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Title: Problems of Genetics
Author: William Bateson
Release Date: January 4, 2014 [EBook #44582]
Language: English
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YALE UNIVERSITY
MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES
PROBLEMS OF GENETICS
SILLIMAN MEMORIAL LECTURES
PUBLISHED BY YALE UNIVERSITY PRESS
ELECTRICITY AND MATTER. _By_ JOSEPH JOHN THOMSON,
D.SC., LL.D., PH.D., F.R.S., _Fellow of Trinity College,
Cambridge, Cavendish Professor of Experimental Physics, Cambridge_.
_Price $1.25 net; postage 10 cents extra._
THE INTEGRATIVE ACTION OF THE NERVOUS SYSTEM.
_By_ CHARLES S. SHERRINGTON,
D.SC., M.D., HON. LL.D., TOR., F.R.S.,
_Holt Professor of Physiology in the University of Liverpool_.
_Price $3.50 net; postage 25 cents extra._
RADIOACTIVE TRANSFORMATIONS. _By_ ERNEST RUTHERFORD,
D.SC., LL.D., F.R.S., _Macdonald Professor of Physics,
McGill University_.
_Price $3.50 net; postage 22 cents extra._
EXPERIMENTAL AND THEORETICAL APPLICATIONS OF
THERMODYNAMICS TO CHEMISTRY.
_By_ DR. WALTHER NERNST, _Professor and Director of the
Institute of Physical Chemistry in the University of Berlin_.
_Price $1.25 net; postage 10 cents extra._
THE PROBLEMS OF GENETICS. _By_ WILLIAM BATESON, M.A.,
F.R.S., _Director of the John Innes Horticultural Institution,
Merton Park, Surrey, England_.
_Price $4.00 net; postage 25 cents extra._
STELLAR MOTIONS.
WITH SPECIAL REFERENCE TO MOTIONS DETERMINED BY MEANS OF
THE SPECTROGRAPH. _By_ WILLIAM WALLACE CAMPBELL, SC.D., LL.D.,
_Director of the Lick Observatory, University of California_.
_Price $4.00 net; postage 30 cents extra._
THEORIES OF SOLUTIONS. _By_ SVANTE AUGUST ARRHENIUS,
PH.D., SC.D., M.D., _Director of the Physico-Chemical
Department of the Nobel Institute, Stockholm, Sweden_.
_Price $2.25 net; postage 15 cents extra._
IRRITABILITY.
A PHYSIOLOGICAL ANALYSIS OF THE GENERAL EFFECT OF
STIMULI IN LIVING SUBSTANCES.
_By_ MAX VERWORN,
_Professor at Bonn Physiological Institute_.
_Price $3.50 net; postage 20 cents extra._
THE EVOLUTION OF MODERN MEDICINE.
_By_ SIR WILLIAM OSLER, BART., M.D., LL.D., SC.D.,
_Regius Professor of Medicine, Oxford University_.
_Price $3.00 net; postage 40 cents extra._
PROBLEMS OF GENETICS
BY
WILLIAM BATESON, M.A., F.R.S.
DIRECTOR OF THE JOHN INNES HORTICULTURAL INSTITUTION,
HON. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE,
AND FORMERLY PROFESSOR OF BIOLOGY IN THE UNIVERSITY
_WITH ILLUSTRATIONS_
[Illustration]
NEW HAVEN: YALE UNIVERSITY PRESS
LONDON: HUMPHREY MILFORD
OXFORD UNIVERSITY PRESS
MCMXIII
Copyright, 1913
By YALE UNIVERSITY
First printed August, 1913, 1000 copies
[** Transcriber's Note:
Underscores "_" before and after a word or phrase indicate ITALICS
in the original text.
Hyphenation was used inconsistently by the author and has been
left as in the original text. ]
THE SILLIMAN FOUNDATION
In the year 1883 a legacy of about eighty-five thousand dollars was left
to the President and Fellows of Yale College in the city of New Haven,
to be held in trust, as a gift from her children, in memory of their
beloved and honored mother, Mrs. Hepsa Ely Silliman.
On this foundation Yale College was requested and directed to establish
an annual course of lectures designed to illustrate the presence and
providence, the wisdom and goodness of God, as manifested in the natural
and moral world. These were to be designated as the Mrs. Hepsa Ely
Silliman Memorial Lectures. It was the belief of the testator that any
orderly presentation of the facts of nature or history contributed
to the end of this foundation more effectively than any attempt to
emphasize the elements of doctrine or of creed; and he therefore
provided that lectures on dogmatic or polemical theology should be
excluded from the scope of this foundation, and that the subjects should
be selected rather from the domains of natural science and history,
giving special prominence to astronomy, chemistry, geology, and anatomy.
It was further directed that each annual course should be made the basis
of a volume to form part of a series constituting a memorial to Mrs.
Silliman. The memorial fund came into the possession of the Corporation
of Yale University in the year 1901; and the present volume constitutes
the fifth of the series of memorial lectures.
PREFACE
This book gives the substance of a series of lectures delivered in Yale
University, where I had the privilege of holding the office of Silliman
Lecturer in 1907.
The delay in publication was brought about by a variety of causes.
Inasmuch as the purpose of the lectures is to discuss some of the wider
problems of biology in the light of knowledge acquired by Mendelian
methods of analysis, it was essential that a fairly full account of
the conclusions established by them should first be undertaken and I
therefore postponed the present work till a book on Mendel's Principles
had been completed.
On attempting a more general discussion of the bearing of the phenomena
on the theory of Evolution, I found myself continually hindered by the
consciousness that such treatment is premature, and by doubt whether
it were not better that the debate should for the present stand
indefinitely adjourned. That species have come into existence by an
evolutionary process no one seriously doubts; but few who are familiar
with the facts that genetic research has revealed are now inclined to
speculate as to the manner by which the process has been accomplished.
Our knowledge of the nature and properties of living things is far too
meagre to justify any such attempts. Suggestions of course can be made:
though, however, these ideas may have a stimulating value in the lecture
room, they look weak and thin when set out in print. The work which may
one day give them a body has yet to be done.
The development of negations is always an ungrateful task apt to be
postponed for the positive business of experiment. Such work is happily
now going forward in most of the centers of scientific life. Of many
of the subjects here treated we already know more than we did in 1907.
The delay in production has made it possible to incorporate these new
contributions.
The book makes no pretence at being a treatise and the number of
illustrative cases has been kept within a moderate compass. A good many
of the examples have been chosen from American natural history, as being
appropriate to a book intended primarily for American readers. The facts
are largely given on the authority of others, and I wish to express my
gratitude for the abundant assistance received from American colleagues,
especially from the staffs of the American Museum in New York, and of
the Boston Museum of Natural History. In connexion with the particular
subjects personal acknowledgments are made.
Dr. F. M. Chapman was so good as to supervise the preparation of the
coloured Plate of _Colaptes_, and to authorize the loan of the Plate
representing the various forms of _Helminthophila_, which is taken from
his _North American Warblers_.
I am under obligation to Messrs. Macmillan & Co., for permission to
reproduce several figures from _Materials for the Study of Variation_,
illustrating subjects which I wished to treat in new associations, and
to M. Leduc for leave to use Fig. 9.
In conclusion I thank my friends in Yale for the high honour they did me
by their invitation to contribute to the series of Silliman Lectures,
and for much kindness received during a delightful sojourn in that
genial home of learning.
TABLE OF CONTENTS.
CHAPTER PAGE
I. INTRODUCTORY. THE PROBLEM OF SPECIES AND VARIETY 1
II. MERISTIC PHENOMENA 31
III. SEGMENTATION, ORGANIC AND MECHANICAL 60
IV. THE CLASSIFICATION OF VARIATION AND THE NATURE
OF SUBSTANTIVE VARIATION 83
NOTE TO CHAPTER IV 94
V. THE MUTATION THEORY 97
NOTE TO CHAPTER V 116
VI. VARIATION AND LOCALITY 118
VII. LOCAL DIFFERENTIATION--_continued_.
OVERLAPPING FORMS 146
VIII. LOCALLY DIFFERENTIATED FORMS--_continued_.
CLIMATIC VARIETIES 164
IX. THE EFFECTS OF CHANGED CONDITIONS 187
X. THE EFFECTS OF CHANGED CONDITIONS--_continued_.
THE CAUSES OF GENETIC VARIATION 212
XI. THE STERILITY OF HYBRIDS. CONCLUDING REMARKS 233
APPENDIX TO CHAPTER X 250
INDEX 251
PROBLEMS OF GENETICS
CHAPTER I
INTRODUCTORY
The purpose of these lectures is to discuss some of the familiar
phenomena of biology in the light of modern discoveries. In the last
decade of the nineteenth century many of us perceived that if any
serious advance was to be made with the group of problems generally
spoken of as the Theory of Evolution, methods of investigation must be
devised and applied of a kind more direct and more penetrating than
those which after the general acceptance of the Darwinian views had been
deemed adequate. Such methods obviously were to be found in a critical
and exhaustive study of the facts of variation and heredity, upon which
all conceptions of evolution are based. To construct a true synthetic
theory of Evolution it was necessary that variation and heredity instead
of being merely postulated as axioms should be minutely examined as
phenomena. Such a study Darwin himself had indeed tentatively begun, but
work of a more thorough and comprehensive quality was required. In the
conventional view which the orthodoxy of the day prescribed, the terms
variation and heredity stood for processes so vague and indefinite that
no analytical investigation of them could be contemplated. So soon,
however, as systematic inquiry into the natural facts was begun it was
at once found that the accepted ideas of variation were unfounded.
Variation was seen very frequently to be a definite and specific
phenomenon, affecting different forms of life in different ways, but
in all its diversity showing manifold and often obvious indications
of regularity. This observation was not in its essence novel. Several
examples of definite variation had been well known to Darwin and
others, but many, especially Darwin himself in his later years, had
nevertheless been disposed to depreciate the significance of such
facts. They consequently then lapsed into general disparagement. Upon
more careful inquiry the abundance of such phenomena proved to be far
greater than was currently supposed, and a discussion of their nature
brought into prominence a consideration of greater weight, namely that
the differences by which these definite or discontinuous variations are
constituted again and again approximate to and are comparable with the
class of differences by which species are distinguished from each other.
The interest of such observations could no longer be denied. The
more they were examined the more apparent it became that by means of
the facts of variation a new light was obtained on the physiological
composition and capabilities of living things. Genetics thus cease to be
merely a method of investigating theories of evolution or of the origin
of species but provide a novel and hitherto untried instrument by which
the nature of the living organism may be explored. Just as in the study
of non-living matter science began by regarding the external properties
of weight, opacity, colour, hardness, mode of occurrence, etc., noting
only such evidences of chemical attributes and powers as chance
spontaneously revealed; and much later proceeded to the discovery that
these casual manifestations of chemical properties, rightly interpreted,
afford a key to the intrinsic nature of the diversity of matter, so in
biology, having examined those features of living things which ordinary
observations can perceive, we come at last to realize that when studied
for their own sake the properties of living organisms in respect of
heredity and variation are indications of their inner nature and provide
evidences of that nature which can be obtained from no other source.
While such ideas were gradually forming in our minds, came the
rediscovery of Mendel's work. Investigations which before had only
been imagined as desirable now became easy to pursue, and questions
as to the genetic inter-relations and compositions of varieties can
now be definitely answered. Without prejudice to what the future may
disclose whether by way of limitation or extension of Mendelian method,
it can be declared with confidence and certainty that we have now the
means of beginning an analysis of living organisms, and distinguishing
many of the units or factors which essentially determine and cause the
development of their several attributes.
Briefly put, the essence of Mendelism lies in the discovery of the
existence of unit characters or factors. For an account of the Mendelian
method, how it is applied and what it has already accomplished,
reference must be made to other works.[1] With this part of the subject
I shall assume a sufficient acquaintance. In these lectures I have
rather set myself the task of considering how certain problems appear
when viewed from the standpoint to which the application of these
methods has led us. It is indeed somewhat premature to discuss such
questions. The work of Mendelian analysis is progressing with great
rapidity and anything I can say may very soon be superseded as out
of date. Nevertheless a discussion of this kind may be of at least
temporary service in directing inquiry to the points of special interest.
THE PROBLEM OF SPECIES AND VARIETY
Nowhere does our new knowledge of heredity and variation apply more
directly than to the problem what is a species and what is a variety? I
cannot assert that we are already in a position to answer this important
question, but as will presently appear, our mode of attack and the
answers we expect to receive are not those that were contemplated by our
predecessors. If we glance at the history of the scientific conception
of Species we find many signs that it was not till comparatively recent
times that the definiteness of species became a strict canon of the
scientific faith and that attempts were made to give precise limits
to that conception. When the diversity of living things began to be
accurately studied in the sixteenth and seventeenth centuries names
were applied in the loosest fashion, and in giving a name to an animal
or a plant the naturalists of those times had no ulterior intention.
Names were bestowed on those creatures about which the writer proposed
to speak. When Gesner or Aldrovandi refer to all the kinds of horses,
unicorns, dogs, mermaids, etc., which they had seen or read of, giving
to each a descriptive name, they do not mean to "elevate" each named
kind to "specific rank"; and if anyone had asked them what they meant by
a species, it is practically certain that they would have had not the
slightest idea what the question might imply, or any suspicion that it
raised a fundamental problem of nature.
Spontaneous generation being a matter of daily observation, then
unquestioned, and supernatural events of all kinds being commonly
reported by many witnesses, transmutation of species had no inherent
improbability. Matthioli,[2] for instance, did not expect to be charged
with heresy when he declared _Stirpium mutatio_ to be of ordinary
occurrence. After giving instances of induced modifications he wrote,
"Tantum enim in plantis naturae germanitas potest, ut non solum saepe
praedictos praestet effectus, sed etiam ut alteram in alteram stirpem
facile vertat, ut cassiam in cinnamomum, sisymbrium in mentham, triticum
in lolium, hordeum in avenam, et ocymum in serpyllum."
I do not know who first emphasized the need for a clear understanding
of the sense in which the term species is to be applied. In the second
half of the seventeenth century Ray shows some degree of concern on
this matter. In the introduction to the _Historia Plantarum_, 1686, he
discusses some of the difficulties and lays down the principle that
varieties which can be produced from the seed of the same plant are to
be regarded as belonging to one species, being, I believe, the first
to suggest this definition. That new species can come into existence
he denies as inconsistent with Genesis 2, in which it is declared that
God finished the work of Creation in six days. Nevertheless he does not
wholly discredit the possibility of a "transmutation" of species, such
that one species may as an exceptional occurrence give rise by seed
to another and nearly allied species. Of such a phenomenon he gives
illustrations the authenticity of which he says he is, against his will,
compelled to admit. He adds that some might doubt whether in the cases
quoted the two forms concerned are really distinct species, but the
passage is none the less of value for it shews that the conception of
species as being distinct unchangeable entities was not to Ray the dogma
sacrosanct and unquestionable which it afterwards became.[3]
In the beginning of the eighteenth century Marchant,[4] having observed
the sudden appearance of a lacinated variety of _Mercurialis_, makes the
suggestion that species in general may have arisen by similar mutations.
Indeed from various passages it is manifest that to the authors of the
seventeenth and early eighteenth centuries species appeared simply as
groups more or less definite, the boundaries of which it was unnecessary
to determine with great exactitude. Such views were in accord with the
general scientific conception of the time. The mutability of species is
for example sometimes likened (see for instance Sharrock, loc. cit.) to
the metamorphoses of insects, and it is to be remembered that the search
for the Philosopher's Stone by which the transmutation of metals was to
be effected had only recently fallen into discredit as a pursuit.
The notion indeed of a peculiar, fixed meaning to be attached to species
as distinct from variety is I think but rarely to be found categorically
expressed in prae-Linnaean writings.
But with the appearance of the _Systema Naturae_ a great change
supervened. Linnaeus was before all a man of order. Foreseeing the
immense practical gain to science that must come from a codification of
nomenclature, he invented such a system.
It is not in question that Linnaeus did great things for us and made
Natural History a manageable and accessible collection of facts instead
of a disorderly heap; but orderliness of mind has another side, and
inventors and interpreters of systems soon attribute to them a force and
a precision which in fact they have not.
The systematist is primarily a giver of names, as Ray with his broader
views perceived. Linnaeus too in the exordium to the _Systema Naturae_
naively remarks, that he is setting out to continue the work which
Adam began in the Golden Age, to give names to the living creatures.
Naming however involves very delicate processes of mind and of logic.
Carried out by the light of meagre and imperfect knowledge it entails
all the mischievous consequences of premature definition, and promotes
facile illusions of finality. So was it with the Linnaean system. An
interesting piece of biological history might be written respecting the
growth and gradual hardening of the conception of Species. To readers
of Linnaeus's own writings it is well known that his views cannot be
summarized in a few words. Expressed as they were at various times
during a long life and in various connexions, they present those divers
inconsistencies which commonly reflect a mind retaining the power of
development. Nothing certainly could be clearer than the often quoted
declaration of the _Philosophia Botanica_, "Species tot numeramus quot
diversae formae in principio sunt creatae," with the associated passage
"Varietates sunt plantae ejusdem speciei mutatae a caussa quacunque
occasionali." Those sayings however do not stand alone. In several
places, notably in the famous dissertation on the peloric _Linaria_
he explicitly contemplates the possibility that new species may arise
by crossing, declaring nevertheless that he thinks such an event to
be improbable. In that essay he refers to Marchant's observation on a
laciniate _Mercurialis_, but though he states clearly that that plant
should only be regarded as a variety of the normal, he does not express
any opinion that the contemporary genesis of new species must be an
impossibility. In the later dissertation on Hybrid Plants he returns to
the same topic. Again though he states the belief that species cannot
be generated by cross-breedings, he treats the subject not as heretical
absurdity but as one deserving respectful consideration.
The significance of the aphorisms that precede the lectures on the
Natural Orders is not easy to apprehend. These are expressed with the
utmost formality, and we cannot doubt that in them we have Linnaeus's
own words, though for the record we are dependent on the transcripts of
his pupils.
The text of the first five is as follows:
1. Creator T. O. in primordio vestiit Vegetabile _Medullare_
principiis constitutivis diversi _Corticalis_ unde tot difformia
individua, quot _Ordines_ Naturales prognata.
2. _Classicas_ has (1) plantas Omnipotens miscuit inter se,
unde tot _Genera_ ordinum, quot inde plantae.
3. _Genericas_ has (2) miscuit Natura, unde tot _Species_
congeneres quot hodie existunt.
4. _Species_ has miscuit Casus, unde totidem quot passim
occurrunt, _Varietates_.
5. Suadent haec (1-4) Creatoris leges a simplicibus ad
Composita.
Naturae leges generationis in hybridis.
Hominis leges ex observatis a posteriori.
I am not clear as to the parts assigned in the first sentence
respectively to the "_Medulla_" and the "_Cortex_," beyond that Linnaeus
conceived that multiformity was first brought about by diversity in
the "_Cortex_." The passage is rendered still more obscure if read in
connection with the essay on "_Generatio Ambigena_," where he expresses
the conviction that the _Medulla_ is contributed by the mother, and the
_Cortex_ by the father, both in plants and animals.[5]
But however that may be, he regards this original diversity as resulting
in the constitution of the Natural Orders, each represented by one
individual.
In the second aphorism the Omnipotent is represented as creating the
genera by intermixing the individual _plantae classicae_, or prototypes
of the Natural Orders.
The third statement is the most remarkable, for in it he declares that
Species were formed by the act of Nature, who by inter-mixing the genera
produced _Species congeneres_, namely species inside each genus, to the
number which now exist. Lastly, Chance or Accident, intermixing the
species, produced as many varieties as there are about us.
Linnaeus thus evidently regarded the intermixing of an originally
limited number of types as the sufficient cause of all subsequent
diversity, and it is clear that he draws an antithesis between
_Creator_, _Natura_, and _Casus_, assigning to each a special part
in the operations. The acts resulting in the formation of genera are
obviously regarded as completed within the days of the Creation, but the
words do not definitely show that the parts played by Nature and Chance
were so limited.
Recently also E. L. Greene[6] has called attention to some curious
utterances buried in the _Species Plantarum_, in which Linnaeus refers
to intermediate and transitional species, using language that even
suggests evolutionary proclivities of a modern kind, and it is not easy
to interpret them otherwise.
Whatever Linnaeus himself believed to be the truth, the effect of his
writings was to induce a conviction that the species of animals and
plants were immutably fixed. Linnaeus had reduced the whole mass of
names to order and the old fantastical transformations with the growth
of knowledge had lapsed into discredit; the fixity of species was
taken for granted, but not till the overt proclamation of evolutionary
doctrine by Lamarck do we find the strenuous and passionate assertions
of immutability characteristic of the first half of the nineteenth
century.
It is not to be supposed that the champions of fixity were unacquainted
with varietal differences and with the problem thus created, but in
their view these difficulties were apparent merely, and by sufficiently
careful observation they supposed that the critical and permanent
distinctions of the true species could be discovered, and the
impermanent variations detected and set aside.
This at all events was the opinion formed by the great body of
naturalists at the end of the eighteenth and beginning of the nineteenth
centuries, and to all intents and purposes in spite of the growth of
evolutionary ideas, it remains the guiding principle of systematists
to the present day. There are 'good species' and 'bad species' and the
systematists of Europe and America spend most of their time in making
and debating them.
In some of its aspects the problem of course confronted earlier
naturalists. Parkinson for instance (1640) in introducing his treatment
of _Hieracium_ wrote, "To set forth the whole family of the Hawkeweedes
in due forme and order is such a world of worke that I am in much doubt
of mine own abilitie, it having lyen heavie on his shoudiers that
hath already waded through them ... for such a multitude of varieties
in forme pertaining to one herbe is not to be found againe in _rerum
natura_ as I thinke," and the same idea, that the difficulty lay rather
in man's imperfect powers of discrimination than in the nature of the
materials to be discriminated, is reflected in many treatises early and
late.
It was however with the great ouburst of scientific activity which
followed Linnaeus that the difficulty became acute. Simultaneously
vast masses of new material were being collected from all parts of the
world into the museums, and the products of the older countries were
re-examined with a fresh zeal and on a scale of quantity previously
unattempted. But the problem how to name the forms and where to draw
lines, how much should be included under one name and where a new name
was required, all this was felt, rather as a cataloguer's difficulty
than as a physiological problem. And so we still hear on the one hand
of the confusion caused by excessive "splitting" and subdivisions, and
on the other of the uncritical "lumpers" who associate together under
one name forms which another collector or observer would like to see
distinguished.
In spite of Darwin's hopes, the acceptance of his views has led to no
real improvement--scarcely indeed to any change at all in either the
practice or aims of systematists. In a famous passage in the _Origin_ he
confidently declares that when his interpretation is generally adopted
"Systematists will be able to pursue their labours as at present; but
they will not be incessantly haunted by the shadowy doubt whether this
or that form be a true species. This, I feel sure, and I speak after
experience, will be no slight relief. The endless disputes whether or
not some fifty species of British brambles are good species will cease."
Those disputes nevertheless proceed almost exactly as before. It is
true that biologists in general do not, as formerly, participate in
these discussions because they have abandoned systematics altogether;
but those who are engaged in the actual work of naming and cataloguing
animals and plants usually debate the old questions in the old way.
There is still the same divergence of opinion and of practice, some
inclining to make much of small differences, others to neglect them.
Not only does the work of the systematists as a whole proceed as if
Darwin had never written but their attitude towards these problems is
but little changed. In support of this statement I may refer to several
British Museum Catalogues, much of the _Biologia Centrali-Americana_,
Ridgway's _Birds of North America_, the _Fauna Hawaiensis_, indeed to
almost any of the most important systematic publications of England,
America, or any other country. These works are compiled by the most
proficient systematists of all countries in the several groups, but
with rare exceptions they show little misgiving as to the fundamental
reality of specific differences. That the systematists consider the
species-unit as of primary importance is shown by the fact that the
whole business of collection and distribution of specimens is arranged
with regard to it.
Almost always the collections are arranged in such a way that the
phenomena of variation are masked. Forms intermediate between two
species are, if possible, sorted into separate boxes under a third
specific name. If a species is liable to be constantly associated
with a mutational form, the mutants are picked out, regardless of the
circumstances of their origin, from the samples among which they were
captured, and put apart under a special name. Only by a minute study
of the original labels of the specimens and by redistributing them
according to locality and dates, can their natural relations be traced.
The published accounts of these collections often take no notice of
variations, others make them the subject of casual reference. Very few
indeed treat them as of much importance. From such indications it is
surely evident that the systematists attach to the conception of species
a significance altogether different from that which Darwin contemplated.
I am well aware that some very eminent systematists regard the whole
problem as solved. They hold as Darwin did that specific diversity
has no physiological foundation or causation apart from fitness, and
that species are impermanent groups, the delimitations of which are
ultimately determined by environmental exigency or "fitness." The
specific diversity of living things is thus regarded as being something
quite different in nature from the specific diversity of inorganic
substances. In practice those who share these opinions are, as might
be anticipated, to be found among the 'lumpers' rather than among the
'splitters.' In their work, certainly, the Darwinian theory is actually
followed as a guiding principle; unanalysed inter-gradations of all
kinds are accepted as impugning the integrity of species; the underlying
physiological problem is forgotten, and while the product is almost
valueless as a contribution to biological research, I can scarcely
suppose that it aids greatly in the advances of other branches of our
science.
But why is it that, with these exceptions, the consequences of the
admittedly general acceptance of a theory of evolution are so little
reflected in the systematic treatment of living things? Surely the
reason is that though the systematist may be convinced of the general
truth of the evolution theory at large, he is still of opinion that
species are really distinct things. For him there are still 'good'
species and 'bad' species and his experience tells him that the
distinction between the two is not simply a question of degree or a
matter of opinion.
To some it may seem that this is mere perversity, a refusal to see
obvious truth, a manifestation of the spirit of the collector rather
than of the naturalist. But while recognising that from a magnification
of the conception of species the systematists are occasionally led
into absurdity I do not think the grounds for their belief have in
recent times been examined with the consideration they deserve. The
phenomenon of specific diversity is manifested to a similar degree by
living things belonging to all the great groups, from the highest to the
lowest, Vertebrates, Invertebrates, Protozoa, Vascular Plants, Algae,
and Bacteria, all present diversities of such a kind that among them the
existence of specific differences can on the whole be recognised with
a similar degree of success and with very similar limitations. In all
these groups there are many species quite definite and unmistakable, and
others practically indefinite. The universal presence of specificity,
as we may call it, similarly limited and characterised, is one of its
most remarkable features. Not only is this specificity thus universally
present among the different forms of life, but it manifests itself
in respect of the most diverse characteristics which living things
display. Species may thus be distinguished by peculiarities of form,
of number, of geometrical arrangement, of chemical constitution and
properties, of sexual differentiation, of development, and of many
other properties. In any one or in several of these features together,
species may be found distinguished from other species. It is also to be
observed that the definiteness of these distinctions has no essential
dependence on the nature of the characteristic which manifests them.
It is for example sometimes said that colour-distinctions are of small
systematic importance, but every systematist is familiar with examples
(like that of the wild species of _Gallus_) in which colours though
complex, show very little variation. On the other hand features of
structure, sexual differentiation, and other attributes which by our
standards are estimated as essential, may be declared to show much
variation or little, not according to any principle which can be
detected, but simply as the attention happens to be applied to one
species or group of species, or to another. In many groups of animals
and plants observers have hit upon characters which were for a time
thought to be finally diagnostic of species. The Lepidoptera and Diptera
for instance, have been re-classified according to their neuration.
Through a considerable range of forms determinations may be easily made
on these characters, but as is now well known, neuration is no more
immune from variation than any other feature of organisation, and in
some species great variability is the rule. Again it was once believed
by some that the genitalia of the Lepidoptera provided a basis of final
determination--with a similar sequel. In some groups, for example
the Lycaenidae, or the Hesperidae, there are forms almost or quite
indistinguishable on external examination, but a glance at the genitalia
suffices to distinguish numerous species, while on the contrary among
Pieridae a great range of species show scarcely any difference in these
respects: and again in occasional species the genitalia show very
considerable variations.
The proposition that animals and plants are on the whole divisible into
definite and recognisable species is an approximation to the truth.
Such a statement is readily defensible, whereas to assert the contrary
would be palpably absurd. For example, a very competent authority
lately wrote: "In the whole Lepidopterous fauna of England there is no
species of really uncertain limits."[7] Others may be disposed to make
certain reservations, but such exceptions would be so few as scarcely to
impair the validity of the general statement. The declaration might be
extended to other orders and other lands.
We know, of course, that the phenomenon of specific diversity is
complicated by local differentiation: that, in general, forms which
cannot disperse themselves freely exhibit a multitude of local races,
and that of these some are obviously adaptative, and that a few even owe
their peculiarity to direct environmental effects. Every systematist
also is perfectly aware that in dealing with collections from little
explored countries the occurrence of polymorphism or even of sporadic
variation may make the practical business of distinguishing the species
difficult and perhaps for the time impossible; still, conceding that a
great part of the diversity is due to geographical differentiation, and
that some is sporadic variation, our experience of our own floras and
faunas encourages the belief that if we were thoroughly familiar with
these exotic productions it would usually be possible to assign their
specific limitations with an approach to certainty.
For apart from any question of the justice of these wider inferences,
if we examine the phenomenon of specificity as it appears in those
examples which are nearest to hand, surely we find signs in plenty that
specific distinction is no mere consequence of Natural Selection. The
strength of this proposition has lain mainly in the appeal to ignorance.
Steadily with the growth of knowledge has its cogency diminished, and
such a belief could only have been formulated at a time when the facts
of variation were unknown.
In Darwin's time no serious attempt had been made to examine the
manifestations of variability. A vast assemblage of miscellaneous facts
could formerly be adduced as seemingly comparable illustrations of the
phenomenon "Variation." Time has shown this mass of evidence to be
capable of analysis. When first promulgated it produced the impression
that variability was a phenomenon generally distributed amongst living
things in such a way that the specific divisions must be arbitrary.
When this variability is sorted out, and is seen to be in part a result
of hybridisation, in part a consequence of the persistence of hybrids
by parthenogenetic reproduction, a polymorphism due to the continued
presence of individuals representing various combinations of Mendelian
allelomorphs, partly also the transient effect of alteration in external
circumstances, we see how cautious we must be in drawing inferences as
to the indefiniteness of specific limits from a bare knowledge that
intermediates exist. Conversely, from the accident of collocation or
from a misleading resemblance in features we deem essential, forms
genetically distinct are often confounded together, and thus the
divergence of such forms in their other features, which we declare
to be non-essential, passes as an example of variation. Lastly, and
this is perhaps the most fertile of all the sources of confusion, the
impression of the indefiniteness of species is created by the existence
of numerous local forms, isolated geographically from each other, forms
whose differences may be referable to any one of the categories I have
enumerated.
The advance has been from many sides. Something has come from the work
of systematists, something from cultural experiments, something from
the direct study of variation as it appears in nature, but progress
is especially due to experimental investigation of heredity. From all
these lines of inquiry we get the same answer; that what the naturalists
of fifty years ago regarded as variation is not one phenomenon but
many, and that what they would have adduced as evidence against the
definiteness of species may not in fact be capable of this construction
at all.
If we may once more introduce a physical analogy, the distinctions with
which the systematic naturalist is concerned in the study of living
things are as multifarious as those by which chemists were confronted in
the early days of their science. Diversities due to mechanical mixtures,
to allotropy, to differences of temperature and pressure, or to degree
of hydration, had all to be severally distinguished before the essential
diversity due to variety of chemical constitution stood out clearly,
and I surmise that not till a stricter analysis of the diversities of
animals and plants has been made on a comprehensive scale, shall we be
in a position to declare with any confidence whether there is or is not
a natural and physiological distinction between species and variety.
As I have said above, it is in the cases nearest to hand that the
problem may be most effectively studied. Comparison between forms from
dissimilar situations contributes something; but it is by a close
examination of the behaviour, especially the genetic behaviour, of
familiar species when living in the presence of their nearest allies
that the most direct light on the problem is to be obtained. I cannot
understand the attitude of those who, contemplating such facts as this
examination elicits, can complacently declare that specific difference
is a mere question of degree. With the spread of evolutionary ideas to
speak much of the fixity of species has become unfashionable, and yet
how striking and inscrutable are the manifestations of that fixity!
Consider the group of species composing the _agrestis_ section of the
genus _Veronica_, namely _Tournefortii_, _agrestis_, and _polita_.
These three grow side by side in my garden, as they do in suitable
situations over a vast area of the temperate regions. I have for years
noticed them with some care and become familiar with their distinctions
and resemblances. Never is there any real doubt as to the identity of
any plant. The species show some variability, but I have never seen
one which assumed any of the distinguishing features of the others. A
glance at the fruits decides at once to which species a plant belongs. I
find it impossible to believe that the fixity of these distinctions is
directly dependent on their value as aids in the struggle for existence.
The mode of existence of the three forms in so far as we can tell is
closely similar. By whatever standard we reckon systematic affinity I
suppose we shall agree that these species come very near indeed to each
other. Bentham even takes the view that _polita_ is a mere variety of
_agrestis_.
Now in such cases as this it has been argued that the specific
features of the several types have been separately developed in as
many distinct localities, and that their present association is due
to subsequent redistribution. Of these Veronicas indeed we know that
one, _Tournefortii_ (= _Buxbaumii_) is as a matter of fact a recent
introduction from the east.[8] But this course of argument leads to
still further difficulties. For if it is true that the peculiarities
of the several species have been perfected and preserved on account
of their survival-value to their possessors, it follows that there
must be many ways of attaining the same result. But since sufficient
adaptation may be ensured in so many ways, the disappearance of the
common parent of these forms is difficult to understand. Obviously it
must have been a plant very similar in general construction to its
modern representatives. Like them it must have been an annual weed, with
an organisation conformable to that mode of life. Why then, after having
been duly perfected for that existence should it have been entirely
superseded in favour of a number of other distinct contrivances for
doing the same thing, and--if a gradual transition be predicated--not
only by them, but by each intermediate stage between them and the
original progenitor? Surely the obvious inference from such facts is
that the burden cast upon the theory of gradual selection is far greater
than it can bear; that adaptation is not in practice a very close fit,
and that the distinctions between these several species of Veronica have
not arisen on account of their survival-value but rather because none
of their diversities was so damaging as to lead to the extermination
of its possessor. When we see these various Veronicas each rigidly
reproducing its parental type, all comfortably surviving in competition
with each other, are we not forced to the conclusion that _tolerance_
has as much to do with the diversity of species as the stringency of
Selection? Certainly these species owe their continued existence to the
fact that they are each good enough to live, but how shall we refer the
distinctions between them directly or indirectly to the determination of
Natural Selection?
The control of Selection is loose while the conformity to specific
distinction is often very strict and precise, and no less so even when
several closely related species co-exist in the same area and in the
same circumstances.
The theory of Selection fails at exactly the point where it was devised
to help: _Specific_ distinction.
Let us examine a somewhat different set of facts in the case of another
pair of nearly allied species _Lychnis diurna_ and _vespertina_. The
two plants have much in common. Both are dioecious perennials, with
somewhat similar flowers, the one crimson, the other white. Each however
has its peculiarities which are discernible in almost any part of its
structure, whether flower, leaf, fruit or seed, distinctions which
would enable a person thoroughly familiar with the plants to determine
at once from which species even a small piece had been taken. There is
so much resemblance however as readily to support the surmise that the
two were mere varieties of one species. Bentham, following Linnaeus,
in fact actually makes this suggestion, with what propriety we will
afterwards consider. Now this case is typical of many. The two forms
have a wide distribution, occurring sometimes separately, sometimes
in juxtaposition. _L. diurna_ is a plant of hedgerows and sheltered
situations. _L. vespertina_ is common in fields and open spaces, where
_diurna_ is hardly ever found; but not rarely _vespertina_ occurs in
association with _diurna_ in the places which that plant frequents. In
this case I do not doubt that we have to do with organisms of somewhat
different aptitudes. That _L. vespertina_ has powers which _diurna_
has not is shown very clearly by the fact that _diurna_ is sometimes
entirely absent from areas where _vespertina_ can abound.[9] But in
order to understand the true genetic relations of the two plants to
each other it is necessary to observe their behaviour when they meet
as they not unfrequently do. If the _Lychnis_ population of such a
locality be examined it will be found to consist of many undoubted
and unmodified _diurna_, a number--sometimes few, sometimes many--of
similarly unmodified _vespertina_, and an uncertain but usually rather
small proportion of plants obviously hybrids between the two. How
is it possible to reconcile these facts with the view that specific
distinction has no natural basis apart from environmental exigency?
Darwinian orthodoxy suggests that by a gradual process of Natural
Selection either one of these two types was evolved from the other,
or both from a third type. I cannot imagine that anyone familiar with
the facts would propose the first hypothesis in the case of _Lychnis_,
nor can I conceive of any process, whether gradual or sudden, by
which _diurna_ could have come out of _vespertina_, or _vespertina_
out of _diurna_. Both however may no doubt have been derived from
some original third type. It is conceivable that _Lychnis macrocarpa_
of Boissier, a native of Southern Spain and Morocco, may be this
original form. This species is said to combine a white flower (like
that of _L. vespertina_), with capsule-teeth rolled back (like those
of _diurna_).[10] But whatever the common progenitor may have been, if
we are to believe that these two species have been evolved from it by
a gradual process of Natural Selection based on adaptation, enormous
assumptions must be made regarding the special fitness of these two
forms and the special unfitness of the common parent, and these
assumptions must be specially invoked and repeated for each several
feature of structure or habits distinguishing the three forms.
Why, if the common parent was strong enough to live to give rise to
these two species, is it either altogether lost now, or at least absent
from the whole of Northern Europe? Its two putative descendants, though
so distinct from each other, are, as we have seen, able often to
occupy the same ground. If they were gradually derived from a common
progenitor--necessarily very like themselves--can we believe that
this original form should always, in all the diversities of soil and
situation which they inhabit, be unable to exist? Some one may fancy
that the hybrids which are found in the situations occupied by both
forms are this original parental species. But nothing can be more
certain than that these plants are simply heterozygous combinations
made by the union of gametes bearing the characters of _diurna_ and
_vespertina_.[11] For they may be reproduced exactly in F_{1} or in
later generations of that cross when it is artificially made; when bred
from their families exhibit palpable phenomena of segregation more or
less complex; and usually, if perhaps not always, they are partially
sterile.[12] In a locality on the Norfolk coast that I know well,
there is a strip of rough ground chiefly sand-bank, which runs along
the shore. This ground is full of _vespertina_. Not a hundred yards
inland is a lane containing _diurna_, and among the _vespertina_ on
the sand-bank are always some of the hybrid form, doubtless the result
of fertilisation from the neighbouring _diurna_ population. Seed saved
from these hybrids gave _vespertina_ and hybrids again, having obviously
been fertilised by other _vespertina_ or by other hybrids, and I have no
doubt that such hybrid plants if fertilised by _diurna_ would have shown
some _diurna_ offspring. The absence of _diurna_ in such localities may
fairly be construed as an indication that _diurna_ is there at a real
disadvantage in the competition for life.
But if, admitting this, we proceed to consider how the special aptitude
of _vespertina_ is constituted, or what it is that puts _diurna_ at
a disadvantage, we find ourselves quite unable to show the slightest
connexion between the success of one or the failure of the other on the
one hand, and _the specific characteristics_ which distinguish the two
forms on the other. The orthodox Selectionist would, as usual, appeal to
ignorance. We ask what can _vespertina_ gain by its white flowers, its
more lanceolate leaves, its grey seeds, its almost erect capsule-teeth,
its longer fruits, which _diurna_ loses by reason of its red flowers,
more ovate leaves, dark seeds, capsule-teeth rolled back, and shorter
fruits? We are told that each of these things _may_ affect the viability
of their possessors. We cannot assert that this is untrue, but we should
like to have evidence that it is true. The same problem confronts us
in thousands upon thousands of examples, and as time goes on we begin
to feel that speculative appeals to ignorance, though dialectically
admissible, provide an insufficient basis for a proposition which,
if granted, is to become the foundation of a vast scheme of positive
construction.
One thing must be abundantly clear to all, that to treat two forms so
profoundly different as one, because intermediates of unknown nature can
be shown to exist between them, is a mere shirking of the difficulties,
and this course indeed creates artificial obstacles in the way of those
who are seeking to discover the origin of organic diversity.
In the enthusiasm with which evolutionary ideas were received the
specificity of living things was almost forgotten. The exactitude with
which the members of a species so often conform in the diagnostic,
specific features passed out of account; and the scientific world by
dwelling with a constant emphasis on the fact of variability, persuaded
itself readily that species had after all been a mere figment of
the human mind. Without presuming to declare what future research
only can reveal, I anticipate that, when variation has been properly
examined and the several kinds of variability have been successfully
distinguished according to their respective natures, the result will
render the natural definiteness of species increasingly apparent.
Formerly in such a case as that of the two _Lychnis_ species, the series
of "intermediates" was taken to be a palpable proof that _vespertina_
"graded" to _diurna_. It is this fact, doubtless, upon which Bentham
would have relied in suggesting that both may be one species.[13]
Genetic tests, though as yet imperfectly applied, make it almost certain
that these inter-grading forms are not in any true sense variations
from either species in the direction of the other, but combinations of
elements derived from both.
The points in which very closely allied species are distinguished
from each other may be found in the most diverse features of their
organisation. Sometimes specific difference is to be seen in a
character which we can believe to be important in the struggle, but at
least as often it is some little detail that we cannot but regard as
trivial which suffices to differentiate the two species. Even when the
diagnostic point is of such a nature that we can imagine it to make a
serious difference in the economy we are absolutely at a loss to suggest
why this feature should be a necessity to species A and unnecessary to
species B its nearest ally. The house sparrow (_Passer domesticus_) is
in general structure very like the tree sparrow (_P. montanus_). They
differ in small points of colour. For instance _montanus_ has a black
patch on the cheek which is absent in _domesticus_. The presence in the
one species and the absence in the other are equally definite, and in
both cases we are equally unable to suggest any consideration of utility
in relation to these features. The two species are distinguished also by
a characteristic that may well be supposed to be of great significance.
In _domesticus_ the two sexes are strongly differentiated, the cock
being more ornate than the hen. On the other hand the two sexes in
_montanus_ are alike, and, if we take a standard from _domesticus_,
we may fairly say that in _montanus_ the hen has the colouration of
the male. It is not unreasonable to suppose that such a distinction
may betoken some great difference in physiological economy, but the
economical significance of this perhaps important distinction is just as
unaccountable as that of the seemingly trivial but equally diagnostic
colour-point.
I have spoken of the fixed characteristics of the two species. If
we turn to a very different feature, their respective liability to
albinistic variation, we find ourselves in precisely similar difficulty.
_Passer domesticus_ is a species in which individuals more or less pied
occur with especial frequency, but in _P. montanus_ such variation is
extremely rare if it occurs at all. The writer of the section on Birds
in the _Royal Natural History_ (III., 1894-5, p. 393) calls attention to
this fact and remarks that in that species he knows no such instance.
The two species therefore, apart from any differences that we can
suppose to be related to their respective habits, are characterised by
small fixed distinctions in colour-markings, by a striking difference
in secondary sexual characters, and by a difference in variability. In
all these respects we can form no surmise as to any economic reason why
the one species should be differentiated in the one way and the other in
the other way, and I believe it is mere self-deception which suggests
the hope that with fuller knowledge reasons of this nature would be
discovered.
The two common British wasps, _Vespa vulgaris_ and _Vespa germanica_,
are another pair of species closely allied although sharply
distinguished, which suggest similar reflexions. Both usually make
subterranean nests but of somewhat different materials. _V. vulgaris_
uses rotten wood from which the nest derives a characteristic yellow
colour, while _V. germanica_ scrapes off the weathered surfaces of
palings and other exposed timber, material which is converted into
the grey walls of the nest. The stalk by which the nest is suspended
(usually to a root) in the case of _germanica_ passes freely through
a hole in the external envelope, but _vulgaris_ unites this external
wall solidly to the stalk. In bodily appearance and structure the
two species are so much alike that they have often been confounded
even by naturalists, and to the untrained observer they are quite
indistinguishable. There are nevertheless small points of difference
which almost though not quite always suffice to distinguish the
two forms. For example the yellow part of the sinus of the eyes is
emarginate in _vulgaris_ but not emarginate in _germanica_. _V.
vulgaris_ often has black spots on the tibiae while in _germanica_ the
tibiae are usually plain yellow. In both species there is a horizontal
yellow stripe on the thorax, but whereas in _vulgaris_ this is a plain
narrow stripe, it is in _germanica_ enlarged downwards in the middle.
These and other apparently trivial details of colouration, though not
absolutely constant, are yet so nearly constant that irregularities
in these respects are quite exceptional. Lastly the genitalia of the
males, though not very different, present small structural points
of distinction which are enough to distinguish the two species at a
glance.[14]
In considering the meaning of the distinctions between these two wasps
we meet the old problem illustrated by the Sparrows. The two species
have somewhat different habits of life and we should readily expect
to find differences of bodily organisation corresponding with the
differences of habits. But is that what we do find? Surely not. To
suppose that there is a correspondence between the little points of
colour and structure which we see and the respective modes of life of
the two species is perfectly gratuitous. We have no inkling of the
nature of such a correspondence, how it can be constituted, or in what
it may consist.
Is it not time to abandon these fanciful expectations which are never
realised? Everywhere both among animals and plants does the problem of
specific difference reiterate itself in the same form. In view of such
facts as I have related and might indefinitely multiply, the fixity of
specific characters cannot readily be held to be a measure of their
economic importance to their possessors. The incidence of specific
fixity is arbitrary and capricious, sometimes lighting on a feature or
a property which can be supposed to matter much, but as often is it
attached to the most trifling of superficial peculiarities.
The incidence of _variability_ is no less paradoxical, and without
investigation of the particular case no one can say what will be found
to show much or little variability. The very characteristic which in
one species may exhibit extreme variability may in an allied species
show extreme constancy. Illustrations will occur to any naturalist, but
nowhere is this truth more strikingly presented than in the British
Noctuid Moths. Many are so variable that, in the common phrase,
"scarcely two can be found alike," while others show comparatively
slight variation. It need scarcely be remarked that, in the instances
I have in mind, the evidence of great variability is in no way due to
the abundance with which the particular species occurs, for common
species may show constancy, and less abundant species may show great
variability. The polymorphism seems to be now at least a general
property of the variable species, as the fixity is a property of the
fixed species. In illustration I may refer to the following examples.
_Dianthoecia capsincola_ is a common and widely distributed moth which
feeds on _Lychnis_. It shows little variation. _Dianthoecia carpophaga_
is another species which feeds chiefly on _Silene_. Its habits are
very similar to those of _capsincola_. Like that species it has a wide
geographical range and is abundant in its localities, but in contrast
to the fixity of _capsincola_, _carpophaga_ exhibits a complex series
of varieties. _Agrotis suffusa_ (= _ypsilon_) is a moth widely spread
through the southern half of England. It is very constant in colour and
markings. _Agrotis segetum_ and _tritici_ are excessively variable both
in ground colour and markings, being found in an immense profusion of
dissimilar forms throughout their distribution. Of these and several
other species of _Agrotis_ there are many named varieties, some of
which have by various writers been regarded as specifically distinct.
Of the genus _Noctua_ many species (e. g. _festiva_) show a similar
polymorphism, but _N. triangulum_, though showing some variation in
certain respects, is usually very constant to its type, and the same is
true of _N. umbrosa_.
In several species of _Taeniocampa_, especially _instabilis_, the
multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is
a comparatively constant species. The genus _Plusia_ contains a number
of constant species, but in _Plusia interrogationis_ we meet the fact
that the central silvery mark undergoes endless variation. "Truly no
two are alike," says Mr. Tutt, "and to look down a long series of
_interrogationis_ is something like looking at a series of Chinese
characters." In contrast to this we have the fact that in _Plusia gamma_
the very similar silvery mark is by no means variable.
I have taken this series of cases from the Noctuid moths, but it would
be as easy to illustrate the same proposition from the Geometridae or
the Micro-Lepidoptera.[15] I have a long series of _Peronea cristana_,
for example, which was given to me by Mr. W. H. B. Fletcher, of Bognor.
All were beaten out of the same hedge, and their polymorphism is such
that no one unaccustomed to such examples could suppose that they
belonged to a single species. Another common form, _P. schalleriana_,
which lives in similar circumstances, exhibits comparatively slight
variability.
It should be expressly noted that the variation of which I am speaking
is a genuine polymorphism. Several of the species enumerated exhibit
also geographical variation, possessing definite and often strikingly
distinct races peculiar to certain localities; but apart from the
existence of such local differentiation, stands out the fact upon which
I would lay stress, that some species are excessively variable while
others are by comparison constant, in circumstances that we may fairly
regard as comparable.
This fact is difficult to reconcile with the conventional view that
specific type is directly determined by Natural Selection and that
the precision with which a species conforms to its pattern is an
indication of the closeness of that control. Anyone familiar with the
characteristics of Moths will agree that the Noctuids, Geometrids and
Tortricids are creatures whose existence depends in some degree on the
success with which they can escape detection by their enemies in the
imaginal state. We are therefore not surprised to find that some species
of these orders exhibit definite geographical variation in conformity
with the character of the ground, which may reasonably be supposed to
aid in their protection. If this were all, there would be nothing to
cause surprise. We might even be disposed to allow that variability
might contribute to the perpetuation of animals so situated, on the
principle that among a variety of surroundings some would probably be
in harmony with the objects on which they rest. But we cannot admit
the plausibility of an argument which demands on the one hand that
the extreme precision with which species A adheres in the minutest
details of its colour and pattern to a certain type shall be ascribed
to the protective fitness of those details, and on the other hand that
the abundant variability of species B shall be ascribed to the same
determination. If it is absolutely necessary for A to conform to one
type how comes it that B may range through some twenty distinct forms,
any two of which differ more from each other than the regular species
of many other genera? The only reply I can conceive is a suggestion
that there _may_ be some circumstance which differentiates the various
classes of cases, that the exigencies of the fixed species _may_ be
different from those of the variable. Those who make such appeals to
ignorance do not always perhaps realise whither this course of reasoning
may lead. If admissible here the same argument would lead us to suggest
that because albino moles have for an indefinite period occurred on a
certain land near Bath there may be something in the soil or in the
conditions of life near Bath which requires a proportion of albinos in
its mole population. Or again, because the butterfly _Thais rumina_
in one locality, Digne in the south of France, has a percentage of
individuals of the variety _Honoratii_ (with certain normally yellow
spots on the hind wing coloured bright red) and nowhere else throughout
its distribution, that therefore we may suggest that there is some
difference in the condition of life at Digne which makes the continuance
of _Honoratii_ there possible and beneficial.
A polymorphism offering a parallel to that of the variable moths is
afforded by the breeding plumage of the Ruff, the male of _Machetes
pugnax_. The variety of plumage which these cocks exhibit is such
that the statement that no two can be found alike is only a venial
exaggeration. Newton remarks[16] "that all this wonderful 'show' is
the consequence of the polygamous habit of the Ruff can scarcely
be doubtful"; but even if it be conceded that the great external
differentiation of the cocks may be a result of sexual selection, the
problem of their _polymorphism_ remains unsolved, for, as we are well
aware, polygamy is not usually associated with polymorphism of the male.
The Black Cock (_Tetrao tetrix_), for example, is as polygamous as the
Ruff, but in that and countless other cases, both sexes are constant to
one type of plumage.
When we thus compare the polymorphism of one species with the fixity
of another, and attempt to determine the causes which have led to
these extraordinary contrasts, two distinct lines of argument are
open to us. We may ascribe the difference either to causes external
to the organisms, primarily, that is to say, to a difference in the
exigencies of Adaptation under Natural Selection; or on the other hand
we may conceive the difference as due to innate distinctions in the
chemical and physiological constitutions of the fixed and the variable
respectively. There is truth undoubtedly in both conceptions. If the
mole were physiologically incapable of producing an albino that variety
would not have come into being, and if the albino were totally incapable
of getting its living it would not be able to hold its own. Were
_Plotheia frontalis_ constructed on a chemical plan which admitted of no
variation, the countless varieties would not have been produced; and if
one of its varieties had an overwhelming success out of all proportion
to that of the rest, then the species would soon become monomorphic
again. We cannot declare that Natural Selection has no part in the
determination of fixity or variability; nevertheless looking at the
whole mass of fact which a study of the incidence of variation provides,
I incline to the view that the variability of polymorphic forms should
be regarded rather as a thing tolerated than as an element contributing
directly to their chances of life; and on the other hand that the fixity
of the monomorphic forms should be looked upon not so much as a proof
that Natural Selection controls them with a greater stringency, but
rather as evidence of a natural and intrinsic stability of chemical
constitution.
Compare the condition of a variable form like the male Ruff (or in
a less degree the Red Grouse in both its sexes) with that of the
common Pheasant which is comparatively constant. In the Pheasant no
doubt variations do occur as in other wild birds, but apart from the
effects of mongrelisation the species is unquestionably uniform. Could
it seriously be proposed that we should regard the constancy of the
pheasant's plumage in this country as depending on the special fitness
of that type of colouration? Even if the pheasant be not an alien in
Western Europe, it has certainly been protected for centuries, and for a
considerable period has existed in a state of semi-domestication. Such
conditions should give good opportunity for polymorphism to be produced.
In some coverts various aberrations do of course occur and persist,
yet there is nothing indicative of a general relaxation of the fixity
of the specific type, and the pheasant remains substantially a fixed
species.[17] The common pheasant (_Phasianus colchicus_) even shows
little of that disposition to form local races which appears in the
species of Further India. Are we not then on safer ground in regarding
the fixity of our species as a property inherent in its own nature
and constitution? Just as in ages of domestication no rose has ever
given off a blue variety so has the pheasant never broken out into the
polymorphism of the Ruff.
As soon as it is realised how largely the phenomena of variation and
stability must be an index of the internal constitution of organisms,
and not mere consequences of their relations to the outer world, such
phenomena acquire a new and more profound significance.
FOOTNOTES:
[1] In _Mendel's Principles of Heredity_ (Cambridge University Press,
1909) I have dealt with this subject, giving an account of the principal
facts discovered up to the beginning of 1909.
[2] Matthioli Opera, Ed. 1598, p. 8, originally published 1565.
[3] Ray's instances relate to Kales, and in most of these examples
we can see that there was no question of mutation or transmutation
at all, but that the occurrence was due either to mistake or to
cross-fertilisation. Sharrock, to whom Ray refers, was inclined to
discredit stories of transmutation, but he has also this passage
(_History of the Propagation and Improvement of Vegetables by the
Concurrence of Art and Nature_, Oxford, 1660, p. 29):
"It is indeed growen to be a great question, whether the transmutation
of a species be possible either in the vegetable, Animal, or Minerall
Kingdome. For the possibility of it in the vegetable; I have heard _Mr.
Bobart_ and his _Son_ often report it, and proffer to make oath that the
Crocus and Gladiolus, as likewise the Leucoium, and Hyacinths by a long
standing without replanting have in his garden changed from one kind to
the other: and for satisfaction about the curiosity in the presence of
_Mr. Boyle_ I tooke up some bulbs of the very numericall roots whereof
the relation was made, though the alteration was perfected before, where
we saw the diverse bulbs growing as it were on the same stoole, close
together, but no bulb half of the one kind, and the other half of the
other: But the changetime being past it was reason we should believe the
report of good artists in matters of their own faculty."
Robert Sharrock was a fellow of New College, Oxford. Both the Bobarts
were professional botanists, the father was author of a Catalogue of
the plants in the Hortus Medicus at Oxford, and the son was afterwards
Curator of the Oxford Garden.
[4] _Mém. Ac. roy. des Sci._ for 1719 (1721), p. 59.
[5] _Amoen. Acad._, 1789, vol. 6. I do not know whether attention has
been called to the curious mistake which Linnaeus makes in the course of
this argument. He cites the differences between the Mule and the Hinny
in illustration of his thesis, pointing out that the Mule is externally
more like a horse and the Hinny more like an ass. This, he says, is
because the Mule has the horse for a father, and the Hinny the ass, thus
inverting the actual facts!
[6] _Proc. Washington Ac. Sci._, 1909, XI, pp. 17-26.
[7] J. W. Tutt, in _Ent. Rec._, 1909, XXI, p. 185.
[8] E. Lehmann (_Bull. l'Herb. Boissier_, Ser. 2, VIII, 1908, p. 229)
has published an admirable paper on the interrelationships of these
species and has instituted cultural experiments which will probably
much elucidate the nature of their specific distinctness. As regards
the existence of intermediate forms he comes to the conclusion that
two only can be so regarded. The first was described by Kuntze from
specimens found on a flower-pot on board a Caspian steamer, from which
Lehmann proposes the new specific name _Siaretensis_. This comes between
_polita_ and _filiformis_, a close ally of _Tournefortii_. The other,
which combines some of the features of both _polita_ and _Tournefortii_,
was found in the province of Asterabad.
[9] In Cambridgeshire for example _vespertina_ is common but _diurna_ is
absent. Whether this absence is connected with the general presence of
chalk I cannot say. When introduced artificially _diurna_ establishes
itself, for a time at least, without any apparent difficulty and
occasionally escapes from the garden on to the neighbouring roadside.
