially to wet weather.
_Oenothera gigas_ and _O. rubrinervis_, or the giant, and the red-veined
evening-primroses, are the names given to two robust and stout species,
which seem to be equal in vigor to the parent-plant, while diverging
from it in striking characters. Both are true elementary species,
differentiated from _lamarckiana_ in nearly all their organs and
qualities, but not showing any preponderating character of a retrograde
nature. Their differences may be compared with those of the elementary
species of other genera, as for instance, of _Draba_, or of violets, as
will be seen by their description.
The giant evening-primrose, though not taller in stature than _O.
lamarckiana_, deserves its name because it is so much stouter in all
respects. [534] The stems are robust, often with twice the diameter of
_lamarckiana_ throughout. The internodes are shorter, and the leaves
more numerous, covering the stems with a denser foliage. This shortness
of the internodes extends itself to the spike, and for this reason the
flowers and fruits grow closer together than on the parent-plant. Hence
the crown of bright flowers, opening each evening, is more dense and
more strikingly brilliant, so much the more so as the individual flowers
are markedly larger than those of the parents. In connection with these
characters, the flower-buds are seen to be much stouter than those of
_lamarckiana_. The fruits attain only half the normal size, but are
broader and contain fewer, but larger seeds.
The _rubrinervis_ is in many respects a counterpart to the _gigasv, but
its stature is more slender. The spikes and flowers are those of the
_lamarckianav, but the bracts are narrower. Red veins and red streaks on
the fruits afford a striking differentiating mark, though they are not
absolutely lacking in the parent-species. A red hue may be seen on the
calyx, and even the yellow color of the petals is somewhat deepened in
the same way. Young plants are often marked by the pale red tinge of the
mid-veins, but in adult rosettes, or from lack of sunshine, this hue is
often very faint.
[535] The leaves are narrow, and a curious feature of this species is
the great brittleness of the leaves and stems, especially in annual
individuals, especially in those that make their stem and flowers in the
first year. High turgidity and weak development of the mechanical and
supporting tissues are the anatomical cause of this deficiency, the
bast-fibers showing thinner walls than those of the parent-type under
the microscope. Young stems of _rubrinervis_ may be broken off by a
sharp stroke, and show a smooth rupture across all the tissues, while
those of _lamarckiana_ are very tough and strong.
Both the giant and the red-veined species are easily recognized in the
rosette-stage. Even the very young seedlings of the latter are clearly
differentiated from the _lamarckiana_, but often a dozen leaves are
required, before the difference may be seen. Under such circumstances
the young plants must reach an age of about two months before it is
possible to discern their characters, or at least before these
characters have become reliable enough to enable us to judge of each
individual without doubt. But the divergencies rapidly become greater.
The leaves of _O. gigas_ are broader, of a deeper green, the blade more
sharply set off against the stalk, all the rosettes [536] becoming stout
and crowded with leaves. Those of _O. rubrinervis_ on the contrary are
thin, of a paler green and with a silvery white surface; the blades are
elliptic, often being only 2 cm. or less in width. They are acute at the
apex and gradually narrowed into the petiole.
It is quite evident that such pale narrow leaves must produce smaller
quantities of organic food than the darker green and broad organs of the
_gigas_. Perhaps this fact is accountable partly, at least, for the more
robust growth of the giant in the second year. Perhaps also some
relation exists between this difference in chemical activity and the
tendency to become annual or biennial. The _gigas_, as a rule, produces
far more, and the _rubrinervis_ far less biennial plants than the
_lamarckiana_. Annual culture for the one is as unreliable as biennial
culture for the other. _Rubrinervis_ may be annual in apparently all
specimens, in sunny seasons, but _gigas_ will ordinarily remain in the
state of rosettes during the entire first summer. It would be very
interesting to obtain a fuller insight into the relation of the length
of life to other qualities, but as yet the facts can only be detailed as
they stand.
Both of these stout species have been found [537] quite constant from
the very first moment of their appearance. I have cultivated them from
seed in large numbers, and they have never reverted to the
_lamarckiana_. From this they have inherited the mutability or the
capacity of producing at their turn new mutants. But they seem to have
done so incompletely, changing in the direction of more absolute
constancy. This was especially observed in the case of _rubrinervis_,
which is not of such rare occurrence as _O. gigas_, and which it has
been possible to study in large numbers of individuals. So for instance,
the "red-veins" have never produced any dwarfs, notwithstanding they are
produced very often by the parent-type. And in crossing experiments also
the red-veins gave proof of the absence of a mutative capacity for their
production.
Leaving the robust novelties, we may now take up a couple of forms,
which are equally constants and differentiated from the parent species
in exactly the same manner, though by other characters, but which are so
obviously weak as to have no manifest chance of self maintenance in the
wild state. These are the whitish and the oblong-leaved
evening-primroses or the _Oenothera albida_ and _oblonga_.
_Oenothera albida_ is a very weak species, with whitish, narrow leaves,
which are evidently incapable [538] of producing sufficient quantities
of organic food. The young seedling-plants are soon seen to lag behind,
and if no care is taken of them they are overgrown by their neighbors.
It is necessary to take them out, to transplant them into pots with
richly manured soil, and to give them all the care that should be given
to weak and sickly plants. If this is done fully grown rosettes may be
produced, which are strong enough to keep through the winter. In this
case the individual leaves become stronger and broader, with oblong
blades and long stalks, but retain their characteristic whitish color.
In the second year the stems become relatively stout. Not that they
become equal to those of _lamarckiana_, but they become taller than
might have been expected from the weakness of the plants in the previous
stages. The flowers and racemes are nearly as large as those of the
parent-form, the fruits only a little thinner and containing a smaller
quantity of seed. From these seeds I have grown a second and a third
generation, and observed that the plants remain true to their type.
_O. oblonga_ may be grown either as an annual, or as a biennial. In the
first case it is very slender and weak, bearing only small fruits and
few seeds. In the alternative case however, it [539] becomes densely
branched, bearing flowers on quite a number of racemes and yielding a
full harvest of seeds. But it always remains a small plant, reaching
about half the height of that of _lamarckiana_.
When very young it has broader leaves, but in the adult rosettes the
leaves become very narrow, but fleshy and of a bright green color. They
are so crowded as to leave no space between them unoccupied. The
flowering spikes of the second year bear long leaf-like bracts under the
first few flowers, but those arising later are much shorter. Numerous
little capsules cover the axis of the spike after the fading away of the
petals, constituting a very striking differentiating mark. This species
also was found to be quite constant, if grown from pure seed.
We have now given the descriptions of seven new forms, which diverge in
different ways from the parent-type. All were absolutely constant from
seed. Hundreds or thousands of seedlings may have arisen, but they
always come true and never revert to the original _O. lamarckiana_ type.
From this they have inherited the condition of mutability, either
completely or partly, and according to this they may be able to produce
new forms themselves. But this occurs only rarely, and combinations of
more than one [540] type in one single plant seem to be limited to the
admixture of the dwarf stature with the characters of the other new
species.
These seven novelties do not comprise the whole range of the new
productions of my _O. lamarckiana_. But they are the most interesting
ones. Others, as the _O. semilata_ and the _O. leptocarpa_ are quite as
constant and quite as distinct, but have no special claims for a closer
description. Others again were sterile, or too weak to reach the adult
stage and to yield seeds, and no reliable description or appreciation
can be given on the ground of the appearance of a single individual.
Contrasted with these groups of constant forms are three inconstant
types which we now take up. They belong to two different groups,
according to the cause of their inconstancy. In one species which I call
_O. lata_, the question of stability or instability must remain wholly
unsolved, as only pistillate flowers are produced, and no seed can be
fertilized save by the use of the pollen of another form, and therefore
by hybridization. The other head comprises two fertile forms, _O.
scintillans_ and _O. elliptica_, which may easily be fertilized with
their own pollen, but which gave a progeny only partly similar to the
parents.
The _Oenothera lata_ is a very distinct form [541] which was found more
than once in the field, and recently (1902) in a luxuriant flowering
specimen. It has likewise been raised from seeds collected in different
years at the original station. It is also wholly pistillate. Apparently
the anthers are robust, but they are dry, wrinkled and nearly devoid of
contents. The inner wall of cells around the groups of pollen grow out
instead of being resorbed, partly filling the cavity which is left free
by the miscarriage of the pollen-grains. This miscarriage does not
affect all the grains in the same degree, and under the microscope a few
of them with an apparently normal structure may be seen. But the
contents are not normally developed, and I have tried in vain to obtain
fertilization with a large number of flowers. Only by
cross-fertilization does _O. lata_ produce seeds, and then as freely as
the other species when self-fertilized. Of course its chance of ever
founding a wild type is precluded by this defect.
_O. lata_ is a low plant, with a limp stem, bent tips and branches, all
very brittle, but with dense foliage and luxuriant growth. It has bright
yellow flowers and thick flower-buds. But for an unknown reason the
petals are apt to unfold only partially and to remain wrinkled
throughout the flowering time. The stigmas are slightly divergent from
the normal type, [542] also being partly united with one another, and
laterally with the summit of the style, but without detriment to their
function.
Young seedlings of _lata_ may be recognized by the very first leaves.
They have a nearly orbicular shape and are very sharply set off against
their stalk. The surface is very uneven, with convexities and
concavities on both sides. This difference is lessened in the later
leaves, but remains visible throughout the whole life of the plant, even
during the flowering season. Broad, sinuate leaves with rounded tips are
a sure mark of _O. lata_. On the summits of the stems and branches they
are crowded so as to form rosettes.
Concerning inheritance of these characteristics nothing can be directly
asserted because of the lack of pollen. The new type can only be
perpetuated by crosses, either with the parent form or some other
mutant. I have fertilized it, as a rule, with _lamarckiana_ pollen, but
have often also used that from _nanella_ and others. In doing so, the
_lata_ repeats its character in part of its offspring. This part seems
to be independent of the nature of the pollen used, but is very variable
according to external circumstances. On the average one-fourth of the
offspring become _lata_, the others assuming the type of the
pollen-parent, if this was a _lamarckiana_ or [543] partly this type and
partly that of any other of the new species derived from _lamarckiana_,
that might have been used as the pollen-parent. This average seems to be
a general rule, recurring in all experiments, and remaining unchanged
through a long series of successive generations. The fluctuations around
this mean go up to nearly 50% and down nearly to 1%, but, as in other
cases, such extreme deviations from the average are met with only
exceptionally.
The second category includes the inconstant but perfectly fertile
species. I have already given the names of the only two forms, which
deserve to be mentioned here.
One of them is called _scintillans_ or the shiny evening-primrose,
because its leaves are of a deep green color with smooth surfaces,
glistening in the sunshine. On the young rosettes these leaves are
somewhat broader, and afterwards somewhat narrower than those of _O.
lamarckiana_ at the corresponding ages. The plants themselves always
remain small, never reaching the stature of the ancestral type. They are
likewise much less branched. They can easily be cultivated in annual
generations, but then do not become as fully developed and as fertile,
as when flowering in the second year. The flowers have the same
structure as those of the _lamarckiana_, but are of a smaller size.
[544] Fertilizing the flowers artificially with their own pollen,
excluding the visiting insects by means of paper bags, and saving and
sowing the seed of each individual separately, furnishes all the
requisites for the estimation of the degree of stability of this
species. In the first few weeks the seed-pans do not show any
unequality, and often the young plants must be replanted at wider
intervals, before anything can be made out with certainty. But as soon
as the rosettes begin to fill it becomes manifest that some of them are
more backward than others in size. Soon the smaller ones show their
deeper green and broader leaves, and thereby display the attributes of
the _scintillans_. The other grow faster and stronger and exhibit all
the characteristics of ordinary _lamarckiana_s.
The numerical proportion of these two groups has been found different on
different occasions. Some plants give about one-third _scintillans_ and
two-thirds _lamarckiana_, while the progeny of individuals of another
strain show exactly the reverse proportion.
Two points deserve to be noticed. First the progeny of the _scintillans_
appears to be mutable in a large degree, exceeding even the
_lamarckiana_. The same forms that are produced most often by the
parent-family are also most ordinarily [545] met with among the
offspring of the shiny evening-primrose. They are _oblonga_, _lata_ and
_nanella_. _Oblonga_ was observed at times to constitute as much as 1%
or more of the sowings of _scintillans_, while _lata_ and _nanella_ were
commonly seen only in a few scattering individuals, although seldom
lacking in experiments of a sufficient size.
Secondly the instability seems to be a constant quality, although the
words themselves are at first sight, contradictory. I mean to convey the
conception that the degree of instability remains unchanged during
successive generations. This is a very curious fact, and strongly
reminds us of the hereditary conditions of striped-flower varieties.
But, on the contrary, the atavists, which are here the individuals with
the stature and the characteristics of the _lamarckiana_, have become
_lamarckiana_s in their hereditary qualities, too. If their seed is
saved and sown, their progeny does not contain any _scintillans_, or at
least no more than might arise by ordinary mutations.
One other inconstant new species is to be noted, but as it was very rare
both in the field and in my cultures, and as it was difficult of
cultivation, little can as yet be said about it. It is the _Oenothera
elliptica_, with narrow elliptical leaves and also with elliptical
petals. It repeats [546] its type only in a very small proportion of its
seed.
All in all we thus have a group of a dozen new types, springing from an
original form in one restricted locality, and seen to grow there, or
arising in the garden from seeds collected from the original locality.
Without any doubt the germs of the new types are fully developed within
the seed, ready to be evolved at the time of germination. More favorable
conditions in the field would no doubt allow all of the described new
species to unfold their attributes there, and to come into competition
with each other and with the common parents. But obviously this is only
of secondary importance, and has no influence on the fact that a number
of new types, analogous to the older swarms of _Draba_, _Viola_ and of
many other polymorphous species, have been seen to arise directly in the
wild state.
[547]
LECTURE XIX
EXPERIMENTAL PEDIGREE-CULTURES
The observation of the production of mutants in the field at Hilversum,
and the subsequent cultivation of the new types in the garden at
Amsterdam, gives ample proof of the mutability of plants. Furthermore it
furnishes an analogy with the hypothetical origin of the swarms of
species of _Draba_ and _Viola_. Last but not least important it affords
material for a complete systematic and morphologic study of the newly
arisen group of forms.
The physiologic laws, however, which govern this process are only very
imperfectly revealed by such a study. The instances are too few.
Moreover the seeds from which the mutants spring, escape observation. It
is simply impossible to tell from which individual plants they have been
derived. The laevifolia and the brevistylis have been found almost every
year, the first always recurring on the same spot, the second on various
parts of the original field. It is therefore allowable to assume a
common [548] origin for all the observed individuals of either strain.
But whether, besides this, similar strains are produced anew by the old
_lamarckiana_ group, it is impossible to decide on the sole ground of
these field-observations.
The same holds good with the other novelties. Even if one of them should
germinate repeatedly, without ever opening its flowers, the possibility
could not be excluded that the seeds might have come originally from the
same capsule but lain dormant in the earth during periods of unequal
length.
Other objections might be cited that can only be met by direct and fully
controlled experiments. Next to the native locality comes the
experimental garden. Here the rule prevails that every plant must be
fertilized with pollen of its own, or with pollen of other individuals
of known and recorded origin. The visits of insects must be guarded
against, and no seeds should be saved from flowers which have been
allowed to open without this precaution. Then the seeds of each
individual must be saved and sown separately, so as to admit of an
appreciation, and if necessary, a numerical determination of the nature
of its progeny. And last but not least the experiments should be
conducted in a similar manner during a series of successive years.
[549] I have made four such experiments, each comprising the handling of
many thousands of individual plants, and lasting through five to nine
generations. At the beginning the plants were biennial, as in the native
locality, but later I learned to cultivate them in annual generations.
They have been started from different plants and seeds, introduced from
the original field into my garden at Amsterdam.
It seems sufficient to describe here one of these pedigree-cultures, as
the results of all four were similar. In the fall of 1886 I took nine
large rosettes from the field, planted them together on an isolated spot
in the garden, and harvested their seeds the next year. These nine
original plants are therefore to be considered as constituting the first
generation of my race. The second generation was sown in 1888 and
flowered in 1889. It at once yielded the expected result. 15,000
seedlings were tested and examined, and among them 10 showed diverging
characters. They were properly protected, and proved to belong to two
new types. 5 of them were _lata_ and 5 _nanella_. They flowered next
year and displayed all the characters as described in our preceding
lecture. Intermediates between them and the general type were not found,
and no indication of their appearance was noted in their parents. [550]
They came into existence at once, fully equipped, without preparation or
intermediate steps. No series of generations, no selection, no struggle
for existence was needed. It was a sudden leap into another type, a
sport in the best acceptation of the word. It fulfilled my hopes, and at
once gave proof of the possibility of the direct observation of the
origin of species, and of the experimental control thereof.
The third generation was in the main a repetition of the second. I tried
some 10,000 seedlings and found three _lata_ and three _nanella_, or
nearly the same proportion as in the first instance. But besides these a
_rubrinervis_ made its appearance and flowered the following year. This
fact at once revealed the possibility that the instability of
_lamarckiana_ might not be restricted to the three new types now under
observation. Hence the question arose how it would be possible to obtain
other types or to find them if they were present. It was necessary to
have better methods of cultivation and examination of the young plants.
Accordingly I devoted the three succeeding years to working on this
problem.
I found that it was not at all necessary to sow any larger quantities of
seed, but that the young plants must have room enough to develop into
full and free rosettes. Moreover I observed [551] that the attributes of
_lata_ and _nanella_, which I now studied in the offspring of my first
mutants, were clearly discernible in extreme youth, while those of
_rubrinervis_ remained concealed some weeks longer. Hence I concluded
that the young plants should be examined from time to time until they
proved clearly to be only normal _lamarckiana_. Individuals exhibiting
any deviation from the type, or even giving only a slight indication of
it, were forthwith taken out of the beds and planted separately, under
circumstances as favorable as possible. They were established in pots
with well-manured soil and kept under glass, but fully exposed to
sunshine. As a rule they grew very fast, and could be planted out early
in June. Some of them, of course, proved to have been erroneously taken
for mutants, but many exhibited new characters.
All in all I had 334 young plants which did not agree with the parental
type. As I examined some 14,000 seedlings altogether, the result was
estimated at about 2.5%. This proportion is much larger than in the
yields of the two first generations and illustrates the value of
improved methods. No doubt many good mutations had been overlooked in
the earlier observations.
As was to be expected, _lata_ and _nanella_ [552] were repeated in this
third generation (1895). I was sure to get nearly all of them, without
any important exceptions, as I now knew how to detect them at almost any
age. In fact, I found many of them; as many as 60 _nanella_ and 73
_lata_, or nearly 5% of each. _Rubrinervis_ also recurred, and was seen
in 8 specimens. It was much more rare than the two first-named types.
But the most curious fact in that year was the appearance of _oblonga_.
No doubt I had often seen it in former years, but had not attached any
value to the very slight differences from the type, as they then seemed
to me. I knew now that any divergence was to be esteemed as important,
and should be isolated for further observation. This showed that among
the selected specimens not less than 176, or more than 1% belonged to
the _oblonga_ type. This type was at that time quite new to me, and it
had to be kept through the winter, to obtain stems and flowers. It
proved to be as uniform as its three predecessors, and especially as
sharply contrasted with _lamarckiana_. The opportunity for the discovery
of any intermediates was as favorable as could be, because the
distinguishing marks were hardly beyond doubt at the time of the
selection and removal of the young plants. But no connecting links were
found.
[553] The same holds good for _albida_, which appeared in 15 specimens,
or in 0.1%, of the whole culture. By careful cultivation these plants
proved not to be sickly, but to belong to a new, though weak type. It
was evident that I had already seen them in former years, but having
failed to recognize them had allowed them to be destroyed at an early
age, not knowing how to protect them against adverse circumstances. Even
this time I did not succeed in getting them strong enough to keep
through the winter.
Besides these, two new types were observed, completing the range of all
that have since been recorded to regularly occur in this family. They
were _scintillans_ and _gigas_. The first was obtained in the way just
described. The other hardly escaped being destroyed, not having showed
itself early enough, and being left in the bed after the end of the
selection. But as it was necessary to keep some rosettes through the
winter in order to have biennial flowering plants to furnish seeds, I
selected in August about 30 of the most vigorous plants, planted them on
another bed and gave them sufficient room for their stems and branches
in the following summer. Most of them sent up robust shoots, but no
difference was noted till the first flowers opened. One plant had a much
larger crown of bright blossoms than any of the others. [554] As soon as
these flowers faded away, and the young fruits grew out, it became clear
that a new type was showing itself. On that indication I removed all the
already fertilized flowers and young fruits, and protected the buds from
the visits of insects. Thus the isolated flowers were fertilized with
their own pollen only, and I could rely upon the purity of the seed
saved. This lot of seeds was sown in the spring of 1897 and yielded a
uniform crop of nearly 300 young _gigas_ plants.
Having found how much depends upon the treatment, I could gradually
decrease the size of my cultures. Evidently the chance of discovering
new types would be lessened thereby, but the question as to the repeated
production of the same new forms could more easily and more clearly be
answered in this way. In the following year (1896) I sowed half as many
seeds as formerly, and the result proved quite the same. With the
exception of _gigas_ all the described forms sprang anew from the purely
fertilized ancestry of normal _lamarckiana_s. It was now the fifth
generation of my pedigree, and thus I was absolutely sure that the
descendants of the mutants of this year had been pure and without
deviation for at least four successive generations.
Owing partly to improved methods of selection, [555] partly no doubt to
chance, even more mutants were found this year than in the former. Out
of some 8,000 seedlings I counted 377 deviating ones, or nearly 5%,
which is a high proportion. Most of them were _oblonga_ and _lata_, the
same types that had constituted the majority in the former year.
_Albida_, _nanella_ and _rubrinervis_ appeared in large numbers, and
even _scintillans_, of which I had but a single plant in the previous
generation, was repeated sixfold.
New forms did not arise, and the capacity of my strain seemed exhausted.
This conclusion was strengthened by the results of the next three
generations, which were made on a much smaller scale and yielded the
same, or at least the mutants most commonly seen in previous years.
Instead of giving the figures for these last two years separately, I
will now summarize my whole experiment in the form of a pedigree. In
this the normal _lamarckiana_ was the main line, and seeds were only
sown from plants after sufficient isolation either of the plants
themselves, or in the latter years by means of paper bags enclosing the
inflorescences. I have given the number of seedlings of _lamarckiana_
which were examined each year in the table below. Of course by far the
largest number of them were [556] thrown away as soon as they showed
their differentiating characters in order to make room for the remaining
ones. At last only a few plants were left to blossom in order to
perpetuate the race. I have indicated for each generation the number of
mutants of each of the observed forms, placing them in vertical columns
underneath their respective heads. The three first generations were
biennial, but the five last annual.
PEDIGREE OF A MUTATING FAMILY
OF _OENOTHERA LAMARCKIANA_ IN THE
EXPERIMENTAL GARDEN AT AMSTERDAM
Gener: O.gig. albida obl. rubrin. Lam. nanella lata. scint.
VIII. 5 1 0 1700 21 1
VII. 9 0 3000 11
VI. 11 29 3 1800 9 5 1
V. 25 135 20 8000 49 142 6
IV. 1 15 176 8 14000 60 73 1
III. 1 10000 3 3
II. 15000 5 5
I. 9
It is most striking that the various mutations of the evening-primrose
display a great degree of regularity. There is no chaos of forms, no
indefinite varying in all degrees and in all directions. Quite on the
contrary, it is at once evident that very simple rules govern the whole
phenomenon.
I shall now attempt to deduce these laws from [557] my experiment.
Obviously they apply not only to our evening-primroses, but may be
expected to be of general validity. This is at once manifest, if we
compare the group of new mutants with the swarms of elementary forms
which compose some of the youngest systematic species, and which, as we
have seen before, are to be considered as the results of previous
mutations. The difference lies in the fact that the evening-primroses
have been seen to spring from their ancestors and that the _drabas_ have
not. Hence the conclusion that in comparing the two we must leave out
the pedigree of the evening-primroses and consider only the group of
forms as they finally show themselves. If in doing so we find sufficient
similarity, we are justified in the conclusion that the _drabas_ and
others have probably originated in the same way as the
evening-primroses. Minor points of course will differ, but the main
lines cannot have complied with wholly different laws. All so-called
swarms of elementary species obviously pertain to a single type, and
this type includes our evening-primroses as the only controlled case.
Formulating the laws of mutability for the evening-primroses we
therefore assume that they hold good for numerous other corresponding
cases.
[558] I. The first law is, that new elementary species appear suddenly,
without intermediate steps.
This is a striking point, and the one that is in the most immediate
contradiction to current scientific belief. The ordinary conception
assumes very slow changes, in fact so slow that centuries are supposed
to be required to make the differences appreciable. If this were true,
all chance of ever seeing a new species arise would be hopelessly small.
Fortunately the evening-primroses exhibit contrary tendencies. One of
the great points of pedigree-culture is the fact that the ancestors of
every mutant have been controlled and recorded. Those of the last year
have seven generations of known _lamarckiana_ parents preceding them. If
there had been any visible preparation towards the coming mutation, it
could not have escaped observation. Moreover, if visible preparation
were the rule, it could hardly go on at the same time and in the same
individuals in five or six diverging directions, producing from one
parent, _gigas_ and _nanella_, _lata_ and _rubrinervis_, _oblonga_ and
_albida_ and even _scintillans_.
On the other hand the mutants, that constitute the first representatives
of their race, exhibit all the attributes of the new type in full
display at once. No series of generations, no selection, [559] no
struggle for existence are needed to reach this end. In previous
lectures I have mentioned that I have saved the seeds of the mutants
whenever possible, and have always obtained repetitions of the prototype
only. Reversions are as absolutely lacking as is also a further
development of the new type. Even in the case of the inconstant forms,
where part of the progeny yearly return to the stature of _lamarckiana_,
intermediates are not found. So it is also with _lata_, which is
pistillate and can only be propagated by cross-fertilization. But though
the current belief would expect intermediates at least in this case,
they do not occur. I made a pedigree-culture of lata during eight
successive generations, pollinating them in different ways, and always
obtained cultures which were partly constituted of _lata_ and partly of
_lamarckiana_ specimens. But the _lata_s remained _lata_ in all the
various and most noticeable characters, never showing any tendency to
gradually revert into the original form.
Intermediate forms, if not occurring in the direct line from one species
to another, might be expected to appear perhaps on lateral branches. In
this case the mutants of one type, appearing in the same year, would not
be a pure type, but would exhibit different degrees of deviation from
the parent. The best would then have to [560] be chosen in order to get
the new type in its pure condition. Nothing of the kind, however, was
observed. All the _oblonga_-mutants were pure _oblongas_. The pedigree
shows hundreds of them in the succeeding years, but no difference was
seen and no material for selection was afforded. All were as nearly
equal as the individuals of old elementary species.
II. New forms spring laterally from the main stem.
The current conception concerning the origin of species assumes that
species are slowly converted into others. The conversion is assumed to
affect all the individuals in the same direction and in the same degree.
The whole group changes its character, acquiring new attributes. By
inter-crossing they maintain a common line of progress, one individual
never being able to proceed much ahead of the others.
The birth of the new species necessarily seemed to involve the death of
the old one. This last conclusion, however, is hard to understand. It
may be justifiable to assume that all the individuals of one locality
are ordinarily intercrossed, and are moreover subjected to the same
external conditions. They might be supposed to vary in the same
direction if these conditions were changed slowly. But this could of
course have no possible influence on the plants of the [561] same
species growing in distant localities, and it would be improbable they
should be affected in the same way. Hence we should conclude that when a
species is converted into a new type in one locality this is only to be
considered as one of numerous possible ones, and its alteration would
not in the least change the aspect of the remainder of the species.
But even with this restriction the general belief is not supported by
the evidence of the evening-primroses. There is neither a slow nor a
sudden change of all the individuals. On the contrary, the vast majority
remain unchanged; thousands are seen exactly repeating the original
prototype yearly, both in the native field and in my garden. There is no
danger that _lamarckiana_ might die out from the act of mutating, nor
that the mutating strain itself would be exposed to ultimate destruction
from this cause.
In older swarms, such as _Draba_ or _Helianthemum_, no such center,
around which the various forms are grouped, is known. Are we to conclude
therefore that the main strain has died out? Or is it perhaps concealed
among the throng, being distinguished by no peculiar character? If our
_gigas_ and _rubrinervis_ were growing in equal numbers with the
_lamarckiana_ in the native field, would it be possible to decide [562]
which of them was the progenitor of the others? Of course this could be
done by long and tedious crossing experiments, showing atavism in the
progeny, and thereby indicating the common ancestor. But even this
capacity seems to be doubtful and connected only with the state of
mutability and to be lost afterwards. Therefore if this period of
mutation were ended, probably there would be no way to decide concerning
the mutual relationship of the single species.
Hence the lack of a recognizable main stem in swarms of elementary
species makes it impossible to answer the question concerning their
common origin.
Another phase of the opposition between the prevailing view and my own
results seems far more important. According to the current belief the
conversion of a group of plants growing in any locality and flowering
simultaneously would be restricted to one type. In my own experiments
several new species arose from the parental form at once, giving a wide
range of new forms at the same time and under the same conditions.
III. New elementary species attain their full constancy at once.
Constancy is not the result of selection or of improvement. It is a
quality of its own. It can neither be constrained by selection if it is
absent [563] from the beginning, nor does it need any natural or
artificial aid if it is present. Most of my new species have proved
constant from the first. Whenever possible, the original mutants have
been isolated during the flowering period and artificially
self-fertilized. Such plants have always given a uniform progeny, all
children exhibiting the type of the parent. No atavism was observed and
therefore no selection was needed or even practicable.
Briefly considering the different forms, we may state that the full
experimental proof has been given for the origin of _gigas_ and
_rubrinervis_, for _albida_ and _oblonga_, and even for _nanella_, which
is to be considered as of a varietal nature; with _lata_ the decisive
experiment is excluded by its unisexuality. _laevifolia_ and
_brevistylis_ were found originally in the field, and never appeared in
my cultures. No observations were made as to their origin, and seeds
have only been sown from later generations. But these have yielded
uniform crops, thereby showing that there is no ground for the
assumption that these two older varieties might behave otherwise than
the more recent derivatives.
_Scintillans_ and _elliptica_ constitute exceptions to the rule given.
They repeat their character, from pure seed, only in part of the
offspring. I have tried to deliver the _scintillans_ from this [564]
incompleteness of heredity, but in vain. The succeeding generations, if
produced from true representatives of the new type, and with pure
fertilization, have repeated the splitting in the same numerical
proportions. The instability seems to be here as permanent a quality as
the stability in other instances. Even here no selection has been
adequate to change the original form.
IV. Some of the new strains are evidently elementary species, while
others are to be considered as retrograde varieties.
It is often difficult to decide whether a given form belongs to one or
another of these two groups. I have tried to show that the best and
strictest conception of varieties limits them to those forms that have
probably originated by retrograde or degressive steps. Elementary
species are assumed to have been produced in a progressive way, adding
one new element to the store. Varieties differ from their species
clearly in one point, and this is either a distinct loss, or the
assumption of a character, which may be met with in other species and
genera. _laevifolia_ is distinguished by the loss of the crinkling of
the leaves, _brevistylis_ by the partial loss of the epigynous qualities
of the flowers, and _nanella_ is a dwarf. These three new forms are
therefore [565] considered to constitute only retrograde steps, and no
advance. This conclusion has been fully justified by some crossing
experiments with _brevistylis_, which wholly complies with Mendel's law,
and in one instance with _nanella_, which behaves in the same manner, if
crossed with _rubrinervis_.
On the other hand, _gigas_ and _rubrinervis_, _oblonga_ and _albida_
obviously bear the characters of progressive elementary species. They
are not differentiated from _lamarckiana_ by one or two main features.
They diverge from it in nearly all organs, and in all in a definite
though small degree. They may be recognized as soon as they have
developed their first leaves and remain discernible throughout life.
Their characters refer chiefly to the foliage, but no less to the
stature, and even the seeds have peculiarities. There can be little
doubt but that all the attributes of every new species are derived from
one principal change. But why this should affect the foliage in one
manner, the flowers in another and the fruits in a third direction,
remains obscure. To gain ever so little an insight into the nature of
these changes, we may best compare the differences of our
evening-primroses with those between the two hundred elementary species
of _Draba_ and other similar instances. In doing so we find the same
main [566] feature, the minute differences in nearly all points.
V. The same new species are produced in a large number of individuals.
This is a very curious fact. It embraces two minor points, viz: the
multitude of similar mutants in the same year, and the repetition
thereof in succeeding generations. Obviously there must be some common
cause. This cause must be assumed to lie dormant in the _Lamarckiana_s
of my strain, and probably in all of them, as no single parent-plant
proved ever to be wholly destitute of mutability. Furthermore the
different causes for the sundry mutations must lie latent together in
the same parent-plant. They obey the same general laws, become active
under similar conditions, some of them being more easily awakened than
others. The germs of the _oblonga_, _lata_ and _nanella_ are especially
irritable, and are ready to spring into activity at the least summons,
while those of _gigas_, _rubrinervis_ and _scintillans_ are far more
difficult to arouse.
These germs must be assumed to lie dormant during many successive
generations. This is especially evident in the case of _lata_ and
_nanella__, which appeared in the first year of the pedigree culture and
which since have been repeated yearly, and have been seen to arise by
mutation [567] also during the last season (1903). Only _gigas_ appeared
but once, but then there is every reason to assume that in larger
sowings or by a prolongation of the experiments it might have made a
second appearance.
Is the number of such germs to be supposed to be limited or unlimited?
My experiment has produced about a dozen new forms. Without doubt I
could easily have succeeded in getting more, if I had had any definite
reason to search for them. But such figures are far from favoring the
assumption of indefinite mutability. The group of possible new forms is
no doubt sharply circumscribed. Partly so by the morphologic
peculiarities of _lamarckiana_, which seem to exclude red flowers,
composite leaves, etc. No doubt there are more direct reasons for these
limits, some changes having taken place initially and others later,
while the present mutations are only repetitions of previous ones, and
do not contribute new lines of development to those already existing.
This leads us to the supposition of some common original cause, which
produced a number of changes, but which itself is no longer at work, but
has left the affected qualities, and only these, in the state of
mutability.
In nature, repeated mutations must be of far greater significance than
isolated ones. How [568] great is the chance for a single individual to
be destroyed in the struggle for life? Hundreds of thousands of seeds
are produced by _lamarckiana_ annually in the field, and only some slow
increase of the number of specimens can be observed. Many seeds do not
find the proper circumstances for germination, or the young seedlings
are destroyed by lack of water, of air, or of space. Thousands of them
are so crowded when becoming rosettes that only a few succeed in
producing stems. Any weakness would have destroyed them. As a matter of
fact they are much oftener produced in the seed than seen in the field
with the usual unfavorable conditions; the careful sowing of collected
seeds has given proof of this fact many times.
The experimental proof of this frequency in the origin of new types,
seems to overcome many difficulties offered by the current theories on
the probable origin of species at large.
VI. The relation between mutability and fluctuating variability has
always been one of the chief difficulties of the followers of Darwin.
The majority assumed that species arise by the slow accumulation of
slight fluctuating deviations, and the mutations were only to be
considered as extreme fluctuations, obtained, in the main, by a
continuous selection of small differences in a constant direction.
[569] My cultures show that quite the opposite is to be regarded as
fact. All organs and all qualities of _lamarckiana_ fluctuate and vary
in a more or less evident manner, and those which I had the opportunity
of examining more closely were found to comply with the general laws of
fluctuation. But such oscillating changes have nothing in common with
the mutations. Their essential character is the heaping up of slight
deviations around a mean, and the occurrence of continuous lines of
increasing deviations, linking the extremes with this group. Nothing of
the kind is observed in the case of mutations. There is no mean for them
to be grouped around and the extreme only is to be seen, and it is
wholly unconnected with the original type. It might be supposed that on
closer inspection each mutation might be brought into connection with
some feature of the fluctuating variability. But this is not the case.
The dwarfs are not at all the extreme variants of structure, as the
fluctuation of the height of the _lamarckiana_ never decreases or even
approaches that of the dwarfs. There is always a gap. The smallest
specimens of the tall type are commonly the weakest, according to the
general rule of the relationship between nourishment and variation, but
the tallest dwarfs are of course the most robust specimens of their
group. [570] Fluctuating variability, as a rule, is subject to
reversion. The seeds of the extremes do not produce an offspring which
fluctuates around their parents as a center, but around some point on
the line which combines their attributes with the corresponding
characteristic of their ancestors, as Vilmorin has put it. No reversion
accompanies mutation, and this fact is perhaps the completest contrast
in which these two great types of variability are opposed to each other.
The offspring of my mutants are, of course, subject to the general laws
of fluctuating variability. They vary, however, around their own mean,
and this mean is simply the type of the new elementary species.
VII. The mutations take place in nearly all directions.
Many authors assume that the origin of species is directed by unknown
causes. These causes are assumed to work in each single case for the
improvement of the animals and plants, changing them in a manner
corresponding in a useful way to the changes that take place in their
environment. It is not easy to imagine the nature of these influences
nor how they would bring about the desired effect.
This difficulty was strongly felt by Darwin, and one of the chief
purposes of his selection theory may be said to have been the attempt
[571] to surmount it. Darwin tried to replace the unknown cause by
natural agencies, which lie under our immediate observation. On this
point Darwin was superior to his predecessors, and it is chiefly due to
the clear conception of this point that his theory has gained its
deserved general acceptance. According to Darwin, changes occur in all
directions, quite independently of the prevailing circumstances. Some
may be favorable, others detrimental, many of them without significance,
neither useful nor injurious. Some of them will sooner or later be
destroyed, while others will survive, but which of them will survive, is
obviously dependent upon whether their particular changes agree with the
existing environic conditions or not. This is what Darwin has called the
struggle for life. It is a large sieve, and it only acts as such. Some
fall through and are annihilated, others remain above and are selected,
as the phrase goes. Many are selected, but more are destroyed; daily
observation does not leave any doubt upon this point.
How the differences originate is quite another question. It has nothing
to do with the theory of natural selection nor with the struggle for
life. These have an active part only in the accumulation of useful
qualities, and only in so [572] far as they protect the bearers of such
characters against being crowded out by their more poorly constituted
competitors.
However, the differentiating characteristics of elementary species are
only very small. How widely distant they are from the beautiful
adaptative organizations of orchids, of insectivorous plants and of so
many others! Here the difference lies in the accumulation of numerous
elementary characters, which all contribute to the same end. Chance must
have produced them, and this would seem absolutely improbable, even
impossible, were it not for Darwin's ingenious theory. Chance there is,
but no more than anywhere else. It is not by mere chance that the
variations move in the required direction. They do go, according to
Darwin's view, in all directions, or at least in many. If these include
the useful ones, and if this is repeated a number of times, cumulation
is possible; if not, there is simply no progression, and the type
remains stable through the ages. Natural selection is continually acting
as a sieve, throwing out the useless changes and retaining the real
improvements. Hence the accumulation in apparently predisposed
directions, and hence the increasing adaptations to the more specialized
conditions of life. It must be obvious to any one who can free himself
from the current ideas, [573] that this theory of natural selection
leaves the question as to how the changes themselves are brought about,
quite undecided. There are two possibilities, and both have been
propounded by Darwin. One is the accumulation of the slight deviations
of fluctuating variability, the other consists of successive sports or
leaps taking place in the same direction.
In further lectures a critical comparison of the two views will be
given. Today I have only to show that the mutations of the
evening-primroses, though sudden, comply with the demands made by Darwin
as to the form of variability which is to be accepted as the cause of
evolution and as the origin of species.
Some of my new types are stouter and others weaker than their parents,
as shown by _gigas_ and _albida_. Some have broader leaves and some
narrower, _lata_ and _oblonga_. Some have larger flowers (_gigas_) or
deeper yellow ones (_rubrinervis_), or smaller blossoms (_scintillans_),
or of a paler hue (_albida_). In some the capsules are longer
(_rubrinervis_), or thicker (_gigas_), or more rounded (_lata_), or
small (_oblonga_), and nearly destitute of seeds (_brevistylis_). The
unevenness of the surface of the leaves may increase as in _lata_, or
decrease as in _laevifolia_. The tendency to become annual prevails in
_rubrinervis_, but _gigas_ tends to become [574] biennial. Some are rich
in pollen, while _scintillans_ is poor. Some have large seeds, others
small. _Lata_ has become pistillate, while _brevistylis_ has nearly lost
the faculty to produce seeds. Some undescribed forms were quite sterile,
and some I observed which produced no flowers at all. From this
statement it may be seen that nearly all qualities vary in opposite
directions and that our group of mutants affords wide material for the
sifting process of natural selection. On the original field the
_laevifolia_ and _brevistylis_ have held their own during sixteen years
and probably more, without, however, being able to increase their
numbers to any noticeable extent. Others perish as soon as they make
their appearance, or a few individuals are allowed to bloom, but
probably leave no progeny.
But perhaps the circumstances may change, or the whole strain may be
dispersed and spread to new localities with different conditions. Some
of the latter might be found to be favorable to the robust _gigas_, or
to _rubrinervis_, which requires a drier air, with rainfall in the
springtime and sunshine during the summer. It would be worth while to
see whether the climate of California, where neither _O. lamarckiana_
nor _O. biennis_ are found wild, would not exactly [575] suit the
requirements of the new species _rubrinervis_ and _gigas_.
