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Title: On the Origin of Species
1st Edition
Author: Charles Darwin
Release Date: March, 1998 [eBook #1228]
[Most recently updated: September 6, 2021]
Language: English
Produced by: Sue Asscher and David Widger
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On
the Origin of Species
BY MEANS OF NATURAL SELECTION,
OR THE
PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.
By Charles Darwin, M.A.,
Fellow Of The Royal, Geological, Linnæan, Etc., Societies;
Author Of ‘Journal Of Researches During H.M.S. Beagle’s Voyage
Round The World.’
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1859.
“But with regard to the material world, we can at least go so far as
this—we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws.”
W. WHEWELL: _Bridgewater Treatise_.
“To conclude, therefore, let no man out of a weak conceit of sobriety, or
an ill-applied moderation, think or maintain, that a man can search too far or
be too well studied in the book of God’s word, or in the book of
God’s works; divinity or philosophy; but rather let men endeavour an
endless progress or proficience in both.”
BACON: _Advancement of Learning_.
_Down, Bromley, Kent,
October_, 1_st_, 1859.
Contents
INTRODUCTION.
1. VARIATION UNDER DOMESTICATION.
2. VARIATION UNDER NATURE.
3. STRUGGLE FOR EXISTENCE.
4. NATURAL SELECTION.
5. LAWS OF VARIATION.
6. DIFFICULTIES ON THEORY.
7. INSTINCT.
8. HYBRIDISM.
9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
11. GEOGRAPHICAL DISTRIBUTION.
12. GEOGRAPHICAL DISTRIBUTION—_continued_.
13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
14. RECAPITULATION AND CONCLUSION.
INDEX
DETEAILED CONTENTS. ON THE ORIGIN OF SPECIES.
INTRODUCTION.
CHAPTER I. VARIATION UNDER DOMESTICATION.
Causes of Variability.
Effects of Habit.
Correlation of Growth.
Inheritance.
Character of Domestic Varieties.
Difficulty of distinguishing between Varieties and Species.
Origin of Domestic Varieties from one or more Species.
Domestic Pigeons, their Differences and Origin.
Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection.
Unknown Origin of our Domestic Productions.
Circumstances favourable to Man’s power of Selection.
CHAPTER 2. VARIATION UNDER NATURE.
Variability.
Individual Differences.
Doubtful species.
Wide ranging, much diffused, and common species vary most.
Species of the larger genera in any country vary more than the
species of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.
CHAPTER 3. STRUGGLE FOR EXISTENCE.
Bears on natural selection.
The term used in a wide sense.
Geometrical powers of increase.
Rapid increase of naturalised animals and plants.
Nature of the checks to increase.
Competition universal.
Effects of climate.
Protection from the number of individuals.
Complex relations of all animals and plants throughout nature.
Struggle for life most severe between individuals and varieties of
the same species; often severe between species of the same genus.
The relation of organism to organism the most important of all
relations.
CHAPTER 4. NATURAL SELECTION.
Natural Selection: its power compared with man’s selection, its power
on characters of trifling importance, its power at all ages and on
both sexes.
Sexual Selection.
On the generality of intercrosses between individuals of the same
species.
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals.
Slow action.
Extinction caused by Natural Selection.
Divergence of Character, related to the diversity of inhabitants of
any small area, and to naturalisation.
Action of Natural Selection, through Divergence of Character and
Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.
CHAPTER 5. LAWS OF VARIATION.
Effects of external conditions.
Use and disuse, combined with natural selection; organs of flight and
of vision.
Acclimatisation.
Correlation of growth.
Compensation and economy of growth.
False correlations.
Multiple, rudimentary, and lowly organised structures variable.
Parts developed in an unusual manner are highly variable: specific
characters more variable than generic: secondary sexual characters
variable.
Species of the same genus vary in an analogous manner.
Reversions to long-lost characters.
Summary.
CHAPTER 6. DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification.
Transitions.
Absence or rarity of transitional varieties.
Transitions in habits of life.
Diversified habits in the same species.
Species with habits widely different from those of their allies.
Organs of extreme perfection.
Means of transition.
Cases of difficulty.
Natura non facit saltum.
Organs of small importance.
Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced
by the theory of Natural Selection.
CHAPTER 7. INSTINCT.
Instincts comparable with habits, but different in their origin.
Instincts graduated.
Aphides and ants.
Instincts variable.
Domestic instincts, their origin.
Natural instincts of the cuckoo, ostrich, and parasitic bees.
Slave-making ants.
Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts.
Neuter or sterile insects.
Summary.
CHAPTER 8. HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication.
Laws governing the sterility of hybrids.
Sterility not a special endowment, but incidental on other
differences.
Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing.
Fertility of varieties when crossed and of their mongrel offspring
not universal.
Hybrids and mongrels compared independently of their fertility.
Summary.
CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
On the vast lapse of time, as inferred from the rate of deposition
and of denudation.
On the poorness of our palæontological collections.
On the intermittence of geological formations.
On the absence of intermediate varieties in any one formation.
On the sudden appearance of groups of species.
On their sudden appearance in the lowest known fossiliferous strata.
CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species.
On their different rates of change.
Species once lost do not reappear.
Groups of species follow the same general rules in their appearance
and disappearance as do single species.
On Extinction.
On simultaneous changes in the forms of life throughout the world.
On the affinities of extinct species to each other and to living
species.
On the state of development of ancient forms.
On the succession of the same types within the same areas.
Summary of preceding and present chapters.
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in
physical conditions.
Importance of barriers.
Affinity of the productions of the same continent.
Centres of creation.
Means of dispersal, by changes of climate and of the level of the
land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.
CHAPTER 12. GEOGRAPHICAL DISTRIBUTION—_continued_.
Distribution of fresh-water productions.
On the inhabitants of oceanic islands.
Absence of Batrachians and of terrestrial Mammals.
On the relation of the inhabitants of islands to those of the nearest
mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.
CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS.
CLASSIFICATION, groups subordinate to groups.
Natural system.
Rules and difficulties in classification, explained on the theory of
descent with modification.
Classification of varieties.
Descent always used in classification.
Analogical or adaptive characters.
Affinities, general, complex and radiating.
Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual.
EMBRYOLOGY, laws of, explained by variations not supervening at an
early age, and being inherited at a corresponding age.
RUDIMENTARY ORGANS; their origin explained.
Summary.
CHAPTER 14. RECAPITULATION AND CONCLUSION.
Recapitulation of the difficulties on the theory of Natural
Selection.
Recapitulation of the general and special circumstances in its
favour.
Causes of the general belief in the immutability of species.
How far the theory of natural selection may be extended.
Effects of its adoption on the study of Natural history.
Concluding remarks.
ON THE ORIGIN OF SPECIES.
INTRODUCTION.
When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America,
and in the geological relations of the present to the past inhabitants
of that continent. These facts seemed to me to throw some light on the
origin of species—that mystery of mysteries, as it has been called by
one of our greatest philosophers. On my return home, it occurred to me,
in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years’ work I allowed
myself to speculate on the subject, and drew up some short notes; these
I enlarged in 1844 into a sketch of the conclusions, which then seemed
to me probable: from that period to the present day I have steadily
pursued the same object. I hope that I may be excused for entering on
these personal details, as I give them to show that I have not been
hasty in coming to a decision.
My work is now nearly finished; but as it will take me two or three
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been
induced to do this, as Mr. Wallace, who is now studying the
natural history of the Malay archipelago, has arrived at almost exactly
the same general conclusions that I have on the origin of species. Last
year he sent to me a memoir on this subject, with a request that I
would forward it to Sir Charles Lyell, who sent it to the Linnean
Society, and it is published in the third volume of the Journal of that
Society. Sir C. Lyell and Dr. Hooker, who both knew of my work—the
latter having read my sketch of 1844—honoured me by thinking it
advisable to publish, with Mr. Wallace’s excellent memoir, some brief
extracts from my manuscripts.
This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors will have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only
the general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing
in detail all the facts, with references, on which my conclusions have
been grounded; and I hope in a future work to do this. For I am well
aware that scarcely a single point is discussed in this volume on which
facts cannot be adduced, often apparently leading to conclusions
directly opposite to those at which I have arrived. A fair result can
be obtained only by fully stating and balancing the facts and arguments
on both sides of each question; and this cannot possibly be here done.
I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however,
let this opportunity pass without expressing my deep obligations to Dr.
Hooker, who for the last fifteen years has aided me in every possible
way by his large stores of knowledge and his excellent judgment.
In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on
their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that each species had not been independently created, but
had descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world have
been modified, so as to acquire that perfection of structure and
coadaptation which most justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one very limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
misseltoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of
external conditions, or of habit, or of the volition of the plant
itself.
The author of the ‘Vestiges of Creation’ would, I presume, say that,
after a certain unknown number of
generations, some bird had given birth to a woodpecker, and some plant
to the misseltoe, and that these had been produced perfect as we now
see them; but this assumption seems to me to be no explanation, for it
leaves the case of the coadaptations of organic beings to each other
and to their physical conditions of life, untouched and unexplained.
It is, therefore, of the highest importance to gain a clear insight
into the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been
disappointed; in this and in all other perplexing cases I have
invariably found that our knowledge, imperfect though it be, of
variation under domestication, afforded the best and safest clue. I may
venture to express my conviction of the high value of such studies,
although they have been very commonly neglected by naturalists.
From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible, and, what
is equally or more important, we shall see how great is the power of
man in accumulating by his Selection successive slight variations. I
will then pass on to the variability of species in a state of nature;
but I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues
of facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the Struggle for
Existence amongst all organic beings throughout the world, which
inevitably follows from their high geometrical powers of
increase, will be treated of. This is the doctrine of Malthus, applied
to the whole animal and vegetable kingdoms. As many more individuals of
each species are born than can possibly survive; and as, consequently,
there is a frequently recurring struggle for existence, it follows that
any being, if it vary however slightly in any manner profitable to
itself, under the complex and sometimes varying conditions of life,
will have a better chance of surviving, and thus be _naturally
selected_. From the strong principle of inheritance, any selected
variety will tend to propagate its new and modified form.
This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less improved
forms of life and induces what I have called Divergence of Character.
In the next chapter I shall discuss the complex and little known laws
of variation and of correlation of growth. In the four succeeding
chapters, the most apparent and gravest difficulties on the theory will
be given: namely, first, the difficulties of transitions, or in
understanding how a simple being or a simple organ can be changed and
perfected into a highly developed being or elaborately constructed
organ; secondly the subject of Instinct, or the mental powers of
animals, thirdly, Hybridism, or the infertility of species and the
fertility of varieties when intercrossed; and fourthly, the
imperfection of the Geological Record. In the next chapter I shall
consider the geological succession of organic beings throughout time;
in the eleventh and twelfth, their geographical distribution throughout
space; in the thirteenth, their classification or mutual affinities,
both when mature and in an embryonic condition. In the last chapter I
shall give a
brief recapitulation of the whole work, and a few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due
allowance for our profound ignorance in regard to the mutual relations
of all the beings which live around us. Who can explain why one species
ranges widely and is very numerous, and why another allied species has
a narrow range and is rare? Yet these relations are of the highest
importance, for they determine the present welfare, and, as I believe,
the future success and modification of every inhabitant of this world.
Still less do we know of the mutual relations of the innumerable
inhabitants of the world during the many past geological epochs in its
history. Although much remains obscure, and will long remain obscure, I
can entertain no doubt, after the most deliberate study and
dispassionate judgment of which I am capable, that the view which most
naturalists entertain, and which I formerly entertained—namely, that
each species has been independently created—is erroneous. I am fully
convinced that species are not immutable; but that those belonging to
what are called the same genera are lineal descendants of some other
and generally extinct species, in the same manner as the acknowledged
varieties of any one species are the descendants of that species.
Furthermore, I am convinced that Natural Selection has been the main
but not exclusive means of modification.
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of Variability. Effects of Habit. Correlation of Growth.
Inheritance. Character of Domestic Varieties. Difficulty of
distinguishing between Varieties and Species. Origin of Domestic
Varieties from one or more Species. Domestic Pigeons, their Differences
and Origin. Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection. Unknown Origin of our Domestic
Productions. Circumstances favourable to Man’s power of Selection.
When we look to the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us, is, that they generally differ much more from each other,
than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and animals
which have been cultivated, and which have varied during all ages under
the most different climates and treatment, I think we are driven to
conclude that this greater variability is simply due to our domestic
productions having been raised under conditions of life not so uniform
as, and somewhat different from, those to which the parent-species have
been exposed under nature. There is, also, I think, some probability in
the view propounded by Andrew Knight, that this variability may be
partly connected with excess of food. It seems pretty clear that
organic beings must be exposed during several generations to the new
conditions of life to cause any appreciable amount of variation; and
that when the organisation has once begun to vary, it generally
continues to vary for many generations.
No case is on record of a variable being ceasing to be variable under
cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable
of rapid improvement or modification.
It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire’s experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by
any clear line of distinction from mere variations. But I am strongly
inclined to suspect that the most frequent cause of variability may be
attributed to the male and female reproductive elements having been
affected prior to the act of conception. Several reasons make me
believe in this; but the chief one is the remarkable effect which
confinement or cultivation has on the functions of the reproductive
system; this system appearing to be far more susceptible than any other
part of the organisation, to the action of any change in the conditions
of life. Nothing is more easy than to tame an animal, and few things
more difficult than to get it to breed freely under confinement, even
in the many cases when the male and female unite. How many animals
there are which will not breed, though living long under not very close
confinement in their native country! This is generally attributed to
vitiated instincts; but how many cultivated plants display the utmost
vigour, and yet rarely or never seed! In some few such cases it has
been found out that very trifling changes, such as a little more or
less water at some particular period of growth, will determine whether
or not the plant sets a seed. I cannot here enter on the copious
details which I have collected on
this curious subject; but to show how singular the laws are which
determine the reproduction of animals under confinement, I may just
mention that carnivorous animals, even from the tropics, breed in this
country pretty freely under confinement, with the exception of the
plantigrades or bear family; whereas, carnivorous birds, with the
rarest exceptions, hardly ever lay fertile eggs. Many exotic plants
have pollen utterly worthless, in the same exact condition as in the
most sterile hybrids. When, on the one hand, we see domesticated
animals and plants, though often weak and sickly, yet breeding quite
freely under confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature, perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail in acting, we need not be surprised at
this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.
Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility; and
variability is the source of all the choicest productions of the
garden. I may add, that as some organisms will breed most freely under
the most unnatural conditions (for instance, the rabbit and ferret kept
in hutches), showing that their reproductive system has not been thus
affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly—perhaps hardly more than in a state
of nature.
A long list could easily be given of “sporting plants;” by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant.
Such buds can be propagated by grafting, etc., and sometimes by seed.
These “sports” are extremely rare under nature, but far from rare under
cultivation; and in this case we see that the treatment of the parent
has affected a bud or offset, and not the ovules or pollen. But it is
the opinion of most physiologists that there is no essential difference
between a bud and an ovule in their earliest stages of formation; so
that, in fact, “sports” support my view, that variability may be
largely attributed to the ovules or pollen, or to both, having been
affected by the treatment of the parent prior to the act of conception.
These cases anyhow show that variation is not necessarily connected, as
some authors have supposed, with the act of generation.
Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Müller has remarked, have apparently been exposed
to exactly the same conditions of life; and this shows how unimportant
the direct effects of the conditions of life are in comparison with the
laws of reproduction, and of growth, and of inheritance; for had the
action of the conditions been direct, if any of the young had varied,
all would probably have varied in the same manner. To judge how much,
in the case of any variation, we should attribute to the direct action
of heat, moisture, light, food, etc., is most difficult: my impression
is, that with animals such agencies have produced very little direct
effect, though apparently more in the case of plants. Under this point
of view, Mr. Buckman’s recent experiments on plants seem extremely
valuable. When all or nearly all the individuals exposed to certain
conditions are affected in the same way, the change at first appears to
be directly due to such conditions; but in some cases it can be shown
that quite opposite conditions produce
similar changes of structure. Nevertheless some slight amount of change
may, I think, be attributed to the direct action of the conditions of
life—as, in some cases, increased size from amount of food, colour from
particular kinds of food and from light, and perhaps the thickness of
fur from climate.
Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has
a more marked effect; for instance, I find in the domestic duck that
the bones of the wing weigh less and the bones of the leg more, in
proportion to the whole skeleton, than do the same bones in the
wild-duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild
parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison with
the state of these organs in other countries, is another instance of
the effect of use. Not a single domestic animal can be named which has
not in some country drooping ears; and the view suggested by some
authors, that the drooping is due to the disuse of the muscles of the
ear, from the animals not being much alarmed by danger, seems probable.
There are many laws regulating variation, some few of which can be
dimly seen, and will be hereafter briefly mentioned. I will here only
allude to what may be called correlation of growth. Any change in the
embryo or larva will almost certainly entail changes in the mature
animal. In monstrosities, the correlations between quite distinct parts
are very curious; and many instances are given in Isidore Geoffroy St.
Hilaire’s great work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head. Some instances of
correlation are quite whimsical; thus
cats with blue eyes are invariably deaf; colour and constitutional
peculiarities go together, of which many remarkable cases could be
given amongst animals and plants. From the facts collected by
Heusinger, it appears that white sheep and pigs are differently
affected from coloured individuals by certain vegetable poisons.
Hairless dogs have imperfect teeth; long-haired and coarse-haired
animals are apt to have, as is asserted, long or many horns; pigeons
with feathered feet have skin between their outer toes; pigeons with
short beaks have small feet, and those with long beaks large feet.
Hence, if man goes on selecting, and thus augmenting, any peculiarity,
he will almost certainly unconsciously modify other parts of the
structure, owing to the mysterious laws of the correlation of growth.
The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth while
carefully to study the several treatises published on some of our old
cultivated plants, as on the hyacinth, potato, even the dahlia, etc.;
and it is really surprising to note the endless points in structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic,
and tends to depart in some small degree from that of the parental
type.
Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both those
of slight and those of considerable physiological importance, is
endless. Dr. Prosper Lucas’s treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical
writers alone. When a
deviation appears not unfrequently, and we see it in the father and
child, we cannot tell whether it may not be due to the same original
cause acting on both; but when amongst individuals, apparently exposed
to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent—say,
once amongst several million individuals—and it reappears in the child,
the mere doctrine of chances almost compels us to attribute its
reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, etc., appearing in several
members of the same family. If strange and rare deviations of structure
are truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the
whole subject, would be, to look at the inheritance of every character
whatever as the rule, and non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say why
the same peculiarity in different individuals of the same species, and
in individuals of different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or other much more remote ancestor; why
a peculiarity is often transmitted from one sex to both sexes or to one
sex alone, more commonly but not exclusively to the like sex. It is a
fact of some little importance to us, that peculiarities appearing in
the males of our domestic breeds are often transmitted either
exclusively, or in a much greater degree, to males alone. A much more
important rule, which I think may be trusted, is that, at whatever
period of life a peculiarity first appears, it tends to appear in the
offspring at a corresponding age, though sometimes earlier. In many
cases this could
not be otherwise: thus the inherited peculiarities in the horns of
cattle could appear only in the offspring when nearly mature;
peculiarities in the silkworm are known to appear at the corresponding
caterpillar or cocoon stage. But hereditary diseases and some other
facts make me believe that the rule has a wider extension, and that
when there is no apparent reason why a peculiarity should appear at any
particular age, yet that it does tend to appear in the offspring at the
same period at which it first appeared in the parent. I believe this
rule to be of the highest importance in explaining the laws of
embryology. These remarks are of course confined to the first
_appearance_ of the peculiarity, and not to its primary cause, which
may have acted on the ovules or male element; in nearly the same manner
as in the crossed offspring from a short-horned cow by a long-horned
bull, the greater length of horn, though appearing late in life, is
clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists—namely, that our domestic
varieties, when run wild, gradually but certainly revert in character
to their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be great
difficulty in proving its truth: we may safely conclude that very many
of the most strongly-marked domestic varieties could not possibly live
in a wild state. In many cases we do not know what the aboriginal stock
was, and so could not tell whether or not nearly perfect reversion had
ensued. It would be quite necessary, in order to prevent the effects of
intercrossing, that only a
single variety should be turned loose in its new home. Nevertheless, as
our varieties certainly do occasionally revert in some of their
characters to ancestral forms, it seems to me not improbable, that if
we could succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very
poor soil (in which case, however, some effect would have to be
attributed to the direct action of the poor soil), that they would to a
large extent, or even wholly, revert to the wild aboriginal stock.
Whether or not the experiment would succeed, is not of great importance
for our line of argument; for by the experiment itself the conditions
of life are changed. If it could be shown that our domestic varieties
manifested a strong tendency to reversion,—that is, to lose their
acquired characters, whilst kept under unchanged conditions, and whilst
kept in a considerable body, so that free intercrossing might check, by
blending together, any slight deviations of structure, in such case, I
grant that we could deduce nothing from domestic varieties in regard to
species. But there is not a shadow of evidence in favour of this view:
to assert that we could not breed our cart and race-horses, long and
short-horned cattle, and poultry of various breeds, and esculent
vegetables, for an almost infinite number of generations, would be
opposed to all experience. I may add, that when under nature the
conditions of life do change, variations and reversions of character
probably do occur; but natural selection, as will hereafter be
explained, will determine how far the new characters thus arising shall
be preserved.
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already
remarked, less uniformity of character than in true species. Domestic
races of
the same species, also, often have a somewhat monstrous character; by
which I mean, that, although differing from each other, and from the
other species of the same genus, in several trifling respects, they
often differ in an extreme degree in some one part, both when compared
one with another, and more especially when compared with all the
species in nature to which they are nearest allied. With these
exceptions (and with that of the perfect fertility of varieties when
crossed,—a subject hereafter to be discussed), domestic races of the
same species differ from each other in the same manner as, only in most
cases in a lesser degree than, do closely-allied species of the same
genus in a state of nature. I think this must be admitted, when we find
that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by some competent judges as mere
varieties, and by other competent judges as the descendants of
aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not
differ from each other in characters of generic value. I think it could
be shown that this statement is hardly correct; but naturalists differ
most widely in determining what characters are of generic value; all
such valuations being at present empirical. Moreover, on the view of
the origin of genera which I shall presently give, we have no right to
expect often to meet with generic differences in our domesticated
productions.
When we attempt to estimate the amount of structural difference between
the domestic races of the same species, we are soon involved in doubt,
from not knowing whether they have descended from one or several
parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
their kind so truly, were the offspring of any single species, then
such facts would have great weight in making us doubt about the
immutability of the many very closely allied and natural species—for
instance, of the many foxes—inhabiting different quarters of the world.
I do not believe, as we shall presently see, that all our dogs have
descended from any one wild species; but, in the case of some other
domestic races, there is presumptive, or even strong, evidence in
favour of this view.
It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates? Has the little variability of the ass
or guinea-fowl, or the small power of endurance of warmth by the
rein-deer, or of cold by the common camel, prevented their
domestication? I cannot doubt that if other animals and plants, equal
in number to our domesticated productions, and belonging to equally
diverse classes and countries, were taken from a state of nature, and
could be made to breed for an equal number of generations under
domestication, they would vary on an average as largely as the parent
species of our existing domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants, I
do not think it is possible to come to any definite conclusion, whether
they have descended from one or several species. The argument mainly
relied on by those who believe in the multiple origin
of our domestic animals is, that we find in the most ancient records,
more especially on the monuments of Egypt, much diversity in the
breeds; and that some of the breeds closely resemble, perhaps are
identical with, those still existing. Even if this latter fact were
found more strictly and generally true than seems to me to be the case,
what does it show, but that some of our breeds originated there, four
or five thousand years ago? But Mr. Horner’s researches have rendered
it in some degree probable that man sufficiently civilized to have
manufactured pottery existed in the valley of the Nile thirteen or
fourteen thousand years ago; and who will pretend to say how long
before these ancient periods, savages, like those of Tierra del Fuego
or Australia, who possess a semi-domestic dog, may not have existed in
Egypt?