[10] Conceivably however it may be a segregated combination. For an
account of this plant see Boissier, _Voy. Bot. Midi de l'Espagne_, 1839,
II, 722.
[11] A discussion of this subject with references to literature is
given by Rolfe, in an excellent paper on "Hybridisation viewed from the
standpoint of Systematic Botany" (_Jour. R. Hort. Soc._, XXIV, 1900, p.
197). He concludes: "The simple fact is that the two plants (_L. diurna_
and _vespertina_) are thoroughly distinct in numerous particulars, and
affect such different habitats that in some localities one or the other
of them is completely wanting. But when their stations are adjacent they
hybridise together very readily, and it is here that these intermediate
forms occur which have puzzled botanists so much." The same paper
contains valuable information concerning several cognate illustrations.
[12] In only two cases have I seen such plants (both females) completely
sterile.
[13] As is well known, in an even more notorious example, he proposed
to unite _Primula vulgaris_, _P. elatior_, and _P. acaulis_, similarly
relying on the existence of "intermediates," which we now well know to
be mongrels between the species.
[14] For an account of the distinctions between _Vespa vulgaris_ and
_germanica_ see Ch. Janet, _Études sur les Fourmis, les Guêpes et les
Abeilles_, 11^e, Note. Sur _Vespa germanica_ et _V. vulgaris_. Limoges
(Ducourtieux), 1895; and R. du Buysson, Monographie des Guêpes, _Ann.
Soc. Ent. France_, 1903, Vol. LXXII, p. 603, Pl. VIII.
[15] The statements made above are for the most part taken from Barrett,
C. G., _Lepidoptera of the British Islands_, and from Tutt, J. W., _The
British Noctuae and their Varieties_. The reader who is unfamiliar
with the amazing polymorphism exhibited by some of these moths should
if possible take an opportunity of looking over a long series in a
collection, or, if that be impossible, refer to the admirable coloured
plates published by Barrett. It may not be superfluous to observe that
plenty of similar examples are known in other countries. For instance
_Plotheia frontalis_, a Noctuid which often abounds in Ceylon, shows
an equally bewildering wealth of forms. If a dozen specimens of such a
species were to be brought home from some little known country, each
individual would almost certainly be described as the type of a distinct
species. (See the coloured plate published by Sir G. Hampson, Cat. Brit.
Mus., Heterocera, Vol. IX.)
[16] _Dict. of Birds_, p. 800. It would be interesting and profitable
to attempt in a long series of Ruffs to determine the Mendelian factors
which by their combinations give rise to this complex assemblage
of varietal forms. A few such factors both of colour and pattern
can be at once distinguished, and it is noticeable that some of the
resulting types of barring, spangling and penciling show a perceptible
correspondence with some of the types of colouration found in the breeds
of domestic fowls.
[17] Howard Saunders (_Illust. Manual of British Birds_, 1899, p. 499)
states that there is evidence that the pheasant had become naturalized
in the south of England before the Norman invasion. He adds, "little, if
any, deviation from the typical _P. colchicus_ took place up to the end
of last century, when the introduction of the Chinese Ring-necked _P.
torquatus_ commenced, which has left almost indelible marks, especially
with regard to the characteristic white collar."
CHAPTER II
MERISTIC PHENOMENA
Twenty years ago in describing the facts of Variation, argument was
necessary to show that these phenomena had a special value in the
sciences of Zoology and Botany. This value is now universally understood
and appreciated. In spite however of the general attention devoted to
the study of Variation, and the accumulation of material bearing on the
problem, no satisfactory or searching classification of the phenomena is
possible. The reason for this failure is that a real classification must
presuppose knowledge of the chemistry and physics of living things which
at present is quite beyond our reach.
It is however becoming probable that if more knowledge of the chemical
and physical structure of organisms is to be attained, the clue will
be found through Genetics, and thus that even in the uncoordinated
accumulation of facts of Variation we are providing the means of
analysis applicable not only to them, but to the problems of normality
also.
The only classification that we can yet institute with any confidence
among the phenomena of Variation is that which distinguishes on the one
hand variations in the processes of division from variations in the
nature of the substances divided.
Variations in the processes of division are most often made apparent by
a change in the number of the parts, and are therefore called _Meristic_
Variations, while the changes in actual composition of material are
spoken of as _Substantive_ Variations. The Meristic Variations form on
the whole a natural and fairly well defined group, but the Substantive
Variations are obviously a heterogeneous assemblage.
Though this distinction does not go very far, it is useful, and in
all probability fundamental. It is of value inasmuch as it brings
into prominence the distinct and peculiar part which the process
of division, or, more generally, repetition of parts, plays in the
constitution of the forms of living things.
That there may be a real independence between the Meristic and the
Substantive phenomena is evident from the fact both that Meristic
changes may occur without Substantive Variation, and that the substances
composing an organism may change without any perceptible alteration
in its meristic structure. When the distinction between these two
classes of phenomena is perceived it will be realised that the study
of genetics has on the one hand a physical, or perhaps more strictly
a mechanical aspect, which relates to the manner in which material is
divided and distributed; and also a chemical aspect, which relates
to the constitution of the materials themselves. Somewhat as the
philosophers of the seventeenth and eighteenth centuries were awaiting
both a chemical and a mechanical discovery which should serve as a
key to the problems of unorganised matter, so have biologists been
awaiting two several clues. In Mendelian analysis we have now, it is
true, something comparable with the clue of chemistry, but there is
still little prospect of penetrating the obscurity which envelops the
mechanical aspect of our phenomena. To make clear the application of
the terms chemical and mechanical to the problem of Genetics the nature
of that problem must be more fully described. In its most concrete form
this problem is expressed in the question, how does a cell divide? If
the organism is unicellular, and the single cell is the whole body,
then the process of heredity is accomplished in the single operation of
cell-division. Similarly in animals and plants whose bodies are made
up of many cells, the whole process of heredity is accomplished in the
cell-divisions by which the germ-cells are formed. When therefore we see
a cell dividing, we are witnessing the process by which the form and the
properties of the daughter-cells are determined.
Now this process has the two aspects which I have called mechanical
and chemical. The term "_Entwicklungsmechanik_" has familiarised us
with the application of the word mechanics to these processes, but on
reflexion it will be seen that this comprehensive term includes two
sorts of events which are sometimes readily distinguishable. There
is the event by which the cell _divides_, and the event by which the
two halves or their descendants are or may be _differentiated_. It
is common knowledge that in some cell-divisions two similar halves,
indistinguishable in appearance, properties, and subsequent fate, may
be produced, while in other divisions daughter-cells with distinct
properties and powers are formed. We cannot imagine but that in the
first case, when the resulting cells are identical, the division is a
mechanical process by which the mother-cell is simply cut in two; while
in order that two differentiated halves may be produced, some event must
have taken place by which a chemical distinction between the two halves
is effected.[1] In any ordinary Mendelian case we have a clear proof
that such a chemical difference may be established between germ-cells.
The facts of colour-inheritance for instance prove that germ-cells,
otherwise identical, may be formed _possessing_ the chromogen-factor
which is necessary to the formation of colour in the flowers, or
_destitute_ of that factor. Similarly the germ-cells may possess the
ferment which, by its action on the chromogenic substance, produces the
colour, or they may be without that ferment. The same line of argument
applied to a great range of cases. Nevertheless, though differences
in chemical properties are often thus constituted by cell-divisions,
and though we are thus able to make a quasi-chemical analysis of the
individual by determining and enumerating these properties, yet it is
evident that the distribution of these factors is not itself a chemical
process. This is proved by the fact that similar divisions may be
effected between halves which are exactly alike, and also by the fact
that the numbers in which the various types of germ-cells are formed
negative any suggestion of valency between them. The recognition of the
unit-factors may lead--indeed must lead--to great advances in chemical
physiology which without that clue would have been impossible, but
in causation the chemical phenomena of heredity must be regarded as
secondary to the physical or mechanical phenomena by which the cells
and their constituents are divided and separated. When therefore we
speak of the _essential_ phenomena of heredity we mean the mechanics
of division, especially, though not, as we shall see, exclusively, of
_cell_-division; and in the relation between the two halves of the
dividing cell we have the problem presented in what seems to be its
simplest form.
In attempting to form some conception of the processes by which bodily
characteristics are transmitted, or--to avoid that confusing metaphor
of "transmission"--how it comes about that the offspring can grow to
resemble its parent, continuity of the germ-substance which in some
animals is a visible phenomenon,[2] gives at least apparent help. An egg
for example on becoming adult develops in certain parts a particular
pigment. The eggs of that adult when they reach the appropriate age
develop the same pigment. We have no clear picture of the mechanism by
which this process is effected, but when we realise that the pigment
results from the interaction of certain substances, and that since
all the eggs are in reality pieces of the same material, it seems,
unless we inquire closely, not unnatural that the several pieces of the
material should exhibit the same colours at the same periods of their
development. The continuity of the material of the germs suggests that
there is a continuity of the materials from which the pigment is formed,
and that thus an actual bit of those substances passes into each egg
ready at the appropriate moment to generate the pigment. The argument
thus outlined applies to all _substantive_ characteristics. In each case
we can imagine, if we will, the appearance of that characteristic as due
to the contribution of its rudiment from the germ tissues.
When we consider more critically it becomes evident that the aid given
by this mental picture is of very doubtful reality, for even if it were
true that any predestined particle actually corresponding with the
pigment-forming materials is definitely passed on from germ to germ,
yet the power of increase which must be attributed to it remains so
incomprehensible that the mystery is hardly at all illuminated.
When however we pass from the substantive to the meristic characters,
the conception that the character depends on the possession by the germ
of a particle of a specific material becomes even less plausible. Hardly
by any effort of imagination can we see any way by which the division
of the vertebral column into _x_ segments or into _y_ segments, or of a
Medusa into 4 segments or into 6, can be determined by the possession
or by the want of a material particle. The distinction must surely be
of a different order. If we are to look for a physical analogy at all
we should rather be led to suppose that these differences in segmental
numbers corresponded with changes in the amplitude or number of dividing
waves than with any change in the substance or material divided.
PHENOMENA OF DIVISION
I have said that in the division of a cell we seem to see the problem in
its simplest form, but it is important to observe that the problem of
division may be presented by the bodies of animals and plants in forms
which are independent of the divisions between cells. The existence of
pattern implies a repetition of parts, and repetition of parts when
developed in a material originally homogeneous can only be created by
division. Cell-division is probably only a special case of a process
similar to that by which the pattern of the skeleton is laid down in
a unicellular body such as that of a Radiolarian or Foraminiferan.
Attempts have lately been made to apply mathematical treatment to
problems of biology. It has sometimes seemed to me that it is in the
geometrical phenomena of life that the most hopeful field for the
introduction of mathematics will be found. If anyone will compare one
of our animal patterns, say that of a zebra's hide, with patterns known
to be of purely mechanical production, he will need no argument to
convince him that there must be an essential similarity between the
processes by which the two kinds of patterns were made and that parts
at least of the analysis applicable to the mechanical patterns are
applicable to the zebra stripes also. Patterns mechanically produced are
of many and very diverse kinds. One of the most familiar examples, and
one presenting some especially striking analogies to organic patterns,
is that provided by the ripples of a mackerel sky, or those made in a
flat sandy beach by the wind or the ebbing tide. With a little search
we can find among the ripple-marks, and in other patterns produced by
simple physical means, the closest parallels to all the phenomena of
striping as we see them in our animals. The forking of the stripes, the
differentiation of two "faces," the deflections round the limbs and
so forth, which in the body we know to be phenomena of division, are
common both to the mechanical and the animal patterns. We cannot tell
what in the zebra corresponds to the wind or the flow of the current,
but we can perceive that in the distribution of the pigments, that
is to say, of the chromogen-substances or of the ferments which act
upon them, a rhythmical disturbance has been set up which has produced
the pattern we see; and I think we are entitled to the inference that
in the formation of patterns in animals and plants mechanical forces
are operating which ought to be, and will prove to be, capable of
mathematical analysis. The comparison between the striping of a living
organism and the sand-ripples will serve us yet a little farther,
for a pattern may either be formed by actual cell-divisions, and
the distribution of differentiation coincidently determined, or--as
visibly in the pigmentation of many animal and plant tissues--the
pattern may be laid down and the pigment (for example) distributed
through a tissue across or independently of the cell-divisions of the
tissue. Our tissues therefore are like a beach composed of sands of
different kinds, and different kinds of sands may show distinct and
interpenetrating ripples. When the essential analogy between these
various classes of phenomena is perceived, no one will be astonished at,
or reluctant to admit, the reality of discontinuity in Variation, and if
we are as far as ever from knowing the actual causation of pattern we
ought not to feel surprised that it may arise suddenly or be suddenly
modified in descent. Biologists have felt it easier to conceive the
evolution of a striped animal like a zebra from a self-coloured type
like a horse (or of the self-coloured from the striped) as a process
involving many intergradational steps; but so far as the _pattern_ is
concerned, the change may have been decided by a single event, just as
the multitudinous and ordered rippling of a beach may be created or
obliterated at one tide.
[Illustration: FIG. 1. Tusk of Indian elephant, showing an abnormal
segmentation.]
This point is well illustrated by the tusk of an Indian elephant which
I lately found in a London sale-room. This tusk is by some unknown
cause, presumably a chronic inflammation, thrown up into thirteen
well-marked ridges which closely simulate a series of segments (Fig. 1).
Whatever the cause the condition shows how easily a normally unsegmented
structure may be converted into a series of repeated parts.
The spread of segmentation through tissues normally unsegmented is very
clearly exemplified in the skates' jaws shown in Fig. 2. The right
side of the upper figure shows the normal arrangement in the species
_Rhinoptera jussieui_, but the structure on the left side is very
different. The probable relations of the several rows of teeth to the
normal rows is indicated by the lettering, but it is evident that by
the appearance of new planes of division constituting separate centers
of growth, the series has been recast. The pattern of the left side is
so definite that had the variation affected the right side also, no
systematist would have hesitated to give the specimen a new specific
name. The other two drawings show similar variations of a less extensive
kind, the nature of which is explained by the lettering of the rows of
teeth.
[Illustration: FIG. 2. Jaws of Skates (_Rhinoptera_) showing meristic
variation. (For a detailed discussion see _Materials for the Study of
Variation_, p. 259.)]
This power to divide is a fundamental attribute of life, and of that
power cell-division is a special example. In regard to almost all the
chief vital phenomena we can say with truth that science has made some
progress. If I mention respiration, metabolism, digestion, each of these
words calls to mind something more than a bare statement that such
acts are performed by an animal or a plant. Each stands for volumes of
successful experiment and research, But the expression cell-division,
the fundamental act which typifies the rest, and on which they all
depend, remains a bare name. We can see with the microscope the outward
symptoms of division, but we have no surmise as to the nature of the
process by which the division is begun or accomplished. I know nothing
which to a man well trained in scientific knowledge and method brings
so vivid a realisation of our ignorance of the nature of life as the
mystery of cell-division. What is a living thing? The best answer in
few words that I know is one which my old teacher, Michael Foster, used
to give in his lectures introductory to biology. "A living thing is a
vortex of chemical and molecular change." This description gives much,
if not all, that is of the essence of life. The living thing is unlike
ordinary matter in the fact that, through it, matter is always passing.
Matter is essential to it; but, provided that the flow in and out is
unimpeded, the life-process can go on so far as we know indefinitely.
Yet the living "vortex" differs from all others in the fact that it
can divide and throw off other "vortices," through which again matter
continually swirls.
We may perhaps take the parallel a stage further. A simple vortex, like
a smoke-ring, if projected in a suitable way will twist and form two
rings. If each loop as it is formed could grow and then twist again to
form more loops, we should have a model representing several of the
essential features of living things.
It is this power of spontaneous division which most sharply
distinguishes the living from the non-living. In the excellent book
dealing with the problems of development, lately published by Mr.
Jenkinson a special emphasis is very properly laid on the distinction
between the processes of division, and those of differentiation. Too
often in discussions of the developmental processes the distinction
is obscured. He regards differentiation as the "central difficulty."
"Growth and division of the nucleus and the cells," he tells us,
are side-issues. This view is quite defensible, but I suspect that
the division _is_ the central difficulty, and that if we could get
a rationale of what is happening in cell-division we should not be
long before we had a clue to the nature of differentiation. It may be
self-deception, but I do not feel it impossible to form some hypothesis
as to the mode of differentiation, but in no mood of freest speculation
are we ever able to form a guess as to the nature of the division. We
see differentiations occurring in the course of chemical action, in
some phenomena of vibration and so forth: but where do we see anything
like the spontaneous division of the living cell? Excite a gold-leaf
electroscope, and the leaves separate, but we know that is because they
were double before. In electrolysis various substances separate out at
the positive and negative poles respectively. Now if in cell-division
the two daughter-cells were always dissimilar--that is to say, if
differentiation always occurred--we could conceive some rough comparison
with such dissociations. But we know the dissimilarity between
daughter-cells is not essential. In the reproduction of unicellular
organisms and many other cases, the products formed at the two poles
are, so far as we can tell, identical. Any assumption to the contrary,
if we were disposed to make it, would involve us in difficulties still
more serious. At any rate, therefore, if differentiation be really the
central difficulty in development, it is division which is the essential
problem of heredity.
Sir George Darwin and Professor Jeans tell us that "gravitational
instability" consequent on the condensation of gases is "the primary
agent at work in the actual evolution of the universe," which has led to
the division of the heavenly bodies. The greatest advance I can conceive
in biology would be the discovery of the nature of the instability which
leads to the continual division of the cell. When I look at a dividing
cell I feel as an astronomer might do if he beheld the formation of a
double star: that an original act of creation is taking place before me.
Enigmatical as the phenomenon seems, I am not without hope that, if it
were studied for its own sake, dissociated from the complications which
obscure it when regarded as a mere incident in development, some hint as
to the nature of division could be found. It is I fear a problem rather
for the physicist than for the biologist. The sentiment may not be a
popular one to utter before an assembly of biologists, but looking at
the truth impersonally I suspect that when at length minds of first rate
analytical power are attracted to biological problems, some advance will
be made of the kind which we are awaiting.
The study of the phenomena of bodily symmetry offers perhaps the
most hopeful point of attack. The essential fact in reproduction is
cell-division, and the essential basis of hereditary resemblance is
the symmetry of cell-division. The phenomena of twinning provide a
convincing demonstration that this is so. By twinning we mean the
production of equivalent structures by division. The process is one
which may affect the whole body of an animal or plant, or certain of
its parts. The term twin as ordinarily used refers to the simultaneous
birth of two individuals. Those who are naturalists know that such twins
are of two kinds, (1) twins that are not more alike than any other
two members of the same family, and (2) twins that are so much alike
that even intimate friends mistake them. These latter twins, except in
imaginative literature, are always of the same sex.
It is scarcely necessary for me to repeat the evidence from which it has
been concluded that without doubt such twins arise by division of the
same fertilised ovum. There is a perfect series of gradations connecting
them with the various forms of double monsters united by homologous
parts. They have been shown several times to be enclosed in the same
chorion, and the proofs of experimental embryology show that in several
animals by the separation of the two first hemispheres of a dividing egg
twins can be produced. Lastly we have recently had the extraordinarily
interesting demonstration of Loeb, to which I may specially refer.
Herbst some years ago found that in sea water, from which all lime salts
had been removed, the segments of the living egg fall apart as they are
formed. Using this method Loeb has shown that a temporary immersion in
lime-free sea water may result in the production of 90 per cent. of
twins. We are therefore safe in regarding the homologous or "identical"
twins as resulting from the divisions of one fertilised egg, while the
non-identical or "fraternal" twins, as they are called, arise by the
fertilisation of two separate ova.[3]
In the resemblance of identical twins we have an extreme case of
hereditary likeness[4] and a proof, if any were needed, that the
cause of individual variation is to be sought in the differentiation
of germ-cells. The resemblance of identical twins depends on two
circumstances, First, since only two germ-cells take part in their
production, difference between the germ cells of the same individual
cannot affect them. Secondly the division of the fertilised ovum,
the process by which they became two instead of one, must have been
a symmetrical division. The structure of twins raises however one
extremely significant difficulty, which as yet we cannot in any way
explain. The resemblance between twins is a phenomenon of symmetry,
like the resemblance between the two sides of a bilaterally symmetrical
body. Not only is the general resemblance readily so interpreted, but
we know also that in double monsters, namely unseparated twins, various
anatomical abnormalities shown by the one half-body are frequently shown
by the other half-also.[5] The two belong to one system of symmetry.
How then does it happen that the body of one of a pair of twins does
not show a transposition of viscera? We know that the relation of right
and left implies that the one should be the mirror-image of the other.
Such a relation of images may be maintained even in minute details.
For example if the same pattern of finger-print is given by the fingers
of the two hands, one is the reverse of the other. In double monsters,
namely unseparated twins, there is evidence that an inversion of viscera
does occur with some frequency. Evidence from such cases is not so clear
and simple as might be expected, because as a matter of fact, the heart
and stomach, upon which the asymmetry of the viscera chiefly depend, are
usually common to the two bodies. Duplicity generally affects either
the anterior end alone, or the posterior end alone. The division is
generally _from the heart forwards_, giving two heads and two pairs of
anterior limbs on a common trunk, or _from the heart backwards_, giving
two pairs of posterior limbs with the anterior body common. In either
case, though the bodies may be grouped in a common system of symmetry,
neither can be proved to show definite reversal of the parts. To see
that reversal recourse must be had to more extreme duplications, such as
the famous Siamese Twins. They, as a matter of fact, were an excellent
instance of the proposition that twins are related as mirror-images,
for both of them had eleven pairs of ribs instead of the normal twelve,
and one of them had a partial reversal of viscera.[6] (Küchenmeister,
_Verlagerung_, etc., p. 204.)
If anyone could show how it is that neither of a pair of twins has
transposition of viscera the whole mystery of division would, I expect,
be greatly illuminated.[7] At present we have simply to accept the fact
that twins, by virtue of their detachment from each other, have the
power of resuming the polarity which is proper to any normal individual.
It was nevertheless with great interest that I read Wilder's recent
observation[8] that occasionally in identical twins the finger-print of
one or both the index-fingers may be reversed, showing that there is
after all some truth in the notion that reversal should occur in them.
There is another phenomenon by twinning which, if we could understand
it, might help. I refer to the free-martin, the subject of one of John
Hunter's masterpieces of anatomical description. In horned cattle twin
births are rare, and when twins of opposite sexes are born, the male
is perfect and normal, but the reproductive organs of the female are
deformed and sterile, being known as a free-martin. The same thing
occasionally happens in sheep, suggesting that in sheep also twins may
be formed by the division of one ovum; for it is impossible to suppose
that mere development in juxtaposition can produce a change of this
character. I mention the free-martin because it raises a question of
absorbing interest. It is conceivable that we should interpret it by
reference to the phenomenon of gynandromorphism, seen occasionally in
insects, and also in birds as a great rarity. In the gynandromorph one
side of the body is male, the other female. A bullfinch for instance has
been described with a sharp line of division down the breast between the
red feathers of the cock on one side and the brown feathers of the hen
on the other. (Poll, H., _SB. Ges. Nat. Fr._, Berlin, 1909, p. 338.) In
such cases neither side is sexually perfect. If the halves of such a
gynandromorph came apart, perhaps one would be a free-martin.
The behaviour of homologous twinning in heredity has been little
studied. It does not exist as a normal feature in any animal which
is amenable to experiment, and we cannot positively assert that a
comparable phenomenon exists in plants; for in them--the Orange, for
example--polyembryony may evidently be produced by a parthenogenetic
development of nucellar tissue. It is possible that in Man twinning
is due to a peculiarity of the mother, not of the father. It may and
not rarely does descend from mother to daughter, but whether it can be
passed on through a male generation to a daughter again, there is not
sufficient evidence to show. The facts as far as they go are consistent
with the inference which may be drawn from Loeb's experiment, that the
twinning of a fertilized ovum may be determined not by the germ-cells
which united to form it, but by the environment in which it begins to
develop. The opinion that twinning may descend through the male directly
has been lately expressed by Dr. J. Oliver in the _Eugenics Review_
(1912), on the evidence of cases in which twins had occurred among the
relations of fathers of twins, but I do not know of any comprehensive
collection of evidence bearing on the subject.
Besides twinning of the whole body a comparable duplicity of various
parts of the same body may occur. Such divisions affect especially those
organs which have an axis of bilateral symmetry, such as the thumb, a
cotyledon, a median petal, the frond of a fern or the anal fin of a
fish. From the little yet known it is clear that the genetic analysis
of these conditions must be very difficult, but evidence of any kind
regarding them will be valuable. We want especially to know whether
these divisions are due to the _addition_ of some factor or power which
enables the part to divide, or whether the division results from the
_absence_ of something which in the normal body prevents the part from
dividing. Breeding experiments, so far as they go, suggest that the less
divided state is usually dominant to the more divided.[9] The two-celled
Tomato fruit is dominant to the many-celled type. The Manx Cat's tail,
with its suppression of caudal segmentation is a partial dominant over
the normal tail. The tail of the Fowl in what is called the "Rumpless"
condition is at least superficially comparable with that of the Manx
Cat, and though the evidence is not wholly consistent, Davenport
obtained facts indicating that this suppressed condition of the caudal
vertebrae is an imperfect dominant.[10]
Some evidence may also be derived from other examples of differences
which at first sight appear to be substantive though they are more
probably meristic in ultimate nature. The distinction between the
normal and the "Angora" hair of the Rabbit is a case in point. We can
scarcely doubt that one of the essential differences between these two
types is that in the Angora coat the hair-follicles are more finely
divided than they are in the normal coat, and we know that the normal,
or less-divided condition, is dominant to the Angora, or more finely
divided.
[Illustration: FIG. 3. _I_, _II_, _III_, various degrees of syndactyly
affecting the medius and annularis in the hand; _IV_, syndactyly
affecting the index and medius in the foot. (After Annandale.)]
In the case of the solid-hoofed or "mule-footed" swine, the evidence
shows, as Spillman has lately pointed out,[11] that the condition
behaves as a dominant. The essential feature of this abnormality is
that the digits III and IV are partially united. The union is greatest
peripherally. Sometimes the third phalanges only are joined to form one
bone, but the second and even the first phalanges may also be compounded
together. Here the variation is obviously meristic and consists in a
failure to divide, the normal separation of the median digits of the
foot being suppressed.
[Illustration: FIG. 4. Case of complete syndactyly in the foot. _II_ and
_III_, digit apparently representing the index and medius. _c_^{2} +
_c_^{3}, bone apparently representing the middle and external cuneiform;
_cb_, cuboid; _c_^{1}, internal cuneiform. (After Gruber.)]
Webbing between the digits, in at least some of its manifestations, is
a variation of similar nature. The family recorded by Newsholme[12]
very clearly shows the dominance of this condition. The case is
morphologically of great interest and must undoubtedly have a bearing on
the problems of the mechanics of Division. In discussing the phenomena
of syndactylism I pointed out some years ago that the digits most
frequently united in the human hand are III and IV, while in the foot,
union most frequently takes place between II and III.[13] In Newsholme's
family the union was always between II and III of the foot, except in
the case of one male who had the digits III and IV of the right _hand_
alone webbed together. There can be little doubt that the geometrical
system on which the foot is planned has an axis of symmetry passing
between the digits II and III, while the corresponding axis in the hand
passes between III and IV. Union between such digits may therefore be
regarded as comparable with any non-division or "coalescence" of lateral
structures in a middle line, and when as in these examples such a
condition is shown to be a dominant we cannot avoid the inference that
some concrete factor has the power of suppressing or inhibiting this
division. Figs. 3 and 4 illustrate degrees of union between digits in
the human hand and foot.
It is not in question that various other forms of irregular webbing
and coalescence of digits exist, and respecting the genetic behaviour
of these practically nothing is as yet known. Such a case is described
by Walker,[14] in which the first and second metacarpals of both
feet were fused in mother and daughter, and several more are found
in literature. Contrasted with these phenomena we have the curious
fact that in the Pigeon, Staples-Browne found webbing of the toes a
_recessive_ character. The question thus arises whether this webbing is
of the same nature as that shown to be a dominant in Man, and indeed
whether the phenomenon in pigeons is really meristic at all. There is
some difference perceptible between the two conditions; for in Man
there is not so much a development of a special web-like skin uniting
the digits as a want of proper division between the digits themselves,
and in extreme cases two digits may be represented by a single one. In
the Pigeon I am not aware that a real union of this kind has ever been
observed, and though the web-like skin may extend the whole length of
the digits and be so narrow as to prevent the spread of the toes, it
may, I think, be maintained that the unity of the digits is unimpaired.
For the present the nature of this variation in the pigeon's feet must
be regarded as doubtful, and we should note that if it is actually an
example of a more perfect division being dominant to a less perfect
division, the case is a marked exception to the general rule that
non-division is dominant to division.
Reference must also be made to the phenomenon of fasciation in the stems
of plants. As Mendel showed in the case of _Pisum_ this condition is
often a recessive. The appearances suggest that the difference between a
normal and a fasciated plant consists in the inability of the fasciated
plant to separate its lateral branches. The nature of the condition is
however very obscure and it is equally likely that some multiplication
of the growing point is the essential phenomenon.[15]
Stockard's interesting experiments[16] illustrate this question. He
showed that by treating the embryos of a fish (_Fundulus heteroclitus_)
with a dilute solution of magnesium salts, various cyclopian
monstrosities were frequently produced. These have been called cases
of _fusion_ of the optic vesicles. I would prefer to regard them as
cases of a division suppressed or restricted by the control of the
environment. Conversely, the splendid discovery of Loeb, that an
unfertilised egg will divide and develop parthenogenetically without
fertilisation, as a consequence of exposure to various media, may be
interpreted as suggesting that the action of those media releases
the strains already present in the ovum, though I admit that an
interpretation based on the converse hypothesis, that the medium acts as
a stimulus, is as yet by no means excluded.
In these cases we come nearest to the direct causation or the direct
inhibition of a division, but the meaning of the evidence is still
ambiguous. I incline to compare Loeb's parthenogenesis with the
development (and of course accompanying cell-division) of dormant buds
on stems which have been cut back.
It is interesting to note that sometimes as an abnormality, the
faculty of division gets out of hand and runs a course apparently
uncontrolled. A remarkable instance of this condition is seen in
_Begonia_ "_phyllomaniaca_", which breaks out into buds at any point on
the stem, petioles, or leaves, each bud having, like other buds, the
power of becoming a new plant if removed. We would give much to know the
genetic properties of _B. phyllomaniaca_, and in conjunction with Mr. W.
O. Backhouse I have for some time been experimenting with this plant.
It proved totally sterile. Its own anthers produce no pollen, and all
attempts to fertilise it with other species failed though the pollen of
a great number of forms was tried.
Recently however we have succeeded in making plants which are in every
respect _Begonia phyllomaniaca_, so far as the characters of stems and
leaves are concerned. These plants, of which we have sixteen, were made
by fertilising _B. heracleifolia_ with _B. polyantha_. They are all
beginning to break out in "phyllomania." As yet they have not flowered,
but as they agree in all details with _phyllomaniaca_ there can be
little doubt that the original plant bearing that name was a hybrid
similarly produced. The production of "phyllomania" on a hybrid Begonia
has also been previously recorded by Duchartre.[17] In this case the
cross was made between _B. incarnata_ and _lucida_. The synonymy of
the last species is unfortunately obscure, and I have not succeeded in
repeating the experiment.
[Illustration: FIG. 5. Piece of petiole of _Begonia phyllomaniaca_. The
proximal end is to the right of the figure.]
From these facts it seems practically certain that the condition is one
which is due to the meeting of complementary factors. At first sight
we may incline to think that the phyllomania is in some way due to the
sterility. This however cannot be seriously maintained; for not only is
sterility in plants not usually associated with such manifestations, but
we know a Begonia called "Wilhelma" which is exactly _phyllomaniaca_ and
equally sterile, though it has no trace of phyllomania. This plant arose
in the nurseries of MM. P. Bruant of Poitiers, and has generally been
described as a seedling of _phyllomaniaca_, but from the total sterility
of that form this account of its origin must be set aside.
[Illustration: FIG. 6. Two right hind feet of polydactyle cats. _II_
shows the lowest development of the condition yet recorded. The digit,
_d_^{1}, which stands as hallux is fully formed and has three phalanges.
Both it and the digit marked _d_^{2} are formed as _left_ digits. In the
normal hind foot of the cat the hallux is represented by a rudiment only.
_I_ shows a further development of the condition. In this foot there are
_six_ digits. _d_^{1} has two phalanges, but both it and _d_^{2} and
_d_^{3} are shaped as left digits. Thus _d_^{3}, which in the normal
foot would be shaped as a right digit, is transformed so as to look like
a _left_ digit.]
The phenomenon in this case can hardly be regarded as due to the
excitation of dormant buds, for it is apparent on examination that
the new growths are not placed in any fixed geometrical relation to
the original plant. They arise on the petiole, for example, as small
green outgrowths each of which gradually becomes a tiny leaf. The
attitude of these leaves is quite indeterminate, and they may point
in any direction, some having their apices turned peripherally, some
centrally, and others in various oblique or transverse positions (Fig.
5). These little leaves are thus comparable with seedlings, in that
their polarity is not related to, or consequent upon that of the parent
plant. They have in fact that "individuality," which we associate with
germinal reproduction.
There are many curious phenomena seen in the behaviour of parts normally
repeated in bilateral symmetry which may some day guide us towards
an understanding of the mechanics of division. A part like a hand,
which needs the other hand to complete its symmetry, cannot twin by
mere division, yet by proliferation and special modifications on the
radial side of the same limb, even a hand may be twinned. In the well
known polydactyle cats a change of this kind is very common and indeed
almost the rule. When extra digits appear at the inner (tibial) side
of the limb, they are shaped as digits of the other side, and even the
normal digit II (index) is usually converted into the mirror-image
of its normal self. The limb then develops a new symmetry in itself.
Nevertheless it is not easy to interpret these facts as meaning that
there has been some interruption in the control which one side of the
body exercises over the other. The heredity of polydactylism is complex
but there is little doubt that the condition familiar in the Cat is a
dominant. In some human cases also the descent is that of a dominant,
but irregularities are so frequent that no general rule can yet be
perceived. The dominance of such a condition is an exception to the
principle that the less-divided is usually dominant to the more-divided,
a fact which probably should be interpreted as meaning that divisions
are of more than one kind.
Among ordinary somatic divisions, whether of organs, cells, or patterns
of differentiation, the control of symmetry is usually manifested. There
is however one class of somatic differentiations which are exceptionally
interesting from the fact that they may show a complete independence
of such geometrical control. The most familiar examples of these
geometrically uncontrolled Variations are to be seen in bud-sports.
The normal differentiation of the organs of a plant is arranged on
a definite geometrical system, which to those who have never given
special attention to such things before, will often seem surprisingly
precise. The arrangement of the leaves on uninjured, free-growing shoots
can generally be seen to follow a very definite order, just as do the
flowers or the parts of the flowers. If however bud sports occur,
then though the parts included in the sports show all the geometrical
peculiarities proper to the sport-variety, yet the sporting-buds
themselves are not related to each other according to any geometrical
plan.
A very familiar illustration is provided by the distribution of colour
in those Carnations that are not self-coloured. The pigment may, as in
Picotees, be distributed peripherally with great regularity to the edges
of the petals; or, as in Bizarres and Flakes, it may be scattered in
radial sectors which show no geometrical regularity. Now in this case
the pigments are the same in both types of flower, and the chemical
factors concerned in their production must surely be the same. The
difference must lie in the mechanical processes of distribution of
the pigment. In the Picotee we see the orderly differentiation which
we associate with normality; in the Bizarre we see the disorderly
differentiation characteristic of bud-sports. The distribution of colour
in this case lies outside the scheme of symmetry of the plant.
Such a distribution is characteristic of bud-sports, and of certain
other differentiations in both plants and animals, which I cannot on
this occasion discuss. Now reflexion will show that these facts have
an intimate bearing on the mechanical problems of heredity. For first
in the bud-sports we are witnessing the distribution of factors which
distinguish genetic varieties. We do not know the physical nature of
those factors, but if we must give them a name, I suppose we should call
them "ferments" exactly as Boyle did in 1666. He is discussing how it
comes about that a bud, budded on a stock, becomes a branch bearing the
fruit of its special kind. He notes that though the bud inserted be "not
so big oftentimes as a Pea," yet "whether by the help of some peculiar
kind of Strainer or by the Operation of some powerful Ferment lodged in
it, or by both these, or some other cause," the sap is "so far changed
as to constitute a Fruit quite otherwise qualify'd."[18] We can add
nothing to his speculation, and we believe still that by a differential
distribution of "ferments" the sports are produced. All the factors are
together present in the normal parts; some are left out in the sport. In
an analogous case however, that of a variegated _Pelargonium_ which has
green and also albino shoots, Baur proved that the shoots pure in colour
are also pure in their posterity. There can be no doubt that the sports
of Carnations, Azaleas, Chrysanthemums, etc., would behave in the same
way.
The well-known Azaleas Perle de Ledeburg, President Kerchove,
and _Vervaeana_ are familiar illustrations. Perle de Ledeburg is
predominantly white, but it has red streaks in some of its flowers. It
not very rarely gives off a self-red sport. This is evidently due to
the development of a bud in a red-bearing area of the stem. The red in
this plant is not under "geometrical control." Many plants have white
flowers with no markings, but if the red markings are geometrically
ordered differentiations, no self-coloured sports are formed. The case
of _Vervaeana_ is a good illustration of this proposition. It has white
flowers with red markings arranged in an orderly manner on the lower
parts of the petals, especially on the dorsal petals. This is one of the
Azaleas most liable to have red sports, and at first sight it might seem
that the sport represented the red of the central marks. Examination
however of a good many flowers shows that irregular red streaks like
those of Perle de Ledeburg occur, about as commonly as in that variety.
_Vervaeana_ in fact is Perle de Ledeburg with _definite_ red markings
added, and its red sports obviously are those branches the germs of
which came in a patch of the stem bearing these red elements. That this
is the true account is rendered quite obvious by the fact that the red
of the sport is a colour somewhat different from that of the definite
marks, and that these marks are still present on the red ground of the
sporting flowers.
It will be understood that these remarks apply to those cases in which
the production of sports is habitual or frequent, and I imagine in
all such examples it will be found that there are indications of
irregularity in the distribution of the differentiations such as to
justify the view that they are not under that geometrical control which
governs the normal differentiation of the parts. The question next
arises whether these considerations apply also to the production of a
bud-sport as a rare exception, but by the nature of the case it is not
possible to say positively whether the appearance of an exceptional
sport is due to the unsuspected presence of a pre-existing fragment of
material having a special constitution, or to the origin, _de novo_, of
such a material. For instance one of the garden forms of _Pelargonium_
known as _altum_ is liable perhaps once in some hundreds of flowers to
have one or two magenta petals. The normal colour is a brilliant red;
and as we may be fairly sure that this red is recessive to magenta the
interpretation would be quite different according as the appearance of
the magenta is regarded as due to the presence of small areas endowed
with magentaness, or to the spontaneous generation of the factor for
that pigment. Either interpretation is possible on the facts, but the
view that the whole plant has in it scarce mosaic particles of magenta
seems on the whole more consistent with present knowledge.
In _Pelargonium altum_ the enzyme causing the magenta colours must
be distributed in very small areas, but a case in which the magenta
is similarly arranged in a much coarser patchwork may be seen in the
_Pelargonium_ "Don Juan," which often bears whole trusses or branches of
red flowers upon plants having the normal dominant magenta trusses. In
most cases there is little doubt that though the magenta flowered parts
can "sport" to red, the red parts could not produce the magenta flowers.
The asymmetrical, or to speak more precisely, the disorderly, mingling
of the colours in the somatic parts is thus an indication of a similarly
disorderly mixing of the factors for those colours in the germ-tissues,
so that some of the gametes bear enough of the colour-factors to make a
self-coloured plant, while others bear so little that the plant to which
they give rise is a patchwork. If this view is correct we may extend it
so far as to consider whether the fineness or coarseness of the mixture
visible in the flowers or leaves may not give an indication of the
degree to which the factors are subdivided among the germ-cells. We know
very little about the genetic properties of striped varieties. In both
_Antirrhinum_ and _Mirabilis_ it has been found that the striped may
occasionally and irregularly throw self-coloured plants, and therefore
the striping cannot be regarded simply as a recessive character. On the
other hand in _Primula Sinensis_ there are well-known flaked varieties
which ordinarily at least breed true. Whether these ever throw selfs I
do not know, but if they do it must be quite exceptionally. The power
of these flaked plants to breed true is, I suspect, connected with the
fact that in their flowers the coloured and white parts are _intimately_
mixed, this intimate mixture thus being an indication of a similarly
intimate mixture in the germ-cells. It would be important to ascertain
whether self-fertilised seed from the occasional flowers in which the
colour has run together to join a large patch gives more self-coloured
plants than the intimately flaked flowers do.
The next fact may eventually prove of great importance. We have seen
that in bud-sports the differentiation is of the same nature as
that between pure types, and also that in the sporting plant this
differentiation is distributed without any reference to the plant's
axis, or any other consideration of symmetry. Now among the germ-cells
of a Mendelian hybrid exactly such characters are being distributed
allelomorphically, and there again we have strong evidence for believing
that the distribution obeys no pattern. For example, we can in the case
of seeds still _in situ_ perceive how the characters were distributed
among the germ-cells, and there is certainly no obvious pattern
connecting them, nor can we suppose that there is an actual pattern
obscured.
Of this one illustration is especially curious. Individual plants of
the same species are, as regards the decussations of their leaves and
in other respects, _either rights or lefts_. The fact is not emphasized
in modern botany and is in some danger of being forgotten. When, as
in the flowers of Arum, some _Gladioli_, _Exacum_, _St. Paulia_, or
the fruits of _Loasa_, rights and lefts occur on the same stem,
they come off alternately. But if, as in the seedlings of Barley the
twist of the first leaf be examined, it will be seen to be either a
right-or left-handed screw. An ear of barley, say a two-row barley,
is a definitely symmetrical structure. The seeds stand in their
envelopes back to back in definite positions. Each has its organs
placed in perfectly definite places. _If these seeds were buds_ their
differentiations would be grouped into a common plan. One might expect
that the differentiations of these embryos would still fall into the
pattern; but they do not, and so far as I have tested them, any one
may be a right or a left, just as each may carry any of the Mendelian
allelomorphs possessed by the parent plant, without reference to the
differentiation of any other seed. The fertilisation may be responsible,
but our experience of the allelomorphic characters suggest that the
irregularity is in the egg-cells themselves.[19]
_Germ cells thus differ from somatic cells in the fact that their
differentiations are outside the geometrical order which governs
the differentiation of the somatic cells._ I can think of possible
exceptions, but I have confidence that the rule is true and I regard it
as of great significance.
The old riddle, what is an individual, finds at least a partial solution
in the reply that an individual is a group of parts differentiated in a
geometrically interdependent order. With the germ-cell a new geometrical
order, with independent polarity is almost if not quite always, begun,
and with this geometrical independence the power of rejuvenescence may
possibly be associated.
The problems thus raised are unsolved, but they do not look insoluble.
The solution may be nearer than we have thought. In a study of the
geometry of differentiation, germinal and somatic, there is a way
of watching and perhaps analyzing what may be distinguished as the
mechanical phenomena of heredity. If any one could in the cases of
the Picotee and the Bizarre Carnation, respectively, detect the real
distinction between the two types of distribution, he would make a
most notable advance. Any one acquainted with mechanical devices can
construct a model which will reproduce some of these distinctions more
or less faithfully. The point I would not lose sight of is that the
analogy with such models must for a long way be a true and valuable
guide. I trust that some one with the right intellectual equipment will
endeavor to follow this guide; and I am sanguine enough to think that a
comprehensive study of the geometrical phenomena of differentiation will
suggest to a penetrative mind that critical experiment which may one day
reveal the meaning of spontaneous division, the mystery through which
lies the road, perhaps the most hopeful, to a knowledge of the nature of
life.
FOOTNOTES:
[1] In saying this we make no assumption as to the particular
cell-division at which differentiation occurs. This may be one of the
maturation-divisions, or it may perhaps be much earlier.
[2] From the recent discoveries of Erwin Baur we are led to surmise
that in the flowering plants the sub-epidermal layer, or some of its
elements, may legitimately be regarded as a similar germ-substance,
continuous in Weismann's sense.
[3] These fraternal twins, which show no special resemblance to each
other, are like the multiple births of other animals, and there is no
disposition for them to be of the same sex. In the sheep, for example,
statistics show that the frequency of pairs of twins, male and female,
is approximately double that of the frequency of pairs, both male or
both female, as it should be if the sex-distribution were fortuitous.
For instance Bernadin (_La Bergerie de Rambouillet_, 1890, p. 100)
gives the following figures for twin-lambs in Merinos: both male, 87;
both female, 83; sexes mixed, 187. The 9-banded Armadillo (_Dasypus
novemcinctus_), in which the young born in one litter are said to be
always of one sex, is the only known exception in Vertebrates, and
is presumably a genuine case of normal polyembryony (see especially,
Rosner, _Bull. Ac. Soc. Cracovie_, 1901, p. 443, and Newman and
Patterson, _Biol. Bull._, XVII, 1909, p. 181), and an important paper
lately published by H. H. Newman and J. T. Patterson, _Jour. Morph._,
1911, XXII, p. 855.
[4] A good collection of evidence as to disease in homologous twins was
lately published by E. A. Cockayne, _Brit. Jour. Child. Diseases_, Nov.,
1911.
[5] Cp. Windle, B. C. A., _Jour. Anal. Phys._, XXVI, p. 295.
[6] Mr. E. Nettleship tells me that in the course of collecting
pedigrees of families containing colour-blind members he has discovered
two cases (shortly to be published) of pairs of twins, which on account
of their very close resemblances must be deemed homologous, one of each
pair being colour-blind and the other normal. Such a distinction between
closely similar twins is most curious and unexpected.
[7] Another paradoxical phenomenon of the same nature occurs in the
Narwhal The males normally have the _left_ tusk alone developed, the
corresponding right tusk remaining as an undeveloped rudiment in its
socket. The left tusk is a left-handed screw. Occasionally the right
tusk is also developed and grows to the same length as that of the
left side, but in such specimens the right tusk is also a left-hand
screw like the tusk of the other side, instead of being reversed as we
should certainly have expected. It need scarcely be remarked that in the
case of the horns of antelopes, and in other examples of spiral organs
arranged in pairs, that of one side of the body is the mirror image of
that on the other side. The Narwhal's tusks in being both twisted in the
same direction are thus highly anomalous, and are comparable with pairs
of twins.
[8] Wilder, H. H., _Amer. Jour. Anat._, 1904, III, p. 452.
[9] Polydactylism which is often a dominant and the web-foot of Pigeons
which is recessive should be remembered as possible exceptions (see p.
49).
[10] Davenport inclined at first to regard rumplessness as a recessive,
but in his latest publication on the subject he definitely concludes
that it is an imperfect dominant. This conclusion accords well with
evidence quoted by Darwin (_An. and Plts._, II, ed. 2, p. 4) that
rumpless fowls may throw tailed offspring. (_Amer. Nat._, 1910, XLIV, p.
134.)
[11] Spillman, W. J., _Amer. Breeders Mag._, 1910, I, p. 178.
[12] Newsholme, _Lancet_, December 10, 1910, p. 1690.
[13] _Materials for the Study of Variation_, 1894, p. 358.
[14] Walker, G., _Johns Hopkins Hospital Bulletin_, XII, 1901, p. 129.
[15] Cp. R. H. Compton, _New Phytologist_, 1911, p. 249.
[16] _Arch. f. Entwickelungsmech._, 1907, XXIII, p. 249.
[17] Bull. Soc. Bot. de France, xxxiv, 1887, p. 182.
[18] R. Boyle, _The Origine of Formes and Qualities_, Oxford, 1666.
[19] Remarkable experiments on this question have lately been carried
out by R. H. Compton (_Camb. Phil. Soc._, XV, 1910, p. 495), showing
that in a certain Barley, "Plumage Corn," the average ratio of left to
right is about 1.5. A fuller paper has since been published by Compton,
_Jour. Genetics_, 1912, II, I, p. 53.
CHAPTER III
SEGMENTATION, ORGANIC AND MECHANICAL
Models may be and often have been devised imitating some of the
phenomena of division, but none of them have reproduced the peculiarity
which characterises divisions of living tissues, that _the position
of chemical differentiation_ is _determined by those divisions_. For
example, models of segmentation, whether radial or linear, may be made
by the vibration of plates as in the familiar Chladni figures of the
physical laboratory, or by the bowing of a tube dusted on the inside
with lycopodium powder, and in various other ways. The sand or the
powder will be heaped up in the nodes or regions of least movement, and
the patterns thus formed reproduce many of the geometrical features
of segmentation. But in the segmentations of living things the nodes
and internodes, once determined by the dividing forces, would each
become the seat of appropriate and distinct chemical processes leading
to the differentiation of the parts, and the deposition of the bones,
petals, spines, hairs, and other organs in relation to the meristic
ground-plan. The "ripples" of meristic division not merely divide but
differentiate, and when a "ripple" forks the result is not merely a
division but a reduplication of the organ through which the fork runs.
An example illustrating such a consequence is that of the half-vertebrae
of the Python. On the left side the vertebra is single (Fig. 7) and
bears a single rib, but on the right side a division has occurred with
the result that two half-vertebrae, each bearing a rib, are formed, one
standing in succession to the other. We cannot, indeed, imagine any
operation of physiological division carried out in such an organ as a
vertebra, passing through a plane at right angles to the long axis of
the body, which does not necessarily involve the further process of
reduplication.
As the meristic system of distribution spreads through the body,
chemical differentiations follow in its track, with segmentation and
pattern as the visible result. Could we analyse these simultaneous
phenomena and show how it is that the places of chemical differentiation
are determined by the system of division, progress would then be rapid.
It is here that all speculation fails.
[Illustration: FIGS. 7 and 8. Two examples of imperfect division in the
vertebræ of a python. _I_, the vertebræ 147-150 from the right side,
showing imperfect division between the 148th and 149th. The condition on
the left side of this vertebra was the same. _II_, the dorsal surface of
vertebræ 165-167. On the right side the 166th is double and bears two
ribs, but on the left side it is normal and has one rib only.]
Many attempts have been made to interpret the processes of division
and repetition, in terms of mechanics, or at least to refer them to
their nearest mechanical analogies, so far with little success. The
problem is beset with difficulties as yet insurmountable and of these
one must be especially noticed. In the living thing the process by which
repetition and patterns come into being consists partly in division but
partly also in growth. We have no means of studying the phenomena of
pattern-formation except in association with that of growth. Growth soon
ceases unless division takes place, and if growth is impossible division
soon ceases also. In consequence of this fact that the final pattern
is partly a product of growth, it can never be used as unimpeachable
evidence of the primary geometrical relations of the members as laid
down in the divisions.
In the last chapter in referring to the problem of repetition I
introduced an analogy, comparing the patterns of the organic world
with those produced in unorganised materials by wave-motion. In the
preliminary stage of ignorance, having no more trustworthy clue, I do
not think it wholly unprofitable to consider the applicability of this
analogy somewhat more fully. It possesses, as I hope to show, at least
so much validity as to encourage the belief that morphology may safely
discard one source of long-standing error and confusion.
Those who have studied the structure of parts repeated in series will
have encountered the old morphological problem of "Serial Homology,"
which has absorbed so much of the attention of naturalists and
especially of zoologists at various periods. This problem includes
two separate questions. The first of these is the origin in evolution
of the resemblance between two organs occurring in a repeated series,
of which the fore and hind limbs of Vertebrates are the prerogative
instance. From the fact that these resemblances can be traced very far,
often into minute details of structure, many anatomists have inclined to
the opinion that the resemblance must originally have been still more
complete, and that the two limbs, for instance, must have acquired their
present forms by the differentiation of two identical groups of parts.
Similar questions arise whenever parts are repeated in series, whether
the series be linear or radial, and, though less obviously, even when
the repetition is bilateral only. In each such example the question
arises, is the resemblance between the parts the remains of a still
closer resemblance, or is differentiation original? Sometimes the
view that these parts have arisen by the differentiation of a series
of identical parts is plausible enough, as for example when the
peculiarities of various appendages of a Decapod Crustacean are referred
to modifications of the Phyllopod series. In application to other cases
however we soon meet with difficulty, and the suggestion that the
segments of a vertebrate were originally all alike is seen at once to be
absurd, for the reason that a creature so constituted could not exist,
and that, differentiation of at least one anterior and one posterior
segment, is an essential condition of a viable organism consisting of
parts repeated in a linear series. Between these two terminal segments
it is possible to imagine the addition of one segment, or of a series
of approximately similar segments; but when once it is realised that
the terminals must have been differentiated from the beginning, it
will be seen that the problem of the origin of the resemblance between
segments is not rendered more comprehensible by the suggestion that
even the intervening members were originally alike. Seeing indeed that
some differentiation must have existed primordially it is as easy to
imagine that the original body was composed of a series grading from the
condition of the anterior segment to that of the posterior, as any other
arrangement. The existence of a linear or successive series in fact
postulates a polarity of the whole, and in such a system the conception
of an ideal segment containing all the parts represented in the others
has manifestly no place. The introduction of that conception though
sanctioned by the great masters of comparative anatomy, has, as I think,
really delayed the progress of a rational study of the phenomena of
division. The same notion has been applied to every class of repetition
both in animals and plants, generally with the same unhappy results. In
the cruder forms in which this doctrine was taught thirty years ago it
is now seldom expressed, but modified presentations of it still survive
and confuse our judgments.
The process of repetition of parts in the bodies of organisms is
however a periodic phenomenon. This much, provided we remain free from
prejudice as to the nature and causation of the period or rhythm, we
may safely declare, and a comparison may thus be instituted between the
consequences of meristic repetition in the bodies of living things and
those repetitions which in the inorganic world are due to rhythmical
processes. Of such processes there is a practically unlimited diversity
and we have nothing to indicate with which of them our repetitions
should rather be compared.
[Illustration: FIG. 9. Osmotic growths simulating segmentation. (After
Leduc.)]
In some respects perhaps the best models of living organisms yet made
are the "osmotic growths" produced by Leduc.[1] These curious structures
were formed by placing a fragment of a salt, for instance calcium
chloride, in a solution of some colloidal substance. As the solid takes
up water from the solution a permeable pellicle or membrane is formed
around it. The vesicle thus enclosed grows by further absorption of
water, often extending in a linear direction, and in many examples this
growth occurs by a series of rhythmically interrupted extensions. Some
of the growths thus formed are remarkably like organic structures,
and might pass for a series of antennary segments or many other
organs consisting of a linear series of repeated parts. In admitting
the essential resemblance between these "osmotic growths" and living
bodies or their organs I lay less stress on the general conformation
of the growths, which often as Leduc points out, recall the forms
of fungi or hydroids, but rather on the fact that the interruptions
in the development of these systems are so closely analogous to the
segmentations or repetitions of parts characteristic of living things
(Fig. 9). In the same way I am less impressed by Leduc's models of
Karyokinesis, wonderful as they nevertheless are, for the division is
here imitated by putting separate drops on the gelatine film. What we
most want to know is how in the living creature one drop becomes two.
The models of linear segmentation have the remarkable merit that they do
in some measure imitate the process of actual division or repetition.
So in a somewhat modified method Leduc, by causing the diffusion
of a solution in a gelatine film, produced rhythmical or periodic
precipitations strikingly reminiscent of various organic tissues, for
here also the process of periodic repetition is imitated with success.
It is a feature common to these and to all other rhythmical repetitions
produced by purely mechanical forces that there is resemblance between
the members of the series, and that this similarity of conformation may
be maintained in most complex detail. When however in the mechanical
series some of the members differ from the rest we have no difficulty
in recognising that these differences--which correspond with the
differentiations of the organic series--are due to special heterogeneity
in the conditions or in the materials, and it never occurs to us to
suppose that all the members must have been primordially alike. For
example, in the case of ripple-marks on the sand, which I choose as one
of the most familiar and obvious illustrations of a repeated series
due to mechanical agencies, if we notice one ripple different in form
from those adjacent to it, we do not suppose that this variation must
have been brought about by deformation of a ripple which was at first
formed like the others, but we ascribe it to a difference in the sand at
that point, or to a difference in the way in which the wind or the tide
dealt with it. We may press the analogy further by observing that in as
much as such a series of waves has a beginning and an end, it possesses
polarity like that of the various linear series of parts in organisms,
and even the formation of each member must influence the shape of its
successor. Since in an organism the beginning and end of the series
are always included, some differentiation among the repetitions must
be inevitable. If therefore it be conceded, as I think it must, that
segmentation and pattern are the consequence of a periodic process we
realize that it is at least as easy to imagine the formation of such a
series of parts having family likeness combined with differentiation
as it would be to conceive of their arising primordially as a series
of identical repetitions. The suggestion that the likenesses which we
now perceive are the remains of a still more complete resemblance is a
substitution of a more complex conception for a simpler one.