NOTE. _Oenothera_s are native to America and all of the species growing
in Europe have escaped from gardens directly, or may have arisen by
mutation, or by hybridization of introduced species. A fixed hybrid
between _O. cruciata_ and _O. biennis_ constituting a species has been
in cultivation for many years. The form known as _O. biennis_ in Europe,
and used by de Vries in all of the experiments described in these
lectures, has not yet been found growing wild in America and is not
identical with the species bearing that name among American botanists.
Concerning this matter Professor de Vries writes under date of Sept. 12,
1905: "The '_biennis_' which I collected in America has proved to be a
motley collection of forms, which at that time I had no means of
distinguishing. No one of them, so far as they are now growing in my
garden is identical with our _biennis_ of the sand dunes." The same
appears to be the case with _O. muricata_. Plants from the Northeastern
American seaboard, identifiable with the species do not entirely agree
with those raised from seed received from Holland.
_O. lamarckiana_ has not been found growing wild in America in recent
years although the evidence at hand seems to favor the conclusion that
it was seen and collected in the southern states in the last century.
(See MacDougal, Vail, Shull, and Small: Mutants and Hybrids of the
_Oenotheras_. Publication 24. Carnegie Institution. Washington, D.C.,
1905.) EDITOR.
[576]
LECTURE XX
THE ORIGIN OF WILD SPECIES AND VARIETIES
New species and varieties occur from time to time in the wild state.
Setting aside all theoretical conceptions as to the common origin of
species at large, the undoubted fact remains that new forms are
sometimes met with. In the case of the peloric toad-flax the mutations
are so numerous that they seem to be quite regular. The production of
new species of evening-primroses was observed on the field and
afterwards duplicated in the garden. There is no reason to think that
these cases are isolated instances. Quite on the contrary they seem to
be the prototypes of repeated occurrences in nature.
If this conception is granted, the question at once arises, how are we
to deal with analogous cases, when fortune offers them, and what can we
expect to learn from them?
A critical study of the existing evidence seems to be of great
importance in order to ascertain the best way of dealing with new facts,
and of estimating the value of the factors concerned. [577] It is
manifest that we must be very careful and conservative in dealing with
new facts that are brought to our attention, and every effort should be
made to bring additional evidence to light. Many vegetable anomalies are
so rare that they are met with only by the purest chance, and are then
believed to be wholly new. When a white variety of some common plant is
met with for the first time we generally assume that it originated on
that very spot and only a short time previously. The discovery of a
second locality for the same variety at once raises the question as to a
common origin in the two instances. Could not the plants of the second
locality have arisen from seeds transported from the first?
White varieties of many species of blue-bells and gentians are found not
rarely, white-flowering plants of heather, both of _Erica Tetralix_ and
_Calluna vulgaris_ occur on European heaths; white flowers of _Brunella
vulgaris_, _Ononis repens_, _Thymus vulgaris_ and others may be seen in
many localities in the habitats of the colored species. Pelories of
labiates seem to occur often in Austria, but are rare in Holland; white
bilberries (_Vaccinium Myrtillus_) have many known localities throughout
Europe, and nearly all the berry-bearing species in the large heath
family are recorded as having white varieties.
[578] Are we to assume a single origin for all the representatives of
such a variety, as we have done customarily for all the representatives
of a wild species? Or can the same mutation have been repeated at
different times and in distant localities? If a distinct mutation from a
given species is once possible, why should it not occur twice or thrice?
A variety which seems to be new to us may only appear so, because the
spot where it grows had hitherto escaped observation. _Lychnis preslii_
is a smooth variety of _Lychnis diurna_ and was observed for the first
time in the year 1842 by Sekera. It grew abundantly in a grove near
Munchengratz in southern Hungary. It was accompanied by the ordinary
hairy type of the species. Since then it has been observed to be quite
constant in the same locality, and some specimens have been collected
for me there lately by Dr. Nemec, of Prague. No other native localities
of this variety have been discovered, and there can be no doubt that it
must have arisen from the ordinary campion near the spot where it still
grows. But this change may have taken place some years before the first
discovery, or perhaps one or more centuries ago. This could only be
known if it could be proved that the locality had been satisfactorily
investigated previously, and that the variety had not [579] been met
with. Even in this case only something would be discovered about the
time of the change, but nothing about its real nature.
So it is in many cases. If a variety is observed in a number of
specimens at the time of its first discovery, and at a locality not
studied previously, it takes the aspect of an old form of limited
distribution, and little can be learned as to the circumstances under
which it arose. If on the contrary it occurs in very small numbers or
perhaps even in a single individual, and if the spot where it is found
is located so that it could hardly have escaped previous observation,
then the presumption of a recent origin seems justified.
What has to be ascertained on such occasions to give them scientific
value? Three points strike me as being of the highest importance. First,
the constancy of the new type; secondly, the occurrence or lack of
intermediates, and last, but not least, the direct observation of a
repeated production.
The first two points are easily ascertained. Whether the new type is
linked with its more common supposed ancestor by intermediate steps is a
query which at once strikes the botanist. It is usually recorded in such
cases, and we may state at once that the general result is, that such
intermediates do not occur. This is [580] of the highest importance and
admits of only two explanations. One is that intermediates may be
assumed to have preceded the existent developed form, and to have died
out afterwards. But why should they have done so, especially in cases of
recent changes? On the other hand the intermediates may be lacking
because they have never existed, the change having taken place by a
sudden leap, such as the mutations described in our former lectures. It
is manifest that the assumption of hypothetical intermediates could only
gain some probability if they had been found in some instance. Since
they do not occur, the hypothesis seems wholly unsupported.
The second point is the constancy of the new type. Seeds should be saved
and sown. If the plant fertilizes itself without the aid of insects, as
do some evening-primroses, the seed saved from the native locality may
prove wholly pure, and if it does give rise to a uniform progeny the
constancy of the race may be assumed to be proved, provided that
repeated trials do not bring to light any exceptions. If the offspring
shows more than one type, cross-fertilization is always to be looked to
as the most probable cause, and should be excluded, in order to sow pure
seeds. Garden-experiments of this kind, and repeated trials, should
always be combined [581] with the discovery of a presumed mutation. In
many instances the authors have realized the importance of this point,
and new types have been found constant from the very beginning. Many
cases are known which show no reversions and even no partial reversions.
This fact throws a distinct light on our first point, as it makes the
hypothesis of a slow and gradual development still more improbable.
My third point is of quite another nature and has not as yet been dealt
with. But as it appeals to me as the very soul of the problem, it seems
necessary to describe it in some detail. It does not refer to the new
type itself, nor to any of its morphologic or hereditary attributes, but
directly concerns the presumed ancestors themselves.
The peloric toad-flax in my experiment was seen to arise thrice from the
same strain. Three different individuals of my original race showed a
tendency to produce peloric mutations, and they did so in a number of
their seeds, exactly as the mutations of the evening-primroses were
repeated nearly every year. Hence the inference, that whenever we find a
novelty which is really of very recent date, the parent-strain which has
produced it might still be in existence on the same spot. In the case of
shrubs or perennials the very parents might yet be found. [582] But it
seems probable, and is especially proved in the case of the
evening-primroses, that all or the majority of the representatives of
the whole strain have the same tendency to mutate. If this were a
general rule, it would suffice to take some pure seeds from specimens of
the presumed parents and to sow and multiply the individuals to such an
extent that the mutation might have a chance to be repeated.
Unfortunately, this has not as yet been done, but in my opinion it
should be the first effort of any one who has the good luck to discover
a new wild mutation. Specimens of the parents should be transplanted
into a garden and fertilized under isolated conditions. Seeds saved from
the wild plant would have little worth, as they might have been partly
fertilized by the new type itself.
After this somewhat lengthy discussion of the value of observations
surrounding the discovery of new wild mutations, we now come to the
description of some of the more interesting cases. As a first example, I
will take the globular fruited shepherd's purse, described by Solms
Laubach as _Capsella heegeri_. Professor Heeger discovered one plant
with deviating fruits, in a group of common shepherd's purses in the
market-place near Landau in Germany, in the fall of 1897. They were
nearly spherical, [583] instead of flat and purse-shaped. Their valves
were thick and fleshy, while those of the ordinary form are
membranaceous and dry. The capsules hardly opened and therefore differed
in this point from the shepherd's purse, which readily loosens both its
valves as soon as it is ripe.
Only one plant was observed; whence it came could not be determined, nor
whether it had arisen from the neighboring stock of C_apsella_ or not.
The discoverer took some seed to his garden and sent some to the
botanical garden at Strassburg, of which Solms-Laubach is the director.
The majority of the seeds of course were sowed naturally on the original
spot. The following year some of the seeds germinated and repeated the
novelty. The leaves, stems and flowers were those of the common
shepherd's purse, but no decision could be reached concerning the type
of this generation before the first flowers had faded and the rounded
capsules had developed. Then it was seen that the _heegeri_ came true
from seed. It did so both in the gardens and on the market-place, where
it was observed to have multiplied and spread in some small measure. The
same was noted the following year, but then the place was covered with
gravel and all the plants destroyed. It is not recorded to have been
seen wild since.
[584] Intermediate forms have not been met with. Some slight reversions
may occur in the autumn on the smallest and weakest lateral branches.
Such reversions, however, seem to be very rare, as I have tried in vain
to produce them on large and richly branched individuals, by applying
all possible inducements in the form of manure and of cutting, to
stimulate the production of successive generations of weaker side
branches.
This constancy was proved by the experiments of Solms-Laubach, which I
have repeated in my own garden during several years with seed received
from him. No atavists or deviating specimens have been found among many
hundreds of flowering plants.
It is important to note that within the family of the crucifers the form
of the capsule and the attributes of the valves and seeds are usually
considered to furnish the characteristics of genera, and this point has
been elucidated at some length by Solms-Laubach. There is, however, no
sufficient reason to construe a new genus on the ground of Heeger's
globular fruited shepherd's purse; but as a true elementary species, and
even as a good systematic species it has proved itself, and as such it
is described by Solms-Laubach, who named it in honor of its discoverer.
Exactly analogous discoveries have been [586] instead of displaying a
bright yellow cup. _O. cruciata_ grows in the Adirondack Mountains, in
the states of New York and Vermont, and seems to be abundant there. It
has been introduced into botanical gardens and yielded a number of
hybrids, especially with _O. biennis and _O. lamarckiana_, and the
narrow petals of the parent-species may be met with in combination with
the stature and vegetative characteristics of these last named species.
_O. cruciata_ has a purple foliage, while _biennis_ and _lamarckiana_
are green, and many of the hybrids may instantly be recognized by their
purple color.
The curious attribute of the petals is not to be considered simply as a
reduction in size. On anatomical inquiry it has been found that these
narrow petals bear some characteristics which, on the normal plants, are
limited to the calyx. Stomata and hairs, and the whole structure of the
surface and inner tissues on some parts of these petals are exactly
similar to those of the calyx, while on others they have retained the
characteristics of petals. Sometimes there may even be seen by the naked
eye green longitudinal stripes of calyx-like structure alternating with
bright yellow petaloid parts. For these reasons the cruciata character
may be considered as a case of sepalody of the petals, or of the petals
being partly converted into sepals.
[587] It is worth while to note that as a monstrosity this occurrence is
extremely rare throughout the whole vegetable kingdom, and only very few
instances have been recorded.
Two cases of sudden mutations have come to my knowledge, producing this
same anomaly in allied species. One has been already alluded to; it
pertains to the common evening-primrose or _Oenothera biennis_, and one
is a species belonging to another genus of the same family, the great
hairy willow-herb or _Epilobium _hirsutum_. I propose to designate both
new forms by the varietal name of _cruciata_, or _cruciatum_.
_Oenothera biennis cruciata_ was found in a native locality of the _O.
biennis itself. It consisted of only one plant, showing in all its
flowers the _cruciata_ marks. In all other respects it resembled wholly
the _biennis_, especially in the pure green color of its foliage, which
at once excluded all suspicion of hybrid origin with the purple _O.
cruciata_. Moreover in our country this last occurs only in the
cultivated state in botanical gardens.
Intermediates were not seen, and as the plant bore some pods, it was
possible to test its constancy. I raised about 500 plants from its
seeds, out of which more than 100 flowered in the first year. The others
were partly kept through the winter and flowered next year. Seeds saved
in [588] both seasons were sown on a large scale. Both the first and the
succeeding generations of the offspring of the original plant came true
without any exception. Intermediates are often found in hybrid cultures,
and in them the character is a very variable one, but as yet they were
not met with in progeny of this mutant. All these plants were exactly
like _O. biennis_, with the single exception of their petals.
_Epilobium hirsutum cruciatum_ was discovered by John Rasor near
Woolpit, Bury St. Edmunds, in England. It flowered in one spot,
producing about a dozen stems, among large quantities of the
parent-species, which is very common there, as it is elsewhere in
Europe. This species is a perennial, multiplying itself by underground
runners, and the stems of the new variety were observed to stand so
close to each other that they might be considered as the shoots of one
individual. In this case this specimen might probably be the original
mutant, as the variety had not been seen on that spot in previous years,
even as it has not been found elsewhere in the vicinity.
Intermediates were not observed, though the difference is a very
striking one. In the cruciate flowers the broad and bright purple petals
seem at first sight to be wholly wanting. They are too weak to expand
and to reflex the calyx [589] as in the normal flowers of the species.
The sepals adhere to one another, and are only opened at their summit by
the protruding pistils. Even the stamens hardly come to light. At the
period of full bloom the flowers convey only the idea of closed buds
crowned by the conspicuous white cross of the stigma. Any intermediate
form would have at once betrayed itself by larger colored petals, coming
out of the calyx-sheath. The cruciate petals are small and linear and
greenish, recalling thereby the color of the sepals.
Mr. Rasor having sent me some flowers and some ripe capsules of his
novelty, I sowed the latter in my experimental garden, where the plant
flowered in large numbers and with many thousands of flowers both in
1902 and 1903. All of these plants and all of these flowers repeated the
cruciate type exactly, and not the slightest impurity or tendency to
partial reversion has been observed.
Thus true and constant cruciate varieties have been produced from
accidentally observed initial plants, and because of their very curious
characters they will no doubt be kept in botanical gardens, even if they
should eventually become lost in their native localities.
At this point I might note another observation made on the wild species
of _Oenothera cruciata_ [590] from the Adirondacks. Through the kindness
of Dr. MacDougal, of the New York Botanical Garden, I received seeds
from Sandy Hill near Lake George. When the plants, grown from these
seeds, flowered, they were not a uniform lot, but exhibited two distinct
types. Some had linear petals and thin flower-buds, and in others the
petals were a little broader and the buds more swollen. The difference
was small, but constant on all the flowers, each single plant clearly
belonging to one or the other of the two types. Probably two elementary
species were intermixed here, but whether one is the systematic type and
the other a mutation, remains to be seen.
Nor seem these two types to exhaust the range of variability of
_Oenothera cruciata_. Dr. B.L. Robinson of Cambridge, Mass., had the
kindness to send me seeds from another locality in the same region. The
seeds were collected in New Hampshire and in my garden produced a true
and constant _cruciata_, but with quite different secondary characters
from both the aforesaid varieties. The stems and flower-spikes and even
the whole foliage were much more slender, and the calyx-tubes of the
flowers were noticeably more elongated. It seems not improbable that
_Oenothera cruciata_ includes a group of lesser unities, and may prove
to comprise a [591] swarm of elementary species, while the original
strain might even now be still in a condition of mutability. A close
scrutiny in the native region is likely to reveal many unexpected
features.
A very interesting novelty has already been described in a former
lecture. It is the _Xanthium wootoni_, discovered in the region about
Las Vegas, New Mexico, by T.D.A. Cockerell. It is similar in all
respects to _X. commune_, but the burrs are more slender and the
prickles much less numerous, and mostly stouter at their base. It grows
in the same localities as the _X. commune_, and is not recorded to occur
elsewhere. Whether it is an old variety or a recent mutation it is of
course impossible to decide. In a culture made in my garden from the
seed sent me by Mr. Cockerell, I observed (1903) that both forms had a
subvariety with brownish foliage, and, besides this, one of a pure
green. Possibly this species, too, is still in a mutable condition.
Perhaps the same may be asserted concerning the beautiful shrub,
_Hibiscus Moscheutos_, observed in quite a number of divergent types by
John W. Harshberger. They grew in a small meadow at Seaside Park, New
Jersey, in a locality which had been undisturbed for years. They
differed from each other in nearly all the [592] organs, in size, in the
diameter of the stems, which were woody in some and more fleshy in
others, in the shape of the foliage and in the flowers. More than twenty
types could be distinguished and seeds were saved from a number of them,
in order to ascertain whether they are constant, or whether perhaps a
main stem in a mutating condition might be found among them. If this
should prove to be the case, the relations between the observed forms
would probably be analogous to those between the _O. lamarckiana_ and
its derivatives.
Many other varieties have sprung from the type-species under similar
conditions from time to time. A fern-leaved mercury, _Mercurialis annua
laciniata_, was discovered in the year 1719 by Marchant. The type was
quite new at the time and maintained itself during a series of years.
The yellow deadly nightshade or _Atropa Belladonna lutea_ was found
about 1850 in the Black Forest in Germany in a single spot, and has
since been multiplied by seeds. It is now dispersed in botanical
gardens, and seems to be quite constant. A dwarf variety of a bean,
_Phaseolus lunatus_, was observed to spring from the ordinary type by a
sudden leap about 1895 by W.W. Tracy, and many similar cases could be
given.
The annual habit is not very favorable for [593] the discovery of new
forms in the wild state. New varieties may appear, but may be crowded
out the first year. The chances are much greater with perennials, and
still greater with shrubs or trees. A single aberrant specimen may live
for years and even for centuries, and under such conditions is pretty
sure to be discovered sooner or later. Hence it is no wonder that many
such cases are on record. They have this in common that the original
plant of the variety has been found among a vast majority of
representatives of the corresponding species. Nothing of course is
directly known about its origin. Intermediate links have as a rule been
wanting, and the seeds, which have often been sown, have not yielded
reliable results, as no care was taken to preserve the blossoms from
intercrossing with their parent-forms.
Stress should be laid upon one feature of these curious occurrences.
Relatively often the same novelty has been found twice or thrice, or
even more frequently, and under conditions which make it very improbable
that any relation between such occurrences might exist. The same
mutation must have taken place more than once from the same main stem.
The most interesting of these facts are connected with the origin of the
purple beech, which [594] is now so universally cultivated. I take the
following statements from an interesting historical essay of Prof.
Jaggi. He describes three original localities. One is near the Swiss
village, Buch am Irchel, and is located on the Stammberg. During the
17th century five purple beeches are recorded to have grown on this
spot. Four of them have died, but one is still alive. Seedlings have
germinated around this little group, and have been mostly dug up and
transplanted into neighboring gardens. Nothing is known about the real
origin of these plants, but according to an old document, it seems that
about the year 1190 the purple beeches of Buch were already enjoying
some renown, and attracting large numbers of pilgrims, owing to some old
legend. The church of Embrach is said to have been built in connection
with this legend, and was a goal for pilgrimages during many centuries.
A second native locality of the purple beech is found in a forest near
Sondershausen in Thuringen, Germany, where a fine group of these trees
is to be seen. They were mentioned for the first time in the latter half
of the eighteenth century, but must have been old specimens long before
that time. The third locality seems to be of much later origin. It is a
forest near Roveredo in South Tyrol, where a new [595] university is
being erected. It is only a century ago that the first specimens of the
purple beech were discovered there.
As it is very improbable that the two last named localities should have
received their purple beeches from the first named forest, it seems
reasonable to assume that the variety must have been produced at least
thrice.
The purple beech is now exceedingly common in cultivation. But Jaggi
succeeded in showing that all the plants owe their origin to the
original trees mentioned above, and are, including nearly all cultivated
specimens with the sole exception of the vicinity of Buch, probably
derived from the trees in Thuringen. They are easily multiplied by
grafting, and come true from seed, at least often, and in a high
proportion. Whether the original trees would yield a pure progeny if
fertilized by their own pollen has as yet not been tested. The young
seedlings have purple seed-leaves, and may easily be selected by this
character, but they seem to be always subjected in a large measure to
vicinism.
Many other instances of trees and shrubs, found in accidental specimens
constituting a new variety in the wild state, might be given. The
oak-leaved beech has been found in a forest of Lippe-Detmold in Germany
and near Versailles, [596] whence it was introduced into horticulture by
Carriere. Similarly divided and cleft leaves seem to have occurred more
often in the wild state, and cut-leaved hazels are recorded from Rouen
in France, birches and alders from Sweden and Lapland, where both are
said to have been met with in several forests. The purple barberry was
found about 1830 by Bertin, near Versailles. Weeping varieties of ashes
were found wild in England and in Germany, and broom-like oaks, _Quercus
pedunculata fastigiata_, are recorded from Hessen-Darmstadt, Calabria,
the Pyrenees and other localities. About the real origin of all these
varieties nothing is definitely known.
The "single-leaved" strawberry is a variety often seen in botanical
gardens, as it is easily propagated by its runners. It was discovered
wild in Lapland at the time of Linnaeus, and appeared afterwards
unexpectedly in a nursery near Versailles. This happened about the year
1760 and Duchesne tested it from seeds and found it constant. This
strain, however, seems to have died out before the end of the 18th
century. In a picture painted by Holbein (1495-1543), strawberry leaves
can be seen agreeing exactly with the monophyllous type. The variety may
thus be assumed to have arisen independently [597] at least thrice, at
different periods and in distant localities.
From all these statements and a good many others which can be found in
horticultural and botanical literature, it may be inferred that
mutations are not so very rare in nature as is often supposed. Moreover
we may conclude that it is a general rule that they are neither preceded
nor accompanied by intermediate steps, and that they are ordinarily
constant from seed from the first.
Why then are they not met with more often? In my opinion it is the
struggle for life which is the cause of this apparent rarity; which is
nothing else than the premature death of all the individuals that so
vary from the common type of their species as to be incapable of
development under prevailing circumstances. It is obviously without
consequence whether these deviations are of a fluctuating or of a
mutating nature. Hence we may conclude that useless mutations will soon
die out and will disappear without leaving any progeny. Even if they are
produced again and again by the same strain, but under the same
unfavorable conditions, there will be no appreciable result.
Thousands of mutations may perhaps take place yearly among the plants of
our immediate vicinity without any chance of being discovered. [598] We
are trained to the appreciation of the differentiating marks of
systematic species. When we have succeeded in discerning these as given
by our local flora lists, we rest content. Meeting them again we are in
the habit of greeting them with their proper names. Such is the
satisfaction ensuing from this knowledge that we do not feel any
inclination for further inquiry. Striking deviations, such as many
varietal characters, may be remarked, but then they are considered as
being of only secondary interest. Our minds are turned from the
delicately shaded features which differentiate elementary species.
Even in the native field of the evening-primroses, no botanist would
have discovered the rosettes with smaller or paler leaves, constituting
the first signs of the new species. Only by the guidance of a distinct
theoretical idea were they discovered, and having once been pointed out
a closer inspection soon disclosed their number.
Variability seems to us to be very general, but very limited. The limits
however, are distinctly drawn by the struggle for existence. Of course
the chance for useful mutations is a very small one. We have seen that
the same mutations are as a rule repeated from time to time by the same
species. Now, if a useful mutation, [599] or even a wholly indifferent
one, might easily be produced, it would have been so, long ago, and
would at the present time simply exist as a systematic variety. If
produced anew somewhere the botanist, would take it for the old variety
and would omit to make any inquiry as to its local origin.
Thousands of seeds with perhaps wide circles of variability are ripened
each year, but only those that belong to the existing old narrow circles
survive. How different would Nature appear to us if she were free to
evolve all her potentialities!
Darwin himself was struck with this lack of harmony between common
observations and the probable real state of things. He discussed it in
connection with the cranesbill of the Pyrenees (_Geranium pyrenaicum_).
He described how this fine little plant, which has never been
extensively cultivated, had escaped from a garden in Staffordshire and
had succeeded in multiplying itself so as to occupy a large area.
In doing so it had evidently found place for an uncommonly large number
of plantlets from its seeds and correspondingly it had commenced to vary
in almost all organs and qualities and nearly in all imaginable
directions. It displayed under these exceptional circumstances a
capacity which never had been exceeded and [600] which of course would
have remained concealed if its multiplication had been checked in the
ordinary way.
Many species have had occasion to invade new regions and cover them with
hundreds of thousands of individuals. First are to be cited those
species which have been introduced from America into Europe since the
time of Columbus, or from Europe into this country. Some of them have
become very common. In my own country the evening-primroses and Canada
fleabane or are examples, and many others could be given. They should be
expected to vary under these circumstances in a larger degree. Have they
done so? Manifestly they have not struck out useful new characters that
would enable their bearers to found new elementary species. At least
none have been observed. But poor types might have been produced, and
periods of mutability might have been gone through similar to that which
is now under observation for Lamarck's evening primrose in Holland.
From this discussion we may infer that the chances of discovering new
mutating species are great enough to justify the utmost efforts to
secure them. It is only necessary to observe large numbers of plants,
grown under circumstances which allow the best opportunities for [601]
all the seeds. And as nature affords such opportunities only at rare
intervals, we should make use of artificial methods. Large quantities of
seed should be gathered from wild plants and sowed under very favorable
conditions, giving all the nourishment and space required to the young
seedlings. It is recommended that they be sown under glass, either in a
glass-house or protected against cold and rain by glass-frames. The same
lot of seed will be seen to yield twice or thrice as many seedlings if
thus protected, compared with what it would have produced when sown in
the field or in the garden. I have nearly wholly given up sowing seeds
in my garden, as circumstances can be controlled and determined with
greater exactitude when the sowing is done in a glasshouse.
The best proof perhaps, of the unfavorable influence of external
conditions for slightly deteriorated deviations is afforded by
variegated leaves. Many beautiful varieties are seen in our gardens and
parks, and even corn has a variety with striped leaves. They are easily
reproduced, both by buds and by seeds, and they are the most ordinary of
all varietal deviations. They may be expected to occur wild also. But no
real variegated species, nor even good varieties with this attribute
occurs in nature. [602] On the other hand occasional specimens with a
single variegated leaf, or with some few of them, are actually met with,
and if attention is once drawn to this question, perhaps a dozen or so
instances might be brought together in a summer. But they never seem to
be capable of further evolution, or of reproducing themselves
sufficiently and of repeating their peculiarity in their progeny. They
make their appearance, are seen during a season, and then disappear.
Even this slight incompleteness of some spots on one or two leaves may
be enough to be their doom.
It is a common belief that new varieties owe their origin to the direct
action of external conditions and moreover it is often assumed that
similar deviations must have similar causes, and that these causes may
act repeatedly in the same species, or in allied, or even systematically
distant genera. No doubt in the end all things must have their causes,
and the same causes will lead under the same circumstances to the same
results. But we are not justified in deducing a direct relation between
the external conditions and the internal changes of plants. These
relations may be of so remote a nature that they cannot as yet be
guessed at. Therefore only direct experience may be our guide. Summing
up the result of our facts and discussions [603] we may state that wild
new elementary species and varieties are recorded to have appeared from
time to time. Invariably this happened by sudden leaps and without
intermediates. The mutants are constant when propagated by seed, and at
once constitute a new race. In rare instances this may be of sufficient
superiority to win a place for itself in nature, but more often it has
qualities which have led to its introduction into gardens as an
ornamental plant or into botanical gardens by reason of the interest
afforded by their novelty, or by their anomaly.
Many more mutations may be supposed to be taking place all around us,
but artificial sowings on a large scale, combined with a close
examination of the seedlings and a keen appreciation of the slightest
indications of deviation seem required to bring them to light.
[604]
LECTURE XXI
MUTATIONS IN HORTICULTURE
It is well known that Darwin based his theory of natural selection to a
large extent upon the experience of breeders. Natural and artificial
selection exhibit the same general features, yet it was impossible in
Darwin's time to make a critical and comparative analysis of the two
processes.
In accordance with our present conception there is selection of species
and selection within the species. The struggle for life determines which
of a group of elementary species shall survive and which shall
disappear. In agricultural practice the corresponding process is usually
designated by the name of variety-testing. Within the species, or within
the variety, the sieve of natural selection is constantly eliminating
poor specimens and preserving those that are best adapted to live under
the given conditions. Some amelioration and some local races are the
result, but this does not appear to be of much importance. On the
contrary, the selection [605] within the race holds a prominent place in
agriculture, where it is known by the imposing term, race-breeding.
Experience and methods in horticulture differ from those in agriculture
in many points. Garden varieties have been tested and separated for a
long time, but neither vegetables nor flowers are known to exhibit such
motley groups of types as may be seen in large forage crops.
New varieties which appear from time to time may be ornamental or
otherwise in flowers, and more or less profitable than their parents in
vegetables and fruits. In either case the difference is usually
striking, or if not, its culture would be unprofitable.
The recognition of useful new varieties being thus made easy, the whole
attention of the breeder is reduced to isolating the seeds of the
mutants that are to be saved and sown separately, and this process must
be repeated during a few years, in order to produce the quantity of seed
that is needed for a profitable introduction of the variety into
commerce. In proportion to the abundance of the harvest of each year
this period is shorter for some and longer for other species.
Isolation in practice is not so simple nor so easy an affair as it is in
the experimental garden. Hence we have constant and nearly unavoidable
[606] cross-fertilizations with the parent form or with neighboring
varieties, and consequent impurity of the new strain. This impurity we
have called vicinism, and in a previous lecture have shown its effects
upon the horticultural races on one hand, and on the other, on the
scientific value that can be ascribed to the experience of the breeder.
We have established the general rule that stability is seldom met with,
but that the observed instability is always open to the objection of
being the result of vicinism. Often this last agency is its sole cause;
or it may be complicated with other factors without our being able to
discern them.
Though our assertion that the practice of the horticulturist in
producing new varieties is limited to isolation, whenever chance affords
them, is theoretically valid, it is not always so. We may discern
between the two chief groups of varieties. The retrograde varieties are
constant, the individuals not differing more from one another than those
of any ordinary species. The highly variable varieties play an important
part in horticulture. Double flowers, striped flowers, variegated leaves
and some others yield the most striking instances. Such forms have been
included in previous lectures among the ever-sporting varieties, because
their peculiar characters oscillate between two extremes, viz: [607] the
new one of the variety and the corresponding character of the original
species.
In such cases isolation is usually accompanied by selection: rarely has
the first of a double, striped or variegated race well filled or richly
striped flowers or highly spotted leaves. Usually minor degrees of the
anomaly are seen first, and the breeder expects the novelty to develop
its features more completely and more beautifully in subsequent
generations. Some varieties need selection only in the beginning, in
others the most perfect specimens must be chosen every year as
seed-bearers. For striped flowers, it has been prescribed by Vilmorin,
that seeds should be taken only from those with the smallest stripes,
because there is always reversion. Mixed seed or seed from medium types
would soon yield plants with too broad stripes, and therefore less
diversified flowers.
In horticulture, new varieties, both retrograde and ever-sporting, are
known to occur almost yearly. Nevertheless, not every novelty of the
gardener is to be considered as a mutation in the scientific sense of
the word. First of all, the novelties of perennial and woody species are
to be excluded. Any extreme case of fluctuating variability may be
preserved and multiplied in the vegetative way. Such types are
designated [608] in horticulture as varieties, though obviously they are
of quite another nature than the varieties reproduced by seed. Secondly,
a large number, no doubt the greater number of novelties, are of hybrid
origin. Here we may discern two cases. Hybrids may be produced by the
crossing of old types, either of two old cultivated forms or newly
introduced species, or ordinarily between an old and an introduced
variety. Such novelties are excluded from our present discussion.
Secondly, hybrids may be produced between a true, new mutation and some
of the already existing varieties of the same species. Examples of this
obvious and usual practice will be given further on, but it must be
pointed out now that by such crosses a single mutation may produce as
many novelties as there are available varieties of the same species.
Summarizing these introductory remarks we must lay stress on the fact
that only a small part of the horticultural novelties are real
mutations, although they do occur from time to time. If useful, they are
as a rule isolated and multiplied, and if necessary, improved by
selection. They are in many instances, as constant from seed as the
unavoidable influence of vicinism allows them to be. Exact observations
on the origin, or on the degree of constancy, are usually lacking, [609]
the notes being ordinarily made for commercial purposes, and often only
at the date of introduction into trade, when the preceding stages of the
novelty may have been partly forgotten.
With this necessary prelude I will now give a condensed survey of the
historical facts relating to the origin of new horticultural varieties.
An ample description has been given recently by Korshinsky, a Russian
writer, who has brought together considerable historical material as
evidence of the sudden appearance of novelties throughout the whole
realm of garden plants.
The oldest known, and at the same time one of the most accurately
described mutations is the origin of the cut-leaved variety of the
greater celandine or _Chelidonium majus_. This variety has been
described either as such, or as a distinct species, called _Chelidonium
laciniatum_ Miller.
It is distinguished from the ordinary species, by the leaves being cut
into narrow lobes, with almost linear tips, a character which is, as we
have seen on a previous occasion, repeated in the petals. It is at
present nearly as commonly cultivated in botanical gardens as the _C.
majus_, and has escaped in many localities and is observed to thrive as
readily as the native wild [610] plants. It was not known until a few
years before the close of the 16th century. Its history has been
described by the French botanist, Rose. It was seen for the first time
in the garden of Sprenger, an apothecary of Heidelberg, where the _C.
majus_ had been cultivated for many years. Sprenger discovered it in the
year 1590, and was struck by its peculiar and sharply deviating
characters. He was anxious to know whether it was a new plant and sent
specimens to Clusius and to Plater, the last of whom transmitted them to
Caspar Bauhin. These botanists recognized the type as quite new and
Bauhin described it some years afterwards in his Phytopinax under the
name of _Chelidonium majus foliis quernis_, or oak-leaved celandine. The
new variety soon provoked general interest and was introduced into most
of the botanical gardens of Europe. It was recognized as quite new, and
repeated search has been made for it in a wild state, but in vain. No
other origin has been discovered than that of Sprenger's garden.
Afterwards it became naturalized in England and elsewhere, but there is
not the least doubt as to its derivation in all the observed cases.
Hence its origin at Heidelberg is to be considered as historically
proven, and it is of course only legitimate to assume that it originated
in [611] the year 1590 from the seeds of the _C. majus_. Nevertheless,
this was not ascertained by Sprenger, and some doubt as to a possible
introduction from elsewhere might arise. If not, then the mutation must
have been sudden, occurring without visible preparation and without the
appearance of intermediates.
From the very first, the cut-leaved celandine has been constant from
seed. Or at least it has been propagated by seed largely and without
difficulty. Nothing, however, is known about it in the first few years
of its existence. Later careful tests were made by Miller, Rose and
others and later by myself, which have shown its stability to be
absolute and without reversion, and it has probably been so from the
beginning. The fact of its constancy has led to its specific distinction
by Miller, as varieties were in his time universally, and up to the
present time not rarely, though erroneously, believed to be less stable
than true species.
Before leaving the laciniate celandine it is to be noted that in crosses
with _C. majus_ it follows the law of Mendel, and for this reason should
be considered as a retrograde variety, the more so, as it is also
treated as such from a morphological point of view by Stahl and others.
We now come to an enumeration of those cases in which the date of the
first appearance [612] of a new horticultural variety has been recorded,
and I must apologize for the necessity of again quoting many variations,
which have previously been dealt with from another point of view. In
such cases I shall limit myself as closely as possible to historical
facts. They have been recorded chiefly by Verlot and Carriere, who wrote
in Paris shortly after the middle of the past century, and afterwards by
Darwin, Korshinsky, and others. It is from their writings and from
horticultural literature at large that the following evidence is brought
together.
A very well-known instance is that of the dwarf variety of _Tagetes
signata_, which arose in the nursery of Vilmorin in the year 1860. It
was observed for the first time in a single individual among a lot of
the ordinary _Tagetes signata_. It was found impossible to isolate it,
but the seeds were saved separately. The majority of the offspring
returned to the parental type, but two plants were true dwarfs. From
these the requisite degree of purity for commercial purposes was
reached, the vicinists not being more numerous than 10% of the entire
number. The same mutation had been observed a year earlier in the same
nursery in a lot of _Saponaria calabrica_. The seeds of this dwarf
repeated the variety in the next generation, but in the third none were
observed. Then the variety was [613] thought to be lost, and the culture
was given up, as the Mendelian law of the splitting of varietal hybrids
was not known. According to our present knowledge we might expect the
atavistic descendants of the first dwarf to be hybrids, and to be liable
to split in their progeny into one-fourth dwarfs and three-fourths
normal specimens. From this it is obvious that the dwarfs would have
appeared a second time if the strain had been continued by means of the
seeds of the vicinistic progeny.
In order to avoid a return to this phase of the question, another use of
the vicinists should at once be pointed out. It is the possibility of
increasing the yield of the new variety. If space admits of sowing the
seeds of the vicinists, a quarter of the progeny may be expected to come
true to the new type, and if they were partly pollinated by the dwarfs,
even a larger number would do so. Hence it should be made a rule to sow
these seeds also, at least when those of the true representatives of the
novelty do not give seed enough for a rapid multiplication.
Other dwarfs are recorded to have sprung from species in the same sudden
and unexpected manner, as for instance _Ageratum coeruleum_ of the same
nursery, further _Clematis Viticella nana_ and _Acer campestre nanum_.
_Prunus Mahaleb nana_ was discovered in 1828 in one [614] specimen near
Orleans by Mme. LeBrun in a large culture of Mahaleb. _Lonicera tatarica
nana_ appeared in 1825 at Fontenay-aux Roses. A tall variety of the
strawberry is called "Giant of Zuidwijk" and originated at Boskoop in
Holland in the nursery of Mr. van de Water, in a lot of seedlings of the
ordinary strawberry. It was very large, but produced few runners, and
was propagated with much difficulty, for after six years only 15 plants
were available. It proved to be a late variety with abundant large
fruit, and was sold at a high price. For a long time it was prominent in
cultures in Holland only.
Varieties without prickles are known to have originated all of a sudden
in sundry cases. _Gleditschia sinensis_, introduced in 1774 from China,
gave two seedlings without spines in the year 1823, in the nursery of
Caumzet. It is curious in being one of the rare instances where a
simultaneous mutation in two specimens is acknowledged, because as a
rule, such records comply with the prevailing, though inexact, belief
that horticultural mutations always appear in single individuals.
From Korshinsky's survey of varieties with cut leaves or laciniate forms
the following cases may be quoted. In the year 1830 a nurseryman named
Jacques had sown a large lot of elms, [615] _Ulmus pedunculata_. One of
the seedlings had cut leaves. He multiplied it by grafting and gave it
to the trade under the name of _U. pedunculata urticaefolia_. It has
since been lost.
Laciniate alders seem to have been produced by mutation at sundry times.
Mirbel says that the _Alnus glutinosa laciniata_ is found wild in
Normandy and in the forests of Montmorency near Paris. A similar variety
has been met with in a nursery near Orleans in the year 1855. In
connection with this discovery some discussion has arisen concerning the
question whether it was probable that the Orleans strain was a new
mutation, or derived in some way from the trees cited by Mirbel. Of
course, as always in such cases, any doubt, once pronounced, affects the
importance of the observation for all time, since it is impossible to
gather sufficient historical evidence to fully decide the point. The
same variety had appeared under similar circumstances in a nursery at
Lyons previously (1812).
Laciniated maples are said to be of relatively frequent occurrence in
nurseries, among seedlings of the typical species. Loudon says that once
100 laciniated seedlings were seen to originate from seed of some normal
trees. But in this case it is rather probable that the presumed [616]
normal parents were in reality hybrids between the type and the
laciniated form, and simply split according to Mendel's law. This
hypothesis is partly founded on general considerations and partly on
experiments made by myself with the cut-leaved celandine, previously
alluded to, which I crossed with the type. The hybrids repeated the
features of the species and showed no signs of their internal hybrid
constitution. But the following year one-fourth of their progeny
returned to the cut-leaved form. If the same thing has taken place in
the case of Loudon's maples, but without their hybrid origin being
known, the result would have been precisely what he observed.
_Broussonetia papyriffera dissecta_ originated about 1830 at Lyons, and
a second time in 1866 at Fontenay-aux-Roses. The cut-leaved hazelnuts,
birches, beeches and others have mostly been found in the wild state, as
I have already pointed out in a previous lecture. A similar variety of
the elder, _Sambucus nigra laciniata_, and its near ally, _Sambucus
racemosa laciniata_, are often to be seen in our gardens. They have been
on record since 1886 and come true from seed, but their exact origin
seems to have been forgotten. Cut-leaved walnuts have been known since
1812; they come true from seed, but are extremely liable to vicinism, a
nuisance which is [617] ascribed by some authors to the fact that often
on the same tree the male catkins flower and fall off several weeks
before the ripening of the pistils of the other form of flowers.
Weeping varieties afford similar instances. _Sophora japonica pendula_
originated about 1850, and _Gleditschia triacanthos pendula_ some time
later in a nursery at Chateau-Thierry (Aisne, France). In the year 1821
the bird's cherry, or _Prunus Padus_, produced a weeping variety, and in
1847 the same mutation was observed for the allied _Prunus Mahaleb_.
Numerous other instances of the sudden origin of weeping trees, both of
conifers and of others, have been brought together in Korshinsky's
paper. This striking type of variation includes perhaps the best
examples of the whole historical evidence. As a rule they appear in
large sowings, only one, or only a few at a time. Many of them have not
been observed during their youth, but only after having been planted out
in parks and forests, since the weeping characters show only after
several years.