The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical and
other considerations, I think it highly probable that our domestic dogs
have descended from several wild species. In regard to sheep and goats
I can form no opinion. I should think, from facts communicated to me by
Mr. Blyth, on the habits, voice, and constitution, etc., of the humped
Indian cattle, that these had descended from a different aboriginal
stock from our European cattle; and several competent judges believe
that these latter have had more than one wild parent. With respect to
horses, from reasons which I cannot give here, I am doubtfully inclined
to believe, in opposition to several authors, that all the races have
descended from one wild stock. Mr. Blyth, whose opinion, from his large
and varied stores of knowledge, I should value more than that of almost
any one, thinks that all the breeds of poultry have proceeded from the
common wild
Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the
breeds of which differ considerably from each other in structure, I do
not doubt that they all have descended from the common wild duck and
rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats in Europe alone, and
several even within Great Britain. One author believes that there
formerly existed in Great Britain eleven wild species of sheep peculiar
to it! When we bear in mind that Britain has now hardly one peculiar
mammal, and France but few distinct from those of Germany and
conversely, and so with Hungary, Spain, etc., but that each of these
kingdoms possesses several peculiar breeds of cattle, sheep, etc., we
must admit that many domestic breeds have originated in Europe; for
whence could they have been derived, as these several countries do not
possess a number of peculiar species as distinct parent-stocks? So it
is in India. Even in the case of the domestic dogs of the whole world,
which I fully admit have probably descended from several wild species,
I cannot doubt that there has been an immense amount of inherited
variation. Who can believe that animals closely resembling the Italian
greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.—so
unlike all wild Canidæ—ever existed freely in a state of nature? It has
often been loosely said that all our races of dogs have been produced
by the crossing of a few aboriginal species; but by crossing we can get
only forms in some degree intermediate between their parents; and if we
account for our several domestic races by this process, we must admit
the former existence of the most extreme forms, as the Italian
greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover, the
possibility of making distinct races by crossing has been greatly
exaggerated. There can be no doubt that a race may be modified by
occasional crosses, if aided by the careful selection of those
individual mongrels, which present any desired character; but that a
race could be obtained nearly intermediate between two extremely
different races or species, I can hardly believe. Sir J. Sebright
expressly experimentised for this object, and failed. The offspring
from the first cross between two pure breeds is tolerably and sometimes
(as I have found with pigeons) extremely uniform, and everything seems
simple enough; but when these mongrels are crossed one with another for
several generations, hardly two of them will be alike, and then the
extreme difficulty, or rather utter hopelessness, of the task becomes
apparent. Certainly, a breed intermediate between _two very distinct_
breeds could not be got without extreme care and long-continued
selection; nor can I find a single case on record of a permanent race
having been thus formed.
_On the Breeds of the Domestic Pigeon_.—Believing that it is always
best to study some special group, I have, after deliberation, taken up
domestic pigeons. I have kept every breed which I could purchase or
obtain, and have been most kindly favoured with skins from several
quarters of the world, more especially by the Honourable W. Elliot from
India, and by the Honourable C. Murray from Persia. Many treatises in
different languages have been published on pigeons, and some of them
are very important, as being of considerable antiquity. I have
associated with several eminent fanciers, and have been permitted to
join two
of the London Pigeon Clubs. The diversity of the breeds is something
astonishing. Compare the English carrier and the short-faced tumbler,
and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier, more especially
the male bird, is also remarkable from the wonderful development of the
carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a
wide gape of mouth. The short-faced tumbler has a beak in outline
almost like that of a finch; and the common tumbler has the singular
and strictly inherited habit of flying at a great height in a compact
flock, and tumbling in the air head over heels. The runt is a bird of
great size, with long, massive beak and large feet; some of the
sub-breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the
carrier, but, instead of a very long beak, has a very short and very
broad one. The pouter has a much elongated body, wings, and legs; and
its enormously developed crop, which it glories in inflating, may well
excite astonishment and even laughter. The turbit has a very short and
conical beak, with a line of reversed feathers down the breast; and it
has the habit of continually expanding slightly the upper part of the
oesophagus. The Jacobin has the feathers so much reversed along the
back of the neck that they form a hood, and it has, proportionally to
its size, much elongated wing and tail feathers. The trumpeter and
laugher, as their names express, utter a very different coo from the
other breeds. The fantail has thirty or even forty tail-feathers,
instead of twelve or fourteen, the normal number in all members of the
great pigeon family; and these feathers are kept expanded, and are
carried so erect that in good birds the head and tail
touch; the oil-gland is quite aborted. Several other less distinct
breeds might have been specified.
In the skeletons of the several breeds, the development of the bones of
the face in length and breadth and curvature differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The number of the caudal and
sacral vertebræ vary; as does the number of the ribs, together with
their relative breadth and the presence of processes. The size and
shape of the apertures in the sternum are highly variable; so is the
degree of divergence and relative size of the two arms of the furcula.
The proportional width of the gape of mouth, the proportional length of
the eyelids, of the orifice of the nostrils, of the tongue (not always
in strict correlation with the length of beak), the size of the crop
and of the upper part of the oesophagus; the development and abortion
of the oil-gland; the number of the primary wing and caudal feathers;
the relative length of wing and tail to each other and to the body; the
relative length of leg and of the feet; the number of scutellæ on the
toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage
is acquired varies, as does the state of the down with which the
nestling birds are clothed when hatched. The shape and size of the eggs
vary. The manner of flight differs remarkably; as does in some breeds
the voice and disposition. Lastly, in certain breeds, the males and
females have come to differ to a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which if shown
to an ornithologist, and he were told that they were wild birds, would
certainly, I think, be ranked by him as well-defined species. Moreover,
I do not believe that any ornithologist would place touch; the
oil-gland is quite aborted. Several other less distinct breeds might
have been specified.
the English carrier, the short-faced tumbler, the runt, the barb,
pouter, and fantail in the same genus; more especially as in each of
these breeds several truly-inherited sub-breeds, or species as he might
have called them, could be shown him.
Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely,
that all have descended from the rock-pigeon (Columba livia), including
under this term several geographical races or sub-species, which differ
from each other in the most trifling respects. As several of the
reasons which have led me to this belief are in some degree applicable
in other cases, I will here briefly give them. If the several breeds
are not varieties, and have not proceeded from the rock-pigeon, they
must have descended from at least seven or eight aboriginal stocks; for
it is impossible to make the present domestic breeds by the crossing of
any lesser number: how, for instance, could a pouter be produced by
crossing two breeds unless one of the parent-stocks possessed the
characteristic enormous crop? The supposed aboriginal stocks must all
have been rock-pigeons, that is, not breeding or willingly perching on
trees. But besides C. livia, with its geographical sub-species, only
two or three other species of rock-pigeons are known; and these have
not any of the characters of the domestic breeds. Hence the supposed
aboriginal stocks must either still exist in the countries where they
were originally domesticated, and yet be unknown to ornithologists; and
this, considering their size, habits, and remarkable characters, seems
very improbable; or they must have become extinct in the wild state.
But birds breeding on precipices, and good fliers, are unlikely to be
exterminated; and the common rock-pigeon, which has the same habits
with the domestic breeds, has not been exterminated
even on several of the smaller British islets, or on the shores of the
Mediterranean. Hence the supposed extermination of so many species
having similar habits with the rock-pigeon seems to me a very rash
assumption. Moreover, the several above-named domesticated breeds have
been transported to all parts of the world, and, therefore, some of
them must have been carried back again into their native country; but
not one has ever become wild or feral, though the dovecot-pigeon, which
is the rock-pigeon in a very slightly altered state, has become feral
in several places. Again, all recent experience shows that it is most
difficult to get any wild animal to breed freely under domestication;
yet on the hypothesis of the multiple origin of our pigeons, it must be
assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.
An argument, as it seems to me, of great weight, and applicable in
several other cases, is, that the above-specified breeds, though
agreeing generally in constitution, habits, voice, colouring, and in
most parts of their structure, with the wild rock-pigeon, yet are
certainly highly abnormal in other parts of their structure: we may
look in vain throughout the whole great family of Columbidæ for a beak
like that of the English carrier, or that of the short-faced tumbler,
or barb; for reversed feathers like those of the jacobin; for a crop
like that of the pouter; for tail-feathers like those of the fantail.
Hence it must be assumed not only that half-civilized man succeeded in
thoroughly domesticating several species, but that he intentionally or
by chance picked out extraordinarily abnormal species; and further,
that these very species have since all become extinct or unknown. So
many strange contingencies seem to me improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white rump
(the Indian sub-species, C. intermedia of Strickland, having it
bluish); the tail has a terminal dark bar, with the bases of the outer
feathers externally edged with white; the wings have two black bars;
some semi-domestic breeds and some apparently truly wild breeds have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of the
outer tail-feathers, sometimes concur perfectly developed. Moreover,
when two birds belonging to two distinct breeds are crossed, neither of
which is blue or has any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly
black barbs, and they produced mottled brown and black birds; these I
again crossed together, and one grandchild of the pure white fantail
and pure black barb was of as beautiful a blue colour, with the white
rump, double black wing-bar, and barred and white-edged tail-feathers,
as any wild rock-pigeon! We can understand these facts, on the
well-known principle of reversion to ancestral characters, if all the
domestic breeds have descended from the rock-pigeon. But if we deny
this, we must make one of the two following highly improbable
suppositions. Either, firstly, that all the several imagined aboriginal
stocks were coloured and marked like the rock-pigeon, although no other
existing species is thus coloured and marked, so that in each separate
breed there might be a tendency to revert to the very same colours and
markings. Or, secondly,
that each breed, even the purest, has within a dozen or, at most,
within a score of generations, been crossed by the rock-pigeon: I say
within a dozen or twenty generations, for we know of no fact
countenancing the belief that the child ever reverts to some one
ancestor, removed by a greater number of generations. In a breed which
has been crossed only once with some distinct breed, the tendency to
reversion to any character derived from such cross will naturally
become less and less, as in each succeeding generation there will be
less of the foreign blood; but when there has been no cross with a
distinct breed, and there is a tendency in both parents to revert to a
character, which has been lost during some former generation, this
tendency, for all that we can see to the contrary, may be transmitted
undiminished for an indefinite number of generations. These two
distinct cases are often confounded in treatises on inheritance.
Lastly, the hybrids or mongrels from between all the domestic breeds of
pigeons are perfectly fertile. I can state this from my own
observations, purposely made on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals _clearly distinct_ being themselves perfectly
fertile. Some authors believe that long-continued domestication
eliminates this strong tendency to sterility: from the history of the
dog I think there is some probability in this hypothesis, if applied to
species closely related together, though it is unsupported by a single
experiment. But to extend the hypothesis so far as to suppose that
species, aboriginally as distinct as carriers, tumblers, pouters, and
fantails now are, should yield offspring perfectly fertile, _inter se_,
seems to me rash in the extreme.
From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed
species of pigeons to breed freely under domestication; these supposed
species being quite unknown in a wild state, and their becoming nowhere
feral; these species having very abnormal characters in certain
respects, as compared with all other Columbidæ, though so like in most
other respects to the rock-pigeon; the blue colour and various marks
occasionally appearing in all the breeds, both when kept pure and when
crossed; the mongrel offspring being perfectly fertile;—from these
several reasons, taken together, I can feel no doubt that all our
domestic breeds have descended from the Columba livia with its
geographical sub-species.
In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
breeds, more especially those brought from distant countries, we can
make an almost perfect series between the extremes of structure.
Thirdly, those characters which are mainly distinctive of each breed,
for instance the wattle and length of beak of the carrier, the
shortness of that of the tumbler, and the number of tail-feathers in
the fantail, are in each breed eminently variable; and the explanation
of this fact will be obvious when we come to treat of selection.
Fourthly, pigeons have been watched, and tended with the utmost care,
and loved by many people. They have been domesticated for thousands of
years in several quarters of the world; the earliest known record of
pigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr. Birch informs me that
pigeons are given in a bill
of fare in the previous dynasty. In the time of the Romans, as we hear
from Pliny, immense prices were given for pigeons; “nay, they are come
to this pass, that they can reckon up their pedigree and race.” Pigeons
were much valued by Akber Khan in India, about the year 1600; never
less than 20,000 pigeons were taken with the court. “The monarchs of
Iran and Turan sent him some very rare birds;” and, continues the
courtly historian, “His Majesty by crossing the breeds, which method
was never practised before, has improved them astonishingly.” About
this same period the Dutch were as eager about pigeons as were the old
Romans. The paramount importance of these considerations in explaining
the immense amount of variation which pigeons have undergone, will be
obvious when we treat of Selection. We shall then, also, see how it is
that the breeds so often have a somewhat monstrous character. It is
also a most favourable circumstance for the production of distinct
breeds, that male and female pigeons can be easily mated for life; and
thus different breeds can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some, yet
quite insufficient, length; because when I first kept pigeons and
watched the several kinds, knowing well how true they bred, I felt
fully as much difficulty in believing that they could ever have
descended from a common parent, as any naturalist could in coming to a
similar conclusion in regard to the many species of finches, or other
large groups of birds, in nature. One circumstance has struck me much;
namely, that all the breeders of the various domestic animals and the
cultivators of plants, with whom I have ever conversed, or whose
treatises I have read, are firmly convinced that the several breeds to
which each has attended, are descended from so many aboriginally
distinct species.
Ask, as I have asked, a celebrated raiser of Hereford cattle, whether
his cattle might not have descended from long horns, and he will laugh
you to scorn. I have never met a pigeon, or poultry, or duck, or rabbit
fancier, who was not fully convinced that each main breed was descended
from a distinct species. Van Mons, in his treatise on pears and apples,
shows how utterly he disbelieves that the several sorts, for instance a
Ribston-pippin or Codlin-apple, could ever have proceeded from the
seeds of the same tree. Innumerable other examples could be given. The
explanation, I think, is simple: from long-continued study they are
strongly impressed with the differences between the several races; and
though they well know that each race varies slightly, for they win
their prizes by selecting such slight differences, yet they ignore all
general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May not
those naturalists who, knowing far less of the laws of inheritance than
does the breeder, and knowing no more than he does of the intermediate
links in the long lines of descent, yet admit that many of our domestic
races have descended from the same parents—may they not learn a lesson
of caution, when they deride the idea of species in a state of nature
being lineal descendants of other species?
_Selection_.—Let us now briefly consider the steps by which domestic
races have been produced, either from one or from several allied
species. Some little effect may, perhaps, be attributed to the direct
action of the external conditions of life, and some little to habit;
but he would be a bold man who would account by such agencies for the
differences of a dray and race horse, a greyhound and bloodhound, a
carrier and tumbler pigeon. One of the most remarkable features in our
domesticated races
is that we see in them adaptation, not indeed to the animal’s or
plant’s own good, but to man’s use or fancy. Some variations useful to
him have probably arisen suddenly, or by one step; many botanists, for
instance, believe that the fuller’s teazle, with its hooks, which
cannot be rivalled by any mechanical contrivance, is only a variety of
the wild Dipsacus; and this amount of change may have suddenly arisen
in a seedling. So it has probably been with the turnspit dog; and this
is known to have been the case with the ancon sheep. But when we
compare the dray-horse and race-horse, the dromedary and camel, the
various breeds of sheep fitted either for cultivated land or mountain
pasture, with the wool of one breed good for one purpose, and that of
another breed for another purpose; when we compare the many breeds of
dogs, each good for man in very different ways; when we compare the
game-cock, so pertinacious in battle, with other breeds so little
quarrelsome, with “everlasting layers” which never desire to sit, and
with the bantam so small and elegant; when we compare the host of
agricultural, culinary, orchard, and flower-garden races of plants,
most useful to man at different seasons and for different purposes, or
so beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed, in
several cases, we know that this has not been their history. The key is
man’s power of accumulative selection: nature gives successive
variations; man adds them up in certain directions useful to him. In
this sense he may be said to make for himself useful breeds.
The great power of this principle of selection is not hypothetical. It
is certain that several of our eminent breeders have, even within a
single lifetime, modified to
a large extent some breeds of cattle and sheep. In order fully to
realise what they have done, it is almost necessary to read several of
the many treatises devoted to this subject, and to inspect the animals.
Breeders habitually speak of an animal’s organisation as something
quite plastic, which they can model almost as they please. If I had
space I could quote numerous passages to this effect from highly
competent authorities. Youatt, who was probably better acquainted with
the works of agriculturalists than almost any other individual, and who
was himself a very good judge of an animal, speaks of the principle of
selection as “that which enables the agriculturist, not only to modify
the character of his flock, but to change it altogether. It is the
magician’s wand, by means of which he may summon into life whatever
form and mould he pleases.” Lord Somerville, speaking of what breeders
have done for sheep, says:—“It would seem as if they had chalked out
upon a wall a form perfect in itself, and then had given it existence.”
That most skilful breeder, Sir John Sebright, used to say, with respect
to pigeons, that “he would produce any given feather in three years,
but it would take him six years to obtain head and beak.” In Saxony the
importance of the principle of selection in regard to merino sheep is
so fully recognised, that men follow it as a trade: the sheep are
placed on a table and are studied, like a picture by a connoisseur;
this is done three times at intervals of months, and the sheep are each
time marked and classed, so that the very best may ultimately be
selected for breeding.
What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds;
all the best breeders are strongly opposed to this practice, except
sometimes amongst closely allied sub-breeds. And when a cross has been
made, the closest selection is far more indispensable even than in
ordinary cases. If selection consisted merely in separating some very
distinct variety, and breeding from it, the principle would be so
obvious as hardly to be worth notice; but its importance consists in
the great effect produced by the accumulation in one direction, during
successive generations, of differences absolutely inappreciable by an
uneducated eye—differences which I for one have vainly attempted to
appreciate. Not one man in a thousand has accuracy of eye and judgment
sufficient to become an eminent breeder. If gifted with these
qualities, and he studies his subject for years, and devotes his
lifetime to it with indomitable perseverance, he will succeed, and may
make great improvements; if he wants any of these qualities, he will
assuredly fail. Few would readily believe in the natural capacity and
years of practice requisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists; but the variations
are here often more abrupt. No one supposes that our choicest
productions have been produced by a single variation from the
aboriginal stock. We have proofs that this is not so in some cases, in
which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be
quoted. We see an astonishing improvement in many florists’ flowers,
when the flowers of the present day are compared with drawings made
only twenty or thirty years ago. When a race of plants is once pretty
well established, the seed-raisers do not pick out the best plants, but
merely go over their seed-beds, and pull up the “rogues,” as they call
the plants that deviate from the proper standard. With animals this
kind of selection is, in fact, also followed; for hardly any one is so
careless as to allow his worst animals to breed.
In regard to plants, there is another means of observing the
accumulated effects of selection—namely, by comparing the diversity of
flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen-garden, in comparison with the flowers
of the same varieties; and the diversity of fruit of the same species
in the orchard, in comparison with the leaves and flowers of the same
set of varieties. See how different the leaves of the cabbage are, and
how extremely alike the flowers; how unlike the flowers of the
heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and
hairiness, and yet the flowers present very slight differences. It is
not that the varieties which differ largely in some one point do not
differ at all in other points; this is hardly ever, perhaps never, the
case. The laws of correlation of growth, the importance of which should
never be overlooked, will ensure some differences; but, as a general
rule, I cannot doubt that the continued selection of slight variations,
either in the leaves, the flowers, or the fruit, will produce races
differing from each other chiefly in these characters.
It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century;
it has certainly been more attended to of late years, and many
treatises have been published on the subject; and the result, I may
add, has been, in a corresponding degree, rapid and important. But it
is very far from true that the principle is a modern discovery. I could
give several references to the full acknowledgment of the importance of
the principle in works of high antiquity. In rude and
barbarous periods of English history choice animals were often
imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may be
compared to the “roguing” of plants by nurserymen. The principle of
selection I find distinctly given in an ancient Chinese encyclopædia.
Explicit rules are laid down by some of the Roman classical writers.
From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes
cross their dogs with wild canine animals, to improve the breed, and
they formerly did so, as is attested by passages in Pliny. The savages
in South Africa match their draught cattle by colour, as do some of the
Esquimaux their teams of dogs. Livingstone shows how much good domestic
breeds are valued by the negroes of the interior of Africa who have not
associated with Europeans. Some of these facts do not show actual
selection, but they show that the breeding of domestic animals was
carefully attended to in ancient times, and is now attended to by the
lowest savages. It would, indeed, have been a strange fact, had
attention not been paid to breeding, for the inheritance of good and
bad qualities is so obvious.
At the present time, eminent breeders try by methodical selection, with
a distinct object in view, to make a new strain or sub-breed, superior
to anything existing in the country. But, for our purpose, a kind of
Selection, which may be called Unconscious, and which results from
every one trying to possess and breed from the best individual animals,
is more important. Thus, a man who intends keeping pointers naturally
tries to get as good dogs as he can, and afterwards breeds from his own
best dogs, but he has no wish or expectation of permanently altering
the breed. Nevertheless I cannot
doubt that this process, continued during centuries, would improve and
modify any breed, in the same way as Bakewell, Collins, etc., by this
very same process, only carried on more methodically, did greatly
modify, even during their own lifetimes, the forms and qualities of
their cattle. Slow and insensible changes of this kind could never be
recognised unless actual measurements or careful drawings of the breeds
in question had been made long ago, which might serve for comparison.
In some cases, however, unchanged or but little changed individuals of
the same breed may be found in less civilised districts, where the
breed has been less improved. There is reason to believe that King
Charles’s spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competent authorities are
convinced that the setter is directly derived from the spaniel, and has
probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly effected by crosses
with the fox-hound; but what concerns us is, that the change has been
effected unconsciously and gradually, and yet so effectually, that,
though the old Spanish pointer certainly came from Spain, Mr. Borrow
has not seen, as I am informed by him, any native dog in Spain like our
pointer.
By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size
the parent Arab stock, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights they carry. Lord Spencer
and others have shown how the cattle of England have increased in
weight and in early maturity, compared with the stock formerly kept in
this country. By comparing the accounts given in old pigeon treatises
of carriers
and tumblers with these breeds as now existing in Britain, India, and
Persia, we can, I think, clearly trace the stages through which they
have insensibly passed, and come to differ so greatly from the
rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so far
that the breeders could never have expected or even have wished to have
produced the result which ensued—namely, the production of two distinct
strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr.
Burgess, as Mr. Youatt remarks, “have been purely bred from the
original stock of Mr. Bakewell for upwards of fifty years. There is not
a suspicion existing in the mind of any one at all acquainted with the
subject that the owner of either of them has deviated in any one
instance from the pure blood of Mr. Bakewell’s flock, and yet the
difference between the sheep possessed by these two gentlemen is so
great that they have the appearance of being quite different
varieties.”
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one
animal particularly useful to them, for any special purpose, would be
carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case there
would be a kind of unconscious selection going on. We see the value set
on animals even by the barbarians of Tierra del Fuego, by their killing
and devouring their old women, in times of dearth, as of less value
than their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked
at their first appearance as distinct varieties, and whether or not two
or more species or races have become blended together by crossing, may
plainly be recognised in the increased size and beauty which we now see
in the varieties of the heartsease, rose, pelargonium, dahlia, and
other plants, when compared with the older varieties or with their
parent-stocks. No one would ever expect to get a first-rate heartsease
or dahlia from the seed of a wild plant. No one would expect to raise a
first-rate melting pear from the seed of a wild pear, though he might
succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears,
from Pliny’s description, to have been a fruit of very inferior
quality. I have seen great surprise expressed in horticultural works at
the wonderful skill of gardeners, in having produced such splendid
results from such poor materials; but the art, I cannot doubt, has been
simple, and, as far as the final result is concerned, has been followed
almost unconsciously. It has consisted in always cultivating the best
known variety, sowing its seeds, and, when a slightly better variety
has chanced to appear, selecting it, and so onwards. But the gardeners
of the classical period, who cultivated the best pear they could
procure, never thought what splendid fruit we should eat; though we owe
our excellent fruit, in some small degree, to their having naturally
chosen and preserved the best varieties they could anywhere find.
A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known fact,
that in a vast number of cases we cannot recognise, and therefore do
not know, the wild parent-stocks of the plants which have been longest
cultivated in our flower and kitchen gardens. If it has taken centuries
or thousands of years to improve
or modify most of our plants up to their present standard of usefulness
to man, we can understand how it is that neither Australia, the Cape of
Good Hope, nor any other region inhabited by quite uncivilised man, has
afforded us a single plant worth culture. It is not that these
countries, so rich in species, do not by a strange chance possess the
aboriginal stocks of any useful plants, but that the native plants have
not been improved by continued selection up to a standard of perfection
comparable with that given to the plants in countries anciently
civilised.
In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
“natural selection,” as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains what has
been remarked by some authors, namely, that the varieties kept by
savages have more of the character of species than the varieties kept
in civilised countries.