The other question raised by the problem of Serial Homology is how far
there is a correspondence between individual members of series when
the series differ from each other either in the number of parts, or in
the mode of distribution of differentiation among them. Students, for
example, of vertebrate morphology debate whether the _n_th vertebra
which carries the pelvic girdle in Lizard A is individually homologous
with the _n_ + _x_th vertebra which fulfils this function in Lizard B,
or whether it is not more truly homologous with the vertebra standing in
the _n_th ordinal position, though that vertebra in Lizard B is free.
In various and more complex aspects the same question is debated in
regard to the cranial and spinal nerves, the branches of the aorta,
the appendages of Arthropoda, and indeed in regard to all such series
of differentiated parts in linear or successive repetition. Persons
exercised with these problems should before making up their minds
consider how similar questions would be answered in the case of any
series of rhythmical repetitions formed by mechanical agencies. In the
case of our illustration of the ripples in the sand, given the same
forces acting on the same materials in the same area, the number of
ripples produced will be the same, and the _n_th ripple counting from
the end of the series will stand in the same place whenever the series
is evoked. If any of the conditions be changed, the number and shapes
can be changed too, and a fresh "distribution of differentiation"
created. Stated in this form it is evident that the considerations
which would guide the judgment in the case of the sand ripples are not
essentially different from those which govern the problem of individual
homology in its application to vertebrae, nerves, or digits.
The fact that the unit of repetition is also the unit of growth is the
source of the obscurity which veils the process. When we compare the
skeleton of a long-tailed monkey with that of a short-tailed or tailless
ape we see at once how readily the additional series of caudal segments
may be described as a consequence of the propagation of the "waves" of
segmentation beyond the point where they die out in the shorter column,
and we see that with an extension of the series of repetitions there is
growth and extension of material.
The considerations which apply to this example will be found operating
in many cases of the variation of terminal members of linear series.
Some of these series, like the teeth of the dog, end in a terminal
member of a size greatly reduced below that of the next to it. Even when
there is thus a definite specialisation of the last member of the series
it not infrequently happens that the addition, by variation, of a member
beyond the normal terminal, is accompanied by a very palpable increase
in size of the member which stands numerically in the place of the
normal terminal.[2] So also with variation in the number of ribs, when
a lumbar vertebra varies homoeotically into the likeness of the last
dorsal and bears a rib, the rib placed next in front of this, which in
the normal trunk is the last, shows a definite increase in development.
The consequences of such homoeoses are sometimes very extensive,
involving readjustments of differentiation affecting a long series of
members, as may easily be seen by comparing the vertebral columns of
several individual Sloths[3] (whether _Bradypus_ or _Choloepus_) to take
a specially striking example.
It may be urged that no feature as yet enables us to perceive wherein
lies the primary distinction which determines such variation, whether it
is due to a difference in the dividing forces or in the material to be
divided. If for instance we were to imitate such a series of segments
by pressing hanging drops of a viscous fluid out of a paint-tube by
successive squeezes, the number of times the tube is contracted before
it is empty will give the number of the segments, but their size may
depend either on the force of the contractions or on the capacity of
the tube, or on various other factors. Nevertheless in the case of the
variation of terminal members, whatever be the nature of the rhythmical
impulse which produces the series of organs, the elevation of the
normally terminal member in correspondence with the addition of another
is what we should expect.
If the organism acquired its full size first and the delimitation of
the parts took place afterwards, there might be some hope that the
resemblance between living patterns and those mechanically caused by
wave-motion might be shown to be a consequence of some real similarity
of causation, but in view of the part played by growth, appeal to these
mechanical phenomena cannot be declared to have more than illustrative
value. Similarly in as much as living patterns appear, and almost
certainly do in reality come into existence by a rhythmical process,
comparisons of these patterns with those developed in crystalline
structures, and in the various fields of force are, as it seems to me,
inadmissible, or at least inappropriate.
However their intermittence be determined, the rhythms of division must
be looked upon as the immediate source of those geometrically ordered
repetitions universally characteristic of organic life. In the same
category we may thus group the segmentation of the Vertebrates and of
the Arthropods, the concentric growth of the Lamellibranch shells or of
Fishes' scales, the ripples on the horns of a goat, or the skeletons of
the Foraminifera or of the Heliozoa. In the case of plant-structures
Church[4] has admirably shown, with an abundance of detail, how on
analysis the definiteness of phyllotaxis is an expression of such
rhythm in the division of the apical tissues, and how the spirals
and "orthostichies" displayed in the grown plant are its ultimate
consequences. The problem thus narrows itself down to the question of
the mode whereby these rhythms are determined.
It is natural that we should incline to refer them to a chemical source.
If we think of the illustration just given, of the segmentation of a
viscous fluid into drops by successive contractions of a soft-walled
tube we can, I think, conceive of such rhythmic contractions as due to
summations of chemical stimuli, somewhat as are the beats of the heart.
But when we recognize the vast diversity of materials the distribution
of which is determined by an ostensibly similar rhythmic process it
seems hopeless to look forward to a directly chemical solution. That the
chemical degradation of protoplasm or of materials which it contains is
the source of the energy used in the divisions cannot be in dispute, but
that these divisions can be themselves the manifestations of chemical
action seems in the highest degree improbable.
We may therefore insist with some confidence on the distinction between
the Meristic and the substantive constitution of organisms, between,
that is to say, the system according to which the materials are divided
and the essential composition of the materials, conscious of the fact
that the energy of division is supplied from the materials, and that
in the ontogeny the manner in which the divisions are effected must
depend secondarily on the nature of the substances to be divided.
The mechanical processes of division remain a distinguishable group
of phenomena, and variations in the substances to be distributed in
division may be independent of variations in the system by which the
distribution is effected.
Modern genetic analysis supplies many remarkable examples of this
distinction. When formerly we compared the leaves of a normal palmatifid
Chinese Primula with the pinnatifid leaves[5] of its fern-leaved variety
we were quite unable to say whether the difference between the two types
of leaf was due to a difference in the material cut up in the process
of division or to a difference in that process itself. Knowledge that
the distinction is determined by a single segregable factor tends to
prove that the critical difference is one of substance. So also in the
Silky fowl we know that the condition of its feathers is due to the
absence of some one factor present in the normal form. We may conceive
such differences as due to change of form in the successive "waves" of
division, but we cannot yet imagine segregation otherwise than as acting
by the removal or retention of a material element. Future observation
by some novel method may suggest some other possibility, but such cases
bring before us very clearly the difficulties by which the problem is
beset.
[Illustration: FIG. 10. The palm-and fern type of leaf in _Primula
Sinensis_. The palm is dominant and the fern is recessive.]
In another region of observation phenomena occur which as it seems to
me put it beyond question that the meristic forces are essentially
independent of the materials upon which they act, save, in the remoter
sense, in so far as these materials are the sources of energy. The
physiology of those regenerations and repetitions which follow upon
mutilation supplies a group of facts which both stimulate and limit
speculation. No satisfactory interpretations of these extraordinary
occurrences has ever been found, but we already know enough to feel
sure that in them we are witnessing indications which should lead
to the discovery of the true mechanics of repetition and pattern.
The consequences of mutilation in causing new growth or perhaps more
strictly in enabling new growth to take place, are such that they cannot
be interpreted as responses to chemical stimuli in any sense which
the word chemical at present connotes. Powers are released by mutilation
of which in the normal conditions of life no sign can be detected. All
who have tried to analyse the phenomena of regeneration are compelled
to have recourse to the metaphor of equilibrium, speaking of the normal
body as in a state of strain or tension (Morgan) which when disturbed by
mutilation results in new division and growth. The forces of division
are inacessible to ordinary means of stimulation. Applications, for
example, of heat or of electricity excite no responses of a positive
kind unless the stimuli are so violent as to bring about actual
destruction.[6] These agents do not, to use a loose expression, come
into touch with the meristic forces. Changes in the chemical environment
of cells may, as in the experiments of Loeb and of Stockard produce
definite effects, but the facts suggest that these effects are due
rather to alterations in the living material than to influence exerted
directly on the forces of division themselves.
By destruction of tissue however the forces both of growth and of
division also may often be called into action with a resulting
regeneration. Interruption of the solid connexion between the parts may
produce the same effects, as for example when the new heads or tails
grow on the divided edges of Planarians (Morgan), or when from each half
embryo partially separated from its normally corresponding half, a new
half is formed with a twin monster as the result.
Often classed with regenerations but in reality quite distinct from
them are those special and most interesting examples where the growth
of a _paired_ structure is excited by a simple wound. Some of the best
known of these instances are presented by the paired extra appendages
of Insects and Crustacea. Some years ago I made an examination of all
the examples of such monstrosities to which access was to be obtained,
and it was with no ordinary feeling of excitement that I found that
these supernumerary structures were commonly disposed on a recognizable
geometrical plan, having definite spatial relations both to each other
and to the normal limb from which they grew. The more recent researches
of Tornier[7] and especially his experiments on the Frog have shown
that a cut into the posterior limb-bud induces the outgrowth of such a
_pair_ of limbs at the wounded place. Few observations can compare with
this in novelty or significance; and though we cannot yet interpret
these phenomena or place them in their proper relations with normal
occurrences, we feel convinced that here is an observation which is no
mere isolated curiosity but a discovery destined to throw a new light on
biological mechanics. The supernumerary legs of the Frog are evidently
grouped in a system of symmetry similar to that which those of the
Arthropods exhibit, and though in Arthropods paired repetitions have not
been actually produced by injury under experimental conditions we need
now have no hesitation in referring them to these causes as Przibram has
done.
At this point some of the special features of the supernumerary
appendages become important. First they may arise at any point on the
normal limb, being found in all situations from the base to the apex.
Nor are they limited as to the surface from which they spring, arising
sometimes from the dorsal, anterior, ventral, or posterior surfaces, or
at points intermediate between these principal surfaces.
With rare and dubious exceptions, the parts which are contained in these
extra appendages are only those which lie _peripheral to their point of
origin_. Thus when the point of origin is in the apical joint of the
tarsus, the extra growth if completely developed consists of a double
tarsal apex bearing two pairs of claws. If they arise from the tibia,
two complete tarsi are added. If they spring from the actual base of
the appendage then two complete appendages may be developed in addition
to the normal one. We must therefore conclude that in any point on a
normal appendage the power exists which, if released, may produce a bud
containing in it a paired set of the parts peripheral to this point.
[Illustration: FIG. 11. Diagrams of the geometrical relations which are
generally exhibited by extra pairs of appendages in Arthropoda. The
sections are supposed to be those of the apex of a tibia in a beetle.
_A_, anterior, _P_, posterior, _D_, dorsal, _V_, ventral. _M_^{1},
_M_^{2} are the imaginary planes of reflexion. The shaded figure is in
each case a limb formed like that of the other side of the body, and the
outer unshaded figures are shaped like the normal for the side on which
the appendages are. On the several radii are shown the extra pairs in
their several possible relations to the normal from which they arise.
The normal is drawn in thick lines in the center.]
Next the geometrical relations of the halves of the supernumerary pair
are determined by the position in which they stand in regard to the
original appendage. These relations are best explained by the diagram
(Fig. 11), from which it will be seen that the two supernumerary
appendages stand as images of each other; and, of them, that which is
adjacent to the normal appendage forms an image of it. Thus if the
supernumerary pair arise from a point on the dorsal surface of the
normal appendage, the two _ventral_ surfaces of the extra pair will
face each other. If they arise on the anterior surface of the normal
appendage, their morphologically posterior surfaces will be adjacent,
and so on.
These facts give us a view of the relations of the two halves of a
dividing bud very different from that which is to be derived from the
exclusive study of normal structures. Ordinary morphological conceptions
no longer apply. The distribution of the parts shows that the bud or
rudiment which becomes the supernumerary pair may break or open out in
various ways according to its relations to the normal limb. Its planes
of division are decided by its geometrical relations to the normal body.
Especially curious are some of the cases in which the extra pair are
imperfectly formed. The appearance produced is then that of two limbs
in various stages of coalescence, though in reality of course they are
stages of imperfect separation. The plane of "coalescence" may fall
anywhere, and the two appendages may thus be compounded with each other
much as an object partially immersed in mercury "compounds" with its
optical image reflected from the surface.
Supernumerary paired structures are not usually, if ever, formed when
an appendage is simply amputated. Cases occasionally are seen which
nevertheless seem to be of this nature. Borradaile,[8] for example,
described a crab (_Cancer pagurus_) having in place of the right chela
three _small_ chelae arising from a common base, where the appearances
suggested that the three reduced limbs replaced a single normal limb.
From the details reported however it seems still possible that one of
the chelae (that lettered F. I in Borradaile's figure) may be the normal
one, and the other two an extra pair. The chela which I suspect to be
the normal is in several respects deformed as well as being reduced in
size, and this deformity may perhaps have ensued as a consequence of the
same wound which excited the growth of the extra pair. Its reduced size
may be due to the same injury, which may quite well have checked its
growth to full proportions.
Admitting doubt in these ambiguous cases it seems to be a general rule
that for the production of the extra pair the normal limb should persist
in connexion with the body. Moreover it is practically certain that
in no case can a _single_, viz. an unpaired, duplicate of the normal
appendage grow from it. Many examples have been described as of this
nature, but all of them may be with confidence regarded as instances of
a supernumerary pair in which only the two morphologically anterior or
the two morphologically posterior surfaces are developed. We have thus
the paradox that a limb of one side of the body, say the right, has in
it the power to form a pair of limbs, right and left, as an outgrowth of
itself, but cannot form a second left limb alone.
A very interesting question arises whether it is strictly correct to
describe the extra pair as a right and a left, or whether they are not
rather two lefts or two rights of which one is reversed. This question
did not occur to me when in former years I studied these subjects. It
was suggested to me by Dr. Przibram. The answer might have an important
bearing on biological mechanics, but I know no evidence from which the
point can be determined with certainty. In order to decide this question
it would be necessary to have cases in which the paired repetition
affected a limb markedly differentiated on the two sides of the body,
and of course the development of the extra parts in order to be decisive
must be fairly complete. One example only is known to me which at all
satisfies these requirements, that of the lobster's chela figured (after
Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig.
184, III.
Here the drawing distinctly suggests that one of the extra
dactylopodites, namely that lettered R, is differentiated as a left
and not merely a reversed right. For the teeth on this dactylopodite
are those of a cutting claw, not of a crushing claw, whereas the
dactylopodites R' and L' bear crushing teeth. The figure makes it fairly
certain also that the limb affected was a crushing claw. Accepting this
interpretation, we reach the remarkable conclusion that the bud of new
growth consisted of halves differentiated into cutter and crusher as the
normal claws are, and that the extra crusher is geometrically a left
but physiologically a right. Though shaped as a left in respect of the
direction in which it points, the extra crusher is really an optically
reversed right, while the dactylopodite R, which is placed pointing like
a right, is really a reversed left (Fig. 12).
[Illustration: FIG. 12. Right claw of lobster bearing a pair of extra
dactylopodites (after van Beneden). The fine toothing on R suggests that
this is part of a cutting claw, though the limb bearing it is a crusher.]
If these indications are reliable[9] and are established by further
observation we shall be led to the conclusion that the bud which
becomes an extra pair of limbs does not merely contain the parts proper
to the side on which it grows, but is comparable with the original
zygotic cell, and consists not simply of two halves, but of two halves
differentiated as a right and a left like the two halves of the normal
body.
Phenomena of this kind, evoked by mutilation or injury, together with
the cognate observations on regeneration throw very curious lights
on the nature of living things. To an understanding of the nature of
the mechanics of living matter and its relation to matter at large
they offer the most hopeful line of approach. I allude especially to
the examples in which it has been established that the part which is
produced after mutilation is a structure different from that which was
removed. The term "regeneration" was introduced before such phenomena
were discovered, and though every one recognizes its inapplicability
to these remarkable cases, the word still misleads us by presenting
a wrong picture to the mind. The expression "heteromorphosis" (Loeb)
has been appropriately applied to various phenomena of this kind, and
Morgan has given the name "morphallaxis" to another group of cases in
which the renewal occurs by the transformation of a previously existing
part.[10] But we must continually remember that all these occurrences
which we know only as abnormalities and curiosities must in reality be
exemplifications of the normal mechanics of division and growth. The
conditions needed to call them forth are abnormal, but the responses
which the system makes are evidences of its normal constitution. When
therefore, for example, the posterior end of a worm produces a reversed
tail from its cut end we have a proof that there must be in the normal
body forces ready to cause this outgrowth. The new structure is not
an ill-shaped head-end, for, as Morgan shows, the nephridial ducts
have their funnels perforating the segments in a reversed direction.
The "tension" of growth is actually reversed.[11] So also when in a
Planarian amputation of the body immediately behind the head leads to
the formation of a new reversed head at the back of the normal head,
while amputation further back leads to the regeneration of a new tail,
these responses give indications of forces normally present in the
body of the Planarian. Such facts open up a great field of speculation
and research. Especially important it would be to determine where the
critical region may be at which the one response is replaced by the
other. I suppose it is even possible that there is some neutral zone in
which neither kind of response is made.
Physical parallels to the phenomena of regeneration are not easy
to find and we still cannot penetrate beyond the empirical facts.
Przibram has laid stress on the general resemblance between the new
growth of an amputated part in an animal and the way in which a broken
crystal repairs itself when placed in the mother-solution. That the
two processes have interesting points of likeness cannot be denied.
It must however never be forgotten that there is one feature strongly
distinguishing the two; for I believe it is universally recognized
by physicists that all the phenomena of geometrical regularity which
crystals display are ultimately dependent on the forms of the particles
of the crystalline body. This cannot in any sense be supposed to hold
in regard to protoplasm or its constituents. The definiteness of
crystals is also an unlikely guide for the reason that it is absolute
and perfect, or in other words because this kind of regularity cannot
be disturbed at all without a change so great that the substance itself
is altered; whereas we know that the forms of living things are capable
of such changes, great and small, that we must regard perfection of
form, whether manifested in symmetry or in number, as an ideal which
will only be produced in the absence of disturbance. The symmetry of the
living things is like the symmetry of the concentric waves in a pool
caused by a splash. Perfect circles are made only in the imaginary case
of mathematical uniformity, but the system maintains an approximate
symmetry though liable to manifold deformation.
Since the geometrical order of the living body cannot be a direct
function of the materials it must be referred to some more proximate
control. In renewing a part the body must possess the power of seizing
particles of many dissimilar kinds, and whirl them into their several
and proper places. The action in renewal, like that of original growth,
may be compared--very crudely--with the action of a separator which
simultaneously distributes a variety of heterogeneous materials in an
orderly fashion; but in the living body the thing distributed must
rather be the _appetency_ for special materials, not the materials
themselves.
If the analogy of crystals be set aside and we seek for other parallels
to regeneration there are none very obvious. I have sometimes wondered
whether it might not be possible to institute a fruitful comparison
between the renewal of parts and the reformation of waves of certain
classes after obliteration. In several respects, as I have already said,
some curious resemblances with the repetitions formed by wave-motion
are to be traced in our organic phenomena, and though admitting that
I cannot develop these comparisons, I think nevertheless they may be
worth bearing in mind. When, after obliteration, an eddy in a stream,
or a ripple-mark (a more complex case of eddy-formation) in blown sand
is re-formed, we have an example in which pattern is reconstituted and
growth takes place not by virtue of the composition of the materials--in
this case the water or the sand--but by the way in which they are acted
upon by extraneous forces.
A feature in the actual mode by which ripple-marks are reconstituted may
not be without interest in connexion with our phenomena of regeneration.
When, for example, the wind is blowing steadily over a surface of fine,
dry sand, the familiar ripple-marks are formed by a heaping of the sand
in lines transverse to the direction of the wind. The heaping is due to
the formation of eddies corresponding with positions of instability.
When the wind is steady and the sand homogeneous, the distances between
the ripples, or wave-lengths, are sensibly equal. If while the wind
continues to blow, the ripples are obliterated with a soft brush they
will quickly be re-formed over the whole area, but I have noticed that
at first their wave-length is approximately half that of the ripples
in the undisturbed parts of the system.[12] The normal wave-length is
restored by the gradual accentuation of alternate ripples. Of course
the sand-ripples are in reality slowly travelling forward in the
direction towards which the wind is blowing, and for this our living
segmentations afford no obvious parallel, but the appearances in the
area of reformation, and especially the forking of the old ridges where
they join the new ones, are curiously reminiscent of the irregularities
of segmentation seen in regenerated structures. The value of the
considerations adduced in the chapter is, I admit, very small. The
utmost that can be claimed for them is that mechanical segmentations,
like those seen in ripple-mark, or in Leduc's osmotic growths, show
how by the action of a continuous force in one direction, repeated
and serially homologous divisions can be produced having features of
similarity common to those repetitions by which organic forms and
patterns are characterised. The analogy supplies a vicarious picture of
the phenomena which in default of one more true may in a slight degree
assist our thoughts. It suggests that the rhythms of segmentation may be
the consequence of a single force definite in direction and continuously
acting during the time of growth. The polarity of the organism would
thus be the expression of the fact that this meristic force is
definitely directed after it has once been excited, and the reversal
seen in some products of regeneration suggest further that it is capable
of being reflected. This polarity cannot be a property of the material,
as such, but is determined by a force acting on that material, just as
the polarity of a magnet is not determined by the arrangement of its
particles, but by the direction in which the current flows.
To some it may appear that even to embark on such discussions as this is
to enter into a perilous flirtation with vitalistic theories. How, they
may ask, can any force competent to produce chemical and geometrical
differentiation in the body be distinguished from the "Entelechy" of
Driesch? Let me admit that in this reflexion there is one element of
truth. If those who proclaim a vitalistic faith intend thereby to
affirm that in the processes by which growth and division are effected
in the body, a part is played by an orderly force which we cannot
_now_ translate into terms of any known mechanics, what observant man
is not a vitalist? Driesch's first volume, putting as it does into
intelligible language that positive deduction from the facts--especially
of regeneration--should carry a vivid realisation of this truth to any
mind. If after their existence is realised, it is desired that these
unknown forces of order should have a name, and the word entelechy is
proposed, the only objection I have to make is that the adoption of a
term from Aristotelian philosophy carries a plain hint that we propose
to relegate the future study of the problem to metaphysic.
From this implication the vitalist does not shrink. But I cannot find in
the facts yet known to us any justification of so hopeless a course. It
was but yesterday that the study of _Entwicklungsmechanik_ was begun,
and if in our slight survey we have not yet seen how the living machine
is to be expressed in terms of natural knowledge that is poor cause for
despair. Driesch sums up his argument thus:[13]
"It seems to me that there is only one conclusion possible. If we are
going to explain what happens in our harmonious-equipotential systems
by the aid of causality based upon the constellation of single chemical
factors and events, there _must_ be some such thing as a machine. Now
the assumption of the existence of a machine proves to be absolutely
absurd in the light of the experimental facts. _Therefore there can
be neither any sort of a machine nor any sort of causality based upon
constellation underlying the differentiation of harmonious-equipotential
systems._"
"For a machine, typical with regard to the three chief dimensions
of space, cannot remain itself if you remove parts of it or if you
rearrange its parts at will."
To the last clause a note is added as follows:
"The pressure experiments and the dislocation experiments come into
account here; for the sake of simplicity they have not been alluded to
in the main line of our argument."
I doubt whether any man has sufficient knowledge of all possible
machines to give reality to this statement. In spite also of the
astonishing results of experiments in dislocation, doubt may further be
expressed as to whether they have been tried in such variety or on such
a scale as to justify the suggestion that the living organism remains
itself if its parts are rearranged at will. All we know is that it can
"remain itself" when much is removed, and when much rearrangement has
been affected, which is a different thing altogether.
I scarcely like to venture into a region of which my ignorance is so
profound, but remembering the powers of eddies to re-form after partial
obliteration or disturbance, I almost wonder whether they are not
essentially machines which remain themselves when parts of them are
removed.
Real progress in this most obscure province is not likely to be made
till it attracts the attention of physicists; and though they for long
may have to forego the application of exact quantitative methods, I
confidently anticipate that careful comparison between the phenomena
of repetition formed in living organisms and the various kinds of
segmentation produced by mechanical agencies would be productive of
illuminating discoveries.
FOOTNOTES:
[1] Stéphane Leduc, _Théorie Physico-Chymique de la Vie_, Paris, 1910.
[2] _Materials for the Study of Variation_, No. 249, p. 217; and p. 272.
[3] _Materials_, p. 118.
[4] Church, A. H., _On the Relation of Phyllotaxis to Mechanical Laws_,
London, 1904.
[5] It is a question whether the dominance of the palmatifid leaf over
the pinnatifid is not really an example of the dominance of a lower
number of segmentations over a higher. From the uncertainty whether two
given leaves of two separate plants are actually comparable one cannot
institute quite satisfactory numerical comparisons, but I think the view
that the "Fern" leaf has more lobes than an otherwise similar "Palm"
leaf may be fairly maintained. If this be admitted, the "Palm" leaf
represents the dominant low number and its round shape is a consequence
of the greater powers of growth which are so often possessed by the
members of a shorter series.
[6] It is perhaps of importance to remember that in certain species
of bacteria (e. g. _Bacillus Anthracis_) division may cease where the
organism is cultivated under certain artificial conditions though growth
continues. In this way very long unsegmented threads are produced.
[7] _Arch. f. Entwm._, XX, 1905, p. 76; _Sitzungsb. d. Ges. Naturf._,
Berlin, 1907, p. 41, etc.
[8] Borradaile, L. A., _Jour. Marine Zool._, 1897, No. 8.
[9] Dr. Przibram, I should mention, concludes that on the whole the
facts are against this interpretation, but as more evidence is certainly
required, I call attention to the possibility.
[10] Morgan, T. H., _Regeneration_, 1901.
[11] It would be interesting to know whether growth continues at the
original posterior end after the new "posterior" end has been formed in
front.
[12] In the actual case observed, the ripples unsmoothed had a
wave-length of about 2-1/2 inches; and when the new ones were first
formed, there were about 30 ridges in the length originally traversed by
15 or 16.
[13] _The Science and Philosophy of the Organism_; Gifford Lectures,
1907. London, 1908, p. 141.
CHAPTER IV
THE CLASSIFICATION OF VARIATION AND THE NATURE OF
SUBSTANTIVE FACTORS
We have now seen that among the normal physiological processes the
phenomena of division form a recognisable, and in all likelihood a
naturally distinct group. Variations in these respects may thus be
regarded as constituting a special class among variations in general.
The substantive variations have only one property in common--the
negative one that they are not Meristic. The work of classifying them
and distinguishing them according to their several types demands a
knowledge of the chemistry of life far higher than that to which science
has yet attained. In reference to some of the simplest variations Garrod
has introduced the appropriate term "Chemical sports." The condition in
man known as Alkaptonuria in which the urine is red is due especially
to the absence of the enzyme which decomposes the excretory substance,
alkapton. The "chemical sport" here consists in the inability to break
up the benzene ring. The chemical feature which distinguishes and is the
proximate cause of several colour-varieties can now in a few cases be
declared. The work of Miss Wheldale has shown that colour-varieties may
be produced by the absence of the chromogen compound the oxidation of
which gives rise to sap-colours, by differences in the completeness of
this process of oxidation, and by a process of reduction supervening on
or perhaps suppressing the oxidation. Some of these processes moreover
may be brought about by the combined action of two bodies, the one an
enzyme, for example an oxygenase, and the other a substance regarded as
a peroxide, contributing the oxygen necessary for the oxidation to take
place. Variation in colour may thus be brought about by the addition or
omission of any one of the bodies concerned in the action.
Similar variations, or rather similar series of variations will
undoubtedly hereafter be identified in reference to all the various
kinds of chemical processes upon which the structure and functions of
living things depend. The identification of these processes and of
the bodies concerned in them will lead to a real classification of
Substantive Variations.
To forecast the lines on which such classification will proceed is to
look too far ahead. We may nevertheless anticipate with some confidence
that future analysis will recognise among the contributing elements,
some which are intrinsic and inalienable, and others which are extrinsic
and superadded.
We already know that there may be such interdependence among the
substantive characters that to disentangle them will be a work of
extreme difficulty. The mere fact that in our estimation characters
belong to distinct physiological systems is no proof of their actual
independence. In illustration may be mentioned the sap-colour in Stocks
and the development of hoariness on the leaves and stems, which Miss
Saunders's experiments have shown to be intimately connected, so that
in certain varieties no hoariness is produced unless the elements for
sap-colour are already present in the individual plant.
The first step in the classification of substantive variations is
therefore to determine which are due to the addition of new elements
or factors, and which are produced by the omission of old ones. _A
priori_ there is no valid criterion by which this can be known, and
actual experiments in analytical breeding can alone provide the
knowledge required. Some very curious results have by this method been
obtained, which throw an altogether unexpected light on these problems.
For example, in order that the remarkable development of mesoblastic
black pigment characteristic of the Silky Fowl should be developed, it
is practically certain that two distinct variations from such a type
as _Gallus bankiva_ must have occurred. I assume, as is reasonable,
that _G. bankiva_ has genetic properties similar to those of the Brown
Leghorn breed which has been used in the experiments which Mr. Punnett
and I have conducted. _Gallus bankiva_ was not available but the Brown
Leghorn agrees with it very closely in colouration, and probably in
the general physiology of its pigmentation. Setting aside the various
structural differences between the two breeds, the Silky is immediately
distinguished from the Leghorn by the fact that the skin of the whole
body including that of the face and comb appears to be of a deep
purplish colour. The face and comb of the Leghorn are red and the skin
of the body is whitish yellow. On examination it is found that the
purple colour of the Silky is in reality due to the distribution of a
deep black pigment in the mesoblastic membranes throughout the body. The
somatopleura, the pleura, _pia mater_, the dermis, and in most organs
the connective tissue and the sheaths of the blood-vessels, are thus
impregnated with black. No such pigmentation exists in the Leghorn.
As the result of an elaborate series of experimental matings we have
proved that the distinction between the Leghorn and the Silky consists
primarily in the fact that the Silky possesses a pigment-producing
factor, _P_, which is not present in the Leghorn.
This variation must undoubtedly have been one of _addition_. But besides
this there is another difference of an altogether dissimilar nature; for
the Brown Leghorn possesses a factor which has the power of partially or
completely restricting the operation of the pigment-producing factor,
_P_. Moreover in respect of this pigment-restricting factor which we
may call _D_, the sexes of the Brown Leghorn differ, for the male is
homozygous or _DD_, but the female is heterozygous, _Dd_. Thus in order
that the black-skinned breed could be evolved from such a type as a
Brown Leghorn it must be necessary _both_ that _P_ should be added and
that _D_ should drop out. We have not the faintest conception of the
process by which either of these events have come to pass, but there is
no reasonable doubt that in the evolution of the Silky fowl they did
actually happen.
We may anticipate that numerous interdependences of this kind will be
discovered.
Before any indisputable progress can be made with the problem of
evolution it is necessary that we should acquire some real knowledge
of the genesis of that class of phenomena which formed the subject of
the last chapter. So long as the process of division remains entirely
mysterious we can form no conception even of the haziest sort as
to the nature of living organisms, or of the proximate causes which
determine their forms, still less can we attempt any answer to those
remoter questions of origin and destiny which form the subject of the
philosopher's contemplation. It is in no spirit of dogmatism that I
have ventured to indicate the direction in which I look for a solution,
though I have none to offer. It may well be that before any solution is
attained, our knowledge of the nature of unorganised matter must first
be increased. For a long time yet we may have to halt, but we none
the less do well to prepare ourselves to utilise any means of advance
that may be offered, by carefully reconnoitering the ground we have to
traverse. The real difficulty which blocks our progress is ignorance of
the nature of division, or to use the more general term, of repetition.
Let us turn to the more familiar problem of the causes of variation. Now
since variation consists as much in meristic change as in alteration in
substance or material, there is one great range of problems of causation
from which we are as yet entirely cut off. We know nothing of the
causation of division, and we have scarcely an observation, experiment
or surmise touching the causes by which the meristic processes may be
altered.
Of the way in which variations in the substantive composition of
organisms are caused we have almost as little real evidence, but we are
beginning to know in what such variations must consist. These changes
must occur either by the addition or loss of factors.
We must not lose sight of the fact that though the factors operate by
the production of enzymes, of bodies on which these enzymes can act, and
of intermediary substances necessary to complete the enzyme-action, yet
these bodies themselves can scarcely be themselves genetic factors, but
consequences of their existence. What then are the factors themselves?
Whence do they come? How do they become integral parts of the organism?
Whence, for example, came the power which is present in a White Leghorn
of destroying--probably reducing--the pigment in its feathers? That
power is now a definite possession of the breed, present in all its
germ-cells, male and female, taking part in their symmetrical divisions,
and passed on equally to all as much as is the protoplasm or any other
attribute of the breed. From the body of the bird the critical and
efficient substance could in all likelihood be isolated by suitable
means, just as the glycogen of the liver can be. But even when this
extraction has been accomplished and the reducing body isolated, we
shall know no more than we did before respecting the mode by which
the power to produce it was conferred on the fowl, any more than we
know how the walls of its blood-vessels acquired the power to form a
fibrin-ferment.
It is when the scope of such considerations as this are fully grasped
that we realise the fatuousness of the conventional treatment which the
problem of the causes of variation commonly receives. Environmental
change, chemical injury, differences in food supply, in temperature,
in moisture, or the like have been proposed as "causes." Admitting
as we must do, that changes may be produced--usually inhibitions of
development--by subjecting living things to changes in these respects,
how can we suppose it in the smallest degree likely that very precise,
new, and adaptative powers can be conferred on the germs by such
treatment? Reports of positive genetic consequences observed comparable
with those I have mentioned, become from time to time current. We
should I think regard them with the gravest doubt. Few, so far as I am
aware, have ever been confirmed, though clear and repeated confirmation
should be demanded before we suffer ourselves at all to build upon such
evidence. In a subsequent chapter some of these cases will be considered
in detail.
In no class of cases would the transmission of an acquired character
superficially appear so probable as in those where power of resisting
the attack of a pathogenic organism is acquired in the lifetime of
the zygote. The possession of such a power is moreover a distinction
comparable with those which differentiate varieties and species. It
is due to the development in the blood of specific substances which
pervade the whole fluid. This development is exactly one of those
"appropriate responses to stimuli" which naturalists who incline to
regard adaptation as a direct consequence of an environmental influence
might most readily invoke as an illustration of their views. And yet all
evidence is definitely unfavourable to the suggestion of an inheritance
of the acquired power of resistance. Such change as can be perceived
in the virulence of the attacks on successive generations may be most
easily regarded as due to the extermination of the more susceptible
strains, and perhaps in some measure to variation in the invading
organisms themselves, an "acquired character" of quite different import.
The specific "anti-body" may have been produced in response to the
stimulus of disease, but the power to produce it without this special
stimulus is not included in the germ-cells any more than a pigment.
All that they bear is the _power to produce_ the anti-bodies when the
stimulus is applied.
If we could conceive of an organism like one of those to which disease
may be due becoming actually incorporated with the system of its host,
so as to form a constituent of its germ-cells and to take part in the
symmetry of their divisions, we should have something analogous to the
case of a species which acquires a new factor and emits a dominant
variety. When we see the phenomenon in this light we realise the
obscurity of the problem. The appearance of recessive varieties is
comparatively easy to understand. All that is implied is the omission of
a constituent. How precisely the omission is effected we cannot suggest,
but it is not very difficult to suppose that by some mechanical fault
of cell-division a power may be lost. Such variation by unpacking,
or analysis of a previously existing complex, though unaccountable,
is not inconceivable. But whence come the new dominants? Whether we
imagine that they are created by some rearrangement or other change
internal to the organism, or whether we try to conceive them as due to
the assumption of something from without we are confronted by equally
hopeless difficulty.
The mystery of the origin of a dominant increases when it is realised
that there is scarcely any recent and authentic account of such an event
occurring under critical observation, which can be taken as a basis for
discussion. The literature of horticulture for example abounds in cases
alleged, but I do not think anyone can produce an illustration quite
free from doubt. Such evidence is usually open to the suspicion that the
plant was either introduced by some accident, or that it arose from a
cross with a pre-existing dominant, or that it owed its origin to the
meeting of complementary factors. In medical literature almost alone
however, there are numerous records of the spontaneous origin of various
abnormal conditions in man which habitually behave as dominants, and of
the authenticity of some of these there can be no doubt.
When we know that such conditions as hereditary cataract or various
deformities of the fingers behave as dominants, we recognize that
those conditions must be due to the addition of some element to the
constitution of the normal man. In the collections of pedigrees relating
to such pathological dominants there are usually to be found alleged
instances of the origin of the condition _de novo_. Not only do these
records occur with such frequency that they cannot be readily set aside
as errors, but from general considerations it must be obvious that as
these malformations are not common to normal humanity they must at
some moment of time have been introduced. The lay reader may not be
so much impressed with the difficulty as we are. He is accustomed to
regard the origin of _any_ new character as equally mysterious, but when
once dominants are distinguished from recessives the problem wears a
new aspect. Thus the appearance of high artistic gifts, whether as an
attribute of a race or as a sporadic event among the children of parents
destitute of such faculties, is not very surprising, for we feel fairly
sure that the faculty is a recessive, due to the loss of a controlling
or inhibiting factor; but the _de novo_ origin of brachydactylous
fingers in a child of normal parents is of quite a different nature, and
must indicate the action of some new specific cause.
Whether such evidence is applicable to the general problem of evolution
may with some plausibility be questioned; but there is an obvious
significance in the fact that it is among these pathological occurrences
that we meet with phenomena most nearly resembling the spontaneous
origin of dominant factors, and I cannot see such pedigrees as these
without recalling Virchow's aphorism that every variation owes its
origin to some pathological accident. In the evolution of domestic
poultry, if _Gallus bankiva_ be indeed the parent form of all our
breeds, at least some half dozen new factors must have been added during
the process. In _bankiva_ there is, for example, no factor for rose
comb, pea comb, barring on the feathers, or for the various dominant
types of dark plumage. Whence came all these? It is, I think, by no
means impossible that some other wild species now extinct did take
part in the constitution of domestic poultry. It seems indeed to me
improbable that the heavy breeds descend from _bankiva_. Both in regard
to domestic races of fowls, pigeons, and some other forms, the belief in
origin within the period of human civilization from one simple primitive
wild type seems on a balance of probabilities insecurely founded, but
allowing something for multiplicity of origin we still fall far short
of the requisite total of factors. Elements exist in our domesticated
breeds which we may feel with confidence have come in since their
captivity began. Such elements in fowls are dominant whiteness, extra
toe, feathered leg, frizzling, etc., so that even hypothetical extension
of the range of origin is only a slight alleviation of the difficulty.
Somehow or other, therefore, we must recognize that dominant factors
do arise. Whether they are created by internal change, or whether, as
seems to me not wholly beyond possibility, they obtain entrance from
without, there is no evidence to show. If they were proved to enter from
without, like pathogenic organisms, we should have to account for the
extraordinary fact that they are distributed with fair constancy to half
the gametes of the heterozygote.
In proportion as the nature of dominants grows more clear so does it
become increasingly difficult to make any plausible suggestion as to
their possible derivation. On the other hand the origin of a recessive
variety by the loss of a factor is a process so readily imagined that
our wonder is rather that the phenomenon is not observed far more
often. Some slip in the accurate working of the mechanical process of
division, and a factor gets left out, the loss being attested by the
appearance of a recessive variety in some subsequent generation.
Consistently with this presentation of the facts we find that, as in our
domesticated animals and plants, a diversity of recessives may appear
within a moderately short period, and that when variations come they
often do not come alone. Witness the cultural history of the Sweet Pea,
_Primula Sinensis_, _Primula obconica_, _Nemesia strumosa_ and many such
examples in which variation when it did come was abundant. The fact
cannot be too often emphasized that in the vast proportion of these
examples of substantive variation under domestication, as well as of
substantive variation in the natural state, the change has come about by
omission, not by addition. To take, for example, the case of the Potato,
in which so many spontaneous bud-variations have been recorded, East
after a careful study of the evidence has lately declared his belief
that all are of this nature, and the opinion might be extended to many
other groups of cases whether of bud or seminal variation. Morgan draws
the same conclusion in reference to the many varieties he has studied in
_Drosophila_.
In the Sweet Pea, a form which is beyond suspicion of having been
crossed with anything else, and has certainly produced all the multitude
of types which we now possess by variations from one wild species,
there is only one character of the modern types which could, with any
plausibility, be referred to a factor not originally forming part
of the constituents of the wild species. This is the waved edge, so
characteristic of the "Spencer" varieties; for the cross between a
smooth-edged and a waved type gives an intermediate not unfrequently.
Nevertheless there is practically no doubt that this is merely an
imperfection in the dominance of the smooth edge, and we may feel sure
that any plant homozygous for smooth edge would show no wave at all.
Hence it is quite possible that even the appearance of the original
waved type, Countess Spencer, was due to the loss of one of the factors
for smooth edge at some time in the history of the Sweet Pea.
In the case of the Chinese Primrose (_Primula Sinensis_) one dominant
factor has been introduced in modern times, probably within the last six
years at most. This is the factor which causes suppression of the yellow
eye, giving rise to the curious type known as "Queen Alexandra." Mr. R.
P. Gregory's experiments proved that this was a very definite dominant,
and the element responsible for this development is undoubtedly an
addition to the original ingredient-properties, with which the species
was endowed. Unfortunately, as happens in almost every case of the kind,
the origin of this important novelty appears to be lost. Its behaviour,
however, when crossed with various other types is that of a simple
dominant giving an ordinary 3:1 ratio. There is therefore no real doubt
that it came into existence by the definite addition of a new factor,
for if it was simply a case of the appearance of a new character made by
combination of two previously existing complementary factors we should
expect that when Queen Alexandra was self-fertilised a 9:7 ratio would
be a fairly common result, which is not in practice found.
In _Oenothera_ Gates[1] has observed the appearance, in a large
sowing of about 1,000 _Oenothera rubrinervis_, of a single individual
having considerably more red pigment in the calyx than is usual in
_rubrinervis_. The whole of the hypanthium in the flowers of this
plant was red instead of green as in _rubrinervis_, and the whole of
the sepals were red in the bud-stage, except for small green areas at
the base. This type behaved as a dominant over _rubrinervis_, but so
far a pure-breeding individual was not found. Admittedly the variation
of this plant from the type of _rubrinervis_ can be represented as
one of degree, though there is a very sensible gap in the series
between the new form which Gates names "_rubricalyx_" and the reddest
_rubrinervis_ seen in his cultures. It must certainly be recognised as
a new dominant. Gates, rightly as I consider, regards the distinction
between _rubrinervis_ and _rubricalyx_ as a quantitative one, and the
same remark applies to certain other types differing in the amount
of anthocyanin which they produce. I do not understand the argument
which Gates introduces to the effect that the difference between
such quantitative types cannot be represented in terms of presence
and absence. We are quite accustomed to the fact that in the rabbit
self-colour segregates from the Dutch-marked type. These two types
differ in a manner which we may reasonably regard as quantitative. It
is no doubt possible that the self-coloured type contains an ingredient
which enables the colour to spread over the whole body, but it is, I
think, perhaps more easy to regard the Dutch type as a form from which a
part of the colour is absent. It may be spoken of in terms I have used,
as a _subtraction-stage_ in colour. Following a similar method we may
regard _rubricalyx_ as an addition-stage in colour-variation. The fact
that crosses between _rubrinervis_, or _rubricalyx_ and _Lamarckiana_
give a mixture of types in F_{1}, does not I think show, as Gates
declares, that there is any system here at work to which a factorial or
Mendelian analysis does not apply; but that question may be more fitly
discussed in connexion with the other problems raised by the behaviour
of _Oenothera_ species in their crosses.
I do, however, feel that, interesting as this case must be admitted to
be, we cannot quite satisfactorily discuss it as an illustration of
the _de novo_ origin of a dominant factor. The difference between the
novelty and the type is quantitative, and it is not unreasonable to
think of such a difference being brought about by some "pathological
accident" in a cell-division.
Recognition of the distinction between dominant and recessive characters
has, it must be conceded, created a very serious obstacle in the way
of any rational and concrete theory of evolution. While variations
of all kinds could be regarded as manifestations of some mysterious
instability of organisms this difficulty did not occur to the mind of
evolutionists. To most of those who have taken part in genetic analysis
it has become a permanent and continual obsession. With regard to the
origin of recessive variations, there is, as we have seen, no special
difficulty. They are negative and are due to absences, but as soon as it
is understood that dominants are caused by an addition we are completely
at a loss to account for their origin, for we cannot surmise any source
from which they may have been derived. Just as when typhoid fever breaks
out in his district the medical officer of health knows for certain that
the bacillus of typhoid fever has by some means been brought into that
district so do we know that when first dominant white fowls arose in the
evolution of the domestic breeds, by some means the factor for dominant
whiteness got into a bird, or into at least one of its germ-cells.
Whence it came we cannot surmise.
Whether we look to the outer world or to some rearrangement within the
organism itself, the prospect of finding a source of such new elements
is equally hopeless.
Leaving this fundamental question aside as one which it is as yet quite
unprofitable to discuss, we are on safe ground in foreseeing that the
future classification of substantive variations, which genetic research
must before long make possible, will be based on a reference to the
modes of action of the several factors. Some will be seen to produce
their effects by oxidation, some by reduction, some by generating
substances of various types, sugars, enzymes, activators, and so forth.
It may thus be anticipated that the relation of varieties to each other
and to types from which they are derived will be expressible in terms
of definite synthetical formulae. Clearly it will not for an indefinite
time be possible to do this in practice for more than a few species
and for characters especially amenable to experimental tests, but as
soon as the applicability of such treatment is generally understood the
influence on systematics must be immediate and profound, for the nature
of the problem will at length be clear and, though the ideal may be
unattainable, its significance cannot be gainsaid.
* * * * *
_Note._--With hesitation I allow this chapter to appear in the form in
which it was printed a year ago, but in passing it for the press after
that interval I feel it necessary to call attention to a possible line
of argument not hitherto introduced.
In all our discussions we have felt justified in declaring that the
dominance of any character indicates that some factor is present which
is responsible for the production of that character. Where there is no
definite dominance and the heterozygote is of an intermediate nature
we should be unable to declare on which side the factor concerned was
present and from which side it was absent. The degree of dominance
becomes thus the deciding criterion by which we distinguish the
existence of factors. But it should be clearly realized that in any
given case the argument can with perfect logic be inverted. We already
recognize cases in which by the presence of an inhibiting factor a
character may be suppressed and purely as a matter of symbolical
expression we might apply the same conception of inhibition to any
example of factorial influence whatever. For instance we say that in
as much as two normal persons do not have brachydactylous children,
there must be some factor in these abnormal persons which causes the
modification. Our conclusion is based on the observed fact that the
modification is a dominant. But it may be that normal persons are
homozygous in respect of some factor _N_, which prevents the appearance
of brachydactyly, and that in any one heterozygous, _Nn_, for this
inhibiting factor, brachydactyly can appear. Similarly the round pea we
say contains _R_, a factor which confers this property of roundness,
without which its seeds would be wrinkled. But here we know that the
wrinkled seed is in reality one having compound starch-grains, and
that the heterozygote, though outwardly round enough, is intermediate
in that starch-character. If we chose to say that the compoundness of
the grains is due to a factor _C_ and that two doses of it are needed
to make the seed wrinkled, I know no evidence by which such a thesis
could be actually refuted. That such reasoning is seemingly perverse
must be conceded; but when we consider the extraordinary difficulties
which beset any attempt to conceive the mode of origin of a new dominant
factor, we are bound to remember that there is this other line of
argument which avoids that difficulty altogether. In the case of the
"Alexandra"-eye in _Primula_, or the red calyx in Gates's _Oenothera_,
inverting the reasoning adopted in the text, we may see that only the
_Primula_ homozygous for the yellow eye can develop it and that two
doses of the factor for the _rubrinervis_ calyx are required to prevent
that part of the plant from being red.
We may proceed further and extend this mode of reasoning to all cases
of genetic variation, and thus conceive of all alike as due to loss of
factors present in the original complex. Until we can recognize factors
by means more direct than are provided by a perception of their effects,
this doubt cannot be positively removed. For all practical purposes of
symbolic expression we may still continue to use in our analyses the
modes of representation hitherto adopted, but we must not, merely on
the ground of its apparent perversity, refuse to admit that the line of
argument here indicated may some day prove sound.
FOOTNOTES:
[1] Gates, R. R., _Zts. f. Abstammungslehre_, 1911, IV, pp. 341 and 361.
CHAPTER V
THE MUTATION THEORY
When with the thoughts suggested in the last chapter we contemplate
the problem of Evolution at large the hope at the present time of
constructing even a mental picture of that process grows weak almost
to the point of vanishing. We are left wondering that so lately men
in general, whether scientific or lay, were so easily satisfied. Our
satisfaction, as we now see, was chiefly founded on ignorance.
Every specific evolutionary change must represent a definite event in
the construction of the living complex. That event may be a disturbance
in the meristic system, showing itself in a change in the frequency of
the repetitions or in the distribution of differentiation among them, or
again it may be a chemical change, adding or removing some factor from
the sum total.
If an attempt be made to apply these conceptions to an actual series
of allied species the complexity of the problem is such that the mind
is appalled. Ideas which in the abstract are apprehended and accepted
with facility fade away before the concrete case. It is easy to imagine
how Man was evolved from an _Amoeba_, but we cannot form a plausible
guess as to how _Veronica agrestis_ and _Veronica polita_ were evolved,
either one from the other, or both from a common form. We have not even
an inkling of the steps by which a Silver Wyandotte fowl descended from
_Gallus Bankiva_, and we can scarcely even believe that it did. The
Wyandotte has its enormous size, its rose comb, its silver lacing, its
tame spirit, and its high egg production. The tameness and the high egg
production are probably enough both recessives, and though we cannot
guess how the corresponding dominant factors have got lost, it is not
very difficult to imagine that they were lost somehow. But the rose comb
and the silver colour are _dominants_. The heavy weight also appears in
the crosses with Leghorns, but we need not at once conclude that it
depends on a simple dominant factor, because the big size of the crosses
may be a consequence of the cross and may depend on other elements.
Now no wild fowl known to us has these qualities. May we suppose that
some extinct wild species had them? If so, may we again make the same
supposition in all similar cases? To do so is little gain, for we are
left with the further problem, whence did those lost wild species
acquire those dominants? Suppositions of this kind help no more than
did the once famous conjecture as to the origin of living things--that
perhaps they came to earth on a meteorite. The unpacking of an
original complex, the loss of various elements, and the recombination
of pre-existing materials may all be invoked as sources of specific
diversity. Undoubtedly the range of possibilities thus opened up is
large. It will even cover an immense number of actual examples which
in practice pass as illustrations of specific distinction. The Indian
Rock pigeon which has a blue rump may quite reasonably be regarded as
a geographically separated recessive form of our own _Columba livia_,
for as Staples-Browne has shown the white rump of _livia_ is due to
a dominant factor. The various degrees to which the leaves of Indian
Cottons are incised have, as Leake says, been freely used as a means
of classification. The diversities thus caused are very remarkable,
and when taken together with diversities in habit, whether sympodial
or monopodial, the various combinations of points of difference
are sufficiently distinctive to justify any botanist in making a
considerable number of species by reference to them alone. Nevertheless
Leake's work goes far to prove that all of these forms represent the
re-combinations of a very small number of factors. The classical example
of _Primula Sinensis_ and its multiform races is in fact for a long
way a true guide as to the actual interrelations of the species which
systematists have made. That they did make them was due to no mistake
in judgment or in principle, but simply to the want of that extended
knowledge of the physiological nature of the specific cases which we now
know to be a prime necessity.
But will such analysis cover all or even most of the ordinary cases
of specific diversity between near allies? Postponing the problem of
the interrelations of the larger divisions as altogether beyond present
comprehension, can we suppose, that in general, closely allied species
and varieties represent the various consequences of the presence or
absence of allelomorphic factors in their several combinations? The
difficulty in making a positive answer lies in the fact that in most
of the examples in which it has been possible to institute breeding
experiments with a view to testing the question, a greater or less
sterility is encountered. Where, however, no such sterility is met
with, as for instance in the crosses made by E. Baur among the species
of _Antirrhinum_ there is every reason to think that the whole mass of
differences can and will eventually be expressed in terms of ordinary
Mendelian factors. Baur has for example crossed species so unlike as
_Antirrhinum majus_ and _molle_, forms differing from each other in
almost every feature of organisation.[1] The F_{2} generation from this
cross presents an amazingly motley array of types which might easily if
met with in nature be described as many distinct species. Yet all are
fertile and there is not the slightest difficulty in believing that they
can all be reduced to terms of factorial analysis.
If allowance be made for the complicating effects of sterility, is there
anything which prevents us from supposing that such good species as
those of _Veronica_ or of any other genus comprising well-defined forms
may not be similarly related? I do not know any reason which can be
pointed to as finally excluding such a possibility. Nevertheless it has
been urged with some plausibility that good species are distinguished
by _groups_ of differentiating characters, whereas if they were really
related as the terms of a Mendelian F_{2} family are, we should expect
to find not groups of characters in association, but rather series of
forms corresponding to the presence and absence of the integral factors
composing the groups of characters. I am not well enough versed in
systematic work to be able to decide with confidence how much weight
should be attached to this consideration. Some weight it certainly
has, but I cannot yet regard it as forming a fatal objection to the
application of factorial conceptions on the grand scale. It may be
recalled that we are no longer under any difficulty in supposing that
differences of all classes may be caused by the presence or absence of
factors. It seemed at first for example that such characters as those of
leaf shape might be too subtle and complex to be reducible to a limited
number of factors. But first the work of Gregory on _Primula Sinensis_
showed that several very distinct types of leaves were related to each
other in the simplest way. In that particular example, intermediates are
so rare as to be negligible, but subsequently Shull dealing with such
a complicated example as _Capsella_, and Leake in regard to Cottons,
both forms in which intergrades occur in abundance, have shown that a
simple factorial scheme is applicable. We need not therefore, to take
an extreme case, doubt that if it were possible to examine the various
forms of fruit seen in the Squashes by really comprehensive breeding
tests, even this excessive polymorphism in respect of structural
features would be similarly reducible to factorial order.
It must always be remembered also that in a vast number of cases, nearly
allied forms which are distinct, occupy distinct ground. Moreover, by
whatever of the many available mechanisms that end be attained, it is
clear that nature very often does succeed in preventing intercrossing
between distinct forms so far that the occurrence of that phenomenon is
a rarity under natural conditions. The facts may, I think, fairly be
summarized in the statement that species are on the whole distinct and
not intergrading, and that the distinctions between them are usually
such as might be caused by the presence, absence, or inter-combination
of groups of Mendelian factors; but that they are so caused the evidence
is not yet sufficient to prove in more than a very few instances.
The alternative, be it explicitly stated, is not to return to the view
formerly so widely held, that the distinctions between species have
arisen by the accumulation of minute or insensible differences. The
further we proceed with our analyses the more inadequate and untenable
does that conception of evolutionary change become. If the differences
between species have not come about by the addition or loss of factors
one at a time, then we must suppose that the changes have been effected
by even larger steps, and variations including groups of characters,
must be invoked.
That changes of this latter order are really those by which species
arise, is the view with which de Vries has now made us familiar by his
writings on the Mutation Theory. In so far as mutations may consist
in meristic changes of many kinds and in the loss of factors it is
unnecessary to repeat that we have abundant evidence of their frequent
occurrence. That they may also more rarely occur by the addition of a
factor we are, I think, compelled to believe, though as yet the evidence
is almost entirely circumstantial rather than direct. The evidence for
the occurrence of those mutations of higher order, by which new species
characterized by several distinct features are created, is far less
strong, and after the best study of the records which I have been able
to make, I find myself unconvinced. The facts alleged appear capable of
other interpretations.
The most famous and best studied examples are of course the forms
of _Oenothera_ raised by de Vries from _Oenothera Lamarckiana_ in
circumstances well known to all readers of genetic literature. Whatever
be the true significance of these extraordinary "mutations" there can
be no question about the great interest which attaches to them, and the
historical importance which they will long preserve. Apart also from
these considerations it is becoming more and more evident that in their
peculiarities they provide illustrations of physiological phenomena of
the highest consequence in the study of genetics at large.
De Vries found, as is well known, that _Oenothera Lamarckiana_ gives off
plants unlike itself. These mutational forms are of several distinct
and recognizable types which recur, and several of them breed true from
their first appearance. The obvious difficulty, which in my judgment
should make us unwilling at present to accept these occurrences as proof
of the genesis of new species by mutation, is that we have as yet no
certainty that the appearance of the new forms is not an effect of the
recombination of factors, such as is to be seen in so many generations
of plants derived from a cross involving many genetic elements. The
first question is what is _Oenothera Lamarckiana_? Is it itself a plant
of hybrid origin? To this fundamental question no satisfactory answer
has yet been given. All attempts to find it as a wild plant in America
have failed. It existed in Europe in the latter half of the eighteenth
century. Whence it came is still uncertain, but the view that it came
into existence in Europe and perhaps in Paris, seems on the whole the
most probable. The question has been debated by Macdougal, Gates, and
Davis. From historical sources there is little expectation of further
light. Those who favour the notion of a hybrid origin look on _Oenothera
biennis_ as one of the putative parents. It has been conjectured that
a species called _grandiflora_ lately re-discovered on the Alabama
river was the other parent. Experiments have been instituted by Davis
to discover whether _Lamarckiana_ can be made artificially by crossing
these two species. The results so far have shown that while plants
approximating in various respects to _Lamarckiana_ have thus been
produced, none agree exactly with that form. Davis, to whom reference
should be made for a full account of the present state of the enquiry,
points out that there are many strains of _biennis_ in existence and
that it is by no means impossible that by using others of these strains
a still closer approximation can be made. None of Davis's artificial
productions as yet breed at all true, as _Lamarckiana_ on the whole
does. In such a case, however, where several characters are involved,
this is perhaps hardly to be expected.