The monophyllous bastard-acacia originated in the same way. Its
peculiarities will be dealt with on another occasion, but the
circumstances of its birth may as well be given here. In 1855 in the
nursery of Deniau, at Brain-sur-l'Authion (Maine et Loire), it appeared
in a lot of [618] seedlings of the typical species in a single
individual. This was transplanted into the Jardin des Plantes at Paris,
where it flowered and bore seeds in 1865. It must have been partly
pollinated by the surrounding normal representatives of the species,
since the seeds yielded only one-fourth of true offspring. This
proportion, however, has varied in succeeding years. Briot remarks that
the monophyllous bastard acacia is liable to petaloid alterations of its
stamens, which deficiency may encroach upon its fertility and
accordingly upon the purity of its offspring.
Broom-like varieties often occur among trees, and some are known for
their very striking reversions by buds, as we have seen on a previous
occasion. They are ordinarily called pyramidal or fastigiate forms, and
as far as their history goes, they arise suddenly in large sowings of
the normal species. The fastigiate birch was produced in this way by
Baumann, the _Abies concolor fastigiata_ by Thibault and Keteleer at
Paris, the pyramidal cedar by Paillat, the analogous form of
_Wellingtonia_ by Otin. Other instances could easily be added, though of
course some of the most highly prized broom-like trees are so old that
nothing is known about their origin. This, for instance, is the case
with the pyramidal yew-tree, _Taxus baccata fastigiata_. [619] Others
have been found wild, as already mentioned in a former lecture.
An analogous case is afforded by the purpleleaved plums, of which the
most known form is Prunus Pissardi. It is said to be a purple variety of
_Prunus cerasifera_, and was introduced at the close of the seventies
from Persia, where it is said to have been found in Tabris. A similar
variety arose independently and unexpectedly in the nursery of Spath,
near Berlin, about 1880, but it seems to differ in some minor points
from the Persian prototype.
A white variety of _Cyclamen vernum_ made its appearance in the year
1836 in Holland. A single individual was observed for the first time
among a large lot of seedlings, in a nursery near Haarlem. It yielded a
satisfactory amount of seed, and the progeny was true to the new type.
Such plants propagate slowly, and it was only twenty-seven years later
(1863) that the bulbs were offered for sale by the Haarlem firm of
Krelage & Son. The price of each bulb was $5.00 in that year, but soon
afterwards was reduced to $1.00 each, which was about thrice the
ordinary price of the red variety.
The firm of Messrs. Krelage & Son has brought into commerce a wide range
of new bulb-varieties, all due to occasional mutations, some by seed and
others by buds, or to the accidental [620] transference of new qualities
into the already existing varieties by cross-pollination through the
agency of insects. Instead of giving long lists of these novelties, I
may cite the black tulips, which cost during the first few years of
their introduction about $25.00 apiece.
Horticultural mutations are as a rule very rare, especially in genera or
species which have not yet been brought to a high degree of variability.
In these the wide range of varieties and the large scale in which they
are multiplied of course give a greater chance for new varieties. But
then the possibilities of crossing are likewise much larger, and
apparent changes due to this cause may easily be taken for original
mutations.
The rarity of the mutations is often proved by the lapse of time between
the introduction of a species and its first sport. Some instances may be
given. They afford a proof of the length of the period during which the
species remained unaltered, although some of these alterations may be
due to a cross with an allied form. _Erythrina Crista-galli_ was
introduced about 1770, and produced its first sport in 1884, after more
than a century of cultivation. _Begonia semperflorens_ has been
cultivated since 1829, and for half a century before it commenced
sporting. The same length of time has elapsed [621] between the first
culture and the first variation of _Crambe maritima_. Other cases are on
record in which the variability exhibited itself much sooner, perhaps
within a few years after the original discovery of the species. But such
instances seem, as a rule, to be subject to doubt as to the concurrence
of hybridization. So for instance the _Iris lortetii_, introduced in the
year 1895 from the Lebanon, which produced a white variety from its very
first seeds. If by chance the introduced plants were natural hybrids
between the species and the white variety, this apparent and rather
improbable mutation would find a very simple explanation. The length of
the period preceding the first signs of variability is largely, of
course, due to divergent methods of culture. Such species as
_Erythrina_, which are perennial and only sown on a small scale, should
not be expected to show varieties very soon. Annual species, which are
cultivated yearly in thousands or even hundreds of thousands of
individuals, have a much better chance. Perhaps the observed differences
are largely due to this cause.
Monstrosities have, from time to time, given rise to cultivated races.
The cockscomb or _Celosia_ is one of the most notorious instances.
Cauliflowers, turnips and varieties of cabbages are recorded by De
Candolle to have arisen in [622] culture, more than a century ago, as
isolated monstrous individuals. They come true from seed, but show
deviations from time to time which seem to be intimately linked with
their abnormal characters. Apetalous flowers may be considered as
another form of monstrosity, and in _Salpiglossis sinuata_ such a
variety without a corolla made its appearance in the year 1892 in the
nursery of Vilmorin. It appeared suddenly, yielded a good crop of seed
and was constant from the outset, without any sign of vicinism or
impurity.
In several cases the origin of a variety is obscure, while the
subsequent historical evidence is such as to make an original sudden
appearance quite probable. Although these instances offer but indirect
evidence, and will sooner or later lose their importance, it seems
desirable to lay some stress on them here, because most of these cases
are very obvious and more striking than purely historical facts. Sterile
varieties belong to this heading. Sometimes they bear fruit without
kernels, sometimes flowers without sexual organs, or even no flowers at
all. Instances have been given in the lecture on retrograde varieties;
they are ordinarily assumed to have originated by a leap, because it is
not quite clear how a loss of the capacity for the formation of seeds
could have been slowly accumulated [623] in preceding generations. An
interesting case is afforded by a sterile variety of corn, which
originated some time ago in my own pedigree-cultures made for another
purpose, and which had begun with an ear of 1886. The first generation
from the original seeds showed nothing particular, but the second at
once produced quite a number of sterile plants. The sterility was caused
by the total lack of branches, including those bearing the pistillate
flowers. The terminal spikes themselves were reduced to naked spindles,
without branches, without flowers and even almost without bracts.
In some individuals, however, this negative character was seen to give
way at the tip, showing a few small naked branches. Of course it was
impossible to propagate this curious form, but my observations showed
that it sprang into existence from known ancestors by a single step or
sudden leap. This leap, however, was not confined to a single specimen;
on the contrary it affected 40 plants out of a culture of 340
individuals. The same phenomenon was repeated from the seeds of the
normal plants in the following year, but afterwards the monstrosity
disappeared.
The Italian poplar affords another instance. It is considered by some
authors as a distinct species, _Populus italica_, and by others as a
[624] broom-like variety of the _Populus nigra_, from which it is
distinguished by its erect branches and other characters of minor
importance. It is often called the pyramidal or fastigiate poplar. Its
origin is absolutely unknown and it occurs only in the cultivated state.
In Italy it seems to have been cultivated from the earliest historical
times, but it was not introduced into other countries till the
eighteenth century. In 1749 it was brought into France, and in 1758 into
England, and to day it may be seen along roads throughout central Europe
and in a large part of Asia. But the most curious fact is that it is
only observed in staminate specimens; pistillate trees have not been
found, although often sought for. This circumstance makes it very
probable that the origin of the broom-like poplar was a sudden mutation,
producing only one individual. This being staminate, it has been
propagated exclusively by cuttings. It is to be admitted, however, that
no material evidence is at hand to prove that it is not an original wild
species, the pistillate form of which has been lost by vegetative
multiplication. One form only of many dioecious plants is to be found in
cultivation, as, for instance some South American species of _Ribes_.
Total lack of historical evidence concerning [625] the origin of a
variety has sometimes been considered as sufficient proof of a sudden
origin. The best known instance is that of the renowned cactus-dahlia
with its recurved instead of incurved ray-florets. It was introduced
from Mexico into the Netherlands by Van den Berg of Jutphaas, under the
following remarkable circumstances. In the autumn of 1872 one of his
friends had sent him a small case, containing seeds, bulbs and roots
from Mexico. From one of these roots a _Dahlia_ shoot developed. It was
cultivated with great care and bloomed next year. It surprised all who
saw it by the unexpected peculiarity of its large rich crimson flowers,
the rays of which were reversed tubular. The margins of the narrow rays
were curved backwards, showing the bright color of the upper surface. It
was a very showy novelty, rapidly multiplied by cuttings, and was soon
introduced into commerce. It has since been crossed with nearly all
other available varieties of the _Dahlia_, giving a large and rich group
of forms, bound together by the curious curling of the petals. It has
never been observed to grow in Mexico, either wild or in gardens, and
thus the introduced individual has come to be considered as the first of
its race.
I have already mentioned that the rapid production of large numbers of
new varieties, by [626] means of the crossing of the offspring of a
single mutant with previously existing sorts, is a very common feature
in horticultural practice. It warns us that only a small part of the
novelties introduced yearly are due to real mutations. Further instances
of novelties with such a common origin are the purple-leaved dahlias,
the gooseberries without prickles, the double petunias, erect gloxinias
and many others. Accumulation of characters, acquired in different races
of a species, may easily be effected in this way; in fact it is one of
the important factors in the breeding of horticultural novelties.
I have alluded more than once in this lecture to the question, whether
it is probable that mutations occur in one individual or in more. The
common belief among horticulturists is that, as a rule, they appear in a
single plant. This belief is so widespread that whenever a novelty is
seen for the first time in two or more specimens it is at once suggested
that it might have originated and been overlooked in a previous
generation. Not caring to confess a lack of close observation, the
number of mutants in such cases is usually kept secret. At least this
statement has been made to me by some of the horticulturists at Erfurt,
whom I visited some years ago in order to learn as much as [627]
possible about the methods of production of their novelties. Hence it is
simply impossible to decide the question on the basis of the experience
of the breeders. Even in the case of the same novelty arising in sundry
varieties of the same species, the question as to common origin, by
means of crossing, is often hard to decide, as for instance in
moss-roses and nectarines. On the other hand, instances are on record
where the same novelty has appeared at different times, often at long
intervals. Such is the case with the butterfly-cyclamen, a form with
wide-spreading petals which originated in Martin's nursery in England.
The first time it was seen it was thought to be of no value, and was
thrown away, but when appearing for a second time it was multiplied and
eventually placed on the market. Other varieties of _Cyclamen_, as for
instance the crested forms, are also known to have originated
repeatedly.
In concluding this series of examples of horticultural mutations, I
might mention two cases, which have occurred in my own experimental
garden. The first refers to a tubular _Dahlia_. It has ray-florets, the
ligules of which have their margins grown together so as to form tubes,
with the outer surface corresponding to the pale under-surface of the
corolla.
This novelty originated in a single plant in a [628] culture from the
seed of the dwarf variety "Jules Chretien." The seeds were taken from
introduced plants in my garden, and as the sport has no ornamental value
it is uncertain whether this was the first instance or whether it had
previously occurred in the nursery at Lyons, from whence the bulbs were
secured. Afterwards it proved true from seed, but was very variable,
exhibiting rather the features of an ever-sporting variety.
Another novelty was seen the first time in several individuals. It was a
pink sport of the European cranesbill, _Geranium pratense_. It arose
quite unexpectedly in the summer of 1902 from a striped variety of the
blue species. It was seen in seven specimens out of a lot of about a
hundred plants. This strain was introduced into my garden in 1897, when
I bought two plants under the name of _Geranium pratense album_, which
however proved to belong to the striped variety. From their seeds I
sowed in 1898 a first generation, of which a hundred plants flowered the
next year, and from their seeds I sowed in 1900 the lot which produced
the sport. Neither the introduced plants nor their offspring had
exhibited the least sign of a color-variation, besides the blue and
white stripes. Hence it is very probable that my novelty was a true
first mutation, the more probably [629] so since a pink variety would
without doubt have a certain horticultural value and would have been
preserved if it had occurred. But as far as I have been able to
ascertain, it is as yet unknown, nor has it been described until today.
Summing up the results of this long, though very incomplete, list of
horticultural novelties with a more or less well-known origin, we see
that sudden appearances are the rule. Having once sprung into existence
the new varieties are ordinarily constant, except as affected by
vicinism. Details concerning the process are mostly unavailable or at
least are of very doubtful value. And to this it should be added that
really progressive mutations have hardly been observed in horticulture.
Hence the theoretical value of the facts is far less than might have
been expected.
[630]
LECTURE XXII
SYSTEMATIC ATAVISM
The steady cooperation of progression and retrogression is one of the
important principles of organic evolution. I have dwelt upon this point
more than once in previous lectures. I have tried to show that both in
the more important lines of the general pedigree of the vegetable
kingdom, and in the numerous lateral branches ending in the genera and
species within the families, progression and retrogression are nearly
always at work together. Your attention has been directed to the
monocotyledons as an example, where retrogression is everywhere so
active that it can almost be said to be the prevailing movement.
Reduction in the vegetative and generative organs, in the anatomical
structure and growth of the stems, and in sundry other ways is the
method by which the monocotyledons have originated as a group from their
supposed ancestors among the lower dicotyledonous families.
Retrogression is the leading idea in the larger families of the group,
[631] as for instance in the aroids and the grasses. Retrograde
evolution is also typical in the highest and most highly differentiated
family of the monocotyledons, the orchids, which have but one or two
stamens. In the second place I have had occasion more than once to
assert that retrogression, though seemingly consisting in the
disappearance of some quality, need not, as a rule, be considered as a
complete loss. Quite on the contrary, it is very probable that real
losses are extremely rare, if not wholly lacking. Ordinarily the loss is
only apparent, the capacity becomes inactive only, but is not destroyed.
The character has become latent, as it is commonly stated, and therefore
may return to activity and to the full display of its peculiarity,
whenever occasion offers.
Such a return to activity was formerly called atavism. But as we have
seen, when dealing with the phenomena of latency at large, sundry cases
of latency are to be distinguished, in order to get a clear insight into
these difficult processes.
So it is with atavism, too. If any plant reverts to a known ancestor, we
have a positive and simple case. But ancestors with alternate specific
marks are as a rule neither historically nor experimentally manifest.
They are only reputed to be such, and the presumption rests [632] upon
the systematic affinity between the derivative species and its nearest
probable allies. Such reversions are now to be examined at some length
and may be adequately treated under the head of systematic atavism. To
this form of atavism pertain, on the basis of our definition, those
phenomena by which species assume one or more characters of allies, from
which they are understood to have descended by the loss of the character
under discussion. The phenomena themselves consist in the production of
anomalies and varieties, and as the genetic relation of the latter is
often hardly beyond doubt, the anomalies seem to afford the best
instances for the study of systematic atavism. This study has for its
chief aim the demonstration of the presence of the latent characters,
and to show that they return to activity suddenly and not by a slow and
gradual recovery of the former features. It supports the assertion that
the visible elementary characters are essentially an external display of
qualities carried by the bearers of heredity, and that these bearers are
separate entities, which may be mingled together, but are not fused into
a chaotic primitive life-substance. Systematic atavism by this means
leads us to a closer examination of the internal and concealed causes,
which rule the affinities and divergencies of [633] allied species. It
brings before us and emphasizes the importance of the conception of the
so-called unit-characters.
The primrose will serve as an example. In the second lecture we have
seen that the old species of Linnaeus, the _Primula veris_, was split up
by Jacquin into three smaller ones, which are called _P. officinalis_,
_P. elatior_ and _P. acaulis_. From this systematic treatment we can
infer that these three forms are assumed to be derived from a common
ancestor. Now two of them bear their flowers in bracted whorls,
condensed into umbels at the summits of a scape. The scapes themselves
are inserted in the axils of the basal leaves, and produce the flowers
above them. In the third species, _Primula acaulis_, this scape is
lacking and the flowers are inserted singly in the axils on long slender
stalks. For this reason the species is called acaulescent, indicating
that it has no other stem than the subterranean rootstock. But on closer
inspection we observe that the flower stalks are combined into little
groups, each group occupying the aril of one of the basal leaves. This
fact at once points to an analogy with the umbellate allies, and induces
us to examine the insertion of the flowers more critically. In doing so
we find that they are united at their base so as to constitute a sessile
umbel. [634] The scapes are not absolutely lacking, but only reduced to
almost invisible rudiments.
Relying upon this conclusion we infer that all of the three elementary
species have umbels, some pedunculate and the others not. On this point
they agree with the majority of the allied species in the genus and in
other genera, as for instance in _Androsace_. Hence the conclusion that
the common ancestors were perennial plants with a rootstock bearing
their flowers in umbels or whorls on scapes. Lacking in the _Primula
veris_, these scapes must obviously have been lost at the time of the
evolution of this form.
Proceeding on this line of speculation we at once see that a very
adequate opportunity for systematic atavism is offered here. According
to our general conception the apparent loss of a scape is no proof of a
corresponding internal loss, but might as well be caused simply by the
reduction of the scape-growing capacity to a latent or inactive state.
It might be awakened afterwards by some unknown agency, and return to
activity.
Now this is exactly what happens from time to time. In Holland the
acaulescent primrose is quite a common plant, filling the woods in the
spring with thousands of clusters of bright yellow flowers. It is a very
uniform type, but in [635] some years it is seen to return to atavistic
conditions in some rare individuals. More than once I have observed such
cases myself, and found that the variation is only a partial one,
producing one or rarely two umbels on the same plant, and liable to fail
of repetition when the varying specimens are transplanted into the
garden for further observation. But the fact remains that scapes occur.
The scapes themselves are of varying length, often very short, and
seldom long, and their umbels display the involucre of bracts in a
manner quite analogous to that of the _Primula officinalis_ and _P.
elatior_. To my mind this curious anomaly strongly supports the view of
the latent condition of the scape in the acaulescent species, and that
such a dormant character must be due to a descent from ancestors with
active scapes, seems to be in no need of further reiteration. Returning
to activity the scapes at once show a full development, in no way
inferior to that of the allied forms, and only unstable in respect to
their length.
A second example is afforded by the bracts of the crucifers. This group
is easily distinguished by its cruciform petals and the grouping of the
flowers into long racemes. In other families each flower of such an
inflorescence would be subtended by a bract, according to the [636]
general rule that in the higher plants side branches are situated in the
arils of leaves. Bracts are reduced leaves, but the spikes of the
cruciferous plants are generally devoid of them. The flower-stalks, with
naked bases, seem to arise from the common axis at indefinite points.
Hence the inference that crucifers are an exception to a general rule,
and that they must have originated from other types which did comply
with this rule, and accordingly were in the possession of floral bracts.
Or, in other words, that the bracts must have been lost during the
original evolution of the whole family. This conclusion being accepted,
the accidental re-apparition of bracts within the family must be
considered as a case of systematic atavism, quite analogous to the
re-appearance of the scapes in the acaulescent primrose. The systematic
importance of this phenomenon, however, is far greater than in the first
case, in which we had only to deal with a specific character, while the
abolition of the bracts has become a feature of a whole family.
This reversion is observed to take place according to two widely
different principles. On one hand, bracts may be met with in a few stray
species, assuming the rank of a specific character. On the other hand
they may be seen [637] to occur as an anomaly, incompletely developed,
often very rare and with all the appearance of an accidental variation,
but sometimes so common as to seem nearly normal.
Coming now to particular instances, we may turn our attention in the
first place to the genus _Sisymbrium_. This is a group of about 50
species, of wide geographic distribution, among which the hedge mustard
(_S. officinalis_) is perhaps the most common of weeds. Two species are
reputed to have bracts, _Sisymbrium hirsutum_ and _S. supinum_. Each
flower-stalk of their long racemes is situated in the aril of such a
bract, and the peculiarity is quite a natural one, corresponding exactly
to what is seen in the inflorescence of other families. Besides the
_Sisymbrium some six other genera afford similar structures.
_Erucastrum pollichii_ has been already alluded to in a former lecture
when dealing with the same problem from another point of view. As
previously stated, it is one of the most manifest and most easily
accessible examples of a latent character becoming active through
systematic atavism. In fact, its bracts are found so often as to be
considered by some authors as of quite normal occurrence. Contrasted
with those of the above mentioned species of _Sisymbrium_, they are not
seen at the base of all the flower [638] stalks, but are limited to the
lowermost part of the raceme, adorning a few, often ten or twelve, and
rarely more flower-stalks. Moreover they exhibit a feature which is
indicative of the presence of an abnormality. They are not all of the
same size, but decrease in length from the base of the raceme upward,
and finally slowly disappear.
Besides these rare cases there are quite a number of cruciferous species
on record, which have been observed to bear bracts. Penzig in his
valuable work on teratology gives a list of 33 such genera, many of them
repeating the anomaly in more than one species. Ordinary cabbages are
perhaps the best known instance, and any unusual abundance of
nourishment, or anomalous cause of growth seems to be liable to incite
the development of bracts. The hedge garlic or garlic mustard
(_Alliaria_), the shepherd's purse, the wormseed or _Erysimum
cheiranthoides_ and many others afford instances. In my cultures of
Heeger's shepherd's purse, the new species derived at Landau in Germany
from the common shepherd's purse, the anomaly was observed to occur more
than once, showing that the mutation, which changed the fruits, had not
in the least affected this subordinate anomalous peculiarity. In all
these cases the bracts behave as with the Erucastrum, [639] being
limited to the base of the spike, and decreasing in size from the lower
flowers upward. Connected with these atavistic bracts is a feature of
minor importance, which however, by its almost universal accompaniment
of the bracts, deserves our attention, as it is indicative of another
latent character. As a rule, the bracts are grown together with their
axillary flower-stalk. This cohesion is not complete, nor is it always
developed in the same degree. Sometimes it extends over a large part of
the two organs, leaving only their tips free, but on other occasions it
is limited to a small part of the base. But it is very interesting that
this same cohesion is to be seen in the shepherd's purse, in the
wormseed and in the cabbage, as well as in the case of the _Erucastrum_
and most of the other observed cases of atavistic bracts. This fact
suggests the idea of a common origin for these anomalies, and would lead
to the hypothesis that the original ancestors of the whole family,
before losing the bracts, exhibited this peculiar mode of cohesion.
Bracts and analogous organs afford similar cases of systematic atavism
in quite a number of other families. Aroids sometimes produce long
bracts from various places on their spadix, as may be seen in the
cultivated greenhouse species, _Anthurium scherzerianum_. [640] Poppies
have been recorded to bear bracts analogous to the little scales on the
flower-stalks of the pansies, on the middle of their flower stalks. A
similar case is shown by the yellow foxglove or _Digitalis parviflora_.
The foxgloves as a rule have naked flower-stalks, without the two little
opposite leafy organs seen in so many other instances. The yellow
species, however, has been seen to produce such scales from time to
time. The honeysuckle genus is, as a rule, devoid of the stipules at the
base of the petiole, but _Lonicera etrusca_ has been observed to develop
such organs, which were seen to be free in some, but in other specimens
were adnate to the base of the leaf, and even connate with those of the
opposite leaf.
Other instances could be given proving that bracts and stipules, when
systematically lacking, are liable to reappear as anomalies. In doing
so, they generally assume the peculiar characters that would be expected
of them by comparison with allied genera in which they are of normal
occurrence. There can be no doubt that their absence is due to an
apparent loss, resulting from the reduction of a formerly active quality
to inactivity. Resuming this effective state, the case attains the value
and significance accorded to systematic atavism.
A very curious instance of reduced bracts, developing [641] to unusual
size, is afforded by a variety of corn, which is called _Zea Mays
cryptosperma_, or _Zea Mays tunicata_. In ordinary corn the kernels are
surrounded by small and thin, inconspicuous and membranaceous scales.
Invisible on the integrate spikes, when ripe, they are easily detected
by pulling the kernels out. In _cryptosperma_ they are so strongly
developed as to completely hide the kernels. Obviously they constitute a
case of reversion to the characters of some unknown ancestor, since the
corn is the only member of the grass-family with naked kernels. The var.
_tunicata_, for this same reason, has been considered to be the original
wild form, from which the other varieties of corn have originated. But
as no historical evidence on this point is at hand, we must leave it as
it is, notwithstanding the high degree of attractiveness attached to the
suggestion.
The horsetail-family may be taken as a further support of our assertion.
Some species have stems of two kinds, the fertile being brownish and
appearing in early spring before the green or sterile ones. In others
the stems are all alike, green and crowned with a conelike spike of
sporangia-bearing scales. Manifestly the dimorphous cases are to be
considered as the younger ones, partly because they are obvious
exceptions to the common rule, and [642] partly because the division of
labor is indicative of a higher degree of evolution. But sometimes these
dimorphic species are seen to revert to the primary condition,
developing a fertile cone at the summit of the green summer-stem. I have
had the opportunity of collecting an instance of this anomaly on the
tall _Equisetum telmateja_ in Switzerland, and other cases are on record
in teratological literature. It is an obvious example of systematic
atavism, occurring suddenly and with the full development of all the
qualities needed for the normal production of sporangia and spores. All
of these must be concealed in a latent condition within the young
tissues of the green stems.
More than once I have had occasion to deal with the phenomenon of
torsions, as exhibited by the teasels and some other plants. This
anomaly has been shown to be analogous to the cases described as double
adaptations. The capacity of evolving antagonistic characters is
prominent in both. The antagonists are assumed to lie quietly together
while inactive. But as soon as evolution calls them into activity they
become mutually exclusive, because only one of them can come to full
display in the same organ. External influences decide which of the two
becomes dominant and which remains dormant. This decision must take
place separately [643] for each stem and each branch, but as a rule, the
stronger ages are more liable to furnish anomalies than the weaker.
Exactly the same thing is true of double adaptations. Every bud of the
water-persicaria may develop either into an erect or into a floating
stem, according as it is surrounded by water or by relatively dry soil.
In other cases utility is often less manifest, but some use may either
be proved, or shown to be very probable. At all events the term
adaptation includes the idea of utility, and obviously useless
contrivances could hardly be brought under the same head.
We have also dealt with the question of heredity. It is obvious that
from the flowers of the floating and erect stems of the water-persicaria
seeds will result, each capable of yielding both forms. Quite the same
thing was the case with the teasels. Some 40% of the progeny produce
beautifully twisted stems, but whether the seed was saved from the most
completely twisted specimens or from the straight plants of the race was
of no importance.
This phenomenon of twisting may now be considered from quite another
point of view. It is a case of systematic atavism, or of the
reacquirement of some ancient and long-lost quality. This quality is the
alternate position of [644] the leaves, which has been replaced in the
teasel family by a grouping in pairs. In order to prove the validity of
this assertion, it will be necessary to discuss two points separately,
viz.: relative positions of the leaves, and the manner in which the
alternate position causes the stems to become twisted.
Leaves are affixed to their stems and branches in various ways. Among
them one is of wide occurrence throughout the whole realm of the higher
plants, while all the others are more rare. Moreover these subordinate
arrangements are, as a rule, confined to definite systematic groups.
Such groups may be large, as for instance, the monocotyledons, that have
their leaves arranged in two opposite rows in many families, or small,
as genera or subdivisions of genera. Apart from these special cases the
main stem and the greater part of the branches of the pedigree of the
higher plants exhibit a spiral condition or a screw arrangement, all
leaves being inserted at different points and on different sides of the
stem. This condition is assumed to be the original one, from which the
more specialized types have been derived. As is usual with characters in
general, it is seen to vary around an average, the spiral becoming
narrower and looser. A narrow spiral condenses the leaves, while a [645]
loose one disperses them. According to such fluctuating deviations the
number of leaves, inserted upon a given number of spiral circuits, is
different in different species. In a vast majority of cases 13 leaves
are found on 5 circuits, and as we have only to deal with this
proportion in the teasels we will not consider others.
In the teasels this screw-arrangement has disappeared, and has been
replaced by a decussate grouping. The leaves are combined into pairs,
each pair occupying the opposite sides of one node. The succeeding pairs
alternate with one another, so as to place their leaves at right angles.
The leaves are thus arranged on the whole stem in four equidistant rows.
On the normal stem of a teasel the two members of a pair are tied to one
another in a comparatively complicated way. The leaves are broadly
sessile and their bases are united so as to constitute a sort of cup.
The margins of these cups are bent upward, thereby enabling them to hold
water, and after a rainfall they may be seen filled to the brim. It is
believed that these little reservoirs are useful to the plant during the
flowering period, because they keep the ants away from the honey.
Considering the internal structure of the stem at the base of these cups
we find that the vascular bundles of the two opposite leaves are
strongly connected [646] with one another, constituting a ring which
narrowly surrounds the stem, and which would impede an increase in
thickness, if such were in the nature of the plant. But since the stems
end their existence during the summer of their development, this
structure is of no real harm.
The grouping of the leaves in alternate pairs may be seen within the bud
as well as on the adult stems. In order to do this, it is necessary to
make transverse sections through the heart of the rosette of the leaves
of the first year. If cut through the base, the pair exhibit connate
wings, corresponding to the water-cups; if cut above these, the leaves
seem to be free from one another.
In order to compare the position of leaves of the twisted plants with
this normal arrangement, the best way is to make a corresponding section
through the heart of the rosette of the first year. It is not necessary
to make a microscopic preparation. In the fall the changed disposition
may at once be seen to affect the central leaves of the group. All the
rosettes of the whole race commence with opposite leaves; those that are
to produce straight stems remain in this condition, but the preparation
for twisting begins at the end of the first year as shown by a special
arrangement of the leaves. This [647] disposition may then be seen to
extend to the very center of the rosette, by use of microscopical
sections. Examining sections made in the spring, the original
arrangement of the leaves of the stem is observed to continue until the
beginning of the growth of the shoot. It is easy to estimate the number
of leaves corresponding to a given number of spiral circuits in these
sections and the proportion is found to indicate 13 leaves on 5 turns.
These figures are the same as those given above for the ordinary
arrangement of alternate leaves in the main lines of the pedigree of the
vegetable kingdom.
Leaving aside for the moment the subsequent changes of this spiral
arrangement, it becomes at once clear that here we have a case of
systematic atavism. The twisted teasels lose their decussation, but in
doing so the leaves are not left in a disorderly dispersion, but a
distinct new arrangement takes its place, which is to be assumed as the
normal one for the ancestors of the teasel family. The case is to be
considered as one of atavism. Obviously no other explanation is
possible, than the supposition that the 5-13 spiral is still latent,
though not displayed by the teasels. But in the very moment when the
faculty of decussation disappears, it resumes its place, and becomes
[648] as prominent as it must once have been in the ancestors, and is
still in that part of their offspring, which has not become changed in
this respect. Thus the proof of our assertion of systematic atavism is,
in this case, not obtained by the inspection of the adult, but by the
investigation of the conditions in an early stage. It remains to be
explained how the twisting may finally be caused by this incipient
grouping of the leaves. Before doing so, it may be as well to state that
the case of the teasel is not an isolated one, and that the same
conclusions are supported by the valerian, and a large number of other
examples. In early spring some rosettes show a special condition of the
leaves, indicating thereby at once their atavism and their tendency to
become twisted as soon as they begin to expand. The Sweet William or
_Dianthus barbatus_ affords another instance; it is very interesting
because a twisted race is available, which may produce thousands of
instances developed in all imaginable degrees, in a single lot of
plants. _Viscaria oculata_ is another instance belonging to the same
family.
The bedstraw (_Galium_) also includes many species which from time to
time produce twisted stems. I have found them myself in Holland on
_Galium verum_ and _G. Aparine_. Both seem [649] to be of rare
occurrence, as I have not succeeded in getting any repetition by
prolonged culture.
Species, which generally bear their leaves in whorls, are also subjected
to casual atavisms of this kind, as for instance the tall European
horsetail, _Equisetum Telmateja_, which occasionally bears cones on its
green summer stems. Its whorls are changed on the twisted parts into
clearly visible spirals. The ironwood or _Casuarina quadrivalvis_ is
sometimes observed to produce the same anomaly on its smaller lateral
branches.
Coming now to the discussion of the way in which the twisting is the
result of the spiral disposition of the leaves, we may consider this
arrangement on stems in the adult state. These at once show the spiral
line and it is easy to follow this line from the base up to the apex. In
the most marked cases it continues without interruption, not rarely
however, ending in a whorl of three leaves and a subsequent straight
internode, of which there may even be two or three. The spiral exhibits
the basal parts of the leaves, with the axillary lateral branches. The
direction of the screw is opposed to that of the twisting, and the
spiral ribs are seen to cross the line of insertion of the leaves at
nearly right angles. On this line the leaves are nearer [650] to one
another than would correspond to the original proportion of 5 turns for
13 leaves. In fact, 10 or even 13 leaves may not rarely be counted on a
single turn. Or the twist may become so strong locally as to change the
spiral into a longitudinal line. On this line all inserted leaves extend
themselves in the same direction, resembling an extended flag.
The spiral on the stem is simply the continuation of the spiral line
from within the rosettes of the first year. Accordingly it is seen to
become gradually less steep at the base. For this reason it must be one
and the same with this line, and in extreme youth it must have produced
its leaves at the same mutual distances as this line. Transverse
sections of the growing summits of the stems support this conclusion.
From these several facts we may infer that the steepness of the spiral
line increases on the stem, as it is gradually changed into a screw.
Originally 5 turns were needed for 13 leaves, but this number diminishes
and 4 or 3 or even 2 turns may take the same number of foliar organs,
until the screw itself is changed into a straight line.
This change consists in an unwinding of the whole spiral, and in order
to effect this the stem must become wound up in the opposite direction.
The winding of the foliar screw must [651] curve the longitudinal ribs.
The straighter and steeper the screw becomes, the more the ribs will
become twisted. That this happens in the opposite direction is obvious,
without further discussion. The twisting is the inevitable consequence
of the reversal of the screw.
Two points remain to be dealt with. One is the direct proof of the
reversal of the screw, the other the discussion of its cause. The first
may be observed by a simple experiment. Of course it proceeds only
slowly, but all that is necessary is to mark the position of one of the
younger leaves of a growing stem of a twisting individual and to observe
the change in its position in a few hours. It will be seen to have
turned some way around the stem, and finally may be seen to make a
complete revolution in the direction opposite to the screw, and thereby
demonstrating the fact of its uncurling.
The cause of this phenomenon is to be sought in the intimate connection
of the basal parts of the leaves, which we have detailed above. The
fibrovascular strands constitute a strong rope, which is twisted around
the stem along the line on which the leaves are inserted. The
strengthening of the internodes may stretch this rope to some extent,
but it is too strong to be rent asunder. Hence it opposes the normal
growth, and the only manner in which the internodes [652] may adjust
themselves to the forces which tend to cause their expansion is by
straightening the rope. In doing so they may find the required space, by
growing out in an unusual direction, bending their axes and twisting the
ribs.
To prove the validity of this explanation, a simple experiment may be
given. If the fibrovascular rope is the mechanical impediment which
hinders the normal growth, we may try the effect of cutting through this
rope. By this means the hindrance may at least locally be removed. Now,
of course, the operation must be made in an early stage before, or at
the beginning of the period of growth, in every case before the
uncurling of the rope begins. Wounds made at this time are apt to give
rise to malformations, but notwithstanding this difficulty I have
succeeded in giving the necessary proof. Stems operated upon become
straight where the rope is cut through, though above and under the
wounded part they go on twisting in the usual way.
Sometimes the plants themselves succeed in tearing the rope asunder, and
long straight internodes divide the twisted stems in two or more parts
in a very striking manner. A line of torn leaf-bases connects the two
parts of the screw and gives testimony of what has passed within [653]
the tissues. At other times the straightening may have taken place
directly internal to a leaf, and it is torn and may be seen to be
attached to the stem by two distinct bases.
Summing up this description of the hereditary qualities of our twisted
teasels and of their mechanical consequences, we may say that the loss
of the normal decussation is the cause of all the observed changes. This
special adaptation, which places the leaves in alternating pairs,
replaced and concealed the old and universal arrangement on a screw
line. In disappearing, it leaves the latter free, and according to the
rule of systematic atavism, this now becomes active and takes its place.
If the fibrovascular connection of the leaf-bases were lost at the same
time the stems would grow and become straight and tall. This change
however, does not occur, and the bases of the leaves now constitute a
continuous rope instead of separate rings, and thereby impede the
stretching of the internodes. These in their turn avoid the difficulty
by twisting themselves in a direction opposite to that of the spiral of
the leaves.
As a last example of systematic atavism I will refer to the reversionary
changes, afforded by the tomatoes. Though the culture of this plant is a
recent one, it seems to be at present in a state of mutability,
producing new strains, or [654] assuming the features of their
presumable ancestors. In his work "The Survival of the Unlike," Bailey
has given a detailed description of these various types. Moreover, he
has closely studied the causes of the changes, and shown the great
tendency of the tomatoes to vicinism. By far the larger part of the
observed cases of running out of varieties are caused by accidental
crosses through the agency of insects. Even improvements are not rarely
due to this cause. Besides these common and often unavoidable changes,
others of greater importance occur from time to time. Two of them
deserve to be mentioned. They are called the "Upright" and the "Mikado"
types, and differ as much or even more from their parents than the
latter do from any one of their wild congeners. Their characters come
true from seed. The "Mikado" race or the _Lycopersicum grandifolium_
(_L. latifolium_) has larger and fewer leaflets than the slender and
somewhat flimsy foliage of the common form. Flat or plane blades with
decurrent margins constitute another character. This variety, however,
does not concern our present discussion. The upright type has stiff and
self-sustaining stems and branches, resembling rather a potato-plant
than a tomato. Hence the name _Lycopersicum solanopsis_ or _L. validum_,
under which it is usually described. [655] The foliage of the plant is
so distinct as to yield botanical characters of sufficient importance to
justify this specific designation. The leaflets are reduced in numbers
and greatly modified, and the flowers in the inflorescence are reduced
to two or three. This curious race came in suddenly, without any
premonition, and the locality and date of its mutation are still on
record. Until some years ago it had not made its appearance for a second
time. Obviously it is to be considered as a reversionary form. The limp
stems of the common tomatoes are in all respects indicative of the
cultivated condition. They cannot hold themselves erect, but must be
tied up to supports. The color of the leaves is a paler green than
should be expected from a wild plant. Considering other species of the
genus _Solanum_, of which the _Lycopersicum_ is a subdivision, the stems
are as a rule erect and self-supporting, with some few exceptions.
These, however, are special adaptations as shown by the winding stems of
the bitter-sweet.
From this discussion we seem justified in concluding that the original
appearance of the upright type was of the nature of systematic atavism.
It differs however, from the already detailed cases in that it is not a
monstrosity, nor an ever-sporting race, but is as constant a form [656]
as the best variety or species. Even on this ground it must be
considered as a representative of a separate group of instances of the
universal rule of systematic reversions.
Of late the same mutation has occurred in the garden of C.A. White at
Washington. The parent form in this case was the "Acme," of the ordinary
weak and spreading habit of growth. It is known as one of the best and
most stable of the varieties and was grown by Mr. White for many years,
and had not given any sign of a tendency towards change. Seeds from some
of the best plants in 1899 were sown the following spring, and the young
seedlings unexpectedly exhibited a marked difference from their parents.
From the very outset they were more strong and erect, more compact and
of a darker green than the "Acme." When they reached the fruiting stage
they had developed into typical representatives of the _Lycopersicum
solanopsis_ or upright division. The whole lot of plants comprised only
some 30 specimens, and this number, of course, is too small to base
far-reaching conclusions upon. But all of the lot showed this type, no
true "Acme" being seen among them. The fruit differed in flavor,
consistency and color from that of the parent, and it also ripened
earlier than the latter. No seed was saved from [657] these plants, but
the following year the "Acme" was sown again and found true to its type.
Seeds saved from this generation in 1900 have, however, repeated the
mutation, giving rise to exactly the same new upright form in 1901. This
was called by its originator "The Washington." Seeds from this second
mutation were kindly sent to me by Mr. White, and proved true to their
type when sown in my garden.
Obviously it is to be assumed in the case of the tomatoes as well as in
instances from other genera cited, that characters of ancestors, which
are not displayed in their progeny, have not been entirely lost, but are
still present, though in a latent condition. They may resume their
activity unexpectedly, and at once develop all the features which they
formerly had borne.
Latency, from this point of view, must be one of the most common things
in nature. All organisms are to be considered as internally formed of a
host of units, partly active and partly inactive. Extremely minute and
almost inconceivably numerous, these units must have their material
representatives within the most intimate parts of the cells.
[658]
LECTURE XXIII
TAXONOMIC ANOMALIES
The theory of descent is founded mainly on comparative studies, which
have the advantage of affording a broad base and the convincing effect
of concurrent evidence brought together from widely different sources.
The theory of mutation on the other hand rests directly upon
experimental investigations, and facts concerning the actual descent of
one form from another are as yet exceedingly rare. It is always
difficult to estimate the validity of conclusions drawn from isolated
instances selected from the whole range of contingent phenomena, and
this is especially true of the present case. Systematic and physiologic
facts seem to indicate the existence of universal laws, and it is not
probable that the process of production of new species would be
different in the various parts of the animal and vegetable kingdoms.
Moreover the principle of unit-characters, the preeminent significance
of which has come to be more fully recognized of late, is in full
harmony [659] with the theory of sudden mutations. Together these two
conceptions go to strengthen the probability of the sudden origin of all
specific characters.
Experimental researches are limited in their extent, and the number of
cases of direct observation of the process of mutation will probably
never become large enough to cover the whole field of the theory of
descent. Therefore it will always be necessary to show that the
similarity between observed and other cases is such as to lift above all
doubt the assertion of their resulting from the same causes.
Besides the direct comparison of the mutations described in our former
lectures, with the analogous cases of the horticultural and natural
production of species and varieties at large, another way is open to
obtain the required proof. It is the study of the phenomena, designated
by Casimir de Candolle by the name of taxonomic anomalies. It is the
assertion that characters, which are specific in one case, may be
observed to arise as anomalies or as varieties in other instances. If
they can be shown to be identical or nearly so in both, it is obviously
allowable to assume the same origin for the specific character and for
the anomaly. In other terms, the specific marks may be considered as
having originated according to the laws [660] that govern the production
of anomalies, and we may assume them to lie within reach of our
experiments. The experimental treatment of the origin of species may
also be looked upon as a method within our grasp.