On the view here given of the all-important part which selection by man
has played, it becomes at once obvious, how it is that our domestic
races show adaptation in their structure or in their habits to man’s
wants or fancies. We can, I think, further understand the frequently
abnormal character of our domestic races, and likewise their
differences being so great in external characters and relatively so
slight in internal parts or organs. Man can hardly select, or only with
much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal. He
can never act by selection, excepting on variations
which are first given to him in some slight degree by nature. No man
would ever try to make a fantail, till he saw a pigeon with a tail
developed in some slight degree in an unusual manner, or a pouter till
he saw a pigeon with a crop of somewhat unusual size; and the more
abnormal or unusual any character was when it first appeared, the more
likely it would be to catch his attention. But to use such an
expression as trying to make a fantail, is, I have no doubt, in most
cases, utterly incorrect. The man who first selected a pigeon with a
slightly larger tail, never dreamed what the descendants of that pigeon
would become through long-continued, partly unconscious and partly
methodical selection. Perhaps the parent bird of all fantails had only
fourteen tail-feathers somewhat expanded, like the present Java
fantail, or like individuals of other and distinct breeds, in which as
many as seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus,—a habit which is disregarded by
all fanciers, as it is not one of the points of the breed.
Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier’s eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one’s own possession. Nor must the value which would
formerly be set on any slight differences in the individuals of the
same species, be judged of by the value which would now be set on them,
after several breeds have once fairly been established. Many slight
differences might, and indeed do now, arise amongst pigeons, which are
rejected as faults or deviations from the standard of perfection of
each breed. The common goose has not given rise to any marked
varieties; hence the Thoulouse and the common breed, which differ only
in colour, that
most fleeting of characters, have lately been exhibited as distinct at
our poultry-shows.
I think these views further explain what has sometimes been
noticed—namely that we know nothing about the origin or history of any
of our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals
and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a
distinct name, and from being only slightly valued, their history will
be disregarded. When further improved by the same slow and gradual
process, they will spread more widely, and will get recognised as
something distinct and valuable, and will then probably first receive a
provincial name. In semi-civilised countries, with little free
communication, the spreading and knowledge of any new sub-breed will be
a slow process. As soon as the points of value of the new sub-breed are
once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend,—perhaps more at one period than
at another, as the breed rises or falls in fashion,—perhaps more in one
district than in another, according to the state of civilisation of the
inhabitants—slowly to add to the characteristic features of the breed,
whatever they may be. But the chance will be infinitely small of any
record having been preserved of such slow, varying, and insensible
changes.
I must now say a few words on the circumstances, favourable, or the
reverse, to man’s power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply
sufficient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept; and hence this comes to be of
the highest importance to success. On this principle Marshall has
remarked, with respect to the sheep of parts of Yorkshire, that “as
they generally belong to poor people, and are mostly _in small lots_,
they never can be improved.” On the other hand, nurserymen, from
raising large stocks of the same plants, are generally far more
successful than amateurs in getting new and valuable varieties. The
keeping of a large number of individuals of a species in any country
requires that the species should be placed under favourable conditions
of life, so as to breed freely in that country. When the individuals of
any species are scanty, all the individuals, whatever their quality may
be, will generally be allowed to breed, and this will effectually
prevent selection. But probably the most important point of all, is,
that the animal or plant should be so highly useful to man, or so much
valued by him, that the closest attention should be paid to even the
slightest deviation in the qualities or structure of each individual.
Unless such attention be paid nothing can be effected. I have seen it
gravely remarked, that it was most fortunate that the strawberry began
to vary just when gardeners began to attend closely to this plant. No
doubt the strawberry had always varied since it was cultivated, but the
slight varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or better
fruit, and raised seedlings from them, and again picked out the best
seedlings and bred from them, then, there appeared (aided by some
crossing with distinct species) those many admirable varieties of the
strawberry which have been raised during the last thirty or forty
years.
In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races,—at least, in a country which is already stocked with other
races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one
breed of the same species. Pigeons can be mated for life, and this is a
great convenience to the fancier, for thus many races may be kept true,
though mingled in the same aviary; and this circumstance must have
largely favoured the improvement and formation of new breeds. Pigeons,
I may add, can be propagated in great numbers and at a very quick rate,
and inferior birds may be freely rejected, as when killed they serve
for food. On the other hand, cats, from their nocturnal rambling
habits, cannot be matched, and, although so much valued by women and
children, we hardly ever see a distinct breed kept up; such breeds as
we do sometimes see are almost always imported from some other country,
often from islands. Although I do not doubt that some domestic animals
vary less than others, yet the rarity or absence of distinct breeds of
the cat, the donkey, peacock, goose, etc., may be attributed in main
part to selection not having been brought into play: in cats, from the
difficulty in pairing them; in donkeys, from only a few being kept by
poor people, and little attention paid to their breeding; in peacocks,
from not being very easily reared and a large stock not kept; in geese,
from being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of distinct
breeds.
To sum up on the origin of our Domestic Races of animals and plants. I
believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and
necessary contingency, under all circumstances, with all organic
beings, as some authors have thought. The effects of variability are
modified by various degrees of inheritance and of reversion.
Variability is governed by many unknown laws, more especially by that
of correlation of growth. Something may be attributed to the direct
action of the conditions of life. Something must be attributed to use
and disuse. The final result is thus rendered infinitely complex. In
some cases, I do not doubt that the intercrossing of species,
aboriginally distinct, has played an important part in the origin of
our domestic productions. When in any country several domestic breeds
have once been established, their occasional intercrossing, with the
aid of selection, has, no doubt, largely aided in the formation of new
sub-breeds; but the importance of the crossing of varieties has, I
believe, been greatly exaggerated, both in regard to animals and to
those plants which are propagated by seed. In plants which are
temporarily propagated by cuttings, buds, etc., the importance of the
crossing both of distinct species and of varieties is immense; for the
cultivator here quite disregards the extreme variability both of
hybrids and mongrels, and the frequent sterility of hybrids; but the
cases of plants not propagated by seed are of little importance to us,
for their endurance is only temporary. Over all these causes of Change
I am convinced that the accumulative action of Selection, whether
applied methodically and more quickly, or unconsciously and more
slowly, but more efficiently, is by far the predominant Power.
CHAPTER II.
VARIATION UNDER NATURE.
Variability. Individual differences. Doubtful species. Wide ranging,
much diffused, and common species vary most. Species of the larger
genera in any country vary more than the species of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.
Before applying the principles arrived at in the last chapter to
organic beings in a state of nature, we must briefly discuss whether
these latter are subject to any variation. To treat this subject at all
properly, a long catalogue of dry facts should be given; but these I
shall reserve for my future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one
definition has as yet satisfied all naturalists; yet every naturalist
knows vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distinct act of creation. The
term “variety” is almost equally difficult to define; but here
community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but they
graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or
not useful to the species, and not generally propagated. Some authors
use the term “variation” in a technical sense, as implying a
modification directly due to the physical conditions of life; and
“variations” in this sense are supposed not to be inherited: but who
can say that the dwarfed condition of shells in the brackish waters of
the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of
an animal from far northwards, would not in some cases be inherited for
at least some few generations? and in this case I presume that the form
would be called a variety.
Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring
from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species
inhabiting the same confined locality. No one supposes that all the
individuals of the same species are cast in the very same mould. These
individual differences are highly important for us, as they afford
materials for natural selection to accumulate, in the same manner as
man can accumulate in any given direction individual differences in his
domesticated productions. These individual differences generally affect
what naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the
most experienced naturalist would be surprised at the number of the
cases of variability, even in important parts of structure, which he
could collect on good authority, as I have collected, during a course
of years. It should be remembered that systematists are far from
pleased at finding variability in important characters, and that there
are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. I
should never have expected that the branching of the main nerves close
to the great central ganglion of an insect would have been variable in
the same species; I should have expected that changes of this nature
could have been effected only by slow degrees: yet quite recently Mr.
Lubbock has shown a degree of variability in these main nerves in
Coccus, which may almost be compared to the irregular branching of the
stem of a tree. This philosophical naturalist, I may add, has also
quite recently shown that the muscles in the larvæ of certain insects
are very far from uniform. Authors sometimes argue in a circle when
they state that important organs never vary; for these same authors
practically rank that character as important (as some few naturalists
have honestly confessed) which does not vary; and, under this point of
view, no instance of an important part varying will ever be found: but
under any other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seems
to me extremely perplexing: I refer to those genera which have
sometimes been called “protean” or “polymorphic,” in which the species
present an inordinate amount of variation; and hardly two naturalists
can agree which forms to rank as species and which as varieties. We may
instance Rubus, Rosa, and Hieracium amongst plants, several genera of
insects, and several genera of Brachiopod shells. In most polymorphic
genera some of the species have fixed and definite characters. Genera
which are polymorphic in one country seem to be, with some few
exceptions, polymorphic in other countries, and likewise, judging from
Brachiopod shells, at former periods of time. These facts seem to be
very perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that we
see in these polymorphic genera variations in points of structure which
are of no service or disservice to the species, and which consequently
have not been seized on and rendered definite by natural selection, as
hereafter will be explained.
Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are
so closely linked to them by intermediate gradations, that naturalists
do not like to rank them as distinct species, are in several respects
the most important for us. We have every reason to believe that many of
these doubtful and closely-allied forms have permanently retained their
characters in their own country for a long time; for as long, as far as
we know, as have good and true species. Practically, when a naturalist
can unite two forms together by others having intermediate characters,
he treats the one as a variety of the other, ranking the most common,
but sometimes the one first described, as the species, and the other as
the variety. But cases of great difficulty, which I will not here
enumerate, sometimes occur in deciding whether or not to rank one form
as a variety of another, even when they are closely connected by
intermediate links; nor will the commonly-assumed hybrid nature of the
intermediate links always remove the difficulty. In very many cases,
however, one form is ranked as a variety of another, not because the
intermediate links have actually been found, but because analogy leads
the observer to suppose either that they do now somewhere exist, or may
formerly have existed; and here a wide door for the entry of doubt and
conjecture is opened.
Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties, but
which have all been ranked by botanists as species; and in making this
list he has omitted many trifling varieties, but which nevertheless
have been ranked by some botanists as species, and he has entirely
omitted several highly polymorphic genera. Under genera, including the
most polymorphic forms, Mr. Babington gives 251 species, whereas Mr.
Bentham gives only 112,—a difference of 139 doubtful forms! Amongst
animals which unite for each birth, and which are highly locomotive,
doubtful forms, ranked by one zoologist as a species and by another as
a variety, can rarely be found within the same country, but are common
in separated areas. How many of those birds and insects in North
America and Europe, which differ very slightly from each other, have
been ranked by one eminent naturalist as undoubted species, and by
another as varieties, or, as they are often called, as geographical
races! Many years ago, when comparing, and seeing others compare, the
birds from the separate islands of the Galapagos Archipelago, both one
with another, and with those from the American mainland, I was much
struck how entirely vague and arbitrary is the distinction between
species and varieties. On the islets of the little Madeira group there
are many insects which are characterized as varieties in Mr.
Wollaston’s admirable work, but which it cannot
be doubted would be ranked as distinct species by many entomologists.
Even Ireland has a few animals, now generally regarded as varieties,
but which have been ranked as species by some zoologists. Several most
experienced ornithologists consider our British red grouse as only a
strongly-marked race of a Norwegian species, whereas the greater number
rank it as an undoubted species peculiar to Great Britain. A wide
distance between the homes of two doubtful forms leads many naturalists
to rank both as distinct species; but what distance, it has been well
asked, will suffice? if that between America and Europe is ample, will
that between the Continent and the Azores, or Madeira, or the Canaries,
or Ireland, be sufficient? It must be admitted that many forms,
considered by highly-competent judges as varieties, have so perfectly
the character of species that they are ranked by other highly-competent
judges as good and true species. But to discuss whether they are
rightly called species or varieties, before any definition of these
terms has been generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have
been brought to bear on the attempt to determine their rank. I will
here give only a single instance,—the well-known one of the primrose
and cowslip, or Primula veris and elatior. These plants differ
considerably in appearance; they have a different flavour and emit a
different odour; they flower at slightly different periods; they grow
in somewhat different stations; they ascend mountains to different
heights; they have different geographical ranges; and lastly, according
to very numerous experiments made during several years by
that most careful observer Gärtner, they can be crossed only with much
difficulty. We could hardly wish for better evidence of the two forms
being specifically distinct. On the other hand, they are united by many
intermediate links, and it is very doubtful whether these links are
hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents,
and consequently must be ranked as varieties.
Close investigation, in most cases, will bring naturalists to an
agreement how to rank doubtful forms. Yet it must be confessed, that it
is in the best-known countries that we find the greatest number of
forms of doubtful value. I have been struck with the fact, that if any
animal or plant in a state of nature be highly useful to man, or from
any cause closely attract his attention, varieties of it will almost
universally be found recorded. These varieties, moreover, will be often
ranked by some authors as species. Look at the common oak, how closely
it has been studied; yet a German author makes more than a dozen
species out of forms, which are very generally considered as varieties;
and in this country the highest botanical authorities and practical men
can be quoted to show that the sessile and pedunculated oaks are either
good and distinct species or mere varieties.
When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he
knows nothing of the amount and kind of variation to which the group is
subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country, he will soon make up his mind how to rank most of the doubtful
forms. His
general tendency will be to make many species, for he will become
impressed, just like the pigeon or poultry-fancier before alluded to,
with the amount of difference in the forms which he is continually
studying; and he has little general knowledge of analogical variation
in other groups and in other countries, by which to correct his first
impressions. As he extends the range of his observations, he will meet
with more cases of difficulty; for he will encounter a greater number
of closely-allied forms. But if his observations be widely extended, he
will in the end generally be enabled to make up his own mind which to
call varieties and which species; but he will succeed in this at the
expense of admitting much variation,—and the truth of this admission
will often be disputed by other naturalists. When, moreover, he comes
to study allied forms brought from countries not now continuous, in
which case he can hardly hope to find the intermediate links between
his doubtful forms, he will have to trust almost entirely to analogy,
and his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species—that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other in an insensible series; and a series impresses
the mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest to the
systematist, as of high importance for us, as being the first step
towards such slight varieties as are barely thought worth recording in
works on natural history. And I look at varieties which are in any
degree more distinct and permanent, as steps leading to more
strongly marked and more permanent varieties; and at these latter, as
leading to sub-species, and to species. The passage from one stage of
difference to another and higher stage may be, in some cases, due
merely to the long-continued action of different physical conditions in
two different regions; but I have not much faith in this view; and I
attribute the passage of a variety, from a state in which it differs
very slightly from its parent to one in which it differs more, to the
action of natural selection in accumulating (as will hereafter be more
fully explained) differences of structure in certain definite
directions. Hence I believe a well-marked variety may be justly called
an incipient species; but whether this belief be justifiable must be
judged of by the general weight of the several facts and views given
throughout this work.
It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this
incipient state become extinct, or they may endure as varieties for
very long periods, as has been shown to be the case by Mr. Wollaston
with the varieties of certain fossil land-shells in Madeira. If a
variety were to flourish so as to exceed in numbers the parent species,
it would then rank as the species, and the species as the variety; or
it might come to supplant and exterminate the parent species; or both
might co-exist, and both rank as independent species. But we shall
hereafter have to return to this subject.
From these remarks it will be seen that I look at the term species, as
one arbitrarily given for the sake of convenience to a set of
individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again, in
comparison with mere individual differences, is also applied
arbitrarily, and for mere convenience sake.
Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C.
Watson, to whom I am much indebted for valuable advice and assistance
on this subject, soon convinced me that there were many difficulties,
as did subsequently Dr. Hooker, even in stronger terms. I shall reserve
for my future work the discussion of these difficulties, and the tables
themselves of the proportional numbers of the varying species. Dr.
Hooker permits me to add, that after having carefully read my
manuscript, and examined the tables, he thinks that the following
statements are fairly well established. The whole subject, however,
treated as it necessarily here is with much brevity, is rather
perplexing, and allusions cannot be avoided to the “struggle for
existence,” “divergence of character,” and other questions, hereafter
to be discussed.
Alph. De Candolle and others have shown that plants which have very
wide ranges generally present varieties; and this might have been
expected, as they become exposed to diverse physical conditions, and as
they come into competition (which, as we shall hereafter see, is a far
more important circumstance) with different sets of organic beings. But
my tables further show that, in any limited country, the species which
are most common, that is abound most in individuals, and the species
which are most widely diffused within their own country (and this is a
different consideration from wide range, and to a certain extent from
commonness), often give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most
flourishing, or, as they may be called, the dominant species,—those
which range widely over the world, are the most diffused in their own
country, and are the most numerous in individuals,—which oftenest
produce well-marked varieties, or, as I consider them, incipient
species. And this, perhaps, might have been anticipated; for, as
varieties, in order to become in any degree permanent, necessarily have
to struggle with the other inhabitants of the country, the species
which are already dominant will be the most likely to yield offspring
which, though in some slight degree modified, will still inherit those
advantages that enabled their parents to become dominant over their
compatriots.
If the plants inhabiting a country and described in any Flora be
divided into two equal masses, all those in the larger genera being
placed on one side, and all those in the smaller genera on the other
side, a somewhat larger number of the very common and much diffused or
dominant species will be found on the side of the larger genera. This,
again, might have been anticipated; for the mere fact of many species
of the same genus inhabiting any country, shows that there is something
in the organic or inorganic conditions of that country favourable to
the genus; and, consequently, we might have expected to have found in
the larger genera, or those including many species, a large
proportional number of dominant species. But so many causes tend to
obscure this result, that I am surprised that my tables show even a
small majority on the side of the larger genera. I will here allude to
only two causes of obscurity. Fresh-water and salt-loving plants have
generally very wide ranges and are much diffused, but this seems to be
connected with the nature of the stations inhabited by them, and has
little or no relation to the size of the genera to which the species
belong. Again, plants low in the scale of organisation are
generally much more widely diffused than plants higher in the scale;
and here again there is no close relation to the size of the genera.
The cause of lowly-organised plants ranging widely will be discussed in
our chapter on geographical distribution.
From looking at species as only strongly-marked and well-defined
varieties, I was led to anticipate that the species of the larger
genera in each country would oftener present varieties, than the
species of the smaller genera; for wherever many closely related
species (_i.e._ species of the same genus) have been formed, many
varieties or incipient species ought, as a general rule, to be now
forming. Where many large trees grow, we expect to find saplings. Where
many species of a genus have been formed through variation,
circumstances have been favourable for variation; and hence we might
expect that the circumstances would generally be still favourable to
variation. On the other hand, if we look at each species as a special
act of creation, there is no apparent reason why more varieties should
occur in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has
invariably proved to be the case that a larger proportion of the
species on the side of the larger genera present varieties, than on the
side of the smaller genera. Moreover, the species of the large genera
which present any varieties, invariably present a larger average number
of varieties than do the species of the small genera. Both these
results follow when another division is made, and when all the smallest
genera, with from only one to four species, are absolutely excluded
from the tables. These
facts are of plain signification on the view that species are only
strongly marked and permanent varieties; for wherever many species of
the same genus have been formed, or where, if we may use the
expression, the manufactory of species has been active, we ought
generally to find the manufactory still in action, more especially as
we have every reason to believe the process of manufacturing new
species to be a slow one. And this certainly is the case, if varieties
be looked at as incipient species; for my tables clearly show as a
general rule that, wherever many species of a genus have been formed,
the species of that genus present a number of varieties, that is of
incipient species, beyond the average. It is not that all large genera
are now varying much, and are thus increasing in the number of their
species, or that no small genera are now varying and increasing; for if
this had been so, it would have been fatal to my theory; inasmuch as
geology plainly tells us that small genera have in the lapse of time
often increased greatly in size; and that large genera have often come
to their maxima, declined, and disappeared. All that we want to show
is, that where many species of a genus have been formed, on an average
many are still forming; and this holds good.
There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been
found between doubtful forms, naturalists are compelled to come to a
determination by the amount of difference between them, judging by
analogy whether or not the amount suffices to raise one or both to the
rank of species. Hence the amount of difference is one very important
criterion in settling whether two forms should
be ranked as species or varieties. Now Fries has remarked in regard to
plants, and Westwood in regard to insects, that in large genera the
amount of difference between the species is often exceedingly small. I
have endeavoured to test this numerically by averages, and, as far as
my imperfect results go, they always confirm the view. I have also
consulted some sagacious and most experienced observers, and, after
deliberation, they concur in this view. In this respect, therefore, the
species of the larger genera resemble varieties, more than do the
species of the smaller genera. Or the case may be put in another way,
and it may be said, that in the larger genera, in which a number of
varieties or incipient species greater than the average are now
manufacturing, many of the species already manufactured still to a
certain extent resemble varieties, for they differ from each other by a
less than usual amount of difference.
Moreover, the species of the large genera are related to each other, in
the same manner as the varieties of any one species are related to each
other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
certain other species. And what are varieties but groups of forms,
unequally related to each other, and clustered round certain forms—that
is, round their parent-species? Undoubtedly there is one most important
point of difference between varieties and species; namely, that the
amount of difference between varieties, when compared with each other
or with their parent-species, is much less than that between the
species of the same genus. But when we come to discuss the principle,
as I call it, of Divergence of Character,
we shall see how this may be explained, and how the lesser differences
between varieties will tend to increase into the greater differences
between species.
There is one other point which seems to me worth notice. Varieties
generally have much restricted ranges: this statement is indeed
scarcely more than a truism, for if a variety were found to have a
wider range than that of its supposed parent-species, their
denominations ought to be reversed. But there is also reason to
believe, that those species which are very closely allied to other
species, and in so far resemble varieties, often have much restricted
ranges. For instance, Mr. H. C. Watson has marked for me in the
well-sifted London Catalogue of plants (4th edition) 63 plants which
are therein ranked as species, but which he considers as so closely
allied to other species as to be of doubtful value: these 63 reputed
species range on an average over 6.9 of the provinces into which Mr.
Watson has divided Great Britain. Now, in this same catalogue, 53
acknowledged varieties are recorded, and these range over 7.7
provinces; whereas, the species to which these varieties belong range
over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr. Watson as doubtful species, but
which are almost universally ranked by British botanists as good and
true species.
Finally, then, varieties have the same general characters as species,
for they cannot be distinguished from species,—except, firstly, by the
discovery of intermediate linking forms, and the occurrence of such
links cannot affect the actual characters of the forms which they
connect; and except, secondly, by a certain amount of
difference, for two forms, if differing very little, are generally
ranked as varieties, notwithstanding that intermediate linking forms
have not been discovered; but the amount of difference considered
necessary to give to two forms the rank of species is quite indefinite.
In genera having more than the average number of species in any
country, the species of these genera have more than the average number
of varieties. In large genera the species are apt to be closely, but
unequally, allied together, forming little clusters round certain
species. Species very closely allied to other species apparently have
restricted ranges. In all these several respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species have once existed as varieties,
and have thus originated: whereas, these analogies are utterly
inexplicable if each species has been independently created.
We have, also, seen that it is the most flourishing and dominant
species of the larger genera which on an average vary most; and
varieties, as we shall hereafter see, tend to become converted into new
and distinct species. The larger genera thus tend to become larger; and
throughout nature the forms of life which are now dominant tend to
become still more dominant by leaving many modified and dominant
descendants. But by steps hereafter to be explained, the larger genera
also tend to break up into smaller genera. And thus, the forms of life
throughout the universe become divided into groups subordinate to
groups.
CHAPTER III.
STRUGGLE FOR EXISTENCE.
Bears on natural selection. The term used in a wide sense. Geometrical
powers of increase. Rapid increase of naturalised animals and plants.
Nature of the checks to increase. Competition universal. Effects of
climate. Protection from the number of individuals. Complex relations
of all animals and plants throughout nature. Struggle for life most
severe between individuals and varieties of the same species; often
severe between species of the same genus. The relation of organism to
organism the most important of all relations.
Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that amongst
organic beings in a state of nature there is some individual
variability; indeed I am not aware that this has ever been disputed. It
is immaterial for us whether a multitude of doubtful forms be called
species or sub-species or varieties; what rank, for instance, the two
or three hundred doubtful forms of British plants are entitled to hold,
if the existence of any well-marked varieties be admitted. But the mere
existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us
but little in understanding how species arise in nature. How have all
those exquisite adaptations of one part of the organisation to another
part, and to the conditions of life, and of one distinct organic being
to another being, been perfected? We see these beautiful co-adaptations
most plainly in the woodpecker and missletoe; and only a little less
plainly in the humblest parasite which clings
to the hairs of a quadruped or feathers of a bird; in the structure of
the beetle which dives through the water; in the plumed seed which is
wafted by the gentlest breeze; in short, we see beautiful adaptations
everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow inevitably from the struggle for life. Owing to this
struggle for life, any variation, however slight and from whatever
cause proceeding, if it be in any degree profitable to an individual of
any species, in its infinitely complex relations to other organic
beings and to external nature, will tend to the preservation of that
individual, and will generally be inherited by its offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which each
slight variation, if useful, is preserved, by the term of Natural
Selection, in order to mark its relation to man’s power of selection.
We have seen that man by selection can certainly produce great results,
and can adapt organic beings to his own uses, through the accumulation
of slight but useful variations, given to him by the hand of Nature.
But Natural Selection, as we shall hereafter see, is a power
incessantly ready for action, and is as immeasurably superior to man’s
feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for existence.
In my future work this subject shall be treated, as it well deserves,
at much greater length. The elder De Candolle and Lyell have largely
and philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently
the result of his great horticultural knowledge. Nothing is easier than
to admit in words the truth of the universal struggle for life, or more
difficult—at least I have found it so—than constantly to bear this
conclusion in mind. Yet unless it be thoroughly engrained in the mind,
I am convinced that the whole economy of nature, with every fact on
distribution, rarity, abundance, extinction, and variation, will be
dimly seen or quite misunderstood. We behold the face of nature bright
with gladness, we often see superabundance of food; we do not see, or
we forget, that the birds which are idly singing round us mostly live
on insects or seeds, and are thus constantly destroying life; or we
forget how largely these songsters, or their eggs, or their nestlings,
are destroyed by birds and beasts of prey; we do not always bear in
mind, that though food may be now superabundant, it is not so at all
seasons of each recurring year.
I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on another,
and including (which is more important) not only the life of the
individual, but success in leaving progeny. Two canine animals in a
time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said to
struggle for life against the drought, though more properly it should
be said to be dependent on the moisture. A
plant which annually produces a thousand seeds, of which on an average
only one comes to maturity, may be more truly said to struggle with the
plants of the same and other kinds which already clothe the ground. The
missletoe is dependent on the apple and a few other trees, but can only
in a far-fetched sense be said to struggle with these trees, for if too
many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the
same branch, may more truly be said to struggle with each other. As the
missletoe is disseminated by birds, its existence depends on birds; and
it may metaphorically be said to struggle with other fruit-bearing
plants, in order to tempt birds to devour and thus disseminate its
seeds rather than those of other plants. In these several senses, which
pass into each other, I use for convenience sake the general term of
struggle for existence.
A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its life, and during some season or
occasional year, otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no country
could support the product. Hence, as more individuals are produced than
can possibly survive, there must in every case be a struggle for
existence, either one individual with another of the same species, or
with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with manifold
force to the whole animal and vegetable kingdoms; for in this case
there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be
now increasing, more or less rapidly, in numbers, all cannot do so, for
the world would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding man
has doubled in twenty-five years, and at this rate, in a few thousand
years, there would literally not be standing room for his progeny.
Linnæus has calculated that if an annual plant produced only two
seeds—and there is no plant so unproductive as this—and their seedlings
next year produced two, and so on, then in twenty years there would be
a million plants. The elephant is reckoned to be the slowest breeder of
all known animals, and I have taken some pains to estimate its probable
minimum rate of natural increase: it will be under the mark to assume
that it breeds when thirty years old, and goes on breeding till ninety
years old, bringing forth three pair of young in this interval; if this
be so, at the end of the fifth century there would be alive fifteen
million elephants, descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several parts of
the world: if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia, had not
been well authenticated, they would have been quite incredible. So it
is with plants: cases could be given of introduced plants which have
become common throughout whole islands in a period of less than ten
years. Several
of the plants now most numerous over the wide plains of La Plata,
clothing square leagues of surface almost to the exclusion of all other
plants, have been introduced from Europe; and there are plants which
now range in India, as I hear from Dr. Falconer, from Cape Comorin to
the Himalaya, which have been imported from America since its
discovery. In such cases, and endless instances could be given, no one
supposes that the fertility of these animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very favourable,
and that there has consequently been less destruction of the old and
young, and that nearly all the young have been enabled to breed. In
such cases the geometrical ratio of increase, the result of which never
fails to be surprising, simply explains the extraordinarily rapid
increase and wide diffusion of naturalised productions in their new
homes.
In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to increase
at a geometrical ratio, that all would most rapidly stock every station
in which they could any how exist, and that the geometrical tendency to
increase must be checked by destruction at some period of life. Our
familiarity with the larger domestic animals tends, I think, to mislead
us: we see no great destruction falling on them, and we forget that
thousands are annually slaughtered for food, and that in a state of
nature an equal number would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is, that
the slow-breeders would require a few more years to people, under
favourable
conditions, a whole district, let it be ever so large. The condor lays
a couple of eggs and the ostrich a score, and yet in the same country
the condor may be the more numerous of the two: the Fulmar petrel lays
but one egg, yet it is believed to be the most numerous bird in the
world. One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one; but this difference does not determine how
many individuals of the two species can be supported in a district. A
large number of eggs is of some importance to those species, which
depend on a rapidly fluctuating amount of food, for it allows them
rapidly to increase in number. But the real importance of a large
number of eggs or seeds is to make up for much destruction at some
period of life; and this period in the great majority of cases is an
early one. If an animal can in any way protect its own eggs or young, a
small number may be produced, and yet the average stock be fully kept
up; but if many eggs or young are destroyed, many must be produced, or
the species will become extinct. It would suffice to keep up the full
number of a tree, which lived on an average for a thousand years, if a
single seed were produced once in a thousand years, supposing that this
seed were never destroyed, and could be ensured to germinate in a
fitting place. So that in all cases, the average number of any animal
or plant depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every single organic
being around us may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life; that
heavy destruction inevitably falls either on the young or old, during
each generation or at recurrent intervals. Lighten any check, mitigate
the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount. The face of Nature may be
compared to a yielding surface, with ten thousand sharp wedges packed
close together and driven inwards by incessant blows, sometimes one
wedge being struck, and then another with greater force.
What checks the natural tendency of each species to increase in number
is most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will its tendency to increase be still
further increased. We know not exactly what the checks are in even one
single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, so
incomparably better known than any other animal. This subject has been
ably treated by several authors, and I shall, in my future work,
discuss some of the checks at considerable length, more especially in
regard to the feral animals of South America. Here I will make only a
few remarks, just to recall to the reader’s mind some of the chief
points. Eggs or very young animals seem generally to suffer most, but
this is not invariably the case. With plants there is a vast
destruction of seeds, but, from some observations which I have made, I
believe that it is the seedlings which suffer most from germinating in
ground already thickly stocked with other plants. Seedlings, also, are
destroyed in vast numbers by various enemies; for instance, on a piece
of ground three feet long and two wide, dug and cleared, and where
there could be no choking from other plants, I marked all the seedlings
of our native weeds as they came up, and out of the 357 no less than
295 were destroyed, chiefly by slugs and insects. If turf which has
long been mown, and the case would be the same with turf closely
browsed by quadrupeds, be let to grow,
the more vigorous plants gradually kill the less vigorous, though fully
grown, plants: thus out of twenty species growing on a little plot of
turf (three feet by four) nine species perished from the other species
being allowed to grow up freely.
The amount of food for each species of course gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average numbers of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually killed. On the other hand, in some cases, as with the elephant
and rhinoceros, none are destroyed by beasts of prey: even the tiger in
India most rarely dares to attack a young elephant protected by its
dam.
Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought, I believe
to be the most effective of all checks. I estimated that the winter of
1854-55 destroyed four-fifths of the birds in my own grounds; and this
is a tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle
for existence; but in so far as climate chiefly acts in reducing food,
it brings on the most severe struggle between the individuals, whether
of the same or of distinct species, which subsist on the same kind of
food. Even when climate, for instance extreme
cold, acts directly, it will be the least vigorous, or those which have
got least food through the advancing winter, which will suffer most.
When we travel from south to north, or from a damp region to a dry, we
invariably see some species gradually getting rarer and rarer, and
finally disappearing; and the change of climate being conspicuous, we
are tempted to attribute the whole effect to its direct action. But
this is a very false view: we forget that each species, even where it
most abounds, is constantly suffering enormous destruction at some
period of its life, from enemies or from competitors for the same place
and food; and if these enemies or competitors be in the least degree
favoured by any slight change of climate, they will increase in
numbers, and, as each area is already fully stocked with inhabitants,
the other species will decrease. When we travel southward and see a
species decreasing in numbers, we may feel sure that the cause lies
quite as much in other species being favoured, as in this one being
hurt. So it is when we travel northward, but in a somewhat lesser
degree, for the number of species of all kinds, and therefore of
competitors, decreases northwards; hence in going northward, or in
ascending a mountain, we far oftener meet with stunted forms, due to
the _directly_ injurious action of climate, than we do in proceeding
southwards or in descending a mountain. When we reach the Arctic
regions, or snow-capped summits, or absolute deserts, the struggle for
life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other species,
we may clearly see in the prodigious number of plants in our gardens
which can perfectly well endure our climate, but which never become
naturalised, for they cannot compete with our native plants, nor resist
destruction by our native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics—at least, this
seems generally to occur with our game animals—often ensue: and here we
have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of
diffusion amongst the crowded animals, been disproportionably favoured:
and here comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of corn
and rape-seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during winter: but any one who has tried, knows how
troublesome it is to get seed from a few wheat or other such plants in
a garden; I have in this case lost every single seed. This view of the
necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such as that of
very rare plants being sometimes extremely abundant in the few spots
where they do occur; and that of some social plants being social, that
is, abounding in individuals, even on the extreme confines of their
range. For in such cases, we may believe, that a plant could exist only
where the conditions of its life were so favourable that many could
exist together, and thus save each other from utter destruction. I
should add that the good effects of frequent intercrossing, and the ill
effects
of close interbreeding, probably come into play in some of these cases;
but on this intricate subject I will not here enlarge.
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, has interested me. In Staffordshire, on the
estate of a relation where I had ample means of investigation, there
was a large and extremely barren heath, which had never been touched by
the hand of man; but several hundred acres of exactly the same nature
had been enclosed twenty-five years previously and planted with Scotch
fir. The change in the native vegetation of the planted part of the
heath was most remarkable, more than is generally seen in passing from
one quite different soil to another: not only the proportional numbers
of the heath-plants were wholly changed, but twelve species of plants
(not counting grasses and carices) flourished in the plantations, which
could not be found on the heath. The effect on the insects must have
been still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see
how potent has been the effect of the introduction of a single tree,
nothing whatever else having been done, with the exception that the
land had been enclosed, so that cattle could not enter. But how
important an element enclosure is, I plainly saw near Farnham, in
Surrey. Here there are extensive heaths, with a few clumps of old
Scotch firs on the distant hill-tops: within the last ten years large
spaces have been enclosed, and self-sown firs are now springing up in
multitudes, so close together that all cannot live.
When I ascertained that these young trees had not been sown or planted,
I was so much surprised at their numbers that I went to several points
of view, whence I could examine hundreds of acres of the unenclosed
heath, and literally I could not see a single Scotch fir, except the
old planted clumps. But on looking closely between the stems of the
heath, I found a multitude of seedlings and little trees, which had
been perpetually browsed down by the cattle. In one square yard, at a
point some hundred yards distant from one of the old clumps, I counted
thirty-two little trees; and one of them, judging from the rings of
growth, had during twenty-six years tried to raise its head above the
stems of the heath, and had failed. No wonder that, as soon as the land
was enclosed, it became thickly clothed with vigorously growing young
firs. Yet the heath was so extremely barren and so extensive that no
one would ever have imagined that cattle would have so closely and
effectually searched it for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous as
they are, must be habitually checked by some means, probably by birds.
Hence, if certain insectivorous birds (whose numbers are probably
regulated by hawks or beasts of prey) were to increase in Paraguay, the
flies would decrease—then cattle and horses would become feral, and
this would certainly greatly alter (as
indeed I have observed in parts of South America) the vegetation: this
again would largely affect the insects; and this, as we just have seen
in Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity. We began this series by
insectivorous birds, and we have ended with them. Not that in nature
the relations can ever be as simple as this. Battle within battle must
ever be recurring with varying success; and yet in the long-run the
forces are so nicely balanced, that the face of nature remains uniform
for long periods of time, though assuredly the merest trifle would
often give the victory to one organic being over another. Nevertheless
so profound is our ignorance, and so high our presumption, that we
marvel when we hear of the extinction of an organic being; and as we do
not see the cause, we invoke cataclysms to desolate the world, or
invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of
complex relations. I shall hereafter have occasion to show that the
exotic Lobelia fulgens, in this part of England, is never visited by
insects, and consequently, from its peculiar structure, never can set a
seed. Many of our orchidaceous plants absolutely require the visits of
moths to remove their pollen-masses and thus to fertilise them. I have,
also, reason to believe that humble-bees are indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not
visit this flower. From experiments which I have tried, I have found
that the visits of bees, if not indispensable, are at least highly
beneficial to the fertilisation of our clovers; but humble-bees alone
visit the common red clover (Trifolium pratense), as other bees cannot
reach the nectar. Hence I have very little doubt, that if the whole
genus of humble-bees became
extinct or very rare in England, the heartsease and red clover would
become very rare, or wholly disappear. The number of humble-bees in any
district depends in a great degree on the number of field-mice, which
destroy their combs and nests; and Mr. H. Newman, who has long attended
to the habits of humble-bees, believes that “more than two thirds of
them are thus destroyed all over England.” Now the number of mice is
largely dependent, as every one knows, on the number of cats; and Mr.
Newman says, “Near villages and small towns I have found the nests of
humble-bees more numerous than elsewhere, which I attribute to the
number of cats that destroy the mice.” Hence it is quite credible that
the presence of a feline animal in large numbers in a district might
determine, through the intervention first of mice and then of bees, the
frequency of certain flowers in that district!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally the
most potent, but all concurring in determining the average number or
even the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what
we call chance. But how false a view is this! Every one has heard that
when an American forest is cut down, a very different vegetation
springs up; but it has been observed that the trees now growing on the
ancient Indian mounds, in the Southern United States, display the same
beautiful diversity and proportion of kinds as in the surrounding
virgin forests. What a struggle between the several kinds of trees
must here have gone on during long centuries, each annually scattering
its seeds by the thousand; what war between insect and insect—between
insects, snails, and other animals with birds and beasts of prey—all
striving to increase, and all feeding on each other or on the trees or
their seeds and seedlings, or on the other plants which first clothed
the ground and thus checked the growth of the trees! Throw up a handful
of feathers, and all must fall to the ground according to definite
laws; but how simple is this problem compared to the action and
reaction of the innumerable plants and animals which have determined,
in the course of centuries, the proportional numbers and kinds of trees
now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This
is often the case with those which may strictly be said to struggle
with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle almost invariably will be
most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat
be sown together, and the mixed seed be resown, some of the varieties
which best suit the soil or climate, or are naturally the most fertile,
will beat the others and so yield more seed, and will consequently in a
few years quite supplant the other varieties. To keep up a mixed stock
of even such extremely close varieties as the variously coloured
sweet-peas, they must be each year harvested separately, and the seed
then mixed in due proportion,
otherwise the weaker kinds will steadily decrease in numbers and
disappear. So again with the varieties of sheep: it has been asserted
that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same
result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any
one of our domestic plants or animals have so exactly the same
strength, habits, and constitution, that the original proportions of a
mixed stock could be kept up for half a dozen generations, if they were
allowed to struggle together, like beings in a state of nature, and if
the seed or young were not annually sorted.
As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species
of the same genus, when they come into competition with each other,
than between species of distinct genera. We see this in the recent
extension over parts of the United States of one species of swallow
having caused the decrease of another species. The recent increase of
the missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia
the small Asiatic cockroach has everywhere driven before it its great
congener. One species of charlock will supplant another, and so in
other cases. We can dimly see why the competition should be most severe
between allied forms, which fill nearly the same place in the economy
of nature; but probably in no one case could we precisely say why one
species has been victorious over another in the great battle of life.
A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other
organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the tiger; and
in that of the legs and claws of the parasite which clings to the hair
on the tiger’s body. But in the beautifully plumed seed of the
dandelion, and in the flattened and fringed legs of the water-beetle,
the relation seems at first confined to the elements of air and water.
Yet the advantage of plumed seeds no doubt stands in the closest
relation to the land being already thickly clothed by other plants; so
that the seeds may be widely distributed and fall on unoccupied ground.
In the water-beetle, the structure of its legs, so well adapted for
diving, allows it to compete with other aquatic insects, to hunt for
its own prey, and to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds (as peas and
beans), when sown in the midst of long grass, I suspect that the chief
use of the nutriment in the seed is to favour the growth of the young
seedling, whilst struggling with other plants growing vigorously all
around.
Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wished in imagination to give the plant
the power of increasing in number, we should have to give it some
advantage over its competitors, or over the animals which preyed on it.
On the confines of its geographical range, a change of constitution
with respect to climate would clearly be an advantage to our plant; but
we have reason to believe that only a few plants or animals range so
far, that they are destroyed by the rigour of the climate alone. Not
until we reach the extreme confines of life, in the arctic regions or
on the borders of an utter desert, will competition cease. The land may
be extremely cold or dry, yet there will be competition between some
few species, or between the individuals of the same species, for the
warmest or dampest spots.
Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the
same as in its former home, yet the conditions of its life will
generally be changed in an essential manner. If we wished to increase
its average numbers in its new home, we should have to modify it in a
different way to what we should have done in its native country; for we
should have to give it some advantage over a different set of
competitors or enemies.
It is good thus to try in our imagination to give any form some
advantage over another. Probably in no single instance should we know
what to do, so as to succeed. It will convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,
as it seems to be difficult to acquire. All that we can do, is to keep
steadily in mind that each organic being is striving to increase at a
geometrical
ratio; that each at some period of its life, during some season of the
year, during each generation or at intervals, has to struggle for life,
and to suffer great destruction. When we reflect on this struggle, we
may console ourselves with the full belief, that the war of nature is
not incessant, that no fear is felt, that death is generally prompt,
and that the vigorous, the healthy, and the happy survive and multiply.
CHAPTER IV.
NATURAL SELECTION.
Natural Selection: its power compared with man’s selection, its power
on characters of trifling importance, its power at all ages and on both
sexes. Sexual Selection. On the generality of intercrosses between
individuals of the same species. Circumstances favourable and
unfavourable to Natural Selection, namely, intercrossing, isolation,
number of individuals. Slow action. Extinction caused by Natural
Selection. Divergence of Character, related to the diversity of
inhabitants of any small area, and to naturalisation. Action of Natural
Selection, through Divergence of Character and Extinction, on the
descendants from a common parent. Explains the Grouping of all organic
beings.
How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply in nature? I
think we shall see that it can act most effectually. Let it be borne in
mind in what an endless number of strange peculiarities our domestic
productions, and, in a lesser degree, those under nature, vary; and how
strong the hereditary tendency is. Under domestication, it may be truly
said that the whole organisation becomes in some degree plastic. Let it
be borne in mind how infinitely complex and close-fitting are the
mutual relations of all organic beings to each other and to their
physical conditions of life. Can it, then, be thought improbable,
seeing that variations useful to man have undoubtedly occurred, that
other variations useful in some way to each being in the great and
complex battle of life, should sometimes occur in the course of
thousands of generations? If such do occur, can we doubt (remembering
that many more individuals are born than can possibly survive) that
individuals having any advantage, however slight, over others, would
have the best chance of surviving and of procreating their kind? On the
other hand, we may feel sure that any variation in the least degree
injurious would be rigidly destroyed. This preservation of favourable
variations and the rejection of injurious variations, I call Natural
Selection. Variations neither useful nor injurious would not be
affected by natural selection, and would be left a fluctuating element,
as perhaps we see in the species called polymorphic.
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants would
almost immediately undergo a change, and some species might become
extinct. We may conclude, from what we have seen of the intimate and
complex manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of some of the
inhabitants, independently of the change of climate itself, would most
seriously affect many of the others. If the country were open on its
borders, new forms would certainly immigrate, and this also would
seriously disturb the relations of some of the former inhabitants. Let
it be remembered how powerful the influence of a single introduced tree
or mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better
adapted forms could not freely enter, we should then have places in the
economy of nature which would assuredly be better filled up, if some of
the original inhabitants were in some manner modified; for, had the
area been open to immigration, these same
places would have been seized on by intruders. In such case, every
slight modification, which in the course of ages chanced to arise, and
which in any way favoured the individuals of any of the species, by
better adapting them to their altered conditions, would tend to be
preserved; and natural selection would thus have free scope for the
work of improvement.
We have reason to believe, as stated in the first chapter, that a
change in the conditions of life, by specially acting on the
reproductive system, causes or increases variability; and in the
foregoing case the conditions of life are supposed to have undergone a
change, and this would manifestly be favourable to natural selection,
by giving a better chance of profitable variations occurring; and
unless profitable variations do occur, natural selection can do
nothing. Not that, as I believe, any extreme amount of variability is
necessary; as man can certainly produce great results by adding up in
any given direction mere individual differences, so could Nature, but
far more easily, from having incomparably longer time at her disposal.
Nor do I believe that any great physical change, as of climate, or any
unusual degree of isolation to check immigration, is actually necessary
to produce new and unoccupied places for natural selection to fill up
by modifying and improving some of the varying inhabitants. For as all
the inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or
habits of one inhabitant would often give it an advantage over others;
and still further modifications of the same kind would often still
further increase the advantage. No country can be named in which all
the native inhabitants are now so perfectly adapted to each other and
to the physical conditions under which they live, that none of
them could anyhow be improved; for in all countries, the natives have
been so far conquered by naturalised productions, that they have
allowed foreigners to take firm possession of the land. And as
foreigners have thus everywhere beaten some of the natives, we may
safely conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.
As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being which she
tends. Every selected character is fully exercised by her; and the
being is placed under well-suited conditions of life. Man keeps the
natives of many climates in the same country; he seldom exercises each
selected character in some peculiar and fitting manner; he feeds a long
and a short beaked pigeon on the same food; he does not exercise a
long-backed or long-legged quadruped in any peculiar manner; he exposes
sheep with long and short wool to the same climate. He does not allow
the most vigorous males to struggle for the females. He does not
rigidly destroy all inferior animals, but protects during each varying
season, as far as lies in his power, all his productions. He often
begins his selection by some half-monstrous form; or at least by some
modification prominent enough to catch his eye, or to be plainly useful
to him. Under nature, the slightest difference of structure or
constitution may well turn the nicely-balanced scale in the
struggle for life, and so be preserved. How fleeting are the wishes and
efforts of man! how short his time! and consequently how poor will his
products be, compared with those accumulated by nature during whole
geological periods. Can we wonder, then, that nature’s productions
should be far “truer” in character than man’s productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?
It may be said that natural selection is daily and hourly scrutinising,
throughout the world, every variation, even the slightest; rejecting
that which is bad, preserving and adding up all that is good; silently
and insensibly working, whenever and wherever opportunity offers, at
the improvement of each organic being in relation to its organic and
inorganic conditions of life. We see nothing of these slow changes in
progress, until the hand of time has marked the long lapse of ages, and
then so imperfect is our view into long past geological ages, that we
only see that the forms of life are now different from what they
formerly were.
Although natural selection can act only through and for the good of
each being, yet characters and structures, which we are apt to consider
as of very trifling importance, may thus be acted on. When we see
leaf-eating insects green, and bark-feeders mottled-grey; the alpine
ptarmigan white in winter, the red-grouse the colour of heather, and
the black-grouse that of peaty earth, we must believe that these tints
are of service to these birds and insects in preserving them from
danger. Grouse, if not destroyed at some period of their lives, would
increase in countless numbers; they are known to suffer largely from
birds of prey; and hawks are guided by eyesight to their prey,—so much
so, that on
parts of the Continent persons are warned not to keep white pigeons, as
being the most liable to destruction. Hence I can see no reason to
doubt that natural selection might be most effective in giving the
proper colour to each kind of grouse, and in keeping that colour, when
once acquired, true and constant. Nor ought we to think that the
occasional destruction of an animal of any particular colour would
produce little effect: we should remember how essential it is in a
flock of white sheep to destroy every lamb with the faintest trace of
black. In plants the down on the fruit and the colour of the flesh are
considered by botanists as characters of the most trifling importance:
yet we hear from an excellent horticulturist, Downing, that in the
United States smooth-skinned fruits suffer far more from a beetle, a
curculio, than those with down; that purple plums suffer far more from
a certain disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle which
variety, whether a smooth or downy, a yellow or purple fleshed fruit,
should succeed.