One feature of the _Oenotheras_ is very curious. Not only _Lamarckiana_,
but all the allied species so far as I am aware, have a considerable
proportion of bad and shrivelled pollen grains. This is undoubtedly true
of species living in the wild state as well as of those in cultivation.
I have had opportunities of verifying this for myself in the United
States. No one looking at the pollen of an _Oenothera_ would doubt that
it was taken from some hybrid plant exhibiting partial sterility. On the
other hand, it is difficult to suppose that numbers, perhaps all, of
the "species" of the genus are really hybrids, and many of them breed
substantially true. I regard this constant presence of bad pollen grains
as an indication that the genetic physiology of _Oenothera_ is in some
way abnormal, and as we shall presently see, there are several other
signs which point in the same direction.
Discussion of the whole series of phenomena is rendered exceedingly
difficult first, by reason of the actual nature of the material. The
characteristics of many of the types which de Vries has named are
evasive. A few of these types, for instance, _gigas_, _nanella_,
_albida_, _brevistylis_, and perhaps a few more are evidently clear
enough, but we have as yet no figures and descriptions precise enough
to enable a reader to appreciate exactly the peculiarities of the vast
number of forms which have now to be considered in any attempt to gain a
comprehensive view of the whole mass of facts. It is also not in dispute
that the forms are susceptible of great variations due simply to soil
and cultural influences.
The fact that no Mendelian analysis has yet been found applicable to
this group of _Oenotheras_ as a whole is perhaps largely due to the fact
that until recently such analysis has not been seriously attempted.
Following the system which he had adopted before the rediscovery of
Mendelism, or at all events, before the development of that method of
analysis, de Vries has freely applied _names_ to special combinations of
characters and has scarcely ever instituted a factorial analysis. Before
we can get much further this must be attempted. It may fail, but we must
know exactly where and how this failure comes about. There are several
indications that such a recognition of factorial characters, could be
carried some way. For example, the height, the size of the flowers, the
crinkling of the leaves, the brittleness of the stems, perhaps even the
red stripes on stems and fruits, and many more, are all characters which
may or may not depend on distinct factors, but if such characters are
really transmitted in unresolved groups, the limitations of those groups
should be carefully determined. The free use of names for the several
forms, rather than for the characters, has greatly contributed to deepen
the obscurity which veils the whole subject.
I do not mean to suggest that these _Oenotheras_ follow a simple
Mendelian system. All that we know of them goes to show that there
are curious complications involved. One of these, probably the most
important of all, has lately been recognized by de Vries himself,
namely, that in certain types the characters borne by the female and
the male germ-cells of the same plant are demonstrably different. There
can be little doubt that further research will reveal cognate phenomena
in many unsuspected places. The first example in which such a state of
things was proved to exist is that of the Stocks investigated by Miss
Saunders.[2] By a long course of analysis she succeeded in establishing
in 1908 the fact that if a plant of _Matthiola_ is of that eversporting
kind which gives a large proportion of double-flowered plants among
its offspring (produced by self-fertilisation), then the egg-cells of
such a plant are mixed in type, but the pollen of the same plant is
homogeneous. Some of the egg-cells have in them the two factors for
singleness, but some of them are short of one or both of these factors.
The pollen-grains, however, are all recessives, containing neither of
these factors. The egg-cells, in other words, are mixed, "singles"
and "doubles," while the pollen-grains are all "doubles." The same is
true of the factor differentiating "white," or colourless plastids
from cream-coloured plastids in _Matthiola_, the egg-cells being mixed
"whites" and "creams," while the pollen-grains are all "creams," viz:
recessives. Later in the same year (1908) de Vries[3] announced a
remarkable case which will be discussed in detail subsequently. It
relates to certain _Oenotheras_ heterozygous for dwarfness, in which (p.
113) the ovules were mixed, tails and dwarfs, while the pollen is all
dwarf.
Again in _Petunia_ Miss Saunders's[4] work has shown that a somewhat
similar state of things exists, but with this remarkable difference,
that though the egg-cells are mixed, singles and doubles, the
pollen-grains are all _singles_, viz: dominants. All the _Petunias_ yet
examined have been in this condition, including some which in botanic
gardens pass for original species. Whether actual wild plants from
their native habitats are in the same state, is not yet known, but it
is by no means improbable. The case may be compared with that of the
moth _Abraxas grossulariata_ studied by Doncaster and Raynor, in which
the females are all heterozygous, or we may almost say "hybrids" of
_grossulariata_ and the variety _lacticolor_. Similarly we may say that
at least garden Petunias are heterozygous in respect of singleness.
The proof of this is of course that when fertilised with the pollen of
doubles they throw a mixture of doubles and singles. The statements
which de Vries has published regarding the behaviour of several of the
_Oenotheras_ go far to show that they must have a somewhat similar
organisation. On the present evidence it is still quite impossible to
construct a coherent scheme which will represent all the phenomena in
their interrelations, and among the facts are several which, as will
appear, seem mutually incompatible. The first indication that the
_Oenotheras_ may have either mixed ovules or mixed pollen appears in the
fact that _Lamarckiana_ and several of its "mutants" used as males, with
several other forms as females, give a mixed offspring. For example, de
Vries (1907) found that
_biennis_ [F] × _Lamarckiana_ [M]
_biennis cruciata_ [F] × _Lamarckiana_ [M]
_muricata_ [F] × _Lamarckiana_ [M]
_biennis_ [F] × _rubrinervis_ [M]
_biennis cruciata_ [F] × _rubrinervis_ [M]
all give a mixture of two distinct types which he names _laeta_ and
_velutina_, consisting of about equal numbers of each. On account of the
fact that the two forms are produced in association de Vries has called
these forms "twin hybrids," a designation which is not fortunate, seeing
that it is impossible to imagine that any kind of twinning is concerned
in their production. The distinction between these two seems to be
considerable, _laeta_ having leaves broader, bright green in colour, and
flat, with pollen scanty, while _velutina_ has leaves narrower, grayish
green, more hairy, and furrow-shaped, with pollen abundant.
We next meet the remarkable fact that these two forms, _laeta_
and _velutina_ breed true to their respective types, and do not
reproduce the parent-types among their offspring resulting from
self-fertilisation. This statement must be qualified in two respects.
When _muricata_ [M] is fertilised by _brevistylis_ the forms _laeta_
and _velutina_ are produced, but each of them subsequently throws the
short-styled form as a recessive (de Vries, 1907, p. 406). It may be
remembered that de Vries's previous publications had already shown that
the short style of _brevistylis_, one of the _Lamarckiana_ "mutants,"
behaves as a recessive habitually (_Mutationstheorie_, II, p. 178, etc.).
Also when _nanella_, the dwarf "mutant" of _Lamarckiana_ is used as
male on _muricata_ as female, _laeta_ and _velutina_ are produced, but
one only of these, namely, _velutina_, subsequently throws dwarfs on
self-fertilisation. The dwarfs thus thrown are said to form about 50 per
cent. of the families in which they occur (de Vries, 1908, p. 668). The
fact that the two forms, _laeta_ and _velutina_, are produced by many
matings in which _Lamarckiana_ and its mutant _rubrinervis_ are used as
males is confirmed abundantly by Honing, who has carried out extensive
researches on the subject. After carefully reading his paper, I have
failed to understand the main purport of the argument respecting the
"double nature" of _Lamarckiana_ which he founds on these results, but
I gather that in some way _laeta_ is shown to partake especially of the
nature of _Lamarckiana_, while _velutina_ is a form of _rubrinervis_.
The paper contains many records which will be of value in subsequent
analysis of these forms.
Before considering the possible meaning of these facts we must have
in our minds the next and most novel of the recent extensions of
knowledge as to the genetic properties of the _Oenotheras_. In the
previous statement we have been concerned with the results of using
either _Lamarckiana_ itself or one of its "mutants" _rubrinervis_,
_brevistylis_, or _nanella_ as male, on one of the species _biennis_
or _muricata_. The new experiments relate to crosses between the two
species _biennis_ and _muricata_ themselves.
De Vries found:
1. That the reciprocal hybrids from these two species differed,
_biennis_ × _muricata_ producing one type of F_{1} and _muricata_ ×
_biennis_ producing another. Each F_{1} resembled the father more than
the mother.
2. That each of the hybrids so produced breeds true on
self-fertilisation.
3. That if we speak of the hybrid from _biennis_ × _muricata_ as _BM_
and of the reciprocal as _MB_, then
_BM_ × _MB_
gives exclusively offspring of _biennis_ type but that
_MB_ × _BM_
gives exclusively offspring of _muricata_ type. Evidently, apart
from all controversy as to the significance of the "mutants" of
_Lamarckiana_, we have here a series of observations of the first
importance.
The fact that reciprocal crossings give constantly distinct results
must be taken to indicate that the male and female sides of one, if not
of both, of the parents are different in respect of characters which
they bear. This is de Vries's view, and he concludes rightly, I think,
that the evidence from all the experiments shows that both _biennis_
and _muricata_ are in this condition, having one set of characters
represented in their pollen-grains and another in their ovules. The
plants breed true, but their somatic structures are compounded of the
two sets of elements which pass into them from their maternal and
paternal sides respectively. This possibility that species may exist of
which the males really belong to one form and the females to another, is
one which it was evident from the first announcement of the discovery of
Mendelian segregation might be found realised in nature.[5]
_Oe. biennis_ and _muricata_ were crossed reciprocally with each other
and with a number of other species, and the behaviour of each, when
used as mother, was consistently different from its behaviour when
used as father. De Vries is evidently justified by the results of
this series of experiments in stating that the "Bild," as he terms
it, or composition of the male and female sides of these two species,
_biennis_ and _muricata_, are distinct. On the evidence before us it
is not, however, possible to form a perfectly clear idea of each, and
until details are published, a reader without personal knowledge of the
material cannot do more than follow the general course of the argument.
For fuller comprehension a proper analysis of the characters with a
clear statement of how they are distributed among the several types and
crosses is absolutely necessary. According to de Vries the female of
_biennis_ possesses a group of characters which he defines as "_conica_"
in allusion to the shape of the flower-buds. Besides the conical buds,
this group of features includes imperfect development of wood, rendering
the plant very liable to attacks of _Botrytis_, and comparatively narrow
leaves.
The female of _muricata_ carries a group of features which he
calls "_frigida_," and, though this is not quite explicitly stated
in a definition of that type, it is to be inferred[6] that its
characteristics are regarded as greater height, strong development of
wood with comparative resistance to _Botrytis_, and broad leaves.
The characters borne by the male parts of the two species are in general
those by which they are outwardly distinguished. For example, the
leaves of _Oe. biennis_ are comparatively broad and are bright green,
while those of _muricata_ are much narrower and of a glaucous green,
and I understand that de Vries regards these properties as contributed
by the male side in each case and to be carried by the male cells of
each species. The suggestion as regards _biennis_ and _muricata_ comes
near the conception often expressed by naturalists in former times (_e.
g._, Linnaeus) and not rarely entertained by breeders at the present
day, that the internal structure is contributed by the mother and the
external by the father.
On the other hand, the offspring of each species when used as mother is
regarded as possessing in the main the features of the maternal "Bild,"
but the matter is naturally complicated by the introduction of features
from the father's side, and it is here especially that the account
provided is at present unsatisfactory and inconclusive. There seems,
however, to be no serious doubt that _biennis_ and _muricata_ each in
their outward appearance exhibit on the whole the features which their
pollens respectively carry, and that the features borne by their ovules
are in many respects distinct.
The _types_ are thus "hybrids" which breed true. The results of
intercrossing them each way are again "hybrids" which breed true. It
will be remembered that on former occasions de Vries has formulated a
general rule that _species_-hybrids breed true, but that the cross-breds
raised by interbreeding _varieties_ do not. One of these very cases
was quoted[7] as an illustration of this principle, viz: _muricata_ ×
_biennis_. The grounds for this general statement have always appeared
to me insufficient, and with the further knowledge which the new
evidence provides we are encouraged to hope that when a proper factorial
analysis of the types is instituted we shall find that the phenomenon
of a constant hybrid will be readily brought into line with the systems
of descent already worked out for such cases as that of the Stocks, and
others already mentioned.
In further discussion of these facts de Vries makes a suggestion which
seems to me improbable. Since the egg-cells of _muricata_, for instance,
bear a certain group of features which are missing on the male side,
and conversely the pollen bears features absent from the female side,
he is inclined to regard the _bad pollen grains_ as the bearers of the
missing elements of the male side and to infer that there must similarly
be defective ovules representing the missing elements of the female
side. No consideration is adduced in support of this view beyond the
simple fact that the characters borne by male and female are dissimilar,
whereas it would be more in accord with preconception if the same sets
of combinations were represented in each--as in a normal Mendelian case.
There is as yet no instance in which the absence of any particular
class of gametes has been shown with any plausibility to be due to
defective viability, though there are, of course, cases in which certain
classes of zygotes do not survive owing to defective constitution (_e.
g._, the albinos of _Antirrhinum_ studied by Baur, and the homozygous
yellow mice). I am rather inclined to suppose that in these examples of
hybrids breeding true we shall find a state of things comparable with
that to which we formerly applied the terms "coupling" and "repulsion."
In these cases certain of the possible combinations of factors occur
in the gametic series with special frequency, being in excess, while
the gametes representing other combinations are comparatively few.
In a recent paper on these cases Professor Punnett and I have shown
that these curious results vary according to the manner in which the
factors are grouped in the parents. If _A_ and _B_ are two factors which
exhibit these phenomena we find that the gametic series of the double
heterozygote differs according as the combination is made by crossing
_AB × ab_, or by crossing _AB × aB_. In a normal Mendelian case the
F_{1} form, _AaBb_, produces gametes _AB_, _Ab_, _aB_, _ab_, in equal
numbers; but in these peculiar cases those gametes which contain
Gametic series Number of Number of
--------------------- gametes zygotes
AB Ab aB ab in series formed
Partial repulsion { 1 (n-1) (n-1) 1 2n 4n^{2}
from zygote { 1 31 31 1 64 4096
of form { 1 15 15 1 32 1024
Ab×aB { 1 7 7 1 16 256
{ 1 3 3 1 8 64
1 1 1 1 4 16
Partial coupling { 3 1 1 3 8 64
from zygote { 7 1 1 7 16 256
of form { 15 1 1 15 32 1024
AB×ab { 31 1 1 31 64 4096
{ 63 1 1 63 128 16384
{ (n-1) 1 1 (n-1) 2n 4n^{2}
Nature of zygotic series
---------------------------------------
AB Ab aB ab
Partial repulsion { 2n^{2}+1 n^{2}-1 n^{2}-1 1
from zygote { 2049 1023 1023 1
of form { 513 255 255 1
Ab×aB { 129 63 63 1
{ 33 15 15 1
9 3 3 1
{ 41 7 7 9
Partial coupling { 177 15 15 49
from zygote { 737 31 31 225
of form { 3009 63 63 961
AB×ab { 12161 127 127 3969
{ 3n^{2}-(2n - 1) 2n-1 2n-1 n^{2}-(2n-1)
the _parental combinations_ are in excess. This excess almost certainly
follows the system indicated by the accompanying table. In the general
expressions _n_ is half the number of gametes required to express the
whole system. Now if we imagine that sex-factors are involved with the
others concerned in such a relationship as this we have a system of
distribution approximating to that found in _biennis_ and _muricata_.
The difference in reciprocals is represented in a not improbable way.
It cannot yet be said that the rarer terms in the series are formed at
all, and perhaps they are not. As we pointed out in our discussion of
these phenomena, the peculiar distribution of factors in these cases
must be taken to mean that the planes of division at some critical stage
in the segregation are determined with reference to the parental groups
of factors, or in other words, that the whole system has a polarity,
and that the distribution of factors with reference to this polarity
differs according to the grouping of factors in the gametes which united
in fertilization to produce the plant. Subsequent proliferation of
cells representing certain combinations would then lead to excess of
the gametes bearing them. It is on similar lines that I anticipate we
shall hereafter find the interpretation of the curious facts discovered
by de Vries, though it is evident that a long course of experiment and
analysis must be carried through before any certainty is reached. The
work must be begun by a careful study of the descent of some single
factor, for example, that causing the broader leaf of _biennis_, and we
may hope that the study of _Oenothera_ by proper analytical methods will
no longer be deferred.
We have now to return to the relations of _laeta_ and _velutina_.
These two forms, it will be remembered are frequently produced when
_Lamarckiana_ or one of its derivatives is used as male, and the most
unexpected feature in their behaviour is that _both breed true as
regards their essential characteristics, on self-fertilisation_. If one
only bred true the case might, in view of the approximate numerical
equality of the two types, be difficult to interpret on ordinary lines,
but as both breed true it must be clear that some quite special system
of segregation is at work. What this may be cannot be detected on the
evidence, but with the results from the _biennis-muricata_ experiments
before us, it is natural to suspect that we may here again have to
recognise a process of allocation of different factors to the male and
female sides in _laeta_ and _velutina_. That some such system is in
operation becomes the more probable from the new fact which de Vries
states in describing the group of characters which he calls _conica_,
namely that this type is the same as that of _velutina_.
There are many collateral observations recorded both by de Vries and
others which have a bearing on the problems, but they do not yet fall
into a coherent scheme. For example, we cannot yet represent the
formation of _laeta_ and _velutina_ from the various species fertilised
by _Lamarckiana_ [M]. That this is not due to any special property
associated with the pollen of _Lamarckiana_ is shown by the fact that
a species called _Hookeri_ gives _laeta_ and _velutina_ in both its
reciprocal crosses with _Lamarckiana_ (de Vries, 1909, p. 3), and also
by the similar fact that _Lamarckiana_ [F] fertilised by the pollen of
a peculiar race of _biennis_ named _biennis Chicago_ throws the same
types. Before these very complicated phenomena can be usefully discussed
particulars must be provided as to the individuality of the various
plants used. This criticism applies to much of the work which de Vries
has lately published, for, as we now know familiarly, plants to which
the same name applies can be quite different in genetic composition.
Attention should also be called to one curiously paradoxical series of
results. When the dwarf "mutant" of _Lamarckiana_ which de Vries names
"_nanella_" is used as father on _muricata_, F_{1} consists of _laeta_
and _velutina_ in approximately equal numbers. Both forms breed true to
their special characteristics, but _velutina_ throws dwarfs of its own
type, while _laeta_ does not throw dwarfs. Subsequent investigation of
the properties of these types has led to some remarkable conclusions,
and it was in a study of these plants that de Vries first came upon the
phenomena of dissimilarity between the factors borne by the male and
female cells of the same plant, a condition which had been recently
detected in the Stocks as a result of Miss Saunders's investigations.
The details are very remarkable. We have first the fact that _muricata_
[F] × dwarf _nanella_ [M] gives about 50 per cent. _laeta_ and about 50
per cent. of _velutina_.
As regards _Velutina_ it was shown that:
Talls, Dwarfs,
per cent. per cent.
1. Velutina selfed gave 38 62
{Velutina [F] × dwarf nanella [M] gave 39 61
2.{ do. × do. gave 49 51
{ do. × dwarf [M] derived from
velutina gave 43 57
3. Dwarfs × velutina [M] gave -- all dwarfs
The three experiments taken together prove, as de Vries says, that the
ovules of _velutina_ are mixed, talls and dwarfs, and that the pollen is
all dwarf. The condition is almost the same as that of the Stocks. It
may be noted also that in the Stocks the egg-cells of the "double" type
are in excess, being approximately 9 to 7 of the "single" type, but de
Vries regards the two types in _velutina_ as probably equal in number.
The figures (169:231) rather suggest some excess of the recessives,
perhaps 9:7, and the point would be worth a further investigation.
As regards _laeta_, by self-fertilisation _no dwarfs were produced_, but
in all other respects it behaved almost exactly like _velutina_. The
ovules are evidently mixed talls and dwarfs, and whether fertilised by
dwarfs or by the pollen of _velutina_, which is already proved to be all
dwarf, the result was a steady 50 per cent. of talls and 50 per cent. of
dwarfs. The pollen of _laeta_ used on dwarfs gives nothing but dwarfs,
and in three series of such experiments 226 dwarfs were produced.
We are thus faced with this difficulty. Since the egg-cells of _laeta_
are evidently mixed, talls and dwarfs, and the pollen used on dwarfs
gives all dwarfs, why does not self-fertilisation give a mixed result,
talls and dwarfs, instead of _all talls_? De Vries regards the result
of self-fertilisation as showing the real nature of the pollen, and
declares it to be all talls, while he represents the behaviour of
the same pollen used on dwarfs by stating that in these combinations
the dwarf character dominates. This does not seem to me a natural
interpretation. I should regard the pollen of _laeta_ as identical
with that of _velutina_, namely dwarf, and I suspect the difficulty
is really created by the behaviour of _laeta_ on self-fertilisation.
Until a proper analysis is made in which the identity of the different
individuals used is recorded, no further discussion is possible.[8]
Other results of a complicated kind involving production of _laeta_ and
_velutina_ together with a third form have been published by de Vries in
his paper on "Triple Hybrids." To these also the same criticism applies.
Some of the observations seem capable of simple factorial representation
and others are conflicting.
Taking the work on _Oenothera_ as a whole we see in it continually
glimpses of order which further on are still blocked by difficulties
and apparent inconsistencies. Through such a stage all the successful
researches in complicated factorial analysis have passed and I see
no reason for supposing that with the application of more stringent
methods this more difficult set of problems will be found incapable
of similar solutions. To return to the original question whether in
_Oenothera_ we can claim to see a special contemporaneous output of new
species in actual process of creation, it will be obvious that while
the interrelation of the several types is still so little understood,
such a claim has no adequate support. It is true that many of the
"mutants" of _Lamarckiana_ can well pass for species, but this is
equally true of many new combinations of pre-existing factors as we
have seen in _Primula Sinensis_ and other cases. Still less can it be
admitted that these facts of uncertain import supply a justification for
the conception which has played a prominent part in the scheme of the
_Mutationstheorie_, namely that there are special periods of Mutation,
when the parent-species has peculiar genetic properties. To conclude:
The impression which the evidence leaves most definitely on the mind is
that further discussion of the bearing which the _Oenotheras_ may have
on the problem of evolution should be postponed until we have before
us the results of a searching analysis applied to a limited part of
the field. In such an analysis it is to be especially remembered that
we have now a new clue in the well-ascertained fact that the genetic
composition of the male and female germ-cells of the same individual
may be quite different. When with this possibility in view the behaviour
of the types is re-examined I anticipate that many of the difficulties
will be removed.
Outside the evidence from _Oenothera_, which, as we have seen, is
still ambiguous, I know no considerable body of facts favourable to
that special view of Mutation which de Vries has promulgated. Of
variation, or if we will, Mutation, in respect of some one character,
or resulting from recombination, there is proof in abundance; but of
that simultaneous variation in several independent respects to which de
Vries especially attributes the origin of new specific types I know only
casual records which have yet to undergo the process of criticism.
* * * * *
Besides de Vries's "_Mutationstheorie_" and "Species and Varieties"
the chief publications relating to the subject of the behaviour of
_Oenothera_ are the following: (Many other papers relating especially to
the cytology of the forms have appeared.)
Davis, B. M. Genetical Studies on _Oenothera_,
I. _Amer. Nat._, XLIV, 1910, p. 108. Genetical Studies on
_Oenothera_, II. _Ibid._, XLV, 1911, p. 193.
Gates, R. R. An Analytical Key to some of the Segregates of
Oenothera. _Twentieth Annual Report of the Missouri Botanical
Garden_, 1909.
Studies on the Variability and Heritability of Pigmentation in
_Oenothera_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 337.
Honing, J. A. Die Doppelnatur der _Oenothera Lamarckiana_.
_Ztsch. f. Abstammungslehre_, 1911, IV, p. 227.
Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K.
Small). Mutants and Hybrids of the _Oenotheras_. _Carnegie
Institution's Publication_, No. 24, 1905.
Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations,
Variations and Relationships of the _Oenotheras_. _Carnegie
Institution's Publication_, No. 81, 1907.
de Vries, H. On Atavistic Variation in _Oenothera cruciata_.
_Bull. Torrey Club_, 1903, Vol. 30, p. 75.
On Twin Hybrids, _Bot. Gaz._, Vol. 44, 1907, p. 401.
Ueber die Zwillingsbastarde von _Oenothera nanella_. _Ber.
Deut. Bot. Ges._, 1908, XXVI, _a_, p. 667.
Bastarde von _Oenothera gigas_. _Ibid._, p. 754.
On Triple Hybrids. _Bot. Gaz._, 1909, Vol. 47, p. 1.
Ueb. doppeltreziproke Bastarde von _Oenothera biennis_ L. und
_Oenothera muricata_ L. _Biol. Cbltt._, 1911, XXXI, p. 97.
Zeijlstra, H. H. _Oenothera nanella_ de Vries, eine krankhafte
Pflanzenart. _Biol. Cbltt._, 1911, XXXI, p. 129.
NOTE.
Since this chapter was written two contributions of special importance
have been made to the study of the _Oenothera_ problems. The first is
that of Heribert-Nilsson.[9] The author begins by giving a critical
account of the evidence for de Vries's interpretation of the nature of
the mutants. In general this criticism pursues lines similar to those
sketched in the foregoing chapter, concluding, as I have done, that the
chief reason why factorial analysis has been declared to be inapplicable
to the _Oenothera_ mutants is because no one has hitherto set about this
analysis in the right way. He has also himself made a valuable beginning
of such an analysis and gives good evidential reasons for the belief
that at least the red veining depends on a definite factor which also
influences the size of certain parts of the plant. He argues further
that many of the distinctions between the mutants are quantitative
in nature. With great plausibility he suggests that the system of
cumulative factors which Nilsson-Ehle discovered in the case of wheat
(subsequently traced by East in regard to maize) may be operating also
in these _Oenotheras_. According to this system several factors having
similar powers may coexist in the same individual, and together produce
a cumulative effect. Scope would thus be given for the production of
the curious and seemingly irregular numbers so often recorded in the
"mutating" families.
Another remarkable observation relating to the crosses of _muricata_
and _biennis_ has been published by Goldschmidt.[10] He finds that in
the formation of this cross the female pronucleus takes no part in the
development of the zygotic cell, but that when the male pronucleus
enters, the female pronucleus is pushed aside and degenerates. As de
Vries observed, the reciprocal hybrids are in each case very like the
father ("_stark patroklin_"), a consequence which finds a natural
explanation in the phenomenon witnessed by Goldschmidt. The results
of the subsequent matings can also be readily interpreted on the same
lines. Indications of maternal characters are nevertheless mentioned
by de Vries, and if Goldschmidt's account of the cytology is confirmed,
these must presumably be referred to the influence of the maternal
cytoplasm. Clearly this new work opens up lines of exceptional interest.
The interpretation I have offered above must probably be reconsidered.
The distinction between the male and female cells of the types may no
doubt be ultimately factorial, but it is difficult to regard such a
distinction as created by a differential distribution of the ordinary
factors.
FOOTNOTES:
[1] See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
Compare Lecoq on _Mirabilis jalapa_ × _longiflora_, Fécondation des
Végétaux, 1862, p. 311.
[2] _Rep. Evol. Ctee. R. S._, IV, 1908, p. 38.
[3] _Ber. Deut. Bot. Ges._, 1908, XXVI, _a_, p. 672.
[4] _Jour. Genetics_, 1, 1910, p. 57.
[5] In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called to
this possibility, though of course at that date it was in sexual animals
alone that it was supposed to exist. It had not occurred to me that even
a hermaphrodite plant might be in this condition.
[6] From the description of the offspring of _muricata_ used as mother.
[7] de Vries, _Species and Varieties_, 1905, p. 259.
[8] Zeijlstra in a recent paper announces that many _nanella_ plants
are the subject of a bacterial disease to which he attributes their
dwarfness. I gather that this does not apply to all _nanella_ plants
and that some are dwarfs apart from disease. The matter may no doubt be
further complicated from this cause.
[9] _Zts. f. Abstamm._, 1912, VIII.
[10] _Arch. f. Zellforschung_, 1912, IX, p. 331.
CHAPTER VI
VARIATION AND LOCALITY
In all discussions of the modes of Evolution the phenomena of
Geographical Distribution have been admitted to be of paramount
importance. First came the broad question, were the facts of
distribution consistent with the Doctrine of Descent? I suppose all
naturalists are now agreed that they are thus consistent, and that
though some very curious and as yet inexplicable cases remain to be
accounted for, the distribution of animal and plant life on the face
of the earth is much what we might expect as a result of a process of
descent with modification. Passing from this general admission to the
more particular question whether the facts of distribution favour one
special conception of the mode of progress of evolution rather than
another, no agreement has yet been reached. One outstanding feature
is hardly in dispute, namely that prolonged isolation is generally
followed by greater or less change in the population isolated. Groups
of individuals which from various causes are debarred from free
intermixture with other groups almost always exhibit peculiarities,
but on the other hand, cosmopolitan types which range over wide areas
are on the whole uniform, or nearly so throughout their distribution.
Examples of these two categories will be familiar to all naturalists.
The barriers to intercourse may be seas, deserts, prairies,
mountain-chains, or circumstances of a much less obvious character
which isolate quite as effectually. The local unit is not necessarily
an island, a district, or an area of special geological formation,
but may, as every collector knows, be a valley, a pond, a creek, a
"bank" in the sea, a clump of trees, a group of rocks in a bay, or a
particular patch of ground on a mountain side. All the great groups
provide examples of such specially isolated forms. The botanist knows
them well; the conchologist, the entomologist, the ornithologist and the
student of marine life are all equally aware that special varieties or
special species come from special places and from nowhere else. In one
remarkable case the season of appearance plainly acts as the isolating
barrier. _Tephrosia bistortata_ is a small Geometrid moth which has two
broods, appearing in _March_ and _July_ respectively. It is closely
allied to _T. crepuscularia_ which emerges in _May_ and _June_. From the
fact that occasional specimens cannot be quite certainly referred to
one or other of the two, many have held that the two are one species.
Nevertheless, in general they present distinctions which are plain
enough. Some localities have one form only, but in several woods they
co-exist. Experiment has shown that the two can be crossed, and that the
cross-breds can breed _inter se_ and with at least one of the parent
stocks.[1] Some diminution in fertility was observed, but perhaps not
more than is commonly encountered when wild forms are bred in captivity.
In such a case it can scarcely be doubted that the distinctness of
the two forms in the places where they co-exist is maintained by the
seasonal isolation.
Just as the consequences of isolation are to be seen in the most
different forms of life so may they also affect the most diverse
features of organisation, such as size, colour, sculpture, shape, or
number of parts. In the Sloth (_Choloepus_) the geographical races
differ in the number of cervical vertebrae--or in other words, in the
distribution of vertebral differentiation. The geographical races of
_Cistudo_ differ in the number of claws and phalanges.[2]
In Shetland, the males of _Hepialus humuli_ (the Ghost Moth) are
not sharply differentiated in colour from the females, as they are
elsewhere, but in varying degrees resemble them.[3] No such males are
found in other localities, and even in the other Scottish islands they
are normal. In the island of Waigiu the converse phenomenon has been
observed in _Phalanger maculatus_. Generally the male is spotted with
white, and the female is without spots, but in Waigiu the females are
spotted like the males.[4]
The following striking illustration was pointed out to me by Dr. W. D.
Miller. _Euphonia elegantissima_ as it occurs in Mexico and Central
America has the two sexes very distinct from each other. The male has
the lower parts orange and the upper parts a dark indigo blue, with a
bright turquoise-blue head and neck. The female, except for the head,
is of a bright olive green. A form in which the sexes are similarly
differentiated exists in Porto Rico and is known as _E. Sclateri_. But
in many of the other West Indian islands the representative "species"
(_E. flavifrons_) has the two sexes closely resembling the _female_
of _E. elegantissima_. This form is found in Antigua, Barbados, St.
Vincent, and Guadeloupe, from which localities the British Museum has
specimens. All three so-called species are very much alike otherwise.
In the genus _Pyrrhulagra_ (_Loxigilla_) to which Mr. Outram Bangs
called my attention, several distinct and alternative possibilities
occur. The genus has many local species occurring on the various West
Indian islands. These species are characterized by differences in size,
colour, and the shape of the bill. The colours have a narrow range,
being black or greyish, with or without chestnut marks about the head
and throat. In most of the islands the males are in general colour a
full black, and the females are distinctly grey. They are thus found
in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In
Porto Rico we meet the peculiarity that the hens are almost as black
as the males (Ridgway describes the black of the hens as slightly less
intense). This form is called _portoricensis_. A larger type, known
as _grandis_, similarly coloured, inhabits St. Kitt's. Then, on the
contrary, in Barbados, _both sexes_ are a dull blackish grey, like the
hens of the Lesser Antilles in general.
The local species of _Agelaius_ show similarly capricious distinctions.
_A. phoeniceus_ is a widely spread species, found over a great part of
North America. The male is black with red-orange bars on the wings, but
the female is somewhat thrush-like in colour. In the island of Porto
Rico there is a form called _xanthomus_, in which _both sexes_ are like
the males of the mainland. A similar species called _humeralis_, also
with both sexes male-like, lives in Cuba. The island of Cuba, curiously
enough, has also a distinct species named _assimilis_, in which the
female is a dull black all over, though the male is like the mainland
type.
So also may local races differ in respect of variability. _Argynnis
paphia_, the Silver Washed Fritillary, through a great part of its
distribution has only one female form. In the English New Forest a
second female form, _valesina_, co-exists with the ordinary _paphia_
female. But in the southern valleys of the Alps the _valesina_ female
is much the commoner of the two, and indeed in some localities where
the species is abundant, I have seen no _paphia_ females in many days
collecting.
The beetle _Gonioctena variabilis_ furnishes an illustration of a
comparable phenomenon affecting the male sex. In 1894 and 1895 I
studied the curious colour variations of this species especially in the
neighbourhood of Granada, and Mr. Doncaster ten years later repeated the
observations on the same ground, and also collected the insect in other
places in the south of Spain. The distinctions are not easy to give in
words and the reader is referred to the colour plate accompanying my
paper.[5] The essential fact is that the males commonly have the elytra
_red with black spots_ and the females for the most part have greenish
grey elytra with black stripes. In some localities a large minority of
males closely resemble the female type, being identical in colour and
then only distinguishable by structural differences. In two Granada
localities I found the proportion of such males quite different. In the
Darro valley about 38 per cent. (in 718) were of this feminine type,
but on the hills some 300 feet above only 19 per cent. (in 3,230) were
like the females. At Castillejo, not far from Toledo I found no such
male in 75 specimens.
Mr. Doncaster collected from several localities, especially from two
areas near Malaga, about 5 miles apart. In one of these the female-like
males were, as usual, in a minority, but in the other these were
actually in great excess, amounting to about 81 per cent. in the 173
taken. Doncaster found a doubtful indication that the composition of the
population varies with the season, which is quite possible, but it is
most interesting to note that in my chief locality after the lapse of
ten years he found the proportions very much the same as I had done at
the same season, for where I had 19 per cent. of the female-like males
his collecting gave 16 per cent. In other respects also, his statistics
corresponded very closely with mine.[6]
The various forms of _Heliconius erato_ are well known to entomologists.
They are strikingly distinguished by the colours of the strong comb-like
marking on the hind wing, which may be red, yellow, green or blue. In
various parts of the distribution in South America sometimes two and
sometimes three of these distinct types co-exist.[7]
The distribution of the varieties of _Noctua castanea_ typifies a large
range of cases. The form which is reckoned the normal of the species
has red fore-wings. It is practically restricted to Great Britain and
Germany, according to Tutt. The other common form, _neglecta_, has grey
fore-wings, and in this pattern it ranges through West Central Europe
from North Italy to Germany. In the British Isles it extends up to
Orkney. In Britain this grey form is by far the commoner, occurring
wherever the species is found. The red form is much scarcer in England,
and does not occur at all in many localities where the grey form is
common. Mr. Woodforde, from whom this account is taken,[8] states that
in August, 1899, he saw considerably over a hundred of the grey in the
New Forest at sugar, but only two red ones. In Staffordshire however the
red is proportionately more numerous and he estimates them as 40 per
cent. of the population. Lastly a form has been taken in Staffordshire
as a rarity in which the red is replaced by yellow, and this has
hitherto been seen nowhere else. It is beyond our immediate purposes
to discuss the genetic relationships of such forms, but the details
of this case are interesting as making fairly clear the fact that the
distinctions between _castanea_ and _neglecta_ are due to combinations
of the presence of and absence of two pairs of factors, of which one
produces a red pigment in the ground colour of the forewing and the
other irrorates the same region with black scales. Mr. Woodforde states
that all intermediates exist, and that in Staffordshire the greys always
have a pinkish tinge. The yellow is doubtless another recessive to the
red.
Species which are uniform in some localities may be polymorphic in
others. Such a phenomenon is well exemplified by the orchid _Aceras
hircina_. Of this species distinct varieties had previously been known
in Germany, but Gallé[9] has lately given a detailed account of a
number of most diverse forms found growing in a district of Eastern
France. Without reference to his plates it is impossible to give any
adequate conception of the profusion of types which the flowers of
the species there assume. In some the lip is elongated to many times
its usual length, twisting and dividing in a fashion suggesting some
of the strangest of the Tropical Orchids. In others the labellum
and the lateral petals are all comparatively short and wide (Fig.
13). Intermediates, combining these qualities in various degrees,
were abundant, and the condition of the species, which was the only
representative of the genus in the locality, recalls the extreme
polymorphism of many of the Noctuid Moths.
[Illustration: FIG. 13. Various forms of _Aceras hircina_. (After
Gallé.) This figure only shows a few of the more striking forms
illustrated in Gallé's plates.]
Somewhat comparable variability has been seen in another Orchid genus
_Ophrys_. In Great Britain the species _apifera_, _aranifera_ and
_muscifera_ though variable are fairly distinct, but Moggridge has
published two series of plates[10] showing a very different state of
things as regards the _Ophrys_ population of the Riviera. Here the
outward diversity is such that the ordinary specific names cannot be
applied with any confidence and the limits of the species are quite
uncertain. It may well be supposed that these Riviera plants are
interbreeding, and indeed we may safely assume that they are. It is,
however, to be remembered that Darwin showed _apifera_ in this country
to be habitually self-fertilised, so that the different behaviour on
the Riviera may itself constitute a local peculiarity. Moreover it is
to be gathered from Moggridge's account that in the districts which he
examined the condition was not to be described by the statement that
our three types were there co-existing and hybridising, but rather we
should say that the population was polymorphic, containing these three
types amongst others. Conchologists are aware that on the Dogger Bank
_Modiola_ attains a size unparalleled elsewhere. The same is true of
the sponges _Grantia compressa_ and _Grantia ciliata_ in the estuary
of the Orwell.[11] Conversely, as we know so well in the case of Man,
dwarf races occur in several special localities. Such examples may be
multiplied indefinitely.
The relation of local forms to species has often been discussed from
many points of view, but I know no treatment of the subject clearer or
more comprehensive than an excellent account of some of the various
manifestations of local differentiation as they appear in Helicidæ
published by Coutagne[12] and a reader interested in the problem which
they raise would do well to make himself acquainted with the original
from which the following notes are taken. He speaks for example of
_Helix lapicida_. This is on the whole a constant form ranging up to the
altitude of 1,300 m., common all over France except at great heights
and in the Olive regions where it is restricted to moist places. Though
subjected to such diverse conditions it shows only trivial variations in
colour and other respects throughout its distribution, excepting that
on both sides of the Pyrenees it has a very distinct sporadic variety
called _Andorrica_ or _microporus_. This variety occurs here and there,
together with the type-form sometimes in colonies (pp. 26-30 and 86).
_Bulimus detritus_ though more restricted in geographical range is a
much more variable form. It exhibits great variations in colour, form,
and size, and as Coutagne well insists, these are independent of each
other. Foreshadowing the methods of factorial analysis he suggests that
distinctions in each respect, the "modes" as he calls them, should
be denoted by a letter, or if desired, by a name, and the several
combinations of differences might thus be most logically and usefully
expressed. Of such combinations he says there are at least 18, all of
which can be found. The whole possible series does not necessarily
occur in the same place, and various localities are characterised by
the presence or absence of certain of the combinations as Coutagne
calls them, and by the relative frequency with which they occur. The
ideas thus enunciated are much in advance of the ordinary practice of
systematists, who give names to forms which are nothing but accidental
combinations of factors, just as the horticulturists for practical
reasons give names to similar combinations, which as we now know are
merely specially noticeable terms in a long series of possibilities.
In each case it is rather the _factors_ which should be named than the
forms which are constituted by their casual collocation. In this special
example of _Bulimus detritus_ the 18 forms are made by the combinations
of three pairs of independent factors. Besides these combinations which
may occur anywhere or almost anywhere in the distribution there are
two more distinct local forms, each of which is regarded by Coutagne
as probably constituting a fresh "mode," perhaps compatible with the
others.
_Helix striata_ (Draparnauld)[13] is truly polymorphic; and its various
forms have been described under various specific names. It abounds in
the calcareous hills of Provence and Languedoc, disappearing in the
alluvial lowlands and equally in the upper levels at about 800-1,000 m.
From this district it extends through regions of similar altitude over a
great part of France (details given).
Locard in his monograph of this group, which he calls collectively the
group of _Helix Heripensis_, tabulates 27 distinct named forms. The
characteristics in which these forms differ have been reckoned as 17,
and as several of these vary in degree of development, the number of
modes may be increased to 109. For practical purposes however Coutagne
considers that the various developments of 7 characteristics in their
several combinations are enough to express the various forms, and he
gives examples of this method of definition. As he observes, though
names may be required to define the modes, no one need be alarmed at
that, for the same names of modes will be applicable to a great range of
distinct species, and the formulae expressing their combinations will
replace the varietal names.
This particular example of polymorphism is but little limited by
locality. Occasional colonies present some special physiognomy which may
in a given place seem almost invariable, though in this very respect the
colonies found elsewhere may be highly variable, but such limitations
are exceptional for _H. striata_.
Some distinct and obvious susceptibilities to the influence of soil
and climate are however noticeable. For example on siliceous ground
the shells are thinner, while on calcareous soils they are thicker;
similarly those from the Northern districts attain a larger size
than those from further South. Moreover those subjected to curtailed
development, whether from drought, heat or cold often show a shortening
of the spire. In contrast with this case Coutagne describes the
varieties of _Helix caespitum_, which he says are for the most part
localised, quoting many illustrative cases.
Another remarkable case in which locality plays a curious part is
provided by the two species _Helix trochoides_ and _pyramidata_. In
France generally they are distinct enough from each other, _trochoides_
being smaller and having a characteristic keel. Coutagne says that after
having collected these species from more than a score of localities he
came upon a colony of _trochoides_ on the island of Pomègues in which
the shells were relatively enormous, most of them having only a slight
keel, and a few none at all. On the other hand he received a consignment
of _pyramidata_ from four localities in Sicily, all small, and one of
them exactly like the _trochoides_ from Pomègues. Judging by the samples
received from Sicily, _trochoides_ is there not more variable than it is
in Provence, while the Sicilian _pyramidata_ is protean.
The relations of the two species _Helix nemoralis_ and _hortensis_
provide an illustration of another kind of manifestation of local
peculiarity. _H. hortensis_ and _nemoralis_ as usually met with, are
two very distinct forms. _H. hortensis_ is smaller and duller, and its
peristome is white. _H. nemoralis_ is larger and more shiny, and its
peristome is brown. In several anatomical points, moreover, especially
in the shape of the dart, there are great differences. For a full
account of these peculiarities of the two forms and a discussion of
their inter-relations the reader is referred to the elaborate work of
A. Lang[14] who has studied them extensively and has also succeeded in
experimentally raising hybrids between them. These hybrids were in a
slight degree fertile with both the parent species, but up to the time
of publication no young had been reared from hybrids _inter se_.
Coutagne describes the result of collections made in 62 French
localities. Some had exclusively _hortensis_, some exclusively
_nemoralis_, and in some the two were found in association. He gives
details of five of these collections from which I take the following
summary of the more essential facts, omitting much that is almost
equally significant.
_Locality A_, near Honfleur. Both forms present, each sharply and
normally distinguished, without any intermediates. They are thus found
in many places. Coutagne instances Müller's observations in Denmark, his
own series from the Jura, etc.
_Locality B._ Vonges (Côte d'Or), 242 _hortensis_ taken at random,
showed 128 with light peristomes (either more or less pinkish or
quite white) and 114 with dark _brown_ peristomes; together with 26
_nemoralis_ all with the usual brown peristomes.
Of the _hortensis_ 50 were in ground-colour _opalescens_ and 1 _roseus_;
and in shape 5 were _umbilicatus_.
_Locality C_, about 3 kilometres from _B_. There were found 35
_hortensis_, of which 20 had light peristomes and 15 brown; together
with 7 _nemoralis_.
Of the _hortensis_ none were _opalescens_; 18 were _roseus_ and none has
the shape of _umbilicatus_.
_Locality D_, about 1,200 metres from _B_. 147 _hortensis_, of which 4
had light peristomes and 143 had brown. No _nemoralis_ were found.
None of the _hortensis_ were _opalescens_ or _roseus_, but 30 were
_umbilicatus_.
In these localities intermediates of every grade existed between the
well-characterised _opalescens_, _roseus_, or _umbilicatus_, and the
other forms, but there were no intergrades between the other _nemoralis_
and the smaller _hortensis_, about which there was no hesitation. In the
next locality a very different state of things was found.
_Locality E._ Banks of the Yvette at Orsay (Seine-et-Oise). The
actual numbers are not given, but we are told that 58 per cent. were
_hortensis_, 33 per cent. _nemoralis_, and 9 per cent. intermediate. As
at Honfleur, the _hortensis_ had white peristomes, and the _nemoralis_
brown. Coutagne's visits to this locality were in 1878 and 1880, and he
calls attention to the fact that Pascal found similar intermediates in
the same neighbourhood in 1873.
The two species, in Coutagne's view, when they occur together, can
generally be sorted from each other with perfect confidence, and it is
only in exceptional localities that these intermediates occur. Whether
they are hybrids, or whether sometimes the species in their variations
transgress their usual limitations is regarded both by Coutagne and
by Lang as a question not yet answerable with certainty. Coutagne
moreover lays stress on the fact that although each species may be
easily known from the other _in its own district_, yet when shells from
different districts are brought together it is sometimes impossible to
sort them. He mentions an example of such casual intermixture occurring
under natural conditions on an island in the Rhone, to which it may
well be supposed that floods had brought immigrants from miscellaneous
localities. This population contained a very large number of uncertain
specimens, and as he says, it was much as if he were to mix the shells
from his 62 localities, after which it would certainly be impossible to
separate the two species again.[15]
Further evidence is given in the same treatise as to other examples
of polymorphism, especially in the genus _Anodonta_, of which Locard
made 251 species for France alone. Here again are cases like those
already given, and many forms or "modes" are found restricted to special
localities, while occasionally in the same locality dissimilar forms are
found, collectively forming a colony, without intermediates.
Taken as a whole the evidence shows the following conclusions to be
true. Local races, whether of animals or plants, may be distinguished
by characters which we are compelled to regard as trivial, or again by
features of such magnitude that if they were known to us only as the
characteristics of a uniform species they would certainly be assumed
without hesitation to be essential for its maintenance. Local forms
may be sharply differentiated from the corresponding populations of
other localities or they may be connected with them by numbers of
intermediates. Not rarely also we find a fact which has always seemed to
me of special significance, that the peculiarity of the local population
or colony may show itself in a special liability to variation, and
this variability may show itself in one of many degrees, either in the
constant possession of a definite aberration, in a dimorphism, or in an
extreme polymorphism.
At this stage attention should be called to two points. First, that
when the details of the geographical distribution of any variable
species are studied in that thorough and minute fashion which is
necessary for any true knowledge of the interrelations of the several
forms, the conception of a species invented by the popular expositions
of Evolution under Selection is found to be rarely if ever realised in
nature.
A species in this generalised sense is an aggregate of individuals,
none exactly alike, but varying round a normal type, the characters of
which are fixed in so far as they are adapted to environmental exigency.
In nature, however, the occurrence of the varieties, and even the
occurrence of the variability is sporadic. In one place a population may
be perfectly uniform. In another it may be again uniform but distinct.
In others the two forms may occur together, sometimes with and sometimes
without intergrades. In some localities a sporadic variety may be an
element of the population, persisting through long periods of time.
In other localities there may be several such aberrations occurring
together which are absent elsewhere.
Secondly, I would remind the reader that in the light of genetic
analysis we know that intergrades, when they do occur, cannot be assumed
to represent conditions through which the species must pass or has
passed on its way to the extreme and definite forms.
Often, perhaps generally, they are nothing but heterozygous forms,
and often also they are conditions corresponding with the presence of
factors in their reduction-stages.
A broad survey of the facts shows beyond question that it is impossible
to reconcile the mode of distribution of local forms with any belief
that they are on the whole adaptational. Their peculiarities are
occasionally the result of direct environmental influence, as we shall
hereafter notice in certain cases, but none can attribute such sporadic
and irregular phenomena to causes uniformly acting.
Writers on systematics, especially those of former generations often
conjecture or assert that local distinctions are caused by "differences
of climate, soil, food, etc.," in vague general terms. It is usually
safe to assume that these remarks do not represent conclusions drawn
from actual evidence, for only rarely can they be translated into more
precise language. So thoroughly have the biological sciences become
permeated with the belief that all distinctions are dependent upon
adaptation, that the mere existence of definite distinctions is felt
by many to be sufficient ground to warrant an assumption that these
distinctions are directly or indirectly due to special local conditions.
For example, Dr. J. A. Allen, who has done so much careful and valuable
work in delimiting the local forms of the United States fauna, writes of
the Ground Squirrels (Tamias)[16] as follows:--
"From the extreme susceptibility of this plastic
group to the influences of environment, it is one of the most
instructive and fascinating groups among North American mammals.
No one can doubt its comparatively recent differentiation from
a common stock, and its dispersion from some common centre.
Whether the type originated at some point in North America, or in
the Northern part of Eurasia, it is perhaps idle to speculate,
but that it has increased, multiplied, spread, and become
differentiated to a wonderful degree in North America is beyond
question; as it is found from the Arctic regions to the high
mountain ranges of Central Mexico, and has developed some twenty
to thirty very palpable local phases."
"Some of them easily take rank as species, others as
subspecies. Probably a more striking illustration of evolution by
environment cannot be cited."
He proceeds to point out that the habits of these creatures are such as
lead to isolation. This may well be admitted, and indeed no exception
can possibly be taken to the passage as a whole, save in the one
respect that there is no real proof that the local diversity is due to
"evolution by environment" or an indication of "susceptibility to the
influences of environment."
Dr. Allen does indeed adduce the fact that California "extending through
800 miles of latitude, with numerous sharply contrasted physiographic
regions, has apparently no less than six strongly differentiated forms,
while the region east of the Rocky Mountains from a little below the
northern boundary of the United States northward to the limit of
trees--a slightly diversified region of at least ten times the area of
California--has only one"! But when one comes to ask how the various
forms are adaptational, and how the influences of environment have led
to their production, only conjectures of a preliminary and tentative
character could be expected in reply. Desert forms are no doubt pallid
as in so many instances, and forest forms are more fully coloured, and
we may readily enough accept such facts as indications of a connection
between bodily features and the conditions of life, but further than
that no one can go; so that when we find size, length of ears or of
tail, the number of dorsal stripes, the pattern of the colours, not to
speak of differences in the pigments themselves, all exhibiting large
modifications, we cannot refer these peculiarities to the causation
of environmental difference, save as a simple expression of faith. I
incline far more to agree with Gulick who, after years of study of the
local variations of the Achatinellidae, came to the conclusion that it
was useless to expect that such local differentiation can be referred to
adaptation in any sense.[17] Even the most convinced Selectionist must
hesitate before such facts as those related by A. G. Mayer regarding
the distribution of _Partula otaheitana_, one of these Achatinellidae.
The island of Tahiti has been scored by erosion so that a series of
separated valleys radiate to the coast. From four successive valleys
Mayer collected the species, and found that in the first (Tipaerui)
valley all the shells were dextral (115, containing 73 young); in the
second valley (Fautaua) 54 per cent. of adults and 55.5 per cent. of
the young contained were sinistral; in the third valley (Hamuta) 69
per cent. of adults and 73 per cent. of young contained in them were
sinistral; and lastly, in the fourth valley (Pirae) all the shells
(131, containing 62 young) were sinistral.[18] In connection with
these observations I may mention the fact that in a certain pond in
the North of England[19] the sinistral form of _Limnaea peregra_ has
been known to occur for about fifty years. Visiting it lately I found
the left-handed shells to be about 3 per cent. of the population. The
species is the commonest British freshwater shell, but left-handed
specimens are exceedingly rare. Will anyone ask us to suppose that the
persistence of a percentage of this rarity in the same place is an
indication of some specially favouring circumstance in the waters of
that pond? It is a horse-pond to all appearances exactly like any other
horse-pond; and I believe that in perfect confidence we may accept the
suggestion of common sense, which teaches us that there is nothing
particular in the circumstances which either calls such varieties into
existence or contributes in any direct way to their survival. Had the
phenomenon of local variation been studied in detail before Darwin
wrote, the attempt to make selection responsible for fixity wherever
found, could never have been made. The proposition that not only the
definiteness of local forms but their variability also is sporadic,
can be established by countless illustrations taken from any group of
either the animal or the vegetable kingdoms. Only exceptionally can the
fixed differences be even suspected of contributing to adaptation, and
sporadic variability, which is a no less positive fact, must manifestly
lie outside the range of such suspicions. It is open to any one to
suggest speculatively that the persistence of special varieties or of
special variability in special places is an indication that in those
places the conditions of life are such that the forms in question are
tolerated though elsewhere the same types are exterminated; but that
consideration, even if it could be proved to be well founded, is not
one which lends much force to the thesis that definiteness of type is
a consequence of Natural Selection. On the contrary, recourse to such
reasoning implies the inevitable but very damaging admission that the
stringency of Selection is frequently so far relaxed that two or more
equally definite forms of the same species can persist side by side.
There is no doubt that this is the simple truth, but when once that
truth is perceived it is useless to invoke the control of Selection as
the factor to which definiteness of type in general must be referred.
The genetic relations of local forms to each other cannot in the
absence of actual breeding experiments be often ascertained. Standfuss
formerly enunciated as a general principle that when two forms
co-exist in the same locality and are able to interbreed, they do not
produce intermediates; but that when the forms are geographically
separated as local races, crosses between them result in a series of
intermediates.[20] In this aphorism there is a good deal of truth, but
if in the light of Mendelian principles we examine the two statements
we see now that the first is in reality only another way of saying
that the distinctness of an aberrational form co-existing with another
is due to segregation, accompanied by some degree of dominance of one
type. Whether, however, one geographically isolated race will give
intermediates when bred with another must depend entirely on the genetic
physiology of the special case, and no general rule can be laid down.
It may well be that, inasmuch as the distinctness of the variety is
maintained by isolation, the difference in factorial composition between
it and the representative form in another area is neither simple nor
sharp; but when two varieties co-exist, though interbreeding, it is now
clear that their differences must depend on the segregation of simple
factors. Plainly such aberrations may in one place co-exist with another
type, and elsewhere be separated from it as local races.
Excellent illustrations of these two stages in evolution are provided
by the melanic varieties of British Lepidoptera. The fact that black or
blackish varieties of many species especially of Geometridae have come
into existence in recent years is well known to British collectors,
and it is not in dispute that they have in several instances replaced
the older type more or less completely in certain districts. In the
year 1900 the Evolution Committee of the Royal Society instituted a
collective inquiry as to the contemporary distribution of these dark
varieties. As the change had happened within living memory and had
greatly progressed in recent years it was hoped that a record of the
existing distribution would serve as a point of departure for future
comparison. The records thus obtained were tabulated by Mr. L.
Doncaster.[21] From that account and from the statements in Barrett's
British Lepidoptera[22] this description of some of the more notable
cases is taken.
The most striking and familiar case is that of _Amphidasys betularia_,
of which only the ordinary type was known in any locality until about
1848-1850, when the totally black var. _doubledayaria_ first appeared
in the neighbourhood of Manchester. This black form was subsequently
recorded in Huddersfield between 1860 and 1870; Kendal about 1870;
Cannock Chase, 1878; Berkshire, 1885; Norfolk, Essex and Cambridge about
1892; Suffolk, 1894; London, 1897. For the Southern Counties of England,
except in the London district, there are still very few records. It
cannot of course be asserted positively that the variety spread from its
place of first appearance into the other localities, and that it did not
arise _de novo_ in them, but there can be little doubt that the process
was one of colonisation. On the European Continent the first records are
from Hanover in 1884, Belgium 1886 and 1894, Crefeld 188-, Berlin 1903,
Dresden about the same date.