The validity and the significance of these considerations will at once
become clear, if we choose a definite example. The broadest and most
convincing one appears to me to be afforded by the cohesion of the
petals in gamopetalous flowers. According to the current views the
families with the petals of their flowers united are regarded as one or
two main branches of the whole pedigree of the vegetable kingdom.
Eichler and others assume them to constitute one branch, and therefore
one large subdivision of the system. Bessey, on the other hand, has
shown the probability of a separate origin for those groups which have
inferior ovaries. Apart from such divergencies the connation of the
petals is universally recognized as one of the most important systematic
characters.
How may this character have originated? The heath-family or the
Ericaceae and their nearest allies are usually considered to be the
lowest of the gamopetalous plants. In them the cohesion of the petals is
still subject to reversionary exceptions. Such cases of atavism may
[661] be observed either as specific marks, or in the way of anomalies.
_Ledum_, _Monotropa_ and _Pyrola_, or the Labrador tea, the Indian pipe
and wintergreen are instances of reversionary gamopetalism with free
petals. In heaths (_Erica Tetralix_) and in rhododendrons the same
deviation is observed to occur from time to time as an anomaly, and even
the common _Rhododendron ponticum_ of our gardens has a variety in which
the corolla is more or less split. Sometimes it exhibits five free
petals, while at other times only one or two are entirely free, the
remaining four being incompletely loosened.
Such cases of atavism make it probable that the coherence of the petals
has originally arisen by the same method, but by action in the opposite
direction. The direct proof of this conclusion is afforded by a curious
observation, made by Vilmorin upon the bright and large-flowered
garden-poppy, _Papaver bracteatum_. Like all poppies it has four petals,
which are free from one another. In the fields of Messrs. Vilmorin,
where it is largely cultivated for its seeds, individuals occur from
time to time which are anomalous in this respect. They exhibit a
tendency to produce connate petals. Their flowers become monopetalous,
and the whole strain is designated by the name of _Papaver_ [662]
_bracteatum monopetalum_. Henry de Vilmorin had the kindness to send me
some of these plants, and they have flowered in my garden during several
years. The anomaly is highly variable. Some flowers are quite normal,
exhibiting no sign of connation; others are wholly gamopetalous, the
four petals being united from their base to the very margin of the cup
formed. In consequence of the broadness of the petals however, this cup
is so wide as to be very shallow.
Intermediate states occur, and not infrequently. Sometimes only two or
three petals are united, or the connation does not extend the entire
length of the petals. These cases are quite analogous to the imperfect
splitting of the corolla of the rhododendron. Giving free rein to our
imagination, we may for a moment assume the possibility of a new
subdivision of the vegetable kingdom, arising from Vilmorin's poppy and
having gamopetalous flowers for its chief character. If the character
became fixed, so as to lose its present state of variability, such a
group of supposititious gamopetalous plants might be quite analogous to
the corresponding real gamopetalous families. Hence there can be no
objection to the view, that the heaths have arisen in an analogous
manner from their polypetalous ancestors. Other species of [663] the
same genus have also been recorded to produce gamopetalous flowers, as
for instance, _Papaver hybridum_, by Hoffmann. Poppies are not the sole
example of accidental gamopetaly. Linnaeus observed the same deviation
long ago for _Saponaria officinalis_, and since, it has been seen in
_Clematis Vitalba_ by Jaeger, in _Peltaria alliacea_ by Schimper, in
_Silene annulata_ by Boreau and in other instances. No doubt it is not
at all of rare occurrence, and the origin of the present gamopetalous
families is to be considered as nothing extraordinary. It is, as a
matter of fact, remarkable that it has not taken place in more numerous
instances, and the mallows show that such opportunities have been
available at least more than once.
Other instances of taxonomic anomalies are afforded by leaves. Many
genera, the species of which mainly bear pinnate or palmate leaves, have
stray types with undivided leaves. Among the brambles, _Rubus odoratus_
and _R. flexuosus_ may be cited, among the aralias, _Aralia crassifolia_
and _A. papyrifera_, and among the jasmines, the deliciously scented
sambac (_Jasminum Sambac_). But the most curious instance is that of the
telegraph-plant, or _Desmodium gyrans_, each complete leaf of which
consists of a large terminal leaflet and two little lateral ones. These
latter keep up, [664] night and day, an irregular jerking movement,
which has been compared to the movements of a semaphore. _Desmodium_ is
a papilionaceous plant and closely allied to the genus _Hedysarum_,
which has pinnate leaves with numerous pairs of leaflets. Its place in
the system leaves no doubt concerning its origin from pinnate-leaved
ancestors. At the time of its origination its leaves must have become
reduced as to the number of the blades, while the size of the terminal
leaflet was correspondingly increased.
It might seem difficult to imagine this great change taking place
suddenly. However, we are compelled to familiarize ourselves with such
hypothetical assumptions. Strange as they may seem to those who are
accustomed to the conception of continuous slow improvements, they are
nevertheless in complete agreement with what really occurs. Fortunately
the direct proof of this assertion can be given, and in a case which is
narrowly related, and quite parallel to that of the _Desmodium_, since
it affects a plant of the same family. It is the case of the
monophyllous variety of the bastard-acacia or _Robinia Pseud-Acacia_. In
a previous lecture we have seen that it originated suddenly in a French
nursery in the year 1855. It can be propagated by seed, and exhibits a
curious degree [665] of variability of its leaves. In some instances
these are one-bladed, the blade reaching a length of 15 cm., and hardly
resembling those of the common bastard-acacia. Other leaves produce one
or two small leaflets at the base of the large terminal one, and by this
contrivance are seen to be very similar to those of the _Desmodium_,
repeating its chief characters nearly exactly, and only differing
somewhat in the relative size of the various parts. Lastly real
intermediates are seen between the monophyllous and the pinnate types.
As far as I have been able to ascertain, these are produced on weak
twigs under unfavorable conditions; the size of the terminal leaflet
decreases and the number of the lateral blades increases, showing
thereby the presence of the original pinnate type in a latent condition.
The sudden origin of this "one-leaved" acacia in a nursery may be taken
as a prototype of the ancient origin of _Desmodium_. Of course the
comparison only relates to a single character, and the movements of the
leaflets are not affected by it. But the monophylly, or rather the size
of the terminal blade and the reduction of the lateral ones, may be held
to be sufficiently illustrated by the bastard-acacia. It is worth while
to state, that analogous varieties have also arisen in other genera. The
"one-leaved" [666] strawberry has already been referred to. It
originated from the ordinary type in Norway and at Paris. The walnut
likewise, has its monophyllous variety. It was mentioned for the first
time as a cultivated tree about 1864, but its origin is unknown. A
similar variety of the walnut, with "one-bladed" leaves but of varying
shapes, was found wild in a forest near Dieppe in France some years ago,
and appeared to be due to a sudden mutation.
Something more is known concerning the "one-bladed" ashes, varieties of
which are often seen in our parks and gardens. The common form has broad
and deeply serrate leaves, which are far more rounded than the leaflets
of the ordinary ash. The majority of the leaves are simple, but some
produce one or two smaller leaflets at their base, closely corresponding
in this respect to the variations of the "one-bladed" bastard-acacia,
and evidently indicating the same latent and atavistic character. In
some instances this analogy goes still further, and incompletely pinnate
leaves are produced with two or more pairs of leaflets. Besides this
variable type another has been described by Willdenow. It has single
leaves exclusively, never producing smaller lateral leaflets, and it is
said to be absolutely constant from seed, while the more variable types
[667] seem to be also more inconstant when propagated sexually. The
difference is so striking and affords such a reliable feature that Koch
proposed to make two distinct varieties of them, calling the pure type
_Fraxinus excelsior monophylla_, and the varying trees _F. excelsior
exheterophylla_. Some writers, and among them Willdenow, have preferred
to separate the "one-leaved" forms from the species, and to call them
_Fraxinus simplicifolia_.
According to Smith and to Loudon, the "one-leaved" ashes are found wild
in different districts in-England. Intermediate forms have not been
recorded from these localities. This mode of origin is that already
detailed for the laciniate varieties of alders and so many other trees.
Hence it may be assumed that the "one-leaved" ashes have sprung suddenly
but frequently from the original pinnate species. The pure type of
Willdenow should, in this case, be considered as due to a slightly
different mutation, perhaps as a pure retrograde variety, while the
varying strains may only be eversporting forms. This would likewise
explain part of their observed inconstancy.
In this respect the historic dates, as collected by Korshinsky, are not
very convincing. Vicinism has of course, almost never been excluded, and
part of the multiformity of the offspring [668] must obviously be due to
this most universal agency. Indirect vicinism also plays some part, and
probably affords the explanation of some reputed mutative productions of
the variety. So, for instance, in the case of Sinning, who after sowing
the seeds of the common ash, got as large a proportion as 2% of
monophyllous trees in a culture of some thousand plants. It is probable
that his seeds were taken partly from normal plants, and partly from
hybrids between the normal and the "one-bladed" type, assuming that
these hybrids have pinnate leaves like their specific parent, and bear
the characters of the other parent only in a latent condition.
Our third example relates to peltate leaves. They have the stalk
inserted in the middle of the blade, a contrivance produced by the
connation of the two basal lobes. The water-lilies are a well known
instance, exhibiting sagittate leaves in the juvenile stage and changing
in many species, into nearly circular peltate forms, of which _Victoria
regia_ is a very good example, although its younger stages do not always
excite all the interest they deserve. The Indian cress (_Tropaeolum_),
the marsh pennywort or _Hydrocotyle_, and many other instances could be
quoted. Sometimes the peltate leaves are not at all orbicular, but are
elongated, oblong or elliptic, and with only the lobes [669] at the base
united. The lemon-scented _Eucalyptus citriodora_ is one of the most
widely known cases. In other instances the peltate leaves become more or
less hollow, constituting broad ascidia as in the case of the
crassulaceous genus _Umbilicus_.
This connation of the basal lobes is universally considered as a good
and normal specific character. Nevertheless it has its manifest analogy
in the realm of the anomalies. This is the pitcher or ascidium. On some
trees it is of quite common occurrence, as on the lime-tree (_Tilia
parvifolia_) and the magnolia (_Magnolia obovata_ and its hybrids). It
is probable that both these forms have varieties with, and others
without, ascidia. Of the lime-tree, instances are known of single trees
which produce hundreds of such anomalous leaves yearly, and one such a
tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I
have alluded to these cases more than once, but on this occasion a
closer inspection of the structure of the ascidium is required. For this
purpose we may take the lime-tree as an example. Take the shape of the
normal leaves in the first place. These are cordate at their base and
mainly inequilateral, but the general shape varies to a considerable
extent. This variation is closely related to the position of the leaves
on the twigs, and shows [670] distinct indications of complying with the
general law of periodicity. The first leaves are smaller, with more
rounded lobes, the subsequent leaves attain a larger size, and their
lobes slightly change their forms. In the first leaves the lobes are so
broad as to touch one another along a large part of their margins, but
in organs formed later this contact gradually diminishes and the typical
leaves have the lobes widely separated. Now it is easily understood that
the contact or the separation of the lobes must play a part in the
construction of the ascidia, as soon as the margins grow together.
Leaves which touch one-another, may be affected by the connation without
any further malformation. They remain flat, become peltate and exhibit a
shape which in some way holds a middle position between the pennyworts
and the lemon-scented eucalyptus. Here we have the repetition of the
specific characters of these plants by the anomaly of another. Whenever
the margins are not in contact, and become connate, notwithstanding
their separation, the blade must be folded together in some slight
degree, in order to produce the required contact. This is the origin of
the ascidium. It is quite superfluous to insist upon the fact that their
width or narrowness must depend upon the corresponding normal form. The
more distant the [671] lobes, the deeper the ascidium will become. It
should be added that this explanation of the different shapes of ascidia
is of general validity.
Ascidia of the snake-plantain or _Plantago lanceolata_ are narrow tubes,
because the leaves are oblong or lanceolate, while those of the broad
leaved species of arrowhead, as for instance, the _Sagittaria japonica_,
are of a conical shape.
From the evidence of the lime-tree we may conclude that normal peltate
leaves may have originated in the same way. And from the fact that
pitchers are one of the most frequent anomalies, we may conclude that
the chance of producing peltate leaves must have been a very great one,
and wholly sufficient to account for all observed cases. In every
instance the previously existing shape of the leaf must have decided
whether peltate or pitcher-like leaves would be formed. As far as we can
judge peltate anomalies are quite uninjurious, while ascidia are forms
which must impede the effect of the light on the leaf, as they conceal
quite an important part of the upper surface. In this way it is easily
conceivable that peltate leaves are a frequent specific character, while
ascidia are not, as they only appear in the special cases of limited
adaptation, as in the instances of the so called pitcher-plants. The
genera _Nepenthes_, [672] _Sarracenia_ and some others are very well
known and perhaps even the bladderworts or _Utricularia_ might be
included here.
The reproduction of specific characters by anomalous ascidia is not at
all limited to the general case as described above. More minute details
may be seen to be duplicated in the same way. Proofs are afforded on one
side by incomplete ascidia, and on the other by the double cups.
Incomplete ascidia are those of the _Nepenthes_. The leaf is divided
into three parts, a blade, a tendril and the pitcher. Or in other words,
the limb produces a tendril at its summit, by means of which the plant
is enabled to fasten itself to surrounding shrubs and to climb between
their branches. But the end of this tendril bears a well-formed urn,
which however, is produced only after the revolving and grasping
movements of the tendril have been made. Some species have more rounded
and some more elongated ascidia and often the shape is seen to change
with the development of the stem. The mouth of the urn is strengthened
by a thick rim and covered with a lid. Numerous curious contrivances in
these structures to catch ants and other insects have been described,
but as they have no relation to our present discussion, we shall abstain
from dealing with them. [673] Likewise we must refrain from a
consideration of the physiologic qualities of the tendril, and confine
our attention to the combination of a limb, a naked midvein and an
ascidium. This combination is to be the basis of our discussion. It is
liable to be produced all of a sudden. This assertion is proved by its
occurrence as a varietal mark in one of our most ordinary cultivated
plants. It is the group known as _Croton_, belonging to the genus
_Codiaeum_. A variety is called _interruptum_ and another
_appendiculatum_, and these names both relate to the interruption of the
leaves by a naked midvein. The leaves are seen to be built up of three
parts. The lower half retains the aspect of a limb; it is crowned by a
vein without lateral nerves or blade-like expansions, and this stalk in
its turn bears a short limb on its summit. The base of this apical limb
exhibits two connate lobes, forming together a wide cup or ascidium. It
should be stated that these _interruptum_ varieties are highly variable,
especially in the relative size of the three principal parts of the
leaf. Though it is of course conceded that the ascidium of _Nepenthes_
has many secondary devices which are lacking in _Croton_, it seems
hardly allowable to deny the possibility of an analogous origin for
both. Those of the _Croton_, according to our knowledge regarding
similar cases, must [674] have arisen at once, and hence the conclusion
that the ascidia of _Nepenthes_ are also originally due to a sudden
mutation. Interrupted leaves, with an ascidium on a naked prolongation
of the midvein, are by no means limited to the _Croton_ varieties. As
stray anomalies they have often been observed, and I myself had the
opportunity of collecting them on magnolia, on clover and on some other
species. They are additional evidence in support of the explanation
given above.
In the same way double ascidia may be made use of to explain the foliar
cups of the teasels and some other plants, as for instance, some
European snakeroots (_Eryngium maritimum_ and _E. campestre_), or the
floral leaves of the honeysuckle. The leaves on the stems of the teasels
are disposed in pairs, and the bases of the two leaves of each pair are
connate so as to constitute large cups. We have already mentioned these
cups, and recall them in the present connection to use them as a
prototype of the double ascidia. These are constituted of two opposite
leaves, accidentally connated at their base or along some part of their
margins. If the leaves are sessile, the analogy with the teasels is
complete, as shown, for instance, in a case of _Cotyledon_, a
crassulaceous plant which is [675] known to produce such cups from time
to time. They are narrower than those of the teasel, but this depends,
as we have seen for the "one-leaved" ascidium, on the shape of the
original leaf. In other respects they exactly imitate the teasel cups
showing thereby how these cups may probably have originated.
In numerous cases of anomalies some accidental structures are parallel
to specific characters, while others are not, being obviously injurious
to their bearers. So it is also with the double ascidia. In the case of
stalked leaves the two opposite stalks must, of course, constitute a
long and very narrow tube, when growing together. This tube must bear at
its summit the conical ascidium produced by the two connate limbs. At
its base however, it includes the terminal bud of the stem, and
frequently the tube is so narrow as to impede its further development.
By this contrivance the double ascidium assumes a terminal position.
Instances have been observed on magnolia, in _Boehmeria_ and in other
cases.
Flowers on leaves are of rare occurrence. Notwithstanding this, they
constitute specific characters in some instances, accidental anomalies
in others. _Helwingia rusciflora flora_ is the most curious and best
known instance. It is a little shrub, belonging to the Cornaceae, and
[676] has broad elliptical undivided leaves. On the middle of the
midvein these leaves are seen to bear small clusters of flowers; indeed
this is the only place where flowers are produced. Each cluster has from
13-15 flowers, of which some are staminate and borne on stalks, while
others are pistillate and nearly sessile. These flowers are small and of
a pale greenish color and yield small stone-fruits, with a thin coating
of pulpy tissue. As the name indicates, this mode of flowering is
closely similar to that of _Ruscus_, which however, does not bear its
flowers and berries on real leaves, but on leaflike expansions of the
twigs. _Phyllonoma ruscifolia_, a saxifragaceous plant, bears the same
specific name, indicating a similar origin of the flowers. Other
instances have been collected by Casimir de Candolle, but their number
is very small.
As a varietal mark, flowers on leaves likewise rarely occur. One
instance however, is very remarkable, and we have already dealt with it,
when treating of constant varieties, and of the lack of vicinism in the
case of species with exclusive self-fertilization.
It is the "Nepaul-barley" or _Hordeum trifurcatum_. The leaves, which in
this case bear the adventitious flowers, are the inner scales of the
spikelets, and not on green leaves as in the [677] cases already alluded
to. But this of course makes no real difference. The character is
variable to a high degree, and this fact indicates its varietal nature,
though it should be recalled that at least with the _Helwingia_, the
majority of the leaves are destitute of flowers, and that in this way
some degree of variability is present in this normal case too.
All in all there are three sorts of "Nepaul-barley." They have the same
varietal mark, but belong to different species of barley. These are
differentiated according to the number of the rows in which the grains
are seen on the spikes. These numbers may be two, four or six, giving
rise to the specific names of _Hordeum distichum_, _tetrastichum_ and
_hexastichum_. Whether these three varieties are of independent, but
parallel origin, or are to be considered as due to a single mutation and
subsequent crosses is not known, all of them being of ancient origin.
Historic evidence concerning their birth is wholly wanting. From analogy
it would seem probable that the character had arisen by a mutation in
one of the three named species, and had been transferred to others by
means of accidental crosses, even as it has been artificially
transmitted of late to quite a number of other sorts. But however
admissible this conception may seem, there is of course no real
objection [678] to the assumption of independent and parallel mutations.
For the purpose of a comparison with the _Helwingia_ type we are
however, not at all concerned with the species to which the
_trifurcatum_ variety belongs, but only with the varietal mark itself.
The spikelets may be one-, two- or three-flowered, according to the
species. If we choose for further consideration the _hexastichum_ type,
each spikelet produces three normal flowers and afterwards three normal
grains. Morphologically however, the spikelet is not homologous to those
parts of other grasses which have the same name. It is constituted of
three real spikelets, and thus deserves the name of a triple
construction. Each of these three little organs has its normal pair of
outer scales or glumae. These are linear and short, ending in a long and
narrow spine. Those of the middle-most spikelets stand on its outer
side, while those of the lateral part are placed transversely. In this
way they form a kind of involucre around the central parts. The latter
consist of the inner and outer palets or scales, each two of which
include one of the flowers. The outer palet is to be considered as the
metamorphosed leaf, in the aril of which the flower is produced. In the
common sorts of barley it bears a long awn, giving thereby its typical
aspect to the [679] whole spike. The axillary flower is protected on the
opposite side by a two-keeled inner palet. Each flower exhibits three
stamens and an ovary. In the six-rowed barley all the three flowers of a
triple spikelet are fertile, and each of them has a long awn on the top
of the outer palet. But in the two-rowed species only the middle-most
flower is normal and has an awn, the two remaining being sterile and
more or less rudimentary and with only very short awns. From this
description it is easily seen that the species of barley may be
distinguished from one another, even at a casual glance, by the number
of the rows of the awns, and therefore by the shape of the entire
spikes. This striking feature, however, does not exist in the
"Nepaul-barley." The awns are replaced by curiously shaped appendices,
which are three-lobed. The central lobe is oblong and hollow, and forms
a kind of hood, which covers a small supernumerary floret. The two
lateral lobes are narrower, often linear and extended into a smaller or
longer awn. These awns are mostly turned away from the center of the
spike. The central lobe may sometimes bear two small florets, but
ordinarily only one is to be found, and this is often incomplete, having
only one or two stamens, or is different in some other way. [680] These
narrow lateral lobes heighten the abnormal aspect of the whole spike.
They are only produced at a somewhat advanced stage of the development
of the palet, are united to one another and to the central part by
strong veins, which form transversal anastomoses at their insertion. The
length of these awns is very variable, and this quality is perhaps the
most striking of the whole variety. Often they reach only 1-2 mm., or
the majority may become longer and attain even 1 cm., while here and
there, between them, longer ones are inserted, extending in some
instances even as far as 3 cm. from the spike. Their transverse position
in such cases is strikingly contrasted with the ordinary erect type of
the awns.
These lateral lobes are to be regarded, from the morphologic point of
view, as differentiated parts of the blade of the leaf. Before they are
formed, or coincidently with the beginning of their development, the
summit of the central lobe becomes hollow, and the development of the
supernumerary flower commences. In different varieties, and especially
in the most recent crosses of them, this development is excessively
variable.
The accidental flower arises at some distance beneath the summit of the
scale, on its middle [681] vein. The development begins with the
protrusion of a little scale, and the flower itself is situated beneath
this scale, and is to be protected by it and by the primary scale, but
is turned upside down at the same time. Opposite to this organ, which
represents the outer palet of the adventitious flower, two little
swollen bodies are evolved. In the normal flowers of barley and other
grains and grasses their function is to open the flowers by swelling,
and afterwards collapse and allow them to close.
In the adventitious flowers of the "Nepaul-barley," however, this
function is quite superfluous. The stamens occur in varying numbers;
typically there are three, but not rarely less, or more, are seen. In
some instances the complete double whorl of six, corresponding to the
ancestral monocotyledonous type, has been found. This is a very curious
case of systematic atavism, quite analogous to the _Iris pallida
abavia_, previously alluded to, which likewise has six stamens, and to
the cases given in a previous lecture. But for our present discussion it
is of no further interest. The ovary is situated in the middle of the
flower, and in some instances two have been observed. This is also to be
considered as a case of atavism.
All these parts of the adventitious flower are more or less subject to
arrest of development, [682] in a later stage. They may even sometimes
become abnormal. Stamens may unite into pairs, or carpels bear four
stigmas. The pollen-sacs are as a rule barren, the mother-cells
undergoing atrophy, while normal grains are seen but rarely. Likewise
the ovaries are rudimentary, but Wittmack has observed the occasional
production of ripe grains from these abnormal florets.
The scale is seldom seen to extend any farther upwards than the
supernumerary flower. But in the rare instances where it does prolong
its growth, it may repeat the abnormality and bear a second floret above
the first. This of course is generally much weaker, and more
rudimentary.
Raciborsky, who has lately given a full and very accurate description of
this anomaly, lays great stress upon the fact that it is quite useless.
It is perhaps the most obviously useless structure in the whole
vegetable kingdom. Notwithstanding this, it has come to be as completely
hereditary as any of the most beautiful adaptations in nature. Therefore
it is one of the most serious objections to the hypothesis of slow and
gradual improvements on the sole ground of their usefulness. The
struggle for life and natural selection are manifestly inadequate to
give even the slightest indication of [683] an explanation of this case.
It is simply impossible to imagine the causes that might have produced
such a character. The only way out of this difficulty is to assume that
it has arisen at once, in its present apparently differentiated and very
variable condition, and that, being quite uninjurious and since it does
not decrease the fertility of the race, it has never been subjected to
natural selection, and so has saved itself from destruction.
But if we once grant the probability of the origin of the
"Nepaul-barley" by a sudden mutation, we obviously must assume the same
in the case of the _Helwingia_ and other normal instances. In this way
we gain a further support for our assertion, that even the strangest
specific characters may have arisen suddenly.
After having detailed at some length those proofs which seem to be the
most striking, and which had not been previously described with
sufficient detail, we may now take a hasty survey of other contingent
cases. In the first place the cruciate flowers of some onagraceous
plants should be remembered. Small linear petals occur as a specific
character in _Oenothera cruciata_ of the Adirondacks, but have been seen
to arise as sudden mutations in the common evening-primrose (_O.
biennis_) in Holland, and in the willow-herb (_Epilobium hirsutum_) in
England. [684] Leaves placed in whorls of three are very rare. The
oleander, juniper and some few other plants have ternate whorls as a
specific character. As an anomaly, ternate whorls are far more common,
and perhaps any plant with opposite leaves may from time to time produce
them. Races rich in this abnormality are found in the wild state in the
yellow loosestrife or _Lysimachia vulgaris_, in which it is a very
variable specific character, the whorls varying from two to four leaves.
In the cultivated state it is met with in the myrtle or _Myrtus
communis_, where it has come to be of some importance in Israelitic
ritual. Crisped leaves are known in a mallow, _Malva crispa_, and as a
variety in cabbages, parsley, lettuce and others. The orbicular fruits
of Heeger's shepherd's purse (_Capsella heegeri_) recall similar fruits
of other cruciferous genera, as for instance, _Camelina_. Screw-like
stems with wide spirals are specific in the flower-stalks of _Cyclamen_
and _Vallisneria_, varietal in _Juncus effusus spiralis_ and accidental
in _Scirpus lacustris_. Dormant buds or small bulbs in inflorescences
are normal for wild onions, _Polygonum viviparum_ and others, varietal
in _Poa alpina vivipara_ and perhaps in _Agave vivipara_, and accidental
in plantains (_Plantago lanceolata_), _Saxifraga umbrosa_ and others.
[685] Cleft leaves, one of the most general anomalies, are typical in
_Boehmeria biloba_. The adnation of the peduncles of the inflorescences
to the stem is typical in _Solanum_ and accidental in many other cases.
It seems quite superfluous to add further proof. It is a very general
phenomenon that specific characters occur in other genera as anomalies,
and under such circumstances that the idea of a slow evolution on the
ground of utility is absolutely excluded. No other explanation remains
than that of a sudden mutation, and once granted for the abnormal cases,
this explanation must obviously likewise be granted for the analogous
specific characters.
Our whole discussion shows that mutations, once observed in definite
instances, afford the most probable basis for the explanation of
specific characters at large.
[686]
LECTURE XXIV
THE HYPOTHESIS OF PERIODIC MUTATIONS
The prevailing belief that slow and gradual, nearly invisible changes
constitute the process of evolution in the animal and vegetable kingdom,
did not offer a strong stimulus for experimental research. No
appreciable response to any external agency was of course to be
expected. Responses were supposed to be produced, but the corresponding
outward changes would be too small to betray themselves to the
investigator.
The direct observation of the mutations of the evening-primrose has
changed the whole aspect of the problem at once. It is no longer a
matter dealing with purely hypothetical conditions. Instead of the vague
notions, uncertain hopes, and a priori conceptions, that have hitherto
confused the investigator, methods of observation have been formulated,
suitable for the attainment of definite results, the general nature of
which is already known.
To my mind the real value of the discovery [687] of the mutability of
the evening-primrose lies in its usefulness as a guide for further work.
The view that it might be an isolated case, lying outside of the usual
procedure of nature, can hardly be sustained. On such a supposition it
would be far too rare to be disclosed by the investigation of a small
number of plants from a limited area. Its appearance within the limited
field of inquiry of a single man would have been almost a miracle.
The assumption seems justified that analogous cases will be met with,
perhaps even in larger numbers, when similar methods of observation are
used in the investigation of plants of other regions. The mutable
condition may not be predicated of the evening-primroses alone. It must
be a universal phenomenon, although affecting a small proportion of the
inhabitants of any region at one time: perhaps not more than one in a
hundred species, or perhaps not more than one in a thousand, or even
fewer may be expected to exhibit it. The exact proportion is immaterial,
because the number of mutable instances among the many thousands of
species in existence must be far too large for all of them to be
submitted to close scrutiny.
It is evident from the above discussion that next in importance to the
discovery of the prototype of mutation is the formulation of methods
[688] for bringing additional instances to light. These methods may
direct effort toward two different modes of investigation. We may search
for mutable plants in nature, or we may hope to induce species to become
mutable by artificial methods. The first promises to yield results most
quickly, but the scope of the second is much greater and it may yield
results of far more importance. Indeed, if it should once become
possible to bring plants to mutate at our will and perhaps even in
arbitrarily chosen directions, there is no limit to the power we may
finally hope to gain over nature.
What is to guide us in this new line of work? Is it the minute
inspection of the features of the process in the case of the
evening-primroses? Or are we to base our hopes and our methods on
broader conceptions of nature's laws? Is it the systematic study of
species and varieties, and the biologic inquiry into their real
hereditary units? Or is the theory of descent to be our starting-point?
Are we to rest our conceptions on the experience of the breeder, or is
perhaps the geologic pedigree of all organic life to open to us better
prospects of success?
The answer to all such questions is a very simple one. All possibilities
must be considered, and no line of investigation ignored. For myself I
have based my field-researches and my [689] testing of native plants on
the hypothesis of unit-characters as deduced from Darwin's _Pangenesis_.
This conception led to the expectation of two different kinds of
variability, one slow and one sudden. The sudden ones known at the time
were considered as sports, and seemed limited to retrograde changes, or
to cases of minor importance. The idea that sudden steps might be taken
as the principal method of evolution could be derived from the
hypothesis of unit characters, but the evidence might be too remote for
a starting point for experimental investigation.
The success of my test has given proof to the contrary. Hence the
assertion that no evidence is to be considered as inadequate for the
purpose under discussion. Sometime a method of discovering, or of
producing, mutable plants may be found, but until this is done, all
facts of whatever nature or direction must be made use of. A very slight
indication may change forever the whole aspect of the problem.
The probabilities are now greatly in favor of our finding out the causes
of evolution by a close scrutiny of what really happens in nature. A
persistent study of the physiologic factors of this evolution is the
chief condition of success. To this study field-observations may
contribute as well as direct experiments, [690] microscopical
investigations as well as extended pedigree-cultures. The cooperation of
many workers is required to cover the field. Somewhere no doubt the
desired principle lies hidden, but until it is discovered, all methods
must be tried.
With this conception as the best starting point for further
investigation, we may now make a brief survey of the other phase of the
problem. We shall try to connect our observations on the
evening-primroses with the theory of descent at large.
We start with two main facts. One is the mutability of Lamarck's
primrose, and the second is the immutable condition of quite a number of
other species. Among them are some of its near allies, the common and
the small flowered evening-primrose, or _Oenothera biennis_ and _O.
muricata_.
From these facts, a very important question arises in connection with
the theory of descent. Is the mutability of our evening-primroses
temporary, or is it a permanent condition? A discussion of this problem
will give us the means of reaching a definite idea as to the scope of
our inquiries.
Let us consider the present first. If mutability is a permanent
condition, it has of course no beginning, and moreover is not due to the
[691] agency of external circumstances. Should this be granted for the
evening-primrose, it would have to be predicated for other species found
in a mutable state. Then, of course, it would be useless to investigate
the causes of mutability at large, and we should have to limit ourselves
to the testing of large numbers of plants in order to ascertain which
are mutable and which not.
If, on the other hand, mutability is not a permanent feature, it must
once have had a beginning, and this beginning itself must have had an
external cause. The amount of mutability and its possible directions may
be assumed to be due to internal causes. The determination of the moment
at which they will become active can never be the result of internal
causes. It must be assigned to some external factor, and as soon as this
is discovered the way for experimental investigation is open.
In the second place we must consider the past. On the supposition of
permanency all the ancestors of the evening-primrose must have been
mutable. By the alternative view mutability must have been a periodic
phenomenon, producing at times new qualities, and at other times leaving
the plants unchanged during long successions of generations. The present
mutable state must then have been preceded by an immutable [692]
condition, but of course thousands of mutations must have been required
to produce the evening-primroses from their most remote ancestors.
If we take the species into consideration that are not mutable at
present, we may ask how we are to harmonize them with each of the two
theories proposed. If mutability is permanent, it is manifest that the
whole pedigree of the animal and vegetable kingdom is to be considered
as built up of main mutable lines, and that the thousands of constant
species can only be taken to represent lateral branches of the
genealogic tree.
These lateral branches would have lost the capacity of mutating,
possessed by all their ancestors. And as the principle of the hypothesis
under discussion does not allow a resumption of this habit, they would
be doomed to eternal constancy until they finally die out. Loss of
mutability, under this conception, means loss of the capacity for all
further development. Only those lines of the main pedigree which have
retained this capacity would have a future; all others would die out
without any chance of progression.
If, on the other hand, mutability is not permanent, but a periodic
condition, all lines of the genealogic tree must be assumed to show
alternatively [693] mutating and constant species. Some lines may be
mutating at the present moment; others may momentarily be constant. The
mutating lines will probably sooner or later revert to the inactive
state, while the powers of development now dormant may then become
awakened on other branches.
The view of permanency represents life as being surrounded with
unavoidable death, the principle of periodicity, on the contrary,
follows the idea of resurrection, granting the possibility of future
progression for all living beings. At the same time it yields a more
hopeful prospect for experimental inquiry.
Experience must decide between the two main theories. It demonstrates
the existence of polymorphous genera, such as _Draba_ and _Viola_ and
hundreds of others. They clearly indicate a previous state of
mutability. Their systematic relation is exactly what would be expected,
if they were the result of such a period. Perhaps mutability has not
wholly ceased in them, but, might be found to survive in some of their
members. Such very rich genera however, are not the rule, but are
exceptional cases, indicating the rarity of powerful mutative changes.
On the other hand, species may remain in a state of constancy during
long, apparently during indefinite, ages.
[694] Many facts plead in favor of the constancy of species. This
principle has always been recognized by systematists. Temporarily the
current form of the theory of natural selection has assumed species to
be inconstant, ever changing and continuously being improved and adapted
to the requirements of the life-conditions. The followers of the theory
of descent believed that this conclusion was unavoidable, and were
induced to deny the manifest fact that species are constant entities.
The mutation theory gives a clew to the final combination of the two
contending ideas. Reducing the changeability of the species to distinct
and probably short periods, it at once explains how the stability of
species perfectly agrees with the principle of descent through
modification.
On the other hand, the hypothesis of mutative periods is by no means
irreconcilable with the observed facts of constancy. Such casual changes
can be proved by observations such as those upon the evening-primrose,
but it is obvious that a disproof can never be given. The principle
grants the present constancy of the vast majority of living forms, and
only claims the exceptional occurrence of definite changes.
Proofs of the constancy of species have been given in different ways.
The high degree of similarity of the individuals of most of our [695]
species has never been denied. It is observed throughout extended
localities, and during long series of years. Other proofs are afforded
by those plants which have been transported to distant localities some
time since, but do not exhibit any change as a result of this migration.
Widely dispersed plants remain the same throughout their range, provided
that they belong to a single elementary species. Many species have been
introduced from America into Europe and have spread rapidly and widely.
The Canadian horsetail (_Erigeron canadensis_), the evening-primrose and
many other instances could be given. They have not developed any special
European features after their introduction. Though exposed to other
environmental conditions and to competition with other species, they
have not succeeded in developing a new character. Such species as proved
adequate to the new environment have succeeded, while those which did
not have succumbed.
Much farther back is the separation of the species which now live both
in arctic regions and on the summits of our highest mountaintops. If we
compare the alpine flora with the arctic plants, a high degree of
similarity at once strikes us. Some forms are quite identical; others
are slightly different, manifestly representing elementary species of
the same systematic [696] type. Still others are more distant or even
belong to different genera. The latter, and even the diverging, though
nearly allied, elementary species, do not yield adequate evidence in any
direction.
They may as well have lived together in the long ages before the
separation of the now widely distant floras, or have sprung from a
common ancestor living at that time, and subsequently have changed their
habits. After excluding these unreliable instances, a good number of
species remain, which are quite the same in the arctic and alpine
regions and on the summits of distant mountain ranges. As no
transportation over such large distances can have brought them from one
locality to the other, no other explanation is left than that they have
been wholly constant and unchanged ever since the glacial period which
separated them. Obviously they must have been subjected to widely
changing conditions. The fact of their stability through all these
outward changes is the best proof that the ordinary external conditions
do not necessarily have an influence on specific evolution. They may
have such a result in some instances, in others they obviously have not.
Many arctic forms bearing the specific name of _alpinus_ justify this
conclusion. _Astragalus alpinus_, _Phleum alpinum_, _Hieracium alpinum_
and [697] others from the northern parts of Norway may be cited as
examples.
Thus Primula imperialis has been found in the Himalayas, and many other
plants of the high mountains of Java, Ceylon and northern India are
identical forms. Some species from the Cameroons and from Abyssinia have
been found on the mountains of Madagascar. Some peculiar Australian
types are represented on the summit of Kini Balu in Borneo. None of
these species, of course, are found in the intervening lowlands, and the
only possible explanation of their identity is the conception of a
common post-glacial origin, coupled with complete stability. This
stability is all the more remarkable as nearly allied but slightly
divergent forms have also been reported from almost all of these
localities. Other evidence is obtained by the comparison of ancient
plants with their living representatives. The remains in tombs of
ancient Egypt have always afforded strong support of the views of the
adherents of the theory of stability, and to my mind they still do so.
The cereals and fruits and even the flowers and leaves in the funeral
wreaths of Rameses and Amen-Hotep are the same that are still now
cultivated in Egypt. Nearly a hundred or more species have been
identified. Flowers of _Acacia_, leaves of _Mimusops_, [698] petals of
_Nymphaea_ may be cited as instances, and they are as perfectly
preserved as the best herbarium-specimens of the present time. The
petals and stamens retain their original colors, displaying them as
brightly as is consistent with their dry state.
Paleontologic evidence points to the same conclusion. Of course the
remains are incomplete, and rarely adequate for a close comparison. The
range of fluctuating variability should be examined first, but the test
of elementary species given by their constancy from seed cannot, of
course, be applied. Apart from these difficulties, paleontologists agree
in recognizing the very great age of large numbers of species. It would
require a too close survey of geologic facts to go into details on this
point. Suffice it to say that in more recent Tertiary deposits many
species have been identified with living forms. In the Miocene period
especially, the similarity of the types of phanerogamic plants with
their present offspring, becomes so striking that in a large number of
cases specific distinctions rest in greater part on theoretical
conceptions rather than on real facts. For a long time the idea
prevailed that the same species could not have existed through more than
one geologic period. Many distinctions founded on this belief have since
had to be abandoned. [699] Species of algae belonging to the
well-preserved group of the diatoms, are said to have remained unchanged
from the Carboniferous period up to the present time.
Summing up the results of this very hasty survey, we may assert that
species remain unchanged for indefinite periods, while at times they are
in the alternative condition. Then at once they produce new forms often
in large numbers, giving rise to swarms of subspecies. All facts point
to the conclusion that these periods of stability and mutability
alternate more or less regularly with one another. Of course a direct
proof of this view cannot, as yet, be given, but this conclusion is
forced upon us by a consideration of known facts bearing on the
principle of constancy and evolution.
If we are right in this general conception, we may ask further, what is
to be the exact place of our group of new evening-primroses in this
theory? In order to give an adequate answer, we must consider the whole
range of the observations from a broader point of view. First of all it
is evident that the real mutating period must be assumed to be much
longer than the time covered by my observations. Neither the beginning
nor the end have been seen. It is quite obvious that _Oenothera
lamarckiana_ was in a mutating condition when I first [700] saw it,
seventeen years ago. How long had it been so? Had it commenced to mutate
after its introduction into Europe, some time ago, or was it already
previously in this state? It is as yet impossible to decide this point.
Perhaps the mutable state is very old, and dates from the time of the
first importation of the species into Europe.
Apart from all such considerations the period of the direct
observations, and the possible duration of the mutability through even
more than a century, would constitute only a moment, if compared with
the whole geologic time. Starting from this conception the pedigree of
our mutations must be considered as only one small group. Instead of
figuring a fan of mutants for each year, we must condense all the
succeeding swarms into one single fan, as might be done also for _Draba
verna_ and other polymorphous species. In _Oenothera_ the main stem is
prolonged upwards beyond the fan; in the others the main stem is lacking
or at least undiscernable, but this feature manifestly is only of
secondary importance. We might even prefer the image of a fan, adjusted
laterally to a stem, which itself is not interrupted by this branch.
On this principle two further considerations are to be discussed. First
the structure of the [701] fan itself, and secondly the combination of
succeeding fans into a common genealogic tree.