In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem to be quite
unimportant, we must not forget that climate, food, etc., probably
produce some slight and direct effect. It is, however, far more
necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is
modified through variation, and the modifications are accumulated by
natural selection for
the good of the being, will cause other modifications, often of the
most unexpected nature.
As we see that those variations which under domestication appear at any
particular period of life, tend to reappear in the offspring at the
same period;—for instance, in the seeds of the many varieties of our
culinary and agricultural plants; in the caterpillar and cocoon stages
of the varieties of the silkworm; in the eggs of poultry, and in the
colour of the down of their chickens; in the horns of our sheep and
cattle when nearly adult;—so in a state of nature, natural selection
will be enabled to act on and modify organic beings at any age, by the
accumulation of profitable variations at that age, and by their
inheritance at a corresponding age. If it profit a plant to have its
seeds more and more widely disseminated by the wind, I can see no
greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify and
adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation,
the structure of the adult; and probably in the case of those insects
which live only for a few hours, and which never feed, a large part of
their structure is merely the correlated result of successive changes
in the structure of their larvæ. So, conversely, modifications in the
adult will probably often affect the structure of the larva; but in all
cases natural selection will ensure that modifications consequent on
other modifications at a different period of life, shall not be in the
least degree injurious: for if they became so, they would cause the
extinction of the species.
Natural selection will modify the structure of the
young in relation to the parent, and of the parent in relation to the
young. In social animals it will adapt the structure of each individual
for the benefit of the community; if each in consequence profits by the
selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the good
of another species; and though statements to this effect may be found
in works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal’s whole life, if
of high importance to it, might be modified to any extent by natural
selection; for instance, the great jaws possessed by certain insects,
and used exclusively for opening the cocoon—or the hard tip to the beak
of nestling birds, used for breaking the egg. It has been asserted,
that of the best short-beaked tumbler-pigeons more perish in the egg
than are able to get out of it; so that fanciers assist in the act of
hatching. Now, if nature had to make the beak of a full-grown pigeon
very short for the bird’s own advantage, the process of modification
would be very slow, and there would be simultaneously the most rigorous
selection of the young birds within the egg, which had the most
powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be
selected, the thickness of the shell being known to vary like every
other structure.
_Sexual Selection_.—Inasmuch as peculiarities often appear under
domestication in one sex and become hereditarily attached to that sex,
the same fact probably occurs under nature, and if so, natural
selection will be able to modify one sex in its functional relations to
the other sex, or in relation to wholly different habits of life in the
two sexes, as is sometimes the case
with insects. And this leads me to say a few words on what I call
Sexual Selection. This depends, not on a struggle for existence, but on
a struggle between the males for possession of the females; the result
is not death to the unsuccessful competitor, but few or no offspring.
Sexual selection is, therefore, less rigorous than natural selection.
Generally, the most vigorous males, those which are best fitted for
their places in nature, will leave most progeny. But in many cases,
victory will depend not on general vigour, but on having special
weapons, confined to the male sex. A hornless stag or spurless cock
would have a poor chance of leaving offspring. Sexual selection by
always allowing the victor to breed might surely give indomitable
courage, length to the spur, and strength to the wing to strike in the
spurred leg, as well as the brutal cock-fighter, who knows well that he
can improve his breed by careful selection of the best cocks. How low
in the scale of nature this law of battle descends, I know not; male
alligators have been described as fighting, bellowing, and whirling
round, like Indians in a war-dance, for the possession of the females;
male salmons have been seen fighting all day long; male stag-beetles
often bear wounds from the huge mandibles of other males. The war is,
perhaps, severest between the males of polygamous animals, and these
seem oftenest provided with special weapons. The males of carnivorous
animals are already well armed; though to them and to others, special
means of defence may be given through means of sexual selection, as the
mane to the lion, the shoulder-pad to the boar, and the hooked jaw to
the male salmon; for the shield may be as important for victory, as the
sword or spear.
Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject,
believe that there is the severest rivalry between the males of many
species to attract by singing the females. The rock-thrush of Guiana,
birds of Paradise, and some others, congregate; and successive males
display their gorgeous plumage and perform strange antics before the
females, which standing by as spectators, at last choose the most
attractive partner. Those who have closely attended to birds in
confinement well know that they often take individual preferences and
dislikes: thus Sir R. Heron has described how one pied peacock was
eminently attractive to all his hen birds. It may appear childish to
attribute any effect to such apparently weak means: I cannot here enter
on the details necessary to support this view; but if man can in a
short time give elegant carriage and beauty to his bantams, according
to his standard of beauty, I can see no good reason to doubt that
female birds, by selecting, during thousands of generations, the most
melodious or beautiful males, according to their standard of beauty,
might produce a marked effect. I strongly suspect that some well-known
laws with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can be explained on the view
of plumage having been chiefly modified by sexual selection, acting
when the birds have come to the breeding age or during the breeding
season; the modifications thus produced being inherited at
corresponding ages or seasons, either by the males alone, or by the
males and females; but I have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour,
or ornament, such differences have been mainly caused by sexual
selection; that is, individual males have had, in successive
generations, some slight advantage over other
males, in their weapons, means of defence, or charms; and have
transmitted these advantages to their male offspring. Yet, I would not
wish to attribute all such sexual differences to this agency: for we
see peculiarities arising and becoming attached to the male sex in our
domestic animals (as the wattle in male carriers, horn-like
protuberances in the cocks of certain fowls, etc.), which we cannot
believe to be either useful to the males in battle, or attractive to
the females. We see analogous cases under nature, for instance, the
tuft of hair on the breast of the turkey-cock, which can hardly be
either useful or ornamental to this bird;—indeed, had the tuft appeared
under domestication, it would have been called a monstrosity.
_Illustrations of the action of Natural Selection_.—In order to make it
clear how, as I believe, natural selection acts, I must beg permission
to give one or two imaginary illustrations. Let us take the case of a
wolf, which preys on various animals, securing some by craft, some by
strength, and some by fleetness; and let us suppose that the fleetest
prey, a deer for instance, had from any change in the country increased
in numbers, or that other prey had decreased in numbers, during that
season of the year when the wolf is hardest pressed for food. I can
under such circumstances see no reason to doubt that the swiftest and
slimmest wolves would have the best chance of surviving, and so be
preserved or selected,—provided always that they retained strength to
master their prey at this or at some other period of the year, when
they might be compelled to prey on other animals. I can see no more
reason to doubt this, than that man can improve the fleetness of his
greyhounds by careful and methodical selection, or by that unconscious
selection which results from each man trying
to keep the best dogs without any thought of modifying the breed.
Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats
rather than mice is known to be inherited. Now, if any slight innate
change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its
young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent-form of wolf. Or, again, the
wolves inhabiting a mountainous district, and those frequenting the
lowlands, would naturally be forced to hunt different prey; and from
the continued preservation of the individuals best fitted for the two
sites, two varieties might slowly be formed. These varieties would
cross and blend where they met; but to this subject of intercrossing we
shall soon have to return. I may add, that, according to Mr. Pierce,
there are two varieties of the wolf inhabiting the Catskill Mountains
in the United States, one with a light greyhound-like form, which
pursues deer, and the other more bulky, with shorter legs, which more
frequently attacks the shepherd’s flocks.
Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is
effected by glands at the base of the stipules in some Leguminosæ, and
at the back of the leaf of the common laurel. This juice, though small
in quantity, is greedily sought by insects. Let us now suppose a little
sweet juice or nectar to be excreted by the inner bases of the petals
of a flower. In this case insects in seeking the nectar would get
dusted with pollen, and would certainly often transport the pollen from
one flower to the stigma of another flower. The flowers of two distinct
individuals of the same species would thus get crossed; and the act of
crossing, we have good reason to believe (as will hereafter be more
fully alluded to), would produce very vigorous seedlings, which
consequently would have the best chance of flourishing and surviving.
Some of these seedlings would probably inherit the nectar-excreting
power. Those individual flowers which had the largest glands or
nectaries, and which excreted most nectar, would be oftenest visited by
insects, and would be oftenest crossed; and so in the long-run would
gain the upper hand. Those flowers, also, which had their stamens and
pistils placed, in relation to the size and habits of the particular
insects which visited them, so as to favour in any degree the
transportal of their pollen from flower to flower, would likewise be
favoured or selected. We might have taken the case of insects visiting
flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole object of fertilisation, its destruction
appears a simple loss to the plant; yet if a little pollen were
carried, at first occasionally and then habitually, by the
pollen-devouring insects from flower to flower, and a cross thus
effected, although nine-tenths of the pollen were destroyed, it might
still be a great gain to the plant; and those individuals which
produced more and more pollen, and had larger and larger anthers, would
be selected.
When our plant, by this process of the continued preservation or
natural selection of more and more attractive flowers, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that they
can most effectually do this, I could easily show by many striking
instances. I will give only one—not as a very striking case, but as
likewise illustrating one step in the separation of the sexes of
plants, presently to be alluded to. Some holly-trees bear only male
flowers, which have four stamens producing rather a small quantity of
pollen, and a rudimentary pistil; other holly-trees bear only female
flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put
the stigmas of twenty flowers, taken from different branches, under the
microscope, and on all, without exception, there were pollen-grains,
and on some a profusion of pollen. As the wind had set for several days
from the female to the male tree, the pollen could not thus have been
carried. The weather had been cold and boisterous, and therefore not
favourable to bees, nevertheless every female flower which I examined
had been effectually fertilised by the bees, accidentally dusted with
pollen, having flown from tree to tree in search of nectar. But to
return to our imaginary case: as soon as the plant had been rendered so
highly attractive to insects that pollen was regularly carried from
flower to flower, another process might commence. No naturalist doubts
the advantage of what has been called the “physiological division of
labour;” hence we may believe that it would be advantageous to a plant
to produce stamens alone in one flower or on one whole plant, and
pistils alone in
another flower or on another plant. In plants under culture and placed
under new conditions of life, sometimes the male organs and sometimes
the female organs become more or less impotent; now if we suppose this
to occur in ever so slight a degree under nature, then as pollen is
already carried regularly from flower to flower, and as a more complete
separation of the sexes of our plant would be advantageous on the
principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes would be effected.
Let us now turn to the nectar-feeding insects in our imaginary case: we
may suppose the plant of which we have been slowly increasing the
nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give many
facts, showing how anxious bees are to save time; for instance, their
habit of cutting holes and sucking the nectar at the bases of certain
flowers, which they can, with a very little more trouble, enter by the
mouth. Bearing such facts in mind, I can see no reason to doubt that an
accidental deviation in the size and form of the body, or in the
curvature and length of the proboscis, etc., far too slight to be
appreciated by us, might profit a bee or other insect, so that an
individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving descendants.
Its descendants would probably inherit a tendency to a similar slight
deviation of structure. The tubes of the corollas of the common red and
incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty
glance appear to differ in length; yet the hive-bee can easily suck the
nectar out of the incarnate clover, but not out of the common red
clover, which is visited by humble-bees alone; so that whole fields of
the red clover offer in vain an abundant supply of precious nectar to
the hive-bee. Thus it might be a great advantage to the hive-bee to
have a slightly longer or differently constructed proboscis. On the
other hand, I have found by experiment that the fertility of clover
greatly depends on bees visiting and moving parts of the corolla, so as
to push the pollen on to the stigmatic surface. Hence, again, if
humble-bees were to become rare in any country, it might be a great
advantage to the red clover to have a shorter or more deeply divided
tube to its corolla, so that the hive-bee could visit its flowers. Thus
I can understand how a flower and a bee might slowly become, either
simultaneously or one after the other, modified and adapted in the most
perfect manner to each other, by the continued preservation of
individuals presenting mutual and slightly favourable deviations of
structure.
I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which
were at first urged against Sir Charles Lyell’s noble views on “the
modern changes of the earth, as illustrative of geology;” but we now
very seldom hear the action, for instance, of the coast-waves, called a
trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.
_On the Intercrossing of Individuals_.—I must here introduce a short
digression. In the case of animals and plants with separated sexes, it
is of course obvious that two individuals must always unite for each
birth; but in the case of hermaphrodites this is far from obvious.
Nevertheless I am strongly inclined to believe that with all
hermaphrodites two individuals, either occasionally or habitually,
concur for the reproduction of their kind. This view, I may add, was
first suggested by Andrew Knight. We shall presently see its
importance; but I must here treat the subject with extreme brevity,
though I have the materials prepared for an ample discussion. All
vertebrate animals, all insects, and some other large groups of
animals, pair for each birth. Modern research has much diminished the
number of supposed hermaphrodites, and of real hermaphrodites a large
number pair; that is, two individuals regularly unite for reproduction,
which is all that concerns us. But still there are many hermaphrodite
animals which certainly do not habitually pair, and a vast majority of
plants are hermaphrodites. What reason, it may be asked, is there for
supposing in these cases that two individuals ever concur in
reproduction? As it is impossible here to enter on details, I must
trust to some general considerations alone.
In the first place, I have collected so large a body of facts, showing,
in accordance with the almost universal belief of breeders, that with
animals and plants a cross between different varieties, or between
individuals of the same variety but of another strain, gives vigour and
fertility to the offspring; and on the other hand, that _close_
interbreeding diminishes vigour and fertility; that
these facts alone incline me to believe that it is a general law of
nature (utterly ignorant though we be of the meaning of the law) that
no organic being self-fertilises itself for an eternity of generations;
but that a cross with another individual is occasionally—perhaps at
very long intervals—indispensable.
On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any
other view are inexplicable. Every hybridizer knows how unfavourable
exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to
the weather! but if an occasional cross be indispensable, the fullest
freedom for the entrance of pollen from another individual will explain
this state of exposure, more especially as the plant’s own anthers and
pistil generally stand so close together that self-fertilisation seems
almost inevitable. Many flowers, on the other hand, have their organs
of fructification closely enclosed, as in the great papilionaceous or
pea-family; but in several, perhaps in all, such flowers, there is a
very curious adaptation between the structure of the flower and the
manner in which bees suck the nectar; for, in doing this, they either
push the flower’s own pollen on the stigma, or bring pollen from
another flower. So necessary are the visits of bees to papilionaceous
flowers, that I have found, by experiments published elsewhere, that
their fertility is greatly diminished if these visits be prevented.
Now, it is scarcely possible that bees should fly from flower to
flower, and not carry pollen from one to the other, to the great good,
as I believe, of the plant. Bees will act like a camel-hair pencil, and
it is quite sufficient just to touch the anthers of one flower and then
the stigma of another with the same brush to ensure fertilisation; but
it must not be
supposed that bees would thus produce a multitude of hybrids between
distinct species; for if you bring on the same brush a plant’s own
pollen and pollen from another species, the former will have such a
prepotent effect, that it will invariably and completely destroy, as
has been shown by Gärtner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause the
stamens to spring forward, as Kölreuter has shown to be the case with
the barberry; and curiously in this very genus, which seems to have a
special contrivance for self-fertilisation, it is well known that if
very closely-allied forms or varieties are planted near each other, it
is hardly possible to raise pure seedlings, so largely do they
naturally cross. In many other cases, far from there being any aids for
self-fertilisation, there are special contrivances, as I could show
from the writings of C. C. Sprengel and from my own observations, which
effectually prevent the stigma receiving pollen from its own flower:
for instance, in Lobelia fulgens, there is a really beautiful and
elaborate contrivance by which every one of the infinitely numerous
pollen-granules are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready to receive them;
and as this flower is never visited, at least in my garden, by insects,
it never sets a seed, though by placing pollen from one flower on the
stigma of another, I raised plenty of seedlings; and whilst another
species of Lobelia growing close by, which is visited by bees, seeds
freely. In very many other cases, though there be no special mechanical
contrivance to prevent the stigma of a flower receiving its own pollen,
yet, as
C. C. Sprengel has shown, and as I can confirm, either the anthers
burst before the stigma is ready for fertilisation, or the stigma is
ready before the pollen of that flower is ready, so that these plants
have in fact separated sexes, and must habitually be crossed. How
strange are these facts! How strange that the pollen and stigmatic
surface of the same flower, though placed so close together, as if for
the very purpose of self-fertilisation, should in so many cases be
mutually useless to each other! How simply are these facts explained on
the view of an occasional cross with a distinct individual being
advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I have
found, of the seedlings thus raised will turn out mongrels: for
instance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only 78 were true to
their kind, and some even of these were not perfectly true. Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant. How,
then, comes it that such a vast number of the seedlings are
mongrelized? I suspect that it must arise from the pollen of a distinct
_variety_ having a prepotent effect over a flower’s own pollen; and
that this is part of the general law of good being derived from the
intercrossing of distinct individuals of the same species. When
distinct _species_ are crossed the case is directly the reverse, for a
plant’s own pollen is always prepotent over foreign pollen; but to this
subject we shall return in a future chapter.
In the case of a gigantic tree covered with innumerable flowers, it may
be objected that pollen could seldom be carried from tree to tree, and
at most only from flower
to flower on the same tree, and that flowers on the same tree can be
considered as distinct individuals only in a limited sense. I believe
this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, we can see that pollen
must be regularly carried from flower to flower; and this will give a
better chance of pollen being occasionally carried from tree to tree.
That trees belonging to all Orders have their sexes more often
separated than other plants, I find to be the case in this country; and
at my request Dr. Hooker tabulated the trees of New Zealand, and Dr.
Asa Gray those of the United States, and the result was as I
anticipated. On the other hand, Dr. Hooker has recently informed me
that he finds that the rule does not hold in Australia; and I have made
these few remarks on the sexes of trees simply to call attention to the
subject.
Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which offers
so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action of insects
and of the wind in the case of plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here currents in the water offer an obvious means
for an occasional cross. And, as in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities, namely,
Professor Huxley, to discover a single case of an hermaphrodite animal
with the organs of reproduction so perfectly enclosed within the body,
that access from without and the occasional influence of a distinct
individual can be shown to be physically impossible. Cirripedes long
appeared to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance,
elsewhere to prove that two individuals, though both are
self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even of
the same genus, though agreeing closely with each other in almost their
whole organisation, yet are not rarely, some of them hermaphrodites,
and some of them unisexual. But if, in fact, all hermaphrodites do
occasionally intercross with other individuals, the difference between
hermaphrodites and unisexual species, as far as function is concerned,
becomes very small.
From these several considerations and from the many special facts which
I have collected, but which I am not here able to give, I am strongly
inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I
am well aware that there are, on this view, many cases of difficulty,
some of which I am trying to investigate. Finally then, we may conclude
that in many organic beings, a cross between two individuals is an
obvious necessity for each birth; in many others it occurs perhaps only
at long intervals; but in none, as I suspect, can self-fertilisation go
on for perpetuity.
_Circumstances favourable to Natural Selection_.—This
is an extremely intricate subject. A large amount of inheritable and
diversified variability is favourable, but I believe mere individual
differences suffice for the work. A large number of individuals, by
giving a better chance for the appearance within any given period of
profitable variations, will compensate for a lesser amount of
variability in each individual, and is, I believe, an extremely
important element of success. Though nature grants vast periods of time
for the work of natural selection, she does not grant an indefinite
period; for as all organic beings are striving, it may be said, to
seize on each place in the economy of nature, if any one species does
not become modified and improved in a corresponding degree with its
competitors, it will soon be exterminated.
In man’s methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when many
men, without intending to alter the breed, have a nearly common
standard of perfection, and all try to get and breed from the best
animals, much improvement and modification surely but slowly follow
from this unconscious process of selection, notwithstanding a large
amount of crossing with inferior animals. Thus it will be in nature;
for within a confined area, with some place in its polity not so
perfectly occupied as might be, natural selection will always tend to
preserve all the individuals varying in the right direction, though in
different degrees, so as better to fill up the unoccupied place. But if
the area be large, its several districts will almost certainly present
different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will
be intercrossing with the other individuals of the same species on the
confines of each. And in this case the effects of intercrossing can
hardly be counterbalanced
by natural selection always tending to modify all the individuals in
each district in exactly the same manner to the conditions of each; for
in a continuous area, the conditions will generally graduate away
insensibly from one district to another. The intercrossing will most
affect those animals which unite for each birth, which wander much, and
which do not breed at a very quick rate. Hence in animals of this
nature, for instance in birds, varieties will generally be confined to
separated countries; and this I believe to be the case. In
hermaphrodite organisms which cross only occasionally, and likewise in
animals which unite for each birth, but which wander little and which
can increase at a very rapid rate, a new and improved variety might be
quickly formed on any one spot, and might there maintain itself in a
body, so that whatever intercrossing took place would be chiefly
between the individuals of the same new variety. A local variety when
once thus formed might subsequently slowly spread to other districts.
On the above principle, nurserymen always prefer getting seed from a
large body of plants of the same variety, as the chance of
intercrossing with other varieties is thus lessened.
Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly
different seasons, or from varieties of the same kind preferring to
pair together.
Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals
which unite for each birth; but I have already attempted to show that
we have reason to believe that occasional intercrosses take place with
all animals and with all plants. Even if these take place only at long
intervals, I am convinced that the young thus produced will gain so
much in vigour and fertility over the offspring from long-continued
self-fertilisation, that they will have a better chance of surviving
and propagating their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If there exist
organic beings which never intercross, uniformity of character can be
retained amongst them, as long as their conditions of life remain the
same, only through the principle of inheritance, and through natural
selection destroying any which depart from the proper type; but if
their conditions of life change and they undergo modification,
uniformity of character can be given to their modified offspring,
solely by natural selection preserving the same favourable variations.
Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same manner
in relation to the same conditions. Intercrosses, also, with the
individuals of the same species, which otherwise would have inhabited
the surrounding and differently circumstanced districts, will be
prevented. But isolation probably acts more efficiently in checking the
immigration of better adapted organisms, after any physical change,
such as of climate or elevation of the land, etc.; and thus new places
in the natural economy of the country are left open for the old
inhabitants to struggle for, and become adapted to, through
modifications
in their structure and constitution. Lastly, isolation, by checking
immigration and consequently competition, will give time for any new
variety to be slowly improved; and this may sometimes be of importance
in the production of new species. If, however, an isolated area be very
small, either from being surrounded by barriers, or from having very
peculiar physical conditions, the total number of the individuals
supported on it will necessarily be very small; and fewness of
individuals will greatly retard the production of new species through
natural selection, by decreasing the chance of the appearance of
favourable variations.
If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the total
number of the species inhabiting it, will be found to be small, as we
shall see in our chapter on geographical distribution; yet of these
species a very large proportion are endemic,—that is, have been
produced there, and nowhere else. Hence an oceanic island at first
sight seems to have been highly favourable for the production of new
species. But we may thus greatly deceive ourselves, for to ascertain
whether a small isolated area, or a large open area like a continent,
has been most favourable for the production of new organic forms, we
ought to make the comparison within equal times; and this we are
incapable of doing.
Although I do not doubt that isolation is of considerable importance in
the production of new species, on the whole I am inclined to believe
that largeness of area is of more importance, more especially in the
production of species, which will prove capable of enduring for a long
period, and of spreading widely. Throughout a great and open area, not
only will there be a better chance of favourable variations arising
from the large number of individuals of the same species
there supported, but the conditions of life are infinitely complex from
the large number of already existing species; and if some of these many
species become modified and improved, others will have to be improved
in a corresponding degree or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread over
the open and continuous area, and will thus come into competition with
many others. Hence more new places will be formed, and the competition
to fill them will be more severe, on a large than on a small and
isolated area. Moreover, great areas, though now continuous, owing to
oscillations of level, will often have recently existed in a broken
condition, so that the good effects of isolation will generally, to a
certain extent, have concurred. Finally, I conclude that, although
small isolated areas probably have been in some respects highly
favourable for the production of new species, yet that the course of
modification will generally have been more rapid on large areas; and
what is more important, that the new forms produced on large areas,
which already have been victorious over many competitors, will be those
that will spread most widely, will give rise to most new varieties and
species, and will thus play an important part in the changing history
of the organic world.
We can, perhaps, on these views, understand some facts which will be
again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia
have formerly yielded, and apparently are now yielding, before those of
the larger Europæo-Asiatic area. Thus, also, it is that continental
productions have everywhere become so largely naturalised on islands.