As regards the increase of the variety we have the fact that in
Lancashire, Cheshire and the West Riding of Yorkshire the black is now
the prevalent form; and in some places, as for example, Huddersfield,
the black alone is now found, though it was unknown there till between
1860 and 1870. About 1870 at Newport, Monmouth, the two forms were in
about equal numbers, but a few years later the type had almost vanished.
Similarly in Crefeld, where the black form was still very rare in the
eighties, it now forms about 50 per cent. of the population. In the
London district the black remains scarce and at the date of the report
it was still very scarce. From Ireland there is only one record and
there are hardly any from Scotland.
_Boarmia repandata_ is another species which is behaving in a somewhat
similar way. Unlike _betularia_, however, the species is a variable
one, and has several colour-forms, amongst them the banded var.
_conversaria_, and many others. In addition to these there is a
black form in the North of England which seems to be spreading. In
Huddersfield the black was first recorded in 1888, and in 1900 20-25 per
cent. were black. At Rotherham the black or very dark are now prevalent
and have increased in the last 15 years. From the Midlands, East Anglia
and Southern Counties the returns show only the light and medium forms.
Of _Odontoptera bidentata_ several intergrading dark forms exist, and
these are found exclusively in the North and the Midlands. Unicolorous
blacks have been found recently in the Lancashire mosses and at
Wakefield. At Huddersfield 50 years ago the light forms were prevalent,
but now a rather dark brown, not infrequently suffused with black, is
the commonest. In Southern Counties only light forms are known.
_Phigalia pilosaria_ in South England is always light, but in the North
the prevalent form is darker. About 35 years ago a form with unicolorous
sooty fore-wings and dull grey hind wings was first seen in Yorkshire
and a similar form is now taken regularly in South Wales.
In the following cases the dark varieties were found originally only in
the South.
_Boarmia rhomboidaria_ gave rise about 40 years ago to a unicolorous
smoky variety called _perfumaria_. This was at first peculiar to the
London district, but it has since been taken in Birmingham and other
large cities. More lately coal-black specimens have been found at
Norwich, and others similar but hardly so dark were taken in the South
of Scotland and at Cannock Chase.
_Eupithecia rectangulata_ is a similar case. Formerly the light forms
were prevalent but within sixty years they have almost entirely been
replaced in the South of London by a nearly black form.
_Tephrosia_ (_Boarmia_) _consortaria_ and _Tephrosia consonaria_ are
exceptionally interesting, for they have both given off dark forms in
the same wood near Maidstone, which is far from the usual "centres of
melanism." They were discovered in this locality by Mr. E. Goodwin. That
of _consortaria_ is a dark grey, but that of _consonaria_ is a full
black, and nothing like either has been found anywhere else.
These examples are all taken from the Geometridae but others, though
of a less conspicuous kind, could be given from the Noctuidae or the
Micro-Lepidoptera. _Acronycta psi_, for instance, has a suffused form
which is believed to be becoming more frequent in the London district.
_Polia chi_ has two dark forms, _olivacea_, a yellowish grey with dark
markings, and _suffusa_ which is a darker, blackish-slate colour. Both
occur in the North of England, sometimes together, sometimes separately,
or mixed with the type and many intermediates. The distribution is
peculiarly irregular. At Huddersfield, where the very dark form appeared
suddenly about 1890, some 30 per cent. are said to be now dark and about
6-7 per cent. very dark, but at Saddleworth, 12 miles away, only the
pale forms occur.
Several questions of interest arise in regard to this evidence. This
progressive Melanism has arisen in certain families only, and may be
confined to certain species only, within those families. As in almost
all other examples in which variation has been much observed, its
incidence is capricious and specific. A collateral line of inquiry
relates to the degree of discontinuity which the variation manifests.
Here again there is no rule. Generally speaking, in _A. betularia_,
to take the case most fully studied, the variation is discontinuous.
Real intermediates between _betularia_ and _doubledayaria_ are in most
localities absent or rare. The black spots of _betularia_ may often
be larger or more numerous than in the normal, but this variation has
nothing to do with _doubledayaria_, and is not an intermediate stage
towards it, though sometimes wrongly so described. _Doubledayaria_ owes
its characteristic appearance to a factor which blurs the surface of the
wings with a layer of black. Sometimes this blurring is slighter than
in the real _doubledayaria_, and these forms are real intermediates.
Occasionally the fore-wings alone are thus blurred. These intermediates
are clearly due to reduction-stages of the _doubledayaria_ factor,
and are related to it as a blue mouse is to a black, or a dutch
rabbit to a self-colour. It cannot positively be asserted that the
full _doubledayaria_ existed before the intermediate, but it almost
certainly did. In certain places as for instance in Belgium, there is
evidence that intermediates have at various times been fairly abundant,
but they have never become common, nor are they known to exist in the
absence of _doubledayaria_. When the black variety and the light type
breed together they do not usually have intermediates among their
offspring, and the evidence is consistent with the view that the
black is a complete dominant. The same is probably true of _Tephrosia
consonaria_.
In some of the other species we know that the darkest forms did
not appear first. For example in _Phigalia pilosaria_ and _Boarmia
rhomboidaria_ dark forms existed and are believed to have increased in
number before the darkest made its appearance. _Hybernia progemmaria_ is
said to have become darker gradually both in Cheshire and in the West
Riding, and a uniformly smoky variety appeared in South Yorkshire less
than 45 years ago which has spread to neighbouring counties. The dark
medium has become the commonest form in Huddersfield district, where the
very dark variety is now about 20 per cent. of the population, though
the light form is still common.
Taking the evidence together we find it consistent with the view that
dark forms have appeared sporadically, in some species the very dark
appearing first and intermediates later, in others the moderately dark
came first and the darkest later in time. It is practically certain that
the change has in general come about not by a gradual change supervening
on the population at large, but by the sporadic appearance of dark
specimens as a new element in the population, and strains derived from
these dark individuals have gradually superseded the normal type more or
less completely.
If it could be shown that these melanic novelties had a definite
advantage in the struggle for existence they would provide an instance
of evolution proceeding much in the way which Darwin contemplated. The
whole process would differ from that conceived by him as the normal
method of evolution only in so far as the change has come about with
great rapidity and in some instances largely by the appearance and
success of discontinuous varieties. The question, however, must be
asked whether the dark form can reasonably be supposed to have an
advantage by reason of their darkness. Some naturalists believe that
the darkness of the colours does thus definitely contribute to their
protection by making the insects less conspicuous and thus more likely
to escape the search of birds. In support of this view it may be
pointed out that it is in the manufacturing districts of Lancashire and
Yorkshire, and again in the London area that the melanics have attained
their greatest development. Consistently with this argument also, it is
in the neighbourhood of Crefeld and Essen, the black country of Germany,
that they have chiefly established themselves on the Continent, and
_Phigalia pilosaria_ in the black form is now at home in South Wales.
Thus superficially regarded, the evidence looks rather strong, but it is
difficult to apply the reasoning in detail. We have first the difficulty
that the black form of _betularia_ for instance has established itself
in thoroughly rural districts, notably near King's Lynn in Norfolk,
and in the neighbourhood of Kendal and Windermere. The black form
of _consonaria_ and the dark _consortaria_ appeared in a wood near
Maidstone, far from town smoke, and the black _rhomboidaria_ was first
found at Norwich, which, as towns go, is clean. Then again the spread of
the melanics is very irregular and unaccountable. The black _pilosaria_
is found both in the West Riding and in the Swansea district, but
not yet elsewhere. It rapidly increased at Huddersfield, but made no
noticeable progress at Sheffield though recorded there for ten years.
It is also a remarkable fact that no similar melanic development has
been observed in America, and, so far as I am aware, comparable melanic
varieties have not appeared on the European continent except in the case
of the few sorts which possibly may have come from England.
The whole subject is beset with complications. It must not be forgotten
that in a few species of moths there is an obvious and recognised
conformity between the colours of the perfect insect and that of the
soil on which they live, comparable with that which is so striking in
the case of some Oedipodidae and other grasshoppers. Of this phenomenon
the clearest example is _Gnophos obscurata_, which is a most variable
species with many local forms. Of these a well-known dark variety lives
on the peaty heaths of the New Forest and other districts, but on the
chalk hills of Kent, Sussex and Surrey various light varieties are
found, of which one is a bright silvery white, very near in colour to
the colour of a chalky bank. This case does not seem to be one of direct
environmental action,[23] for Poulton found no change induced by rearing
larvae among either white or black surrounding objects. No one however
can doubt that there is some indirect connection between the colour of
the ground and that of the moths.
To my mind there is a serious objection to the theory of protective
resemblance in application to such a case as that of the _betularia_
forms, which arises from the fact that the black _doubledayaria_ is a
fairly conspicuous insect anywhere except perhaps on actually black
materials, which are not common in any locality. Tree trunks and walls
are dirty in smoky districts but they are not often black, and I doubt
whether in the neighbourhood of Rotherham, for instance, which is one
of the great melanic centres, _doubledayaria_ can be harder for a bird
to find than _betularia_ would be. After all, too, many of the species
much affected are not urban insects. They live in country places between
the towns, and the general tone of these places even in Lancashire
and the West Riding is not very different from that of similar places
elsewhere. As against the objection that the black varieties are much
blacker than the case requires it may be replied that we know nothing
of the senses of birds, and that perhaps to their eyes blackness does
constitute a disguise even though the surroundings are much less dark.
This is undeniable, but recourse to such an argument is dangerous; for
if the sight of the insect-eating birds is so dull that it does not
distinguish dark things from dingy grey, we cannot subsequently regard
the keen sight of birds as the sufficient control which has led to the
minute and detailed resemblance of many insects to their surroundings.
Those who see in such cases examples of the omnipotence of Selection
must frequently find themselves in this dilemma.
Taking the evidence as a whole, we may say that it fairly suggests the
existence of some connection between modern urban developments and the
appearance and rise of the melanic varieties. More than that we cannot
yet affirm. It is a subject in which problems open up on every side,
and all of them are profitable subjects for investigation. Unhappily
such animals are difficult to rear successfully in captivity for many
generations, owing to their extreme liability to disease. Not the least
interesting feature of the melanics is the fact that the black varieties
provide about the best and clearest example of a new dominant factor
attaching itself to a wild species in recent times. None of the cases
are satisfactorily recorded or analysed as yet, but the evidence is
clear that _doubledayaria_ is a dominant to its type, and in several
other dark varieties, though the pigment deposited is not black, the
records show that the increased amount of the pigment almost certainly
is due to a positive factor. Of this, _Hemerophila abruptaria_ is a
good example.[24] There are some irregularities in the results, but
taken together they leave little doubt that the dark brown variety is a
dominant and the light, yellowish brown a recessive.
A curious parallel to the rise of the melanic moths in England
is provided by the case of the Honey-creepers or Sugar-birds, in
certain West Indian islands.[25] These birds of the genus _Coereba_
(_Certhiola_) range from Southern Mexico to the Northern parts of
South America and through the whole chain of the West Indian islands
and Bahamas except Cuba. There are numerous local forms, and many of
the islands have types peculiar to themselves, as is usual in such
cases. Some of the types or species range through several islands, but
according to Austin Clark[26] no island has more than one of them.
Cory[27] reckoned twelve such species within the Antillean region. They
are small birds about the size of a nuthatch with a general colouring of
black, yellow, and white. From the island of St. Vincent the Smithsonian
Institution received in the late seventies of last century several
completely black specimens in addition to two of the usual type of
colouring. The black were described by W. N. Lawrence as _atrata_, and
those marked with the usual yellow and white were called _saccharina_.
The collector (Mr. F. A. Ober) reported that the black form was common,
and that the _saccharina_ form was rarer. Lawrence remarks, "Had there
been only a single example (of the black form) I should have considered
it as probably a case of abnormal colouring, but it seems to be a
representative form of the genus in this island."[28] There is of course
no doubt of the correctness of the view taken by Austin Clark that
"_atrata_" is a black variety. The black bird is in every respect, other
than colour, identical with _saccharina_, and it is even possible to
detect a greenish colour in the areas which would normally be yellow,
showing plainly enough the yellow pigment obscured by the black.
We have next the interesting fact that like our melanic moths the dark
form is replacing the "type." At the time of Ober's visit the type was
already in a minority, but now it is nearly or perhaps actually extinct,
though the black form is one of the commonest birds on the island.
Austin Clark found no specimen when he collected there in 1903-4, though
formerly it was not uncommon in the vicinity of Kingston and in the
immediate windward district of St. Vincent.
The Grenadines are geographically just south of St. Vincent, though
separated by a deep channel. In these islands no black forms have
yet been taken, but Grenada, the next island to the south, has both
normals and blacks. There are trifling differences of size between the
Grenada birds and those from St. Vincent, the Grenada specimens being
slightly smaller and for this reason they have received distinct names,
the form marked with yellow and white being called _Godmani_ (Cory)
and the black, _Wellsi_ (Cory), but this merely introduces a useless
complication. There is evidence that in Grenada, as in St. Vincent,
the black is gradually ousting the original type, but the process has
not gone so far as in St. Vincent. Austin Clark very properly compares
this case of the Sugar-birds with that of _Papilio turnus_, which
as is well-known, has a black female in the southern parts of its
distribution, in addition to a female of the yellow type, but in the
Northern States the black female does not occur.
During the present year P. R. Lowe, who lately studied _Coerebas_ on
a large scale in the West Indies, has published an important paper on
the subject.[29] He calls attention to the fact that Cory recently
found a black form of _Coereba_ on Los Roques Islands, and he himself
discovered another on the Testigos Islands. Both localities are on the
coast of Venezuela, far from St. Vincent and Grenada. The whole problem
is thus further complicated by the fact that the black varieties have,
as we are almost driven to admit, arisen independently in remote places.
Improbable as this conclusion may be, it is still more difficult to
regard all the black forms as derived from one source. For first, they
present definite small differences from each other; and secondly we have
to remember a consideration of greater importance, that the very fact
that each island has its own type must be accepted as proving that the
localities are effectively isolated from each other, and that migration
must be a very rare event.
The rarity of such illustrative cases is, I believe, more apparent than
real. It is probably due to the extreme reluctance of systematists to
admit that such things can be, and of course to the almost complete
absence of knowledge as to the genetic behaviour of wild animals and
plants. Only in such examples as this of the _Coereba_, where colour
constitutes the sole difference, or that of the moths which have been
minutely studied by many collectors, does the significance of the facts
appear. The arrangement of catalogues and collections is such that much
practical difficulty of a quite unnecessary kind is introduced. For
example, in this very case of _Coereba_, I find the British Museum has
a fine series from Grenada including 3 normals and 11 black, and also
16 blacks from St. Vincent. If the black specimens from Grenada were
put with the normals which are almost certainly nothing but a recessive
form of the same bird, the variation would strike the eye on even a
superficial glance at the drawer. But following the notions so naively
expressed in the passage quoted above from W. N. Lawrence, the blacks
from Grenada are put apart together with the other blacks from St.
Vincent, though two of them were shot on the same date as one of the
normals.
FOOTNOTES:
[1] For the evidence see Tutt, J. W., _Trans. Ent. Soc._, 1898, p. 17.
Compare the remarkable case given by Gulick (_Evolution Racial and
Habitudinal_, p. 123) of the two races of _Cicada_, which are separated
by reason of their life-cycles, one having a period of 13, the other 17
years.
[2] For references see _Materials_, p. 396, and also G. Baur, _Amer.
Nat._, 1893, July, p. 677.
[3] Jenner Weir, _Entomologist_, 1880, XIII, p. 251.
[4] Jentink, _Notes Leyden Mus._, 1885, VII, p. 111. Specimens
illustrating this peculiarity are in the British Museum.
[5] _Proc. Zool. Soc._, 1895, p. 850. Plate. Many points beyond that
mentioned above are involved in this remarkable case. For example, not
only are there males like females, but a small proportion of females
resemble the ordinary male type. The stripes are not merely the spots
produced, for they occupy different anatomical positions. The spots
almost always go with a black ventral surface, but the striped forms
nearly always have that region testaceous. _Spartium retama_, the
food-plant, will not grow in England, but if it could be naturalised in
America the whole problem might be investigated there and results of
exceptional interest would almost certainly be attained.
[6] Doncaster, L., _Proc. Zool. Soc._, 1905, II, p. 528.
[7] I am not aware that the details of this striking case have ever
been worked out. It should be noted that the green and blue forms are
not due to simple modification of the red pigment; for these colours,
due to interference, fork over the area occupied by the red lines. The
distinctions between these forms cannot therefore be simply chemical,
as we may suppose them to be, for instance, in the case of many red
and yellow forms, and the genetic relationships of the _Heliconid_
varieties would raise many novel problems and be well worth studying
experimentally.
[8] Woodeforde, F. C., _Trans. North Staffordshire Field Club_, XXXV,
1901, Plate.
[9] E. Gallé, _Compte Rendus du Congres Internat. de Bot. a l'Expos.
Univ._, 1900, p. 112.
[10] Flora of Mentone, 1864-8, _Nova Acta Acad. Caes._, XXXV, 1869.
[11] I owe these facts to Canon A. M. Norman, who showed me illustrative
specimens. They were originally described by Bowerbank (_Monogr. Brit.
Spongiadae_, vol. II, pp. 18 and XX; vol. III, Pls. I and III). A
specimen of _G. compressa_ measured 5 inches, with a greatest width of
3-1/4 in. _G. ciliata_ was found measuring 3 in. long and 3/4 in. wide.
These dimensions are many times those of normal specimens.
[12] Coutagne, G., _Recherches sur le Polymorphisme des Mollusques de
France_, _Annales Soc. d'Agric. Sci. et Industr. Lyon_, 1895.
[13] As to the synonymy and references see Coutagne, p. 45.
[14] A. Lang, _Die Bastarde von H. hortensis Muller H. nemoralis L._
Jena, G. Fischer, 1908; with a fine coloured plate showing the varieties
of the species and their hybrids.
[15] With this evidence compare that given by A. Delcourt in his
valuable papers lately published relating to the variations of
_Notonecta_. See especially _Bull. Sci. Fr. Belg._, 1909, XLIII, p. 443;
and _C. R. Soc. Biol._, 1909, LXVI, p. 589.
[16] Allen, J. A., _Bull. Amer. Mus. N. H._, III, 1891, pp. 51-54.
[17] J. T. Gulick, _Evolution, Racial and Habitudinal_, Carnegie
Institution, Publication No. 25, 1905.
[18] A. G. Mayer, _Mem. Mus. Comp. Anat. Harvard_, Vol. XXVI, 1902, p.
117. From the tables given I cannot ascertain the actual numbers from
the two intermediate valleys, but they were considerable.
[19] To which I was very kindly guided by Mr. C. T. Trechmann.
[20] Standfuss, _Handbuch d. paläarkt Gross-schmet_, 1896, p. 321.
[21] _Ent. Rec._, XVIII, No. 7, 1906.
[22] This evidence was largely collected by Mr. G. T. Porritt, who has
given much attention to the subject.
[23] Such direct action has of course been proved to occur in the case
of several dimorphic larvae (_e. g._, _A. betularia_, itself) and pupae.
[24] See Harris, _Proc. Ent. Soc. London_, 1904, p. lxxii, and 1905, p.
lxiii; also Hamling, _Trans. City of London Ent. Soc._, 1905, p. 5.
[25] I am indebted to Mr. Outram Bangs of the Harvard Museum for calling
my attention to this remarkable case.
[26] _Auk_, 1889, VI, p. 219.
[27] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
[28] _Ann. N. Y. Acad. Sci._, 1878, I, p. 149.
[29] _Ibid_, 1912, pp. 523-8.
CHAPTER VII
LOCAL DIFFERENTIATION. _Continued_
OVERLAPPING FORMS
The facts of the distribution of local forms on the whole are consistent
with the view that these forms come into existence by the sporadic
appearance of varieties in a population, rather than by transformation
of the population as a whole. Of such sporadically occurring varieties
there are examples in great abundance, though by the nature of the
case it can be but rarely that we are able to produce evidence of a
previous type being actually superseded by the variety. When the two
forms are found co-existing in the same area they are usually recorded
as one species if intergrades are observed, and as two species if the
intergrades are absent. On the other hand when two forms are found
occupying separate areas, when, that is, the process of replacement is
completed in one of the areas, then forthwith each is named separately
either as species or subspecies. Successive observations carried out
through considerable periods of time would be necessary to establish
beyond question that the history proceeds in one way rather than
another. Such continuity of observation has for the most part never
been attempted. The kind of information wanted has indeed only been
lately recognized, and really critical collecting is a thing of only
the last few decades. The methods of the older collectors, who aimed at
bringing together a few typical specimens of all distinct forms, are of
little service in this class of inquiry, which is better promoted by the
indiscriminate collection of large numbers of common forms from many
localities. When this has been done on a comprehensive scale we shall be
in a position to form much more confident judgments as to the general
theory of evolution.
Some little work of the kind has however been done and the results are
already of great value. Seeing that the differentiation of local forms
is only made possible by isolation, it necessarily happens that the
collector finds one form in one locality and another in a distinct
locality, and there is no evidence as to the behaviour which the two
representative species might exhibit if they came into touch with each
other. In the most familiar examples of such distinction each inhabits
an island, completely occupying it to the exclusion of any other similar
form. It can only be when the two representative species occupy parts
of a continental area connected with each other by regions habitable
for the organism in question, that there is a chance of seeing the two
forms in contact. Often also, even where this condition is satisfied,
the habits, social organisation, or some other special cause may
act as a barrier which prevents the distinguishable forms from ever
coming into such complete contact as to interbreed or to behave as a
genetically continuous race. When genetic continuity is ensured by a
constant diffusion of the population over the whole area which they
inhabit there will manifestly be no formation of local races. The
practical uniformity, for example, of so many species of birds which
inhabit widely extended ranges of Western Europe is doubtless maintained
by such constant diffusion. When, as in the case of the Falcons, many
localities have peculiar forms, the fact may be taken as conclusive
evidence that there is little or no diffusion; and when we find in such
a species as the Goldfinch that in spite of migratory fluctuations there
are nevertheless geographical races fairly well differentiated, it may
similarly be inferred that these fluctuations habitually move up and
down on paths which do not intermingle. There are however a few examples
of animals, not given to much irregular wandering, which occupy a wide
and continuous range of diversified country and are differentiated as
local races in two or more districts, though the distinct races meet
in intervening areas. Of these the most notorious illustration which
has been investigated with any thoroughness is that of the species of
_Colaptes_ (Woodpeckers) known in the United States as Flickers. The
study of the variations of these forms, made by J. A. Allen[1] is an
admirable piece of work, with which every student of variation and
evolutionary problems should make himself familiar. The two forms with
which we are most concerned are known as _C. auratus_ and _C. cafer_,
and are very strikingly different in appearance. In size, proportions,
general pattern of colouration, habits, and notes, the two are alike,
but they differ in the following seven respects as stated by Allen.
_Auratus_ _Cafer_
1. Quills _yellow_. 1. Quills _red_.
2. Male with a _black_ malar 2. Male with a _red_
stripe. malar stripe.
3. Adult female with _no_ 3. Adult female with usually a
malar stripe. brown malar stripe.
4. _A scarlet nuchal crescent 4. No nuchal crescent in
in both sexes._ either sex.
5. Throat and fore neck 5. Throat and fore neck
_brown_. _grey_.
6. Whole top of head and hind neck 6. Whole top of neck and hind
_grey_. neck _brown_.
7. General plumage with an 7. General plumage with a
_olivaceous_ cast. _rufescent_ cast.
These differences are illustrated in the accompanying coloured plate,
which has been most kindly prepared for me under the instructions of
Dr. F. M. Chapman of the American Museum of Natural History. Before
going further it is worth considering the nature of these differences
a little more closely. All but the last are large differences which no
one would overlook even in a hasty glance at the birds. If the only
distinction lay in the colour of the quills we might feel fairly sure
that _auratus_ was a recessive form of _cafer_, and so probably it is in
this respect. Similarly the black malar stripe of _auratus_ is in all
probability recessive to the red malar stripe of _cafer_ and I imagine
the pigments concerned are comparable with those in the Gouldian Finch
(_Poephila gouldiae_) of Australia. Both sexes in that species may have
the head black, red, or, less often, yellow, and though it is not any
longer in question that birds may breed in either plumage, I believe
that the young are always black-headed and I imagine that those which
become red-headed possess a dominant factor absent from the permanently
black-headed birds.[2] Yellow as a recessive form of a red is certainly
very common, but red and black as variants of the same pigment are less
usual. In the Gouldian Finch we seem to have a case where a pigment can
assume all three forms. It would be interesting to know whether the red
of the malar stripes in _Colaptes_ is a pigment of the same nature as
the red of the quills. Both in _Colaptes_ and in _Poephila gouldiae_ I
have seen specimens intermediate between the black and the red, and the
appearance of the part affected was exactly alike in the two cases, red
feathers coming up among the black ones, and many feathers containing
both red and black pigments mixed together. The development of the
scarlet nuchal crescent in _auratus_ and the absence of this conspicuous
mark in _cafer_ constitute from the physiological point of view the most
remarkable pair of differences. When the red crescent is not formed, the
feathers which would bear it are exactly like the rest, and no special
pigment is visible in them which one can regard as ready to be modified
into red. If the crescent is due to a factor it must therefore be
supposed that this factor has the power of modifying the pigment of the
neck in one special place alone. Dr. W. D. Miller called my attention to
the fact that a similar variation occurs in another American woodpecker,
the Sapsucker, _Sphyropicus varius_.[3]
I do not suggest that such variations are without parallel: indeed in
_P. gouldiae_ the factor which turns the black of the head into scarlet
affects one special region of the black only, being sharply distinct
from the unmodified black of the throat. These regions of the head are
however often the seat of special colours in birds.[4] So also may be
instanced the variety of the Common Guillemot (_Uria troile_) which
has a white line round the eyes and at the sides of the head where the
normal has no such mark; but this line is formed in a very special
place, the groove joining the eye to the ear, whereas the feathers
of the nuchal crescent are not ostensibly distinguished from those
adjacent.[5]
The transposition of the brown and the grey on the back and front of the
neck also constitutes a very remarkable difference. If either grey or
brown depends on a factor then it must be supposed that _auratus_ has
one of these factors and _cafer_ the other.
From these several considerations it is quite clear that if _auratus_
and _cafer_ are modifications of the same type produced by presence or
absence of factors, several independent elements must be concerned, and
to unravel their inter-relations would be most difficult even if it were
possible to breed the types under observation, which is of course quite
beyond present possibilities.
The distribution of the two is as follows. On the east side of the
Continent _C. auratus_, relatively pure, occupies the whole of Canada
and the States from the North to Galveston. Westward it extends across
the whole continent in the more northern region to Alaska, but in its
pure form it only reaches down the Pacific coast to about the northern
border of British Columbia. Its southern and western limit is thus
roughly a line drawn from north of Vancouver, southeast to North Dakota
and then south to Galveston. _C. cafer_ in the comparatively pure
form inhabits Mexico, Arizona, California (except Lower California
and the opposite coast), central and western Nevada, Utah, Oregon,
and is bounded on the east by a line drawn from the Pacific south of
Washington, south and eastward through Colorado to the mouth of the
Rio Grande or the Gulf of Mexico. Between the two lines thus roughly
defined is a band of country about 1,200-1,300 miles long and 300-400
miles wide, which contains some normal birds of each type, but chiefly
birds exhibiting the characters of both, mixed together in various and
irregular ways. Even in the areas occupied by the pure forms occasional
birds are recorded with more or less indication of characteristics
of the other form, but within the area in which the two forms are
conterminous, the mixed birds are in the majority. The condition of
these birds of mixed character is described by Allen as follows:
"As has been long known--indeed, as shown by Baird
in 1858--the 'intermediates' or 'hybrids' present ever-varying
combinations of the characters of the two birds, from individuals
of _C. auratus_ presenting only the slightest traces of the
characters of _C. cafer_, or, conversely--individuals of _C.
cafer_ presenting only the slightest traces of the characters
of _C. auratus_--to birds in which the characters of the two
are about equally blended. Thus we may have _C. auratus_ with
merely a few red feathers in the black malar stripe, or with the
quills merely slightly flushed with orange, or _C. cafer_ with
either merely a few black feathers in the red malar stripe, or
a few red feathers at the sides of the nape, or an incipient,
barely traceable scarlet nuchal crescent. Where the blending
of the characters is more strongly marked, the quills may be
orange-yellow or orange-red, or of any shade between yellow and
red, with the other features of the two birds about equally
blended. But such examples are exceptional, an unsymmetrical
blending being the rule, the two sides of the same bird being
often unlike. The quills of the tail, for example, may be part
red and part yellow, the number of yellow or red feathers varying
in different individuals, and very often in the opposite sides of
the tail in the same bird. The same irregularity occurs also, but
apparently less frequently, in the quills of the wings. In such
cases the quills may be mostly yellow with a few red or orange
quills intermixed, or red with a similar mixture of yellow. A
bird may have the general colouration of true _cafer_ combined
with a well-developed nuchal crescent, or nearly pure _auratus_
with the red malar stripes of a _cafer_. Sometimes the body
plumage is that of _C. auratus_ with the head nearly as in pure
_cafer_, or exactly the reverse may occur. Or we may have the
general plumage as in _cafer_ with the throat and crown as in
_auratus_, and the malar stripe either red or black, or mixed
red and black, and so on in almost endless variations, it being
rare to find, even in birds of the same nest, two individuals
alike in all their features of colouration. Usually the first
trace of _cafer_ seen in _auratus_ manifests itself as a mixture
of red in the black malar stripe, either as a few red feathers,
or as a tipping of the black feathers with red, or with merely
the basal portion of the feathers red. Sometimes, however, there
is a mixture of orange or reddish quills, while the malar stripe
remains normal. In _C. cafer_ the traces of _auratus_ are usually
shown by a tendency to an incipient nuchal crescent, represented
often by merely a few red-tipped feathers on the sides of the
nape; at other times by a slight mixture of black in the red
malar stripe."
Such a state of things accords very imperfectly with expectations under
any received theory of Evolution. As in some of the instances discussed
in the first chapter we have here two fairly definite forms, nearly
allied, which on any evolutionary hypothesis must have been evolved
either the one from the other, or both from a third form at a time not
very remote from the present, as time must be measured in evolution. Yet
though intermediates exist in some quantity, no one can for a moment
suggest that they are that definite intermediate from which _auratus_
and _cafer_ descend in common. One cannot imagine that the immediate
ancestor of these birds was a mosaic, made up of asymmetrical patches
of each sort: but that is what many of the intermediates are. It is not
much easier to suppose the ancestor to have been a nondescript, with a
compromise between the developed characters of each, with quills buff,
malar stripes neither black nor red, with a trace of nuchal crescent,
and so on. Such Frankenstein-monsters have played, a considerable part
in the imaginations of evolutionary philosophers, but if it were true
that there was once a population of these monsters capable of successful
existence, surely they should now be found as a population occupying the
neutral zone between the two modern forms. Yet, though much remains to
be done in clearing up the facts, one thing is certain, namely that the
neutral zone has not a definite and normally intermediate population,
but on the contrary it is peopled by fragments of the two definite types
and miscellaneous mongrels between them.
On the other hand, one cannot readily suppose that either form was
the parent of the other. The process must have involved both addition
and loss of factors, for whatever hypothesis be adopted, such changes
must be supposed to have occurred. A careful statistical tabulation of
the way in which the characters are distributed in the population of
the mixed zone would be of great value, and till that has been done
there is little that can be said with certainty as to the genetics of
these characters. In the collection of Dr. Bishop of New Haven I was
very kindly allowed to examine a sample, all taken at random, near
together, in Saskatchewan. There were females 4 adult, 2 young; males
4 adult and 5 young. This number, though of course insufficient, is
enough to give some guide as to the degree of definiteness which the
characters generally show in their variations. Of the 15 birds, 8 had
simply yellow quills; 2 had red; 1 was almost red but had one yellow
tail-quill; 3 were intermediate and 1 was buff. As regards the malar
patch, which can only be determined properly in the adult males, 1 was
red, 1 was approximately red, 2 intermediate. As to nuchal crescent 4
females had none, 2 females very slight; 7 males had it, 1 had only a
slight crescent, and 1 had none. In point of quills therefore 10 were
definite out of 15; in point of crescent, 11 were definite out of 15;
and in point of malar patch 1 only was definite out of 4. The last is a
feature directly dependent on age and so counts for less, but as regards
the other two features there is some indication that the factors show
definiteness in their behaviour. It must be remembered that we have no
knowledge what the heterozygous form may be, and in the case of red
and yellow it is probably a reddish buff. The patch-works are no doubt
to be compared with other well-known pied forms, and in these we must
suppose the active factor broken up, which it probably can be very
easily. The asymmetry, which Allen notices as so marked a feature, in
the distribution of the red and yellow quills of the tail especially,
recalls that of the black markings in the pied Canaries. As is well
known to students of variations _some_ pigment-factors in _some_ animals
are apparently uncontrolled by symmetry, while in other specific cases
symmetry is the rule. On the other hand the blackness or redness of
the malar patches is, I think, as a rule nearly symmetrical. It should
be mentioned that two of Dr. Bishop's young birds belonged to the same
nest, one a female with _red_ quills, the other a male with _yellow_.
Both are without crescent.
As to the question whether certain combinations of characters occur
with special frequency, the evidence is insufficient to give a definite
answer. Among all the birds I have seen in America or in England I
have not yet found one having the malar patches black without any
nuchal crescent. Of Dr. Bishop's 8 adults not one, however, showed the
combination of the three chief features normal for _auratus_ or for
_cafer_.
Besides the two forms that we have hitherto considered, several other
local types exist, and these throw some further light on the problem.
Of these the most important in this connexion is _chrysoides_, which
inhabits the whole of southern California and the mainland opposite.
This remarkable form is as Allen says, very different from _auratus_
except that it has the quills yellow like _auratus_, not red like
_cafer_. So that we find here in the extreme west of the whole
distribution a type agreeing in one of its chief features with the
eastern type. Between this and _cafer_ intergrades have, according to
Allen, not been found. The relations of this _chrysoides_ are, Allen
thinks, rather with _mexicanoides_, a southern, smaller race with
colours more intense, which inhabits Guatemala, but however that may
be, it must be regarded as a _cafer_ which has lost its red quills. The
island of Guadeloupe off Lower California has an island form. Beyond the
other side of the continent there is also an island form of _auratus_,
inhabiting Cuba, so that clearly the yellow quills can extend into the
tropics.
The above account is in many respects incomplete, but it suffices to
give an outline of the chief facts. The whole problem is complicated by
the undoubted effects of an uncertain amount of migration, and in many,
perhaps all, districts, the winter population differs from the summer
population of the same localities. The existence of these seasonal ebbs
and flows is now well known to ornithologists, and most of the bird
species of temperate regions are subject to them.
Difficult as it may be to conceive the actual process of origin of the
two types _auratus_ and _cafer_, it is I think still harder to suggest
any possible circumstance which can have determined their development as
distinct races, or which can maintain that distinctness when created.
Some will no doubt be disposed to appeal once more to our ignorance
and suggest that if we only knew more we should see that the yellow
quills, the black "moustache" and the red crescent, specially qualify
_auratus_ for the north and eastern region, and the red quills, red
"moustache" and absence of crescent fit _cafer_ to the conditions of its
homes. Each can judge for himself, but my own view is that this is a
vain delusion, and that to cherish it merely blunts the receptivity of
the mind, which if unoccupied with such fancies would be more ready to
perceive the truth when at last it shall appear. Think of the range of
conditions prevailing in the country occupied by _auratus_--a triangle
with its apex in Florida and its base the whole Arctic region of North
America. Is it seriously suggested that there is some element common to
the "conditions" of such an area which demands a nuchal crescent in the
Flickers, though the birds of the _cafer_ area, almost equally varied,
can dispense with the same character? Curiously enough, the geographical
variation of _Sphyropicus varius_, another though a very different
Woodpecker[6] shows that conversely the nuchal crescent can be dispensed
with in the Eastern form though it is assumed by the Western.[7]
Allen points out the interesting additional fact that superposed
upon each of the two distinct forms, _auratus_ and _cafer_, are
many geographical variations which can very naturally be regarded
as climatic. Each decreases in size from the North southward, as so
many species do.[8] They become paler in the arid plains, and show
the ordinary phases which are seen in other birds having the same
distribution. Such differences we may well suppose to be determined
directly or indirectly, by environment, and we may anticipate with
fuller knowledge it will be possible to distinguish variations of this
nature as in the broad sense environmental, from the larger differences
separating the two main types of _Colaptes_, which I surmise are
altogether independent of such influences.
It is generally supposed that phenomena like those now so well
established in the case of _Colaptes_ are very exceptional, and as has
already been stated a number of circumstances must combine in order
that they may be produced. I suspect however that the examples are
more numerous than is commonly thought. In all likelihood the three
forms _Sphyropicus varius_, _nuchalis_ and _ruber_ are in a very
similar condition though the details have not, so far as I know, been
worked out. A complex example which is closely parallel to the case of
_Colaptes_ was described by F. M. Chapman[9] at the same date as Allen's
work. This is the case of _Quiscalus_, the Grackles, which in the North
American Continent have three fairly distinct forms which Chapman speaks
of as _Q. aeneus_, _Q. quiscula_, and _Q. quiscula aglaeus_. The birds
are all, so far as pigment is concerned, dark blackish brown, but the
head and mantle have superposed a metallic sheen of interference-colours
which in the various forms take different tints, bluish green, bronze
green, or bronze purple. The details are complicated and difficult
to appreciate without actual specimens, but the two common types are
sufficiently distinct. The birds inhabit the whole area east of the
Rockies, _quiscula aglaeus_ occupying Florida and the Southern States
southwest of a band of country about a hundred miles broad extending
roughly from Connecticut to the mouth of the Mississippi; and _aeneus_
taking the area north and west of this band. In discussing this case
Chapman expresses the same view as Allen does in the _Colaptes_ case,
that there are two distinct populations, substantially fixed, and
that the band of country in which they meet each other has a mongrel
population, with no consistent type, but showing miscellaneous
combinations of the character of the two chief types.
The warblers of the genus _Helminthophila_ provide another illustration
which has points of special interest. The two chief species are _H.
pinus_, which has a yellow mantle and lower parts, white bars on the
wings, a black patch behind the eyes and a broad black mark on the
throat; and _H. chrysoptera_ with dark grey mantle and pale whitish grey
lower parts, yellow bars on the wings, and grey marks on cheeks and
throat where _pinus_ has black. These two birds are exceeding distinct,
and in addition their songs are quite unlike. _H. pinus_ ranges through
the eastern United States up to Connecticut and Iowa. _H. chrysoptera_
is a northern form extending down to Connecticut and New Jersey. Both
are migrants.
In these two States, where the two types overlap, certain forms have
been repeatedly found which have been described as two distinct species,
_Lawrencei_ and _leucobronchialis_. Dr. L. B. Bishop and Mr. Brewster
showed me two long series of _Helminthophila_ containing various
intergrades between the four named kinds, and details regarding these
may be found in Chapman's _North American Warblers_ and in Dr. Bishop's
paper in Auk, 1905, XXII. Though the characters evidently break up to
some extent, the series can be represented as due to recombinations of
definite factors more easily than the others which I have described. The
differentiating characters are:
_Pinus_
1. Mantle and lower parts _yellow_ (Y^1).
2. Wing-bars _white_ (y^2).
3. Cheek and throat _not black_ (b).
_Chrysoptera_
1. Mantle and lower parts _grey_ (y^1).
2. Wing-bars _yellow_ (Y^2).
3. Cheek and throat _black_ (B).
The grey pigment of the mantle is common to both, but is masked by the
yellow in _pinus_, the net result being an olive-green.[10]
I am much indebted to Dr. F. M. Chapman for the loan of the coloured
plate in which these distinctions are shown. It first appeared in his
book, _North American Warblers_.
We cannot tell whether _yellow_ or _not-yellow_ is due to the presence
of a factor, but we may suppose that one or other gives the special
colour to the parts. The black of character 3 is no doubt a dominant.
Thus _pinus_ becomes Y^{1}y^{2}b and _chrysoptera_ in y^{1}Y^{2}B. The
_Lawrencei_ which has the underparts _yellow_, wing-bars _white_, and
_black_ patches is Y^{1}y^{2}B and _leucobronchialis_ which has mantle
and underparts _not-yellow_, wing-bars _yellow_ and _no black patches_
is y^{1}Y^{2}b. This representation, it should be clearly understood,
is tentative and approximate only. The characters are not really sharp,
for there is much grading; but allowing for the effects of heterozygosis
and for some actual breaking-up of factors I believe it gives a fairly
correct view of the case. In particular we can see how it meets the
difficulty which Chapman felt in accepting _leucobronchialis_ as in any
sense derived from _pinus_ which has a yellow breast, and _chrysoptera_
which has a black throat, seeing that _leucobronchialis_ has neither.
We now recognize at once that this form could be produced by ordinary
re-combination of the absence of Y^{1} with the absence of B.
I note also with great interest that the modern observers agree that the
so-called hybrids may have the song either of the one species, or of the
other, or a song intermediate between the two. It may also be added that
these two types have several times been seen, in the breeding season,
paired with each other or with one of the other combinations.
[Illustration: FIG. 1. _Helminthophila pinus_, male.
FIG. 2. _Helminthophila pinus_, female.
FIG. 3. "Lawrence's Warbler," male; one of the integrading forms.
FIG. 4. "Brewster's Warbler," male; another of the integrading forms.
FIG. 5. _Helminthophila chrysoptera_, male.
FIG. 6. _Helminthophila chrysoptera_, female.]
Allen[11] has described another excellent American example, the Tits of
the group _Baeolophus bicolor-atricristatus_. The form _bicolor_ belongs
to the eastern States and ranges from the Atlantic coast to the Great
Plains, and _atricristatus_, of east Mexico, extends from Vera Cruz to
central Texas. In southern and central Texas the breeding ranges adjoin,
and in this country various intermediates occur. The chief types differ
in two main points.
_B. bicolor_
Forehead varies from deep _black_
to dull black, suffused with
rusty brown.
Crown and crest _grey_,
slightly darker than the back.
_B. atricristatus_
Forehead _white_ to buffish white.
Crown and crest _black_, abruptly
contrasting with the back.
The intergrades between the two have, as usual, received specific names.
A detailed description is given by Allen, from which it appears that
the gradation is very complete. In one case a series of 16 adults were
all intermediates. It is not stated whether the collector took these at
random, but from the local lists it is clear that the types are found
not far away from the place where the intergrades were shot.
Another very striking case is that of the Tanagers, of the genus
_Rhamphocoelus_. In this group there are several local forms which
are related to each other in remarkable ways. The forms known as
_passerinii_ and _icteronotus_ exhibit the clearest phenomena of
intergradation. The species _passerinii_ has a brilliant scarlet
and black male, and it inhabits Honduras and Nicaragua. Proceeding
southwards along the isthmus we find next _costaricensis_ which has
a male like that of _passerinii_ (but a female with more orange than
the olive-grey female of _passerinii_). Next we come to Panama which
is occupied by _icteronotus_, sharply distinguished from _passerinii_
by the fact that the _scarlet is replaced by lemon-yellow_. This same
_icteronotus_ occurs again as a pure type in Ecuador and many other
parts of South America; but Colombia, _between Panama and Ecuador_,
contains scarlets like _passerinii_, yellows like _icteronotus_, and
various intergrades of several shades of orange. The _passerinii_ males
from Nicaragua are indistinguishable from those of Colombia, and the
_icteronotus_ of Ecuador are the same as those in Panama. The orange
intergrades, doubtless heterozygous forms, though collected at the
same locality (Medellin in Colombia) as several pure yellows and pure
scarlets, are in the British Museum series sorted out as a separate
species under the name _chrysonotus_! Complications are introduced by
the relations of these forms to another named type, _flammigerus_, but
we may for our purpose leave that out of consideration, and say that the
order of geographical sequence from Honduras to Ecuador is (1) scarlet,
(2) yellow, (3) mixture of types, scarlet, yellow, orange, (4)yellow.
Similar examples exist in the birds of the old world, but I do not know
of any that have been studied so fully as those of America. The best
known is that of the two Rollers, _Coracias indicus_ which spreads from
Asia Minor through Persia, Baluchistan, the Indian Peninsula and Ceylon,
and _affinis_ which ranges from Nepal, through Assam, Tenasserim and
the Indo-Chinese countries. The two types are very different and may be
distinguished as follows:
_C. indicus_
_Mantle_ drab brown-chestnut.
_Breast_ chestnut.
_Throat_ purplish, streaked with white.
_Upper tail-coverts_ indigo.
_C. affinis_
Dark olive-green.
Dull purple brown.
Purple, streaked with blue.
Turquoise.
The wings are the same in both. In the provinces of Nepal, Sikhim, and
Darjiling the two species coexist, with the result that intergrades
have been frequently recorded. The line of intergradation extends to
the coast, and birds showing various combinations of the two types from
the Calcutta district exist in collections.[12] The case is interesting
inasmuch as like that of _Quiscalus_ it shows a series of combinations
of various metallic colours. Some of these are probably evoked by
the development of pigment behind striations or other interferences
already existing, but in the present state of knowledge it would be
quite impossible to suggest what the actual factors producing these
appearances may be.
There are, naturally, many other cases among birds which are suspected
of being in reality comparable, but in most of them the evidence is
still inadequate. Among Lepidoptera also there are a few of these;
perhaps the most striking is that of _Basilarchia "proserpina."_[13] The
genus is well known to European collectors under the name _Limenitis_,
of which we in England have one species, _L. sibylla_, the "White
Admiral." A species very like _sibylla_ in general appearance is common
in the northern parts of the United States, ranging through Canada and
Northern New England, but rarely south of Boston. This species has the
conspicuous white bands across both wings like our _sibylla_.
There is also a more Southern type known as _astyanax_, which is very
different in its appearance, being without the white bands and having
a broad irroration of blue scales on the posterior border of the hind
wings. The two are so distinct that one would not be tempted to suspect
any very close relation between them. In its distribution _astyanax_ is
described by Field as replacing arthemis south of latitude 42°. About
Boston it is much more common than _arthemis_.
The two forms encroach but little on each other's territory, but where
they do coexist, a third form, known as _proserpina_, is found which is
almost intermediate, with the white bands much reduced. There is now
no doubt that this _proserpina_ is a heterozygous form, resulting from
a combination of the characters of _arthemis_ and _astyanax_. Field
succeeded in rearing a brood of 16 from a _proserpina_ mother caught
wild which laid 31 eggs, and of these, nine (five males, four females)
resembled the mother, being _proserpina_, and seven (four males, three
females) were _arthemis_. There can be no question therefore that the
mother had been fertilised by a male _arthemis_ and that _no-white-band_
is a factor partially dominant over the _white band_. Another point of
interest which Field observed was that the _proserpina_ female refused
to lay on birch, poplar or willow, but accepted wild cherry (_Prunus
serotina_) a species on which _astyanax_ can live, though that tree is
not known to be eaten by _arthemis_. Incidentally also the observations
show that sterility cannot be supposed to be the bar which maintains the
distinctness of _arthemis_ and _astyanax_.
In this connection _Papilio oregonia_ and _bairdii_ should be
mentioned.[14] _P. oregonia_ is one of the numerous forms like
_machaon_, but rather paler. It is a northern insect, inhabiting British
Colombia east of the Cascade Range, and reaching to Colorado. _P.
bairdii_ is a much darker butterfly, representing the _asterias_ group
of the genus _Papilio_. Like _asterias_ it has the abdomen spotted at
the sides, not banded as in the _machaon_ group. It belongs to Arizona
and Utah extending into Colorado. From Colorado the form _brucei_ is
described, more or less intermediate, like _bairdii_ but with the
abdomen banded as in _oregonia_. W. H. Edwards records the results of
rearing the offspring of the _bairdii_-like and of the _oregonia_-like
mothers. Each was found able to have offspring of both kinds, that is to
say, _bairdii_ females gave both forms, and _oregonia_ females gave both
forms. It is not possible to say which is dominant, since the fathers
were unknown. On general grounds one may expect that the _bairdii_ form
will be found to dominate, but this is quite doubtful.
From this particular discussion I omit reference to those examples in
which the permanently established types are obviously associated with
special conditions of life. Where considerable climatic differences
exist between localities, or when we pass from South to North, or from
the plains into Alpine levels we often find that in correspondence
with the change of climate there is a change in the characteristics of
a species common to both. When I say "species" in such a connection I
am obviously using the term in the inclusive sense. Some would prefer
to say that in the two sets of conditions two _representative species_
exist. Whichever expression be preferred it is plain that such examples
present another phase of the problem we have been just considering, and
in them also we have an opportunity of observing the consequences of
the overlap of two closely related types, but there are advantages in
considering them separately. In the examples hitherto given, with the
possible exception of the Papilios,[15] the two fixed types severally
range over so extensive a region that it may fairly be supposed that
in the different parts they are subject to considerable diversities of
climate. There is no outstanding difference that we know distinguishing
the habitats of the two forms; but in comparing Alpine with Lowland
forms, or essentially northern with essentially southern forms we do
know an external circumstance, temperature, that may reasonably be
supposed to have an influence, direct or indirect, on the population.
FOOTNOTES:
[1] J. A. Allen, _The North American Species of the Genus Colaptes,
Considered with Special Reference to the Relationships of C. auratus and
C. cafer_. Bull. Am. Mus. Nat. Hist., IV, 1892.
[2] For a case in which a red-headed female × a black-headed male gave a
black-headed female and a red-headed male, see _Avian Mag._, N. S., IV,
pp. 49 and 329
[3] The other variations of this bird are also interesting and
important. The normal male has a red head and a red throat. The female
has a red head and a white throat, but varieties of the female are known
with a black head, thus again illustrating the change from black to
red. It should be noted that this is not a mere retention of a juvenile
character, but, as the birds mature, the red feathers come up, or as an
exception, the black. There is also a western species, _ruber_, in which
both sexes have a great extension of red, and are alike. The male of
_nuchalis_ intergrades with this type, but the female does not.
[4] Dr. W. Brewster, for example, has a remarkable specimen of the Teal
(_Nettion carolinense_) with a white collar strongly developed at the
front and sides of the neck, in a place where the normal has no such
mark.
[5] This variety is spoken of as the Ringed Guillemot and is sometimes
regarded as a distinct species to which the name _ringvia_ was given by
Brünnich. In support of this view Dr. William Brewster, to whom I am
indebted for much assistance in regard to the variation of birds, called
my attention to observations of his own and also of Maynard's, that the
ringed birds were sometimes mated together, though in a small minority
(see Brewster, _Proc. Boston Soc. N. H._, XXII, 1883, p. 410). It
would however be possible to produce many instances of varieties mated
together though surrounded by a typical population (_e. g._, two varying
Blackbirds, _Zoologist_, p. 2765; two varying Nightjars, _ibid._, p.
5278). I am inclined to believe that in nature matings between brothers
and sisters are frequent in many species of animals, and that the
production of sporadically varying colonies is thus greatly assisted.
[6] The Sap-suckers feed on trees and somewhat resemble our Spotted
Woodpeckers in general appearance. _Colaptes_ feeds on the ground and
corresponds perhaps rather with the European Green Woodpecker.
[7] For an introduction to this example I am indebted to Mr. W. D.
Miller of the American Museum of Natural History. Some account of the
facts is given by Baird, Brewer, and Ridgway (_A Hist. of N. Amer.
Birds_. 1874, II, pp. 540, 544, etc.). _S. varius_ occupies the whole
country in suitable places from the Atlantic to the eastern slopes of
the Rockies, and all Mexico to Guatemala. _S. nuchalis_ was first known
from the Southern Rockies only, but many were afterwards taken in Utah.
_S. ruber_ is restricted to the Pacific coast. In Ridgway's opinion all
three are geographical forms of one species. In _ruber_ the sexes are
alike having both a great extension of the red in the throat, and a red
crescent. The male of _nuchalis_ grades to the _ruber_ form, but the
female does not. This female has some red in the throat like the male of
_varius_, whereas the female of _varius_ has a whitish throat.
[8] Not only vertebrates but the marine Crustacea and Mollusca
illustrate this curious "principle" of variation, as Canon Norman
formerly pointed out to me with abundant illustrations. There are of
course cases to the contrary also.
[9] Chapman, F. M., _Bull. Amer. Mus._, IV, 1892, p. 1; see also
Ridgway, _Birds of North and Middle America_, 1902, Part II, p. 214.
[10] It would aid greatly in factorial analysis if the descriptive term
"green" could be avoided in application to cases where the green effect
is due only to a mixture of black and yellow pigments. The absence of
yellow is the sole difference between the mantle and underparts of
_pinus_ and _chrysoptera_.
[11] _Bull. Amer. Mus. Nat. Hist._, XXIII, 1907, p. 467.
[12] References on this subject will be found in _Brit. Mus. Cat.
Birds_, XVII, p. 13.
[13] For these facts I am indebted to Mr. W. L. W. Field, who has
lately published an account of his observations and experiments. See
especially, _Psyche_, 1910, XVII, No. 3, where full references to
previous publications are given.
[14] For the facts and further references see W. H. Edwards,
_Butterflies of N. America_, 2d series, Papilio VII and X; 3d series,
1897, Papilio IV, _Can. Entom._, 1895, XXVII, p. 239.
[15] I think this case is fairly included because the _machaon_ type
is so widespread that it cannot be regarded as a product of a Northern
climate, nor can _asterias_ be claimed as especially a warm country
form, seeing that _brevicauda_, which is scarcely distinguishable from
_asterias_, inhabits Newfoundland (having a curious phase there in which
the yellow is largely replaced by red).
CHAPTER VIII
LOCALLY DIFFERENTIATED FORMS. _Continued._
CLIMATIC VARIETIES
In this chapter we will examine certain cases which illustrate phenomena
comparable with those just considered, though as I have already
indicated, they form to some extent a special group. The outstanding
fact that emerges prominently from the study of the local forms is that
when two definite types, nearly allied, and capable of interbreeding
with production of fertile offspring, meet together in the region
where their distributions overlap, though intergrades are habitually
found, there is no normally or uniformly intermediate population
occupying the area of intergradation. Such phenomena as these must, I
think, be admitted to have great weight in any attempt to construct a
theory of evolution. True we must hesitate in asserting their positive
significance, but I see no escape from the conclusion that they throw
grave doubt on conventional views. Again and again the same question
presents itself. If _A_ and _B_ lately emerged from a common form why is
that common form so utterly lost that it does not even maintain itself
in the region of overlapping? Almost equally difficult is it, in the
cases which I have numerated, to apply concrete suggestions based on
any factorial scheme. We may see that in _Heliconius erato_ the type
with the red mark on the hind wing probably contains a dominant factor,
and that where the red mark is absent the metallic colours are exposed;
and that similarly the green metallic colour may have another factor
which distinguishes it from the blue. In this way we can fairly easily
represent the various types of _erato_ on a factorial system as the
result of the various possible combinations of two pairs of factors. But
there we stop, and we are quite unable to suggest any reason why one
area should have the red and the green type while another should have
the blue also. So again with _Colaptes_ or the Warblers. By application
of a factorial system, admittedly in a somewhat lax fashion, the
genetic interrelations of the types can be represented; but how it comes
about that each type maintains a high degree of integrity in its own
region we can only imagine. Each has in actual fact a stability which
the intermediate forms have not, but we cannot yet analyse the nature
of that stability. Mendelian conceptions show us how by segregation the
integrity of the factors can be in some degree maintained, but not why
certain combinations of factors should be exceptionally stable. All that
is left us to fall back on is the old unsatisfying suggestions that some
combinations _may_ have greater viability than others, that there _may_
be a tendency for like to mate with like, and so forth.
These difficulties acquire more than ordinary force in those cases in
which the two fixed types inhabit regions differing in some respect
so obvious and definite that we are compelled to regard each type as
climatic and as specially adapted to the conditions. When for example
an animal has a distinct type never met with except in Arctic or Alpine
conditions, and another type proper to the plains and temperate regions,
what are the characteristics of the population of intermediate latitudes
or at intermediate levels? Some of the examples discussed in the last
chapter may be instances of this very nature, but even if they are not,
others are forthcoming which certainly are. The evidence of these cases
leads to the suspicion that with further knowledge they will be found
to consist of two classes, some in which the observer as he passes from
the one climate to the other will find the intermediate area actually
occupied by a population of intermediate character, and others in which,
though we may presume the maintenance of intermediate conditions in
the transitional area, there is no definite transitional population.
This interrupted or discontinuous distribution seems, so far as I have
means of judging, to be by far the more common of the two. I do not
doubt that by sufficient search individuals representing every or almost
every transitional form can be found, but it is apparently rare that
_populations_ corresponding to these several grades can be seen. The
question has in few if any cases been studied with precision sufficient
to provide a positive answer; but I suspect that real and complete
continuity, in the sense thus defined, will only be found where the
character of the local populations depends _directly_ on the conditions
of life, and shows an immediate response to changes in them apart from
that postponed response which we suppose to be achieved by selection.
Obviously the character must be one, like size for instance, capable of
sensibly complete gradation.
The only example I have met with of the phenomenon of anything like a
complete intergradation between local types really distinct in kind is
that provided by the butterfly _Pararge egeria_. It is well known to
entomologists that this insect exists in two very different types, a
northern one, the "Speckled Wood" of England, in which the spots are
a pale whitish yellow, and a southern type having the full fulvous
colour that we know as characteristic of _megaera_, the "Gatekeeper."