The composition of the fan as a whole includes more than is directly
indicated by the facts concerning the birth of new species. They arise
in considerable quantities, and each of them in large numbers of
individuals, either in the same or in succeeding years. This multiple
origin must obviously have the effect of strengthening the new types,
and of heightening their chances in the struggle for life. Arising in a
single specimen they would have little chance of success, since in the
field among thousands of seeds perhaps one only survives and attains
complete development. Thousands or at least hundreds of mutated seeds
are thus required to produce one mutated individual, and then, how small
are its chances of surviving! The mutations proceed in all directions,
as I have pointed out in a former lecture. Some are useful, others might
become so if the circumstances were accidentally changed in definite
directions, or if a migration from the original locality might take
place. Many others are without any real worth, or even injurious.
Harmless or even slightly useless ones have been seen to maintain
themselves in the field during the seventeen years of my research, as
proved by _Oenothera laevifolia_ and _Oenothera_ [702] _brevistylis_.
Most of the others quickly disappear.
This failure of a large part of the productions of nature deserves to be
considered at some length. It may be elevated to a principle, and may be
made use of to explain many difficult points of the theory of descent.
If, in order to secure one good novelty, nature must produce ten or
twenty or perhaps more bad ones at the same time, the possibility of
improvements coming by pure chance must be granted at once. All
hypotheses concerning the direct causes of adaptation at once become
superfluous, and the great principle enunciated by Darwin once more
reigns supreme.
In this way too, the mutation-period of the evening-primrose is to be
considered as a prototype. Assuming it as such provisionally, it may aid
us in arranging the facts of descent so as to allow of a deeper insight
and a closer scrutiny. All swarms of elementary species are the remains
of far larger initial groups. All species containing only a few
subspecies may be supposed to have thrown off at the outset far more
numerous lateral branches, out of which however, the greater part have
been lost, being unfit for the surrounding conditions. It is the
principle of the struggle for life between elementary species, followed
by the survival of the [703] fittest, the law of the selection of
species, which we have already laid stress upon more than once.
Our second consideration is also based upon the frequent repetition of
the several mutations. Obviously a common cause must prevail. The
faculty of producing _nanella_ or _lata_ remains the same through all
the years. This faculty must be one and the same for all the hundreds of
mutative productions of the same form. When and how did it originate? At
the outset it must have been produced in a latent condition, and even
yet it must be assumed to be continuously present in this state, and
only to become active at distant intervals. But it is manifest that the
original production of the characters of _Oenothera gigas_ was a
phenomenon of far greater importance than the subsequent accidental
transition of this quality into the active state. Hence the conclusion
that at the beginning of each series of analogous mutations there must
have been one greater and more intrinsic mutation, which opened the
possibility to all its successors. This was the origination of the new
character itself, and it is easily seen that this incipient change is to
be considered as the real one. All others are only its visible
expressions.
Considering the mutative period of our evening-primrose [704] as one
unit-stride section in the great genealogic tree, this period includes
two nearly related, but not identical changes. One is the production of
new specific characters in the latent condition, and the other is the
bringing of them to light and putting them into active existence. These
two main factors are consequently to be assumed in all hypothetic
conceptions of previous mutative periods.
Are all mutations to be considered as limited to such periods? Of course
not. Stray mutations may occur as well. Our knowledge concerning this
point is inadequate for any definite statement. Swarms of variable
species are easily recognized, if the remnants are not too few. But if
only one or two new species have survived, how can we tell whether they
have originated-alone or together with others. This difficulty is still
more pronounced in regard to paleontologic facts, as the remains of
geologic swarms are often found, but the absence of numerous mutations
can hardly be proved in any case.
I have more than once found occasion to lay stress on the importance of
a distinction between progressive and retrograde mutations in previous
lectures. All improvement is, of course, by the first of these modes of
evolution, but apparent losses of organs or qualities are [705] perhaps
of still more universal occurrence. Progression and regression are seen
to go hand in hand everywhere. No large group and probably even no genus
or large species has been evolved without the joint agency of these two
great principles. In the mutation-period of the evening-primroses the
observed facts give direct support to this conclusion, since some of the
new species proved, on closer inspection, to be retrograde varieties,
while others manifestly owe their origin to progressive steps. Such
steps may be small and in a wrong direction; notwithstanding this they
may be due to the acquisition of a wholly new character and therefore
belong to the process of progression at large.
Between them however, there is a definite contrast, which possibly is in
intimate connection with the question of periodic and stray mutations.
Obviously each progressive change is dependent upon the production of a
new character, for whenever this is lacking, no such mutation is
possible. Retrograde changes, on the other hand, do not require such
elaborate preliminary work. Each character may be converted into the
latent condition, and for all we know, a special preparation for this
purpose is not at all necessary. It is readily granted that such special
preparation may occur, because the [706] great numbers in which our
dwarf variety of the _Oenothera_ are yearly produced are suggestive of
such a condition. On the other hand, the _laevifolia_ and _brevistylis_
mutations have not been repeated, at least not in a visible way.
From this discussion we may infer that it is quite possible that a large
part of the progressive changes, and a smaller part of the retrograde
mutations, are combined into groups, owing their origin to common
external agencies. The periods in which such groups occur would
constitute the mutative periods. Besides them the majority of the
retrograde changes and some progressive steps might occur separately,
each being due to some special cause. Degressive mutations, or those
which arise by the return of latent qualities to activity, would of
course belong with the latter group.
This assumption of a stray and isolated production of varieties is to a
large degree supported by experience in horticulture. Here there are no
real swarms of mutations. Sudden leaps in variability are not rare, but
then they are due to hybridization. Apart from this mixture of
characters, varieties as a rule appear separately, often with intervals
of dozens of years, and without the least suggestion of a common cause.
It is quite superfluous to go into details, as we have dealt with the
horticultural [707] mutations at sufficient length on a previous
occasion. Only the instance of the peloric toadflax might be recalled
here, because the historic and geographic evidence, combined with the
results of our pedigree-experiment, plainly show that peloric mutations
are quite independent of any periodic condition. They may occur anywhere
in the wide range of the toad-flax, and the capacity of repeatedly
producing them has lasted some centuries at least, and is perhaps even
as old as the species itself.
Leaving aside such stray mutations, we may now consider the probable
constitution of the great lines of the genealogic tree of the evening
primroses, and of the whole vegetable and animal kingdom at large. The
idea of drawing up a pedigree for the chief groups of living organisms
is originally due to Haeckel, who used this graphic method to support
the Darwinian theory of descent. Of course, Haeckel's genealogic trees
are of a purely hypothetic nature, and have no other purpose than to
convey a clear conception of the notion of descent, and of the great
lines of evolution at large. Obviously all details are subject to doubt,
and many have accordingly been changed by his successors. These changes
may be considered as partial improvements, and the somewhat picturesque
form of Haeckel's pedigree might well be replaced by [708] more simple
plans. But the changes have by no means removed the doubts, nor have
they been able to supplant the general impression of distinct groups,
united by broad lines. This feature is very essential, and it is easily
seen to correspond with the conception of swarms, as we have deduced it
from the study of the lesser groups.
Genealogic trees are the result of comparative studies; they are far
removed from the results of experimental inquiry concerning the origin
of species. What are the links which bind them together? Obviously they
must be sought in the mutative periods, which have immediately preceded
the present one. In the case of the evening-primrose the systematic
arrangement of the allied species readily guides us in the delimitations
of such periods. For manifestly the species of the large genus of
_Oenothera_ are grouped in swarms, the youngest or most recent of which
we have under observation. Its immediate predecessor must have been the
subgenus _Onagra_, which is considered by some authors as consisting of
a single systematic species, _Oenothera biennis_. Its multifarious forms
point to a common origin, not only morphologically but also
historically. Following this line backward or downward we reach another
apparent mutation-period, which includes the origin of [709] the group
called _Oenothera_, with a large number of species of the same general
type as the _Onagra-forms, Still farther downward comes the old genus
_Oenothera_ itself, with numerous subgenera diverging in sundry
characters and directions.
Proceeding still farther we might easily construct a main stem with
numerous succeeding fans of lateral branches, and thus reach, from our
new empirical point of view, the theoretical conclusion already
formulated.
Paleontologic facts readily agree with this conception. The swarms of
species and varieties are found to succeed one another like so many
stories. The same images are repeated, and the single stories seem to be
connected by the main stems, which in each tier produce the whole number
of allied forms. Only a few prevailing lines are prolonged through
numerous geologic periods; the vast majority of the lateral branches are
limited each to its own storey. It is simply the extension of the
pedigree of the evening-primroses backward through ages, with the same
construction and the same leading features. There can be no doubt that
we are quite justified in assuming that evolution has followed the same
general laws through the whole duration of life on earth. Only a moment
of their lifetime is disclosed to us, but it [710] is quite sufficient
to enable us to discern the laws and to conjecture the outlines of the
whole scheme of evolution.
A grave objection which has, often, and from the very outset, been urged
against Darwin's conception of very slow and nearly imperceptible
changes, is the enormously long time required. If evolution does not
proceed any faster than what we can see at present, and if the process
must be assumed to have gone on in the same slow manner always,
thousands of millions of years would have been needed to develop the
higher types of animals and plants from their earliest ancestors.
Now it is not at all probable that the duration of life on earth
includes such an incredibly long time. Quite on the contrary the
lifetime of the earth seems to be limited to a few millions of years.
The researches of Lord Kelvin and other eminent physicists seem to leave
no doubt on this point. Of course all estimates of this kind are only
vague and approximate, but for our present purposes they may be
considered as sufficiently exact.
In a paper published in 1862 Sir William Thomson (now Lord Kelvin) first
endeavored to show that great limitation had to be put upon the enormous
demand for time made by Lyell, Darwin and other biologists. From a
consideration [711] of the secular cooling of the earth, as deduced from
the increasing temperature in deep mines, he concluded that the entire
age of the earth must have been more than twenty and less than forty
millions of years, and probably much nearer twenty than forty. His views
have been much criticised by other physicists, but in the main they have
gained an ever-increasing support in the way of evidence. New mines of
greater depth have been bored, and their temperatures have proved that
the figures of Lord Kelvin are strikingly near the truth. George Darwin
has calculated that the separation of the moon from the earth must have
taken place some fifty-six millions of years ago. Geikie has estimated
the existence of the solid crust of the earth at the most as a hundred
million years. The first appearance of the crust must soon have been
succeeded by the formation of the seas, and a long time does not seem to
have been required to cool the seas to such a degree that life became
possible. It is very probable that life originally commenced in the
great seas, and that the forms which are now usually included in the
plankton or floating-life included the very first living beings.
According to Brooks, life must have existed in this floating condition
during long primeval epochs, and evolved nearly all the main branches of
the animal and vegetable kingdom [712] before sinking to the bottom of
the sea, and later producing the vast number of diverse forms which now
adorn the sea and land.
All these evolutions, however, must have been very rapid, especially at
the beginning, and together cannot have taken more time than the figures
given above.
The agency of the larger streams, and the deposits which they bring into
the seas, afford further evidence. The amount of dissolved salts,
especially of sodium chloride, has been made the subject of a
calculation by Joly, and the amount of lime has been estimated by Eugene
Dubois. Joly found fifty-five and Dubois thirty-six millions of years as
the probable duration of the age of the rivers, and both figures
correspond to the above dates as closely as might be expected from the
discussion of evidence so very incomplete and limited.
All in all it seems evident that the duration of life does not comply
with the demands of the conception of very slow and continuous
evolution. Now it is easily seen, that the idea of successive mutations
is quite independent of this difficulty. Even assuming that some
thousands of characters must have been acquired in order to produce the
higher animals and plants of the present time, no valid objection is
raised. The demands of the biologists and the results of [713] the
physicists are harmonized on the ground of the theory of mutation.
The steps may be surmised to have never been essentially larger than in
the mutations now going on under our eyes, and some thousands of them
may be estimated as sufficient to account for the entire organization of
the higher forms. Granting between twenty and forty millions of years
since the beginning of life, the intervals between two successive
mutations may have been centuries and even thousands of years. As yet
there has been no objection cited against this assumption, and hence we
see that the lack of harmony between the demands of biologists and the
results of the physicists disappears in the light of the theory of
mutation.
Summing up the results of this discussion, we may justifiably assert
that the conclusions derived from the observations and experiments made
with evening-primroses and other plants in the main agree satisfactorily
with the inferences drawn from paleontologic, geologic and systematic
evidence. Obviously these experiments are wonderfully supported by the
whole of our knowledge concerning evolution. For this reason the laws
discovered in the experimental garden may be considered of great
importance, and they may guide us in our further inquiries. Without
doubt many minor [714] points are in need of correction and elaboration,
but such improvements of our knowledge will gradually increase our means
of discovering new instances and, new proofs.
The conception of mutation periods producing swarms of species from time
to time, among which only a few have a chance of survival, promises to
become the basis for speculative pedigree-diagrams, as well as for
experimental investigations.
[715]
LECTURE XXV
GENERAL LAWS OF FLUCTUATION
The principle of unit-characters and of elementary species leads at once
to the recognition of two kinds of variability. The changes of wider
amplitude consist of the acquisition of new units, or the loss of
already existing ones. The lesser variations are due to the degree of
activity of the units themselves.
Facts illustrative of these distinctions were almost wholly lacking at
the time of the first publication of Darwin's theories. It was a bold
conception to point out the necessity for such distinction on purely
theoretical grounds. Of course some sports were well known and
fluctuations were evident, but no exact analysis of the details was
possible, a fact that was of great importance in the demonstration of
the theory of descent. The lack of more definite knowledge upon this
matter was keenly felt by Darwin, [716] and exercised much influence
upon his views at various times.
Quetelet's famous discovery of the law of fluctuating variability
changed the entire situation and cleared up many difficulties. While a
clear conception of fluctuations was thus gained, mutations were
excluded from consideration, being considered as very rare, or
non-existent. They seemed wholly superfluous for the theory of descent,
and very little importance was attached to their study. Current
scientific belief in the matter has changed only in recent years.
Mendel's law of varietal hybrids is based upon the principle of
unit-characters, and the validity of this conception has thus been
brought home to many investigators.
A study of fluctuating or individual variability, as it was formerly
called, is now carried on chiefly by mathematical methods. It is not my
purpose to go into details, as it would require a separate course of
lectures. I shall consider the limits between fluctuation and mutation
only, and attempt to set forth an adequate idea of the principles of the
first as far as they touch these limits. The mathematical treatment of
the facts is no doubt of very great value, but the violent discussions
now going on between mathematicians such as Pearson, Kapteyn and others
should warn biologists to abstain [717] from the use of methods which
are not necessary for the furtherance of experimental work.
Fortunately, Quetelet's law is a very clear and simple one, and quite
sufficient for our considerations. It claims that for biologic phenomena
the deviations from the average comply with the same laws as the
deviations from the average in any other case, if ruled by chance only.
The meaning of this assertion will become clear by a further discussion
of the facts. First of all, fluctuating variability is an almost
universal phenomenon. Every organ and every quality may exhibit it. Some
are very variable, while others seem quite constant. Shape and size vary
almost indefinitely, and the chemical composition is subject to the same
law, as is well known for the amount of sugar in sugar-beets. Numbers
are of course less liable to changes, but the numbers of the rays of
umbels, or ray-florets in the composites, of pairs of blades in pinnate
leaves, and even of stamens and carpels are known to be often
exceedingly variable. The smaller numbers however, are more constant,
and deviations from the quinate structure of flowers are rare.
Complicated structures are generally capable of only slight deviations.
From a broad point of view, fluctuating variability [718] falls under
two heads. They obey quite the same laws and are therefore easily
confused, but with respect to questions of heredity they should be
carefully separated. They are designated by the terms individual and
partial fluctuation. Individual variability indicates the differences
between individuals, while partial variability is limited to the
deviations shown by the parts of one organism from the average
structure. The same qualities in some cases vary individually and in
others partially. Even stature, which is as markedly individual for
annual and biennial plants as it is for man, becomes partially variant
in the case of perennial herbs with numbers of stems. Often a character
is only developed once in the whole course of evolution, as for
instance, the degree of connation of the seed-leaves in tricotyls and in
numerous cases it is impossible to tell whether a character is
individual or partial. Consequently such minute details are generally
considered to have no real importance for the hereditary transmission of
the character under discussion.
Fluctuations are observed to take place only in two directions. The
quality may increase or decrease, but is not seen to vary in any other
way. This rule is now widely established by numerous investigations, and
is fundamental to [719] the whole method of statistical investigation.
It is equally important for the discussion of the contrast between
fluctuations and mutations, and for the appreciation of their part in
the general progress of organization. Mutations are going on in all
directions, producing, if they are progressive, something quite new
every time. Fluctuations are limited to increase and decrease of what is
already available. They may produce plants with higher stems, more
petals in the flowers, larger and more palatable fruits, but obviously
the first petal and the first berry, cannot have originated by the
simple increase of some older quality. Intermediates may be found, and
they may mark the limit, but the demonstration of the absence of a limit
is quite another question. It would require the two extremes to be shown
to belong to one unit, complying with the simple law of Quetelet.
Nourishment is the potent factor of fluctuating variability. Of course
in thousands of cases our knowledge is not sufficient to allow us to
analyze this relation, and a number of phases of the phenomenon have
been discovered only quite recently. But the fact itself is thoroughly
manifest, and its appreciation is as old as horticultural science.
Knight, who lived at the beginning of the last century, has laid great
stress upon it, and it has since influenced practice in a [720] large
measure. Moreover, Knight pointed out more than once that it is the
amount of nourishment, not the quality of the various factors, that
exercises the determinative influence. Nourishment is to be taken in the
widest sense of the word, including all favorable and injurious
elements. Light and temperature, soil and space, water and salts are
equally active, and it is the harmonious cooperation of them all that
rules growth.
We treated this important question at some length, when dealing with the
anomalies of the opium-poppies, consisting of the conversion of stamens
into supernumerary pistils. The dependency upon external influences
which this change exhibited is quite the same as that shown by
fluctuating variability at large. We inquired into the influence of good
and bad soil, of sunlight and moisture and of other concurrent factors.
Especial emphasis was laid upon the great differences to which the
various individuals of the same lot may be exposed, if moisture and
manure differ on different portions of the same bed in a way unavoidable
even by the most careful preparation. Some seeds germinate on moist and
rich spots, while their neighbors are impeded by local dryness, or by
distance from manure. Some come to light on a sunny day, and increase
their first leaves rapidly, while on [721] the following day the weather
may be unfavorable and greatly retard growth. The individual differences
seem to be due, at least in a very great measure, to such apparent
trifles.
On the other hand partial differences are often manifestly due to
similar causes. Considering the various stems of plants, which multiply
themselves by runners or by buds on the roots, the assertion is in no
need of further proof. The same holds good for all cases of artificial
multiplication by cuttings, or by other vegetative methods. But even if
we limit ourselves to the leaves of a single tree, or the branches of a
shrub, or the flowers on a plant, the same rule prevails. The
development of the leaves is dependent on their position, whether
inserted on strong or weak branches, exposed to more or less light, or
nourished by strong or weak roots. The vigor of the axillary buds and of
the branches which they may produce is dependent upon the growth and
activity of the leaves to which the buds are axillary.
This dependency on local nutrition leads to the general law of
periodicity, which, broadly speaking, governs the occurrence of the
fluctuating deviations of the organs. This law of periodicity involves
the general principle that every axis, as a rule, increases in strength
when [722] growing, but sooner or later reaches a maximum and may
afterwards decrease.
This periodic augmentation and declination is often boldly manifest,
though in other cases it may be hidden by the effect of alternate
influences. Pinnate leaves generally have their lower blades smaller
than the upper ones, the longest being seen sometimes near the apex and
sometimes at a distance from it. Branches bearing their leaves in two
rows often afford quite as obvious examples, and shoots in general
comply with the same rule. Germinating plants are very easy of
observation on this point. When they are very weak they produce only
small leaves. But their strength gradually increases and the subsequent
organs reach fuller dimensions until the maximum is attained. The
phenomenon is so common that its importance is usually overlooked. It
should be considered as only one instance of a rule, which holds good
for all stems and all branches, and which is everywhere dependent on the
relation of growth to nutrition.
The rule of periodicity not only affects the size of the organs, but
also their number, whenever these are largely variable. Umbellate plants
have numerous rays on the umbels of strong stems, but the number is seen
to decrease and to become very small on the weakest lateral [723]
branches. The same holds good for the number of ray-florets in the
flower-heads of the composites, even for the number of stigmas on the
ovaries of the poppies, which on weak branches may be reduced to as few
as three or four. Many other instances could be given.
One of the best authenticated cases is the dependency of partial
fluctuation on the season and on the weather. Flowers decline when the
season comes to an end, become smaller and less brightly colored. The
number of ray-florets in the flower-heads is seen to decrease towards
the fall. Extremes become rarer, and often the deviations from the
average seem nearly to disappear. Double flowers comply with this rule
very closely, and many other cases will easily occur to any student of
nature.
Of course, the relation to nourishment is different for individual and
partial fluctuations. Concerning the first, the period of development of
the germ within the seed is decisive. Even the sexual cells may be in
widely different conditions at the moment of fusion, and perhaps this
state of the sexual cells includes the whole matter of the decision for
the average characters of the new individual. Partial fluctuation
commences as soon as the leaves and buds begin to form, and all later
changes in nutrition can only cause partial differences. All leaves,
[724] buds, branches, and flowers must come under the influence of
external conditions during the juvenile period, and so are liable to
attain a development determined in part by the action of these factors.
Before leaving these general considerations, we must direct our
attention to the question of utility. Obviously, fluctuating variability
is a very useful contrivance, in many cases at least. It appears all the
more so, as its relation to nutrition becomes manifest. Here two aspects
are intimately combined. More nutrient matter produces larger leaves and
these are in their turn more fit to profit by the abundance of
nourishment. So it is with the number of flowers and flower-groups, and
even with the numbers of their constituent organs. Better nourishment
produces more of them, and thereby makes the plant adequate to make a
fuller use of the available nutrient substances. Without fluctuation
such an adjustment would hardly be possible, and from all our notions of
usefulness in nature, we therefore must recognize the efficiency of this
form of variability.
In other respects the fluctuations often strike us as quite useless or
even as injurious. The numbers of stamens, or of carpels are dependent
on nutrition, but their fluctuation is not known to have any attraction
for the visiting insects.
[725] If the deviations become greater, they might even become
detrimental. The flowers of the St. Johnswort, or _Hypericum
perforatum_, usually have five petals, but the number varies from three
to eight or more. Bees could hardly be misled by such deviations. The
carpels of buttercups and columbines, the cells in the capsules of
cotton and many other plants are variable in number. The number of seeds
is thereby regulated in accordance with the available nourishment, but
whether any other useful purpose is served, remains an open question.
Variations in the honey-guides or in the pattern of color-designs might
easily become injurious by deceiving insects, and such instances as the
great variability of the spots on the corolla of some cultivated species
of monkey-flowers, for instance, the _Mimulus quinquevulnerus_, could
hardly be expected to occur in wild plants. For here the dark brown
spots vary between nearly complete deficiency up to such predominancy as
almost to hide the pale yellow ground-color.
After this hasty survey of the causes of fluctuating variability, we now
come to a discussion of Quetelet's law. It asserts that the deviations
from the average obey the law of probability. They behave as if they
were dependent on chance only.
Everyone knows that the law of Quetelet can [726] be demonstrated the
most readily by placing a sufficient number of adult men in a row,
arranging them according to their size. The line passing over their
heads proves to be identical with that given by the law of probability.
Quite in the same way, stems and branches, leaves and petals and even
fruits can be arranged, and they will in the main exhibit the same line
of variability. Such groups are very striking, and at the first glance
show that the large majority of the specimens deviate from the mean only
to a very small extent. Wider deviations are far more rare, and their
number lessens, the greater the deviation, as is shown by the curvature
of the line. It is almost straight and horizontal in the middle portion,
while at the ends it rapidly declines, going sharply downward at one
extreme and upward at the other.
It is obvious however, that in these groups the leaves and other organs
could conveniently be replaced by simple lines, indicating their size.
The result would be quite the same, and the lines could be placed at
arbitrary, but equal distances. Or the sizes could be expressed by
figures, the compliance of which with the general law could be
demonstrated by simple methods of calculation. In this manner the
variability of different organs can easily be compared. Another method
of demonstration consists in [727] grouping the deviations into
previously fixed divisions. For this purpose the variations are measured
by standard units, and all the instances that fall between two limits
are considered to constitute one group. Seeds and small fruits, berries
and many other organs may conveniently be dealt with in this way. As an
example we take ordinary beans and select them according to their size.
This can be done in different ways. On a small piece of board a long
wedge-shaped slit is made, into which seeds are pushed as far as
possible. The margin of the wedge is calibrated in such a manner that
the figures indicate the width of the wedge at the corresponding place.
By this device the figure up to which a bean is pushed at once shows its
length. Fractions of millimeters are neglected, and the beans, after
having been measured, are thrown into cylindrical glasses of the same
width, each glass receiving only beans of equal length. It is clear that
by this method the height to which beans fill the glasses is
approximately a measure of their number. If now the glasses are put in a
row in the proper sequence, they at once exhibit the shape of a line
which corresponds to the law of chance. In this case however, the line
is drawn in a different manner from the first. It is to be pointed out
that the glasses may be replaced by lines indicating [728] the height of
their contents, and that, in order to reach a more easy and correct
statement, the length of the lines may simply be made proportionate to
the number of the beans in each glass. If such lines are erected on a
common base and at equal distances, the line which unites their upper
ends will be the expression of the fluctuating variability of the
character under discussion.
The same inquiry may be made with other seeds, with fruits, or other
organs. It is quite superfluous to arrange the objects themselves, and
it is sufficient to arrange the figures indicating their value. In order
to do this a basal line is divided into equal parts, the demarcations
corresponding to the standard-units chosen for the test. The observed
values are then written above this line, each finding its place between
the two demarcations, which include its value. It is very interesting
and stimulating to construct such a group. The first figures may fall
here and there, but very soon the vertical rows on the middle part of
the basal line begin to increase. Sometimes ten or twenty measurements
will suffice to make the line of chance appear, but often indentations
will remain. With the increasing number of the observations the
irregularities gradually [729] disappear, and the line becomes smoother
and more uniformly curved.
This method of arranging the figures directly on a basal line is very
convenient, whenever observations are made in the field or garden. Very
few instances need be recorded to obtain an appreciation of the mean
value, and to show what may be expected from a continuance of the test.
The method is so simple and so striking, and so wholly independent of
any mathematical development that it should be applied in all cases in
which it is desired to ascertain the average value of any organ, and the
measure of the attendant deviations.
I cite an instance, secured by counting the ray-florets on the
flower-heads of the corn-marigold or _Chrysanthemum segetum_. It was
that, by which I was enabled to select the plant, which afterwards
showed the first signs of a double head. I noted them in this way;
47
47 52
41 54 68
44 50 62 75
36 45 58 65 72 __ 99
Of course the figures might be replaced in this work by equidistant dots
or by lines, but experience teaches that the chance of making mistakes
is noticeably lessened by writing down [730] the figures themselves.
Whenever decimals are made use of it is obviously the best plan to keep
the figures themselves. For afterwards it often becomes necessary to
arrange them according to a somewhat different standard.
Uniting the heads of the vertical rows of figures by a line, the form
corresponding to Quetelet's law is easily seen. In the main it is always
the same as the line shown by the measurements of beans and seeds. It
proves a dense crowding of the single instances around the average, and
on both sides of the mass of the observations, a few wide deviations.
These become more rare in proportion to the amount of their divergency.
On both sides of the average the line begins by falling very rapidly,
but then bends slowly so as to assume a nearly horizontal direction. It
reaches the basal line only beyond the extreme instances.
It is quite evident that all qualities, which can be expressed by
figures, may be treated in this way. First, of all the organs occurring
in varying numbers, as for instance the ray-florets of composites, the
rays of umbels, the blades of pinnate and palmate leaves, the numbers of
veins, etc., are easily shown to comply with the same general rule.
Likewise the amount of chemical substances can be expressed in
percentage numbers, as is done on a large [731] scale with sugar in
beets and sugar-cane, with starch in potatoes and in other instances.
These figures are also found to follow the same law.
All qualities which are seen to increase and to decrease may be dealt
with in the same manner, if a standard unit for their measurement can be
fixed. Even the colors of flowers may not escape our inquiry.
If we now compare the lines, compiled from the most divergent cases,
they will be found to exhibit the same features in the main. Ordinarily
the curve is symmetrical, the line sloping down on both sides after the
same manner. But it is not at all rare that the inclination is steep on
one side and gradual on the other. This is noticeably the case if the
observations relate to numbers, the average of which is near zero. Here
of course the allowance for variation is only small on one side, while
it may increase with out distinct limits on the alternate slope. So it
is for instance with the numbers of ray-florets in the example given on
p. 729. Such divergent cases, however, are to be considered as
exceptions to the rule, due to some unknown cause.
Heretofore we have discussed the empirical side of the problem only. For
the purpose of experimental study of questions of heredity this is
ordinarily quite sufficient. The inquiry [732] into the phenomenon of
regression, or of the relation of the degree of deviation of the progeny
to that of their parents, and the selection of extreme instances for
multiplication are obviously independent of mathematical considerations.
On the other hand an important inquiry lies in the statistical treatment
of these phenomena, and such treatment requires the use of mathematical
methods.
Statistics however, are not included in the object of these lectures,
and therefore I shall refrain from an explanation of the method of their
preparation and limit myself to a general comparison of the observed
lines with the law of chance. Before going into the details, it should
be repeated once more that the empirical result is quite the same for
individual and for partial fluctuations. As a rule, the latter occur in
far greater number, and are thus more easily investigated, but
individual or personal averages have also been studied.
Newton discovered that the law of chance can be expressed by very simple
mathematical calculations. Without going into details, we may at once
state that these calculations are based upon his binomium. If the form
(a + b) is calculated for some value of the exponent, and if the values
of the coefficients after development are alone considered, they yield
the basis [733] for the construction of what is called the line or curve
of probability. For this construction the coefficients are used as
ordinates, the length of which is to be made proportionate to their
value. If this is done, and the ordinates are arranged at equal
distances, the line which unites their summits is the desired curve. At
first glance it exhibits a form quite analogous to the curves of
fluctuating variability, obtained by the measurements of beans and in
other instances. Both lines are symmetrical and slope rapidly down in
the region of the average, while with increasing distance they gradually
lose their steep inclination, becoming nearly parallel to the base at
their termination.
This similarity between such empirical and theoretical lines is in
itself an empirical fact. The causes of chance are assumed to be
innumerable, and the whole calculation is based on this assumption. The
causes of the fluctuations of biological phenomena have not as yet been
critically examined to such an extent as to allow of definite
conceptions. The term nourishment manifestly includes quite a number of
separate factors, as light, space, temperature, moisture, the physical
and chemical conditions of the soil and the changes of the weather.
Without doubt the single factors are very numerous, but whether they are
numerous enough to be treated [734] as innumerable, and thereby to
explain the laws of fluctuations, remains uncertain. Of course the
easiest way is to assume that they combine in the same manner as the
causes of chance, and that this is the ground of the similarity of the
curves. On the other hand, it is manifestly of the highest importance to
inquire into the part the several factors play in the determination of
the curves. It is not at all improbable that some of them have a larger
influence on individual, and others on partial, fluctuations. If this
were the case, their importance with respect to questions of heredity
might be widely different. In the present state of our knowledge the
fluctuation-curves do not contribute in any large measure to an
elucidation of the causes. Where these are obvious, they are so without
statistics, exactly as they were, previous to Quetelet's discovery.
In behalf of a large number of questions concerning heredity and
selection, it is very desirable to have a somewhat closer knowledge of
these curves. Therefore I shall try to point out their more essential
features, as far as this can be done without mathematical calculations.
At a first glance three points strike us, the average or the summit of
the curve, and the extremes. If the general shape is once denoted by the
results of observations or by the coefficients [735] of the binomium,
all further details seem to depend upon them. In respect to the average
this is no doubt the case; it is an empirical value without need of any
further discussion. The more the number of the observations increases,
the more assured and the more correct is this mean value, but generally
it is the same for smaller and for larger groups of observations.
This however, is not the case with the extremes. It is quite evident
that small groups have a chance of containing neither of them. The more
the number of the observations increases, the larger is the chance of
extremes. As a rule, and excluding exceptional cases, the extreme
deviations will increase in proportion to the number of cases examined.
In a hundred thousand beans the smallest one and the largest one may be
expected to differ more widely from one another than in a few hundred
beans of the same sample. Hence the conclusion that extremes are not a
safe criterion for the discussion of the curves, and not at all adequate
for calculations, which must be based upon more definite values.
A real standard is afforded by the steepness of the slope. This may be
unequal on the two sides of one curve, and likewise it may differ for
different cases. This steepness is usually measured by means of a point
on the half curve and [736 ] for this purpose a point is chosen which
lies exactly half way between the average and the extreme. Not however
half way with respect to the amplitude of the extreme deviation, for on
this ground it would partake of the uncertainty of the extreme itself.
It is the point on the curve which is surpassed by half the number, and
not reached by the other half of the number of the observations included
in the half of the curve. This point corresponds to the important value
called the probable error, and was designated by Galton as the quartile.
For it is evident that the average and the two quartiles divide the
whole of the observations into four equal parts.
Choosing the quartiles as the basis for calculations we are independent
of all the secondary causes of error, which necessarily are inherent in
the extremes. At a casual examination, or for demonstrative purposes,
the extremes may be prominent, but for all further considerations the
quartiles are the real values upon which to rest calculations.
Moreover if the agreement with the law of probability is once conceded,
the whole curve is defined by the average and the quartiles, and the
result of hundreds of measurements or countings may be summed up in
three, or, in [737] the case of symmetrical curves, perhaps in two
figures.
Also in comparing different curves with one another, the quartiles are
of great importance. Whenever an empirical fluctuation-curve is to be
compared with the theoretical form, or when two or more cases of
variability are to be considered under one head, the lines are to be
drawn on the same base. It is manifest that the averages must be brought
upon the same ordinate, but as to the steepness of the line, much
depends on the manner of plotting. Here we must remember that the mutual
distance of the ordinates has been a wholly arbitrary one in all our
previous considerations. And so it is, as long as only one curve is
considered at a time. But as soon as two are to be compared, it is
obvious that free choice is no longer allowed. The comparison must be
made on a common basis, and to this effect the quartiles must be brought
together. They are to lie on the same ordinates. If this is done, each
division of the base corresponds to the same proportionate number of
individuals, and a complete comparison is made possible.
On the ground of such a comparison we may thus assert that,
fluctuations, however different the organs or qualities observed, are
the same whenever their curves are seen to overlap one [738] another.
Furthermore, whenever an empirical curve agrees in this manner with the
theoretical one, the fluctuation complies with Quetelet's law, and may
be ascribed to quite ordinary and universal causes. But if it seems to
diverge from this line, the cause of this divergence should be inquired
into.
Such abnormal curves occur from time to time, but are rare.
Unsymmetrical instances have already been alluded to, and seem to be
quite frequent. Another deviation from the rule is the presence of more
than one summit. This case falls under two headings. If the ray florets
of a composite are counted, and the figures brought into a curve, a
prominent summit usually corresponds to the average. But next to this,
and on both sides, smaller summits are to be seen. On a close inspection
these summits are observed to fall on the same ordinates, on which, in
the case of allied species, the main apex lies. The specific character
of one form is thus repeated as a secondary character on an allied
species. Ludwig discovered that these secondary summits comply with the
rule discovered by Braun and Schimper, stating the relation of the
subsequent figures of the series. This series gives the terms of the
disposition of leaves in general, and of the bracts and flowers on the
composite flower [739] heads in our particular case. It is the series to
which we have already alluded when dealing with the arrangement of the
leaves on the twisted teasels. It commences with 1 and 2 and each
following figure is equal to the sum of its two precedents. The most
common figures are 3, 5, 8, 13, 18, 21, higher cases seldom coming under
observation. Now the secondary summits of the ray-curves of the
composites are seen to agree, as a rule, with these figures. Other
instances could readily be given.
Our second heading includes those cases which exhibit two summits of
equal or nearly equal height. Such cases occur when different races are
mixed, each retaining its own average and its own curve-summit. We have
already demonstrated such a case when dealing with the origin of our
double corn-chrysanthemum. The wild species culminates with 13 rays, and
the grandiflorum variety with 21. Often the latter is found to be
impure, being mixed with the typical species to a varying extent. This
is not easily ascertained by a casual inspection of the cultures, but
the true condition will promptly betray itself, if curves are
constructed. In this way curves may in many instances be made use of to
discover mixed races. Double curves may also result from the
investigation [740] of true double races, or ever-sporting varieties.
The striped snapdragon shows a curve of its stripes with two summits,
one corresponding to the average striped flowers, and the other to the
pure red ones. Such cases may be discovered by means of curves, but the
constituents cannot be separated by culture-experiments.
A curious peculiarity is afforded by half curves. The number of petals
is often seen to vary only in one direction from what should be expected
to be the mean condition. With buttercups and brambles and many others
there is only an increase above the typical five; quaternate flowers are
wanting or at least are very rare. With weigelias and many others the
number of the tips of the corolla varies downwards, going from five to
four and three. Hundreds of flowers show the typical five, and determine
the summit of the curve. This drops down on one side only, indicating
unilateral variability, which in many cases is due to a very intimate
connection of a concealed secondary summit and the main one. In the case
of the bulbous buttercup, _Ranunculus bulbosus_, I have succeeded in
isolating this secondary summit, although not in a separate variety, but
only in a form corresponding to the type of ever-sporting varieties.
[741] Recapitulating the results of this too condensed discussion, we
may state that fluctuations are linear, being limited to an increase and
to a decrease of the characters. These changes are mainly due to
differences in nourishment, either of the whole organism or of its
parts. In the first case, the deviations from the mean are called
individual; they are of great importance for the hereditary characters
of the offspring. In the second case the deviations are far more
universal and far more striking, but of lesser importance. They are
called partial fluctuations.
All these fluctuations comply, in the main, with the law of probability,
and behave as if their causes were influenced only by chance.
[742]
LECTURE XXVI
ASEXUAL MULTIPLICATION OF EXTREMES
Fluctuating variability may be regarded from two different points of
view. The multiformity of a bed of flowers is often a desirable feature,
and all means which widen the range of fluctuation are therefore used to
enhance this feature, and variability affords specimens, which surpass
the average, by yielding a better or larger product.
In the case of fruits and other cultivated forms, it is of course
profitable to propagate from the better specimens only, and if possible
only from the very best. Obviously the best are the extremes of the
whole range of diverging forms, and moreover the extremes on one side of
the group. Almost always the best for practical purposes is that in
which some quality is strengthened. Cases occur however, in which it is
desirable to diminish an injurious peculiarity as far as possible, and
in these instances the opposite extreme is the most profitable one.
These considerations lead us to a discussion [743] of the results of the
choice of extremes, which it may be easily seen is a matter of the
greatest practical importance. This choice is generally designated as
selection, but as with most of the terms in the domain of variability,
the word selection has come to have more than one meaning. Facts have
accumulated enormously since the time of Darwin, a more thorough
knowledge has brought about distinctions, and divisions at a rapidly
increasing rate, with which terminology has not kept pace. Selection
includes all kinds of choice. Darwin distinguished between natural and
artificial selection, but proper subdivisions of these conceptions are
needed.
In the fourth lecture we dealt with this same question, and saw that
selection must, in the first place, make a choice between the elementary
species of the same systematic form. This selection of species or
species-selection was the work of Le Couteur and Patrick Shirreff, and
is now in general use in practice where it has received the name of
variety-testing. This clear and unequivocal term however, can hardly be
included under the head of natural selection. The poetic terminology of
selection by nature has already brought about many difficulties that
should be avoided in the future. On the other hand, the designation of
the process as a natural [744] selection of species complies as closely
as possible with existing terminology, and does not seem liable to any
misunderstanding.
It is a selection between species. Opposed to it is the selection within
the species. Manifestly the first should precede the second, and if this
sequence is not conscientiously followed it will result in confusion.
This is evident when it is considered that fluctuations can only appear
with their pure and normal type in pure strains, and that each admixture
of other units is liable to be shown by the form of the curves. More
over, selection chooses single individuals, and a single plant, if it is
not a hybrid, can scarcely pertain to two different species. The first
choice therefore is apt to make the strain pure.
In contrasting selection between species with that within the species,
of course elementary species are meant, including varieties. The terms
would be of no consequence if only rightly understood. For the sake of
clearness we might designate the last named process with the term of
intra-specific selection, and it is obvious that this term is applicable
both to natural and to artificial selection.
Having previously dealt with species-selection at sufficient length, we
may now confine ourselves to the consideration of the intra-specific
[745] selection process. In practice it is of secondary importance, and
in nature it takes a very subordinate position. For this reason it will
be best to confine further discussions to the experience of the
breeders.
Two different ways are open to make fluctuating variability profitable.
Both consist in the multiplication of the chosen extremes, and this
increase may be attained in a vegetative manner, or by the use of seeds.
Asexual and sexual propagation are different in many respects, and so
they are also in the domain of variability.
In order to obtain a clear comprehension of this difference, it is
necessary to start from the distinction between individual and partial
fluctuations, as given in the last lecture. This distinction may be
discussed more understandingly if the causes of the variability are
taken into consideration. We have dealt with them at some length, and
are now aware that inner conditions only, determine averages, while some
fluctuation around them is allowable, as influenced by external
conditions. These outward influences act throughout life. At the very
first they impress their stamp on the whole organism, and incite a
lasting change in distinct directions. This is the period of the
development of the germ within the seed; it begins with the fusion of
the sexual cells, and each of them may be influenced [746] to a
noticeable degree before this union. This is the period of the
determination of individual variability. As soon as ramifications begin,
the external conditions act separately on every part, influencing some
to a greater and others to a lesser degree. Here we have the beginning
of partial variability. At the outset all parts may be affected in the
same way and in the same measure, but the chances of such an agreement,
of course, rapidly diminish. This is partly due to differences in
exposure, but mainly to alterations of the sensibility of the organs
themselves.