On a small island, the race for life will have been less severe, and
there will have been less modification and less extermination.
Hence, perhaps, it comes that the flora of Madeira, according to Oswald
Heer, resembles the extinct tertiary flora of Europe. All fresh-water
basins, taken together, make a small area compared with that of the sea
or of the land; and, consequently, the competition between fresh-water
productions will have been less severe than elsewhere; new forms will
have been more slowly formed, and old forms more slowly exterminated.
And it is in fresh water that we find seven genera of Ganoid fishes,
remnants of a once preponderant order: and in fresh water we find some
of the most anomalous forms now known in the world, as the
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a
certain extent orders now widely separated in the natural scale. These
anomalous forms may almost be called living fossils; they have endured
to the present day, from having inhabited a confined area, and from
having thus been exposed to less severe competition.
To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a
large continental area, which will probably undergo many oscillations
of level, and which consequently will exist for long periods in a
broken condition, will be the most favourable for the production of
many new forms of life, likely to endure long and to spread widely. For
the area will first have existed as a continent, and the inhabitants,
at this period numerous in individuals and kinds, will have been
subjected to very severe competition. When converted by subsidence into
large separate islands, there will still exist many individuals of the
same species on each island: intercrossing on the confines of the range
of each species will thus be checked: after physical changes of any
kind, immigration will be prevented,
so that new places in the polity of each island will have to be filled
up by modifications of the old inhabitants; and time will be allowed
for the varieties in each to become well modified and perfected. When,
by renewed elevation, the islands shall be re-converted into a
continental area, there will again be severe competition: the most
favoured or improved varieties will be enabled to spread: there will be
much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be a fair field
for natural selection to improve still further the inhabitants, and
thus produce new species.
That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of
nature, which can be better occupied by some of the inhabitants of the
country undergoing modification of some kind. The existence of such
places will often depend on physical changes, which are generally very
slow, and on the immigration of better adapted forms having been
checked. But the action of natural selection will probably still
oftener depend on some of the inhabitants becoming slowly modified; the
mutual relations of many of the other inhabitants being thus disturbed.
Nothing can be effected, unless favourable variations occur, and
variation itself is apparently always a very slow process. The process
will often be greatly retarded by free intercrossing. Many will exclaim
that these several causes are amply sufficient wholly to stop the
action of natural selection. I do not believe so. On the other hand, I
do believe that natural selection will always act very slowly, often
only at long intervals of time, and generally on only a very few of the
inhabitants of the same region at the same time. I further believe,
that this very slow, intermittent
action of natural selection accords perfectly well with what geology
tells us of the rate and manner at which the inhabitants of this world
have changed.
Slow though the process of selection may be, if feeble man can do much
by his powers of artificial selection, I can see no limit to the amount
of change, to the beauty and infinite complexity of the coadaptations
between all organic beings, one with another and with their physical
conditions of life, which may be effected in the long course of time by
nature’s power of selection.
_Extinction_.—This subject will be more fully discussed in our chapter
on Geology; but it must be here alluded to from being intimately
connected with natural selection. Natural selection acts solely through
the preservation of variations in some way advantageous, which
consequently endure. But as from the high geometrical powers of
increase of all organic beings, each area is already fully stocked with
inhabitants, it follows that as each selected and favoured form
increases in number, so will the less favoured forms decrease and
become rare. Rarity, as geology tells us, is the precursor to
extinction. We can, also, see that any form represented by few
individuals will, during fluctuations in the seasons or in the number
of its enemies, run a good chance of utter extinction. But we may go
further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must
become extinct. That the number of specific forms has not indefinitely
increased, geology shows us plainly; and indeed we can see reason why
they should not have thus increased, for the number of places in the
polity of nature is not indefinitely great,—not that we
have any means of knowing that any one region has as yet got its
maximum of species. Probably no region is as yet fully stocked, for at
the Cape of Good Hope, where more species of plants are crowded
together than in any other quarter of the world, some foreign plants
have become naturalised, without causing, as far as we know, the
extinction of any natives.
Furthermore, the species which are most numerous in individuals will
have the best chance of producing within any given period favourable
variations. We have evidence of this, in the facts given in the second
chapter, showing that it is the common species which afford the
greatest number of recorded varieties, or incipient species. Hence,
rare species will be less quickly modified or improved within any given
period, and they will consequently be beaten in the race for life by
the modified descendants of the commoner species.
From these several considerations I think it inevitably follows, that
as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct. The
forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,—varieties of the same species, and species of the
same genus or of related genera,—which, from having nearly the same
structure, constitution, and habits, generally come into the severest
competition with each other. Consequently, each new variety or species,
during the progress of its formation, will generally press hardest on
its nearest kindred, and tend to exterminate them. We see the same
process of extermination amongst our domesticated productions, through
the selection of improved forms by man. Many curious
instances could be given showing how quickly new breeds of cattle,
sheep, and other animals, and varieties of flowers, take the place of
older and inferior kinds. In Yorkshire, it is historically known that
the ancient black cattle were displaced by the long-horns, and that
these “were swept away by the short-horns” (I quote the words of an
agricultural writer) “as if by some murderous pestilence.”
_Divergence of Character_.—The principle, which I have designated by
this term, is of high importance on my theory, and explains, as I
believe, several important facts. In the first place, varieties, even
strongly-marked ones, though having somewhat of the character of
species—as is shown by the hopeless doubts in many cases how to rank
them—yet certainly differ from each other far less than do good and
distinct species. Nevertheless, according to my view, varieties are
species in the process of formation, or are, as I have called them,
incipient species. How, then, does the lesser difference between
varieties become augmented into the greater difference between species?
That this does habitually happen, we must infer from most of the
innumerable species throughout nature presenting well-marked
differences; whereas varieties, the supposed prototypes and parents of
future well-marked species, present slight and ill-defined differences.
Mere chance, as we may call it, might cause one variety to differ in
some character from its parents, and the offspring of this variety
again to differ from its parent in the very same character and in a
greater degree; but this alone would never account for so habitual and
large an amount of difference as that between varieties of the same
species and species of the same genus.
As has always been my practice, let us seek light on
this head from our domestic productions. We shall here find something
analogous. A fancier is struck by a pigeon having a slightly shorter
beak; another fancier is struck by a pigeon having a rather longer
beak; and on the acknowledged principle that “fanciers do not and will
not admire a medium standard, but like extremes,” they both go on (as
has actually occurred with tumbler-pigeons) choosing and breeding from
birds with longer and longer beaks, or with shorter and shorter beaks.
Again, we may suppose that at an early period one man preferred swifter
horses; another stronger and more bulky horses. The early differences
would be very slight; in the course of time, from the continued
selection of swifter horses by some breeders, and of stronger ones by
others, the differences would become greater, and would be noted as
forming two sub-breeds; finally, after the lapse of centuries, the
sub-breeds would become converted into two well-established and
distinct breeds. As the differences slowly become greater, the inferior
animals with intermediate characters, being neither very swift nor very
strong, will have been neglected, and will have tended to disappear.
Here, then, we see in man’s productions the action of what may be
called the principle of divergence, causing differences, at first
barely appreciable, steadily to increase, and the breeds to diverge in
character both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places
in the polity of nature, and so be enabled to increase in numbers.
We can clearly see this in the case of animals with simple habits. Take
the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If
its natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions)
only by its varying descendants seizing on places at present occupied
by other animals: some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animal became, the more places they
would be enabled to occupy. What applies to one animal will apply
throughout all time to all animals—that is, if they vary—for otherwise
natural selection can do nothing. So it will be with plants. It has
been experimentally proved, that if a plot of ground be sown with one
species of grass, and a similar plot be sown with several distinct
genera of grasses, a greater number of plants and a greater weight of
dry herbage can thus be raised. The same has been found to hold good
when first one variety and then several mixed varieties of wheat have
been sown on equal spaces of ground. Hence, if any one species of grass
were to go on varying, and those varieties were continually selected
which differed from each other in at all the same manner as distinct
species and genera of grasses differ from each other, a greater number
of individual plants of this species of grass, including its modified
descendants, would succeed in living on the same piece of ground. And
we well know that each species and each variety of grass is annually
sowing almost countless seeds; and thus, as it may be said, is striving
its utmost to increase its numbers. Consequently,
I cannot doubt that in the course of many thousands of generations, the
most distinct varieties of any one species of grass would always have
the best chance of succeeding and of increasing in numbers, and thus of
supplanting the less distinct varieties; and varieties, when rendered
very distinct from each other, take the rank of species.
The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets; and so in small ponds of fresh water. Farmers find
that they can raise most food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any
small piece of ground, could live on it (supposing it not to be in any
way peculiar in its nature), and may be said to be striving to the
utmost to live there; but, it is seen, that where they come into the
closest competition with each other, the advantages of diversification
of structure, with the accompanying differences of habit and
constitution, determine that the inhabitants, which thus jostle each
other most closely, shall, as a general rule, belong to what we call
different genera and orders.
The same principle is seen in the naturalisation of
plants through man’s agency in foreign lands. It might have been
expected that the plants which have succeeded in becoming naturalised
in any land would generally have been closely allied to the indigenes;
for these are commonly looked at as specially created and adapted for
their own country. It might, also, perhaps have been expected that
naturalised plants would have belonged to a few groups more especially
adapted to certain stations in their new homes. But the case is very
different; and Alph. De Candolle has well remarked in his great and
admirable work, that floras gain by naturalisation, proportionally with
the number of the native genera and species, far more in new genera
than in new species. To give a single instance: in the last edition of
Dr. Asa Gray’s ‘Manual of the Flora of the Northern United States,’ 260
naturalised plants are enumerated, and these belong to 162 genera. We
thus see that these naturalised plants are of a highly diversified
nature. They differ, moreover, to a large extent from the indigenes,
for out of the 162 genera, no less than 100 genera are not there
indigenous, and thus a large proportional addition is made to the
genera of these States.
By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we can gain some crude idea in what manner some of the
natives would have had to be modified, in order to have gained an
advantage over the other natives; and we may, I think, at least safely
infer that diversification of structure, amounting to new generic
differences, would have been profitable to them.
The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labour
in the organs of the same individual body—a subject so well elucidated
by
Milne Edwards. No physiologist doubts that a stomach by being adapted
to digest vegetable matter alone, or flesh alone, draws most nutriment
from these substances. So in the general economy of any land, the more
widely and perfectly the animals and plants are diversified for
different habits of life, so will a greater number of individuals be
capable of there supporting themselves. A set of animals, with their
organisation but little diversified, could hardly compete with a set
more perfectly diversified in structure. It may be doubted, for
instance, whether the Australian marsupials, which are divided into
groups differing but little from each other, and feebly representing,
as Mr. Waterhouse and others have remarked, our carnivorous, ruminant,
and rodent mammals, could successfully compete with these
well-pronounced orders. In the Australian mammals, we see the process
of diversification in an early and incomplete stage of development.
After the foregoing discussion, which ought to have been much
amplified, we may, I think, assume that the modified descendants of any
one species will succeed by so much the better as they become more
diversified in structure, and are thus enabled to encroach on places
occupied by other beings. Now let us see how this principle of great
benefit being derived from divergence of character, combined with the
principles of natural selection and of extinction, will tend to act.
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal
distances. I have said a large genus, because we have seen in the
second chapter,
that on an average more of the species of large genera vary than of
small genera; and the varying species of the large genera present a
greater number of varieties. We have, also, seen that the species,
which are the commonest and the most widely-diffused, vary more than
rare species with restricted ranges. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in its
own country. The little fan of diverging dotted lines of unequal
lengths proceeding from (A), may represent its varying offspring. The
variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear simultaneously,
but often after long intervals of time; nor are they all supposed to
endure for equal periods. Only those variations which are in some way
profitable will be preserved or naturally selected. And here the
importance of the principle of benefit being derived from divergence of
character comes in; for this will generally lead to the most different
or divergent variations (represented by the outer dotted lines) being
preserved and accumulated by natural selection. When a dotted line
reaches one of the horizontal lines, and is there marked by a small
numbered letter, a sufficient amount of variation is supposed to have
been accumulated to have formed a fairly well-marked variety, such as
would be thought worthy of record in a systematic work.
The intervals between the horizontal lines in the diagram, may
represent each a thousand generations; but it would have been better if
each had represented ten thousand generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely _a_1 and _m_1. These two varieties will
generally continue to be exposed to the same conditions which made
their parents variable,
and the tendency to variability is in itself hereditary, consequently
they will tend to vary, and generally to vary in nearly the same manner
as their parents varied. Moreover, these two varieties, being only
slightly modified forms, will tend to inherit those advantages which
made their common parent (A) more numerous than most of the other
inhabitants of the same country; they will likewise partake of those
more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And these
circumstances we know to be favourable to the production of new
varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety _a_1 is supposed in the
diagram to have produced variety _a_2, which will, owing to the
principle of divergence, differ more from (A) than did variety _a_1.
Variety _m_1 is supposed to have produced two varieties, namely _m_2
and _s_2, differing from each other, and more considerably from their
common parent (A). We may continue the process by similar steps for any
length of time; some of the varieties, after each thousand generations,
producing only a single variety, but in a more and more modified
condition, some producing two or three varieties, and some failing to
produce any. Thus the varieties or modified descendants, proceeding
from the common parent (A), will generally go on increasing in number
and diverging in character. In the diagram the process is represented
up to the ten-thousandth generation, and under a condensed and
simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever goes
on so regularly as is represented in the diagram, though in itself made
somewhat irregular.
I am far from thinking that the most divergent varieties will
invariably prevail and multiply: a medium form may often long endure,
and may or may not produce more than one modified descendant; for
natural selection will always act according to the nature of the places
which are either unoccupied or not perfectly occupied by other beings;
and this will depend on infinitely complex relations. But as a general
rule, the more diversified in structure the descendants from any one
species can be rendered, the more places they will be enabled to seize
on, and the more their modified progeny will be increased. In our
diagram the line of succession is broken at regular intervals by small
numbered letters marking the successive forms which have become
sufficiently distinct to be recorded as varieties. But these breaks are
imaginary, and might have been inserted anywhere, after intervals long
enough to have allowed the accumulation of a considerable amount of
divergent variation.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later and
more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and less
improved branches: this is represented in the diagram by some of the
lower branches not reaching to the upper horizontal lines. In some
cases I do not doubt that the process of modification will be confined
to a single line of descent, and the number of the descendants will not
be increased; although the amount
of divergent modification may have been increased in the successive
generations. This case would be represented in the diagram, if all the
lines proceeding from (A) were removed, excepting that from _a_1 to
_a_10. In the same way, for instance, the English race-horse and
English pointer have apparently both gone on slowly diverging in
character from their original stocks, without either having given off
any fresh branches or races.
After ten thousand generations, species (A) is supposed to have
produced three forms, _a_10, _f_10, and _m_10, which, from having
diverged in character during the successive generations, will have come
to differ largely, but perhaps unequally, from each other and from
their common parent. If we suppose the amount of change between each
horizontal line in our diagram to be excessively small, these three
forms may still be only well-marked varieties; or they may have arrived
at the doubtful category of sub-species; but we have only to suppose
the steps in the process of modification to be more numerous or greater
in amount, to convert these three forms into well-defined species: thus
the diagram illustrates the steps by which the small differences
distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for a greater
number of generations (as shown in the diagram in a condensed and
simplified manner), we get eight species, marked by the letters between
_a_14 and _m_14, all descended from (A). Thus, as I believe, species
are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced,
by analogous steps, after ten thousand generations, either two
well-marked varieties (_w_10 and _z_10) or two species, according to
the amount of change supposed to be represented between
the horizontal lines. After fourteen thousand generations, six new
species, marked by the letters _n_14 to _z_14, are supposed to have
been produced. In each genus, the species, which are already extremely
different in character, will generally tend to produce the greatest
number of modified descendants; for these will have the best chance of
filling new and widely different places in the polity of nature: hence
in the diagram I have chosen the extreme species (A), and the nearly
extreme species (I), as those which have largely varied, and have given
rise to new varieties and species. The other nine species (marked by
capital letters) of our original genus, may for a long period continue
transmitting unaltered descendants; and this is shown in the diagram by
the dotted lines not prolonged far upwards from want of space.
But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and
exterminate in each stage of descent their predecessors and their
original parent. For it should be remembered that the competition will
generally be most severe between those forms which are most nearly
related to each other in habits, constitution, and structure. Hence all
the intermediate forms between the earlier and later states, that is
between the less and more improved state of a species, as well as the
original parent-species itself, will generally tend to become extinct.
So it probably will be with many whole collateral lines of descent,
which will be conquered by later and improved lines of descent. If,
however, the
modified offspring of a species get into some distinct country, or
become quickly adapted to some quite new station, in which child and
parent do not come into competition, both may continue to exist.
If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have
become extinct, having been replaced by eight new species (_a_14 to
_m_14); and (I) will have been replaced by six (_n_14 to _z_14) new
species.
But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally
the case in nature; species (A) being more nearly related to B, C, and
D, than to the other species; and species (I) more to G, H, K, L, than
to the others. These two species (A) and (I), were also supposed to be
very common and widely diffused species, so that they must originally
have had some advantage over most of the other species of the genus.
Their modified descendants, fourteen in number at the
fourteen-thousandth generation, will probably have inherited some of
the same advantages: they have also been modified and improved in a
diversified manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of their country.
It seems, therefore, to me extremely probable that they will have taken
the places of, and thus exterminated, not only their parents (A) and
(I), but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will
have transmitted offspring to the fourteen-thousandth generation. We
may suppose that only one (F), of the two species which were least
closely related to the other nine original species, has transmitted
descendants to this late stage of descent.
The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency
of natural selection, the extreme amount of difference in character
between species _a_14 and _z_14 will be much greater than that between
the most different of the original eleven species. The new species,
moreover, will be allied to each other in a widely different manner. Of
the eight descendants from (A) the three marked _a_14, _q_14, _p_14,
will be nearly related from having recently branched off from _a_10;
_b_14 and _f_14, from having diverged at an earlier period from a5,
will be in some degree distinct from the three first-named species; and
lastly, _0_14, _e_4, and _m_14, will be nearly related one to the
other, but from having diverged at the first commencement of the
process of modification, will be widely different from the other five
species, and may constitute a sub-genus or even a distinct genus.
The six descendants from (I) will form two sub-genera or even genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme points of the original genus, the six descendants
from (I) will, owing to inheritance, differ considerably from the eight
descendants from (A); the two groups, moreover, are supposed to have
gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct,
and have left no descendants. Hence the six new species descended from
(I), and the eight descended from (A), will have to be ranked as very
distinct genera, or even as distinct sub-families.
Thus it is, as I believe, that two or more genera are produced by
descent, with modification, from two or more species of the same genus.
And the two or more
parent-species are supposed to have descended from some one species of
an earlier genus. In our diagram, this is indicated by the broken
lines, beneath the capital letters, converging in sub-branches
downwards towards a single point; this point representing a single
species, the supposed single parent of our several new sub-genera and
genera.
It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case, its affinities to the other fourteen
new species will be of a curious and circuitous nature. Having
descended from a form which stood between the two parent-species (A)
and (I), now supposed to be extinct and unknown, it will be in some
degree intermediate in character between the two groups descended from
these species. But as these two groups have gone on diverging in
character from the type of their parents, the new species (F14) will
not be directly intermediate between them, but rather between types of
the two groups; and every naturalist will be able to bring some such
case before his mind.
In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth’s crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again to this subject,
and I think we shall then see that the diagram throws light on the
affinities of extinct beings, which, though generally belonging to the
same orders, or families, or genera, with those now living, yet are
often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for
the extinct species lived at very ancient epochs when the branching
lines of descent had diverged less.
I see no reason to limit the process of modification, as now explained,
to the formation of genera alone. If, in our diagram, we suppose the
amount of change represented by each successive group of diverging
dotted lines to be very great, the forms marked _a_14 to _p_14, those
marked _b_14 and _f_14, and those marked _o_14 to _m_14, will form
three very distinct genera. We shall also have two very distinct genera
descended from (I) and as these latter two genera, both from continued
divergence of character and from inheritance from a different parent,
will differ widely from the three genera descended from (A), the two
little groups of genera will form two distinct families, or even
orders, according to the amount of divergent modification supposed to
be represented in the diagram. And the two new families, or orders,
will have descended from two species of the original genus; and these
two species are supposed to have descended from one species of a still
more ancient and unknown genus.
We have seen that in each country it is the species of the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts through
one form having some advantage over other forms in the struggle for
existence, it will chiefly act on those which already have some
advantage; and the largeness of any group shows that its species have
inherited from a common ancestor some advantage in common. Hence, the
struggle for the production of new and modified descendants, will
mainly lie between the larger groups, which are all trying to increase
in number. One large group will slowly conquer another large group,
reduce its numbers, and thus lessen its chance of further variation and
improvement. Within the same large
group, the later and more highly perfected sub-groups, from branching
out and seizing on many new places in the polity of Nature, will
constantly tend to supplant and destroy the earlier and less improved
sub-groups. Small and broken groups and sub-groups will finally tend to
disappear. Looking to the future, we can predict that the groups of
organic beings which are now large and triumphant, and which are least
broken up, that is, which as yet have suffered least extinction, will
for a long period continue to increase. But which groups will
ultimately prevail, no man can predict; for we well know that many
groups, formerly most extensively developed, have now become extinct.
Looking still more remotely to the future, we may predict that, owing
to the continued and steady increase of the larger groups, a multitude
of smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one
period, extremely few will transmit descendants to a remote futurity. I
shall have to return to this subject in the chapter on Classification,
but I may add that on this view of extremely few of the more ancient
species having transmitted descendants, and on the view of all the
descendants of the same species making a class, we can understand how
it is that there exist but very few classes in each main division of
the animal and vegetable kingdoms. Although extremely few of the most
ancient species may now have living and modified descendants, yet at
the most remote geological period, the earth may have been as well
peopled with many species of many genera, families, orders, and
classes, as at the present day.
_Summary of the Chapter_.—If during the long course of ages and under
varying conditions of life, organic beings
vary at all in the several parts of their organisation, and I think
this cannot be disputed; if there be, owing to the high geometrical
powers of increase of each species, at some age, season, or year, a
severe struggle for life, and this certainly cannot be disputed; then,
considering the infinite complexity of the relations of all organic
beings to each other and to their conditions of existence, causing an
infinite diversity in structure, constitution, and habits, to be
advantageous to them, I think it would be a most extraordinary fact if
no variation ever had occurred useful to each being’s own welfare, in
the same way as so many variations have occurred useful to man. But if
variations useful to any organic being do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance they
will tend to produce offspring similarly characterised. This principle
of preservation, I have called, for the sake of brevity, Natural
Selection. Natural selection, on the principle of qualities being
inherited at corresponding ages, can modify the egg, seed, or young, as
easily as the adult. Amongst many animals, sexual selection will give
its aid to ordinary selection, by assuring to the most vigorous and
best adapted males the greatest number of offspring. Sexual selection
will also give characters useful to the males alone, in their struggles
with other males.
Whether natural selection has really thus acted in nature, in modifying
and adapting the various forms of life to their several conditions and
stations, must be judged of by the general tenour and balance of
evidence given in the following chapters. But we already see how it
entails extinction; and how largely extinction has acted in the world’s
history, geology plainly declares. Natural selection, also, leads to
divergence of
character; for more living beings can be supported on the same area the
more they diverge in structure, habits, and constitution, of which we
see proof by looking at the inhabitants of any small spot or at
naturalised productions. Therefore during the modification of the
descendants of any one species, and during the incessant struggle of
all species to increase in numbers, the more diversified these
descendants become, the better will be their chance of succeeding in
the battle of life. Thus the small differences distinguishing varieties
of the same species, will steadily tend to increase till they come to
equal the greater differences between species of the same genus, or
even of distinct genera.