It appears that Linnaeus gave the name _egeria_ to the southern
type,[1] and our own is now called _egerides_. Broadly speaking, so
far as Great Britain, France, and the Spanish Peninsula are concerned,
the tawny-coloured _egeria_ occupies Spain and western France up to
the latitude of Poitiers and the pale yellow _egerides_ extends from
Scotland, where it has a scanty distribution, through southern England,
where in suitable localities it is common, and the north of France
to Paris.[2] The two types when placed side by side are strikingly
different from each other, and are an excellent illustration of what is
meant by climatic variation. The insect is not a great traveller and
probably scarcely ever wanders far from its home. It should therefore
be possible by collecting from north to south to find out how the
transition is effected, whether suddenly or gradually. This at various
times I have endeavoured to do, but I am still without exact information
as to the population in certain critical areas. In addition to the
information derived from specimens which I have collected or seen
in the collections of others there is a good account of the general
distribution in Europe given by the Speyers,[3] who evidently paid
more attention to the subject than most lepidopterists have done, and
many more recent records. In particular Oberthür[4] has published many
details as to the distribution in western France and I am especially
indebted to Mr. H. Rowland-Brown for a long series of notes as to the
distribution in France generally, and to Mr. H. E. Page and Dr. T. A.
Chapman, Mr. Oberthür Prof. Arrigoni degli Oddi, Mr. H. Williams and
other correspondents, for showing me forms from many localities. The
butterfly is attached for the most part to woods of deciduous trees and
to country abounding in tall hedges or rough scrub. It is not usually to
be found in highly cultivated districts or in very dry regions. Hence
there is necessarily some want of continuity in the distribution at the
present time and I should think a mile or two of arable land without big
hedges would constitute a barrier hardly ever passed. The larva feeds
on several coarse grasses, especially _Dactylis glomerata_. Barrett
mentions also _Triticum repens_. In this country the winter is usually
passed in the larval stage, but I have found that in captivity, at
least, there is much irregularity. The larvæ feed whenever the weather
is not very cold and may pupate, but if sharp cold comes on when they
are pupating or nearly full-grown they often get killed unless protected.
Some writers speak of a difference between the early and later broods,
but I have never noticed this, and I do not think that the general tone
of the yellow is affected by the seasons (see Tutt, _Ent. Rec._, IX,
1897, p. 37).[5]
Beginning at the south of Spain the thoroughly fulvous type _egeria_
is common at Gibraltar in the Cork woods, at Granada, and doubtless
generally. Lederer is said to have found only this type in Spain
(Speyer), and though I have no precise information as to other places
in the Peninsula north of Jaen I feel tolerably sure that there is no
change from south to north.[6] Immediately north of the Pyrenees we
still meet _egeria_ exclusively, and up to Poitiers at least there is no
noticeable change. But somewhere between Poitiers and the bottom of the
Loire valley at Tours, the genuine southern type comes to an end, and
the whole population begins at the Loire to be of an intermediate type,
easy to distinguish both from _egeria_ and from _egerides_. As to the
exact condition of the species in the fifty miles separating St. Savin
on the Vienne from places on the Loire I have no adequate information. I
have only one small sample from there, but it does contain insects both
of the southern and intermediate types taken on the same day, in a wood
near Preuilly. Oberthür also states that at Nantes the true southern
form exists in company with the northern. From this I infer that the
southern form extends up the coast further than it does inland, but
I imagine the representative spoken of as northern would be of usual
Brittany or intermediate type.
The Vienne river joins the Loire, so the true southern type reaches over
into the basin of the Loire. From the Loire (Tours, Corméry) north to
Calvados (Balleroy) only the intermediate is found, so far as I know,
and the same type extends over Brittany.[7] In general, however, the
woods near Paris have the thoroughly northern type _egerides_, but at
St. Germain-en-Laye and at Etampes (Oberthür) the population approaches
the intermediate type.
On the whole the intermediate type is certainly less homogeneous than
either of the extremes, and females with the two central spots either
paler or more fulvous than the rest are not uncommon, but I have never
taken one on the Loire or in Brittany which I should class with either
of the extreme types.
Before speaking of the distribution in other parts of France and in
Europe generally I will briefly state the results of my breeding
experiments. The work was done many years ago before we had the
Mendelian clue, and it is greatly to be hoped that some one will
find opportunities of repeating it. Crossing the English and the
thoroughly southern type the families produced agree entirely with
the intermediates of Brittany and the Loire. Reciprocals are alike.
Of F_{2} I only succeeded in raising very few and of those that I had
(about 30) nearly all were intermediate in character, though perhaps
rather less uniform than F_{1}. One family alone, containing only 4
specimens, had one _egerides_, and three fulvous intermediates. As the
case stands alone I hesitate whether or not to suppose it due to some
mistake. Moreover from F_{1} crossed back with the respective parental
types I had fairly long series, especially from F_{1} × the southern
type, and looking at these families I cannot see any clear evidence
of segregation. On the contrary, I think that though there are slight
irregularities, they would, taken as a whole, be classed as coming
between the intermediate type and the extreme form used as the second
parent. This at least is true when the second parent was of the southern
type.
On this evidence I have regarded the case as one in which there is
no good evidence of segregation and as conforming most nearly with
the conventional view of gradual transition in response to climatic
influences. Such influence must however be indirect; for I reared five
generations of the northern type in England, and these, though they
included several abnormal-looking specimens in the last generation and
then died out, did not show any noticeable change from the fulvous
colour of the wild type. Merrifield[8] also found that heat applied to
pupae of the northern type produced no approach to the southern type.
Looking at the facts now in the light of more experience it seems to me
just possible that the case may be one in which, as in Nilson-Ehle's
Wheats, the dominant differs from the recessive in having two pairs of
factors with similar effects. The fulvous type for example may have
two or more elements in separate pairs which together produce the full
effect, and the intermediate may have one of these. If this were so,
some segregation should of course eventually be observable, but the
proportion of the various fulvous and fulvous-intermediate individuals
would be large, and the reappearance of actual representatives of the
northern type might be rare. I admit that this is a somewhat strained
interpretation of the facts, and as yet it is not entitled to serious
consideration. Nevertheless I am led to form some such expectation
partly from the great difficulty in the way of any other, partly from
the evidence of the small mixed sample found at Preuilly and partly from
the statements given by Oberthür. There are moreover other features in
the general distribution of the species which make it improbable that
the dependence on climate can after all be so close. Published lists
are unfortunately of little use in deciding which form occurs at a
particular place, because, since the name _Meone_ has ceased to be used
for the southern form, there is no complete unanimity among authors as
to the application of the names _egeria_ and _egerides_, and unless
more particulars are given, either name may be used for either form.
Besides this, difficulty arises from the fact that the intermediate
type is not generally distinguished at all, and English collectors
finding it, may easily record it as the southern type. From Staudinger's
note on the distribution, I gather that he, on the contrary, reckoned
the intermediate with the northern type, as do the Speyers also. The
late Mr. J. W. Tutt was careful to distinguish the three forms and has
left several useful records. Easy therefore as it might seem to be to
make out the distribution of such a familiar insect in its various
modifications, there are serious practical difficulties, and until
long series are brought together with this special object in view many
obscurities will remain.
With only the series from England, the west of France, and Spain before
one it would be easy to regard the successive series of tones as a fair
measure of climate; the brighter the colour, the hotter might one expect
the locality to be. Such rough correspondence is often to be observed
in butterflies and birds. It becomes impossible to take these simple
views in the light of more complete knowledge. Beginning with France
the fulvous _egeria_ occupies the lower valley of the Rhone, probably
from well above Lyon, though I have no exact information respecting the
country above Avignon. According to Speyer it also takes the department
of Lozère. The same authority says that Puy-de-Dôme has "_egeria_,"
meaning perhaps the intermediate form, with the fulvous form much less
commonly. Next comes the curious fact that though the Lower Rhone
(Avignon, Tarascon, Nîmes) has the true fulvous form, Hyères, Cannes,
Grasse, Nice, Digne, and Alassio have _the intermediate_. Savoy has the
intermediate (Chambéry) and even _egerides_ perhaps, though in the same
latitude on the west of France there is nothing but the fulvous type.
At Chalseul and Besançon (Doubs) the ordinary northern type is found.
Switzerland generally, I believe, has the northern type, but Staudinger
gives _egeria_ for Valais and the intermediate occurs in Vaud.[9] The
south side of the Alps has probably colonies of the pale _egerides_, and
of intermediates. Orta, with a very hot summer, has the English type
(Tutt, _Ent. Rec._, XII, 1900, p. 328). Locarno has the intermediate
(_ibid._, XV, 1903, p. 321). North Italy in general and western
Piedmont have the intermediate; but further south _egeria_ begins,
at what region I do not know. Speyer gives on his own authority the
remarkable statement that at Florence both extremes occur, but chiefly
intermediates between the two. Mr. R. Verity however kindly informs
me that in his experience this is not so, and that neither the real
southern type nor the northern occur there. Sardinia, Sicily, Crete all
have the southern type. Greece probably has various types. Staudinger
(_Hor. Ross._, VII, 1870, p. 78) says intermediates resembling Nice
types common everywhere, but from "Greece" the British Museum has a
series that would pass for English specimens; and the same type occurs
near Constantinople. The island of Corfu has a pale intermediate,
distinct from _egerides_ but approaching it. In Roumania all three
forms are recorded from various places: _egeria_ in the Dobrutscha; not
quite typical (presumably an intermediate) at Bukharest; intermediate
in various mountainous localities as well as in Macedonia and Dalmatia;
but _egerides_ in Azuga at about 3,000 feet.[10] Hungary has the true
_egerides_ also. (Cf. Caradja, _Deut. Ent. Zt._, IX, p. 58.) Mathew
records the same from Gallipoli (_E. M. M._, 1881, p. 95). Staudinger
does not distinguish the intermediates from the northern, but he gives
"_egerides_" for Armenia and Fergana (Central Asia). As against the mere
proximity of a great mountain chain being the influence which keeps
the Riviera population intermediate may be mentioned the fact that the
northern foothills of the Pyrenees have the pure southern type, and the
climate of Cambo must surely be far cooler than that of Nice. The exact
locality of the Greek specimens is not given, but there can be no part
of Greece which is not much hotter in summer than Brittany, or Calvados,
which have the intermediate, not the English type.
In face of these facts it can scarcely be maintained that average
temperature is the efficient cause of the particular tone of colour
which the butterfly shows in a given region. Nevertheless it is clear
that climate counts for much in determining the distribution. It is
noticeable that though the pale _egerides_ can be established in a
warm climate we never find _egeria_ in cold climates, and even the
intermediate is not found in places that have a hard winter. I suspect
that the distribution of the broods through the year and the condition
of the animal at the onset of hard frost are features which really
determine whether a strain can live in a particular place or not.
Though the truth of the suggestion cannot be tested by experiments in
captivity, which at once introduce disturbances, I incline to the idea
that _egeria_ has not got the right periodicity for northern climates.
If it could arrange its life so that the population consisted either
of young larvae, or perhaps of thoroughly formed pupae[11] at the
onset of winter, it might, for any obvious reason to the contrary,
be able to live in England. It is irregularly "polyvoltine," as the
silk-worm breeders say, and as soon as a little warmth encourages it,
a new generation starts into being, which if the frost comes at an
untimely moment, is immediately destroyed. Many species are continually
throwing off individuals which feed up fast[12] and emerge at once if
the temperature permits, and I imagine a species of Satyrid wholly
or largely represented by such individuals could scarcely survive in
a country which had a hard winter. For such a climate some definite
periodicity in the appearance of the broods may well be indispensable.
But assuming that _egeria_ is cut off from cold climates for such a
reason, there is nothing yet to connect these habits with the fulvous
colour, and until breeding can be carried out on a satisfactory scale
there is no more to be said.
From time to time records appear of individual specimens more or
less fulvous being caught in southern England, especially in the
New Forest.[13] It would be interesting to know what offspring such
individuals might produce. From the evidence now given some notion
both of the strength and the weakness of the case considered as one of
continuous climatic variation can be formed. I know no other equally
satisfactory. Whether or not definite mixture of the intermediates
with either of the extremes will be proved to occur, the case differs
materially from those considered in the last chapter in the fact that
at all events there is no general overlapping of forms. In a species so
little given to wandering, overlapping could indeed scarcely be expected
to occur. It is this circumstance which makes the species preeminently
suitable as a subject for the study of climatic influences, and I trust
that entomologists with the right opportunities may be disposed to
explore the facts further.
Just as many species, like _egeria_, have varieties which can be
regarded as adapted to northern and southern regions, so there are
also several which have lowland and Alpine forms quite distinct from
each other. Every such case presents an example of the problem we
have been considering. As the collector passes from the plains to the
Alpine region, how will he find the transition from one form to the
other effected? Does the lowland form give place to the Alpine form
suddenly, with a region in which the two are mixed, or will he find a
zone inhabited by an intermediate population? I have spent a good deal
of time examining the facts in the case of _Pieris napi_ and its Alpine
female variety _bryoniae_, and though there are many complications which
still have to be cleared up, no doubt is possible as to the main lines
of the answer. If in any valley in the Alps inhabited by both _napi_ and
_bryoniae_ the collector catches every specimen he can, beginning at
the bottom and working up to 7,000 feet, he will at first get nothing
but _napi_. At about 2,500 feet, he may catch an occasional _bryoniae_
flying with the _napi_. After 3,000 feet _napi_ usually ceases, and
only _bryoniae_ are found. As an exception a colony of _napi_ may be
met with at much greater heights. I once found them in numbers at about
6,000 feet.[14] Not only were they free from any trace of modification
in the direction of _bryoniae_, but they were of the thoroughly southern
type of _napi_, being a late brood of that large and very pale kind
(_meridionalis_) almost destitute both of dark veining above and of
green veining below, which are common on the shores of Lago Maggiore
and in other hot southern localities. Not far off at the same level
were typical _bryoniae_ in fair abundance. Occasionally an intermediate
may be met with. I have taken a few, for example, at Macugnaga and at
Fobello. These, however, in my experience are rarities in the Alps.
Fleck[15] gives notes on the distribution in Roumania which shows the
same state of things. The lowland form is not transformed though found
at great heights, and at Azuga (nearly 3,000 feet) _bryoniae_ occurs
with only occasional "_flavescens_," viz., intermediates of the second
brood.
If this were all the evidence we should be satisfied that the lowland
and Alpine types keep practically distinct, overlapping occasionally,
but rarely interbreeding. The problem would remain, how is the
distinctness of the two types maintained in the region of overlapping?
Nowadays, I suppose, we should incline to answer this question by
reference to segregation, and perhaps by an appeal to selective mating.
The suggestion that segregation does take place is certainly true to
some extent. There are, however, difficulties in the way, and the whole
subject is one of great complexity. My own experiments were made in
pre-Mendelian times and were not arranged with the simplicity which we
now know to be essential. The results are neither extensive enough nor
clear enough to settle the many collateral questions which have to be
considered, and the work ought to be done again. Nevertheless, some
notes of the observations may have a suggestive value.
When I began, I did not sufficiently appreciate that the "_napi_" group,
omitting the North American forms, and the Asiatic representatives, has
at least three chief types in western Europe. The differences we have
to deal with are manifested by the females only, so in this account
particulars as to the males are omitted for the most part. These are (1)
our own British _napi_; (2) the form found in the south, from the Loire
downwards, and in the Italian Alps, which I think may be spoken of as
_meridionalis_; (3) _bryoniae_, which is a form clearly recognizable in
the _female_ only, and is found only in the arctic regions and in the
Alps above 2,500 feet. The first two have several broods, two, three,
or more, according to opportunity, and the first brood is different
from the later ones. In _napi_ the markings on the upper surface are
a dark grey but in _meridionalis_ they are a pale silvery grey and
much less extensive. In the later broods of _napi_ there is much less
general irroration of the veins, and the spots stand out as more defined
and blacker. These differences vary greatly in degree of emphasis. In
_meridionalis_ the later broods are entirely different from the first.
Instead of having silvery markings they have the ground colour quite
white, with the spots large and a full black. On the under side of the
hind wings the usual green veins are almost absent, and I have seen
individuals which could scarcely be distinguished from _rapae_. To these
later broods the term _napaeae_ is sometimes applied, but I here use
_meridionalis_ for the southern race in general as applicable to all
broods.
The female _bryoniae_ is totally unlike the others. The ground colour
is a full yellow, and each nervure is thickly irrorated with a brown
pigment often spreading so far as to hide the ground almost entirely
in the fore-wings. The males corresponding with these females are not
certainly distinguishable from those of our own _napi_. Both sexes have
the green veining of the underside of the hind wing fully developed,
rather more than is usual in the lowland races, but this is not really
diagnostic of the variety. The first serious difficulty arises in regard
to the second brood of _bryoniae_. It is stated that there is only one
brood,[16] but I feel fairly sure that a second brood is sometimes
produced, and that the females with a yellow ground and diminished
irroration of the veins, not very uncommon in the Italian Alps in July
to August, are generally representatives of it. Such insects would of
course be classed with _bryoniae_ in collections.
My experiments began with eggs of true _bryoniae_ females caught at
about 2,500 feet early in July. These emerged in August-September as
intermediates with yellow ground and about half as much black on the
upper surface as _bryoniae_. They are exactly like the intermediates
usually found in nature and in the light of later experience I regard
them as natural F_{1} forms, and I think the mothers had been fertilised
by _napi_ males, though I admit that in view of the rarity of natural
intermediates there is a difficulty in this suggestion. Three of these
females were mated with males raised from thorough _meridionalis_
females, and three families were produced. Two of them showed distinct
evidence of segregation, some being yellow and some white with various
intergrades, some being no blacker than _meridionalis_ and some ranging
up to a dark intermediate type. Part emerged in the same autumn; and
part overwintered, emerging as the spring _meridionalis_ or as the
peculiar type which I afterwards learnt to know as the spring F_{1}
form. The distinctions were fairly sharp between the several forms. But
the offspring of the third female gave a series practically continuous
from _meridionalis_ to the F_{1} type. The work of subsequent years
gave results similarly irregular which could only be described
adequately at great length. The outcome may however be summed up in
the statement that there is evidence that both the yellow ground and
the dark veining are due to factors, but that there are several of
these and that imperfect segregation is not uncommon, producing various
reduction-stages. The yellow ground may be due to one factor, and the
several shades may be the result of irregularities in dominance, but
the black markings when fully developed cannot I think be the result of
less than three factors, one for the basal darkening, one for general
irroration, and one for the margins. Probably also the enlargement of
the spots is produced by a fourth factor.
There was not, in my experience any great difficulty in getting the
various forms to pair in captivity. Some attempts were made to see
whether individuals of either type selected mates of their own type in
preference to those of the other, but the results were inconclusive.
There were some indications of such a preference; though, from
the impossibility of judging how much of this may be due to other
circumstances, I could not come to a positive conclusion on the rather
meagre evidence.
Recently Schima[17] has given a careful and detailed account of all
the forms found in Lower Austria which he enumerates under 14 distinct
varietal names. He gives full references to previous accounts,
especially to the beautiful plates lately published by Roger Verity.[18]
Examination of these and of my own specimens strongly suggests that
the several forms are due to the recombination of the factors I have
named. Among those which I have bred are representatives of most if not
all the types enumerated by Schima in addition to other curious forms.
For example I have _bryoniae_ markings on a ground practically white;
the dark veins with spots almost obsolete; _meridionalis_ on a yellow
ground; the intermediate amount of black on a white ground, etc. The
last-named may occur wild and I have one from Macugnaga as well as one
given me by Mr. F. Gayner from Lulea (Lapmark).
To obtain really exact knowledge of the number of factors and
their properties it would be necessary to repeat the work. After
the beginning, I made a mistake in using British _napi_ instead of
_meridionalis_ and the results were much confused thereby. The contrast
between _meridionalis_ and the various dark forms is much greater and
classification of the types would have been therefore easier. The
British form is presumably _meridionalis_ plus the factor for the basal
pigmentation. The problem is greatly complicated by the differentiation
of the seasonal forms. The first point to be determined is whether
_bryoniae_ is capable of producing a second brood when it is thoroughly
pure-bred, and whether such a second brood is, as I suspect, normally
intermediate in character.
In the Alps generally there is no definitely intermediate population;
nor I believe, is any such population met with in the north where
the arctic _bryoniae_ meets _napi_, but as to this I have no precise
information. One curious fact, however, must be mentioned, namely that
there is a population that can probably be so described with fairness
established at Mödling near Vienna. This is not in any sense an Alpine
locality, and does not, as I am told, differ in any obvious way from
the other suburbs of Vienna. Dr. H. Przibram was so good as to send me
a set taken at this place, representing a second brood, and they were
decidedly heterogeneous, ranging from an intermediate form such as
_bryoniae_ fertilised by _napi_ usually produces, to a light yellowish
second-brood type with little dark pigment. There are also two actual
_bryoniae_. Whether true _napi_ also occur there I do not know, but I
have no doubt they do. It would be well worth while to investigate the
Mödling population statistically, and to breed from the intermediates
which might not impossibly prove to be heterozygotes. There are also
records of such intermediates being occasionally found in some parts
of Ireland, in the north of Scotland, and in south Wales,[19] but I do
not know of any regular colony of these forms. We can scarcely avoid
the inference that one or more of the factors which make up _bryoniae_
may be carried by these intermediates. It is not clear why their
interbreeding does not produce actual _bryoniae_ occasionally. If this
occurred, the probability is that the fact would be known to collectors,
at least in the British localities. The absence of true _bryoniae_ must,
I think, be taken to mean that some essential factor is absent from
these intermediates.
To sum up the evidence, the facts that are clear may be thus enumerated:
1. _Napi_ and _bryoniae_, or in the Italian Alps,
_napaeae_ and _bryoniae_ frequently meet each other.
2. They cross without difficulty, producing fertile offspring.
3. But in the levels at which they overlap there is no
intermediate population, and only occasional intermediate
individuals.
4. In certain parts of the distribution of _napi_ similar
intermediates sometimes occur, and at one place (Mödling) they
are so frequent as apparently to constitute a colony.
5. As to the genetic relations of the two forms there is no
complete certainty. Indications of segregation have been observed
in some cases, but there are several factors concerned and they
are liable to some disintegration.
Another form in which I tried to investigate the same problem is
_Coenonympha arcania_, which has one Alpine form known as _darwiniana_,
and another, _satyrion_. In calling _satyrion_ a form of _arcania_ I
follow Staudinger and other authorities, but I have never been quite
satisfied that it should be so regarded. The differences between
_arcania_ and _darwiniana_ are essentially differences of degree; _C.
arcania_ occurs in places where there is cover, and reaches up the
valleys usually as high as the mixed woods of deciduous trees, which
is about 2,500 feet. The variety _darwiniana_, on the contrary, is an
insect of treeless hillsides, and I regard it as a dwarf and possibly
a stunted form. It would not greatly surprise me to find that with
the application of good conditions _arcania_ could be raised from
_darwiniana_ eggs, or that if _arcania_ larvae were starved they might
give rise to _darwiniana_ butterflies. I have been unsuccessful in
trying to rear the species, having lost the larvae by disease. Usually
one does not catch _arcania_ and _darwiniana_ on the same ground, and
as _Festuca ovina_--a typically hill-side grass--is a common food-plant
of _darwiniana_ there can be little doubt that _arcania_ feeds on
some other grass, probably woodland species. Colonies of _arcania_ of
varying size and brightness are commonly found, and though a sample of
_arcania_, finely grown, from a warm Italian wood, presents a striking
contrast with _darwiniana_ from an Alpine pasture, one certainly may get
samples which fill all the gradations. Generally the sample from a given
locality is fairly homogeneous.
Of _satyrion_ I have little personal experience. I only twice found
it, namely at Zinal, and at Hallstatt in Austria, but it occurs at
Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and
I understand that it is the typical Alpine form in the Engadine. With
its darkened colour and reduced size it might well be expected to be a
still further stunted form of _darwiniana_. Yet I have never found the
one succeed to the other at the higher levels. If _darwiniana_ appears
when Alpine conditions are reached in a valley it will be met with
up to the highest level at which such butterflies live. Tutt was of
opinion that _satyrion_ is a distinct species.[20] I once, at the top
of the Vorderrheinthal caught a sample of _darwiniana_ a few of which
(males) were so dark and had the eye spots so poorly developed that they
looked like transitions to _satyrion_. Otherwise I never found any such
transitional forms and they are certainly exceptional. There is further
a record[21] of _satyrion_ having been taken flying with _arcania_.
This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has
similar specimens from Caud and from St. Anton (Arlberg). The females,
however, both of mine and of Mr. Page's samples are a pale brown, quite
unlike the females both of _arcania_ and of the dark Zinal _satyrion_.
The difficulty thus raised has not I think yet been considered by the
authorities, and it is possible that the Alpine forms of _arcania_ are
in reality three, not two.
The evidence taken together suggests, I think, that _darwiniana_ is
related to _arcania_ much as so many of the Alpine varieties of plants
are to the well-developed individuals of the lower levels. I do not
anticipate that factorial differences will be found in these insects,
and it is by no means impossible that the distinctions between them
are the direct consequences of altered conditions. The relations of
_arcania_ to _satyrion_ are more doubtful, and in that case a factorial
difference may at least be suspected.
The species of the genus _Setina_ have Alpine forms which agree in
possessing a characteristic extension of the black pigment to form
radiating junctions between the spots on the wings. Speyer, who
discussed the interrelations of these forms in detail,[22] lays stress
on the absence of genuine transitional forms between _aurita_ and the
variety _ramosa_. Both are mountain insects but _ramosa_ extends to
levels higher than that at which _aurita_ ceases, which is about 4,000
feet. The two forms are often found flying together. Speyer says that
his brother searched diligently for transitional forms at the level of
overlapping, but found none, so that at least they may be regarded as
rare. The variety _ramosa_ is not infrequent at much lower levels (_e.
g._, Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high
as the permanent snows. In the British Museum collection, however, I
have seen several that I should regard as transitional. Speyer perhaps
would have classed as _ramosa_ all in which the spots of the central
field were united, and it is by no means unlikely that breeding would
prove such individuals to be heterozygous.[23]
There can scarcely be a doubt that the distinction between _aurita_ and
_ramosa_ is factorial, the radiate _ramosa_ probably having the factor
for striping. In support of this view may be mentioned the observation
of Boisduval,[24] respecting a gynandromorphous individual, which was
_aurita_ male on one side, and _ramosa_ female on the other. Speyer
makes another excellent comment. He points out that the simple notion
that the radiation is a mere extension of pigmentation consequent on
the climate of the higher levels, will not fit the facts very easily,
because the size of the spots varies greatly in _aurita_ itself at any
level, and lowland specimens may actually have more black confined
to the spots alone than some _ramosa_ possess on spots and lines
combined.[25]
The two Salamanders, _S. maculosa_ and its Alpine form _atra_, might
not improbably furnish evidence bearing on the same problem. The two
are of course very distinct, not merely in colour (_maculosa_ being
spotted with yellow or orange while _atra_ is entirely black) but also
in the mode of reproduction, a feature to which reference will be made
in the next chapter. I cannot, however, find any evidence as to the
overlapping of the two forms. _S. atra_ occurs from about 3,000 feet
or somewhat less, and reaches great elevations in the Eastern Alps,
but I do not know if the two forms ever occur in the same localities.
Leydig,[26] Boulenger,[27] and most modern authorities regard the two
types as distinct species, but they are in any case closely allied, and
it would be of interest to have exact knowledge of their geographical
delimitations.
The reader who has considered the cases adduced will appreciate the
difficulties which must be faced in any attempt to account for the
facts in a rational way. As always in a problem of Evolution, two
separate questions have to be answered. First how did the form under
consideration come into existence, and secondly, how did it succeed
in maintaining itself so as to become a race? The evidence from the
local forms, though very far from giving complete answers to either of
these questions definitely refutes the popular notion that a new race
comes into existence by transformation of an older race. If a gradual
mass-transformation of this kind took place we should certainly expect
that when two types, nearly allied and capable of interbreeding, overlap
each other in their geographical distribution, a normally intermediate
population would exist. If each type can maintain itself, and if
each came into existence by gradual transformation, then there must
have been an intermediate capable of existing and maintaining itself
as a population; and if this had ever been, surely in the region of
overlapping, that intermediate population should continue. Especially
should such a population be found when the two extreme types are
adaptational forms and the region of overlap is a region of intermediate
conditions. But of the examples we have examined there is only one, that
of _Pararge egeria_ and _egerides_, which can at all be so interpreted,
and even in that case it is not impossible that more minute observation
would reveal discontinuity between the extremes and the admittedly
normal intermediate population. Granting provisionally however that
this example, as it stands, is consistent with the conventional theory
of evolution, I know not where we should look for another case equally
good. When the distinctions are produced by direct influence of
conditions operating during the lifetime of the individuals, examples of
intermediate populations occupying the areas of intermediate conditions
can no doubt be produced. Many turf-like Alpine plants, for instance,
if protected from exposure and properly nourished can grow as large as
those of the same species found in the valleys, and in the case of such
quantitative effects, intermediate conditions can doubtless produce
intermediate characters.
Even these examples however are not very abundant, and often the
intermediate locality has not a form intermediate between those of
the two extreme localities, but some third form distinct from either.
This is the case for instance in the fauna of brackish waters. We are
taught to believe that the fresh water fauna was evolved from the
marine fauna, which it well may have been; but as students of Crustacea
and Mollusca know familiarly, the brackish water forms are not as a
rule intermediates between fresh water species and sea species, but
more usually they are special forms belonging to the brackish waters,
with the peculiar property that they can tolerate a great range of
conditions, and live without ostensible variation in waters of most
various compositions and densities, which very few marine or fresh water
species are able to do.
Sometimes the distinction between local races, as in _Rhamphocoelus
passerinii_ and _icteronotus_ may be regarded with confidence as due to
one simple Mendelian factor possessed by one race and absent from the
other, but I think, more often, as in _Colaptes_ or in the varieties
of _Pieris napi_, the existence of several distinct factors is to
be inferred. As we have seen, the races of _Colaptes_ show almost
beyond doubt that in different areas at least three distinct factorial
combinations can be perpetuated as races.
In the distribution of variability we find, I think, some hint as to
the steps by which the phenomena under consideration have come to their
present stage, and I am disposed to regard the facts so well attested
in the case of our own melanic moths as a true indication of the
process. Following this indication we should regard the change in the
character of a population as beginning sporadically, by the appearance
of varying individuals, possibly only one varying individual, in, it may
be, one place only. As to _why_ a variety should increase in numbers
we have nothing but mere speculation to offer, and for the present we
must simply recognise the fact that it may. That such survival and
replacement may reasonably be taken as an indication that the replacing
race has some superior power of holding its own I am quite disposed to
admit. Nevertheless it seems in the highest degree unlikely that the
outward and perceptible character or characters which we recognise as
differentiating the race should be the actual features which contribute
effectively to that result.
In discussions of geographical distribution in relation to problems of
origin it is generally said that very nearly allied species usually
occupy distinct areas, while other competent observers state the exact
contrary. Lately, for example, Dr. R. G. Leavitt[28] has published an
important collection of evidence upholding the latter proposition, taken
chiefly from the botanical side, showing how in numerous genera two or
more closely allied species coexist, frequently without intermediates,
in the same localities, and may even be thus found in company throughout
their distribution. The difference of opinion evidently arises from a
confusion as to the sense in which the term "species" is understood and
applied. Leavitt, for example, is avowedly following Jordan and, among
moderns, Sargent, in applying a close analysis, and denoting as species
all forms which are distinct and breed true. Against this use of the
term I know no valid objection[29] but it must be obvious that if others
follow a different practice confusion may result when observations are
summarised in general statements. We will consider this subject again
in another place, but here it may be sufficient to say that there can
scarcely now be a doubt that numbers of these associated species, such
as Jordan discriminated, represent various combinations of the presence
and absence of Mendelian factors. This does not in any way weaken the
argument which Leavitt founds upon the facts, namely, that the observed
distribution of these forms is consistent with the supposition of an
evolution largely discontinuous.
On the other hand, those who have come to the opinion that nearly
allied species generally occupy distinct ground are presumably more
impressed by the characters differentiating the geographically distinct
or adaptational races, seeing that genuine intermediates between
them are less commonly found. Those geographical races may no doubt
contain various differentiated forms; but when all live together,
occasional intermediates are usually to be found even in the case of
characters habitually segregating. These segregating forms Jordan would
certainly have determined as species, and it must be conceded that no
physiological definition has yet been drawn which consistently excludes
them.
FOOTNOTES:
[1] Often referred to by older writers as _Meone_, Esper's name.
[2] There are also two distinct island forms, unlike the European,
_Xiphia_ of Madeira, and a smaller variety, _Xiphioides_ of Canary. See
especially, Baker, G. T., _Trans. Ent. Soc. London_, 1891, p. 292.
[3] Speyer, Adolf, and August. _Verbreitung der Schmetterlinge_, 1858,
I, p. 217.
[4] _Lepid. Comparée_, fsc. III, p. 372.
[5] Mr. Rowland-Brown has called my attention to a statement by Dr.
Vaillantin (_Petites Nouv. Ent._, II, 235) that in Indre-et-Cher the
first brood is of the northern type and the second of the southern. My
experience is that in captivity these distinctions do not occur, and I
have true _egeria_ as first brood from Vienne and as the late brood from
the Landes. I never collected in Indre-et-Cher.
[6] I have since seen true _egeria_ from Ferrol in the extreme
northwest, which was in Mr. Tutt's collection.
[7] Mr. G. Wheeler kindly showed me a series identical with this type,
from Guernsey, and others from near Laon.
[8] _Ent. Rec._, V, 1894, p. 134.
[9] Mr. Wheeler has some pale but rather worn specimens from the Rhone
Valley at Vernayaz.
[10] See Fleck, E., Die Macrolep. Rumäniens, _Bul. Soc. Sciinte_, VIII,
1899, p. 720.
[11] My experience agrees with that of Mr. H. Williams (_Ent. Rec._,
VIII, 1896, p. 181) that pupae, well-formed, can stand considerable
frost; but I used to find that half-grown larvae usually died if
unprotected, and I believe that larvae which attempted to pupate in warm
autumn weather and then got caught by frosts, always died. Small larvae
which can creep into shelter at the bottom of the plants survived, and I
expect that in the north the winter is usually passed in that state (see
also Merrifield, F., _Ent. Rec._, VIII, 1896, p. 168, and Carpenter, J.
H., _ibid._).
[12] Some most unlikely species do this. I once had a larva of
_Parnassius delius_, found at about 5,500 feet, which emerged late
in the autumn (in October I believe), a season at which it must have
perished in its own country.
[13] See, for examples, Barrett, G. C., _Lepidoptera of the Brit.
Islands_, I, 1893, p. 229; also Grover, W., _Ent. Rec._, IX, 1897, p.
314; Williams, H., _Proc. Ent. Soc._, 1898, who reared several specimens
from the New Forest which would pass for Bretons, though the rest of the
family were true _egerides_.
[14] Above the Tosa falls.
[15] _Bul. Soc. Sciinte_, VIII, 1899, p. 691.
[16] The fact that Weismann by heating pupæ obtained only one autumn
specimen seems to me to show rather that a second brood can be produced
than that it cannot, which is the inference usually drawn.
[17] Schima, K., _Verh. Zool. bot. Ges. Wien_, LX, 1910, p. 268.
[18] _Rhopalocera Palaearctica_, Florence, 1905-11, especially Pl. XXXII.
[19] See figures in Barrett, G. C., _Lepidoptera of Brit. Islands_, I,
pt. 3, p. 25.
[20] Tutt, J. W., _Ent. Rec._, XVIII, 1905, p. 5. In the same place he
states that on the Mendel Pass _arcania_ "runs into" _darwiniana_ and
that in the Tyrolean localities the transition is especially evident.
Wheeler (_ibid._, XIII, 1901, p. 121) expresses the contrary opinion,
that _satyrion_ does grade to _arcania_.
[21] H. Rowland-Brown, _Ent. Rec._, XI, 1899, p. 293.
[22] Speyer, Stettiner, _Ent. Ztg._, XXXI, 1870, p. 63.
[23] In regard to the closely analogous case of _Spilosoma lubricipeda_,
Standfuss makes a similar statement. He bred the type on a large scale
with the radiate form which he calls _intermedia_, and says that in four
years of miscellaneous crossing he never obtained really transitional
forms. Nevertheless after examining large series, especially those of
Mr. W. H. B. Fletcher, I came to the conclusion that several might be
so classed, but I am quite prepared to find that such specimens are
heterozygous. (See Standfuss, _Handb. d. Gross-Schmet._, 1896, p. 307.)
It is by no means unlikely that various dark forms of _lubricipeda_
correspond with a progressive series of factorial additions. Many of
the stages have been named, and of these the most definite are the
_intermedia_ of Standfuss (probably = _eboraci_ of Tugwell) and the very
dark _Zatima_ of Heligoland, in which only the thorax, the nervures
and a small field in the fore-wings remain yellow. A form was bred
by Deschange from _Zatima_ in which even the field in the forewing
is obliterated. The exact circumstances in which _Zatima_ occurs in
Heligoland would be worthy of special investigation, for the normal
_lubricipeda_ is also found on the island. For references as to the
British occurrences see especially, Hewett, W., _Naturalist_, 1894, p.
353. As to _Zatima_ see especially Krancher, _Soc. Ent._, II, 1887-8, p.
26. I am indebted to Dr. Hartlaub for information as to the Heligoland
types.
[24] Boisduval, _Bull. Soc. Ent. Fr._, III, 1834, p. 5.
[25] The systematics of _Setina_ have been much controverted, but no one
I believe doubts that _aurita_ and _ramosa_ are forms of one species.
See also Chapman, A. T., _Ent. Rec._, XIII, 1901, p. 139.
[26] _Arch. Naturg._, 33, 1867, p. 116.
[27] _Brit. Mus. Cat., Batrachia Gradientia_, 1882.
[28] The Geographical Distribution of nearly related Species. _Amer.
Nat._, XLI. 1907, p. 207.
[29] See later, p. 242.
CHAPTER IX
THE EFFECTS OF CHANGED CONDITIONS: ADAPTATION
In the attempt to conceive a process by which Evolution may have come
about, the first phenomenon to be recognized and accounted for is
specific difference. With that recognition the outline of the problem is
defined. The second prerogative fact is adaptation. Forms of life are
_on the whole_ divided into species, and these species _on the whole_
are adapted and fit the places in which they live. To many students of
Evolution, adaptation has proved so much more interesting and impressive
than specific diversity that they have preferred it to the first place
in their considerations.
Whether this is, as I believe, an inversion of the logical order or
not, there is one most serious practical objection to such preference,
that whereas specific diversity is a subject which can be investigated
both by the study of variation and by the analytical apparatus which
modern genetic science has developed, we have no very effectual means of
directly attacking the problems of Adaptation.
The absence of any definite progress in genetics in the last century was
in great measure due to the exclusive prominence given to the problem
of Adaptation. Almost all debates on heredity centered in that part of
the subject. No one disputes that the adaptation of organisms to their
surroundings is one of the great problems of nature, but it is not the
primary problem of descent. Moreover, until the normal and undisturbed
course of descent under uniform conditions is ascertained with some
exactness, it is useless to attempt a survey of the consequences of
external interference; nor as a rule can it be even possible to decide
with much confidence whether such interferences have or have not
definite consequences. Those, for example, who debated with enthusiasm
whether acquired characters are or are not transmitted were constantly
engaged in discussing occurrences which we now know to be ordinary
features of descent under uniform conditions, and the origin of
variations which were certainly not caused directly by circumstances at
all. In the absence of any factorial analysis, or of any conception of
what factorial composition means and implies, no one knew what varieties
might be expected from given parents. The appearance of any recessive
variety was claimed as a consequence of some treatment which might have
been applied to the parents. There was no possible standard of evidence
or means of controlling it, and thus the discussion was singularly
unfruitful. Before we can tell how the course of descent has departed
from the normal, we must know what the normal would have been if we
had let alone. We are still far from having such knowledge in adequate
measure, but it does now exist in some degree, and we are steadily
approaching a position from which we shall be able to form fairly
sound estimates of the true significance of evidence for or against
the proposition that environmental treatment can produce positive
disturbances in the physiological course of descent.
Thus described, the field for consideration is very wide. Though the
effects of changed conditions were especially studied in the hope of
solving the problem of adaptation by direct observation, that, as all
are now agreed, is but a part of a more general question. We must ask
not only do changed conditions produce an _adaptative_ response on the
part of the offspring, but whether they produce any response on the
part of the offspring at all. It is not in doubt that by violent means,
such as starvation or poisoning of the reproductive cells, effects of a
kind, stunting and deformity for instance, can be made evident, just as
similar effects may follow similar treatment during embryonic or larval
life. Apart from interferences of this class, are there any that may be
reasonably invoked as modifying the course of inheritance?
No epitome of the older evidence for the inheritance of adaptative
changes is here required. That has often been collected, especially
by Weismann, who exposed its weaknesses so thoroughly as to carry
conviction to most minds, and showed that whether the phenomenon
occurs or not, no one can yet prove that it does. Belief in these
transmissions, after being almost universally held, was with singular
unanimity abandoned. This change in opinion, though doing credit to the
faith of the scientific community in evidential reasoning, is the more
remarkable inasmuch as the strength of the idea was not derived from the
minute amounts of supposed facts now demolished. On the contrary, it was
really an instinctive deduction from a wide superficial acquaintance
with the properties of animals and plants. They _can_ accommodate
themselves to circumstances. They _do_ make responses sometimes
marvellously appropriate to demands for which they can scarcely have
been prepared. What more natural than to suppose that the permanent
adaptations have been achieved by inherited summation of such responses?
No one had actually been driven to believe in the inheritance of
adaptative changes because bitches which had been docked had been known
to give birth to tailless puppies, or because certain wheat in Norway
was alleged to have become acclimatized in a few generations. Evidence
of this kind was collected and produced rather as an ornamental appendix
to a proposition already accepted, and held to be plainly demonstrated
by the facts of nature. Looked at indeed in that preliminary and
uncritical way, the case is simply overwhelming. Those who desire to see
how strong it is should turn to Samuel Butler's _Life and Habit_, and
even if in reading they reiterate to themselves that no experimental
evidence exists in support of the propositions advanced, the misgiving
that none the less they may be true is likely to remain. Making every
deduction for the fact that the wonders of adaptation have been grossly
exaggerated, and that marvels of fitness and correspondence between
means and ends have grown out of mere anthropomorphic speculations,
there is much more left to be accounted for than can at all comfortably
be accepted as the product of happy accidents. So oppressive are these
difficulties that we can scarcely blame those who imagine that the study
of heredity is primarily directed to the problem of the transmission of
acquired characters, a preconception still almost universal among the
laity.
But since the belief in transmission of acquired adaptations arose
from preconception rather than from evidence, it is worth observing
that, rightly considered, the probability should surely be the other
way. For the adaptations relate to every variety of exigency. To supply
themselves with food, to find it, to seize and digest it, to protect
themselves from predatory enemies whether by offence or defence, to
counter-balance the changes of temperature, or pressure, to provide for
mechanical strains, to obtain immunity from poison and from invading
organisms, to bring the sexual elements into contact, to ensure the
distribution of the type; all these and many more are accomplished by
organisms in a thousand most diverse and alternative methods. Those are
the things that are hard to imagine as produced by any concatenation
of natural events; but the suggestions that organisms had had from the
beginning innate in them a power of modifying themselves, their organs
and their instincts so as to meet these multifarious requirements does
not materially differ from the more overt appeals to supernatural
intervention.
The conception, originally introduced by Hering and independently by
S. Butler, that adaptation is a consequence or product of accumulated
_memory_ was of late revived by Semon and has been received with some
approval, especially by F. Darwin. I see nothing fantastic in the notion
that memory may be unconsciously preserved with the same continuity
that the protoplasmic basis of life possesses. That idea, though purely
speculative and, as yet, incapable of proof or disproof contains
nothing which our experience of matter or of life at all refutes. On
the contrary, we probably do well to retain the suggestion as a clue
that may some day be of service. But if adaptation is to be the product
of these accumulated experiences, _they must in some way be translated
into terms of physiological and structural change_, a process frankly
inconceivable.
To attempt any representation of heredity as a product of memory is,
moreover, to substitute the more obscure for the less. Both are now
inscrutable; but while we may not unreasonably aspire to analyse
heredity into simpler components by ordinary methods of research, the
case of memory is altogether different. Memory is a mystery as deep
as any that even psychology can propound. Philosophers might perhaps
encourage themselves to attack the problem of the nature of memory by
reflecting that after all the process may in some of its aspects be
comparable with that of inheritance, but the student of genetics, as
long as he can keep in close touch with a profitable basis of material
fact, will scarcely be tempted to look for inspiration in psychical
analogies.
For a summary of the recent evidence I may refer the reader to Semon's
paper[1] where he will find a collection of these observations described
from the standpoint of a convinced believer. At the outset one cannot
help being struck by the fact that of the instances alleged, very few,
even if authentic, show the transmission of acquired modifications
which can in any sense be regarded as adaptative, and many are examples
not so much of a transmission of characters produced in the parents as
of variation induced in the offspring as a consequence of treatment
to which the parents were submitted, the parents themselves remaining
apparently unmodified. No one questions the great importance of evidence
of this latter class as touching the problem of the causes of variation,
but it is not obvious why it is introduced in support of the thesis that
acquired characters are inherited.
It is most difficult to form a clear judgment of the value of the
evidence as a whole. To doubt the validity of testimony put forward
by reputable authors is to incur a charge of obstinacy or caprice;
nevertheless in matters of this kind, where the alleged phenomena are,
if genuine, of such exceptional significance, belief should only be
extended to evidence after every possible source of doubt has been
excluded. We believe such things when we must, but not before. At the
very least we are entitled to require that confirmatory evidence should
be forthcoming from independent witnesses. So far as I have seen, this
requirement is satisfied in scarcely any of the examples that have been
lately published, and until it is, judgment may reasonably be suspended.
In some cases, however, the facts are not doubtful. Standfuss, by
subjecting pupae of _Vanessa urticae_ to cold, produced the now
well-known temperature-aberrations in which the dark pigment is greatly
extended. He put together in a breeding-cage 32 males and 10 females
showing this modification in various degrees. Two of these females died
without leaving young. Seven produced exclusively normal offspring. From
the eighth female 43 butterflies were bred, and of these there were four
(all males) which to a greater or less extent exhibited the aberrational
form.[2] The mother of this family was the most abnormal of the 10
females originally put in.
Fischer's experiment with _Aretia caja_ was on similar lines. From
pupae which had been frozen almost all the moths which emerged showed
aberrational markings. A pair of these mated and produced 173 young
which pupated. Those which emerged early were all normal, but of those
which emerged late, 17 had in various degrees abnormal markings like
those of the parents.[3] In neither of these examples is there any
question as to the facts. Both observers have great experience and give
full details of their work.
As regards _Vanessa urticae_, however, it must be recalled that Fischer
himself showed that in Nymphalids somewhat similar aberrations could be
produced both by heat and by cold, and even by centrifuging the pupae.
Frl. von Linden produced a transitional form of the same aberration
in _V. urticae_ by the action of carbonic acid gas.[4] It is highly
probable that the appearance is due to a morbid change, perhaps an
arrest of development, which may be brought about by a great diversity
of causes. In the experiments the cause probably was a diseased
condition of the tissues of the mother herself. She had been subjected
to freezing sufficiently severe to prevent the proper development of
the pigments and some of the ovarian cells presumably suffered also.
It will be observed that the only specimens which were affected were
the offspring of the most abnormal female, and of them only four out of
forty-three showed any change.
The same interpretation probably applies to the cases in _Arctia caja_.
In this species the markings are well known to be liable to great
variation. As Barrett says, even in nature individuals are rarely quite
alike, and an immense number of strange forms occur in collections.[5]
These are greatly sought after by some collectors, especially in
England, where they fetch high prices at auctions, and it is notorious
that most of them come from Lancashire and the West Riding of Yorkshire.
It is commonly supposed that the breeders of that district subject them
to abnormal conditions, and especially to unnatural feeding, but I know
no clear evidence that this is true. From whatever cause it is certain
that the natural pattern is, in some strains at all events, very easily
disturbed.
The elaborate experiments of Schröder with _Abraxas grossulariata_ are
difficult to follow and are complicated by the fact that the series
which was submitted to abnormal temperatures was derived from an
abnormal original pair. From the evidence given it is not clear to me
whether the temperature had a distinct effect. This insect, like _Arctia
caja_, produces an immense number of variations (especially in the
amount of the black pigment) and as most of these are, I believe, reared
in domestication for sale, it is highly probable that the species is
easily influenced by cultural conditions.
Schröder describes two other experiments which have been accepted
by Semon and other supporters of the view that acquired characters
are transmitted. In the first, _Phratora vitellinae_, a phytophagous
beetle living on the undersides of leaves, was used. It naturally
feeds on _Salix fragilis_, a species without a felt, or tomentum, on
the underside of the leaves. Larvae were transferred to another willow
(near _S. viminalis_) which has the undersides of the leaves felted. The
larvae took readily to the new food, pushing the tomentum before them as
they gnawed the leaves. They came to maturity and when they were about
to lay their eggs they were given a free choice between _S. fragilis_
and the tomentose species. The greater number of ovipositions, 219,
took place on _fragilis_, and there were 127 on the tomentose bush,
which we are told was six times as large as the _fragilis_. The larvae
from _fragilis_ were next put on the tomentose species and reared on
it. When they became imagines they were similarly given their choice,
with the result that there were 104 ovipositions on the tomentose
species and only 83 on _fragilis_. In the next generations there were 48
ovipositions on the tomentose and 11 on _fragilis_. Finally the fourth
generation made 15 ovipositions on the tomentose and none on _fragilis_.
The difficulty about such experiments is obviously that one has no
assurance that the change of instinct, in so far as there is any,
may not be a mere consequence of the captivity. It must, besides, be
extremely difficult to arrange the experiment so that there is really
an equal choice between the two bushes, when one stands beside the
other. Przibram, in quoting this case, considers that as the tomentose
bush was about six times as large as the _fragilis_, some indication of
the relative attractiveness of the two may be obtained by dividing the
ovipositions on the larger bush by six, but I imagine the matter must be
much more complex.
Schröder's second example is not more convincing, in my opinion, though
Semon regards it as one of the most important pieces of evidence. It
concerns a leaf-rolling moth, _Gracilaria stigmatella_, the larva of
which is said normally to make its house by bending over the _tips_ of
the sallow leaves on which it feeds. Schröder placed larvae on leaves
from which the tips had been cut, and these larvae made their houses by
rolling over the _sides_ of the leaves. Their offspring were again fed
on leaves without tips, and as before, they rolled in the leaf-margins
either on one side or both. The offspring of this second generation were
then fed on entire leaves. There were 19 houses made by these (?19)
larvae, and of them 15 were normal, made by folding down the tips of
the leaves, while 4 were abnormal, made by rolling in the leaf-margins.
Schröder says that in nature he has only twice seen abnormal houses; but
it is clearly essential not only that the frequency of such variability
in nature should be thoroughly examined, but also that we should know
whether when the species is bred in captivity these irregularities of
behaviour do or do not occur when the larvae are fed on uninjured leaves.
The famous case of Schübeler's wheat is revived by Semon. The story will
be familiar to most readers of the literature of the subject. Briefly
it is that annuals, especially wheat and maize, raised from seed in
Central Europe take more time in coming to maturity and ripening than
similar plants raised in Norway, where the summer days are much longer.
The received account is that he imported seed especially of maize and
of wheat from Central Europe to Norway and found that in successive
years the period of growth and ripening was increasingly reduced. After
two generations seed of the accelerated wheat was sent back to Breslau
where it was grown, and was found to ripen rather more slowly than in
Norway, but much more quickly than the original stock had done. The
facts recorded by Schübeler[6] are that he received seed from Eldena,
which is on the Baltic near Greifswald. The variety is described as
"_100 tägiger Sommer Weizen_," but no more exact record of its behaviour
in Germany is given. This wheat, grown at Christiania in 1857, took
103 days to harvest. Its seed was again grown in Christiania in 1858,
and took 93 days, and sown again in 1859 it took only 75 days, 28 days
less than in the first year of cultivation in Norway. Seed of the 1858
crop was sent to Breslau, and grown there by Roedelius in 1859; it
took 80 days. Evidently before such a record can be used as proving an
inheritance of acquired characters numbers of particulars should be
forthcoming. The view that Johannsen has taken is that the result was
probably due to unconscious selection of the earlier individuals among a
population consisting of many types of various compositions. Some effect
may no doubt be ascribed to that cause, but I cannot think that alone
it would account for the results. My impression is rather that they
were produced by differences in the cultivation and especially in the
seasons. Research of an elaborate character would be necessary in order
to eliminate the various sources of error, and nothing of the kind has
been done; nor does Semon allude to these difficulties in prominently
adducing Schübeler's evidence. A difference of even three weeks in time
of harvesting may easily be due to variation in the season. It would in
any case be difficult to analyse the meteorological conditions, and to
decide how much effect in postponing or accelerating the harvest might
be due to cold days, to cloudy days, to wet weather, to fluctuations in
average temperature, to hot days, and other such incidents occurring at
the different periods of growth, even if they were specially watched
while the experiments were in progress, and at this distance of time
such analysis is practically impossible. Without careful simultaneous
control-experiments this evidence is almost worthless. The director of
the Meteorological Office[7] has, however, kindly sent me some details
of the weather at Breslau from 1857 to 1860, and I notice that as a
matter of fact July, 1859, was an exceptionally hot month, _having an
average of 2.67° C. above the mean_ for the twenty years 1848-1867. June
in that year was slightly (0.31° C.) below the mean and May slightly
above it (0.18° C.). August was also abnormally hot, 2.35° C. above the
average. The Breslau wheat was sown on _May 19_ and harvested on August
6. There was a cold spell from May 11 to 14, which this wheat escaped,
as it was sown on May 19. In the other years the cold spell came much
later. These elements of the weather may possibly have done something to
hurry the ripening in 1859. It unfortunate that we are not told how long
similar wheat from Breslau seed took to ripen in that year.
As regards the Norway cultivations we have the average monthly
temperatures recorded by Schübeler, though he does not discuss them
in connection with this special problem. It is quite clear that 1857,
in which the period was 103 days, was an exceptionally cold summer,
especially as regards the months of June and July, but though there
was, so far as the temperature records go, no great difference between
1858 and 1859, the year 1859, in which the period of ripening was the
shortest, was somewhat colder in Norway than 1858. But we have the
further difficulty that there were ten days difference in sowing, for
in 1858 the sowing was made on May 14, and in 1859 on May 24. With
all these possibilities uncontrolled, and indeed unconsidered, I am
surprised that Semon should claim these experiments as one of the chief
supports for his views.
Schübeler's other allegations respecting the influence of climate on
plants grown in various places and especially at different elevations
in Norway have been destructively criticised by Wille[8] to whose paper
readers interested in the subject should refer.
Before the appearance of Wille's criticisms Wettstein[9] made a
favourable reference to Schübeler's work, accepting his conclusion. He
states also that he has himself made analogous experiments with flax,
finding that the length of the period of development and a series of
morphological characters show an adaptation to local conditions, and
that on transference of seed to other conditions the previous effects
are maintained. No details, however, are given, and I do not know if
anything more on the subject has appeared since. The other examples
cited by Wettstein, such as the observations of Cieslar on forest-trees
and those of Jakowatz on gentians seem to me open to all the usual
objections applicable to evidence of this kind. Such work, to be of any
value for the purpose to which it is applied, must be preceded by a
study of the normal heredity and of the variations of the species.
Most of the recent writers (Semon, Przibram, etc.) on the inheritance
of acquired characters accept the story of Brown-Séquard's guinea pigs,
which are said to have inherited a liability to peculiar epileptiform
attacks induced in their parents by various nervous lesions.
The question has been often debated and several observers have repeated
the experiments with varying results, some failing to confirm
Brown-Séquard, others finding evidence which in various degrees
supported his conclusions. Recently a new and especially valuable paper
has been published by Mr. T. Graham Brown[10] which goes far towards
settling this outstanding question. He states that "the Brown-Séquard
phenomenon is nothing more or less than a specific instance of the
scratch-reflex," and it is due to a raised excitability of the mechanism
of this reflex. This raised excitability is the character acquired as a
consequence, for instance, of the removal of part of one great sciatic
nerve. The nature of this raised excitability and its causation are
discussed and elucidated, but this part of the work is not essential
to the present consideration. Mr. Graham Brown in his summary of
conclusions remarks that it is very difficult to see how this condition
of raised excitability can be transmitted to the offspring, and this
comment which might be made in reference to any of the alleged cases
certainly applies with special cogency to the present example.
He then calls special attention to three observations:
1. That guinea pigs which had a "trophic" change in the foot, as a
result of division of the great sciatic nerve, have repeatedly been seen
to nibble the feet of other guinea pigs which had this change in the
foot from the same causes.
2. That accidental injury to the toes may be followed by the
Brown-Séquard phenomenon in an otherwise normal animal.