It is difficult to gain a clear conception of the contrast between
individual and partial variability, and neither is it easy to appreciate
their cooperation rightly. Perhaps the best way is to consider their
activity as a gradual narrowing of possibilities. At the outset the
plant may develop its qualities in any measure, nothing being as yet
fixed. Gradually however, the development takes a definite direction,
for better or for worse. Is a direction once taken, then it becomes the
average, around which the remaining possibilities are grouped. The plant
or the organ goes on in this way, until finally it reaches maturity with
one of the thousands of degrees of development, between which at the
beginning it had a free choice.
[747] Putting this discussion in other terms, we find every individual
and every organ in the adult state corresponding with a single ordinate
of the curve. The curve indicates the range of possibilities, the
ordinate shows the choice that has been made. Now it is clear at once
that this choice has not been made suddenly but gradually. Halfway of
the development, the choice is halfway determined, but the other half is
still undefined. The first half is the same for all the organs of the
plant, and is therefore termed individual; the second differs in the
separate members, and consequently is known as partial. Which of the two
halves is the greater and which the lesser, of course depends on the
cases considered.
Finally we may describe a single example, the length of the capsules of
the evening-primrose. This is highly variable, the longest reaching more
than twice the length of the smallest. Many capsules are borne on the
same spike, and they are easily seen to be of unequal size. They vary
according to their position, the size diminishing in the main from the
base upwards, especially on the higher parts. Likewise the fruits of
weaker lateral branches are smaller. Curves are easily made by measuring
a few hundred capsules from corresponding parts of different plants, or
even by limiting the [748] inquiry to a single individual. These curves
give the partial variability, and are found to comply with Quetelet's
law.
Besides this limited study, we may compare the numerous individuals of
one locality or of a large plot of cultivated plants with one another.
In doing so, we are struck with the fact that some plants have large and
others small fruits. We now limit ourselves to the main spike of each
plant, and perhaps to its lower parts, so as to avoid as far as possible
the impression made by the partial fluctuations. The differences remain,
and are sufficient to furnish an easy comparison with the general law.
In order to do this, we take from each plant a definite number of
capsules and measure their average length. In some experiments I took
the twenty lowermost capsules of the main spikes. In this way one
average was obtained for each plant, and combining these into a curve,
it was found that these fluctuations also came under Quetelet's law.
Thus the individual averages, and the fluctuations around each of them,
follow the same rule. The first are a measure for the whole plant, the
second only for its parts. As a general resume we can assert that, as a
rule, a quality is determined in some degree during the earlier stages
of the organism, and that this determination is valid throughout its
[749] life. Afterwards only the minor points remain to be regulated.
This makes it at once clear that the range of individual and partial
variability together must be wider than that of either of them, taken
alone. Partial fluctuations cannot, of course, be excluded. Thus our
comparison is limited to individual and partial variability on one side,
and partial fluctuations alone on the other side.
Intra-specific selection is thus seen to fall under two heads: a
selection between the individuals, and a choice within each of them. The
first affords a wider and the latter a narrower field.
Individual variability, considered as the result of outward influences
operative during extreme youth, can be excluded in a very simple manner.
Obviously it suffices to exclude extreme youth, in other words, to
exclude the use of seeds. Multiplication in a vegetative way, by
grafting and budding, by runners or roots, or by simple division of
rootstocks and bulbs is the way in which to limit variability to the
partial half. This is all we may hope to attain, but experience shows
that it is a very efficient means of limitation. Partial fluctuations
are generally far smaller than individual and partial fluctuations
together.
Individual variability in the vegetable kingdom [750] might be called
seed-variation, as opposed to partial or bud-fluctuation. And perhaps
these terms are more apt to convey a clear conception of the distinction
than any other. The germ within the unripe seed is easily understood to
be far more sensitive to external conditions than a bud.
Multiplication of extremes by seed is thus always counteracted by
individual variability, which at once reopens all, or nearly all, the
initial possibilities. Multiplication by buds is exempt from this danger
and thus leads to a high degree of uniformity. And this uniformity is in
many cases exactly what the breeder endeavors to obtain.
We will treat of this reopening of previous possibilities under the head
of regression in the next lecture. It is not at all absolute, at least
not in one generation. Part of the improvement remains, and favors the
next generation. This part may be estimated approximately as being about
one-third or one-half of the improvement attained. Hence the conclusion
that vegetative multiplication gives rise to varieties which are as a
rule twice or thrice as good as selected varieties of plants propagated
by seeds. Hence, likewise the inference that breeders generally prefer
vegetative multiplication of improved forms, and apply it in all
possible cases. [751] Of course the application is limited, and forage
crops and the greater number of vegetables will always necessarily be
propagated by seed.
Nature ordinarily prefers the sexual way. Asexual multiplications,
although very common with perennial plants, appear not to offer
important material for selection. Hence it follows that in comparing the
work of nature with that of man, the results of selection followed by
vegetative propagation should always be carefully excluded. Our large
bulb-flowers and delicious fruits have nothing in common with natural
products, and do not yield a standard by which to judge nature's work.
It is very difficult for a botanist to give a survey of what practice
has attained by the asexual multiplication of extremes. Nearly all of
the large and more palatable fruits are due to such efforts. Some
flowers and garden-plants afford further instances. By far the greatest
majority of improved asexual varieties, however, are not the result of
pure intra-specific selection. They are due largely to the choice of the
best existing elementary species, and to some extent to crosses between
them, or between distinct systematic species. In practice selection and
hybridization go hand in hand and it is often difficult to ascertain
what part of [752] the result is due to the one, and what to the other
factor.
The scientist, on the contrary, has nothing to do with the industrial
product. His task is the analysis of the methods, in order to reach a
clear appreciation of the influence of all the competing factors. This
study of the working causes leads to a better understanding of the
practical processes, and may become the basis of improvement in methods.
Starting from these considerations, we will now give some illustrative
examples, and for the first, choose one in which hybridization is almost
completely excluded.
Sugar-canes have long been considered to be plants without seed. Their
numerous varieties are propagated only in a vegetative way. The stems
are cut into pieces, each bearing one or two or more nodes with their
buds. An entire variety, though it may be cultivated in large districts
and even in various countries, behaves with respect to variability as a
single individual. Its individual fluctuability has been limited to the
earliest period of its life, when it arose from an unknown seed. The
personal characters that have been stamped on this one seed, partly by
its descent, and partly in the development of its germ during the period
of ripening, have become the indelible characters [753] of the variety,
and only the partial fluctuability, due to the effect of later
influences, can now be studied statistically.
This study has for its main object the production of sugar in the stems,
and the curves, which indicate the percentage of this important
substance in different stems of the same variety, comply with Quetelet's
law. Each variety has its own average, and around this the data of the
majority of the stems are densely crowded, while deviations on both
sides are rare and become the rarer the wider they are. The "Cheribon"
cane is the richest variety cultivated in Java, and has an average of
19% sugar, while it fluctuates between 11% and 28%. "Chunnic" averages
14%, "Black Manilla" 13% and "White Manilla" 10%; their highest and
lowest extremes diverge in the same manner, being for the last named
variety 1% and 15%.
This partial variability is of high practical interest, because on it a
selection may be founded. According to the conceptions described in a
previous lecture, fluctuating variability is the result of those outward
factors that determine the strength of development of the plant or the
organ. The inconstancy of the degree of sensibility, combined with the
ever-varying weather conditions preclude any close proportionality, but
apart from this difficulty there is, in the [754] main, a distinct
relation between organic strength and the development of single
qualities. This correlation has not escaped observation in the case of
the sugar-cane, and it is known that the best grown stocks are generally
the richest in sugar. Now it is evident that the best grown and richest
stems will have the greater chance of transmitting these qualities to
the lateral-buds. This at once gives, a basis for vegetative selection,
upon which it is not necessary to choose a small number of very
excellent stems, but simply to avoid the planting of all those that are
below the average. By this means the yield of the cultures has often
noticeably been enhanced.
As far as experience goes, this sort of selection, however profitable,
does not conduce to the production of improved races. Only temporary
ameliorations are obtained, and the selection must be made in the same
manner every year. Moreover the improvement is very limited and does not
give any promise of further increase. In order to reach this, one has to
recur to the individual fluctuability, and therefore to seed.
Nearly half a century ago, Parris discovered, on the island of Barbados,
that seeds might occasionally be gathered from the canes. These,
however, yielded only grass-like plants of no real value. The same
observation was made [755] shortly afterwards in Java and in other sugar
producing countries. In the year 1885, Soltwedel, the director of one of
the experiment stations for the culture of sugar-cane in Java, conceived
the idea of making use of seedlings for the production of improved
races. This idea is a very practical one, precisely because of the
possibility of vegetative propagation. If individuals would show the
same range as that of partial fluctuability, then the choice of the
extremes would at once bring the average up to the richness of the best
stocks. Once attained, this average would be fixed, without further
efforts.
Unfortunately there is one great drawback. This is the infertility of
the best variety, that of the "Cheribon" cane. It flowers abundantly in
some years, but it has never been known to produce ripe seeds. For this
reason Soltwedel had to start from the second best sort, and chose the
"Hawaii" cane. This variety usually yields about 14% sugar, and
Soltwedel found among his seedlings one that showed 15%. This fact was
quite unexpected at that time, and excited widespread interest in the
new method, and since then it has been applied to numerous varieties,
and many thousands of seedlings have been raised and tested as to their
sugar-production.
[756] From a scientific point of view the results are quite striking.
From the practical standpoint, however, the question is, whether the
"Hawaii" and other fertile varieties are adequate to yield seedlings,
which will surpass the infertile "Cheribon" cane. Now "Hawaii" averages
14% and "Cheribon" 19%, and it is easily understood that a "Hawaii"
seedling with more than 19% can be expected only from very large
sowings. Hundreds of thousands of seedlings must be cultivated, and
their juice tested, before this improvement can be reached. Even then,
it may have no significance for practical purposes. Next to the amount
of sugar comes the resistance to the disease called "Sereh," and the new
race requires to be ameliorated in this important direction, too. Other
qualities must also be considered, and any casual deterioration in other
characters would make all progress illusory. For these reasons much time
is required to attain distinct improvements.
These great difficulties in the way of selecting extremes for vegetative
propagation are of course met with everywhere. They impede the work of
the breeder to such a degree, that but few men are able to surmount
them. Breeding new varieties necessitates the bending of every effort to
this purpose, and a clear conception of [757] the manifold aspects of
this intricate problem. These fall under two heads, the exigencies of
practice, and the physiologic laws of variability. Of course, only the
latter heading comes within the limits of our discussion which includes
two main points. First comes the general law of fluctuation that, though
slight deviations from the average may be found by thousands, or rather
in nearly every individual, larger and therefore important deviations
are very rare. Thousands of seedlings must be examined carefully in
order to find one or two from which it might be profitable to start a
new race. This point is the same for practical and for scientific
investigation. In the second place however, a digression is met with.
The practical man must take into consideration all the varying qualities
of his improved strains. Some of them must be increased and others be
decreased, and their common dependency on external conditions often
makes it very difficult to discover the desired combinations. It is
obvious, however, that the neglect of one quality may make all
improvement of other characters wholly useless. No augmentation of
sugar-percentage, of size and flavor of fruits can counterbalance an
increase in sensitiveness to disease, and so it is with other qualities
also.
[758] Improved races for scientific investigation can be kept free from
infection, and protected against numerous other injuries. In the
experimental garden they may find conditions which cannot be realized
elsewhere. They may show a luxuriant growth, and prove to be excellent
material for research, but have features which, having been overlooked
at the period of selection, would at once condemn them if left to
ordinary conditions, or to the competition of other species.
Considering all these obstacles, it is only natural that breeders should
use every means to reach their goal. Only in very rare instances do they
follow methods analogous to scientific processes, which tend to simplify
the questions as much as possible. As a rule, the practical way is the
combination of as many causes of variability as possible. Now the three
great sources of variability are, as has been pointed out on several
occasions, the original multiformity of the species, fluctuating
variability, and hybridization. Hence, in practical experiments, all
three are combined. Together they yield results of the highest value,
and Burbank's improved fruits and flowers give testimony to the
practical significance of this combination.
From a scientific point of view however, it is [759] ordinarily
difficult, if not impossible, to discern the part which each of the
three great branches of variability has taken in the origination of the
product. A full analysis is rarely possible, and the treatment of one of
the three factors must necessarily remain incomplete.
Notwithstanding these considerations, I will now give some examples in
order to show that fluctuating variability plays a prominent part in
these improvements. Of course it is the third in importance in the
series. First comes the choice of the material from the assemblage of
species, elementary species and varieties. Hybridization comes next in
importance. But even the hybrids of the best parents may be improved,
because they are no less subject to Quetelet's law than any other
strain. Any large number of hybrids of the same ancestry will prove
this, and often the excellency of a hybrid variety depends chiefly, or
at least definitely, on the selection of the best individuals. Being
propagated only in a vegetative way, they retain their original good
qualities through all further culture and multiplication.
As an illustrative example I will take the genus _Canna_. Originally
cultivated for its large and bright foliage only, it has since become a
flowering plant of value. Our garden strains have originated by the
crossing of [760] a number of introduced wild species, among which the
_Canna indica_ is the oldest, now giving its name to the whole group. It
has tall stems and spikes with rather inconspicuous flowers with narrow
petals. It has been crossed with _C. nepalensis_ and _C. warczewiczii_,
and the available historic evidence points to the year 1846 as that of
the first cross. This was made by Annee between the _indica_ and the
_nepalensis_; it took ten years to multiply them to the required degree
for introduction into commerce. These first hybrids had bright foliage
and were tall plants, but their flowers were by no means remarkable.
Once begun, hybridization was widely practiced. About the year 1889
Crozy exhibited at Paris the first beautifully flowering form, which he
named for his wife, "Madame Crozy." Since that time he and many others,
have improved the flowers in the shape and size, as well as in color and
its patterns. In the main, these ameliorations have been due to the
discovery and introduction of new wild species possessing the required
characters. This is illustrated by the following incident. In the year
1892 I visited Mr. Crozy at Lyons. He showed me his nursery and numerous
acquisitions, those of former years as well as those that were quite
new, and which were in the process of rapid [761] multiplication,
previous to being given to the trade. I wondered, and asked, why no pure
white variety was present. His answer was "Because no white species had
been found up to the present time, and there is no other means of
producing white varieties than by crossing the existing forms with a new
white type."
Comparing the varieties produced in successive periods, it is very easy
to appreciate their gradual improvement. On most points this is not
readily put into words, but the size of the petals can be measured, and
the figures may convey at least some idea of the real state of things.
Leaving aside the types with small flowers and cultivated exclusively
for their foliage, the oldest flowers of _Canna_ had petals of 45 mm.
length and 13 mm. breadth. The ordinary types at the time of my visit
had reached 61 by 21 mm., and the "Madame Crozy" showed 66 by 30 mm. It
had however, already been surpassed by a few commercial varieties, which
had the same length but a breadth of 35 mm. And the latest production,
which required some years of propagation before being put on the market,
measured 83 by 43 mm. Thus in the lapse of some thirty years the length
had been doubled and the breadth tripled, giving flowers with broad
corollas and with petals [762] joined all around, resembling the best
types of lilies and _Amaryllis_.
Striking as this result unquestionably is, it remains doubtful as to
what part of it is due to the discovery and introduction of new large
flowered species, and what to the selection of the extremes of
fluctuating variability. As far as I have been able to ascertain
however, and according to the evidence given to me by Mr. Crozy,
selection has had the largest part in regard to the size, while the
color-patterns are introduced qualities.
The scientific analysis of other intricate examples is still more
difficult. To the practical breeder they often seem very simple, but the
student of heredity, who wishes to discern the different factors, is
often quite puzzled by this apparent simplicity. So it is in the case of
the double lilacs, a large number of varieties of which have recently
been originated and introduced into commerce by Lemoine of Nancy. In the
main they owe their origin to the crossing and recrossing of a single
plant of the old double variety with the numerous existing
single-flowered sorts.
This double variety seems to be as old as the culture of the lilacs. It
was already known to Munting, who described it in the year 1671. Two
centuries afterwards, in 1870, a new description [763] was given by
Morren, and though more than one varietal name is recorded in his paper,
it appears from the facts given that even at that time only one variety
existed. It was commonly called _Syringa vulgaris azurea plena_, and
seems to have been very rare and without real ornamental value.
Lemoine, however, conceived the desirability of a combination of the
doubling with the bright colors and large flower-racemes of other
lilacs, and performed a series of crosses. The "_azurea plena_" has no
stamens, and therefore must be used in all crosses as the pistil-parent;
its ovary is narrowly inclosed in the tube of the flower, and difficult
to fertilize. On the other hand, new crosses could be made every year,
and the total number of hybrids with different pollen-parents was
rapidly increased. After five years the hybrids began to flower and
could be used for new crosses, yielding a series of compound hybrids,
which however, were not kept separate from the products of the first
crosses.
Gradually the number of the flowering specimens increased, and the
character of doubling was observed to be variable to a high degree.
Sometimes only one supernumerary petal was produced, sometimes a whole
new typical corolla was extruded from within the first. In the same
[764] way the color and the number of the flowers on each raceme were
seen to vary. Thousands of hybrids were produced, and only those which
exhibited real advantages were selected for trade. These were multiplied
by grafting, and each variety at present consists only of the buds of
one original individual and their products. No constancy from seed is
assumed, many varieties are even quite sterile.
Of course, no description was given of the rejected forms. It is only
stated that many of them bore either single or poorly filled flowers, or
were objectionable in some other way. The range of variability, from
which the choices were made, is obscure and only the fact of the
selection is prominent. What part is due to the combination of the
parental features and what to the individual fluctuation of the hybrid
itself cannot be ascertained.
So it is in numerous other instances. The dahlias have been derived from
three or more original species, and been subjected to cultivation and
hybridization in an ever-increasing scale for a century. The best
varieties are only propagated in the vegetative way, by the roots and
buds, or by grafting and cutting. Each of them is, with regard to its
hereditary qualities, only one individual, and the individual characters
were selected at the same time with the [765] varietal and hybrid
characters. Most of them are very inconstant from seed and as a rule,
only mixtures are offered for sale in seed-lists. Which of their
ornamental features are due to fluctuating deviation from an average is
of course unknown. _Amaryllis_ and _Gladiolus_ are surrounded with the
same scientific uncertainties. Eight or ten, or even more, species have
been combined into one large and multiform strain, each bringing its
peculiar qualities into the mixed mass. Every hybrid variety is one
individual, being propagated by bulbs only. Colors and color-patterns,
shape of petals and other marks, have been derived from the wild
ancestors, but the large size of many of the best varieties is probably
due to the selection of the extremes of fluctuating variability. So it
is with the begonias of our gardens, which are also composite hybrids,
but are usually sown on a very large scale. Flowers of 15 cm. diameter
are very showy, but there can be no doubt about the manner in which they
are produced, as the wild species fall far short of this size.
Among vegetables the potatoes afford another instance. Originally quite
a number of good species were in culture, most of them having small
tubers. Our present varieties are due to hybridization and selection,
each of them being propagated only in the vegetative way.
[766] Selection is founded upon different qualities, according to the
use to be made of the new sort. Potatoes for the factory have even been
selected for their amount of starch, and in this case at least,
fluctuating variability has played a very important part in the
improvement of the race.
Vegetative propagation has the great advantage of exempting the
varieties from regression to mediocrity, which always follows
multiplication by seeds. It affords the possibility of keeping the
extremes constant, and this is not its only advantage. Another, likewise
highly interesting, side of the question is the uniformity of the whole
strain. This is especially important in the case of fruits, though
ordinarily it is regarded as a matter of course, but there are some
exceptions which give proof of the real importance of the usual
condition. For example, the walnut-tree. Thousands of acres of
walnut-orchards consist of seedling trees grown from nuts of unknown
parentage. The result is a great diversity in the types of trees and in
the size and shape of the nuts, and this diversity is an obvious
disadvantage to the industry. The cause lies in the enormous
difficulties attached to grafting or budding of these trees, which make
this method very expensive and to a high degree uncertain and
unsatisfactory.
[767] After this hasty survey of the more reliable facts of the practice
of an asexual multiplication of the extremes of fluctuating variability,
we may now return to the previously mentioned theoretical
considerations. These are concerned with an estimation of the chances of
the occurrence of deviations, large enough to exhibit commercial value.
This chance may be calculated on the basis of Quetelet's law, whenever
the agreement of the fluctuation of the quality under consideration has
been empirically determined. In the discussion of the methods of
comparing two curves, we have pointed to the quartiles as the decisive
points, and to the necessity of drawing the curves so that these points
are made to overlie one another, on each side of the average. If now we
calculate the binomium of Newton for different values of the exponent,
the sum of the coefficients is doubled for each higher unit of the
exponent, and at the same time the extreme limit of the curve is
extended one step farther. Hence it is possible to calculate a relation
between the value of the extreme and the number of cases required. It
would take us too long to give this calculation in detail, but it is
easily seen that for each succeeding step the number of individuals must
be doubled, though the length of the steps, or the amount of increase of
the quality [768] remains the same. The result is that many thousands of
seedlings are required to go beyond the ordinary range of variations,
and that every further improvement requires the doubling of the whole
culture. If ten thousand do not give a profitable deviation, the next
step requires twenty thousand, the following forty thousand, and so on.
And all this work would be necessary for the improvement of a single
quality, while practice requires the examination and amelioration of
nearly all the variable characters of the strain.
Hence the rule that great results can only be obtained by the use of
large numbers, but it is of no avail to state this conclusion from a
scientific point of view. Scientific experimenters will rarely be able
to sacrifice fifty thousand plants to a single selection. The problem is
to introduce the principle into practice and to prove its direct
usefulness and reliability. It is to Luther Burbank that we owe this
great achievement. His principles are in full harmony with the teachings
of science. His methods are hybridization and selection in the broadest
sense and on the largest scale. One very illustrative example of his
methods must suffice to convey an idea of the work necessary to produce
a new race of superlative excellency. Forty thousand blackberry and
raspberry [769] hybrids were produced and grown until the fruit matured.
Then from the whole lot a single variety was chosen as the best. It is
now known under the name of "Paradox." All others were uprooted with
their crop of ripening berries, heaped up into a pile twelve feet wide,
fourteen feet high and twenty-two feet long, and burned. Nothing
remained of that expensive and lengthy experiment, except the one
parent-plant of the new variety. Similar selections and similar amount
of work have produced the famous plums, the brambles and the
blackberries, the Shasta daisy, the peach almond, the improved
blueberries, the hybrid lilies, and the many other valuable fruits and
garden-flowers that have made the fame of Burbank and the glory of
horticultural California.
[770]
LECTURE XXVII
INCONSTANCY OF IMPROVED RACES
The greater advantages of the asexual multiplication of extremes are of
course restricted to perennial and woody plants. Annual and biennial
species cannot as a rule, be propagated in this way, and even with some
perennials horticulturists prefer the sale of seeds to that of roots and
bulbs. In all these cases it is clear that the exclusion of the
individual variability, which was shown to be an important point in the
last lecture, must be sacrificed.
Seed-propagation is subject to individual as well as to fluctuating
variability. The first could perhaps be designated by another term,
embryonic variability, since it indicates the fluctuations occurring
during the period of development of the germ. This period begins with
the fusion of the male and female elements and is largely dependent upon
the vigor of these cells at the moment, and on the varying qualities
they may have acquired. It comprises in the main the time of the
ripening of the seed, and [771] might perhaps best be considered to end
with the beginning of the resting stage of the ripe seed. Hence it is
clear that the variability of seed-propagated annual races has a wider
range than that of perennials, shrubs and trees. At present it is
difficult to discern exactly the part each of these two main factors
plays in the process. Many indications are found however, that make it
probable that embryonic variability is wider, and perhaps of far greater
importance than the subsequent partial fluctuations. The high degree of
similarity between the single specimens of a vegetative variety, and the
large amount of variability in seed-races strongly supports this view.
The propagation and multiplication of the extremes of fluctuating
variability by means of seeds requires a close consideration of the
relation between seedling and parent. The easiest way to get a clear
conception of this relation is to make use of the ideas concerning the
dependency of variability upon nourishment. Assuming these to be correct
in the main, and leaving aside all minor questions, we may conclude that
the chosen extreme individual is one of the best nourished and
intrinsically most vigorous of the whole culture. On account of these
very qualities it is capable of nourishing all of its organs better and
also its seeds. In other words, the seeds [772] of the extreme
individuals have exceptional chances of becoming better nourished than
the average of the seeds of the race. Applying the same rule to them, it
is easily understood that they will vary, by reason of this better
nourishment, in a direction corresponding to that of their parent.
This discussion gives a very simple explanation of the acknowledged fact
that the seeds of the extremes are in the main the best for the
propagation of the race. It does not include however, all the causes for
this preferment. Some are of older date and due to previous influences.
A second point in our discussion is the appreciation of the fact that a
single individual may be chosen to gather the seed from, and that these
seeds, and the young plants they yield, are as a rule, numerous. Hence
it follows that we are to compare their average and their extremes with
the qualities of the parents. Both are of practical as well as of
theoretical interest. The average of the progeny is to be considered as
the chief result of the selection in the previous generation, while the
extremes, at least those which depart in the same direction, are
obviously the means of further improvement of the race.
Thus our discussion should be divided into [773] two heads. One of these
comprises the relation of the average of the progeny to the exceptional
qualities of the chosen parent, and the other the relation of
exceptional offspring to the exceptional parents.
Let us consider the averages first. Are they to be expected to be equal
to the unique quality of the parent, or perhaps to be the same as the
average of the whole unselected race? Neither of these cases occur.
Experience is clear and definite on this important point. Vilmorin, when
making the first selections to improve the amount of sugar in beets, was
struck with the fact that the average of the progeny lies between that
of the original strain and the quality of the chosen parent. He
expressed his observation by stating that the progeny are grouped around
and diverge in all directions from some point, placed on the line which
unites their parent with the type from which it sprang. All breeders
agree on this point, and in scientific experiments it has often been
confirmed. We shall take up some illustrative examples presently, but in
order to make them clear, it is necessary to give a closer consideration
to the results of Vilmorin.
From his experience it follows that the average of the progeny is higher
than that of the race at large, but lower than the chosen parent. [774]
In other words, there is a progression and a regression. A progression
in relation to the whole race, and a regression in comparison with the
parent. The significance of this becomes clear at once, if we recall the
constancy of the variety which could be obtained from the selected
extreme in the case of vegetative multiplication. The progression is
what the breeder wants, the regression what he detests. Regression is
the permanency of part of the mediocrity which the selection was invoked
to overcome. Manifestly it is of the highest interest that the
progression should be as large, and the regression as small as possible.
In order to attain this goal the first question is to know the exact
measure of progression and regression as they are exhibiting themselves
in the given cases, and the second is to inquire into the influences, on
which this proportion may be incumbent.
At present our notions concerning the first point are still very limited
and those concerning the second extremely vague. Statistical inquiries
have led to some definite ideas about the importance of regression, and
these furnish a basis for experimental researches concerning the causes
of the phenomenon. Very advantageous material for the study of
progression and regression in the realm of fluctuating variability is
afforded by the [775] ears of corn or maize. The kernels are arranged in
longitudinal rows, and these rows are observed to occur in varying, but
always even, numbers. This latter circumstance is due to the fact that
each two neighboring rows contain the lateral branches of a single row
of spikelets, the ages of which however, are included in the fleshy body
of the ear. The variation of the number of the rows is easily seen to
comply with Quetelet's law, and often 30 or 40 ears suffice to give a
trustworthy curve. Fritz Muller made some experiments upon the
inheritance of the number of the rows, in Brazil. He chose a race which
averaged 12 rows, selected ears with 14, 16 and 18 rows, etc., and sowed
their kernels separately. In each of-these cultures he counted the rows
of the seeds on the ears of all the plants when ripe, and calculated
their average. This average, of course, does not necessarily correspond
to a whole number, and fractions should not be neglected.
According to Vilmorin's rule he always found some progression of the
average and some regression. Both were the larger, the more the
parent-ear differed from the general average, but the proportion between
both remained the same, and seems independent of the amount of the
deviation. Putting the deviation at 5, the progression calculated from
his figures is [776] 2 and the regression 3. In other words the average
of the progeny has gained over the average of the original variety
slightly more than one-third, and slightly less than one-half of the
parental deviation. I have repeated this experiment of Fritz Miller's
and obtained nearly the same regression of three-fifths, though working
with another variety, and under widely different climatic conditions.
The figures of Fritz Muller were, as given below, in one experiment. In
the last column I put the improvement calculated for a proportion of
two-fifths above the initial average of 12.
Rows on Average of rows 12 + 2/5 of
parent ears of progeny Difference
14 12.6 12.8
16 14.1 13.6
18 15.2 14.4
20 15.8 15.2
22 16.1 16.0
Galton, in his work on natural inheritance, describes an experiment with
the seeds of the sweet pea or _Lathyrus odoratus_. He determined the
average size in a lot of purchased seeds, and selected groups of seeds
of different, but for each group constant, sizes. These were sown, and
the average of the seeds was determined anew in the subsequent harvest
they yielded. These figures agreed with the rule of Vilmorin and were
calculated in the manner [777] given for the test of the corn. The
progression and regression were found to be proportionate to the amount
of the deviation. The progression of the average was one-third, and the
regression in consequence two-thirds of the total deviation. The
amelioration is thus seen to be nearly, though not exactly, the same as
in the previous case.
From the evidence of the other corresponding experiments, and from
various statistical inquiries it seems that the value of the progression
is nearly the same in most cases, irrespective of the species used and
the quality considered. It may be said to be from one-third to one-half
of the parental deviation, and in this form the statement is obviously
of wide and easy applicability.
Our figures also demonstrate the great preeminence of vegetative
varieties above the improved strains multiplied by seeds. They have a
definite relation. Asexually multiplied strains may be said to be
generally two times or even three times superior to the common
offspring. This is a difference of great practical importance, and
should never be lost sight of in theoretical considerations of the
productive capacity of selection. Multiplication by seed however, has
one great advantage over the asexual method; it may be repeated. The
[778] selection is not limited to a single choice, but may be applied in
two or more succeeding generations. Obviously such a repetition affords
a better chance of increasing the progression of the average and of
ameliorating the race to a greater degree than would be possible by a
single choice. This principle of repeated selection is at present the
prominent feature of race improvement. Next to variety-testing and
hybridizing it is the great source of the steady progression of
agricultural crops. From a practical standpoint the method is clear and
as perfect as might be expected, but this is not the side of the problem
with which we are concerned here. The theoretical analysis and
explanation of the results obtained, however, is subject to much doubt,
and to a great divergence of conceptions. So it is also with the
application of the practical processes to those occurring in nature.
Some assume that here repeated selection is only of subordinate
importance, while others declare that the whole process of evolution is
due to this agency. This very important point however, will be reserved
for the next lecture, and only the facts available at present will be
considered here.
As a first example we may take the ray-florets of the composites. On a
former occasion we have dealt with their fluctuation in number and [779]
found that it is highly variable and complies in the main with
Quetelet's law. _Madia elegans_, a garden species, has on the average 21
rays on each head, fluctuating between 16 and 25 or more. I saved the
seeds of a plant with only 17 rays on the terminal head, and got from
them a culture which averaged 19 rays, which is the mean between 21 and
17. In this second generation I observed the extremes to be 22 and 12,
and selected a plant with 13 rays as the parent for a continuation of
the experiment. The plants, which I got from its seeds, averaged 18 and
showed 22 and 13 as extremes. The total progression of the average was
thus, in two generations, from 21 to 18, and the total regression from
13 to 18, and the proportion is thus seen to diminish by the repetition
rather than to increase.
This experiment, however, is of course too imperfect upon which to found
general conclusions. It only proves the important fact that the improved
average of the second generation is not the starting-point for the
further improvement. But the second generation allows a choice of an
extreme, which diverges noticeably more from the mean than any
individual of the first culture, and thereby gives a larger amount of
absolute progression, even if the proportion between progression and
regression remains [780] the same. The repetition is only an easy method
of getting more widely deviating extremes; whether it has, besides this,
another effect, remains doubtful. In order to be able to decide this
question, it is necessary to repeat the selection during a series of
generations. In this way the individual faults may be removed as far as
possible. I chose an experiment of Fritz Muller, relating to the number
of rows of grains on the ears exactly as in the case above referred to,
and which I have repeated in my experimental garden at Amsterdam.
I started from a variety known to fructify fairly regularly in our
climate, and exhibiting in the mean 12-14 rows, but varying between 8
and 20 as exceptional cases. I chose an ear with 16 rows and sowed its
seeds in 1887. A number of plants were obtained, from each of which, one
ear was chosen in order to count its rows. An average of 15 rows was
found with variations complying with Quetelet's law. One ear reached 22
rows, but had not been fertilized, some others had 20 rows, and the best
of these was chosen for the continuation of the experiment. I repeated
the sowing during 6 subsequent generations in the same way, choosing
each time the most beautiful ear from among those with the greatest
number of rows. Unfortunately with the increase of the number the [781]
size of the grains decreases, the total amount of nourishment available
for all of them remaining about the same. Thus the kernels and
consequently the new plants became smaller and weaker, and the chance of
fertilization was diminished in the ears with the highest number of
rows. Consequently the choice was limited, and after having twice chosen
a spike with 20 and once one with 24 rows, I finally preferred those
with the intermediate number of 22.
This repeated choice has brought the average of my race up from 13 to
20, and thus to the extreme limit of the original variety. Seven years
were required to attain this result, or on an average the progression
was one row in a year. This augmentation was accompanied by an
accompanying movement of the whole group in the same direction. The
extreme on the side of the small numbers came up from 8 to 12 rows, and
cobs with 8 or 10 rows did not appear in my race later than the third
generation. On the other side the extreme reached 28, a figure never
reached by the original variety as cultivated with us, and ears with 24
and 26 rows have been seen during the four last generations in
increasing numbers.
This slow and gradual amelioration was partly due to the mode of
pollination of the corn. [782] The pollen falls from the male spikes on
the ears of the same plant, but also is easily blown on surrounding
spikes. In order to get the required amount of seed it is necessary in
our climate to encroach as little as possible upon free pollination,
aiding the self-pollination, but taking no precautions against
intercrossing. It is assumed that the choice of the best ears indicates
the plants which have had the best pollen-parents as well as the best
pistil parents, and that selection here, as in other cases, corrects the
faults of free intercrossing. But it is granted that this correction is
only a slow one, and accounts in a great degree for the slowness of the
progression. Under better climatic conditions and with a more entire
isolation of the individuals, it seems very probable that the same
result could have been reached in fewer generations.
However this may be, the fact is that by repeated selection the strain
can be ameliorated to a greater extent than by a single choice. This
result completely agrees with the general experience of breeders and the
example given is only an instance of a universal rule. It has the
advantage of being capable of being recorded in a numerical way, and of
allowing a detailed and definite description of all the succeeding
generations. The entire harvest of all [783] of them has been counted
and the figures combined into curves, which at once show the whole
course of the pedigree-experiment. These curves have in the main taken
the same shape, and have only gradually been moved in the chosen
direction.
Three points are now to be considered in connection with this
experiment. The first is the size of the cultures required for the
resulting amelioration. In other words, would it have been possible to
attain an average of 20 rows in a single experiment? This is a matter of
calculation, and the calculation must be based upon the experience
related above, that the progression in the case of maize is equal to
two-fifths of the parental deviation. A cob with 20 rows means a
deviation of 7 from the average of 13, the incipient value of my race.
To reach such an average at once, an ear would be required with 7 x 5/2
= 17-1/2 rows above the average, or an ear with 30-32 rows. These never
occur, but the rule given in a previous lecture gives a method of
calculating the probability of their occurrence, or in other words, the
number of ears required to give a chance of finding such an ear. It
would take too long to give this calculation here, but I find that
approximately 12,000 ears would be required to give one with 28 rows,
which was the highest number attained in [784] my experiment, while
100,000 ears would afford a chance of one with 32 rows*. Had I been able
to secure and inspect this number of ears, perhaps I would have needed
only a year to get an average of 20 rows. This however, not being the
case, I have worked for seven years, but on the other hand have
cultivated all in all only about one thousand individuals for the entire
experiment.
Obviously this reduction of the size of the experiment is of importance.
One hundred thousand ears of corn could of course, be secured directly
from trade or from some industrial culture, but corn is cultivated only
to a small extent in Holland, and in most cases the requisite number of
individuals would be larger than that afforded by any single plantation.
Repeated selection is thereby seen to be the means of reducing the size
of the required cultures to possible measures, not only in the
experimental-garden, but also for industrial purposes. A selection from
among 60,000-100,000 individuals may be within reach of Burbank, but of
few others. As a rule they prefer a longer time with a smaller lot of
plants. This
* On about 200 ears the variability ranges from 8-22 rows, and
this leads approximately to one row more by each doubling of
the numbers of instances. One ear with 22 rows in 200 would
thus lead to the expectation of one ear with 32 rows in
100,000 ears.
[785] is exactly what is gained by repeated selections. To my mind this
reduction of the size of the cultures is probably the sole effect of the
repetition. But experience is lacking on this point, and exact
comparisons should be made whenever possible, between the descendants of
a unique but extreme choice, and a repeated but smaller selection. The
effect of the repetition on the nourishment of the chosen
representatives should be studied, for it is clear that a plant with 22
rows, the parents and grandparents of which had the same number,
indicates a better condition of internal qualities than one with the
same number of rows, produced accidentally from the common race. In this
way it may perhaps be possible to explain, why in my experiment an ear
with 22 rows gave an average offspring with 20, while the calculation,
founded on the regression alone would require a parental ear with 32
rows.
However, as already stated, this discussion is only intended to convey
some general idea as to the reduction of the cultures by means of
repeated selections, as the material at hand is wholly inadequate for
any closer calculation. This important point of the reduction may be
illustrated in still another manner.
The sowing of very large numbers is only required because it is
impossible to tell from the [786] inspection of the seeds which of them
will yield the desired individual. But what is impossible in the
inspection of the seeds may be feasible, at least in important measure,
in the inspection of the plants which bear the seeds. Whenever such an
inspection demonstrates differences, in manifest connection with the
quality under consideration, any one will readily grant that it would be
useless to sow the seeds of the worst plants, and that even the whole
average might be thrown over, if it were only possible to point out a
number of the best. But it is clear that by this inspection of the
parent plants the principle of repeated selection is introduced for two
succeeding generations, and that its application to a larger series of
generations is only a question of secondary importance.
Summing up our discussion of this first point we may assert that
repeated selection is only selection on a small and practical scale,
while a single choice would require numbers of individuals higher than
are ordinarily available.
A second discussion in connection with our pedigree-culture of corn is
the question whether the amelioration obtained was of a durable nature,
or only temporary. In other words, whether the progeny of the race would
remain constant, if cultivated after cessation of the selection. In
order to ascertain this, [787] I continued the culture during several
generations, choosing ears with less than the average number of rows.
The excellence of the race at once disappeared, and the ordinary average
of the variety from which I had started seven years before, returned
within two or three seasons. This shows that the attained improvement is
neither fixed nor assured and is dependent on continued selection. This
result only confirms the universal experience of breeders, which teaches
the general dependency of improved races on continued selection. Here a
striking contrast with elementary species or true varieties is obvious.
The strains which nature affords are true to their type; their average
condition remains the same during all the succeeding generations, and
even if it should be slightly altered by changes in the external
conditions, it returns to the type, as soon as these changes come to an
end. It is a real average, being the sum of the contribution of all the
members of the strain. Improved races have only an apparent average,
which is in fact biased by the exclusion of whole groups of individuals.
If left to themselves, their appearance changes, and the real average
soon returns. This is the common experience of breeders.
A third point is to be discussed in connection [788] with the detailed
pedigree-cultures. It is the question as to what might be expected from
a continuation of improvement selection. Would it be possible to obtain
any imaginable deviation from the original type, and to reach
independency from further selection? This point has not until now
attracted any practical interest, and from a practical point of view and
within the limits of ordinary cultures, it seems impossible to obtain a
positive answer. But in the theoretical discussion of the problems of
descent it has become of the highest importance, and therefore requires
a separate treatment, which will be reserved for the next lecture.
Here we come upon another equally difficult problem. It relates to the
proportion of embryonic or individual fluctuation, to partial variation
as involved in the process of selection. Probably all qualities which
may be subjected to selection vary according to both principles, the
embryonic decision giving only a more definite average, around which the
parts of the individual are still allowed to oscillate. It is so with
the corn, and whenever two or more ears are ripening or even only
flowering on the same plant, differences of a partial nature may be seen
in the number of their rows. These fluctuations are only small however,
ordinarily not exceeding two and rarely four [789] rows. Choosing always
the principal ear, the figures may be taken to indicate the degree of
personal deviation from the average of the race. But whenever we make a
mistake, and perchance sow from an ear, the deviation of which was
largely due to partial variation, the regression should be expected to
become considerably larger. Hence it must be conceded that exact
calculations of the phenomena of inheritance are subject to much
uncertainty, resulting from our very imperfect knowledge concerning the
real proportion of the contributing factors, and the difficulty of
ascertaining their influence in any given case. Here also we encounter
more doubts than real facts, and much remains to be done before exact
calculations may become of real scientific value.