We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary
most; and these will tend to transmit to their modified offspring that
superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly
wonderful fact—the wonder of which we are apt to overlook from
familiarity—that all animals and all plants throughout all time and
space should be related to each other in group subordinate to group, in
the manner which we everywhere behold—namely, varieties of the same
species most closely related together, species of the same genus less
closely and unequally related together, forming sections and
sub-genera, species of distinct genera much less closely related, and
genera related in different degrees, forming sub-families, families,
orders, sub-classes, and classes. The several subordinate groups in any
class cannot be
ranked in a single file, but seem rather to be clustered round points,
and these round other points, and so on in almost endless cycles. On
the view that each species has been independently created, I can see no
explanation of this great fact in the classification of all organic
beings; but, to the best of my judgment, it is explained through
inheritance and the complex action of natural selection, entailing
extinction and divergence of character, as we have seen illustrated in
the diagram.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long
succession of extinct species. At each period of growth all the growing
twigs have tried to branch out on all sides, and to overtop and kill
the surrounding twigs and branches, in the same manner as species and
groups of species have tried to overmaster other species in the great
battle for life. The limbs divided into great branches, and these into
lesser and lesser branches, were themselves once, when the tree was
small, budding twigs; and this connexion of the former and present buds
by ramifying branches may well represent the classification of all
extinct and living species in groups subordinate to groups. Of the many
twigs which flourished when the tree was a mere bush, only two or
three, now grown into great branches, yet survive and bear all the
other branches; so with the species which lived during long-past
geological periods, very few now have living and modified descendants.
From the first growth of the tree, many a limb and branch has decayed
and dropped off; and these lost branches of various sizes may represent
those whole orders, families, and genera which have now no living
representatives, and
which are known to us only from having been found in a fossil state. As
we here and there see a thin straggling branch springing from a fork
low down in a tree, and which by some chance has been favoured and is
still alive on its summit, so we occasionally see an animal like the
Ornithorhynchus or Lepidosiren, which in some small degree connects by
its affinities two large branches of life, and which has apparently
been saved from fatal competition by having inhabited a protected
station. As buds give rise by growth to fresh buds, and these, if
vigorous, branch out and overtop on all sides many a feebler branch, so
by generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and
covers the surface with its ever branching and beautiful ramifications.
CHAPTER V.
LAWS OF VARIATION.
Effects of external conditions. Use and disuse, combined with natural
selection; organs of flight and of vision. Acclimatisation. Correlation
of growth. Compensation and economy of growth. False correlations.
Multiple, rudimentary, and lowly organised structures variable. Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable.
Species of the same genus vary in an analogous manner. Reversions to
long lost characters. Summary.
I have hitherto sometimes spoken as if the variations—so common and
multiform in organic beings under domestication, and in a lesser degree
in those in a state of nature—had been due to chance. This, of course,
is a wholly incorrect expression, but it serves to acknowledge plainly
our ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to
produce individual differences, or very slight deviations of structure,
as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe
that deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote
ancestors have been exposed during several generations. I have remarked
in the first chapter—but a long catalogue of facts which cannot be here
given would be necessary to show the truth of the remark—that the
reproductive system is eminently susceptible to changes in the
conditions of life; and to
this system being functionally disturbed in the parents, I chiefly
attribute the varying or plastic condition of the offspring. The male
and female sexual elements seem to be affected before that union takes
place which is to form a new being. In the case of “sporting” plants,
the bud, which in its earliest condition does not apparently differ
essentially from an ovule, is alone affected. But why, because the
reproductive system is disturbed, this or that part should vary more or
less, we are profoundly ignorant. Nevertheless, we can here and there
dimly catch a faint ray of light, and we may feel sure that there must
be some cause for each deviation of structure, however slight.
How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that
of plants. We may, at least, safely conclude that such influences
cannot have produced the many striking and complex co-adaptations of
structure between one organic being and another, which we see
everywhere throughout nature. Some little influence may be attributed
to climate, food, etc.: thus, E. Forbes speaks confidently that shells
at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from
greater depths. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living on islands
or near the coast. So with insects, Wollaston is convinced that
residence near the sea affects their colours. Moquin-Tandon gives a
list of plants which when growing near the sea-shore have their leaves
in some degree fleshy, though not elsewhere fleshy. Several other such
cases could be given.
The fact of varieties of one species, when they range
into the zone of habitation of other species, often acquiring in a very
slight degree some of the characters of such species, accords with our
view that species of all kinds are only well-marked and permanent
varieties. Thus the species of shells which are confined to tropical
and shallow seas are generally brighter-coloured than those confined to
cold and deeper seas. The birds which are confined to continents are,
according to Mr. Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector knows, are
often brassy or lurid. Plants which live exclusively on the sea-side
are very apt to have fleshy leaves. He who believes in the creation of
each species, will have to say that this shell, for instance, was
created with bright colours for a warm sea; but that this other shell
became bright-coloured by variation when it ranged into warmer or
shallower waters.
When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and
better fur the more severe the climate is under which they have lived;
but who can tell how much of this difference may be due to the
warmest-clad individuals having been favoured and preserved during many
generations, and how much to the direct action of the severe climate?
for it would appear that climate has some direct action on the hair of
our domestic quadrupeds.
Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same species
under the same conditions. Such facts show how indirectly into the zone
of habitation of other species, often acquiring in a very slight degree
some of the characters of such species, accords with our view that
species of all kinds are only well-marked and permanent varieties. Thus
the species of shells which are confined to tropical and shallow seas
are generally brighter-coloured than those confined to cold and deeper
seas. The birds which are confined to continents are, according to Mr.
Gould, brighter-coloured than those of islands. The insect-species
confined to sea-coasts, as every collector knows, are often brassy or
lurid. Plants which live exclusively on the sea-side are very apt to
have fleshy leaves. He who believes in the creation of each species,
will have to say that this shell, for instance, was created with bright
colours for a warm sea; but that this other shell became
bright-coloured by variation when it ranged into warmer or shallower
waters.
the conditions of life must act. Again, innumerable instances are known
to every naturalist of species keeping true, or not varying at all,
although living under the most opposite climates. Such considerations
as these incline me to lay very little weight on the direct action of
the conditions of life. Indirectly, as already remarked, they seem to
play an important part in affecting the reproductive system, and in
thus inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.
_Effects of Use and Disuse_.—From the facts alluded to in the first
chapter, I think there can be little doubt that use in our domestic
animals strengthens and enlarges certain parts, and disuse diminishes
them; and that such modifications are inherited. Under free nature, we
can have no standard of comparison, by which to judge of the effects of
long-continued use or disuse, for we know not the parent-forms; but
many animals have structures which can be explained by the effects of
disuse. As Professor Owen has remarked, there is no greater anomaly in
nature than a bird that cannot fly; yet there are several in this
state. The logger-headed duck of South America can only flap along the
surface of the water, and has its wings in nearly the same condition as
the domestic Aylesbury duck. As the larger ground-feeding birds seldom
take flight except to escape danger, I believe that the nearly wingless
condition of several birds, which now inhabit or have lately inhabited
several oceanic islands, tenanted by no beast of prey, has been caused
by disuse. The ostrich indeed inhabits continents and is exposed to
danger from which it cannot escape by flight, but by kicking it can
defend itself from enemies, as well as any of the smaller
quadrupeds. We may imagine that the early progenitor of the ostrich had
habits like those of a bustard, and that as natural selection increased
in successive generations the size and weight of its body, its legs
were used more, and its wings less, until they became incapable of
flight.
Kirby has remarked (and I have observed the same fact) that the
anterior tarsi, or feet, of many male dung-feeding beetles are very
often broken off; he examined seventeen specimens in his own
collection, and not one had even a relic left. In the Onites apelles
the tarsi are so habitually lost, that the insect has been described as
not having them. In some other genera they are present, but in a
rudimentary condition. In the Ateuchus or sacred beetle of the
Egyptians, they are totally deficient. There is not sufficient evidence
to induce us to believe that mutilations are ever inherited; and I
should prefer explaining the entire absence of the anterior tarsi in
Ateuchus, and their rudimentary condition in some other genera, by the
long-continued effects of disuse in their progenitors; for as the tarsi
are almost always lost in many dung-feeding beetles, they must be lost
early in life, and therefore cannot be much used by these insects.
In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the world are very
frequently blown to sea and perish; that the beetles in Madeira, as
observed by Mr. Wollaston, lie much concealed,
until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira
itself; and especially the extraordinary fact, so strongly insisted on
by Mr. Wollaston, of the almost entire absence of certain large groups
of beetles, elsewhere excessively numerous, and which groups have
habits of life almost necessitating frequent flight;—these several
considerations have made me believe that the wingless condition of so
many Madeira beetles is mainly due to the action of natural selection,
but combined probably with disuse. For during thousands of successive
generations each individual beetle which flew least, either from its
wings having been ever so little less perfectly developed or from
indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which
most readily took to flight will oftenest have been blown to sea and
thus have been destroyed.
The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their
wings to gain their subsistence, have, as Mr. Wollaston suspects, their
wings not at all reduced, but even enlarged. This is quite compatible
with the action of natural selection. For when a new insect first
arrived on the island, the tendency of natural selection to enlarge or
to reduce the wings, would depend on whether a greater number of
individuals were saved by successfully battling with the winds, or by
giving up the attempt and rarely or never flying. As with mariners
shipwrecked near a coast, it would have been better for the good
swimmers if they had been able to swim still further, whereas it would
have been better for the bad swimmers if they had not been able to swim
at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered up by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse, but
aided perhaps by natural selection. In South America, a burrowing
rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its
habits than the mole; and I was assured by a Spaniard, who had often
caught them, that they were frequently blind; one which I kept alive
was certainly in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane. As frequent
inflammation of the eyes must be injurious to any animal, and as eyes
are certainly not indispensable to animals with subterranean habits, a
reduction in their size with the adhesion of the eyelids and growth of
fur over them, might in such case be an advantage; and if so, natural
selection would constantly aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind.
In some of the crabs the foot-stalk for the eye remains, though the eye
is gone; the stand for the telescope is there, though the telescope
with its glasses has been lost. As it is difficult to imagine that
eyes, though useless, could be in any way injurious to animals living
in darkness, I attribute their loss wholly to disuse. In one of the
blind animals, namely, the cave-rat, the eyes are of immense size; and
Professor Silliman thought that it regained, after living some days in
the light, some slight power of vision. In the same manner as in
Madeira the wings of some of the insects have been enlarged, and the
wings of others have been reduced by natural selection aided by use and
disuse, so in the case of the cave-rat natural selection seems to have
struggled with the loss of light and
to have increased the size of the eyes; whereas with all the other
inhabitants of the caves, disuse by itself seems to have done its work.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the common
view of the blind animals having been separately created for the
American and European caverns, close similarity in their organisation
and affinities might have been expected; but, as Schiödte and others
have remarked, this is not the case, and the cave-insects of the two
continents are not more closely allied than might have been anticipated
from the general resemblance of the other inhabitants of North America
and Europe. On my view we must suppose that American animals, having
ordinary powers of vision, slowly migrated by successive generations
from the outer world into the deeper and deeper recesses of the
Kentucky caves, as did European animals into the caves of Europe. We
have some evidence of this gradation of habit; for, as Schiödte
remarks, “animals not far remote from ordinary forms, prepare the
transition from light to darkness. Next follow those that are
constructed for twilight; and, last of all, those destined for total
darkness.” By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have more
or less perfectly obliterated its eyes, and natural selection will
often have effected other changes, such as an increase in the length of
the antennæ or palpi, as a compensation for blindness. Notwithstanding
such modifications, we might expect still to see in the cave-animals of
America, affinities to the other inhabitants of that continent, and in
those of Europe, to the inhabitants of the European continent. And this
is the case with some of the American cave-animals, as I hear from
Professor Dana; and some of the European cave-insects are very closely
allied to those of the surrounding country. It would be most difficult
to give any rational explanation of the affinities of the blind
cave-animals to the other inhabitants of the two continents on the
ordinary view of their independent creation. That several of the
inhabitants of the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relationship of most of
their other productions. Far from feeling any surprise that some of the
cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the inhabitants of these
dark abodes will probably have been exposed.
_Acclimatisation_.—Habit is hereditary with plants, as in the period of
flowering, in the amount of rain requisite for seeds to germinate, in
the time of sleep, etc., and this leads me to say a few words on
acclimatisation. As it is extremely common for species of the same
genus to inhabit very hot and very cold countries, and as I believe
that all the species of the same genus have descended from a single
parent, if this view be correct, acclimatisation must be readily
effected during long-continued descent. It is notorious that each
species is adapted to the climate of its own home: species from an
arctic or even from a temperate region cannot endure a tropical
climate, or conversely. So again, many succulent plants cannot endure a
damp climate. But the degree of adaptation of species to the climates
under which they live is often overrated.
We may infer this from our frequent inability to predict whether or not
an imported plant will endure our climate, and from the number of
plants and animals brought from warmer countries which here enjoy good
health. We have reason to believe that species in a state of nature are
limited in their ranges by the competition of other organic beings
quite as much as, or more than, by adaptation to particular climates.
But whether or not the adaptation be generally very close, we have
evidence, in the case of some few plants, of their becoming, to a
certain extent, naturally habituated to different temperatures, or
becoming acclimatised: thus the pines and rhododendrons, raised from
seed collected by Dr. Hooker from trees growing at different heights on
the Himalaya, were found in this country to possess different
constitutional powers of resisting cold. Mr. Thwaites informs me that
he has observed similar facts in Ceylon, and analogous observations
have been made by Mr. H. C. Watson on European species of plants
brought from the Azores to England. In regard to animals, several
authentic cases could be given of species within historical times
having largely extended their range from warmer to cooler latitudes,
and conversely; but we do not positively know that these animals were
strictly adapted to their native climate, but in all ordinary cases we
assume such to be the case; nor do we know that they have subsequently
become acclimatised to their new homes.
As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable of
far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly
fertile (a far severer test) under them, may be used as an argument
that a large proportion of other animals, now in a state of nature,
could easily be brought to bear widely different climates. We must not,
however, push the foregoing argument too far, on account of the
probable origin of some of our domestic animals from several wild
stocks: the blood, for instance, of a tropical and arctic wolf or wild
dog may perhaps be mingled in our domestic breeds. The rat and mouse
cannot be considered as domestic animals, but they have been
transported by man to many parts of the world, and now have a far wider
range than any other rodent, living free under the cold climate of
Faroe in the north and of the Falklands in the south, and on many
islands in the torrid zones. Hence I am inclined to look at adaptation
to any special climate as a quality readily grafted on an innate wide
flexibility of constitution, which is common to most animals. On this
view, the capacity of enduring the most different climates by man
himself and by his domestic animals, and such facts as that former
species of the elephant and rhinoceros were capable of enduring a
glacial climate, whereas the living species are now all tropical or
sub-tropical in their habits, ought not to be looked at as anomalies,
but merely as examples of a very common flexibility of constitution,
brought, under peculiar circumstances, into play.
How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopædias
of China, to be very cautious
in transposing animals from one district to another; for it is not
likely that man should have succeeded in selecting so many breeds and
sub-breeds with constitutions specially fitted for their own districts:
the result must, I think, be due to habit. On the other hand, I can see
no reason to doubt that natural selection will continually tend to
preserve those individuals which are born with constitutions best
adapted to their native countries. In treatises on many kinds of
cultivated plants, certain varieties are said to withstand certain
climates better than others: this is very strikingly shown in works on
fruit trees published in the United States, in which certain varieties
are habitually recommended for the northern, and others for the
southern States; and as most of these varieties are of recent origin,
they cannot owe their constitutional differences to habit. The case of
the Jerusalem artichoke, which is never propagated by seed, and of
which consequently new varieties have not been produced, has even been
advanced—for it is now as tender as ever it was—as proving that
acclimatisation cannot be effected! The case, also, of the kidney-bean
has been often cited for a similar purpose, and with much greater
weight; but until some one will sow, during a score of generations, his
kidney-beans so early that a very large proportion are destroyed by
frost, and then collect seed from the few survivors, with care to
prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has
been published how much more hardy some seedlings appeared to be than
others.
On the whole, I think we may conclude that habit,
use, and disuse, have, in some cases, played a considerable part in the
modification of the constitution, and of the structure of various
organs; but that the effects of use and disuse have often been largely
combined with, and sometimes overmastered by, the natural selection of
innate differences.
_Correlation of Growth_.—I mean by this expression that the whole
organisation is so tied together during its growth and development,
that when slight variations in any one part occur, and are accumulated
through natural selection, other parts become modified. This is a very
important subject, most imperfectly understood. The most obvious case
is, that modifications accumulated solely for the good of the young or
larva, will, it may safely be concluded, affect the structure of the
adult; in the same manner as any malconformation affecting the early
embryo, seriously affects the whole organisation of the adult. The
several parts of the body which are homologous, and which, at an early
embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the same
manner; in the front and hind legs, and even in the jaws and limbs,
varying together, for the lower jaw is believed to be homologous with
the limbs. These tendencies, I do not doubt, may be mastered more or
less completely by natural selection: thus a family of stags once
existed with an antler only on one side; and if this had been of any
great use to the breed it might probably have been rendered permanent
by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of
the petals of the corolla into a tube. Hard parts seem to affect the
form of adjoining soft parts; it is believed by some authors that the
diversity in the shape of the pelvis in birds causes the remarkable
diversity in the shape of their kidneys. Others believe that the shape
of the pelvis in the human mother influences by pressure the shape of
the head of the child. In snakes, according to Schlegel, the shape of
the body and the manner of swallowing determine the position of several
of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure.
M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist,
without our being able to assign any reason. What can be more singular
than the relation between blue eyes and deafness in cats, and the
tortoise-shell colour with the female sex; the feathered feet and skin
between the outer toes in pigeons, and the presence of more or less
down on the young birds when first hatched, with the future colour of
their plumage; or, again, the relation between the hair and teeth in
the naked Turkish dog, though here probably homology comes into play?
With respect to this latter case of correlation, I think it can hardly
be accidental, that if we pick out the two orders of mammalia which are
most abnormal in their dermal coverings, viz. Cetacea (whales) and
Edentata (armadilloes, scaly ant-eaters, etc.), that these are likewise
the most abnormal in their teeth.
I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of utility
and, therefore, of natural selection, than that of the difference
between the outer and inner flowers in some Compositous and
Umbelliferous plants. Every one knows the
difference in the ray and central florets of, for instance, the daisy,
and this difference is often accompanied with the abortion of parts of
the flower. But, in some Compositous plants, the seeds also differ in
shape and sculpture; and even the ovary itself, with its accessory
parts, differs, as has been described by Cassini. These differences
have been attributed by some authors to pressure, and the shape of the
seeds in the ray-florets in some Compositæ countenances this idea; but,
in the case of the corolla of the Umbelliferæ, it is by no means, as
Dr. Hooker informs me, in species with the densest heads that the inner
and outer flowers most frequently differ. It might have been thought
that the development of the ray-petals by drawing nourishment from
certain other parts of the flower had caused their abortion; but in
some Compositæ there is a difference in the seeds of the outer and
inner florets without any difference in the corolla. Possibly, these
several differences may be connected with some difference in the flow
of nutriment towards the central and external flowers: we know, at
least, that in irregular flowers, those nearest to the axis are
oftenest subject to peloria, and become regular. I may add, as an
instance of this, and of a striking case of correlation, that I have
recently observed in some garden pelargoniums, that the central flower
of the truss often loses the patches of darker colour in the two upper
petals; and that when this occurs, the adherent nectary is quite
aborted; when the colour is absent from only one of the two upper
petals, the nectary is only much shortened.
With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C.
C. Sprengel’s idea that the ray-florets serve to attract insects, whose
agency is highly advantageous in the fertilisation of plants of
these two orders, is so far-fetched, as it may at first appear: and if
it be advantageous, natural selection may have come into play. But in
regard to the differences both in the internal and external structure
of the seeds, which are not always correlated with any differences in
the flowers, it seems impossible that they can be in any way
advantageous to the plant: yet in the Umbelliferæ these differences are
of such apparent importance—the seeds being in some cases, according to
Tausch, orthospermous in the exterior flowers and coelospermous in the
central flowers,—that the elder De Candolle founded his main divisions
of the order on analogous differences. Hence we see that modifications
of structure, viewed by systematists as of high value, may be wholly
due to unknown laws of correlated growth, and without being, as far as
we can see, of the slightest service to the species.
We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and,
after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted to a
whole group of descendants with diverse habits, would naturally be
thought to be correlated in some necessary manner. So, again, I do not
doubt that some apparent correlations, occurring throughout whole
orders, are entirely due to the manner alone in which natural selection
can act. For instance, Alph. De Candolle has remarked that winged seeds
are never found in fruits which do not open: I should explain the rule
by the fact that seeds could not gradually become winged through
natural selection, except in fruits which opened; so that the
individual plants producing
seeds which were a little better fitted to be wafted further, might get
an advantage over those producing seed less fitted for dispersal; and
this process could not possibly go on in fruit which did not open.
The elder Geoffroy and Goethe propounded, at about the same period,
their law of compensation or balancement of growth; or, as Goethe
expressed it, “in order to spend on one side, nature is forced to
economise on the other side.” I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part
or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to fatten
readily. The same varieties of the cabbage do not yield abundant and
nutritious foliage and a copious supply of oil-bearing seeds. When the
seeds in our fruits become atrophied, the fruit itself gains largely in
size and quality. In our poultry, a large tuft of feathers on the head
is generally accompanied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature it can hardly be
maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by this same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the
excess of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying
to economise in every part of the organisation. If under
changed conditions of life a structure before useful becomes less
useful, any diminution, however slight, in its development, will be
seized on by natural selection, for it will profit the individual not
to have its nutriment wasted in building up an useless structure. I can
thus only understand a fact with which I was much struck when examining
cirripedes, and of which many other instances could be given: namely,
that when a cirripede is parasitic within another and is thus
protected, it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly extraordinary
manner with the Proteolepas: for the carapace in all other cirripedes
consists of the three highly-important anterior segments of the head
enormously developed, and furnished with great nerves and muscles; but
in the parasitic and protected Proteolepas, the whole anterior part of
the head is reduced to the merest rudiment attached to the bases of the
prehensile antennæ. Now the saving of a large and complex structure,
when rendered superfluous by the parasitic habits of the Proteolepas,
though effected by slow steps, would be a decided advantage to each
successive individual of the species; for in the struggle for life to
which every animal is exposed, each individual Proteolepas would have a
better chance of supporting itself, by less nutriment being wasted in
developing a structure now become useless.
Thus, as I believe, natural selection will always succeed in the long
run in reducing and saving every part of the organisation, as soon as
it is rendered superfluous, without by any means causing some other
part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in
largely developing any organ, without requiring as a necessary
compensation the reduction of some adjoining part.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in
varieties and in species, that when any part or organ is repeated many
times in the structure of the same individual (as the vertebræ in
snakes, and the stamens in polyandrous flowers) the number is variable;
whereas the number of the same part or organ, when it occurs in lesser
numbers, is constant. The same author and some botanists have further
remarked that multiple parts are also very liable to variation in
structure. Inasmuch as this “vegetative repetition,” to use Professor
Owen’s expression, seems to be a sign of low organisation; the
foregoing remark seems connected with the very general opinion of
naturalists, that beings low in the scale of nature are more variable
than those which are higher. I presume that lowness in this case means
that the several parts of the organisation have been but little
specialised for particular functions; and as long as the same part has
to perform diversified work, we can perhaps see why it should remain
variable, that is, why natural selection should have preserved or
rejected each little deviation of form less carefully than when the
part has to serve for one special purpose alone. In the same way that a
knife which has to cut all sorts of things may be of almost any shape;
whilst a tool for some particular object had better be of some
particular shape. Natural selection, it should never be forgotten, can
act on each part of each being, solely through and for its advantage.
Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to
the general subject of rudimentary and aborted organs; and I will here
only add that their variability seems to be owing to their uselessness,
and therefore to natural selection having no power to check deviations
in their structure. Thus
rudimentary parts are left to the free play of the various laws of
growth, to the effects of long-continued disuse, and to the tendency to
reversion.
_A part developed in any species in an extraordinary degree or manner,
in comparison with the same part in allied species, tends to be highly
variable_.—Several years ago I was much struck with a remark, nearly to
the above effect, published by Mr. Waterhouse. I infer also from an
observation made by Professor Owen, with respect to the length of the
arms of the ourang-outang, that he has come to a nearly similar
conclusion. It is hopeless to attempt to convince any one of the truth
of this proposition without giving the long array of facts which I have
collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowance for
them. It should be understood that the rule by no means applies to any
part, however unusually developed, unless it be unusually developed in
comparison with the same part in closely allied species. Thus, the
bat’s wing is a most abnormal structure in the class mammalia; but the
rule would not here apply, because there is a whole group of bats
having wings; it would apply only if some one species of bat had its
wings developed in some remarkable manner in comparison with the other
species of the same genus. The rule applies very strongly in the case
of secondary sexual characters, when displayed in any unusual manner.