3. That in several instances the young of guinea pigs which exhibited
the phenomenon have been noticed to have one or more toes eaten off by
the mother.
Brown-Séquard noticed that almost all his animals in which the great
sciatic was divided acquired the "epilepsy" and nibbled those parts of
their feet in which sensation had been lost. Of the offspring of such
animals he found that a very small proportion exhibited a malformation
of the feet, and of these some showed the "epilepsy." The proportion
which showed the "epilepsy" was one to two per cent. of the offspring.
Morgan[11] is quoted by Graham Brown as having suggested that the
loss of toes in the offspring may have been due to mutilation by the
mother, following his experience in a case in which the tails of mice
in succeeding litters were thus devoured, and there can be little doubt
that in this suggestion lies the clue to the explanation of the whole
mystery. Graham Brown concludes that it may be supposed with every
degree of probability that the "transmission" was due to injuries
inflicted upon the young by their parents. With this conclusion most
people will now be disposed to agree, and we may hope that we shall
hear the last of this curious myth--to the elucidation of which a vast
quantity of research has been devoted.
The series of experiments made by Kammerer with various Amphibia have
attracted much attention and have been acclaimed by Semon and other
believers in the transmission of acquired characters as giving proof of
the truth of their views. With respect to these observations the chief
comment to be made is that they are as yet unconfirmed. Many of the
results that are described, it is scarcely necessary to say, will strike
most readers as very improbable; but coming from a man of Dr. Kammerer's
wide experience, and accepted as they are by Dr. Przibram, under whose
auspices the work was done in the Biologische Vesuchsanstalt at Vienna,
the published accounts are worthy of the most respectful attention.
The evidence relates chiefly to three distinct groups of occurrences:
1. Modification in _Alytes obstetricans_, the Midwife Toad, affecting
both the structure and the mode of reproduction, induced by compulsory
change of habits.
2. Modification in the mode of reproduction of _Salamandra atra_ and
_maculosa_ induced by compulsory change of habits.
3. Modification in the colour of _Salamandra maculosa_ induced by change
in the colour of the soil on which the animals were kept.
1. I will take first the case of _Alytes_,[12] because it is the most
definite example, and because it is the case which most readily admits
of repetition and verification.
The habits of _Alytes obstetricans_ are well known. The animals copulate
on land. As the strings of eggs leave the female they are entangled by
the hind legs of the male, and being adhesive they stick to him and
undergo their development attached to his back and legs. The number of
eggs varies from 18 to 86, a number much smaller than is usual in toads
and frogs which lay their eggs in water. The eggs are large and full of
yolk.
There are two breeding seasons, one about April and the other about
September, and a winter hibernation. Not only animals brought in from
outside, but their offspring reared in domestication maintain these
normal habits in confinement, if the temperature does not exceed 17° C.
(pp. 499 and 534).
If, however, the temperature be artificially raised and kept at 25-30°
C., the males do not attach the eggs to themselves when spawning occurs
on land but let them lie. The adhesion of the eggs is said to be
hindered by the comparatively rapid drying of their surfaces.
More usually in the high temperatures the animals _take to the water_
and copulate there. The eggs are ejected into the water, and as their
gelatinous coverings immediately swell up, they do not stick to the
males.
The offspring thus derived from the parents subjected to heat for one
breeding-period only, whether they were laid in water or on land, did
not show departures from the normal type.
Kammerer states next, however, that in subsequent breeding-periods the
same parents frequently take to the water to breed, though they have
become quite accustomed to the heated chamber; and furthermore that if
such animals, having thus lost their instinct to brood their young, be
transferred to ordinary temperatures they do not readily reassume their
normal habits, but for several breeding seasons--at least four--will
take to the water. These parents lay from 90 to 115 eggs, which are
small and contain little yolk, and the larvae, on hatching, breathe with
their embryonic gills until they are absorbed instead of being broken
off as normally.
The offspring thus abnormally developed when they mature are said never
to brood their eggs. If they are derived from the earlier spawnings of
their parents, before, that is to say, the parents had been submitted
to the changed conditions long enough to transmit their effects, they
lay on land; but if they are derived from the later spawnings, they
lay in the water. These changes of habit are manifested without the
continued application of the abnormal experimental conditions, and, as I
understand the account, in normal conditions of temperature.
If the abnormal experimental conditions are continued, the toads always
lay in water, and their eggs become progressively smaller and more
numerous. The larvae in the fourth generation acquire three pairs of
gills instead of one pair, and are in other respects also different from
the normal form.
Respecting the _Alytes_ bred in this way Kammerer makes the very
striking statement that _the males in the third generation_ (p. 535)
_have roughened swellings on their thumbs and that in the fourth
generation_ (pp. 516 and 535) _these swellings develop black pigment_.
Together with the appearance of this secondary sexual character there
is hypertrophy of the muscles of the fore-arm. To my mind this is
the critical observation. If it can be substantiated it would go far
towards proving Kammerer's case. _Alytes_, among toads and frogs, is
peculiar in that the males do not develop these lumps in the breeding
season, and the fact may no doubt be taken to be correlated with the
breeding habits, copulation occurring on land and not in water as is
usual with Batrachians. It is to be expressly noticed that these lumps
on the thumbs or arms of male toads and frogs are not merely pigmented
swellings, but are pads bearing numerous minute horny black spines,
which are used in holding the females in the water. The figures which
Kammerer gives (Taf. XVI, figs. 26 and 26a) are quite inadequate, and as
they merely indicate a dark patch on the thumbs it is not possible to
form any opinion as to the nature of the structure they represent.
The systematists who have made a special study of Batrachia appear to
be agreed that _Alytes_ in nature does not have these structures; and
when individuals possessing them can be produced for inspection it
will, I think be time to examine the evidence for the inheritance of
acquired characters more seriously. I wrote to Dr. Kammerer in July,
1910, asking him for the loan of such a specimen[13] and on visiting
the Biologische Versuchsanstalt in September of the same year I made
the same request, but hitherto none has been produced. In matters of
this kind much generally depends on interpretations made at the time
of observation; here, however, is an example which could readily be
attested by preserved material. I notice with some surprise that in a
later publication on the same subject no reference to the development of
these structures is made (see below).
The statements here given represent but a small part of Kammerer's
papers on the subject. He gives much further information as to the
course of the experiments, especially in regard to the fate of the eggs
laid on land and the aberrations induced in them by treatment. The
ramifications of the experiments are, however, very difficult to follow,
and as I am not sure that I have always understood them I must refer the
reader to the original.
More recently Kammerer has published[14] a most curious account of
experiments in crossing his modified and abnormal _Alytes_, derived from
the water-eggs, with normal individuals.
In the first case the cross was made between a _normal female_ and an
_abnormal male_. The offspring were normal in their habits. In the next
generation bred from these almost exactly a quarter showed the abnormal
instinct.
The reciprocal cross was made between an _abnormal female_ and a _normal
male_. In this case the offspring were abnormal in their behaviour; but
the second generation bred from them showed three quarters abnormal and
one quarter normal.
Certain details as to numbers and sexes of the various families bred
in the course of this amazing experiment are given in a subsequent
publication.[15] This later paper goes somewhat fully into the question
of the difference in behaviour between the normal and modified
individuals, describing the ways in which the males and females
possessing the acquired character could be recognised from the males and
females which were normal, but in this account I find no reference to
the development of the "_Brunftschwielen_"--the horny pads on the hands
of the males. As these structures would be of special value in such a
diagnosis the omission of any allusion to them calls for explanation.
Kammerer claims the evidence as proof of Mendelian segregation in
regard to an acquired character, the first example recorded. Pending a
repetition of the experiments there is no more to be said.
2. _The Mode of Reproduction of Salamandra atra and
maculosa._[16]--_Salamandra maculosa_, the common lowland form, with
yellow bands or spots, deposits its young in water, generally as
gill-bearing tadpoles, with a wide, swimming tail, though occasionally
they are born still enclosed in the egg-capsule out of which they soon
hatch. Spawning extends over a considerable period, often many weeks,
and during the season one female may bear more than 50 young.
_S. atra_, the black Alpine form, produces its young on land. They are
born without gills, ready to breathe air, and with the rounded tail
of the adult. These differences may, as Kammerer says, naturally be
regarded as adaptations to the Alpine conditions. Moreover, the female
bears _only two_ young in a season, and this reduction in the number
must be taken to be a consequence or condition of viviparity. There are
many eggs in the ovary, but all except the two which are destined to
develop degenerate and form a yolk-material on which these two survivors
feed.
Kammerer gives a long account of the various conditions to which he
subjected both species. The treatment was complicated in many ways, but
the essential statements are, as regards _S. maculosa_, that when no
water was provided in which the young might be born, they were dropped
on land, larger and in a later stage of development and of a darker
colour than is normal; that the larvae so born gradually diminished
in number until only two were deposited in each breeding-period;
that dissection showed that the other ova degenerated to form a
yolk-material. The larvae so produced reached maturity. The summary of
results describes their behaviour, stating that they produced:
(_a_) _In water_, either (1) _very_ advanced, large-headed larvae 45
mm. long (instead of 25-30 mm.) with gills already reduced, which had
awkward, embryo-like movements, and in some few days metamorphosed
into small perfect salamanders; or (2) moderately advanced, properly
proportioned larvae, 40-41 mm. long, provided with large gills of (at
first) intrauterine character, which were reduced during aquatic life.
(_b_) _On land_, small (26 mm. long) larvae with rudimentary gills,
having the body rounded instead of being flattened from above downwards,
and an elongated narrow head, which were unable to live in deep water.
These larvae changed to the salamander colour in 10-12 days, and after
four weeks metamorphosed into salamanders 29 mm. long.
(_c_) In the foregoing cases the experimental conditions were not
continued, or in other words, basins of water were provided in which
they could spawn. But if the experimental conditions are continued,
these _Salamandra maculosa_ which were born newt-like (viz., not in a
larval condition), are themselves newt-bearing from the first time they
give birth, using the dry land, and bringing forth only two young, the
normal number for the births of _S. atra_. These young are 40-41 mm.
long, and are dark-coloured, resembling greatly the normal new-born _S.
atra_.
This epitome of the observations illustrating the inheritance of
acquired characters has been very widely quoted, and may not unnaturally
be taken to summarize a wide experience of the modified animals.
Reference to the details given in the same paper shows that, as alleged,
each of the four types of behaviour enumerated was witnessed _once_ only
in the case of each of four females, no two agreeing with each other.
As to the number of the males or their habits nothing is said. The first
female, _a_ (1), bore five young; the second, _a_ (2), bore two, of
which one was a partial albino; the third, _b_, produced four young; and
the fourth, _c_, two as already stated.
In the case of _c_ the details show that the female gave birth
immediately after being transferred from the open-air terrarium to
one indoors, which contained no basin of water. This is the example
of the consequences which follow on a continuance of the experimental
conditions.[17]
As regards _S. atra_ the converse is reported. Various means were
used to induce them to eject their young prematurely in water, such
as massaging the sides of the mothers, or raising the temperature to
25° or 30° C., with various degrees of success. But afterwards it was
found that specimens collected wild at an elevation of about 1,000
metres responded to much simpler treatment, and gave birth prematurely
in water when they were kept in a large shallow basin of water not so
deep but that they could everywhere touch the bottom with their feet and
keep their heads above the surface. With specimens collected at higher
elevations this treatment was inoperative, and the suggestion is made
that _S. atra_ at the lower confines of its habitat partakes more of the
nature of _maculosa_ than do the individuals from greater heights; for
Kammerer argues that pools suitable for breeding must be more uncommon
at those elevations than they are lower down.
In the earlier paper[18] Kammerer states that newly caught females
of _S. atra_ often give birth in the water, and show an undoubted
preference for doing so. He describes also how he once saw several
females, wild in their natural habitat, lay their young in a rain-puddle
at 1,800 metres elevation, but the larvae thus born were fully formed.
When the deposition of the young as larvae has become "habitual"[19]
with _S. atra_, three to nine larvae may be produced at one spawning
period, from 35 to 45 mm. long, with gills at most 8 mm. long, and a
tail-fin 2-3 mm. broad. Such larvae are generally coffee-brown, or grey
(instead of black), and show other minor differences.
The summary states that when grown to maturity they become in their turn
larva-bearing, and go into the water to bring forth. Their young are
more than two (3 to 5 being the numbers observed) with a length of 33-40
mm. or of 21-23 mm. at birth. They are light grey, spotted (mottled with
lighter and darker colour), have relatively short gills (8 to 9 mm.
at most) and a broad tail-fin (3 mm. wide). At metamorphosis they are
relatively long (44 mm.) and one of them had some yellow pigment.
Here again this summary is, as a matter of fact, describing the
behaviour of two mothers, of which one produced three, and the other
five young.
To my mind these experiments suggest that the reproductive habits
of both species, if closely observed, will be found to be subject
to considerable variation, and I think it not impossible that each
species is, especially in confinement, capable of being a good deal
deflected from its normal behaviour. Moreover, there seems to me no
great improbability in the idea that there is an interdependence
between the number of young and the stage of maturity in which they
are born. But, at the same time, the case as told by Kammerer strikes
me as proving too much. If each species is so sensitive to conditions
that the normal procedure is gravely modified in one generation, and
if that modification can reappear in a pronounced form in the next
generation without a renewal of the disturbing conditions, it becomes
extremely difficult to understand how the regularity which each species
is believed to display in nature can be maintained. Surely both species
might be expected to be in confusion. From a passage in Kammerer's
earlier paper (1904, p. 55) on the subject, I infer that he also would
expect considerable irregularity in the natural behaviour, but that he
has not investigated the point.[20]
3. _Modification of the Colour of Salamandra maculosa induced by Change
in the Colour of the Soil on which the Animals were kept._--Kammerer
speaks of this as the most convincing of all his experiments on the
transmission of acquired characters. So far, however, no full account of
them has been published.[21] The statement is that when salamanders are
kept in yellow surroundings the yellow markings gradually in the course
of years increase in amount relatively to the black ground colour.
Conversely by keeping the animals on black garden soil, the yellow may
be greatly diminished in quantity until it largely disappears. (The
account in _Natur_ adds that very moist conditions also favour the
increase of yellow, and that with less moist conditions the yellow
diminishes.) From each kind, the (induced) yellower and the (induced)
blacker, a second generation was raised, on soil of neutral colour, and
each family was later divided into two parts, half being put on black
and half on yellow ground.
As regards the offspring of those which had lived on _black_ soil no
positive result had been reached up to the date of publication, but it
is stated that these young resembled their parents in having the yellow
distributed in _irregular spots_.
As regards the offspring of those which had lived on yellow soil the
account follows up the story of that part of the offspring which were
put on yellow soil again. It is stated that these, though derived from
parents with irregular spots, _developed the yellow as longitudinal
bands_.
This account is given with slight differences of expression in the
three places to which I have referred. On returning from Vienna in
1910 I consulted Mr. G. A. Boulenger in reference to the subject, and
he very kindly showed me the fine series from many localities in the
British Museum, and pointed out that in nature the colour-varieties
can be grouped into two distinct types, one in which the yellow of
the body is irregularly distributed in spots and one in which this
yellow is arranged for the most part in two longitudinal bands which
may be continuous or interrupted. _The spotted form is, as he showed
me, an eastern variety, and the striped form belongs to western
Europe._ Mr. E. G. Boulenger[22] has since published a careful account
of the distribution of the two forms. The spotted he regards as the
typical form, var. _typica_, and for the striped he uses the name
var. _taeniata_. The typical form occupies eastern Europe in general,
including Austria and Italy, extending as far west as parts of eastern
France. The var. _taeniata_ is found all over France, excepting parts of
the eastern border, Belgium and western Germany, Spain and Portugal. Of
the very large series examined there was only one specimen (Lausanne)
which could not with confidence be referred to one or other of the
two varieties. Mr. E. G. Boulenger points out that both varieties
inhabit very large areas, and live on soils of most different colours
and compositions. Both are liable to variations in the amount and the
shade of the yellow, but that any suggestion that _taeniata_ belongs
especially to yellow soils and _typica_ to black soils is altogether
inadmissible. He expresses surprise that Kammerer should not allude to
these peculiarities in the geographical distribution of the two forms.
He suggests further that it is more likely that some mistake occurred in
Kammerer's observations than that the east European _typica_ should, in
the course of a generation, have been transformed into the west European
_taeniata_ by the influence of yellow clay soil.
In his last paper on the subject Kammerer states incidentally[23] that
he has found the _striped form recessive to the spotted_. No evidence
for this statement is given, and I have not found any other reference
to crosses effected between the two natural types. If, however, this
representation is correct, it is conceivable that the production of
_taeniata_ from _typica_ was in fact the re-appearance of a recessive
form. The plate which Kammerer gives in illustration of his modified
parent figures a single animal at four stages, and though it is
certainly more like the spotted than the striped form, it has a certain
suggestion of the striped arrangement, such as I can well imagine being
produced in the heterozygote.[24]
In continuation[25] of the experiments on the colour of _S. maculosa_
Kammerer publishes an account of elaborate experiments in grafting
ovaries of the various forms, modified and unmodified, into each other,
and describes the offspring which followed. Before pursuing this part of
the inquiry I am disposed to wait until the earlier steps have been made
much more secure than they yet are.
More recently Kammerer has published similar statements in regard to the
inheritance of characters induced in various lizards by keeping them in
abnormal temperatures, high and low. The changes induced affected in
some species the colours, in others the reproductive habits. Respecting
these examples I feel the same scepticism that I have indicated in
regard to the others, somewhat heightened by the fact that insufficient
evidence is given both regarding the behaviour of these various species
in captivity when not subjected to abnormal temperatures, and in the
wild state.
Respecting this part of the evidence Mr. G. A. Boulenger has lately
published a criticism[26] from which I extract the following passages.
Referring to a previous note[27] on the question of the melanism of the
various insular forms of _Lacerta muralis_ he writes: "I also alluded
(_l. c._) to the theories that have been propounded to explain the
melanism of various insular forms. This is a subject which has been
lately taken up by Dr. Kammerer at the Biologische Versuchsanstalt in
Vienna, and he claims to have produced nigrinos artificially by a very
strong elevation of the temperature, accompanied by extreme dryness. Dr.
Werner[28] has already opposed his own experiments to those of Kammerer,
artificial melanism having been produced by him in _Lacerta oxycephala_
by keeping two very light specimens from Ragusa for a whole summer
in very damp conditions. Neither is Kammerer's theory in accordance
with the distribution of the black lizards, as pointed out by Werner.
Kammerer also finds that those forms which are known to produce melanic
races in a state of nature, lend themselves more readily than the others
to the success of his experiments. But he shows himself misinformed
when he states that the variety called _Lacerta fiumana_ belongs to the
category of those of which black forms are not known. He overlooks the
fact, first pointed out by Scherer in 1904, and which I can confirm,
that the black lizard from Melisello near Lissa in the Adriatic is
unquestionably derived from the lizard from Lissa, which he correctly
regards as not separable from _L. fiumana_...."
"Another colour modification which Dr. Kammerer states that he obtained
by raising the temperature is the assumption by the female of the
typical _Lacerta muralis_ of the bright red colour of the lower parts
which often distinguishes the male from the female, and which was not
shown by the individuals of the latter sex kept by him under normal
conditions. He quotes various authorities to show that the lower parts
are never red in the females, but he has omitted to consult others who
say the contrary. Thus Bedriaga (1878 and 1879) remarks that a so-called
var. _rubriventris_ of the typical wall lizard has the lower parts red
in both sexes."[29]
In reading such papers as those of Semon or Kammerer the thought
uppermost in my mind is that to multiply illustrations of supposed
transmission of acquired characters is of little use until some one
example has been thoroughly investigated. If we had certain assurance
that even a single unimpeachable case could be repeated at will, the
whole matter would assume a more serious aspect. If, for instance,
Kammerer were able to show us _Alytes_ males with horny pads on their
hands, it would be something tangible; still more, if the experiment
were repeated by others until no doubt remained that the offspring
of _Alytes_ which had bred in water for some three generations did
acquire these pads and that they could transmit these novelties to
descendants raised in normal conditions. Till evidence of this kind is
published by at least two independent observers investigating similar
material, I find it easier to believe that mistakes of observation or
of interpretation have been made than that any genuine transmission of
acquired characters has been witnessed.
Meanwhile there is no denying that the origin of adaptational features
is a very grave difficulty. With the lapse of time since evolutionary
conceptions have become a universal subject of study that difficulty
has, so far as I see, been in nowise diminished. But I find nothing in
the evidence recently put forward which justifies departure from the
agnostic position which most of us have felt obliged to assume.[30]
APPENDIX TO CHAPTER IX.
Professor G. Klebs, as is well known to students of evolutionary
phenomena, has for several years been engaged in investigations relating
to the inheritance of acquired characters. In his many publications
on the subject the issue has always been represented as more or less
uncertain.
Desiring to know how the matter now stands according to Professor Klebs'
present judgment I wrote to him asking him to favour me with a brief
general statement. This he most kindly sent in a letter dated 8th July,
1912.
As such a statement will be read with the greatest interest by all who
are watching the progress of these studies I obtained permission to
publish it as follows:
8. Juli 1912
Ihre liebenswurdige Anfrage will ich sehr gern
beantworten, obwohl ich sie nicht so beantworten kann wie ich
erwünschte. Ihr Skepticismus in der Frage der Uebertragung
erworbener Charactere auf die Nachkommen ist nur zu berechtigt.
Meine Versuche mit Veronica sind _nicht_ beweisend, da es mir
bisher nicht gelungen ist eine einigermasse konstante Varietät
mit verlaubten Inflorescenze zu erzeugen. In Bezug auf mein
Semper vivum bin ich allerdings noch heute der Meinung dass
die starke künstliche Veränderung der Blüte einen Einfluss auf
einzelnen Nachkommen gehabt hat. Ich habe seither nichts darüber
veröffentlicht: die Mehrzahl der anormalen gefüllten Blüten war
leider steril. Von einem weniger veränderten Exemplar erhielt
ich einige Sämlinge, aber sie haben noch nicht geblüht. Es kann
sich in diesem Falle nur um eine _Nachwirkung in der ersten
Generation_ handeln, vergleichbar jenen Fällen in denen Samen von
Bäumen aus den hohen Alpen in der Ebene gewisse Nachwirkungen
zeigen. Aber es ist bisher kein sicherer. Fall bekannt in den
der kunstliche herbeigeführte Charakter _mehrere Generationen
hindurch unter der gewöhnlichen "normalen" Bedingungen_
übertragen worden ist.
Auf der andere Seite sind diese negativen Resultaten nicht
entscheidend. Denn wie wenig ist in dieser Beziehung überhaupt
ernstlich versucht worden! Und zweifellos geht die Sache nicht so
einfach.
Ich versuche es mit anderen Pflanzen weil ich der Meinung bin
dass es möglich sein müsse wenigstens solche neuen Varietäten zu
erzeugen, wie sie die Gartenvarietäten entsprechen.
Aber bis jetzt leider sind die Versuche nicht gelungen, weder
mir noch irgend einem anderen.
FOOTNOTES:
[1] Semon, R., Der Stand der Frage nach der Vererbung erworbener
Eigenschaften, published in _Fortschr. der naturw. Forschung._, Bd. 11,
1910.
[2] Standfuss, M., _Denks. Schweiz. naturf. Ges._, XXXVI, 1898, p. 32.
[3] Fischer, E., _Allg. Ztschr. f. Entomologie_, Bd. VI, 1901.
[4] Out of 12 pupae treated 8 died and of the 4 survivors, one only was
affected. See M. v. Linden, _Archiv. Rassen. u. Gesells._, 1904, I.
[5] For illustrations see _Oberthur's Études d'Entom._, 1896, where many
of these curious aberrations are represented; also Barrett, _Lepid.
Brit. Islands_, II, pp. 71 and 72.
[6] Schübeler, F. C., _Die Culturpflanzen Norwegens_, 1862, especially
pp. 24 and 28.
[7] I am obliged to him and to Dr. E. Gold for much trouble taken to
answer my questions. Some idea of the kind of weather indicated by an
average of 2.76° C. above the mean may be got from a comparison with the
year 1911, which most people will remember as one of the hottest summers
they have known. The July of that year was in east and southeast England
about 4° F. above the mean but 2.67 C. means about 4.8° F. above the
mean. At Greenwich July, 1859, was about 6.5° F. above the average.
[8] Wille, N., _Biol. Cbltt._, XXV, 1905, p. 521.
[9] Wettstein, R. von. _Der Neo-marckismus u. seine Beziehungen zum
Darwinismus_, Jena, 1903.
[10] T. Graham Brown, _Proc. Roy. Soc._, 1912, vol. 84, B, p. 555. This
paper gives full reference to the previous literature of the subject.
[11] Morgan, T. H., _Evolution and Adaptation_, New York, 1903.
[12] Kammerer's chief paper on this subject is in _Arch. f. Entwm._,
1909, XXVIII, p. 447, and it is to this that the paginal references in
the present text relate. His previous paper appeared, _ibid._, 1906,
XXII, p. 48. An account of his further experiments with _Alytes_ is
given in _Natur_, 1909-10, Heft 6, p. 95.
[13] In reply to my letter Dr. Kammerer who was then away from home
very kindly replied that he was not quite sure whether he had killed
specimens of _Alytes_ with "_Brunftschwielen_" or whether he only
had living males of the fourth generation, but that he would send
illustrative material.
[14] Kammerer, P., _ Natur_, 12 December, 1909, Heft 6, p. 95, repeated
in _12 Flugschrift d. Deutsch Ges. f. Züchtungskunde_, Berlin, 1910.
[15] _Festschrift zum Andenken an Gregor Mendel_, being vol. XLIX of the
_Verh. Naturf. Ver. in Brünn_, 1911, p. 98.
[16] Kammerer's chief papers on this subject are _Archiv fur Entwm._,
XVII, 1904, and _ibid._, XXV, 1907. An epitome of results is also given
by him in _12 Flugschrift d. Deutsch. Ges. f. Züchtungskunde_, Berlin,
1910.
[17] "_Bei Fortdauer der Versuchsbedingungen sind als Vollmolche
geborene Salamandra maculosa_ gleich bei der ersten Geburt _abermals
voll molchgebärend_, benutzen zum Geburtsakt das trockene Land,
und zwar unter Erreichung der (bei _Salamandra atra_ normalen)
_Embryonen-Zweizahl_," Kammerer, 1907, p. 49.
[18] 1904, p. 56.
[19] Throughout Kammerer's papers this is used almost as a technical
term. It means, I presume, that the feature was manifested more than
once.
[20] It should be stated that the papers contain a quantity of detail,
especially descriptive of the state of the larvae, which I have not
attempted to represent, but the account here given contains all that
seemed essential to an understanding of the more important features of
the account.
[21] The first appeared in _Natur_, 1909-10, Heft 6, p. 94; and the
second, which contains coloured plates of the animals, in the lecture
already referred to, _12 Flugschr. d. Deut. Ges. f. Züchtungkunde_,
Berlin, 1910, p. 26. In the paper in _Mendel Festschrift_, 1911, the
subject is continued, but no more is added as to this part of the
experiment.
[22] E. G. Boulenger, _Proc. Zool. Soc._, 1911, p. 323.
[23] _Mendel Festschrift_, 1911, p. 84.
[24] _12 Flugschrift. Deut. Ges. Züchtungskunde_, 1910, Fig. 15, _P.
Reihe_.
[25] _Mendel Festschrift_, 1911, p. 83.
[26] Field, 1912, 30 March.
[27] _Ibid._, 1904, p. 863.
[28] _Mitth. Naturw. Ver. a. d. Univ. Wien_, 1908, p. 53.
[29] As to the variations of _Lacerta muralis_ in Western Europe and
North Africa see Boulenger, G. A., _Trans. Zool. Soc._, 1905, vol. XVII,
p. 351.
[30] As to the experiments of Klebs relating to the transmission of
acquired characters, see Appendix.
CHAPTER X
EFFECTS OF CHANGED CONDITIONS CONTINUED
THE CAUSES OF GENETIC VARIATION
In the last chapter we examined some of the evidence offered in support
of the belief that adaptation in highly organised forms is a consequence
of the inheritance of adaptative changes induced by the influence of
external conditions. The state of knowledge of this whole subject is, as
I have said, most unsatisfactory, chiefly for the reason that in none of
the cases which are alleged to show a positive result have two observers
been over the same ground, or as yet confirmed each other. In the wider
consideration respecting the causes of variation at large we find
ourselves still in the same difficulty. The study has thus far proved
sadly unfruitful. In spite of the considerable efforts lately made by
many observers to induce genetic variation in highly organised plants
or animals, and though successes have occasionally been announced, I
do not know a single case which has been established and confirmed in
such a way that we could with confidence expect to witness the alleged
phenomena if we were to repeat the experiment. Abundant illustrations
are available in which individuals exposed to novel conditions manifest
considerable changes in characters or properties, but as yet there is
no certain means of determining that germ-cells of a new type shall be
formed.
Of the direct effect of conditions the lower organisms, especially
bacteria, offer the best examples, the alterations of virulence which
can be produced in so many distinct ways being the most striking
and familiar. That attenuation of virulence can be produced by high
temperatures or by exposure to chemical agents, and that this diminution
in virulence may remain permanent is, from our point of view, not
surprising; but the fact that in many cases the full virulence can by
suitable cultivation be restored is difficult to understand. Similar
variations have been observed in power of pigment production and other
properties.
These phenomena naturally raise the question whether any cases of
apparent loss of factors in higher forms may be comparable.
The subject of variations in the lower organisms and their dependence on
conditions is a highly special one, and I have no knowledge which can
justify me in offering any discussion of them, but I understand that
hitherto little beyond empirical recognition of the phenomena has been
attempted. A useful summary of observations made by many investigators
was lately published by Hans Pringsheim,[1] who enumerates the different
agencies which have been observed to produce modifications, and the
various ways in which these changes are manifested. One of the most
comprehensive studies of the subject from the genetic point of view is
that made by F. Wolf.[2] In his extensive cultivations of _Bacillus
prodigiosus_, _Staphylococcus pyogenes_ and _Myxococcus_ he succeeded
in producing many strains with modified properties. In most of these
the modifications arose in consequence of the application of high or
low temperatures or of the addition of various chemical substances
to the culture-media. Some of the variations, which are for the most
part in the powers of pigment-formation, persisted when the strains
were returned to normal conditions, and others did not. In reference
especially to the variations witnessed in the Cocci the reader should
consult the critical account of variation in that group published by
the Winslows,[3] where much information on the subject is to be found.
The authors attempted to determine the systematic relationships of the
several forms, as far as possible, by the application of statistical
methods. The result is interesting as showing that the problem of
species in its main features is presented by these organisms in a form
identical with that which we know so well in the higher animals and
plants, whatever properties be selected as the diagnostic characters.
There are many types perfectly distinct and others which intergrade.
Some of the types change greatly with conditions while others do not.
This is exactly what we encounter whenever we study the problem of
species on an extended scale among the higher forms of life.
There is now practically complete agreement among bacteriologists
that the observations made first by Massini on the change in color of
_Bacterium coli mutabile_ grown in Endo's medium, associated with the
acquisition of the power to ferment lactose, are perfectly reliable
and free from possibilities of mistake. The work has been extended and
confirmed by many workers, especially R. Müller, who finds that this
bacterium can similarly acquire and maintain the power to ferment other
sugars. A careful account of the whole subject written by Müller for the
information of biologists will be found in _Zts. für Abstammungsl._,
VIII, 1912. After discussing the biological significance of the facts,
he concludes with a caution to the effect that bacteria are so different
from all other living things that generalizations from their behavior
must not be indiscriminately applied to animals and plants.
In all work with this class of material there is obviously danger of
error through foreign infection of the cultures, but there can be no
doubt that though some of the "mutations" recorded may be due to this
cause, the majority of the instances observed under stringent conditions
are genuine.
Another and equally serious difficulty besetting work with bacteria
and fungi cultivated from spores is that the appearance of variation
may in reality be due to the selection of a special strain previously
living masked among other strains. This possibility must be remembered
especially in those instances which are claimed as exemplifying the
effects of acclimatisation. Manifestly this consideration can be urged
with most force when the strain which gave rise to the novelty was
not raised from a single individual spore. Moreover, when once the
possibility of spontaneous variation is admitted, it must be difficult
to be quite confident that any given variation observed is in reality
due to the novel conditions applied, and as I understand the evidence,
the appearance of the mutational forms does not with any regularity
follow upon the application of the changed conditions.
Researches into the variation of these lower forms will, no doubt, be
continued on a comprehensive scale. So long as the instances recorded
are each isolated examples it is impossible to know what value they
possess. If they could be coordinated in such a way as to provide some
general conception of the types of variation in properties to which
bacteria, or any considerable group of them, are habitually liable, the
knowledge might greatly advance the elucidation of genetic problems.
Of mutational changes directly produced with regularity in
micro-organisms by treatment, the experiments with trypanosomes provide
some of the clearest examples. A summary of the evidence was lately
published by Dobell,[4] from which the present account is taken. The
most definite fact of this kind established is that certain dyes
introduced into the blood of the host have the effect of destroying
the small organ known as the "kinetonucleus" in the trypanosomes. The
trypanosomes thus altered continue to breed, and give rise to races
destitute of kinetonuclei. This observation was originally made by
Werbitzki and has been confirmed by several observers. The exact way
in which this alteration is effected in the trypanosomes is not quite
definitely made out, but there is good reason for supposing that
the dyes have a direct and specific action upon the kinetonucleus
itself, and circumstances make it improbable that in some division a
daughter-organism without that body is produced, or that any selection
of a pre-existing defective variety occurs.
Ehrlich has suggested with great probability that the dyes which possess
this action owe it to the fact that they have the particular chemical
linkage which he calls "ortho-quinoid." In outward respects, such as
motility and general appearance, the modified organisms are unchanged,
but their virulence is diminished. As regards the possibility of the
defective strain reacquiring the kinetonucleus, Werbitzki states
that in one case passage through 50 animals and treatment with dyes
left the strain unaltered; but that in another case at the sixteenth
passage 7 per cent. of the trypanosomes were found to have re-acquired
the organ, and in subsequent passages the percentage increased, until
at the twenty-seventh passage practically all had re-acquired it.
Kudicke, however, in similar experiments did not succeed in causing
re-acquisition by transplantation.
By the action of various drugs and anti-bodies races of trypanosomes
resistant to those substances have been obtained. These breed true, at
least when kept in the same species of animal in which the resistance
was acquired. As to whether change of virulence is produced by passage
through certain animals or not, there is as yet no general agreement.
Other changes, especially in size and some points of structure, are said
to occur when certain trypanosomes proper to mammals are passed through
cold-blooded vertebrates (Wendelstadt and Fellmer), and it is stated
that these changes persist, but the observations have not yet been
confirmed.
Experiments lately conducted by Woltereck with _Daphnia_ are interesting
as having given a definite positive result, in so far, at least, as the
ova were affected by conditions before leaving the bodies of the parent
individuals. The observations relate to the offspring resulting from
_parthenogenetic_ eggs. Females bearing ephippia (fertilised eggs) were
isolated until the ephippia were dropped, and in this way the offspring
of fertilisation were excluded. Males, of course, appeared from time
to time in the cultures, but as fertilised eggs were rejected, their
presence did not disturb the result. The most remarkable observations
related to _Daphnia longispina_.
This species as found in the lower lake at Lunz had the front end of
the body blunt and nearly round in profile; but on being cultivated in
a warm temperature and with abundant nourishment the front end of the
body became produced into an elongated "helmet," as Woltereck calls it.
Experiment showed that the change was primarily due to the abundance of
food, and owing to temperature in a subordinate degree.
This distinction arose as soon as the species was taken into the
hothouse, but when the modified individuals were put back into the
original conditions, a lower temperature and scanty food-supply,
the next generation returned to their original form. After being
cultivated for two years and about 40 generations in the more favourable
conditions, when similarly put back into the lower temperature with
scanty food the _first generation_ born in these conditions was helmeted
like the modified parents. Woltereck is of opinion that the ova were
still unformed at the time the parents were put back, and the influence
of the favourable conditions upon the unformed ova he speaks of as a
"prae-induction." The effect never extended beyond the one generation,
after which the strain returned to its original state.
The fact that the influence on the offspring was not manifested at
first led Woltereck to expect that by more prolonged cultivation in the
favourable conditions a further extension of this influence would be
produced, but this expectation was never fulfilled, though the attempt
was made again and again.
Similar experiments were made with _Hyalodaphnia cucullata_, which is
far more sensitive to cultural influences, and in nature manifests
a considerable elongation of the helmet as a seasonal modification,
but the results were essentially the same as in the preceding case,
no modification extending beyond the first generation born after the
restoration to _normal conditions_.[5]
The only criticism of these extremely interesting results which suggests
itself is that perhaps the original appearance of the modification was
not in reality due to an _accumulated_ effect of the conditions, but
to some change in the conditions themselves which was not noticed. It
is difficult to see how length of time or even the lapse of several
generations could have so specific an effect on the race. It is no
doubt often vaguely supposed by many that a long period of time may be
necessary for the effect of climate or of other environmental conditions
to be produced in an organism which does not thus respond at first. I
have never been able to see any reason for this opinion nor how it is
to be translated into terms of physiological fact, and I imagine that
in those cases in which the lapse of time is really required for the
production of an effect, the influence of the prolongation is rather
on the conditions than on the organisms. The response of the organisms
thus probably indicates not that the creature is at length feeling the
effects because of their accumulated action on itself, but that the
conditions have at length ripened.
As this sheet is passing through the press Agar has published[6] an
abstract of evidence as to another comparable case in a parthenogenetic
strain in the daphnid, _Simocephalus vetulus_. When fed on certain
abnormal foods the shape of the body is changed, the edges of the
carapace being rolled backwards so as to expose the appendages. The
offspring of animals thus modified showed similar modification in the
first, and to a very slight degree, in the second generation, though the
original mothers were removed to normal conditions before their eggs
were laid. In the third generation there was "a very pronounced reaction
in the opposite direction." Agar suggests that the change may be due to
some toxin-like substances, carried on passively by the egg into the
next generation, against which the protoplasm eventually produces an
anti-body.
The experiments which have been in recent years regarded by evolutionary
writers as the most conclusive proof that direct environmental action
may produce germinal variation are those of Professor W. L. Tower, of
Chicago, on _Leptinotarsa_, the potato beetles. This work has attained
considerable celebrity and has been generally accepted as making a
definite extension of knowledge. After frequently reading Tower's papers
and after having been privileged to see some of the experiments in
progress (in 1907) I am still in doubt as to the weight which should be
assigned to this contribution.
The work is described in two chief publications, the first of which
appeared in 1906.[7] This treatise contains a vast amount of information
about numerous species and varieties of these beetles which the author
has observed and bred in many parts of their distribution throughout
the United States, Mexico and Central America. The part of the book
which has naturally excited the greatest interest is that in which Tower
states that by subjecting the beetles to change in temperature and
moisture, he caused them to produce offspring quite unlike themselves,
which in several cases bred true.
It is much to be regretted that the author did not happen to become
acquainted with Mendelian analysis at an earlier stage in the
investigation. The evidence might then have been handled in a much more
orderly and comprehensive way, and a watch would have been kept for
several possibilities of error.
The headquarters of the genus is evidently as Tower states, in Mexico
and the adjoining countries. In this region there is a great profusion
of forms, some very local, some as for instance the well-known
_decemlineata_,[8] more widely spread. The distinctions are almost all
found in peculiarities of colour and pattern, and the limits of species
are even more indefinable than is usual in multiform animals. Tower
arranges the various types into seven groups of which the one most
studied is that which he calls the _lineata_ group. To this group belong
all the forms to which reference is here made, and, as I understand,
they differ among themselves entirely in size, colour and pattern.
There is no suggestion of infertility in the crosses made between the
several forms of the _lineata_ group; in fact they present, like many
Chrysomelidae, a good example of what most of us would now call a
polymorphic species, consisting of many types, some found existing in
the same locality, others being geographically isolated.
A series of experiments was devoted to the attempt to fix strains
corresponding to the extremes of continuous variations. For example,
those with most black pigment and those with least black taken from
a population continuously varying in this respect, were separately
bred; but almost always the selection led to no sensible change in the
position of the mean of the population. The variations in these cases
were evidently fluctuational. In some instances, however, real genetic
differences were met with, and strains exhibiting them were, as usual,
rapidly fixed.
Tower points out that several of the varieties (or species, as he
prefers to call them) were obviously recessive to _decemlineata_. This
is most clearly demonstrated in the case of the form called _pallida_,
which is a pale depauperated-looking creature, with the orange of
the thorax almost white and the eyes devoid of pigment.[9] This form
behaved as an ordinary Mendelian recessive, breeding true whenever it
appeared in the cultures, or when individuals found wild were studied
in captivity. A black form which Tower names _melanicum_ was similarly
shown to be a Mendelian recessive. Wild specimens of this variety of
opposite sexes were not found simultaneously in nature, and there was
thus no opportunity of breeding them together, but the hereditary
behaviour was seen in the F_{2} generation from a _melanicum_ found
coupled with _decemlineata_. Experiments also occurred giving indication
that a variety with the stripes anastomosing in pairs (_tortuosa_), was
another recessive, and that a variety--called "_rubri-vittata_"--gave an
intermediate F_{1} with subsequent segregation. All these are forms of
_decemlineata_ Stål.
Similar observations were made regarding forms recessive to
_multitaeniata_ Stål. Of these two were thrown by _multitaeniata_
itself, namely a form named by Stål _melanothorax_, and regarded by him
as a species, and one which Tower names _rubicunda_ n. sp. The facts
proving the recessive behaviour of their several forms will be found in
the following places in Tower's book:
_pallida_, pp. 273-278.
_melanicum_, p. 279.
_tortuosa_, p. 280.
_rubrivittata_, pp. 280-281.
_melanothorax_ and _rubicunda_, pp. 283-285.
Following this evidence of recessive nature of the six forms
enumerated, Tower describes experiments showing, as he believes, that
some of them may be caused to appear by applying special treatment
to the parents during the "growth and fertilisation" (p. 287) of the
eggs. The most striking example is that in which 4 males and 4 females
of _decemlineata_ were kept very hot (average 35° C.) and dry, and at
low atmospheric pressure (19-21 inches). The eggs laid were restored
to natural conditions. These gave 506 larvae, from which emerged 14
normal, 82 _pallida_ and 2 "_immaculothorax_," viz., without pigment
on the pronotum. The account of the rest of the experiment is somewhat
involved, but I understand that the _pallida_, of which two only
survived, behaved as normal recessives when bred to the type: also that
the parents, after having laid the eggs whose history has been given,
were restored to normal conditions and laid 319 eggs which gave 61
normals.
In another case normal parents laid 409 eggs in the hot and dry
conditions, and on restoration to normal conditions, the same parents
laid 840 eggs. Then 409 eggs gave 64 adults as follows:
_Males_ _Females_
_decemlineata_ 12 8
_pallida_ 10 13
_immaculothorax_ 2 3
_albida_ 9 7
--- ---
33 31
The 840 eggs laid in normal conditions gave 123 normal _decemlineata_.
Similar experiments were made with _multitaeniata_ and gave comparable
results, the two recessives (_melanothorax_, _rubicunda_) being produced
in large numbers when the parents were subjected to heat, but in this
case the atmosphere was kept _saturated_ with moisture, instead of dry,
as in the previous instance. The same parents transferred to normal
conditions gave normals only.
Lastly the form _undecimlineata_ was exposed "to an extreme stimulus of
high temperature, 10° C. above the average," and a dry atmosphere, with
the result that from 190 eggs there emerged 11 beetles, all of the form
_angustovittata_ Jacoby, which subsequently bred true to that type (see
p. 295).
In the results of these experiments, as described, there is one feature
which I regard as quite unaccountable. Tower makes no comment upon
it. Indeed, from the general tenour of the paper, I infer, not only
that he does not perceive that he is recounting anything contrary to
usual experience, but rather that he regards the result as conforming
to expectations previously formed. The point in question is the
genetic behaviour of the dominant normals produced under the abnormal
conditions. These normals were the result of the breeding of parents
declared to be at the same time giving off many recessive gametes. Some
of these normals must be expected therefore to be heterozygous unless
some selective fertilisation occurs. Nevertheless in every case they and
their offspring are reported to have continually bred true. I allude
especially to the tables given on pp. 288, 289, 292, and 293. Tower does
not mention any misgiving about this result, and I think he regards
himself as recounting phenomena in general harmony with the ideas of
mutation expressed by De Vries. This they may be; but to anyone familiar
with analytical breeding the course of these experiments must seem so
surprising as to call for most careful, independent confirmation.
In 1910[10] Tower published an account of further experiments with
_Leptinotarsa_. The work described related to two subjects. Crosses
were made between three forms, _undecimlineata_ Stål, _signaticollis_
Stål and "_diversa_" named by Tower as a new species. The distinctions
between these three depend partly on characters of the adults and
partly on those of the larvae. The adults of _undecimlineata_ and
_diversa_ have the elytra striped, but the elytra of _signaticollis_
are unstriped. The larvae of _signaticollis_ and of _diversa_ are
yellow, but those of _undecimlineata_ are white.[11] Moreover, in
_signaticollis_ and _diversa_ the black increases in the third
stage of the larvae to form transverse bands which are absent in
_undecimlineata_. The general course of the experiments shows that these
differences may be approximately represented as due to the action of
three factors, any of which may be independently present or absent. The
stripings of the elytra and of the larvae are each due to a separate
factor. As regards the distinction between the yellow and the white
larvae the evidence does not prove that there is decided dominance of
either colour and I infer that the heterozygotes are often intermediate.
The chief contribution which this new paper claims to make relates to
differences in the results which ensue from crosses effected between
these three types at different average temperatures.
We are first concerned with four experiments which I number (1), (2),
(3), (4):
1. _Signaticollis_ [F] × _diversa_ [M] bred at an average temperature
of 80º F. by day and 75° F. by night, gave two groups in about equal
numbers. The first (49) was pure _signaticollis_ and bred true. The
second (53) was of an intermediate type, which on being bred together
gave the typical Mendelian result--1 _sig._: 2 _intermediate_: 1 _div_.
2. Next, as the account originally stood in the published paper, we
are told that _sig_ [F] × _div_ [M] bred together at a day-temp.
average 75° F. and night average 50° F. gave an _intermediate_ only,
which subsequently produced a normal 1:2:1 ratio. The two crosses were
repeated eleven times with identical results.
In a further experiment (3) _signaticollis_ [F] × _diversa_ [M] were
bred under the same conditions as those used in expt. (1). They again
gave _sig._ and intermediates as before in fairly equal numbers. The
_sig._ as before bred true, and the intermediate gave 1:2:1, all exactly
as in expt. (1).
In expt. (4) _the same parents used_ in (3) were again mated under
conditions of expt. (2) at the lower temperature, and this time gave
_signaticollis_ exclusively, which bred true for four generations. This
experiment was repeated seven times with uniform results.
Diagrams are given representing all these histories in graphic fashion.
From these observations, Tower concludes that the determination of
dominance, and the ensuing type of behaviour, is clearly a function of
the conditions incident upon the combining germ plasms.
It will be observed that expts. (1) and (3) gave identical results
but (2) and (4), though much the same conditions were applied, are
at variance, for (2) gave all intermediates, while (4) gave all
_signaticollis_. In _Amer. Nat._, XLIV, 1910, p. 747, Professor T.
D. A. Cockerell commented on this paper of Tower's and pointed out
that there must be an error somewhere, for when he discusses these
experiments Tower speaks of (2) and (4) as confirming each other. To
this Tower replied[12] that there had been a mistake. He states that
in preparing the paper "certain minor experiments were taken from
a larger series and combined to illustrate a general point in the
behaviour of alternative characters in inheritance," and that expt. (2)
was introduced inadvertently in place of another which he desires to
substitute. In this, which I number (5), _signaticollis_ [F] × _diversa_
[M] from exactly the same stocks as those used in (1), were mated at the
lower temperatures specified for (2), day average 75° F., night average
50° F. These gave all of the _signaticollis_ type with a narrow range of
variability, which bred true, in some cases to F_{6}. Tower says he has
repeated this experiment six times with identical results.
Nevertheless he proceeds to say that the description of expt. (2), which
was repeated eleven times with identical results, was correct "as far as
given." That experiment was "from a second series of cultures parallel
to the one given, but in which there are other factors involved, which
in H. 410 [my (2)] are productive of a typical Mendelian behaviour." He
adds he does "not care at this time to make any statement of what these
factors are, nor of their relations to the behaviours given in the H.
409, H. 411, H. 409/11 series [my (1), (5) and (3)--(4)] which are the
simplest and most easily presented series obtained in the crossing of
_signaticollis_ and _diversa_."
Professor Cockerell's intervention has thus elicited the fact that
we have as yet only a small selected part of the evidence before us,
even as concerning the effect of temperature on the cross between
_signaticollis_ [F] × _diversa_ [M]. We learn that at the lower
temperatures the result was eleven times the expected one, and six times
an unexpected one; further, that we owe it to the author's inadvertence
that we have come to hear of the expected result at all, and that though
he knows the factors which determine the discrepancy, he declines for
the present to name them. In these circumstances we can scarcely venture
as yet to estimate the significance of these records.
The paper goes on to recount somewhat comparable, but more complex
instances in which the descent of the colour of adults and of larvae
was affected by temperature in crosses between _undecimlineata_ and
_signaticollis_. As they stand the results are very striking and
unexpected, but I think, in view of what has been admitted respecting
the former part of the paper, full discussion may be postponed till
confirmation is forthcoming.
One feature, however, calls for remark. This second paper is written
apparently without any reference to the discoveries related by Tower in
his previous book, to which no allusion is made. This is most noticeable
in the case of an experiment in which (p. 296, H. 700A) _undecimlineata_
[F] (the dominant) was mated to _signaticollis_ [M] with the result
that all the offspring were _undecimlineata_ and bred true to that
type (Parthenogenesis was tested for, but never found to occur). This
experiment was made at a temperature averaging 95° F. ± 3.5° by day
and 89° F. ± 4.8° by night, and in a humidity given as 84 per cent. by
day and 100 per cent. by night; but in the previous book (p. 294) we
are told that pure _undecimlineata_ bred together "under an extreme
stimulus of high temperature, 10° C. above the average" and a relative
humidity of 40 per cent. gave 11 beetles only, all _angustovittata_.
But reference to the Plate 16, Fig. 2, shows that _angustovittata_
must be exceedingly like _signaticollis_, having, like it, the elytral
stripes obsolete, and if there is any marked difference at all, it can
only be in the larvae. It seems strange that if _undecimlineata_ really
gives off ova of this recessive type at high temperatures, the fact
should not be alluded to in connection with expt. H. 700A, where, as
the father was _signaticollis_, having the same recessive character,
their appearance might have been expected not to pass unobserved. The
temperature in the older experiment is, of course, not given with the
great accuracy used in the second, and it may have been higher still.
The humidity also was widely different. Still, in discussing the
phenomena we should expect some reference to the very remarkable and
closely cognate discovery which Tower himself had previously reported in
regard to the same species.[13]
The hesitation which I had come to feel respecting these two
publications of Tower's has been, I confess, increased by the appearance
of a destructive criticism by Gortner[14] who has examined the parts
of Chapter III of Tower's book, in which he discusses at some length
the chemistry of the pigments in _Leptinotarsa_ and other animals. As
Gortner has shown, this discussion, though offered with every show of
confidence, exhibits such elementary ignorance, both of the special
subject and of chemistry in general, that it cannot be taken into
serious consideration.
Some observations made by Dr. W. T. Macdougal[15] have also been
interpreted as showing the actual causation of genetic variation by
chemical treatment. Of these perhaps the least open to objection
were the experiments with _Raimannia odorata_, a Patagonian plant
closely allied to _Oenothera_. The ovaries were injected with various
substances and from some of the seeds which subsequently formed in them
a remarkable new variety was raised. This varying or mutational form
was strikingly different from the parental type, with which it was not
connected by any intergradational forms, and it bred true. It made
no rosette, growing to a much smaller size than the parent, and was
totally glabrous instead of being very hairy as the parental type is.
I was shown specimens of these plants by the kindness of Dr. Britton
in the Bronx Park Botanic Garden in 1907 and can testify to their very
remarkable peculiarities. They had a somewhat weakly look, and might
at first sight be thought to be a pathological product, but they had
bred true for several generations. From the evidence, however, I am by
no means satisfied that their original appearance was a consequence
of the treatment applied. This treatment was of a most miscellaneous
description. Two of the mutants came from an ovary which had been
treated with a ten per cent. sugar solution. Ten came from one into
which a 0.1 per cent. solution of calcium nitrate had been injected. One
was from a capsule which "had been exposed to the action of a radium
pencil." Macdougal speaks of these results as decisive, but clearly
before such evidence can be admitted even for consideration it must be
shown by control experiments that the individual plants which threw
the mutant were themselves breeding true in ordinary circumstances.
Nothing is more likely than that the mutant was an ordinary recessive.
I may add that Mr. R. H. Compton made a number of experiments with
_Raimannia odorata_, raised from seeds kindly given me by Dr. Britton,
injecting the ovaries with a variety of substances, including those
named by Macdougal; but though a numerous progeny was raised from the
ovaries treated, all were normal. Macdougal relates also that some
mutational forms came from ovaries of _Oenothera Lamarckiana_ exposed
to radium pencils, and also from _Oenothera biennis_ injected with
zinc sulphate a peculiar mutant was raised, but taking into account
the frequency of these occurrences in those species, he very properly
regarded this evidence as of doubtful application. In a later paper,[16]
however, he has returned to the subject and affirms his conviction that
the appearance of a mutant among seedlings raised from an ovary of
_Oenothera biennis_ treated with zinc sulphate was really a consequence
of the injection, saying that the variation previously observed in
the species was afterwards shown to be due to fungoid disease. The
circumstances to which he mainly points in support of his view is
that the mutation bred true, but this is only evidence of its genetic
distinctness, which may, of course, be admitted by those who remain
unconvinced as to the original cause of its appearance. He adds that he
is making similar experiments with some twenty genera; but what is more
urgently needed is repeated confirmation of the original observation.
When it has been shown that this mutation can be produced with any
regularity from a plant which does not otherwise produce it on normal
self-fertilisation, the enquiry may be profitably extended to other
plants.
A curious and novel experiment, which however, led ultimately to a
negative result, was made by F. Payne. Many discussions have been held
respecting the blindness of cave animals. The phenomenon is one of the
well-known difficulties, and most of us would admit that the theory of
evolution by the natural selection of small differences does not offer a
really satisfying account of it. Those who believe in the causation of
such modifications by environmental influences and in their hereditary
transmission make, of course, the simple suggestion that the darkness
is the cause of the loss of sight, and that disuse has led to the
reduction of the visual organs. Payne bred _Drosophila ampelophila_,
the pomace-fly (which is easy to keep in confinement, fed on fermenting
bananas), for sixty-nine generations in darkness. At the end of that
period there was no perceptible change in the structure of the eyes, or
in any other respect. The number of generations may possibly be regarded
as insufficient to prove anything, but comparing them, as he does, with
the generations of mankind, we see that they correspond with a period of
about two thousand years, an interval far longer than those which many
writers in particular cases have deemed sufficient.
In his first paper Payne states that, though no structural difference
could be perceived, the flies which had been bred in the dark reacted
less readily to light than those which had been reared under normal
conditions, and he inclined to think that the treatment had thus
produced a definite effect. After more careful tests, however, he
withdrew this opinion. It proved that both individual flies and
individual groups of flies, both of those bred in the light and of
those bred in the dark, differed greatly in their reactions, which were
measured by counting the time that it took for a fly to travel to the
light end of a covered tube, various sources of error being eliminated.
He found further that these differences of behaviour were not inherited
in any simple way, but he is disposed to attribute them to accidental
differences in the nature of the food, an account which seems probable
enough.[17]
In several recent publications Blaringhem[18] has described the origin
of many abnormal forms of plants, especially of maize, which he
attributes to various mutilations practised upon the parents. Respecting
these the same difficulty which has been expressed in other cases
reappears, that before drawing any conclusion as to the value of such
evidence we require to know that the plants treated belong to a really
pure line, which if left to nature in the ordinary circumstances of its
life in that locality would have had normal offspring. Abnormalities
abound in the experience of everyone who examines pans of seedlings
of almost any species of plant, and in maize they are well known to
be exceptionally common. Some of those which we meet with when we
attempt to ripen maize in this country are very similar to those which
Blaringhem describes, consisting in irregularities in the distribution
of the sexes, in the shapes of the panicles, etc. Many of these are
doubtless imperfections of development, due to the dullness of our
climate, but others are presumably genetic and would recur in the
offspring however treated. If some one working in a climate where maize
could be raised in perfection would repeat these experiments, and show
that a strain which was thoroughly reliable and normal in its genetic
behaviour did, after mutilation, throw the miscellaneous types observed
by Blaringhem, that would be evidence at least that the development of
the seed could be so influenced by injury to the parental tissues that
its properties were changed. Such evidence could be used for what it is
worth; but pending an inquiry of this kind I am disposed to regard these
observations of variation following on parental injury as suggestive
rather than convincing.