Returning to the question of the effects of selection in the long run,
two essentially different cases are to be considered. Extremes may be
selected from among the variants of ordinary fluctuating variability, or
from ever-sporting varieties. These last we have shown to be double
races. Their peculiar and wide range of variability is due to the
substitution of two characters, which exclude one another, or if
combined, are diminished in various degrees. Striped flowers and stocks,
"five-leaved" clover, pistilloid opium-poppies and numerous other [790]
monstrosities have been dealt with as instances of such ever-sporting
varieties.
Now the question may be put, what would be the effect of selection if in
long series of years one of the two characters of such a double race
were preferred continuously, to the complete exclusion of the other.
Would the race become changed thereby? Could it be affected to such a
degree as to gradually lose the inactive quality, and cease to be a
double race?
Here manifestly we have a means by which to determine what selection is
able to accomplish. Physiologic experiments may be said to be too short
to give any definite evidence. But cases may be cited where nature has
selected during long centuries and with absolute constancy in her
choice. Moreover unconscious selections by man have often worked in an
analogous manner, and many cultivated plants may be put to the test
concerning the evidence they might give on this point. Stating
beforehand the result of this inquiry, we may assert that long-continued
selection has absolutely no appreciable effect. Of course I do not deny
the splendid results of selection during the first few years, nor the
necessity of continued selection to keep the improved races to the
height of their ameliorated qualities. I only wish to state that the
work [791] of selection here finds its limit and that centuries and
perhaps geologic periods of continued effort in the same direction are
not capable of adding anything more to the initial effect. Some
illustrative examples may suffice to prove the validity of this
assertion. Every botanist who has studied the agricultural practice of
plant-breeding, or the causes of the geographic distribution of plants,
will easily recall to his mind numerous similar cases. Perhaps the most
striking instance is afforded by cultivated biennial plants. The most
important of them are forage-beets and sugar-beets. They are, of course,
cultivated only as biennials, but some annual specimens may be seen each
year and in nearly every field. They arise from the same seed as the
normal individuals, and their number is obviously dependent on external
conditions, and especially on the time of sowing. Ordinary cultures
often show as much as 1% of these useless plants, but the exigencies of
time and available labor often compel the cultivator to have a large
part of his fields sown before spring. In central Europe, where the
climate is unfavorable at this season, the beets respond by the
production of far larger proportions of annual specimens, their number
coming often up to 20% or more, thus constituting noticeable losses in
the product [792] of the whole field. Rimpau, who has made a thorough
study of this evil and has shown its dependency on various external
conditions, has also tried to find methods of selection with the aim of
overcoming it, or at least of reducing it to uninjurious proportions.
But in these efforts he has reached no practical result. The annuals are
simply inexterminable.
Coming to the alternative side of the problem it is clear that annuals
have always been excluded in the selection. Their seeds cannot be mixed
with the good harvest, not even accidentally, since they have ripened in
a previous year. In order to bear seeds in the second year beets must be
taken from the field, and kept free from frost through the winter. The
following spring they are planted out, and it is obvious that even the
most careless farmer is not liable to mix them with annual specimens.
Hence we may conclude that a strict and unexcelled process of selection
has been applied to the destruction of this tendency, not only for
sugar-beets, since Vilmorin's time, when selection had become a well
understood process, but also for forage-beets since the beginning of
beet culture. Although unconscious, the selection of biennials must have
been uninterrupted and strict throughout many centuries.
It has had no effect at all. Annuals are seen [793] to return every
year. They are ineradicable. Every individual is in the possession of
this latent quality and liable to convert it into activity as soon as
the circumstances provoke its appearance, as proved by the increase of
annuals in the early sowings. Hence the conclusion that selection in the
long run is not adequate to deliver plants from injurious qualities.
Other proofs could be given by other biennials, and among them the stray
annual plants of common carrots are perhaps the most notorious. In my
own cultures of evening-primroses I have preferred the annuals and
excluded the biennials, but without being able to produce a pure annual
race. As soon as circumstances are favorable, the biennials return in
large numbers. Cereals give analogous proofs. Summer and winter
varieties have been cultivated separately for centuries, but in trials
it is often easy to convert the one into the other. No real and definite
isolation has resulted from the effect of the long continued unconscious
selection.
Striped flowers, striped fruits, and especially striped radishes afford
further examples. It would be quite superfluous to dwell upon them.
Selection always tends to exclude the monochromatic specimens, but does
not prevent their return in every generation. Numerous [794] rare
monstrosities are in the same category, especially when they are of so
rare occurrence as not to give any noticeable contribution to the
seed-production, or even if they render their bearers incapable of
reproduction. In such cases the selection of normal plants is very
severe or even absolute, but the anomalies are by no means exterminated.
Any favorable circumstances, or experimental selection in their behalf
shows them to be still capable of full development. Numerous cases of
such subordinate hereditary characters constitute the greater part of
the science of vegetable teratology.
If it should be objected that all these cases cover too short a time to
be decisive, or at least fail in giving evidence relative to former
times, alpine plants afford a proof which one can hardly expect to be
surpassed. During the whole present geologic epoch they have been
subjected to the never failing selection of their climate and other
external conditions. They exhibit a full and striking adaptation to
these conditions, but also possess the latent capacity for assuming
lowland characters as soon as they are transported into such
environment. Obviously this capacity never becomes active on the
mountains, and is always counteracted by selection. This agency is
evidently without any effect, for as we have seen when dealing [795]
with the experiments of Nageli, Bonnier and others, each single
individual may change its habits and its aspect in response to
transplantation. The climate has an exceedingly great influence on each
individual, but the continuance of this influence is without permanent
result.
So much concerning ever-sporting varieties and double adaptations. We
now come to the effects of a continuous selection of simple characters.
Here the sugar-beets stand preeminent. Since Vilmorin's time they have
been selected according to the amount of sugar in their roots, and the
result has been the most striking that has ever been attained, if
considered from the standpoint of practice. But if critically examined,
with no other aim than a scientific appreciation of the improvement in
comparison with other processes of selection, the support of the
evidence for the theory of accumulative influence proves to be very
small.
The amount of sugar is expressed by percentage-figures. These however,
are dependent on various causes, besides the real quantity of sugar
produced. One of these causes is the quantity of watery fluid in the
tissues, and this in its turn is dependent on the culture in dryer or
moister soil, and on the amount of moisture in the air, and the same
variety of sugar-beets [796] yields higher percentage-figures in a dry
region than in a wet one. This is seen when comparing, for instance, the
results of the analyses from the sandy provinces of Holland with those
from the clay-meadows, and it is very well known that Californian beets
average as high as 26% or more, while the best European beets remain at
about 20%. As far as I have been able to ascertain, these figures
however, are not indicative of any difference of race, but simply direct
responses to the conditions of climate and of soil.
Apart from these considerations the improvement reached in half a
century or in about twenty to thirty generations is not suggestive of
anything absolute. Everything is fluctuating now, even as it was at the
outset, and equally dependent on continual care. Vilmorin has given some
figures for the beets of the first generations from which he started his
race. He quotes 14% as a recommendable amount, and 7 and 21 as the
extreme instances of his analyses. However incorrect these figures may
be, they coincide to a striking degree with the present condition of the
best European races. Of course minor values are excluded each year by
the selection, and in consequence the average value has increased. For
the year 1874 we find a standard of 10-14% considered as normal, [797]
bad years giving 10%, good years from 12% to 14% in the average. Extreme
instances exceeded 17%. From that time the practice of the polarization
of the juice for the estimate of the sugar has rapidly spread throughout
Europe, and a definite increase of the average value soon resulted. This
however, often does not exceed 14%, and beets selected in the field for
the purpose of polarization come up to an average of 15 to 16%, varying
downward to less than 10% and upward to 20 and 21%. In the main the
figures are the same as those of Vilmorin, the range of variability has
not been reduced, and higher extremes are not reached. An average
increase of 1% is of great practical importance, and nothing can excel
the industry and care displayed in the improvement of the beet-races.
Notwithstanding this a lasting influence has not been exercised; the
methods of selection have been improved, and the number of polarized
beets has been brought up to some hundreds of thousands in single
factories, but the improvement is still as dependent upon continuous
selection as it was half a century ago.
The process is practically very successful, but the support afforded by
it to the selection theory vanishes on critical examination.
[798]
LECTURE XXVIII
ARTIFICIAL AND NATURAL SELECTION
The comparison of artificial and natural selection has furnished
material support for the theory of descent, and in turn been the object
of constant criticism since the time of Darwin. The criticisms, in
greater part, have arisen chiefly from an imperfect knowledge of both
processes. By the aid of distinctions recently made possible, the
contrast between elementary species and improved races has become much
more vivid, and promises to yield better results on which to base
comparisons of artificial and natural selection.
Elementary species, as we have seen in earlier lectures, occur in wild
and in cultivated plants. In older genera and systematic species they
are often present in small numbers only, but many of the more recent
wild types and also many of the cultivated forms are very rich in this
respect. In agriculture the choice of the most adequate elementary forms
for any special purpose is acknowledged [799] as the first step in the
way of selection, and is designated by the name of variety-testing,
applying the term variety to all the subdivisions of systematic species
indiscriminately. In natural processes it bears the title of survival of
species. The fact that recent types show large numbers, and in some
instances even hundreds of minor constant forms, while the older genera
are considerably reduced in this respect, is commonly explained by the
assumption of extinction of species on a correspondingly large scale.
This extinction is considered to affect the unfit in a higher measure
than the fit. Consequently the former vanish, often without leaving any
trace of their existence, and only those that prove to be sufficiently
adapted to the surrounding external conditions, resist and survive.
This selection exhibits far-reaching analogies between the artificial
and the natural processes, and is in both cases of the very highest
importance. In nature the dying out of unfit mutations is the result of
the great struggle for life. In a previous lecture we have compared its
agency with that of a sieve. All elements which are too small or too
weak fall through, and only those are preserved which resist the sifting
process. Reduced in number they thrive and multiply and are thus enabled
to [800] strike out new mutative changes. These are again submitted to
the sifting tests, and the frequent repetition of this process is
considered to give a good explanation of the manifold, highly
complicated, and admirable structures which strike the beginner as the
only real adaptations in nature.
Exactly in the same way artificial selection isolates and preserves some
elementary species, while it destroys others. Of course the time is not
sufficient to secure new mutations, or at least these are only rare at
present, and their occurrence is doubtful in historic periods. Apart
from this unavoidable difference the analogy between natural and
artificial selection appears to me to be very striking.
This form of selection may be termed selection between species. Opposed
to it stands the selection within the elementary species or variety. It
has of late, alone come to be known as selection, though in reality it
does not deserve this distinction. I have already detailed the
historical evidence which gives preference to selection between species.
The process can best be designated by the name of intraspecific
selection, if it is understood that the term intraspecific is meant to
apply to the conception of small or elementary species.
I do not wish to propose new terms, but [801] I think that the principal
differences might better become understood by the introduction of the
word election into the discussion of questions of heredity. Election
meant formerly the preferential choice of single individuals, while the
derivation of the word selection points to a segregation of assemblies
into their larger parts. Or to state it in a shorter way, individual
selection is exactly what is usually termed election. Choosing one man
from among thousands is to elect him, but a select party is a group of
chosen persons. There would be no great difficulty in the introduction
of the word election, as breeders are already in the habit of calling
their choice individuals "elite," at least in the case of beets and of
cereals.
This intraspecific selection affords a second point for the comparison
between natural and artificial processes. This case is readily granted
to be more difficult than the first, but there can be no doubt that the
similarity is due to strictly comparable causes. In practice this
process is scarcely second in importance to the selection between
species, and in numerous cases it rests upon it, and crowns it, bringing
the isolated forms up to their highest possible degree of usefulness. In
nature it does quite the same, adapting strains of individuals to the
local conditions of their environment. Improved [802] races do not
generally last very long in practice; sooner or later they are surpassed
by new selections. Exactly so we may imagine the agency of natural
intraspecific selection. It produces the local races, the marks of which
disappear as soon as the special external conditions cease to act. It is
responsible only for the smallest lateral branches of the pedigree, but
has nothing in common with the evolution on the main stems. It is of
very subordinate importance.
These assertions of course, are directly opposed to the current run of
scientific belief, but they are supported by facts. A considerable part
of the evidence has already been dealt with and for our closing
discussion only an exact comparison remains to be made between the two
detailed types of intraspecific selection. In coming to this I will
first dwell upon some intermediate types and conclude with a critical
discussion of the features of artificial selection, which to my mind
prove the invalidity of the conclusions drawn from it in behalf of an
explanation of the processes of nature.
Natural selection occurs not only in the wild state, but is also active
in cultivated fields. Here it regulates the struggle of the selected
varieties and improved races with the older types, and even with the
wild species. In a previous [803] lecture I have detailed the rapid
increase of the wild oats in certain years, and described the
experiments of Risler and Rimpau in the running out of select varieties.
The agency is always the same. The preferred forms, which give a larger
harvest, are generally more sensitive to injurious influences, more
dependent on rich manure and on adequate treatment. The native varieties
have therefore the advantage, when climatic or cultural conditions are
unfavorable for the fields at large. They suffer in a minor degree, and
are thereby enabled to propagate themselves afterwards more rapidly and
to defeat the finer types. This struggle for life is a constant one, and
can easily be followed, whenever the composition of a strain is noted in
successive years. It is well appreciated by breeders and farmers,
because it is always liable to counteract their endeavors and to claim
their utmost efforts to keep their races pure. There can be no doubt
that exactly the same struggle exempt from man's intrusion is fought out
in the wild state.
Local races of wild plants have not been the object for field
observations recently. Some facts however, are known concerning them. On
the East Friesian Islands in the North Sea the flowers are strikingly
larger and brighter colored than those of the same species on the [804]
neighboring continent. This local difference is ascribed by Behrens to a
more severe selection by the pollinating insects in consequence of their
lesser frequency on these very windy isles. Seeds of the pines from the
Himalayas yield cold-resisting young plants if gathered from trees in a
high altitude, while the seeds of the same species from lower regions
yield more sensitive seedlings. Similar instances are afforded by
_Rhododendron_ and other mountain species. According to Cieslar
corresponding differences are shown by seeds of firs and larches from
alpine and lowland provinces.
Such changes are directly dependent on external influences. This is
especially manifest in experiments extending the cultures in higher or
in more northern regions. The shorter summer is a natural agent of
selection; it excludes all individuals which cannot ripen their seeds
during so short a period. Only the short lived ones survive. Schubeler
made very striking experiments with corn and other different cereals,
and has succeeded in making their culture possible in regions of Norway
where it formerly failed. In the district of Christiania, corn had
within some few years reduced its lifetime from 123 to 90 days, yielding
smaller stems and fewer kernels, but still sufficient to make its
culture profitable under the existing conditions. [805] This change was
not permanent, but was observed to diminish rapidly and to disappear
entirely, whenever the Norwegian strain was cultivated in the southern
part of Germany. It was a typical improved race, dependent on continual
selection by the short summers which had produced it. Similar results
have been reached by Von Wettstein in the comparison of kinds of flax
from different countries. The analogy between such cultivated local
races and the local races of nature is quite striking. The practice of
seed exchange rests for a large part on the experience that the
characters, acquired under the definite climatic and cultural conditions
of some select regions, hold good for one or two, and sometimes even
more generations, before they decrease to practical uselessness. The
Probstei, the Hanna and other districts owe their wealth to this
temporary superiority of their wheat and other cereals.
Leaving these intermediate forms of selection, we now come to our
principal point. It has already been discussed at some length in the
previous lecture, but needs further consideration. It is the question
whether intraspecific selection may be regarded as a cause of lasting
and ever-increasing improvement. This is assumed by biologists who
consider fluctuating variability as the main source of progression [806]
in the organic world. But the experience of the breeders does not
support this view, since the results of practice prove that selection
according to a constant standard soon reaches a limit which it is not
capable of transgressing. In order to attain further improvements the
method of selection itself must be improved. A better and sharper method
assures the choice of more valuable representatives of the race, even if
these must be sought for in far larger numbers of individuals, as is
indicated by the law of Quetelet.
Continuous or even prolonged improvement of a cultivated race is not the
result of frequently repeated selection, but of the improvement of the
standard of appreciation. Nature, as far as we know, changes her
standard from time to time only in consequence of the migrations of the
species, or of local changes of climate. Afterwards the new standard
remains unchanged for centuries.
Selection, according to a constant standard, reaches its results in few
generations. The experience of Van Mons and other breeders of apples
shows that the limit of size and lusciousness may be soon attained.
Vilmorin's experiments with wild carrots and those of Carriere with
radishes lead to the same conclusion as regards roots. Improvements of
flowers in [807] size and color are usually easy and rapid in the
beginning, but an impassable limit is soon reached. Numerous other
instances could be given.
Contrasted with these simple cases is the method of selecting sugar
beets. More than once I have alluded to this splendid example of the
influence of man upon domestic races, and tried to point out how little
support it affords to the current scientific opinion concerning the
power of natural selection. For this reason it is interesting to see how
a gradual development of the methods of selection has been, from the
very outset, one of the chief aims of the breeders. None of them doubts
that an improvement of the method alone is adequate to obtain results.
This result, in the main, is the securing of a few percent more of
sugar, a change hardly comparable with that progress in evolution, which
our theories are destined to explain.
Vilmorin's original method was a very simple one. Polarization was still
undiscovered in his time. He determined the specific weight of his
beets, either by weighing them as a whole, or by using a piece cut from
the base of the roots and deprived of its bark, in order to test only
the sugar tissues. The pieces were floated in solutions of salt, which
were diluted until the pieces [808] began to sink. Their specific weight
at that moment was determined and considered to be a measure of the
corresponding value of the beet. This principle was afterwards improved
in two ways. The first was a selection after the salt solution method,
but performed on a large scale. After some few determinations, a
solution was made of such strength as to allow the greater number of the
beets to float, and only the best to sink down. In large vessels
thousands of beets could be tested in this way, to select a few of the
very heaviest. The other improvement was the determination of the
specific weight of the sap, pressed out from the tissue. It was more
tedious and more expensive, but more direct, as the influence of the air
cavities of the tissue was excluded. It prepared the way for
polarization.
This was introduced about the year 1874 in Germany, and soon became
generally accepted. It allowed the amount of sugar to be measured
directly, and with but slight trouble. Thousands of beets could be
tested yearly by this method, and the best selected for the production
of seed. In some factories a standard percentage is determined by
previous inquiries, and the mass of the beets is tested only by it. In
others the methods of taking samples and clearing the sap have been
improved so far as to allow the [809] exact determination of three
hundred thousand polarization values of beets within a few weeks. Such
figures give the richest material for statistical studies, and at once
indicate the best roots, while they enable the breeder to change his
standard in accordance with the results at any time. Furthermore they
allow the mass of the beets to be divided into groups of different
quality, and to produce, besides the seeds for the continuation of the
race, a first class and second-class product and so on. In the factory
of Messrs. Kuhn & Co., at Naarden, Holland, the grinding machine has
been markedly improved, so as to tear all cell walls asunder, open all
cells, and secure the whole of the sap within less than a minute, and
without heating.
It would take too long to go into further details, or to describe the
simultaneous changes that have been applied to the culture of the elite
strains. The detailed features suffice to show that the chief care of
the breeder in this case is a continuous amelioration of the method of
selecting. It is manifest that the progression of the race is in the
main due to great technical improvements, and not solely to the
repetition of the selection.
Similar facts may be seen on all the great lines of industrial
selection. An increasing appreciation [810] of all the qualities of the
selected plants is the common feature. Morphological characters, and the
capacity of yielding the desired products, are the first points that
strike the breeder. The relation to climate and the dependence on manure
soon follow; but the physiological and chemical sides of the problem are
usually slow of recognition in the methods of selection. When visiting
Mr. de Vilmorin at Paris some years ago, I inspected his laboratory for
the selection of potatoes. In the method in use, the tubers were rubbed
to pulp and the starch was extracted and measured. A starch percentage
figure was determined for each plant, and the selection of the tubers
for planting was founded upon this result. In the same way wheat has
been selected by Dippe at Quedlinburg, first by a determination of its
nitrogenous contents in general, and secondly by the amount of the
substances which determine its value for baking purposes.
The celebrated rye of Schlanstedt was produced by the late Mr. Rimpau in
a similar manner and was put on the market between 1880 and 1890 and was
received with great favor throughout central Europe, especially in
Germany and in France. It is a tall variety, with vigorous stems and
very long heads, the kernels of which are nearly double the size of
those of the [811] ordinary rye, and are seen protruding, when ripe,
from between the scales of the spikelets. It is unfit for poor soils,
but is one of the very best varieties for soils of medium fertility in a
temperate climate. It is equal in the production of grain to the best
French sorts, but far surpassing them in its amount of straw. It was
perfected at the farm of Schlanstedt very slowly, according to the
current conceptions of the period. The experiment was started in the
year 1866, at which time Rimpau collected the most beautiful heads from
among his fields, and sowed their kernels in his experiment garden. From
this first culture the whole race was derived. Every year the best ears
of the strain were chosen for repeated culture, under experimental care,
while the remainder was multiplied in a field to furnish the seeds for
large and continually increasing areas of his farms.
Two or three years were required to produce the quantity of seed of each
kind required for all the fields of Schlanstedt. The experiment garden,
which through the kindness of Mr. Rimpau I had the good fortune of
visiting more than once between 1875 and 1878, was situated in the
middle of his farm, at some distance from the dwellings. Of course it
was treated with more care, and especially kept [812] in better
conditions of fertility than was possible for the fields at large. A
continued study of the qualities and exigencies of the elite plants
accompanied this selection, and gave the means of gradually increasing
the standard. Resistance against disease was observed and other
qualities were ameliorated in the same manner. Mr. Rimpau repeatedly
told me that he was most anxious not to overlook any single character,
because he feared that if any of them might become selected in the wrong
way, perchance unconsciously, the whole strain might suffer to such a
degree as to make all the other ameliorations quite useless. With this
purpose the number of plants per acre was kept nearly the same as those
in the fields, and the size of the culture was large enough every year
to include the best kernels of quite a number of heads. These were never
separated, and exact individual pedigrees were not included in the plan.
This mixture seemed to have the advantage of keeping up an average value
of the larger number of the characters, which either from their nature
or from their apparent unimportance had necessarily to be neglected.
After ten years of continuous labor, the rye of Rimpau caught the
attention of his neighbors, being manifestly better than that of
ordinary [813] sowings. Originally he had made his cultures for the
improvement of his own fields only. Gradually however, he began to sell
his product as seed to others, though he found the difference still very
slight. After ten years more, about 1886, he was able to sell all his
rye as seed, thereby making of course large profits. It is now
acknowledged as one of the best sorts, though in his last letter Mr.
Rimpau announced to me that the profits began to decline as other
selected varieties of rye became known. The limit of productiveness was
reached, and to surmount this, selection had to be begun again from some
new and better starting point.
This new starting point invokes quite another principle of selection, a
principle which threatens to make the contrast between artificial and
natural selection still greater. In fact it is nothing new, being in use
formerly in the selection of domestic animals, and having been applied
by Vilmorin to his sugar beets more than half a century ago. Why it
should ever have been overlooked and neglected in the selection of sugar
beets now is not clear.
The principle in itself is very simple. It agrees that the visible
characters of an animal or a plant are only an imperfect measure for
its hereditary qualities, instead of being the real criterion to be
relied upon, as is the current belief. [814] It further reasons that a
direct appreciation of the capacity of inheritance can only be derived
from the observation of the inheritance itself. Hence it concludes that
the average value of the offspring is the only real standard by which
to judge the representatives of a race and to found selection upon.
These statements are so directly opposed to views prevalent among plant
breeders, that it seems necessary to deal with them from the theoretical
and experimental, as well as from the practical side.
The theoretical arguments rest on the division of the fluctuating
variability into the two large classes of individual or embryonic, and
of partial deviations. We have dealt with this division at some length
in the previous lecture. It will be apparent at once, if we choose a
definite example. Let us ask what is the real significance of the
percentage figure of a single plant in sugar beets. This value depends
in the first place, on the strain or family from which the beet has been
derived, but this primary point may be neglected here, because it is the
same for all the beets of any lot, and determines the average, around
which all are fluctuating.
The deviation of the percentage figure of a single beet depends on two
main groups of external [815] causes. First come those that have
influenced the young germs of the plant during its most sensitive
period, when still an embryo within the ripening seed. They give a new
limitation to the average condition, which once and forever becomes
fixed for this special individual. In the second place the young
seedling is affected during the development of its crown of leaves, and
of its roots, by numerous factors, which cannot change this average, but
may induce deviations from it, increasing or decreasing the amount of
sugar, which will eventually be laid down in the root. The best young
beet may be injured in many ways during periods of its lifetime, and
produce less sugar than could reasonably be expected from it. It may be
surpassed by beets of inferior constitution, but growing under more
favorable circumstances.
Considered from this point of view the result of the polarization test
is not a single value, but consists of at least two different factors.
It may be equal to the algebraic sum of these, or to their difference,
according to whether the external conditions on the field were locally
and individually favorable or unfavorable. A large amount of sugar may
be due to high individual value, with slight subsequent deviation from
it, [816] or to a less prominent character combined with an extreme
subordinate deviation.
Hence it is manifest that even the results of such a highly improved
technical method do not deserve the confidence usually put in them. They
are open to doubt, and the highest figures do not really indicate the
best representatives of the race. In order to convey this conception to
you in a still stronger manner, let us consider the partial variability
as it usually shows itself. The various leaves of a plant may noticeably
vary in size, the flowers in color, the fruits in flavor. They fluctuate
around an average, which is assumed to represent the approximate value
of the whole plant. But if we were allowed to measure only one leaf, or
to estimate only one flower or fruit, and be compelled to conclude from
it the worth of the whole plant, what mistakes we could make! We might
indeed hit upon an average case, but we might as easily get an extreme,
either in the way of increase or of decrease. In both cases our judgment
would be badly founded. Now who can assure us that the single root of a
given beet is an average representative of the partial variability? The
fact that there is only one main root does not prove anything. An annual
plant has only one stem, but a perennial species has many. The average
height of the last is a [817] reliable character, but the casual height
of the former is very uncertain.
So it is with the beets. A beet may be divided by its buds and give
quite a number of roots, belonging to the same individual. These
secondary roots have been tested for the amount of sugar, and found to
exhibit a manifest degree of variability. If the first root corresponded
to their average, it might be considered as reliable, but if not anyone
will grant that an average is more reliable than a single determination.
Deviations have as a fact been observed, proving the validity of our
assertion. These considerations at once explain the disappointment so
often experienced by breeders. Some facts may be quoted from the Belgian
professor of agriculture at Gembloux, the late Mr. Laurent. He selected
two beets, from a strain, with the exceptional amount of 23% sugar, but
kept their offspring separate and analyzed some 60 of each. In both
groups the average was only 11-12%, the extremes not surpassing 14-15%.
Evidently the choice was a bad one, notwithstanding the high
polarization value of the parent. Analogous cases are often observed,
and my countrymen, Messrs. Kuhn & Co., go so far as to doubt all
excessive variants, and to prefer beets with high, but less
extraordinary percentages. Such are to be had in larger numbers [818]
and their average has a good chance of exemption from a considerable
portion of the doubts adhering to single excessive cases.
It is curious to note here what Louis de Vilmorin taught concerning this
point in the year 1850. I quote his own words: "I have observed that in
experiments on heredity it is necessary to individualize as much as
possible. So I have taken to the habit of saving and sowing separately
the seeds of every individual beet, and I have always found that among
the chosen parent plants some had an offspring with a better average
yield than others. At the end I have come to consider this character
only, as a standard for amelioration."
The words are clear and their author is the originator of the whole
method of plant breeding selection. Yet the principle has been
abandoned, and nearly forgotten under the impression that polarization
alone was the supreme guide to be relied upon. However, if I understand
the signs rightly, the time is soon coming when Vilmorin's experience
will become once more the foundation for progress in breeding.
Leaving the theoretical and historical aspects of the problem, we will
now recall the experimental evidence, given in a former lecture, dealing
with the inheritance of monstrosities. I have shown that in many
instances monstrosities [819] constitute double races, consisting of
monstrous and of normal individuals. At first sight one might be induced
to surmise that the monstrous ones are the true representatives of the
race, and that their seeds should be exclusively sown, in order to keep
the strain up to its normal standard. One might even suppose that the
normal individuals, or the so-called atavists, had really reverted to
the original type of the species and that their progeny would remain
true to this.
My experiments, however, have shown that quite the contrary is the case.
No doubt, the seeds of the monstrous specimens are trustworthy, but the
seeds of the atavists are not less so. Fasciated hawkweeds and twisted
teasels gave the same average constitution of the offspring from highly
monstrous, and from apparently wholly normal individuals. In other words
the fullest development of the visible characteristic was not in the
slightest degree an indication of better hereditary tendencies. In
unfavorable years a whole generation of a fasciated race may exhibit
exclusively normal plants, without transmitting a trace of this
deficiency to the following generation. As soon as the suitable
conditions return, the monstrosity reassumes its full development. The
accordance of these facts with the experience [820] of breeders of
domestic animals, and of Louis de Vilmorin, and with the result of the
theoretical considerations concerning the factors of fluctuation has led
me to suggest the method of selecting, which I have made use of in my
experiments with tricotyls and syncotyls.
Seedling variations afford a means of counting many hundreds of
individuals in a single germinating pan. If seed from one parent plant
is sown only in each pan, a percentage figure for the amount of
deviating seedlings may be obtained. These figures we have called the
hereditary percentages. I have been able to select the parent plants
after their death on the sole ground of these values. And the result has
been that from varieties which, on an average, exhibited 50-55%
deviating seedlings, after one or two years of selection this proportion
in the offspring was brought up to about 90% in most of the cases.
_Phacelia_ and mercury with tricotylous seedlings, and the Russian
sunflower with connate seed leaves, may be cited as instances.
Besides these tests, others were performed, based only on the visible
characters of the seedlings. The result was that this characteristic was
almost useless as a criterion. The atavists gave, in the main, nearly
the same hereditary percentages as the tricotyls and syncotyls, and
[821] their extremes were in each case far better constituted than the
average of the chosen type. Hence, for selection purposes, the atavists
must be considered to be in no way inferior to the typical specimens.
If it had been possible to apply this principle to twisted and fasciated
plants, and perhaps even to other monstrosities, I think that it will
readily be granted that the chance of bringing even these races up to a
percentage of 90% would have been large enough. But the large size of
the cultures required for the counting of numerous groups of offspring
in the adult state has deterred me from making such trials. Recently
however, I have discovered a species, _Viscaria oculata_ which allows of
counting twisted specimens in the pans, and I may soon be able to obtain
proofs of this assertion. The validity of the hereditary percentage as a
standard of selection has, within the last few years, been recognized
and defended by two eminent breeders, W.A. Hays in this country and Von
Lochow in Germany. Both of them have started from the experience of
breeders of domestic animals. Von Lochow applied the principle to rye.
He first showed how fallacious the visible characters often are. For
instance the size of the kernels is often dependent on their number in
the head, and if this number is [822] reduced by the injurious varietal
mark of lacunae (Luckigkeit), the whole harvest will rapidly deteriorate
by the selection of the largest kernels from varieties which are not
quite free from this hereditary deficiency.
In order to estimate the value of his rye plants, he gathers the seed of
each one separately and sows them in rows. Each row corresponds to a
parent plant and receives 200 or 150 seeds, according to the available
quantity. In this way from 700 to 800 parent plants are tested yearly.
Each row is harvested separately. The number of plants gives the average
measure of resistance to frost, this being the only important cause of
loss. Then the yield in grain and straw is determined and calculated,
and other qualities are taken into consideration. Finally one or more
groups stand prominent above all others and are chosen for the
continuation of the race. All other groups are wholly excluded from the
"elite," but among them the best groups and the very best individuals
from lesser groups are considered adequate for further cultivation, in
order to produce the commercial product of the race.
As a matter of fact the rye of Von Lochow is now one of the best
varieties, and even surpasses the celebrated variety of Schlanstedt. It
was only after obtaining proof of the validity [823] of his method that
Von Lochow decided to give it to the public.
W.M. Hays has made experiments with wheat at the Minnesota Agricultural
Experiment Station. He chose a hundred grains as a proper number for the
appreciation of each parent plant, and hence has adopted the name of
"centgener power" for the hereditary percentage.
The average of the hundred offspring is the standard to judge the parent
by. Experience shows at once that this average is not at all
proportional to the visible qualities of the parent. Hence the
conclusion that the yield of the parent plant is a very uncertain
indication of its value as a parent for the succeeding generation. Only
the parents with the largest power in the centgener of offspring are
chosen, while all others are wholly discarded. Afterwards the seeds of
the chosen groups are propagated in the field until the required
quantities of seed are obtained.
This centgener power, or breeding ability, is tested and compared for
the various parent plants as to yield, grade, and percentage of
nitrogenous content in the grain, and as to the ability of the plant to
stand erect, resist rust, and other important qualities. It is evident
that by this test of a hundred specimens a far better [824] and much
more reliable determination can be made than on the ground of the
minutest examination of one single plant. From this point of view the
method of Hays commands attention. But the chief advantage lies in the
fact that it is a direct proof of that which it is desired to prove,
while the visible marks give only very indirect information.
Thus the results of the men of practice are in full accordance with
those of theory and scientific experiment, and there can be little doubt
that they open the way for a rapid and important improvement. Once
attained, progress however, will be dependent on the selection
principle, and the hereditary percentage, or centgener power or breeding
ability, must be determined in each generation anew. Without this the
race would soon regress to its former condition.
To return to our starting point, the comparison of artificial and
natural selection. Here we are at once struck by the fact that it is
hardly imaginable, how nature can make use of this principle. In some
measure the members of the best centgener will manifestly be at an
advantage, because they contain more fit specimens than the other
groups. But the struggle for existence goes on between individuals, and
not between groups of brethren against groups of [825] cousins. In every
group the best adapted individuals will survive, and soon the breeding
differences between the parents must vanish altogether. Manifestly they
can, as a rule, have no lasting result on the issue of the struggle far
existence.
If now we remember that in Darwin's time this principle, breeding
ability, enjoyed a far more general appreciation than at present, and
that Darwin must have given it full consideration, it becomes at once
clear that this old, but recently revived principle, is not adequate to
support the current comparison between artificial and natural selection.
In conclusion, summing up all our arguments, we may state that there is
a broad analogy between breeding selection in the widest sense of the
word, including variety testing, race improvement and the trial of the
breeding ability on one side, and natural selection on the other. This
analogy however, points to the importance of the selection between
elementary species, and the very subordinate role of intraspecific
selection in nature. It strongly supports our view of the origin of
species by mutation instead of continuous selection. Or, to put it in
the terms chosen lately by Mr. Arthur Harris in a friendly criticism of
my views: "Natural selection may explain the survival [826] of the
fittest, but it cannot explain the arrival of the fittest."
A
_Abies concolor fastigiata_, 618
_Acacia_, 176, 196, 217, 458, 697
bastard, 343, 617, 618, 664, 665, 666
_Acer compestre nanum_, 612
_Achillea millefolium_, 131, 132, 441
Adaptation, 702
double, 430, 451, 452, 454, 455, 457, 458, 642
_Aegilops ovata_, 265
_speltaeformis_, 265
_Agave vivipara_, 684
_Ageratum coeruleum_, 612
_Agrostemma Coronaries bicolor_, 125
_Githago_, 282
_nicaeensis_, 162
_Agrotis_, 204
Alder, cut-leaved, 147, 596
Alfalfa, 264
Algae, 699
Allen, Grant, 237
_Alliaria_, 638
_Alnus glutinosa laciniata_, 615
Alpine plants, 437, 695, 794
_Althaea_, 490
Amaranth, 282, 452
_Amaranthus caudatus_, 282
_Amaryllis_, 272, 275, 762
brasiliensis_, 275
leopoldi_, 275
pardina_, 275
psittacina_, 275
vittata_, 275
Amen-Hotep, 697
_Ampelopsis_, 239
_Amygdalus persica laevis_, 126
_Anagallis arvensis_, 162
_Androsace_, 634
_Anemone_, 266, 331
_coronaria_, 241, 491
var. "Bride," 510
_magellanica_, 266
_sylvestris_, 266
_Anemone_, garden, 241
Annee, 760
Anomalies, taxonomic, 658, 685
_Anthemis_, 236
_nobilis_, 130
_Anthurium scherzerianum_, 639
_Antirrhinum majus_, 315
_luteum rubro-striatum_, 315
Apetalous flowers, 622
Apples, 134, 240, 328, 454, 806
elementary species, 75
method of cultivating, 76
origin of cultivated varieties, 73
use by the Romans, 74
"Wealthy," 78, 79
wild, 73, 74, 75, 76
_Aquilegia chrysantha_, 161
_Arabis ciliata glabrata_
_hirsuta glaberrima_, 126
_Aralia crassifolia_, 662
Arbres fruitiers ou Pomonomie belge, 76
_Aralia papyrifera_, 662
Arctic flora, 695
_Arnica_, 494
_montana_, 236
Aroids, 222, 631, 639
Artemisias, 131
Artificial selection, 18, 71, 77, 93, 95, 743, 744, 798, 826
first employed, 72, 92
nature of, 19
_Arum maculatum immaculatum_, 125
Ascidia, 310, 366, 367, 427, 428, 669, 670, 671, 672, 673,
674, 675
Ash, 135, 341
one-bladed, 666, 667
weeping, 196, 596
Ashe, 343
Aster, 132, 152, 242
seashore, 200, 282
_Aster Tripolium_, 132, 200, 236, 282, 410
_Astragalus alpinus_, 696
Atavism, 154, 170, 172, 175, 176, 178, 182, 185, 187, 188,
198, 220, 222, 226, 235, 344, 354, 399, 405, 411,
660, 661
bud, 183, 226
definition of, 170, 631
false, 185, 187
negative, 344
positive, 344
seed, 176
systematic, 174, 222, 630-657
Atavists, 156, 201
heredity of, 412
_Atropa Belladonna lutea_, 592
_Aubretia_, 241
_Avena fatua_, 100, 207
_Azalea_, 178, 322
_Azolla caroliniana_, 239
B
Babington, _Manual of British Botany_, 36,
Bailey, 78, 306, 684
Balsams, 334
Bananas, 90, 134
Banyan, 244
Barberry, 133, 180
European, 270
purple, 596
_Barbarea vulgaris_, 427
Barley, 98, 105, 133, 203, 678, 679
"Nepaul," 203, 676, 677, 679, 681, 682
Bastard-acacia, 133, 136, 140
Bateson, 250
Bauhin, Caspar, 72, 610
Baumann, 618
Beans, 90, 152, 327, 727, 735
Bedstraw, 648
Beech, 133, 135, 242
cut-leaved, 179, 196, 616
laciniated, 196
oak-leaved, 595
purple, 196, 593, 595
Beeches, 427
fern-leaved, 147
Beets, 68, 72, 92, 93, 792, 796, 801, 815, 817, 818
Californian, 796
European, 796
forage, 71, 72, 791
salad, 71
Beet-sugar, 67, 68, 69, 70, 71, 109, 165, 717, 791, 807,
813, 814
_Begonia_, 218, 366, 509, 765
ever-flowering, 148
tuberous, 272
_clarkii_, 272
_davisii_, 272
_rosiflora_, 272
_sedeni_, 273
_semperflorens_, 133, 148, 620
_Begonia_
bulbous, 372
_veitchi_, 272
Behrens, 804
Belladonna, 145
_Bellis perennis_, 236
_perennis plena_, 195
Bentham, 237
Bentham & Hooker, _Handbook of British Flora_, 36
_Berberis_, 133, 180, 455
_ilicifolia_, 270
_vulgaris_, 270
Bertin, 596
_Berula angustifolia_, 457
Bessey, 660
_Beta maritima_, 69
_patula_, 69, 70
_vulgaris_, 69, 70
_Betula_, 132
Between-race, 358
Bewirkung, Theorie der directen (Nageli), 448
_Biastrepsis_, 402
_Bidens_, 131
_atropurpurea_, 131
_cernua_, 131, 158
_leucantha_, 131
_tripartite_, 131
Bilberries, 577
Bindweed, 41924
Binomium, of Newton, 767
Birch, 133, 243
cut-leaved, 596, 616
fastigiate, 618
fern-leaved, 179
_Bisoutella_, 282
_laevigata glabra_, 125
Bitter-sweet, 125
Blackberry, 268, 768
"Paradox," 769
Blue-bells, variation in, 54, 491, 577
Blueberries, 769
Blue-bottle, 499, 507, 509, 510
Blueflag, atavism of, 172
_Boehmeria_, 675
_bilboa_, 685
Bonnier, 439, 441, 442, 444, 451, 795
Boreau, 663
Brambles, 126, 127, 147, 239, 244, 245, 268, 740,
769, 663
_Brassica_, 244
Braun, 738
Braun and Schimper, 494
Bread-fruits, 90
Briot, 618
Britton and Brown's Flora, 162
Brooks, 711
Broom, 140
prickly, 217
Broom-rape, 220
_Broussonetia papyifera dissecta_, 616
_Brunella_, 146, 268
_vulgaris_, 577
_vulgaris alba_, 201
_Bryophyllum calycinum_, 218
Buckwheat, 452
Bud-variation, 750
Buds, adventitious, 218
Burbank, Luther, 57, 79, 116, 134, 268, 758, 768,
769, 784
Buttercup, 331, 357, 410, 725, 740
Asiatic, 241
C
Cabbages, 428, 684
atavism in, 638
origin of varieties, 621
Cactuses, 444
Cactus-dahlia, 625
_Calamintha Acinos_, 437, 452
Calamus root, 222
_Calendula officinalis_, 502
_Calliopsis tinctoria_, 195
_Calluna_, 146
_vulgaris_, 437, 577
_Caltha_, 490
_palustris_, 331
_Camelina_, 684
_Camellia_, 178, 323
_japonica_, 368
Camellias, 331
Camomile, 130, 132, 156, 366, 494, 503, 509, 512
_Campanula persicifolia_, 151, 234
_rotundifolia_, 437
Campion, 283, 302, 304
evening, 281
red, 238
_Canna_, 751, 759, 761
_indica_, 760
"Madame Crozy," 760, 761
_nepalensis_, 760
_warczewiczii_, 760
_Capsella Bursa-pastoris apetala_, 585
_heegeri_, 22, 582, 583, 684
_Carex_, 53
Carnation, 178, 241, 491
wheat-ear, 227
_Carpinus Betulus heterophylla_, 180
Carriere, 491, 596, 612, 806
Carrots, 806
Catch-fly, 419
Carboniferous period, 699
_Casuarina quadrivalvis_, 649
Cauliflowers, origin of, 621
Caumzet, 614
Causation, theory of direct, (Nageli), 448
Cedar, pyramidal, 618
Celandine, 147, 245, 280, 365
oak-leaved, 603, 610, 611
_Celosia_, 621
_Celosia cristata_, 327, 411
_Centaurea_, 242
Centgener power, 20, 822
_Centranthus macrosiphon_, 424
_Cephalotaxus_, 170, 226
_pedunculata fastigiata_, 169
Cereals, 105, 106, 107, 119, 801, 804
origin of cultivation, 104
Character-units, 632
Charlock, 424
_Cheiranthus_, 490
_Cheiri_, 370
_Cheiri gynantherus_, 371
_Chelidonium laciniatum_, 22, 609
_majus_, 147, 365, 600, 610, 611
_majus foliis quernis_, 610
Cherries, 79
Cherry, bird's, 617
Chestnuts, 427
Chromosomes, 306
_Chrysanthemum_, 178, 274
corn, 739
_Chrysanthemum carinatum_, 494
_coronarium_, 161, 202, 510
_grandiflorum_, 739
_imbricatum_, 494
_indicum_, 490
_inodorum_, 503
_inodorum plenissimum_, 336
new double, 501
_segetum_, 202, 493, 504, 729
_segetum_, var. _grandiflorum_, 43, 495, 498, 504,
504
_Chrysopogon montanus_, 450
Cieslar, 804
_Cineraria cruenta_, 514
Cinquefoil, 52
_Clarkia_, 420
_elegans_, 198
_pulchella_, 282
_pulchella carnea_, 162
_Clematis Vitalba_, 662
_Viticella nana_, 612
Clover, 80, 102, 674
crimson (Italian), 353, 358, 359, 360
five-leaved, 340, 362, 374, 431, 509, 789
four-leaved, 340, 346, 352
red, 235, 281
white, 133, 366
Clusius, 610
_Cochlearia anglica_, 52
_danica_, 52
_officinalis_, 52
Coconut, 67, 82, 83, 87, 88, 89
dispersal of, 85, 89
geographic origin of, 88,89
Coconut-palm, 84, 88
Cockerell, T.D.A., 139, 140, 591
Cocklebur, 139
Cockscomb, 165, 327, 356, 411, 621
_Cocos nucifera stupposa_, 83, 84
_cupuliformis_, 82
_rutila_, 82
_Codiaeum appendicularum_, 673
_Colchicum_, 490
_Coleus_, 132
Columbine, 725
yellow, 161
Columbus, 89, 118
Columella, 106
Composites, 130, 131, 336, 723, 778
Conifers, 168, 226, 239, 455
weeping, 617
Connation, of petals, 660, 661
"Conquests," 242
Contra-selection, 425
Cook, 84, 86, 88, 89
Corn, 81, 90, 118, 119, 135, 283, 287, 288, 775,
786, 788, 804
American, 205
Corn-cockle, 162
Corn-chrysanthemum, 739
Corn-flowers, 491, 92
Corn, "Forty-day," 118
"Harlequin," 327
sterile variety of, 622
sugar, 135, 158
"Tuscarora," 205
Corn-marigold, 493, 494
Cornel berry, yellow, 196
Cornaceae, 675
_Cornu_, 338
_Cornus Mas_, 196
Correlation, 142
_Corylus_, 133
_Avellana_, 181
_tubulosa_, 181
Cotton, 725
Cotyledon, 674
variation in, 416
_Crambe maritima_, 621
Cranesbill, 599
European, 628
meadow, 322
_Crataegus_, 196
_oxyacantha_, 132
Crowfoot, 331
corn, 283
_Crepis biennis_, 410, 411
Cress, Indian, 192
Crosses
bisexual, 255, 276, 294, 298
reciprocal, 279
unisexual, 255, 261
varietal (see Hybrids)
_Croton_, 673, 674
Crozy, 760, 762
Crucifers, 222, 635
_Cryptomeria_, 169, 226
_japonica_, 239
Cucumbers, 118
_Cucumis_, 52
_Cucurbita_, 52
Cultivated plants, 65, 66
elementary species of, 62
improvement of, 92
mixed nature of, 96, 118
origin of, 91
Currants, 79
Californian, 270
flowering, 166
"Gordon's," 270
Missouri, 270
white, 158
white-flowered, 167
Cuttings, 721
_Cyclamen_, 323, 355, 627, 684
Butterfly, 627
_vernum_, 619
_Cypripedium caudatum_, 487
_Cytisus adami_, 271
_candicans Attleyanus_, 367
_Laburnum_, 271
_prostratus_, 139
_prostratus ciliata_, 125
_purpureus_, 271
_spinescens_, 139
D
_Dahlia_, 131, 241, 272, 625
cactus, 625
"Jules Chretien," 628
purple-leaved, 626
"surprise," 230
tubular, 627
[sic] 274, 490, 764
first double ones, 490
green, 227, 229, 230
Daisies, 131, 132, 494
double, 195
hen-and-chicken, 514
ox-eye, 202
Shasta, 769
yellow, 202
Dandelion, 411
parthenogenesis, 61
variations in, 60
Daphne Mezereum, 146
Darwin, 1, 2, 3, 4, 5, 6, 7, 18, 76, 85, 93, 109,
110, 180, 196, 205, 206, 242, 306, 324, 338,
448, 571, 604, 612, 689, 702, 710, 715, 743,
798, 825
Darwin, George, 711
Darwinian theory, 461
basis of, 5
Date, 134
_Datura Stramonium_, 139, 142
_Stramonium inermis_, 300
_Tatula_, 139, 142, 300
Dead-nettle, 237
De Bary, 38, 47, 49
De Candolle, 76, 84, 85, 89, 228, 370, 403, 621
Alphonse, 74, 129, 226
A.P., 129
Casimir, 659, 676
De Graaff, 275
_Delphinium Ajacis_, 192
Deniau, 617
Descent, theory of, 690, 694, 702, 707, 716, 798
De Serres, Olivier, 72
_Desmodium gyrans_, 655, 656, 663, 664, 65
Dewberry, California, 269
_Dianthus barbatus_, 322, 648
twisted variety, 408
Diatoms, 699
Dictoyledons
ancestors of monocotyledons, 15
_Digitalis parviflora_, 161, 640
_purpurea_, 483
pelorism of, 482
Dimorphism, 445, 447, 454, 457, 458
Dippe, 810
_Dipsacus fullonum_, 402
sylvestris_, 402, 402
Dominant character, 280
Double flowers
poppies 490
production of, 489
types of, 330
Double races (see also ever-sporting varieties),
419, 427, 428
Dubois, Eugene, 712
Duchesne, 185, 188, 596
Duckweed, 222
_Draba_, 692, 693
verna, 47, 50, 51, 53, 125, 126, 518, 533,
546, 547, 561
_Dracocephalum moldavicum_, 419
Dragon-head, 419
_Drosera anglica_, 268
_filiformis_, 268
_intermedia_, 268
_obovata_, 267
_rotundifolia_, 268
E
Earth, age of, 710
Edelweiss, 438
Eichler, 660
Election, 801
Electric light, growth in, 442
Elementary species, 11, 13, 32, 67, 74, 76, 77,
78, 79, 91, 95, 116, 119, 124, 126, 128, 129,
207, 238, 252, 256, 307, 430, 435, 695, 696,
698, 702, 715, 787, 798, 800, 825
apples, 75
coconut, 82
corn, 81
cultivated plants, 62
definition of, 12, 35, 127
flax, 80
how produced, 16, 248
hybrids of, 253, 255
mutation of, 141
origin of, 459, 603
origin of, how studied, 463
selection of, 92
varieties vs., 14, 15, 141, 152, 224, 243,
247, 251, 495
Elm, 136, 219, 239, 427
_Epilobium_, 268
_hirsutum_, 683
_hirsutum cruciatum_, 588
_montanum_, 269
_tetragonum_, 269
_Equisetum Telmateja_, 642, 649
_Erica Tetralix_, 577, 661
Ericaceae, 146, 660
_Erigeron _Asteroides_, 450
_canadensis_, 132, 236, 453, 600, 695
_Erodium_, 146
_cicutarium album_, 161
_Erucastrum_, 630, 638, 639
_pollichii_, 222, 637
_Eryngium campestre_, 674
_maritimum_, 674
_Erysimum cheiranthoides_, 638
_Erythraea pulchella_, 452
_Erythrina_, 621
_Crista-galli_, 620
Eschcholtzias, 59
Esimpler, 337
_Eucalyptus citriodora_, 669
_Globulus_, 217
_Euphorbia Ipecacuanha_, 55
Evening-primrose, 62, 204, 256, 424, 686, 687,
688, 690, 691, 694, 695, 699, 702, 703,
705, 707, 708, 713, 747, 793
Evolution, 93, 685, 686, 689, 704, 707, 709,
710, 713, 718
degressive, 222, 223, 249
progression in, 630
progressive, 221, 222, 223, 248
regression in, 630
regressive, 221, 222; 223, 24
retrograde, 221, 631
Extremes, asexual multiplication of, 742, 769
F
Fabre, 265
_Fagus_, 133
_Fagus sylvatica pectinata_, 179
Fan, genealogical, 700
Fasciated stems, 409, 412
Ferns, 63
cristate, 427
plumose, 427
_Ficaria_, 53
_Ficus radicans_, 436
_religiosus_, 244
_repens_, 436
_stipulata_, 436
_ulmifolia_, 436
Figs, 436
_Filago_, 52
Fir, 134, 804
Fittest, survival of, 826
Flax, 80, 805
springing, 80
threshing, 80
white-flowered, 158, 160
Fleabane, Canada, 132, 236
Flowers, gamopetalous, 660
Fluctuability
embryonic, see Fluctuation, individual
Fluctuation, 708, 715, 716, 718, 719, 724, 737, 741
curves of, 729, 794
defined, 191
individual, 718, 723, 732, 741, 745, 749, 788
mutation vs. 7, 16, 719
partial, 718, 723, 732, 741, 745, 748, 749,
771
inadequate for evolution, in elementary species,
19
nature of, 18
specific and varietal characters vs. 17
Forget-me-not, 368
Fothergill, John, 521
Foxglove, 163
peloric, 164, 356, 367
yellow, 161, 640
_Fraxinus excelsior monophylla_, 667
_exheterophylla_, 667
_simplici folio_, 667
French flora (Grenier and Godron), 433
Fries
on _Hieracium_, 60
Frostweed, 440
species of, 52
_Fuchsia_, 272, 355
Fuchsias, 491
G
Gaertner, 279
_Galeopsis Ladanum canescens_, 139
_Galium_, 648
_Aparine_, 409, 648
_elatum_, 52
_erectum_, 52
_Mollugo_, 62
_verum_, 648
Gallesio, 138
Galton, 736, 776
Gamopetaly, 662
Garden-pansy, origin of, 38
Garlic, 638
Gauchery, 452
Geikie, 711
Genera
artificial character of, 36
polymorphous, 692
_Gentiana punctata concolor_, 125
Gentians, 577
Georgics (Vergil), 106
_Geranium pratense_, 323, 628
_album_, 628
_pyreniacum_, 599
German flora (Koth), 432
Geum, 282
Gherkins, 118
Gideon, Peter M., 78
Glacial period, 696
_Gladiolus_, 241, 272, 274, 368, 765
_cardinalis_, 275
_gandavensis_, 275
_psittacinus_, 275
_purpureo-auratus_, 275
_Glaucium_, 241
_Gleditschia sinensis_, 614
_triacanthos pendula_, 617
_Gloxinia_, 282, 485
erect, 626
_Gloxinia erecta_, 485
peloric variety, 485
_Gnaphalium Leontopodium_, 438
_Godetia amoena_, 161
Godetias, 59, 232
Godron, 265, 432
Goeppert, 370
Gooseberry, 79, 140, 626
red, 133, 165, 241
Grapes, 90, 158, 328
Grape-hyacinth, _plumosa_, 134
Grasses, 102, 631, 681
Grenier, 433
Groundsel, 132
Growth, nutrition and, 714, 720, 722
Guelder-rose, 134, 239
Gum-tree, Australian, 217
_Gypsophila paniculata_
twisted variety, 409
H
Haeckel, 707
Half-races, 358, 372, 409, 419, 424, 427, 428
Hall, 444
Hallet, F.F., 109
Harebell, 232
peach-leaved, 234
Harris, Arthur, 825
Harshberger, John W., 591
on _Euphorbia_ in New Jersey, 55
Hawksbeard, 410, 411, 412
Hawkweed, 411, 439, 443, 819
Hawkweeds
seeding without fertilization, 61
Hawthorn, white, 132
Hays, W.M.
on individual selection, 20, 94, 95, 117,
821, 823, 824
Hazelnut, 133, 181, 242
Hazels, cut-leaved, 596,-616
Heath family, 146, 222, 660
Heaths, origin of, 662
Heather, 577
_Hedera Helix arborea_, 437
Hedgehog burweed, 140
_Hedys_Arum_, 664
Heeger, 582
Heer, Oswald, 74, 105
Heinricher, 172, 173, 174
_Helianthemum_, 53, 125, 126, 561
_apenninum_, 52
_pilosum_, 52
_polifolium_, 52
_pulverulentum_, 52
_vulgare_, 440
_Helichrysum_, 420
_Helwingia_, 678, 678, 682
_rusciflora_, 675
Hemp, 419
Henbane, 282
_Hepatica_, 322, 490
Heredity, 731, 734, 818
bearers of, 632
in teasels, 642
_Hesperis_, 241, 322
_matronalis_, 323, 411
_Heylandia latebrosa_, 450
_Hibiscus Moscheutos_, 591
_Hieracium_, 59, 439
_alpinum_, 696
Hildebrand, 160, 240, 241
Hoffman, 160, 662
Hofmeister, 160, 370, 480
Holbein, 164, 596
Holly, 140, 196
Holtermann, 449, 451
Hollyhock, 427
Honeysuckle, 674
ground, 443
_Hordeum distichum_, 677
_hexastichum_, 677, 678
_tetrastichum_, 677
_trifurcatum_, 676, 678
_vulgare trifurcatum_, 203
Hornbeam, European, 180
Horse-chestnut, 219
thornless, 234
Horsetail, Canadian, 695
European, 649
Horsetail, family, 641
Horse-weed, 132
Canadian, 452
_Hortensia_, 134, 181
Horticulture, mutations in, 604
Houseleek, 370, 371
Hunneman, John, 521
Hyacinths, 178, 322
white, 160
Hybrids, 58, 201, 202, 206, 250, 575
between elementary species, 253
constant, 263, 264, 265, 266, 267, 268, 269
law of varietal, 716
Mendelian, 324
nature of, 20
species, 256, 260
splitting of, 210
varietal, 208, 209, 247, 277, 278, 279, 281,
285, 293, 294
Hybridization, 706, 751, 752, 758, 759, 764
_Hydrocotyle_, 668
_Hyoscyamus niger_, 282
_pallidus_, 283
_Hypericum perforatum_, 725
_Hyssopus officinalis_, 161
I
_Iberis umbellata rosea_, 195
Improved races, inconstancy of 770-797
Indian cress, 668
pelorism of, 485
Indian pipe, 661
Ipecac spurge, 55
_Iris_, 456
_falcifolis_, 172
_kaempferi_, 174
_lortetii_, 521
_pallida_, 172
_pallida abavia_, 681
Isolation, 108
Ivy, 436
J
Jacob's ladder, 200, 202
Jacques, 614
Jacquin, 52, 632
Jaggi, 594, 595
Jaeger, 228, 662
Jalappa, 165
Janczewski, 266
Japanese plum, 58
_Jasminum Sambac_, 662
Joly, 712
Jordan, Alexis, 45, 47, 49, 50, 129
experiments with species, 37, 40
_Juncus effusus spiralis_, 684
Juniper, 684
K
Kapteyn, 716
Kelvin, Lord, 720, 711
Kerner von Marilaun, 266, 267
Keteleer, 618
Knight, 390, 719, 720
Koch, 433, 667
Koelreuter, 279
Korshinsky, 609, 612, 614, 617, 667
Krelage, 510, 619
Kuhn & Co., Messrs., 801, 809, 817
L
_Labiates_, 237
pelories of, 577
_Labiatiflorae_, pelorism of, 468
Labrador tea, 661
_Laburnum_, 270, 284, 342
oak-leaved 147, 179
pelorism of, 485
_Lactuca_, 52
_Scariola_, 456
Lagasca, Mariano, 96, 97, 114
Lamarck, 1, 447, 461, 522, 522
Lamarckism
objections to, 449
_Lamium album_, 237
_maculatum_, 237
pelorism of, 486
_purpureum_, 237
Larch, 804
Larkspur, 124, 192, 311, 452
hybrid, 213
white, 160
Latency, 657
individual, 219
specific, 246
systematic, 219, 220, 235
varietal, 246
Latent characters, 216
_Lathyrus odoratus_, 776
_Laurea pinnatifida_, 450
Laurel, lady's, 146
Laurent, 802
Leaves, cleft, 685
variegated, 426, 431
LeBrun, Mme., 614
Le Couteur, 96, 97, 107, 108, 114, 115, 116, 742
_Ledum_, 222, 661
_Lemna_, 222
Lemoine, 762, 762
Lettuce, 684
crisped, 158
prickly, 456
Life, struggle for, 103, 119, 120
Lilacs, 59, 769
double, 762
_Lilium candidum flore pleno_, 331
_pardalium_, 116
Lime-tree, 355, 366, 428, 669
fern-leaved, 147
_Linaria_, 467, 471, 480
_dalmatica_, 482
_genistifolia_, 267
_italica_, 267
_vulgaris_, 267, 471
_vulgaris peloria_, 464
Lindley, 63, 129, 506
Linnaeus, 32, 33, 129, 132, 256, 663
on the idea of species, 11, 13
on origin of species, 2, 34
on primroses, 52
_Linum angustifolium_, 80
_crepitans_, 81
_usitatissimum_, 80, 161
Link, 466
Liver-leaf, 322
_Lobelia syphilitica_, 161
_Lonicera etrusca_, 640
_tartarica nana_, 614
Lorenz, Chr., 482
Lothelier, 454
_Lotus corniculatus_, 442
_corniculatus hirsutus_, 139
London, 615, 616, 667
Lucerne, 264
Ludwig, 738
Lupines, 90
_Lychnis_, 282
_chalcedonica_, 161
_diurna_, 238, 578
_preslii_, 578
_vespertina_, 238, 281, 585
_Lycium_, 455
_Lycopersicum_, 655
_grandifolium_, 654
_latifolium_ (see _L. grandifolium_).
_solanopsis_, 854, 656
_validum_ (see _L. solanopsis_).
Lyell, 1, 710
_Lysimachia vulgaris_, 684
M
MacDougal, D.T., 62, 575, 590
Macfarlane, 56, 255, 268
_Madia elegans_, 779
_Magnolia_, 355, 366, 428, 674, 675
_obovata_, 355, 669
_Magnus_, 228
_Mahonia aquifolia_, 270
Maize, 134, 775
"Cuzco," 152
European, 206
"Gracillima," 152
"Horse-dent," 152
"Quarantino," 118
Mallow, 663, 684
_Malva crispa_, 684
Maples, laciniate, 615
Marchant, 592
Marigold, 131, 158
corn, 729
field, 503, 505, 508
garden, 503
Japanese, 490, 494, 495
Marsh-marigold, 331
Martinet, 80
Measart, 434
Masters, 228, 370, 372
_Matricaria Chamomilla_, 130
_Chamomilla discoidea_, 156
Matricaria discoidea, D.C., 157
May-thorn, red, 196
_Medicago media_, 264
_falcata_, 264
_Melanium_, 39
Melons, 118
Mendel, 6, 210, 294, 296, 306, 308
Mendel's law, 276, 293, 294, 298, 299, 300, 301,
307, 612, 613, 616, 716
Mendelism, 307
Mentha, 52
_Mercurialis annua_, 420
_annua laciniata_, 592
Mercury, 420, 422, 425, 820
Methods of investigation, 21
Metzger, 205, 206
Milde, 38
Milfoil, 441
Millardet, 266
Miller, 611
Millet, 105
_Mimulus_, 151
_quinquevulnerus_, 725
_Mimusops_, 697
Miocene period, 698
Miquel, 83
_Mirabilis_, 241
_Jalappa_, 322
Mirbel, 615
_Monardella macrantha_, 444
Monstrosities, 400, 401, 445, 446, 447
Monkey-flower, 725
Monocotyledons
ancestry of, 1, 5
regression in, 630
_Monotropa_, 222, 661
Morphologic units, 145, 152
Monstrosities, 818
Morgan
on mutation-theory, 9
Morren, 244, 762
Mountain-ash, 342
Muller, Fritz, 775, 776, 780
Multiplication, vegetative (see Asexual propagation)
Munting, Abraham, 164, 165, 490, 762
Munting's drawings, 512
Murr, 158, 236
_Muscari comosam_, 134
Museum d'Histoire Naturelle, Paris, 522
Mutability vs. fluctuating variability, 568
Mutation, 659, 674, 677, 685, 686, 694, 713, 716,
825
absence of intermediate steps in, 474, 480
conditions for observing, 601
decided within the seed, 28
definition of, 7
easily observed, 30
experimental, 688
few observations of, 8
fluctuation vs., 7, 16, 719
influence of on variability, 335
iterative nature of, 476, , 703
laws of, 556, 558, 560, 562, 564, 566, 568,
570
limited in time, 29
observation of, 16
in _Oenothera_, 521, 525, 690
oldest known, 609
oldest recorded, 22
periodic, 690, 692, 694
perodicity of, 519
progressive, 307
repetition of, 476
in _Saponaria calabrica_, 612
simultaneous, 614
in tomato, 655
Mutations, 141, 275, 280, 445, 449, 573, 608, 620,
626, 678, 685, 686, 701, 704, 712, 713, 716,
800
artificial, 402
chance for useful, 598
defined, 191
frequency of, 597
in garden-flowers, 488
in horticulture, 604, 706
latent, 703
mode of appearance, 517
numerical proportion of, 475
original production of, 702
peloric, 707
periodic, 686, 705
progressive, 704
retrograde, 704
stray, 704, 705, 706
synonyms of, 191
Mutation-period, 714
_Myosotis azorica_, 368
_Myrtus communis_, 684
N
Nageli, 60, 439, 443, 448, 795
Nagelian principle, 448, 450, 451
Natural selection, 18, 119, 120, 445, 456, 682,
694, 703, 743, 744, 798-826
basis, 604
nature of, 6, 19
Naudin, 118
Nectarines, 137, 138, 226, 627
Nemec, 578
Neo-Lamarckians
principle of, 8
Neo-Lamarckism 447
_Nepenthes_, 671, 672, 673, 674
Newton, 1, 732, 767
_Nicandra_, 152
_Nigella_, 134
Nightshade, 298
black, 282
Nourishment
meaning of, 732
variability and 771
_Nuphar_, 268
Nutrition and growth, 720, 722
_Nymphaea_, 698
O
Oats, 98, 100, 101, 105, 112, 113, 115, 119, 133,
452
"Early Angus," 115
"Early Fellow," 115
"Fine Fellow," 115
"Hopetown," 112
"Longfellow," 115
"Make-him-rich," 112
wild, 207, 803
Oak, 136, 239
_Oenothera_, 260, 262, 279, 700, 706, 708, 709
European species, source of, 575
mutation in, 521, 525, 585, 690, 708
new species of, 516-546
_albida_, 537, 553, 555, 563, 565, 573
_biennia_, 82, 205, 256, 257, 258; 259, 262,
263, 264, 521, 524, 527, 574, 575, 586,
587, 683, 690, 708
_biennis cruciata_, 22, 587
_brevistylis_, 263, 280, 526, 529, 530, 547,
563, 564, 565, 573, 574, 702, 706
_cruciata_, 575, 585, 586, 589, 590, 683
_elliptica_, 540, 545, 555, 562
_gigas_, 533, 534, 535, 536; 537, 553, 554,
563, 565, 566, 567, 573, 574, 702
_glauca_, 424
_hirtella_, 262
_laevifolia_, 526, 528, 529, 547, 563, 564,
573, 574, 701, 706
_lamarckiana_, 17, 262, 262, 522, 523, 527,
528, 529,, 533, 574, 575, 586, 690, 699
pollination of, 524
_lata_, 540, 541, 542, 549, 550, 551, 552,
555, 559, 563, 566, 573, 574, 702
_leptocarpa_, 540
_muricata_, 256, 257, 258, 259, 262, 263,
264, 513, 575, 690
pollination of, 524
_nanella_, 526, 531, 549, 50, 551, 552, 555,
563, 564, 565, 566, 703
_oblonga_, 537, 538, 552, 555, 563, 565, 566,
572
_rubrinervis_, 533, 534, 536, 537, 550, 551,
552, 555, 563, 565, 568, 573, 574
_scintillans_, 540, 543, 553, 555, 563, 566,
573, 574
mutability of, 544
_semilata_, 540
_suaveolens_, 521
_Oleander_, 684
_Onagra_, 262, 708, 709
Onions, wild, 684
_Ononis repens_, 577
Orange, 90, 133, 134
Orchids, 631
Origin of species (Darwin), 109
_Orobanche_, 220
_Othonna crassifolia_, 442
Otin, 618
Oviedo, 89
P
_Paeonia corallina leiocarpa_, 126
Paillat, 618
Pangenes, 306
Pangenesis, 306, 689
_Panicum_, 105
Pansies, 640
Pansy, 118, 121
_Papaver alpinum_, 139
_bracteatum_, 661
_bracteatum monopetalum_, 661
_commutatum_, 357
_dubium glabrum_, 126
hybridism, 662
_somniferum Danebrog, 162
_somniferum monstruosum_, 371
_somniferum polycephalum_, Parris, 754
Parsley
crisped, 158, 181
Parsnip, water, 457
Pea-family, 344
Peach, 138, 226, 240
Peach-almond, 769
Pears, 79, 90, 134, 147, 152, 203, 283
Pearson, Karl, 716
Peas, sugar, 135, 158
_Pedicularis_, 410
_palustris_, 410
Pedigree-culture, 109
experimental, 547
_Pelargonium_, 272, 355
Peloria, definition of, 164
Peloric toad-flax
first record of, 466
origin of, 459, 464, 472
sterility of, 467
Pelorism
_Antirrhinum majus_ (see snapdragon)
_Digitalis purpurea_, 482
_Gloxinia_, 484, 485
labiates, 486
_Laburnum_, 485
_Lamium_, 486.
_Linaria_, see Toad-flax
_Linaria dalmatica_, 482
_Linaria vulgaris_, 464
orchids, 479, 486, 487
_Salvia_, 486
_Scrophularia nodosa_, 486
snapdragon, 481
toad-flax, 459-487
_Tropaeolum majus_, 485
_Uropedium Lindenii_, 487
wild sage, 486
_Peltaria alliacea_, 663
Pennywort, marsh, 668
Penzig, 638
Periodicity, law of, 365, 368, 721, 722
Periods, mutative, 706, 708
Periwinkles, 322
Persicaria, water, 433, 434, 435, 643
Petalomany, 330
Petunia, 491, 626
_Phacelia_, 420, 422, 820
_Phaseolus lunatus_, 592
_multiflorus_, 202
_nanus_, 202
_Phleum alpinum_, 696
_Phlox_, 232
_drummondi_, 161
_Phyllonoma ruscifolia_, 676
Physiologic units, 144, 153, 249
_Picris hieraoioides_, 411
Pimpernel, scarlet, 162
Pinacothec, Munich, 164
Pine, 368, 804
Pine-apples, 90, 134
Pinks, 178
_Pinus sylvestris_, 368
Pistillody in poppies, 369, 370, 372
Pitcher-plants, 671
Plankton, 711
_Plantago_, 53
lanceolata_, 520, 671, 684
Plantain, 684
Plater, 610
Plum, 79, 134, 789
beach, 58
Japanese, 58
purple-leaved, 619
_Plusia_, 204
_Poa alpina vivipara_, 684
_Podocarpus koraiana_, 169
_Polemonium coeruleum_, 282
_coeruleum album_, 200
_dissectum_, 161, 202
_Polygala_, 242
_Polygonum amphibium_, 432
var. _natans_ Moench, 433, 434
var. _terrestris_ Wench, 433, 434
_Convolvulus_, 419, 424
_viviparum_, 684
Polymorphy, 188
Pomegranate, 90
Pond-lily, yellow, 268
Poplar, fastigiate, 623, 624
Italian, 623
_Populus italica_, 622
_nigra_, 624
Poppy, 146, 151, 152, 163, 165, 241, 356, 640,
723
"Danebrog," 283, 291
garden, 661
"Mephisto," 283, 291
opium, 89, 189, 195, 198, 282, 291, 369,
371, 373, 379, 383, 391, 405, 406, 420, 452,
720, 789
pistillody in, 369
pistilloid, 508
polycephalous, 405
Potatoes, 765, 810
_Potentilla Tormentilla_, 52
Pre-Linnean attitude, 2
Primrose, 268, 372, 410
evening (see evening-primrose).
_Primula acaulis_, 52, 632
_elatior_, 52, 633, 635
_grandiflora_, 268
_imperialis_, 697
_japonica_, 410
_officinalis_, 52, 268, 633, 635
_variabilis_, 268
_veris_, 52, 633, 634
Prodromus (De Candolle) 370
Progression, 430, 705, 774, 775, 777, 779, 805
in evolution, 630
Propagation
asexual, 745, 751, 766, 767, 770, 774, 777
sexual, 745, 777
vegetative (see asexual)
Proskowetz, Em. von, 70
Prototype
definition of, 170
_Prunus_, 52
_cerasifera_, 619
_Mahaleb_, 617
_nana_, 612
_maritima_, 59
_Padus_, 617
_Pissardi_, 619
variation in, 56
_Pyrethrum roseum_, 511
_Pyrola_, 222, 661
Q
Quartile, 736, 737, 767
_Quercus pedunculata fastigata_, 596
Quetelet's law, 463, 716, 717, 725, 730, 734,
738, 748, 753, 759, 767, 775, 779, 780, 806
R
Races, inconstancy of improved, 770-797
Raciborsky, 682
Radishes, 325, 806
Ragwort, tansy, 157
Raisins, 134
Rameses, 697
_Ranunculus_, 331
_acris_, 331
_arvensis_, 282
_arvensis inermis_, 125
_asiaticus_, ,241
_bulbosus_, 357, 410, 740
Ra-n-Woser, King, 104
_Raphanus Raphanistrum_, 202, 424,520
_caudatus_, 202
Rasor, John, 588, 589
Raspberry, 268, 768
"Phenomenal," 268
"Primus," 269
Siberian, 269
Ratzeburg, 467
Raunkiaer
on variation in _Taraxacum_, 60
Recessive character, 280
Sports, 191, 715, 689
bud, 427
S
Sprenger, 610, 611
Stability, 155
Stahl, 611
_Stellaria Holostea apetala_, 585
Stocks, 146, 322, 328, 329, 332, 334, 336, 338,
432
Stock
"Brompton," 329
chamois-colored, 198
"Queen," 324
white, 160
Stork's-bill, white hemlock, 161
Strasburger, 196, 448
Strawberry, 158, 266, 342
"Gaillon," 135
"Giant of Zuidwijk," 614
one-leaved, 164, 596, 666
white, 158, 165
Striped flowers, 309, 374, 431, 606, 607
races, types of, 328
Struggle for life, 674, 571, 682, 702, 799, 803,
824, 825
St. Johnswort, 725
St. Sebastian, 164
Sub-species (see also Elementary species), 224, 225
Sugar-beets (see Beets, sugar)
Sugar-cane, 731, 752
"Black Manilla," 753
"Cheribon," 753, 755, 756
"Chunnic," 753
"Hawaii," 755, 756
seeds of, 754
"White Manilla," 752
Sundew, 268
Sunflower, 410, 425, 820
Sweet-flag, 222
Sweet-pea, 160, 776
Sweet William, 163, 282, 322, 648
twisted variety, 408, 648
Syncotyls, 417, 424
_Syringa vulgaris axurea plena_, 763
Systematic species, 12, 64, 101, 128
nature of, 54, 62
Systematic units, 61, 91
T
_Tagetes africana_, 510
_signata_, 612
"Talavera de Bellevue," 97
_Tanacetum vulgare_, 131, 132, 236
Tansy, 131, 132, 236
_Taraxacum_, 125, 126
officinale, 59, 411
_Tares_, 105
_Taxus_, 136
_baccata_, 169
_baccata fastigiata_, 170, 618
_minor_, 169
Teasels, 402, 642, 645, 674, 675
twisted, 405, 412, 446, 447, 643, 646, 647,
648, 819
_Tetragonia expansa_, 162
Theatre d'Agriculture, 72
Thibault, 618
Thomson, Sir William (see Kelvin, Lord)
Thorn-apples, 139, 142, 143, 145, 238, 283, 300,
452
thornless, 234
Thorn-broom, 457
_Thrincia hirta_, 411
Thuret, 38, 47, 49
Thyme, white creeping, 201
_Thymus Serphyllum album_, 201
_vulgaris_, 577
_Tilia parvifolia_, 355, 669
Toad-flax, 267, 282, 707
cross pollination of, 471
experiment with, described, 468
invisible dimorphous state of, 470, 471, 478
latent tendency to mutation in, 479
peloric, see Peloric toad flax
sterility of mutants, 477
unusual pelorism, 486
Tomato, 653
"Acme," 656, 657
"Mikado," 654
mutation of, 655
upright, 654
"Washington," 657
Tournefort
author of genera, 32
Tracy, W.W. 592
Trees, genealogic, 707, 708
Tricotyls, 416, 419, 420
_Trifolium incarnatum_, 352
_Triticum dicoccum_, 105
_Tropaeolum_, 193, 668
_majus_, pelorism of, 485
"True Exercises with Plants" (hunting), 490
Tulips, 149, 178, 274, 322
black, 620
Turnip, 244, 621
Twisted stems, 402, 403, 405, 413
Twisted varieties
atavists of, 406
U
_Ulex europaeus_, 140, 217
_Ulmus pedunculata_, 615
_pedunculata urticaefolia_, 615
Umbellifers, 457
_Umbilicus_, 669
Unger, 105
Unit-characters, 249, 261, 306, 307, 313, 658,
689, 715, 716
Urban, 265
_Uropedium lindenii_, 487
Utility, 685, 724
Utricularia, 672
V
_Vaccinium Myrtillus_, 577
Valerian, 402, 409, 648
twisted, 403
_Valeriana officinalis_, 402
_Vallisneria_, 684.
Van den Berg, 625
Van de Water, 614
Van Mons, 76, 77, 78, 806
Variability (see also Fluctuation ), 188, 190, 191
analogous, 244
apple, 75
asexual, 320
correlative, 142, 143, 148, 167
cultivated plants, 66
embryonic, 770, 771, 814
ever-recurring, 190
fluctuating (see also individual), 62, 142,
190, 233, 375, 416, 454, 698, 759, 762, 765,
766, 767, 770, 771, 789, 805, 814
fluctuating vs. mutability 569
homologous, 244
individual (see also fluctuating), 190, 716,
718, 746, 749, 770, 814
influence of mutation on, 335
kinds of, 715
nutrition and, 390, 391, 719, 771
parallel, 243
partial, 440, 444, 718, 746, 748, 753, 814,
816
repeated, 242
restricted, 598
sectional, 317
sexual, 320
sources of, 758
Variation
bud, 176, 178, 180, 284, 317, 318, 321, 338,
427, 750
definition of, 188
partial, 788, 789
seed, 750
spontaneous, 191
use of term, 189
Variegation, 426, 427
Varietal marks, origin of, 275
Varieties, 84, 95, 126, 127, 128, 129, 132, 142
broom-like, 618, 624
constancy of, 532
constant, 135
crosses of species with, 247, 277, 278, 281
elementary species vs. 459
ever-sporting, 178, 309, 310, 311, 312, 313,
321, 324, 328, 329, 332, 333, 334, 350, 358,
365, 368, 372, 399, 413, 420, 430, 431, 432,
434, 445, 606, 607, 628, 740, 789, 790, 795
fasciated (see Fasciated stems).
groups of, 606
horticultural, 607, 609
hybrid, 122, 190, 608
hybrids of, 210, 254, 255
inconstant, 135, 154; 155, 161
mutation of, 141
negative (retrogressive), 131, 132, 134, 224,
226, 238, 245, 277
positive, 131, 132, 134, 224, 238, 245
pure, 122, 190
retrograde, 14, 15, 16, 95, 121, 208, 430,
435, 606, 607
retrogressive (see negative).
seed, 122
single, 191
spontaneous crosses, 209
sporting (see inconstant)
stability of, 207
sterile, 622
types of, 142
variable, 606
vegetative, 122
weeping, 617
Variety, 130
definition of, 11, 12
elementary species vs. 141, 152, 154, 224,
243, 247, 251
origin of, 141, 152, 224
use of term, 189, 435
Variety-testing, 95, 97, 116, 119, 743, 799, 825
Varro, 106
Veitch & Sons, 272
Venus' looking-glass, 367
Verlot, 186, 612
Vernon, 132
_Vernonia cinerea_, 450
_Veronica longifolia_, 282, 284
_scutellata_, 139
_spicata nitens_, 126
_Viburnum Opulus_, 134, 239
Vicinism, 185, 188, 203, 205, 206, 213, 214, 776
definition of, 188, 192, 606
Vicinist, 199, 201
_Vicoa aurioulata_, 450
_Victoria regia_, 668
Villars
on _Draba verna_, 49
Vilmorin, 570, 607, 612, 622, 661, 662, 773, 775,
776; 792, 795, 796, 797, 806, 807, 810, 813,
818, 820
Vilmorin, Louis de, 72, 92, 93, 97, 108, 109, 110,
114, 185, 818
Vilmorin, Messrs., 322
_Vinca_, 242, 490
_minor_, 322
Vine, parsley-leaved, 179
_Viola_, 126, 546, 547, 692
_agrestis_, 45
_alpestris_, 40
_altaica_, 39
_anopetala_, 44
_arvensis_, 39, 40, 41, 44
_curtisepala_, 45
_striolata_, 45
_aurobadia_, 44
_caloarata_, 39
_cornuta_, 39, 281
_lutea_, 38
_lutescens_, 44
_nemausensis_, 45
_ornatissima_, 44
_palescens_, 45
_patens_, 45
_roseola_, 44
_segetatis_, 45
_stenochila_, 41
_tricolor_, 38, 40, 41, 44, 46
_ammotropha_, 41
_coniophila_, 41
_genuina_, 42
_versicolor_, 42
Violets, 63, 232, 233, 490
Violet, dame's, 322, 323, 411
long-spurred, 281
Virgil, 105, 106, 108
_Viscaria oculata_, 4, 648, 821
twisted variety, 408
_Vitis_, 52
Volckamer, 228
Von Lochow, 821, 822, 822
Von Rumker, 94
Von Wettstein, 448, 805
Vrolik, 164, 483
W
"Waare Oeffeninge der Planten" (Munting), 490
Wallace, 5, 7, 8, 30, 205
Wall-flower, 370, 371
Walnut, 243, 766
cut-leaved, 616
one-bladed, 666
Water-lilies, 668
Weber, 228
Weeping-willow, 180
crisped, 181
Weigelias, 740
_Wellingtonia_, 618
Wheat, 96, 98, 105, 113, 119, 283, 810, 823
bearded, 98
"Blue-stem," 117
"Galland," 100, 207
"Hopetown," 112, 112
"Hunter's," 111, 112
"Minnesota No. 169," 117
"Mungoswell's," 110, 111
"Pedigree," 109
"Pringle's," 114
"Rivett's bearded," 207
"Sheriff's bearded red," 114
"Sheriff's bearded white," 114
"White Hunter's," 112
Wheat-ear carnation, 227
White, C.A., 656, 657
White varieties, 577
Whitlow-grasses, 63, 118, 119
Whorls, ternate, 684
Wild sage (see Salvia)
Willdenow, 468, 666, 667
Williamson, 491
Willows, 135, 267
Willow
weeping (see Weeping-Willow)
Willow-herb, 268, 269, 682
Wintercress, 427
Wintergreen, 661
Wittmack, 682
Wittrock, 38, 40, 41, 42, 43, 44, 45, 46
Wooton, E.O., 140
Wormseed, 638
X
_Xanthium canadense_, 140
_commune_, 140, 152, 591
_commune Wootoni_, 22
Wootoni, 140, 152, 591
Y
Yarrow, 131, 132
Yew, 136, 169
pyramidal, 618
Z
_Zea Mays cryptosperma_, 641
_tunicata_, 641
_Zinnia_, 490
Zioberg, 466
Zocher & Co., 230
End of the Project Gutenberg EBook of Species and Varieties, Their Origin by
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