Some evidence of a remarkably interesting kind has been collected by
J. H. Powers[19] respecting the structure and habits of _Amblystoma
tigrinum_, which led him to the conclusion that striking differences
in the form, anatomy, and developmental processes could be effected
directly by change in the conditions of life. It is well known that
a profusion of forms, distinct in various degrees, is grouped round
_Amblystoma tigrinum_. Some of these are believed to be geographically
isolated, others occur together in the same waters, and, as usual,
authorities have differed greatly as to the number of names to be given.
These forms were studied in detail by Cope who described them in the
_Batrachia of North America_. The view which he inclined to take was
that the individual variations of _Amblystoma tigrinum_ resulted from
variations in the time and completeness of the metamorphosis, and these
were regarded as due to external causes, such as differences in season,
temperature, and geographical conditions. Powers, however, states that
collecting within a radius of six or eight miles he found almost if
not quite the whole "gamut of recorded variation in this species."
Some, however, as he states, occurred rarely except under experimental
conditions, but considerable differences in temperature were not found
necessary in producing them. Every year, he says, he has been able to
add to the number of peculiar types found in the same small area in
nature, until the amount of natural variation at least equals that seen
by Cope in the collections of the National Museum and those of the
Philadelphia Academy.
Powers states that his observations by no means confirm Cope's view
that these differences are in the main referable to variation in
the completeness of metamorphosis, and on the contrary, he regards
metamorphosis as on the whole a levelling process, tending to obliterate
diversity. The enormous differences in size and proportions which he
describes can only be appreciated by reference to his figures. They
affect almost all features of bodily organisation. These striking
differences he looks upon as brought about by differences in nutrition,
"diversities in habitual locomotion," and diversity in the age at which
metamorphosis occurs, and to sexual difference. Apart from sexual
difference he regards the chief distinctions, in brief, as "acquired
variations of the larva."
As an example he gives the great elongation of some of the forms as
"due first to slow growth, second to the free-swimming habit, third
to the prolongation of larval life, and finally to the assumption of
sexual maturity as males," either in the branchiate or non-branchiate
condition. He describes the rapid growth of some and the slow growth
of others. A larva of intermediate type may grow about a centimeter a
month, but a rapidly growing specimen may grow more than four times
as much. The slower rate of growth may, he says, be induced by winter
feeding, and other treatment.[20]
When, however, he goes on to describe the influences which he regards
as exerted by the habit of freely swimming, I am led to wonder whether
after all in most of these illustrations, the primary distinctions
are not in reality genetic. "Specimens raised in the same aquarium or
in similar aquaria, side by side with all conditions as uniform as it
is possible to make them, seldom fail to furnish striking examples of
broad-headed, short-bodied, and short-tailed types which are habitually
found at the bottom, while others, slender and elongated, are free
swimmers, and maintain themselves in almost as continual suspension
and motion as does a gold fish." Later, again, he writes, "Yet despite
the uniformity of these favourable conditions, the larvae soon began
to split up into two noticeably distinct groups, the one of unusually
compact proportions, the other of uniform intermediate build, such
is most commonly met with." It is to my mind scarcely possible to
resist the inference that, though there may be definite responses
to certain conditions, yet the chief distinctions are genetic, and
that it is these distinctions which confer the power to respond. The
parts respectively played by cause and effect are always difficult to
assign; but when it is stated that "a weak-limbed, long-bodied and
long-tailed animal becomes well nigh perforce an undulatory swimmer,
while the strong-limbed, short-tailed, heavy-bodied specimen, when
these characteristics are rapidly forced upon it, is, under certain
circumstances, just as forcibly induced to become a crawler," we feel
how erroneous any estimates of causation are likely to be.
One of the most remarkable and interesting sections of Powers' paper
is that in which he describes the differences in bodily structure and
habits which he attributes to cannibalism, and the whole account of the
phenomena should be read in the original. It appears that there are
two extremely distinct types of larvae, those with narrow heads and
slender bodies which live for the most part on small Crustacea such
as _Daphnias_, and those with huge mouths and very wide heads, which
disregard such small animals altogether and live on amphibian larvae,
whether of their own or other species. As the illustrations show, the
differences between these two types are very great, and the differences
in instinct and behaviour are no less. The cannibals take no heed of the
pelagic crustacea, lying sluggishly at the bottom, rousing themselves
immediately to a violent attack on the larger living things which
approach them. Nothing but the most incontrovertible evidence based on
abundant control experiments should convince us that such differences
are not primarily genetic, and in the present state of knowledge I
incline to think that the families really consist of individuals which
are ready to assume the cannibal habit if opportunity offers, and
others which are congenitally incapable of it. It may readily be that
if all chance of cannibal diet be excluded, the full development of the
wide head and mouth, or the other peculiarities, would never become
pronounced, but I doubt whether such change could be induced in any
individual taken at random.
FOOTNOTES:
[1] Pringsheim, H., _Die Variabilität niederer Organismen_, Berlin, 1910.
[2] F. Wolf, Modifikationen u. Mutationen von Bakterien, _Zts. F.
indukt. Abstam. u. Vererbungslehre_, II, 1909, p. 90.
[3] Winslow, C. E. A. and A. R., _Systematic Relationships of the
Coccaceae_. New York. 1909.
[4] C. C. Dobell, _Jour. Genetics_, 1912, II, p. 201, where full
references are given.
Still more recently the same author has contributed an excellent summary
of the evidence relating to bacteria (_ibid._, II. 1913, p. 325).
[5] See Woltereck, _Verh. d. Deut. Zool. Ges._, 1909, p. 110; and 1911,
p. 142. This is a subject which can only be properly appreciated on
reference to the original papers. Several complications are involved to
which I have not here alluded.
[6] _Proc. Roy. Soc._, B, Vol. 86, 1913, p. 113.
[7] _An Investigation of Evolution in Chrysomelid Beetles of the Genus
Leptinotarsa_, Carnegie Publications, 1906, No. 48.
[8] This is the famous Colorado beetle or potato-bug, which has caused
such serious destruction in potato crops. There seems to be no doubt
that this insect, formerly unknown in the eastern States, made its way
east along the mining trails when the west was opened up.
[9] This is indicated in the coloured plate, but I have not found any
explicit statement to this effect in the text, and am not sure if the
absence of pigment was regarded as complete.
[10] _Biol. Bull._, XVIII, 1910, p. 285.
[11] This description does not quite agree with the representation of
the larvae in Pl. 17 of the book _Evolution in the Genus Leptinotarsa_
for there the larva of _undecimlineata_ is shown as white in the second
stage, but yellowish in the third stage; perhaps there is an error in
printing.
[12] _Biol. Bull._, XX, 1910, p. 67.
[13] As to the interrelations of these three forms, Tower states (1906,
p. 18) that _angustovittata_, which he reared from _undecimlineata_, is
intermediate between it and _signaticollis_. Compare Stål, "_Monogr.
des Chrysomélides_," 1862, p. 163; and Jacoby, _Biol. Centr. Amer.
Celeopt._, vi, Pt. 1, p. 234, Pl. xiii, fig. 20; Tab. 41, fig. 15;
_ibid._, Suppl., p. 253. All these forms are evidently very closely
related, and the delimitation of species is quite arbitrary. Jacoby
indeed suggests that _undecimlineata_ may be a variety of _decemlineata_.
[14] Gortner, _Amer. Nat._, Dec., 1911, XLV, p. 743.
[15] _Mutations, Variations, and Relationships of the Oenotheras_,
Carnegie Institution Publication No. 81, 1907, pp. 61-64.
[16] Macdougal, D. T., "Alterations in Heredity induced by Ovarial
Treatments", _Bot. Gaz._, vol. 51, 1911, p. 241.
[17] Payne, Fernandus, _Biol. Bull._, XVIII, 1910, p. 188, and _ibid._,
XXI, 1911, p. 297.
[18] See especially, _Mutation et Traumatismes_, Paris, Felix Alcan,
1908.
[19] J. H. Powers, "Morphological Variation and its Causes in
_Amblystoma tigrinum_." _Studies from the Zoological Laboratory. _ The
University of Nebraska, No. 71, 1907.
[20] In connexion with this case I would refer the reader to some
remarkable observations of Dr. T. A. Chapman on various types of larvae
which he reared from the moth _Arctia caja_ (_Ent. Rec._, IV, 1893,
p. 265, and following parts). From a single mother he raised a great
diversity of forms, some which fed up rapidly and passed through their
development without assuming certain stages, and others which were, as
he called them, "laggards," moulting more times than their brethren and
developing at a much slower rate. It is greatly to be hoped that such a
case may be critically investigated by analytical breeding.
CHAPTER XI.
STERILITY OF HYBRIDS. CONCLUDING REMARKS.
When we consider the bearing of recent discoveries on those
comprehensive schemes of evolution with which we were formerly
satisfied, we find that certain details of the process are more easy
to imagine. We readily now understand how varieties once formed,
can persist, but at the same time difficulties hitherto faced with
complacency become formidable in the light of the new knowledge. So
generally is this admitted by those familiar with modern genetic
research that most are rightly inclined to postpone the discussion. The
premisses, indeed, on which such a discussion must be based are almost
wholly wanting.
The difficulties to which I chiefly refer are not those created by the
phenomena of adaptation, though they are serious enough. In treating
of that subject I have felt obliged to express scepticism as to the
validity of nearly all the new evidence for the transmission of acquired
characters. At the present time the utmost we are bound to accept is the
proof that (1) in some parthenogenetic forms variations, or perhaps we
may say malformations, produced in response to special conditions, recur
in one or perhaps two generations asexually produced after removal to
other conditions. (2) That violent maltreatment may in rare instances
so affect the germ-cells contained in the parents as to cause the
individuals resulting from the fertilisation of those cells to exhibit
an arrest of development similar to that which their parents underwent.
I do not doubt that evidence of this type will be greatly extended. As
a contribution to genetic physiology these facts are very important
and interesting, but I cannot think that any one, on reflexion, will
feel encouraged by such indications to revive old beliefs in the direct
origin of adaptations.
In these respects we are simply left where we were. The force of
objections based upon the existence of adaptative mechanisms is
no greater than it has always been. On the contrary the fact that
variations can now so generally be recognized as definite is some
alleviation of the difficulty. We can moreover disabuse ourselves of
the notion that for all characters which are definite or fixed, some
utilitarian rationale may be presumed. Upon that point the study of
variation has provided a perfectly clear answer.
In frankly recognizing that the fixity of characters in general need
not connote usefulness to their possessors we deliver ourselves of a
distracting pre-occupation and prepare our minds for an investigation
of the properties of living organisms in the same spirit as that
in which the chemist and the physicist examine the properties of
unorganized materials. The creature persists not merely by virtue of its
characteristics but in spite of them, and the fact of its persistence
proves no more than that on the whole the balance of its properties
leaves something in its favour.
It may be noted by the way that the fact that the structures of living
things are on the whole adaptative was not always obvious. Though to
naturalists of this generation it is a truism, we have only to turn to
Buffon to find that in his philosophy of nature it played no essential
part. The passage in which Buffon describes what he regards as the
forlorn and degraded condition of the Woodpecker is well known. We have
come to think of the Woodpecker as a capital example of adaptation to
the mode of life; but Buffon after enumerating the hard features of
the bird's existence, forced to earn its living by piercing the bark
of trees in an attitude of perpetual constraint, remarks[1] "Tel est
l'instinct étroit et grossier d'un oiseau borné a une vie triste et
chétive. Il a reçu de la Nature des organes et des instrumens appropriés
a cette destinée _ou plutôt il tient cette destinée même des organes
avec lesquels il est né_" (my italics). His reflexions on the Stilt
(_Himantopus_) read even more strangely to us, accustomed as we are to
see in the prodigious length and thinness of the shanks and in the other
features of its organisation palpable adaptations to a wading life. For
Buffon, however, this curious bird seemed a poor, neglected production,
extravagant in its disproportions, one of the misfits of creation,
left as a shadow in the picture composed of nature's more successful
efforts.[2] This theme he develops at some length, being evidently well
pleased with the idea.
Our way of regarding these things is doubtless sounder and more fruitful
than Buffon's, but it is well to remember that what seems so obvious to
us looked quite differently to other excellent observers; and stupid
as it may have been to have overlooked plain examples of adaptation,
it is a far worse mistake to see adaptation everywhere. I do not seek
to minimise the real and permanent difficulty which the existence of
adaptations creates, but by the suggestion that all normal specific
differences are adaptational that difficulty was quite gratuitously
increased.
In these respects it may be claimed that progress has been made, even if
that progress seem outwardly of small account.
But all constructive theories of evolution have been built on the
understanding that what we know of the relation of varieties to species
justifies the assumption that the one phenomenon is a phase of the
other, and that each species arises or has arisen from another species
either by one or several genetic steps. In the varieties we have
accustomed ourselves to think that we see those steps. We still know
little enough of the mode of occurrence of variation, but we do begin to
know something, and if we ask ourselves whether our knowledge, such as
it is, conforms at all readily with our former expectations, we cannot
with any confidence assert that it does. Among the plants and animals
genetically investigated are many illustrations of very striking and
distinct varieties. Many of these might readily enough be accepted as
species by even the most exacting systematists, and not a few have
been so treated in classification; but when we have examined their
relationship to each other we feel not merely that they are not species
in any strict sense but that the distinctions they present cannot be
regarded as stages in the direction of specific difference. Complete
fertility of the results of inter-crossing is and I think must rightly
be regarded as inconsistent with actual specific difference; and of
variations leading to that consequence no clear indication has yet been
found. As an example of possible exceptions mention should perhaps be
made of the case of a giant form of _Primula sinensis_ investigated by
Keeble.[3] It arose from a "Star" Primula of normal size, and though
fertile with its own pollen all attempts to fertilise it with the pollen
of other forms failed. Miss Pellew, who did these fertilisations,
tells me that very extensive trials were made, and repeated in several
seasons. Ultimately two plants were raised from it fertilised with a
plant of the strain from which it sprang, and these proved sterile.
In the light of modern experience the significance of such isolated
instances is doubtful.
All the strains known as "Giants" are, as Messrs. Sutton have always
found, more or less sterile, and their sterility is presumably due to
some negative defect.
In regard to the fertility of Primula species there are several
paradoxes. For example the long-styled varieties, apart from giants, are
fertile with their own pollen, and for many years short-styled plants
have not been used in most strains. Auriculas and Polyanthuses, on the
contrary, are generally if not always bred from short-styled plants,
as the florists have decided that the long-styled are inadmissible.
Mr. R. P. Gregory tells me that, though most strains of _P. sinensis_
give seed enough when only long-styled plants are used, he finds
nevertheless that when a "legitimate" union is made the amount of seed
usually increases much as Darwin observed. Darwin's statement that
plants of "illegitimate" origin are less fertile than the "legitimately"
raised plants is also in general confirmed by his experience. To
this rule there were some marked exceptions in strains derived from
_long_-styled plants, which though illegitimate showed a high degree
of fertility, but illegitimate unions between _short_-styled plants
always produced comparatively sterile offspring. I have no records of
the behavior of Auriculas and Polyanthuses. It would be interesting to
know whether among them pure strains of short-styled plants (dominants)
have appeared, and, if so, how their fertility is affected. Without
much more critical data I suppose no one would nowadays be inclined
to follow Darwin in instituting a comparison between the sterility
of hybrids and that of illegitimately raised plants of heterostyle
species.[4] It is even difficult to imagine any essential resemblance
between these two phenomena, nor has evidence ever been produced to
show that illegitimately raised plants have bad pollen grains, which is
the usual symptom of sterility in hybrid plants and the consequence,
as we believe, of failure of some essential division in the process of
maturation.
The difficulty that we have no knowledge of the contemporary origin of
forms, from a common stock, which when crossed together give a sterile
product, is one of the objections constantly and prominently adduced
from the time of the first promulgation of evolutionary ideas. In the
light of recent work the objection has gathered strength. Why, if
we are able to produce instances of variation colourably simulating
specific difference in almost all other respects, do we never find
an original appearance of this most widely spread of all specific
characteristics? No doubt all breeders know that sterile animals
and plants occasionally appear in their cultures, but it is more in
accordance with probability that the sterility in these sporadic
instances should be regarded as due to defect than that it should be
thought comparable with that of the sterile hybrids. For their sterility
must, by all analogy with results elsewhere seen, be attributed not
to the absence of something, but to the presence and operation of
complementary factors leading to the production of inhibition of
division; and consistently with that interpretation, we find that when
from a partially sterile hybrid comparatively fertile offspring can be
raised, their comparative fertility continues in the posterity generally
if not always without diminution. The distinction between these several
kinds of sterility was of course not understood in Darwin's time. The
comparison, for example, which he instituted[5] between the sterility
of "contabescent" anthers and that of hybrids no longer holds, for at
least in those cases in which the nature of contabescent anthers have
been genetically investigated (Sweet Pea, _Tropaeolum_) they proved
to be a simple recessive character. Nor can we now easily suppose that
the attempt there made by Darwin to suggest resemblance between the
sterility produced by unnatural conditions and that of hybrids has any
physiological justification.
In regarding the power to produce a sterile or partially sterile hybrid
as a distinction in kind, of a nature other than those which we perceive
among our varieties, I am aware that I am laying stress on an impression
which may hereafter prove false. The distinction nevertheless is so
striking and so continually before the eyes of a practical breeder that
he can scarcely avoid the inference that when he meets a considerable
degree of sterility in a cross-bred he is dealing with something
belonging to a distinct category, and not merely a varietal feature of
an exceptional kind.
Besides the sterility of hybrids appeal has often been made to the
phenomenon of incompatibility, in its several stages of completeness,
as distinguishing species. No one doubts that incompatibility may
arise from a variety of causes of most diverse degrees of importance,
but though sometimes referred to as an extreme case of interspecific
sterility, it is really a very different matter. In regard to one phase
of this incompatibility, that associated with self-sterility, some
progress has been made, and we are not wholly without experimental
evidence of its being within the range of contemporary variation.
Given the outline of Mendelian teaching as to gametic differentiation
and the classification of individuals in a mixed population, it
seemed highly probable that what we call self-sterility must mean
that the species really consisted of _classes_, some of which are
capable of interbreeding with others while others are not. According
to the received account every individual, though incapable of
fertilising itself, was supposed to be able both to fertilise and to
be fertilised by any other individual. This notion has always seemed
to me a self-evident absurdity, for it would imply that there can be
as many categories as individuals. Such experiments, however, as I
made did certainly give results consistent with that belief. I first
tried Cinerarias, which are usually self-sterile, but I found no
incompatible pairs of plants. Whether I was deceived by the consequences
of apogamy, or whether the pollen of certain plants may belong to more
than one class I do not know. The results were confused in various
ways. Usually the self-fertilised plants set little or nothing, and
cross-fertilised they set fully with such uniformity that the few
failures could plausibly be attributed to mistakes in manipulation
or to other extraneous causes. Later de Vries announced[6] (without
giving particulars) that he had proved the existence of such classes in
_Linaria vulgaris_; but on making experiments with that species I again
got no positive results, and I came to the conclusion that in spite of
inherent improbability the conventional belief must be substantially
true. At last, however, the work of Correns, lately published,[7] does
definitely show that in one species, _Cardamine pratensis_, classes of
individuals exist such that individuals of the same class are incapable
of fertilising themselves or each other, but fertilisation made between
the classes is usually completely effective. Many complications were
encountered and some contradictory evidence is recorded, but the general
bearing of the results was positive and indubitable.
We know far too little of this phenomenon as yet to be able to
understand its significance, but I suppose we may anticipate with some
confidence that it will be found to be a manifestation of dissimilarity
between the male and female gametes of the same individual, comparable
with that first seen in the Stocks (_Matthiola_) which throw doubles--a
state of things in all likelihood to be found widely spread among
hermaphrodite organisms. Whether the incompatibility between species
is to be associated with that of the self-steriles also cannot be
positively asserted, though it seems not unreasonable to expect that
such an association will be discovered.
The case of the apple and the pear is an impressive illustration of
this possibility. The two species are of course exceedingly alike in
all outward respects, but nevertheless the pollen of each is entirely
without effect on the other. Presumably we should interpret this fact
as meaning not so much that the apple and the pear are in reality
very wide apart, but rather that either, each is lacking in one of
two complementary elements, or that each possesses a factor with an
inhibitory effect. Their incompatibility may well be of the same nature
as that of the classes in _Cardamine pratensis_.
Returning now to the problem of inter-specific sterility; we note,
as I have said, the absence of contemporary evidence that variation
can confer on a variety the power to form a sterile hybrid with the
parent species. The considerations based on this want of evidence have
for a long while been familiar to all who have discussed evolutionary
theories, and it is worth observing the exact reason why the difficulty
strikes us now with a new and special force. In pre-Mendelian times
all that was known was that some forms could freely interbreed without
diminution of fertility in the product, while others could not. But now
we find that, by virtue of segregation, from one and the same pair of
parents, or even, in the case of hermaphrodites, from one and the same
individual, offspring commonly arises showing among themselves exactly
such differences as distinguish species--and very good species too. This
we see happening again and again. But to forms capable of arising as
brethren in one family the title species has never been meant to apply,
and if we are going to use the term in application to fraternal groups
we must definitely recognise that by "specific" difference is to be
understood simply _difference_, without any immediate or even ulterior
physiological limitation whatever. Naturally, therefore, we begin to
think of the appearance of sterility in crosses as something apart, and
as a manifestation which distinguishes certain kinds of unions in a very
special way.
I am perfectly aware that there are gradations in the sterility of
hybrids as in every other characteristic upon which it has been proposed
to base specific definitions; but, as also so often happens in the
matter of defining intergrading categories, the difficulty in practice
is not often such as to lead to actual ambiguity. I am speaking of
course of those examples which are amenable to genetic experiment.
As to the rest there is complete and permanent uncertainty. But the
experience of the practical breeder does, I think, on the whole, support
the contention to which systematists have so steadily clung under all
the assaults of evolutionary philosophers, that, though we cannot
strictly define species, they yet have properties which varieties have
not, and that the distinction is not merely a matter of degree.
The first step is to discover the nature of the factors which by their
complementary action inhibit the critical divisions and so cause
the sterility of the hybrid. Thus expressed, we see the problem of
inter-specific sterility in its right place; and the question why we
do not now find contemporary instances of varieties lately arisen in
domestication, which when crossed back with their parents, or with their
coderivatives, can produce sterile products, is perceived to be only a
special case of a problem which in its more general form is that of the
origin of new and additional factors.
For the requisite evidence no comprehensive search has been made, but
perhaps it will yet be found. All that we can say at the present time
is that the incidence both of hybrid sterility, and of incompatibility
also, is most capricious; and provided that two forms have such features
in common that a cross between them seems not altogether out of the
question, no one can predict without experiment whether such a cross
is feasible, and if feasible whether the product will be fertile, or
sterile more or less completely. For instance, though probably all
the British and some Foreign Finches (Fringillidae) have been crossed
together, and some of these crosses, as for instance, the various
Canary-mules have been made in thousands, I believe no quite clear
example of a fertile hybrid can be produced. Many species of Anatidae
cross readily and produce fertile hybrids: others give results uniformly
sterile. Though most of the Equidae can be crossed and some of the
hybrids are among the commonest of domesticated animals there is no
certain record of a fertile mule. Among the Canidae the dogs, wolves and
jackals all give fertile hybrids, but there is no clearly authenticated
instance of a cross between any of these forms and the European fox.
In spite of their close anatomical resemblance it is doubtful if the
rabbit and the hare have ever interbred. Many of the wild species of
_Bos_ have been crossed and recrossed both with each other and with
many domesticated races, but I understand that no cross with the Indian
buffalo (_Bos bubalus_) has yet been successful even in producing a
live calf.[8] In the genus _Primula_ many hybrids are known and several
of them occur in nature, but hitherto no certain hybrid between _P.
sinensis_ and any other species has been made, in spite of repeated
attempts.
In _Nicotiana_ many--doubtless all--the various forms of _N. tabacum_
can be crossed together without diminution of fertility, though some
are very distinct in appearance, but crosses between _tabacum_ and
_sylvestris_ are highly sterile (in my experience totally sterile[9]),
though the distinctions between them are not to outward observation
nearly so great as those which can be found between the various races of
_Primula sinensis_.
Recently some remarkable experiments bearing closely on these questions
have been published by F. Rosen.[10] They concern the forms of _Erophila
(Draba) verna_, celebrated in the history of evolutionary theory as the
plants especially chosen by Alexis Jordan for the exposition of his
views on these subjects.
The "species" contains a profusion of forms dissimilar in many
structural characters, such as the size and shape of leaves, flowers,
fruits, etc. Of these forms many grow in association. Jordan found, on
experiment, that each, to the number of some two hundred, bred true, and
that therefore, the conventional assumption that polymorphism of this
kind must mean great contemporary variability had no foundation in fact.
So far indeed is the evidence from favouring the belief that such forms
are in any way transitional or indeterminate, that, as is well known,
Jordan used it with every plausibility to support the doctrine of the
fixity of species. To certain aspects of Jordan's work we will return
later in this chapter, but the matter is in the present connection
of especial interest for the reason that Rosen has lately found by
experiment that some of these presumably very closely allied forms,
crossed together, gave hybrids more or less sterile. In the case of the
offspring of one pair of forms only (_E. cochleata_ and _stricta_) was
the fertility undiminished, and the various degrees of sterility found
in the other crosses ranged up to the extreme infertility of the hybrids
between _E. stricta_ × _elata_. From this cross ten plants were bred.
Of these the four strongest were chosen to breed from, but two of the
four proved totally sterile; one had only bad seeds; and from the fourth
a single seedling was raised which in its turn proved to be sterile.
From the less sterile hybrids F_{2} families were raised, with the
usual experience that in this and subsequent generations the sterility
diminished among extracted forms, new and true-breeding types with
complete fertility being thus derived from the original cross.[11]
The production of sterility as a consequence of crossing plants so
nearly approaching each other as these _Erophila_ "species" do is
not a little interesting, and the fact well exemplifies the futility
of the various attempts to frame general expressions as to specific
properties or behaviour. Commenting on his results Rosen argues that
the polymorphic group commonly called by systematists _Erophila (Draba)
verna_ may now be regarded as having arisen by crossing, as did his own
types mentioned above. The question, however, _what_ species were the
original progenitors of the group cannot be answered. Rosen considers
that no form which he knows satisfies the requirements, and that
it or they must be supposed to be lost. This conclusion will recall
the similar problem raised by the _Oenothera_ mutants (Chap. V); and
unsatisfactory as it may be to have recourse to such hypotheses we
must remember the possibility that as a consequence of hybridisation,
subsequent segregation and recombination of factors, species may
have thus actually, as we may say, exploded, and left nothing but a
polymorphic group of miscellaneous types to represent them in posterity.
If this way of regarding the phenomena be a true one, the sterility
now seen when some of the group are re-crossed, becomes analogous to
that "reversion or crossing" which we now so well understand to be
a consequence of the recombination of characters separated at some
previous point in the history of descent. In the partial sterility of
the contemporary hybrid we see this character reappearing, formed now
as it was on the occasion of the original cross, by the meeting of
complementary factors.
Another case that may be mentioned in this connection is that of the
crosses between various culinary peas (_Pisum sativum_) and a peculiar
form found by Mr. Arthur Sutton growing ostensibly in a wild state in
Palestine. This Palestine Pea is low growing, rarely reaching 18 inches.
It is in general appearance like a small and poorly grown field pea.
The stems are thin and rather hard. The most obvious differences which
distinguish this from other field peas are the marked serration of the
stipules, and the development of pith in the pods. Such pith is often
present in the pods of peas more or less, but in the Palestines it is
so strongly developed as almost to form a lomentum. Curiously enough,
though the flowers are purple much as those of ordinary field peas,
there is no coloured spot in the axils. On the other hand, the stems
have coloured stripes running up from the axils. Though this plant
differs so little from domesticated peas, all crosses with them either
failed, or produced hybrids quite or almost quite sterile. This was Mr.
Sutton's experience, and on repeating the experiments with material
kindly given by him I found the same result.[12]
In a large series of crosses some seeds died or gave rise to feeble
plants. Of the plants which lived, few gave any seed. The seed, however,
that was obtained from F_{1} plants grew well enough, and the F_{2}
plants proved, as often in such cases, fertile. In these, indeed, no
sign of sterility was noticeable. The experiment is being repeated in
various ways, for, as the genetic behaviour of peas is comparatively
well known, the subject is an exceptionally favourable one for these
investigations.
Such an example shows the confusion produced the moment we attempt to
harmonize conceptions of specific difference with results attained by
experimental methods. It has been usual to regard the field pea (_P.
arvense_) as a species distinct from the edible pea (_P. sativum_).
De Candolle and others regard the field pea as derived from a form
wild in Italy, but the origin of the edible pea is considered to be
unknown. From breeding experiments we find no sterility whatever in the
crosses between the various _arvense_ and _sativum_ types, nor in the
crosses made between them and several other peculiar types from various
countries; whereas this Palestine Pea, which only differs from a small
_arvense_ in what might have been thought trivial characters,[13] either
fails to cross altogether or gives a sterile product, whatever type be
chosen as the other parent.
Examples of this kind have at least the merit that they lead to more
precise delimitations of the problem. We are confronted with two
distinct alternatives.
1. We may apply the term Species promiscuously to all distinct forms.
If we do so it must be clearly understood that we cannot even rule out
the several combinations of "presences and absences" represented by the
various types whether wild or domesticated. For we may feel perfectly
assured that at least all the _arvense_ and all the _sativum_ types yet
subjected to experimental tests are on precisely the same level in this
respect. There is no distinction, logical or physiological, to be drawn
between them. Some contain more factors, and others contain fewer. In
some the re-combinations have been brought about by natural variation or
crossing, while the same consequences in the others have resulted from
man's interference.
2. We may follow the conventions of systematists and distinguish the
outstanding or conspicuous forms such as _arvense_, _quadratum_,
_sativum_ and perhaps a few more as species, and leave the rest
unheeded. If this course is followed it must be clearly understood and
permitted as a piece of pure pragmatism, deliberately adopted for the
convenience of cataloguers and collectors, without regard to any natural
fact or system whatsoever.
But while following either the one plan or the other we shall be still
awaiting the answer, which only genetic experiment can provide, to the
question whether among the various types there are some which differ
from the rest in a peculiar way: whether by having groups of characters
linked together in especially durable combinations, or by possessing
ingredients which cause greater or less disturbance in the processes of
cell-division, and especially in the processes of gametic maturation,
when they are united by fertilisation with complementary ingredients.
Before any but the vaguest ideas regarding the nature and significance
of inter-specific sterility can be formed, a vast amount of detailed
work must be done. Sterility as a result of crossing, as well as
that which is alleged sometimes to arise in consequence of changed
conditions, is at best a negative characteristic, and there are endless
opportunities for mistake and misinterpretation in studying features
of this kind. No one, I suppose, would now feel any great confidence
in most of the data which from time to time are resuscitated for the
purpose of such discussions. Even the best collections of evidence, such
as those given by Darwin in _Forms of Flowers_, cannot be regarded as
critical when judged by present-day standards. Nothing short of the most
familiar acquaintance with the habitual behaviour of individuals, and of
strains kept under constant scrutiny for several years would enable the
experimenter to form reliable judgments as to the value to be attached
to observations of this class.
The admission must, however, be faced that nothing in recent work
materially tends to diminish the surprise which has always been felt
at the absence of sterility in the crosses between co-derivatives. We
should expect such groups of forms to behave like the _Erophila_ types,
and frequently to produce sterile products on crossing. Whatever be
the explanation, the fact remains that such evidence is wanting almost
completely. In spite of all that we know of variability nothing readily
comparable with the power to produce a sterile hybrid on crossing with
a near ally, has yet been observed spontaneously arising, though that
characteristic of specificity is one of the most widely distributed
in nature. It may be that the lacuna in our evidence is due merely to
want of attention to this special aspect of genetic inquiry, and on the
whole that is the most acceptable view which can be proposed. But seeing
that naturalists are more and more driven to believe the domesticated
animals and plants to be poly-phyletic in origin--the descendants, that
is to say, of several wild forms--the difficulty is proportionately
greater than it was formerly, when variation spontaneously occurring was
regarded as a sufficient account of their diversity.
CONCLUDING REMARKS.
The many converging lines of evidence point so clearly to the central
fact of the origin of the forms of life by an evolutionary process
that we are compelled to accept this deduction, but as to almost all
the essential features, whether of cause or mode, by which specific
diversity has become what we perceive it to be, we have to confess an
ignorance nearly total. The transformation of masses of population by
imperceptible steps guided by selection, is, as most of us now see, so
inapplicable to the facts, whether of variation or of specificity, that
we can only marvel both at the want of penetration displayed by the
advocates of such a proposition, and at the forensic skill by which it
was made to appear acceptable even for a time.
In place of this doctrine we have little teaching of a positive kind
to offer. We have direct perception that new forms of life may arise
sporadically, and that they differ from their progenitors quite
sufficiently to pass for species. By the success and maintenance of
such sporadically arising forms, moreover, there is no reasonable doubt
that innumerable strains, whether in isolation or in community with
their co-derivatives, have as a fact arisen, which now pass in the
lists of systematists as species. For an excellent account of typical
illustrations I would refer the reader to the book lately published by
R. E. Lloyd[14] on the rat-population of India. The observations there
recorded are typical of the state of things disclosed whenever the
variations of large numbers of individuals are closely investigated,
whether in domestication or in natural conditions.
Guided by such clues we may get a good way into the problem. We see
the origin of colourable species in abundance. Then, however, doubt
arises whether though these new forms are as good species as many
which are accepted as such by even cautious systematists, there may
not be a stricter physiological sense in which the term species can be
consistently used, which would exclude the whole mass of these _petites
espèces_.
If further we find that we have, with certain somewhat doubtful
exceptions, never seen the contemporary origin of a dominant factor, or
of inter-racial sterility between indubitable co-derivatives, it needs
no elaboration of argument to show that the root of the matter has not
been reached.
Examination of the inter-relations of unquestionably distinct species
nearly allied, such as the two common species of _Lychnis_, leads to
the same disquieting conclusion, and the best suggestion we can make
as to their origin is that _conceivably_ they may have arisen as two
re-combinations of factors brought together by the crossing of parent
species, one or both of which must be supposed to be lost.
All this is, as need hardly be said, an unsatisfying conclusion. To
those permanently engaged in systematics it may well bring despair.
The best course for them is once for all to recognise that whether
or no specific distinction may prove hereafter to have any actual
physiological meaning, it is impossible for the systematist with the
means at his disposal to form a judgment of value in any given case.
Their business is purely that of the cataloguer, and beyond that they
cannot go. They will serve science best by giving names freely and by
describing everything to which their successors may possibly want to
refer, and generally by subdividing their material into as many species
as they can induce any responsible society or journal to publish.
Between Jordan with his 200 odd species for _Erophila_, and Grenier and
Godron with one, there is no hesitation possible. Jordan's view, as he
again and again declares with vehemence, is at least a view of natural
facts, whereas the collective species is a mere abstraction, convenient
indeed for librarians and beginners, but an insidious misrepresentation
of natural truth, perhaps more than any other the source of the
plausible fallacies regarding evolution that have so long obstructed
progress.
Nevertheless though we have been compelled to retreat from the
speculative position to which scientific opinion had rashly advanced,
the prospect of permanent progress is greatly better than it was. With
the development of genetic research clear conceptions have at length
been formed of the kind of knowledge required and of the methods by
which it is to be attained. If we no longer see how varieties give
rise to species, we may feel confident that a minute study of genetic
physiology of varieties and species is the necessary beginning of any
critical perception of their inter-relations. It is little more than a
century since no valid distinction between a mechanical mixture and a
chemical combination could be perceived, and in regard to the forms of
life we may well be in a somewhat similar confusion.
As yet the genetic behaviour of animals and plants has only been
sampled. When the work has been done on a scale so large as to provide
generalisations, we may be in a position to declare whether specific
difference is or is not a physiological reality.
FOOTNOTES:
[1] Buffon, _Hist. Nat._, Oiseaux, 1780, VII, p. 3.
[2] Ibid., VIII, p. 115.
[3] Keeble, _Jour. Gen._, 1912, II, p. 173.
[4] _Animals and Plants_, ed. 1, 1868, II, pp. 180-5.
[5] _Animals and Plants_, ed. 1, 1868, II, p. 165.
[6] _Species and Varieties_, 1905, p. 471.
[7] Correns, _Festschr. med.-nat. Ges. zur 84 Versamml. Deutsch. Naturf.
u. Aertze. Münster i. W._, 1912.
[8] This is a case of a somewhat different order and I mention it partly
for that reason as an illustration of the complexity which such negative
instances may present. The difficulty is that though the buffalo and
the zebu can breed together, the foetus is too large to be born alive.
(See Ackermann _Ber. d. Ver. f. Naturk._, Kassel, 1898, p. 69. Prof. S.
Nathusius, of Halle, who has great experience in crossing Bovidae, tells
me that he has always failed to cross the buffalo with other species.)
[9] In a paper to be published in the Report of the Genetic Conference,
Paris, 1911, Bellair states that he obtained some partially fertile
hybrids in the cross _N. sylvestris_ × _tabacum_. As to the various
degrees of sterility in hybrids between _Nicotiana_ species see Lock, R.
H., _Ann. Roy. Bot. Gardens_. Peradeniya, IV, 1909, p. 195.
[10] _Beitrage zur Biol. der Pflanzen._, X, 1911, p. 379.
[11] One very peculiar feature was observed, namely, that all the new
forms in F_{2} which were bred from came true. As I understand, this
statement applied to five such new types, and they were represented
by 76 individuals in F_{3}, but further details on this point are
desirable. Another curious fact was observed, namely that one of the
F_{1} forms (_cochleata_ × _radiata_) when fertilised by _cochleata_
gave a highly polymorphic family, but fertilised by _radiata_ the
resulting offspring were almost uniform.
[12] I also had a few F_{1} seeds given me by Mr. R. H. Lock.
[13] In a paper about to appear in _Jour. Linn. Soc._ Mr. A. W. Sutton
identifies this Palestine pea as _Pisum humile_ of Boissier and Noé.
[14] Lloyd, R. E., _The Growth of Groups in the Animal Kingdom_, London,
1912.
INDEX OF SUBJECTS
PAGE
Abraxa grossulariata, 105,193
Aceras hircina, local variability, 123
Achatinellidae, local forms of, 133
Acquired characters, inheritance of, 188 et seq.,217,233
Acronycta psi, melanic, 138
Adaptation, problem of, 187,234
Agelaius, local forms, 120
Agrotis, fixed and variable species, 25
Alkaptonuria, 83
Alpine Plants, growing larger, if protected, 183
Alpine Varieties, 165
Alytes obstetricans, Kammerer's experiments on, 199,210
Amblystoma, races of, 230
Amphidasys betularia, melanic form, 136,138
dimorphic larvae, 141
Anodonta, polymorphism of, 130
Antirrhinum, striped, 57
species-hybrids, 99
albinos, 110
Apple, will not cross with pear, 239
Arctia caja, effects of temperature, 192
larval variation in, 231
Arctic varieties, 165
Argynnis paphia and valesina in Italy, 121
Armadillo, polyembryony, 42
Artistic faculty, 89
Arum, rights and lefts, 57
Auriculas, short-styled selected, 236
Axis of symmetry in hand and foot, 48
Axolotl, alleged effect of conditions, 230
Azalea, bud-sports, 55
Bacillus anthracis, unsegmented form, 71
Bacillus prodigiosus, variation in, 213
Bacteria, variation in, 212
Bacterium coli, variation in, 214
Baeolophus, geographical races of, 159
Barley, right and left-handed, 58
Basilarchia, geographical races of, 161
Begonia phyllomaniaca, 50
hybrids, 51
Bizarre Carnation, genetics of, 54
Black, as a variation from red, 148
Blackbird, varying, 150
Black Cock, fixity of, 28
Boarmia repandata, melanic form, 136
rhomboidaria, 137,139
Botrytis susceptibility to, 108
Bovidae, hybrid, 242
Brachydactyly, 89,95
Bradypus, vertebral variation, 68
Bud-sports geometrically irregular, 54-57
Buffalo, attempts to hybridize, 242
Bullfinch, gynandromorph, 45
Bulimus detritus, local variation of, 126
Canary, asymmetrical markings in, 154
Canidae, hybrid, 241
Capsella, 100
Cardamine pratensis, 239
Cat, Polydactylism, 53
Carnation, Picotees and bizarres compared, 54,58
Cataract, hereditary, 89
Certhiola, melanic, 142
Chladni figures, 60
Choloepus, vertebral variation in, 68
local variation in, 119
Cinerarias, self-sterility in, 238
Cistudo, local variation in, 119
Climatic varieties, 164
Coccaceae, variation in, 213
Coenonympha arcania, climatic forms of, 179
satyrion, 180
Coereba, melanic, 142
Colaptes, geographical races, 147 et seq.
chrysoides, 154
Colloids, growth in, 65
Colorado beetles, experiments on, 218
Colour blindness in twins, 44
Continuous variation, possible example of, 173
Coracias, geographical races of, 160
Cotton, genetics of, 98,100
Coupling, 110
Crab, extra claws, 74
Crustacean appendages and Serial Homology, 63
Crystals, analogy with, 78
Cyclopian monsters, artificial, 50
Daphnia, changed by environment, 216
Dasypus, polyembryony, 42
Dianthoecia, fixed and variable species, 25
Disease-resistance, 87
Division, power of,
a fundamental attribute of living things, 38
Genetics of, 46,50
Dogger Bank, large varieties on, 125
Dogs, hybrid, 241
Dominance, nature of, 95
Dominants, origin of new, 88,90,95
Double monsters, 42
Draba, experiments with, 242
Drosophila, 91
Payne's experiments on, 228
Earthworm, regeneration, 77
Elephant, tusk segmented, 38
Entelechy, 80
Environmental treatment, effects of, 188 et seq.
Enzymes and genetic factors, 86
Epilepsy, inheritance of traumatic, 197
Equidae, sterility of hybrid, 241
Erophila, experiments with, 242
species, 249
Exacum, right and left, 57
Euphonia elegantissima, local forms, 120
Eupithecia rectangulata, melanic form, 137
Factors, new, 88
loss of, 96
Factorial representation of varieties, 158,165
Falcons, geographical races, 147
Fasciation, 49
Ferments, Boyle on, 54
Finger-prints of twins, 44
Fixity and Variability in species, 25
Flax, climatic experiments, 197
Fowl, Silky, 84
Leghorn, 85,90
Dominant white, 94
Wyandotte, 97
Rumpless, 46
Foxes, incompatibility with dogs, 241
Free-martin, 44
Fringillidae, sterility of hybrid, 241
Fundulus, cyclopian, 50
Gallus, invariability of wild species, 13
and origin of poultry, 90,97
Genitalia, a basis for classification in insects, 13
Gentians, climatic experiments, 197
Geometrical structure and differentiation, 54,56
Geometrical distinction between germ-cells
and somatic cells, 58
Gladiolus, right and left, 57
Gnophus obscurata, protective colouring, 141
Goldfinch, geographical races, 147
Gonioctena variabilis, variation in sexes of, 121
Gouldian Finch, polymorphism, 148,149
Gracilaria stigmatella, experiments on, 193
Grantia, large varieties of, 125
Ground-Squirrels, local forms of, 132
Grouse, red, variation, 29
Guillemot, Ringed, 150
Guinea-pig, Brown-Séquard's experiments on, 198
Gynandromorphs, 45
Heliconius erato, forms of, 122,164
Helix lapicida, local variation of, 126
striata, 127
Heripensis, 127
Caespitum, 127
trochoides, 127
nemoralis and hortensis, 128
Helminthophila, geographical races of, 157
Hemerophila abruptaria, melanic, 142
Hepialus humuli, in Shetland, 119
Heterostyle plants, 236
Hieracium, 9
Himantopus, 234
Homoeosis, 68
Hybernia progemmaria, 139
Hybrids, sterility of, 233 et seq.
Incompatibility between certain allied species, 239
Individual, geometrical independence of, 58
Inhibiting Factors, 95
Intermediates, nature of, 131,135
Isolation, consequences of, 118
Lacerta muralis, Kammerer's experiments on, 209
fiumana, 210
Leptinotarsa, Power's experiments on, 218
Limbs, extra, in pairs, 72
Limnaea, sinistral, 134
Linaria vulgaris, self-sterility, 239
Loasa fruits, right and left, 57
Lobster, extra claws, 76
Locality, variation connected with, 14,118,146 et seq.,208
Lumbricus, regeneration, 77
Lychnis dioica and vespertina, inter-relations of, 18
macrocarpa, possibly a common parent of, 19
Machetes pugnax, polymorphism of male, 28
Maize, Blaringhem's experiments on, 229
Maize, cumulative factors in, 116
Malformations, dominants, arising de novo, 89
Manx Cat, heredity, 46
Matthiola, 84,104,113
Melanic varieties, 135 et seq.
Memory, analogy with heredity, 190
Meristic variation, 69,83,86
Mirabilis, striped, 57
Models of segmentation, 59,60
"Modes," Coutagne's conception of, 126
Mödling, peculiar race of _Pieris napi_ at, 178
Mole, albino, 27,28
Mule, Linnaeus on, 8
Mutation, Matthioli on, 4
in Mercurialis, 5
in Kales, 5
alleged in bulbs, 5
Theory, 97
periods of, 114
in Bacteria, 214
Mutilation, consequences of, 71
alleged effect of, on offspring, 229
Myxococcus, variation in, 213
Narwhal, asymmetry of tusks, 44
Nemesia strumosa, 91
Neuration, a basis for classification, 13
Nicotiana, sterility of hybrid, 242
Nightjars, varying, 150
Noctuidae, fixity and variability, 25
Noctua, polymorphic and fixed species, 25
Noctua castanea, local forms of, 122
Nomenclature, future of, 94,245
Notonecta, variations of, 130
Odontoptera bidentata, melanic form, 137
Oedipodidae, protectively coloured, 140
Oenothera, new dominant in, 92
rubricalyx and rubrinervis, 92,95
Lamarckiana, 92,101
origin of, 102,244
has bad pollen-grains, 102
factorial analysis of, 103
pollen and egg-cells genetically dissimilar, 104
Oenothera, "twin hybrids", 105
laeta and velutina, 105
reciprocal crosses in, 105 et seq.
possible coupling in, 111
dwarfs, 112,114
"Triple hybrids", 114
alleged variation due to treatment, 227
Ophrys, local variability, 125
Orange, polyembryony, 45
Osmotic growth, 65
Overlapping forms, 146,174
Papilio, geographical races of, 162
Papilio turnus, variation of, 144
Pararge egeria, geographical forms, 166 et seq.
Parthenogenesis, 50
Partula, local forms of, 133
Passer domesticus and montanus, distinctions, 22
Pea, round and wrinkled, 95
Pear, will not cross with apple, 239
Pelargonium, variegated, 55
bud-sports, 56
Periodic phenomena in structure, 63
Peronea, fixed and variable species, 26
"Petites espèces", 248
Petunia, double, 104
Phalanger maculatus, local variation, 119
Pheasant, fixity of, 29
Phigalia pilosaria, melanic, 139,140
Phratora vitellinae, experiments on, 193
Phyllotaxis, 69
Pied varieties common in Passer domesticus
unknown in Montanus, 23
Pieris napi and bryoniae, 174 et seq.
Pig, mule-footed, 46
Pigeon, web-footed, 46,49
Indian Rock, a recessive form, 98
Pigments, nature of, 83
Pisum humile, hybrids with culinary peas, 244
species, 246
Planarian, regeneration of, 71,77
Plotheia frontalis, polymorphic, 26,29
Plusia, fixity and variation in, 26
Poephila gouldiae, variation of, 148,149
Polarity of individual, 44
Polia chi, melanic, 138
Polyanthus, short-styled selected, 236
Polydactylism in Cat, 52,53
Polyembryony, 45
Potato, variation in, 91
Poultry, evolution of, 90
Primula obconica, 91
Primula sinensis, flaked, 57
Leaf-shapes, 70
new dominant in, 92
sterility in, 236
"Giants", 236
Primula, species-hybrids, 242
Protective coloration, 140
Pyrrhulagra, local forms, 120
Python, twin-vertebrae, 60
Quiscalus, geographical races of, 156
Rabbit, Angora, 46
colours of, 93
Incompatibility with hare, 242
Raimannia odorata, Macdougal's experiments on, 226
Rats, Variation in, 248
Recessives, origin of, 90
Reciprocal crosses, giving distinct results, 105 et seq.
Regeneration, 70
Repulsion, 110
Reversal on Regeneration, 77
Rhamphocoelus, geographical forms, 159,184
Rhinoptera, variation in jaws of, 38
Rhythm in repetition, 69
Ribs, variation of, 68
Rights and Lefts, 57-58
Ripples, analogous to segments, 60,66,67
regeneration of, 79
Rollers, geographical races of, 160
Ruff, polymorphism of male, 28
Salamandra, maculosa and atra, 182,199,203
spotted and striped, 207
geographical variation of, 208
Segmentation, nature of, 63
simulated mechanically, 64
compared with rippling, 65
analogies with, 68
Segmentation of normally unsegmented structures, 38
Selection, Natural, an insufficient
cause of definiteness of types, 17,134,142
Sempervivum, 250
Serial Homology, the true nature of, 62,66
Setina, Alpine varieties, 181
Sex of Twins, 44
Sex-factors, possible coupling of, 111
Sexual characters, variation in, 119 et seq.
Siamese twins, 44
Silky Fowl, 84,85
Simocephalus, changed by environment, 218
Sinistral forms, 33-34
Situs transversus, 43
Skate's jaws, variation in, 38
Sloths, vertebral variation, 68
Species, conceptions of, 3,94,99,240,245
allied, distribution of, 185
alternative uses of the term, 245
Specific difference, universality of, 12
of organisms compared with those
of inorganic materials, 15
failure of theory of Selection
to explain, 18,134,247
Sphyropicus varius, 149,156
Spilosoma lubricipeda, varieties of, 181
Zatima, Heligoland form, 181
Spinal nerves, segmentation of, 67
Sporadic variation, 131,134,248
Squashes, polymorphism of, 100
Staphylococcus pyogenes, variation in, 213
Sterility of hybrids, in general, 233
in Lychnis hybrids, 20 et seq.
in crossing forms of Draba, 243
Significance of, 244
Self, 238
Stilt, 234
Stocks, 84,104,113
Striped varieties, 57
Substantive variation, 84
Subtraction-stages, 93
Supernumerary limbs, 72-76
Sweet pea, variation of, 91
sterile anthers in, 237
Symmetry compared with heredity, 41
Symmetry of body approximate, 78
Syndactyly, 47
in foot, 48
Synthetic formulae, in nomenclature, 94
Taeniocampa, fixed and variable species, 25
Tamias, local forms of, 132
Tanagers, geographical races of, 159
Teeth, variation in, 67,39
Tephrosia consortaria and consonaria, 137,139,140
Tephrosia species, separated by season, 119
Terminal members, variation of, 68
Thais rumina, local variation in, 27
Tolerance, persistence of diversity due to, 17,134
Tomato, number of cells in fruit, 46
Transitional populations, rarity of, 165
an example, 178
Tropaeolum, sterile anthers in, 237
Trypanosomes, variation in, 215
Tusk, of Elephant, segmented, 38
of Narwhal, 44
Twinning, 41,44,71
heredity of, 45
in organs, 46
Uria troile, variety of, 150
Vanessa urticae, effects of temperature, 191
Variation, a medley of phenomena, 14,15
sporadic, 131,134
and locality, 118
Causes of genetic, 86,87,131,212
Substantive and meristic, 83
Veronica, specific difference in, 16
intermediates between species, 17
Vertebrae, division in, 60,61
homologies of, 66
Vespa, specific difference in, 23
Vortex, living organism compared with, 40
Wave-motion compared with repetition of parts, 62,67,79
Wheat, cumulative factors in, 116
climatic experiments on, 195
Woodpecker, 234
Zebra, pattern of stripes compared with ripples, 38
INDEX OF PERSONS
PAGE
Ackermann, 242
Agar, 218
Allen, J. A., 132,147,159
Annandale, 47
Arrigoni degli Oddi, 167
Backhouse, 50
Baker, G. T., 166
Bangs, Outram, 120,142,155
Barrett, 26,136,167,173,178,193
Baur, E., 55,99
Baur, G., 119
Beneden, van, 75
Bentham, on species of Veronica, 16
Lychnis, 21
Primula, 22
Bernadin, 42
Bishop, L. B., 153,157
Blaringhem, 229
Bobart, 5
Boisduval, 182
Boissier, 19
Borradaile, 74,75
Boulenger, E. G., 208
Boulenger, G. A., 182,207,209
Boyle, 5,54
Brewster, W., 149,150
Britton, 227
Brown, T. Graham, 198
Brown-Séquard, 197 et seq.
Bruant, P., 51
Buffon, 234
Butler, S., 189,190
Buysson, R. du, 24
Candolle, de, 245
Carpenter, J. H., 172
Chapman, F. M., 148,156,157,158
Chapman, T. A., 13,167,182,231
Church, A. H., 69
Cieslar, 197
Clark, Austin, 142,144
Cockayne, E. A., 43
Cockerell, T. D. A., 224
Compton, R. H., 50,58,227
Cope, 230
Cory, 142
Correns, 239
Coutagne, 125 et seq.
Darwin, on Variation, 1,2
Systematics, 10
Selection, 134,139
Heterostyle plants, 236,237
Darwin, F., 190
Darwin, Sir G., 41
Davenport, 46
Davis, H. M., 102
Delcourt, 130
Deschange, 181
Dobell, 215
Doncaster, 105,121,136
Driesch, 80,81
Duchartre, 51
East, 91,116
Edwards, W. H., 162
Ehrlich, 215
Fellmer, 215
Field, W. L. W., 161
Fischer, E., 192
Fleck, 171,174
Fletcher, W. H. B., 26,181
Foster, Sir N., 39
Gallé, 123
Garrod, 83
Gates, 92,95,102
Gayner, F., 177
Godron, 249
Gold, E., 196
Goldschmidt, 116
Goodwin, E., 137
Gortner, 226
Greene, E. L., 8
Gregory, R. P., 92,100,236
Grenier, 249
Grover, 173
Gruber, 48
Gulick, 119,133
Hamling, 142
Hampson, Sir G., 26
Harris, 142
Hartlaub, 182
Herbst, 42
Heribert-Nilsson, 116
Hewett, 182
Honing, 105
Hunter, John, 44
Jakowatz, 197
Janet, 24
Jeans, 41
Jenkinson, 40
Jentink, 120
Johannsen, 195
Jordan, 185,242,249
Kammere, 199 et seq.
Keeble, 236
Klebs, 250
Krancher, 182
Küchenmeister, 44
Kudicke, 215
Lamarck, 9
Lang, A., 128
Lawrence, W. N., 142,145
Leake, H. Martin, 98,100
Leavitt, 185
Lecoq, 99
Lederer, 167
Leduc, 64,65,80
Leydig, 182
Linden, M. von, 192
Linnaeus, 6,7,8
Lloyd, R. E., 248
Locard, 130
Lock, R. H., 242,244
Loeb, 42,45,50,71,77
Lotsy, 99
Lowe, P. R., 143
Macdougal, W. T., 102,226
Marchant, 7
Mathew, 171
Matthioli, 4
Mayer, A. G., 133
Mendel, Rediscovery of, 2
On Fasciation, 49
Merrifield, 169, 172
Miller, W. D., 120,149
Morgan, 71,77,91,198
Moggridge, 125
Nathusius, S., 242
Nettleship., 44
Newman, H. H., 42
Newsholme, 48
Nilsson-Ehle, 116,169
Norman, A. M., 125,156
Ober, 142
Oberthür, 168,170,193
Oliver, J., 45
Page, H. E., 167,180
Patterson, J. T., 42
Payne, F., 278
Pellew, 236
Poll, 45
Porritt, 136
Poulton, 141
Powers, J. H., 230
Pringsheim, H., 213
Przibram, 72,78,178,194,197,199
Punnett, 110
Ray, 4,5
Raynor, 105
Ridgway, 10,120
Roedelius, 195
Rolfe, 20
Rosen, F., 242
Rosner, 42
Rowland-Brown, H., 167,180
Sargent, 185
Saunders, E. R., 84,104,112
Schima, 177
Schröder, 193,194
Schübeler, 195
Semon, R., 190 et seq.
Sharrock, 5
Shull, 100
Speyer, A., 166,170,181
Spillman, 47
Standfuss, 135,181,191
Staples-Browne, 49,98
Staudinger, 170,179
Stockard, 50,71
Sutton, 236,244
Tornier, 72
Tower, W. L., 218-226
Trechmann, 133
Tugwell, 181
Tutt, J. W. On Definiteness of Species, 13
On Plusia interrogationis, 26
On Tephrosia, 119
On N. castanea, 122
On Pararge egeria, 167 et seq.
Verity, R., 171,177
Vries, H. de, 101-115,222,239
Walker, G, 49
Weir, Jenner, 119
Weismann, 176,188
Wendelstadt, 215
Werbitzki, 215
Werner, 209
Wettstein, 197
Wheeler, G., 168,171
Wheldale, 83
Wilder, 44
Wille, 197
Williams, H., 167,172
Windle, B. C. A., 43
Winslow, 213
Wolf, F., 213
Woodforde, 123
Woltereck, 215
Zeijlstra, 114
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