The term, secondary sexual characters, used by Hunter, applies to
characters which are attached to one sex, but are not directly
connected with the act of reproduction. The rule applies to males and
females; but as females more rarely offer remarkable secondary sexual
characters, it applies
more rarely to them. The rule being so plainly applicable in the case
of secondary sexual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual manner—of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse’s remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma,
these valves present a marvellous amount of diversification: the
homologous valves in the different species being sometimes wholly
unlike in shape; and the amount of variation in the individuals of
several of the species is so great, that it is no exaggeration to state
that the varieties differ more from each other in the characters of
these important valves than do other species of distinct genera.
As birds within the same country vary in a remarkably small degree, I
have particularly attended to them, and the rule seems to me certainly
to hold good in this class. I cannot make out that it applies to
plants, and this would seriously have shaken my belief in its truth,
had not the great variability in plants made it particularly difficult
to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or
manner in any species, the fair
presumption is that it is of high importance to that species;
nevertheless the part in this case is eminently liable to variation.
Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see
no explanation. But on the view that groups of species have descended
from other species, and have been modified through natural selection, I
think we can obtain some light. In our domestic animals, if any part,
or the whole animal, be neglected and no selection be applied, that
part (for instance, the comb in the Dorking fowl) or the whole breed
will cease to have a nearly uniform character. The breed will then be
said to have degenerated. In rudimentary organs, and in those which
have been but little specialised for any particular purpose, and
perhaps in polymorphic groups, we see a nearly parallel natural case;
for in such cases natural selection either has not or cannot come into
full play, and thus the organisation is left in a fluctuating
condition. But what here more especially concerns us is, that in our
domestic animals those points, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to
variation. Look at the breeds of the pigeon; see what a prodigious
amount of difference there is in the beak of the different tumblers, in
the beak and wattle of the different carriers, in the carriage and tail
of our fantails, etc., these being the points now mainly attended to by
English fanciers. Even in the sub-breeds, as in the short-faced
tumbler, it is notoriously difficult to breed them nearly to
perfection, and frequently individuals are born which depart widely
from the standard. There may be truly said to be a constant struggle
going on between, on the one hand, the tendency to reversion to a less
modified state, as well as an innate tendency to further
variability of all kinds, and, on the other hand, the power of steady
selection to keep the breed true. In the long run selection gains the
day, and we do not expect to fail so far as to breed a bird as coarse
as a common tumbler from a good short-faced strain. But as long as
selection is rapidly going on, there may always be expected to be much
variability in the structure undergoing modification. It further
deserves notice that these variable characters, produced by man’s
selection, sometimes become attached, from causes quite unknown to us,
more to one sex than to the other, generally to the male sex, as with
the wattle of carriers and the enlarged crop of pouters.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has undergone
an extraordinary amount of modification, since the period when the
species branched off from the common progenitor of the genus. This
period will seldom be remote in any extreme degree, as species very
rarely endure for more than one geological period. An extraordinary
amount of modification implies an unusually large and long-continued
amount of variability, which has continually been accumulated by
natural selection for the benefit of the species. But as the
variability of the extraordinarily-developed part or organ has been so
great and long-continued within a period not excessively remote, we
might, as a general rule, expect still to find more variability in such
parts than in other parts of the organisation, which have remained for
a much longer period nearly constant. And this, I am convinced, is the
case. That the struggle between natural selection on the one hand, and
the tendency to reversion and variability on the other hand, will in
the
course of time cease; and that the most abnormally developed organs may
be made constant, I can see no reason to doubt. Hence when an organ,
however abnormal it may be, has been transmitted in approximately the
same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to my theory, for an
immense period in nearly the same state; and thus it comes to be no
more variable than any other structure. It is only in those cases in
which the modification has been comparatively recent and
extraordinarily great that we ought to find the _generative
variability_, as it may be called, still present in a high degree. For
in this case the variability will seldom as yet have been fixed by the
continued selection of the individuals varying in the required manner
and degree, and by the continued rejection of those tending to revert
to a former and less modified condition.
The principle included in these remarks may be extended. It is
notorious that specific characters are more variable than generic. To
explain by a simple example what is meant. If some species in a large
genus of plants had blue flowers and some had red, the colour would be
only a specific character, and no one would be surprised at one of the
blue species varying into red, or conversely; but if all the species
had blue flowers, the colour would become a generic character, and its
variation would be a more unusual circumstance. I have chosen this
example because an explanation is not in this case applicable, which
most naturalists would advance, namely, that specific characters are
more variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera.
I believe this explanation is partly, yet only indirectly, true; I
shall, however, have to return
to this subject in our chapter on Classification. It would be almost
superfluous to adduce evidence in support of the above statement, that
specific characters are more variable than generic; but I have
repeatedly noticed in works on natural history, that when an author has
remarked with surprise that some _important_ organ or part, which is
generally very constant throughout large groups of species, has
_differed_ considerably in closely-allied species, that it has, also,
been _variable_ in the individuals of some of the species. And this
fact shows that a character, which is generally of generic value, when
it sinks in value and becomes only of specific value, often becomes
variable, though its physiological importance may remain the same.
Something of the same kind applies to monstrosities: at least Is.
Geoffroy St. Hilaire seems to entertain no doubt, that the more an
organ normally differs in the different species of the same group, the
more subject it is to individual anomalies.
On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same part
in other independently-created species of the same genus, be more
variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view of species being only strongly marked and fixed varieties, we
might surely expect to find them still often continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the case
in another manner:—the points in which all the species of a genus
resemble each other, and in which they differ from the species of some
other genus, are called generic characters; and these characters in
common I attribute to inheritance from a common
progenitor, for it can rarely have happened that natural selection will
have modified several species, fitted to more or less widely-different
habits, in exactly the same manner: and as these so-called generic
characters have been inherited from a remote period, since that period
when the species first branched off from their common progenitor, and
subsequently have not varied or come to differ in any degree, or only
in a slight degree, it is not probable that they should vary at the
present day. On the other hand, the points in which species differ from
other species of the same genus, are called specific characters; and as
these specific characters have varied and come to differ within the
period of the branching off of the species from a common progenitor, it
is probable that they should still often be in some degree variable,—at
least more variable than those parts of the organisation which have for
a very long period remained constant.
In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also
will be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts
of their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference
between their females; and the truth of this proposition will be
granted. The cause of the original variability of secondary sexual
characters is not manifest; but we can see why these characters should
not have been rendered as constant and uniform as other parts of the
organisation; for secondary sexual characters have been accumulated by
sexual selection, which
is less rigid in its action than ordinary selection, as it does not
entail death, but only gives fewer offspring to the less favoured
males. Whatever the cause may be of the variability of secondary sexual
characters, as they are highly variable, sexual selection will have had
a wide scope for action, and may thus readily have succeeded in giving
to the species of the same group a greater amount of difference in
their sexual characters, than in other parts of their structure.
It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of the
same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my list;
and as the differences in these cases are of a very unusual nature, the
relation can hardly be accidental. The same number of joints in the
tarsi is a character generally common to very large groups of beetles,
but in the Engidæ, as Westwood has remarked, the number varies greatly;
and the number likewise differs in the two sexes of the same species:
again in fossorial hymenoptera, the manner of neuration of the wings is
a character of the highest importance, because common to large groups;
but in certain genera the neuration differs in the different species,
and likewise in the two sexes of the same species. This relation has a
clear meaning on my view of the subject: I look at all the species of
the same genus as having as certainly descended from the same
progenitor, as have the two sexes of any one of the species.
Consequently, whatever part of the structure of the common progenitor,
or of its early descendants, became variable; variations of this part
would it is highly probable, be taken advantage of by natural and
sexual selection, in
order to fit the several species to their several places in the economy
of nature, and likewise to fit the two sexes of the same species to
each other, or to fit the males and females to different habits of
life, or the males to struggle with other males for the possession of
the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;—that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in
its congeners; and the not great degree of variability in a part,
however extraordinarily it may be developed, if it be common to a whole
group of species;—that the great variability of secondary sexual
characters, and the great amount of difference in these same characters
between closely allied species;—that secondary sexual and ordinary
specific differences are generally displayed in the same parts of the
organisation,—are all principles closely connected together. All being
mainly due to the species of the same group having descended from a
common progenitor, from whom they have inherited much in common,—to
parts which have recently and largely varied being more likely still to
go on varying than parts which have long been inherited and have not
varied,—to natural selection having more or less completely, according
to the lapse of time, overmastered the tendency to reversion and to
further variability,—to sexual selection being less rigid than ordinary
selection,—and to variations in the same parts having been accumulated
by natural and sexual selection, and thus adapted for secondary sexual,
and for ordinary specific purposes.
_Distinct species present analogous variations; and a variety of one
species often assumes some of the characters of an allied species, or
reverts to some of the characters of an early progenitor_.—These
propositions will be most readily understood by looking to our domestic
races. The most distinct breeds of pigeons, in countries most widely
apart, present sub-varieties with reversed feathers on the head and
feathers on the feet,—characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more
distinct races. The frequent presence of fourteen or even sixteen
tail-feathers in the pouter, may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a common
parent the same constitution and tendency to variation, when acted on
by similar unknown influences. In the vegetable kingdom we have a case
of analogous variation, in the enlarged stems, or roots as commonly
called, of the Swedish turnip and Ruta baga, plants which several
botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
not to the vera causa of community of descent, and a consequent
tendency to vary in a like manner, but to three separate yet closely
related acts of creation.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, a white
rump, a bar at the end of the tail, with the outer feathers externally
edged near their bases with white. As all these marks are
characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may I think confidently
come to this conclusion, because, as we have seen, these coloured marks
are eminently liable to appear in the crossed offspring of two distinct
and differently coloured breeds; and in this case there is nothing in
the external conditions of life to cause the reappearance of the
slaty-blue, with the several marks, beyond the influence of the mere
act of crossing on the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations—some say, for a dozen or even a
score of generations. After twelve generations, the proportion of
blood, to use a common expression, of any one ancestor, is only 1 in
2048; and yet, as we see, it is generally believed that a tendency to
reversion is retained by this very small proportion of foreign blood.
In a breed which has not been crossed, but in which _both_ parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might be, as
was formerly remarked, for all that we can see to the contrary,
transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations
distant, but that in each successive generation there has been a
tendency to reproduce the character in question, which at last, under
unknown favourable conditions, gains an ascendancy. For instance, it is
probable that in each generation of the barb-pigeon, which produces
most rarely a blue and black-barred bird, there has been a tendency in
each generation in the plumage to assume this colour. This view is
hypothetical, but could be supported by some facts; and I can see no
more abstract improbability in a tendency to produce any character
being inherited for an endless number of generations, than in quite
useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce
a rudiment inherited: for instance, in the common snapdragon
(Antirrhinum) a rudiment of a fifth stamen so often appears, that this
plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they
would occasionally vary in an analogous manner; so that a variety of
one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent
variety. But characters thus gained would probably be of an unimportant
nature, for the presence of all important characters will be governed
by natural selection, in accordance with the diverse habits of the
species, and will not be left to the mutual action of the conditions of
life and of a similar inherited constitution. It might further be
expected that the species of the same genus would occasionally exhibit
reversions to lost ancestral characters. As, however, we never know the
exact character of the common ancestor of a group, we could not
distinguish these two
cases: if, for instance, we did not know that the rock-pigeon was not
feather-footed or turn-crowned, we could not have told, whether these
characters in our domestic breeds were reversions or only analogous
variations; but we might have inferred that the blueness was a case of
reversion, from the number of the markings, which are correlated with
the blue tint, and which it does not appear probable would all appear
together from simple variation. More especially we might have inferred
this, from the blue colour and marks so often appearing when distinct
breeds of diverse colours are crossed. Hence, though under nature it
must generally be left doubtful, what cases are reversions to an
anciently existing character, and what are new but analogous
variations, yet we ought, on my theory, sometimes to find the varying
offspring of a species assuming characters (either from reversion or
from analogous variation) which already occur in some other members of
the same group. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable species
in our systematic works, is due to its varieties mocking, as it were,
some of the other species of the same genus. A considerable catalogue,
also, could be given of forms intermediate between two other forms,
which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as
independently created species, that the one in varying has assumed some
of the characters of the other, so as to produce the intermediate form.
But the best evidence is afforded by parts or organs of an important
and uniform nature occasionally varying so as to acquire, in some
degree, the character of the same part or organ in an allied species. I
have collected a long list of such cases; but
here, as before, I lie under a great disadvantage in not being able to
give them. I can only repeat that such cases certainly do occur, and
seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly under
nature. It is a case apparently of reversion. The ass not rarely has
very distinct transverse bars on its legs, like those on the legs of a
zebra: it has been asserted that these are plainest in the foal, and
from inquiries which I have made, I believe this to be true. It has
also been asserted that the stripe on each shoulder is sometimes
double. The shoulder stripe is certainly very variable in length and
outline. A white ass, but _not_ an albino, has been described without
either spinal or shoulder-stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan of
Pallas is said to have been seen with a double shoulder-stripe. The
hemionus has no shoulder-stripe; but traces of it, as stated by Mr.
Blyth and others, occasionally appear: and I have been informed by
Colonel Poole that the foals of this species are generally striped on
the legs, and faintly on the shoulder. The quagga, though so plainly
barred like a zebra over the body, is without bars on the legs; but Dr.
Gray has figured one specimen with very distinct zebra-like bars on the
hocks.
With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of _all_
colours; transverse bars on the legs are not rare in duns, mouse-duns,
and in one instance in a chestnut: a faint shoulder-stripe may
sometimes be seen in duns, and I have seen a trace in a
bay horse. My son made a careful examination and sketch for me of a dun
Belgian cart-horse with a double stripe on each shoulder and with
leg-stripes; and a man, whom I can implicitly trust, has examined for
me a small dun Welch pony with _three_ short parallel stripes on each
shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined the
breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes
quite disappear in old horses. Colonel Poole has seen both gray and bay
Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I
may state that I have collected cases of leg and shoulder stripes in
horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago in
the south. In all parts of the world these stripes occur far oftenest
in duns and mouse-duns; by the term dun a large range of colour is
included, from one between brown and black to a close approach to
cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse have descended
from several aboriginal species—one of which, the dun, was striped; and
that the above-described appearances are all due to ancient
crosses with the dun stock. But I am not at all satisfied with this
theory, and should be loth to apply it to breeds so distinct as the
heavy Belgian cart-horse, Welch ponies, cobs, the lanky Kattywar race,
etc., inhabiting the most distant parts of the world.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and
horse is particularly apt to have bars on its legs. I once saw a mule
with its legs so much striped that any one at first would have thought
that it must have been the product of a zebra; and Mr. W. C. Martin, in
his excellent treatise on the horse, has given a figure of a similar
mule. In four coloured drawings, which I have seen, of hybrids between
the ass and zebra, the legs were much more plainly barred than the rest
of the body; and in one of them there was a double shoulder-stripe. In
Lord Moreton’s famous hybrid from a chestnut mare and male quagga, the
hybrid, and even the pure offspring subsequently produced from the mare
by a black Arabian sire, were much more plainly barred across the legs
than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr. Gray (and he informs
me that he knows of a second case) from the ass and the hemionus; and
this hybrid, though the ass seldom has stripes on its legs and the
hemionus has none and has not even a shoulder-stripe, nevertheless had
all four legs barred, and had three short shoulder-stripes, like those
on the dun Welch pony, and even had some zebra-like stripes on the
sides of its face. With respect to this last fact, I was so convinced
that not even a stripe of colour appears from what would commonly be
called an accident, that I was led solely from the occurrence of the
face-stripes on this hybrid from the ass and hemionus,
to ask Colonel Poole whether such face-stripes ever occur in the
eminently striped Kattywar breed of horses, and was, as we have seen,
answered in the affirmative.
What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like an
ass. In the horse we see this tendency strong whenever a dun tint
appears—a tint which approaches to that of the general colouring of the
other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We see
this tendency to become striped most strongly displayed in hybrids from
between several of the most distinct species. Now observe the case of
the several breeds of pigeons: they are descended from a pigeon
(including two or three sub-species or geographical races) of a bluish
colour, with certain bars and other marks; and when any breed assumes
by simple variation a bluish tint, these bars and other marks
invariably reappear; but without any other change of form or character.
When the oldest and truest breeds of various colours are crossed, we
see a strong tendency for the blue tint and bars and marks to reappear
in the mongrels. I have stated that the most probable hypothesis to
account for the reappearance of very ancient characters, is—that there
is a _tendency_ in the young of each successive generation to produce
the long-lost character, and that this tendency, from unknown causes,
sometimes prevails. And we have just seen that in several species of
the horse-genus the stripes are either plainer or appear more commonly
in the young than in the old. Call the breeds of pigeons, some of which
have bred true for centuries, species; and how exactly parallel is the
case with that of the species of the horse-genus!
For myself, I venture confidently to look back thousands on thousands
of generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our
domestic horse, whether or not it be descended from one or more wild
stocks, of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency, when
crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents, but
other species of the genus. To admit this view is, as it seems to me,
to reject a real for an unreal, or at least for an unknown, cause. It
makes the works of God a mere mockery and deception; I would almost as
soon believe with the old and ignorant cosmogonists, that fossil shells
had never lived, but had been created in stone so as to mock the shells
now living on the sea-shore.
_Summary_.—Our ignorance of the laws of variation is profound. Not in
one case out of a hundred can we pretend to assign any reason why this
or that part differs, more or less, from the same part in the parents.
But whenever we have the means of instituting a comparison, the same
laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between
species of the same genus. The external conditions of life, as climate
and food, etc., seem to have induced some slight modifications. Habit
in producing constitutional differences,
and use in strengthening, and disuse in weakening and diminishing
organs, seem to have been more potent in their effects. Homologous
parts tend to vary in the same way, and homologous parts tend to
cohere. Modifications in hard parts and in external parts sometimes
affect softer and internal parts. When one part is largely developed,
perhaps it tends to draw nourishment from the adjoining parts; and
every part of the structure which can be saved without detriment to the
individual, will be saved. Changes of structure at an early age will
generally affect parts subsequently developed; and there are very many
other correlations of growth, the nature of which we are utterly unable
to understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialised to
any particular function, so that their modifications have not been
closely checked by natural selection. It is probably from this same
cause that organic beings low in the scale of nature are more variable
than those which have their whole organisation more specialised, and
are higher in the scale. Rudimentary organs, from being useless, will
be disregarded by natural selection, and hence probably are variable.
Specific characters—that is, the characters which have come to differ
since the several species of the same genus branched off from a common
parent—are more variable than generic characters, or those which have
long been inherited, and have not differed within this same period. In
these remarks we have referred to special parts or organs being still
variable, because they have recently varied and thus come to differ;
but we have also seen in the second Chapter that the same principle
applies to the whole individual; for in a district where many species
of any genus are found—that is, where there has been much former
variation and differentiation, or where the manufactory of new specific
forms has been actively at work—there, on an average, we now find most
varieties or incipient species. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same
group. Variability in the same parts of the organisation has generally
been taken advantage of in giving secondary sexual differences to the
sexes of the same species, and specific differences to the several
species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through an
extraordinary amount of modification since the genus arose; and thus we
can understand why it should often still be variable in a much higher
degree than other parts; for variation is a long-continued and slow
process, and natural selection will in such cases not as yet have had
time to overcome the tendency to further variability and to reversion
to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants—which on my view must be a very slow process, requiring a
long lapse of time—in this case, natural selection may readily have
succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar
influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters of
their ancient progenitors. Although new and important modifications may
not arise from reversion and analogous variation, such modifications
will add to the beautiful and harmonious diversity of nature.
Whatever the cause may be of each slight difference in the offspring
from their parents—and a cause for each must exist—it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face
of this earth are enabled to struggle with each other, and the best
adapted to survive.
CHAPTER VI.
DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification. Transitions.
Absence or rarity of transitional varieties. Transitions in habits of
life. Diversified habits in the same species. Species with habits
widely different from those of their allies. Organs of extreme
perfection. Means of transition. Cases of difficulty. Natura non facit
saltum. Organs of small importance. Organs not in all cases absolutely
perfect. The law of Unity of Type and of the Conditions of Existence
embraced by the theory of Natural Selection.
Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so
grave that to this day I can never reflect on them without being
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to my theory.
These difficulties and objections may be classed under the following
heads:—
Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?
Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the
modification of some animal with wholly different habits? Can we
believe that natural selection could produce, on the one hand, organs
of trifling importance, such as the tail of a giraffe, which serves as
a fly-flapper, and, on the other hand, organs of
such wonderful structure, as the eye, of which we hardly as yet fully
understand the inimitable perfection?
Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct as that which
leads the bee to make cells, which have practically anticipated the
discoveries of profound mathematicians?
Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?
The two first heads shall be here discussed—Instinct and Hybridism in
separate chapters.
_On the absence or rarity of transitional varieties._—As natural
selection acts solely by the preservation of profitable modifications,
each new form will tend in a fully-stocked country to take the place
of, and finally to exterminate, its own less improved parent or other
less-favoured forms with which it comes into competition. Thus
extinction and natural selection will, as we have seen, go hand in
hand. Hence, if we look at each species as descended from some other
unknown form, both the parent and all the transitional varieties will
generally have been exterminated by the very process of formation and
perfection of the new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record;
and I will here only state that I believe the answer mainly lies in the
record being incomparably less perfect than is generally supposed; the
imperfection of the record being chiefly due to organic beings not
inhabiting
profound depths of the sea, and to their remains being embedded and
preserved to a future age only in masses of sediment sufficiently thick
and extensive to withstand an enormous amount of future degradation;
and such fossiliferous masses can be accumulated only where much
sediment is deposited on the shallow bed of the sea, whilst it slowly
subsides. These contingencies will concur only rarely, and after
enormously long intervals. Whilst the bed of the sea is stationary or
is rising, or when very little sediment is being deposited, there will
be blanks in our geological history. The crust of the earth is a vast
museum; but the natural collections have been made only at intervals of
time immensely remote.
But it may be urged that when several closely-allied species inhabit
the same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals
with closely allied or representative species, evidently filling nearly
the same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and
rarer, the other becomes more and more frequent, till the one replaces
the other. But if we compare these species where they intermingle, they
are generally as absolutely distinct from each other in every detail of
structure as are specimens taken from the metropolis inhabited by each.
By my theory these allied species have descended from a common parent;
and during the process of modification, each has become adapted to the
conditions of life of its own region, and has supplanted and
exterminated its original parent and all the transitional varieties
between its past and present states. Hence we ought not to expect at
the
present time to meet with numerous transitional varieties in each
region, though they must have existed there, and may be embedded there
in a fossil condition. But in the intermediate region, having
intermediate conditions of life, why do we not now find closely-linking
intermediate varieties? This difficulty for a long time quite
confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during a
long period. Geology would lead us to believe that almost every
continent has been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been
separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition than
at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the
formerly broken condition of areas now continuous has played an
important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes
it is quite remarkable how abruptly, as Alph. De Candolle has observed,
a common alpine species disappears. The same fact has been noticed by
Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the
all-important elements of distribution, these facts ought to cause
surprise, as climate and height or depth graduate away insensibly. But
when we bear in mind that almost every species, even in its metropolis,
would increase immensely in numbers, were it not for other competing
species; that nearly all either prey on or serve as prey for others; in
short, that each organic being is either directly or indirectly related
in the most important manner to other organic beings, we must see that
the range of the inhabitants of any country by no means exclusively
depends on insensibly changing physical conditions, but in large part
on the presence of other species, on which it depends, or by which it
is destroyed, or with which it comes into competition; and as these
species are already defined objects (however they may have become so),
not blending one into another by insensible gradations, the range of
any one species, depending as it does on the range of others, will tend
to be sharply defined. Moreover, each species on the confines of its
range, where it exists in lessened numbers, will, during fluctuations
in the number of its enemies or of its prey, or in the seasons, be
extremely liable to utter extermination; and thus its geographical
range will come to be still more sharply defined.
If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each
has a wide range, with a comparatively narrow neutral territory between
them, in which they become rather suddenly rarer and rarer; then, as
varieties do not essentially differ from species,
the same rule will probably apply to both; and if we in imagination
adapt a varying species to a very large area, we shall have to adapt
two varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist
in lesser numbers from inhabiting a narrow and lesser area; and
practically, as far as I can make out, this rule holds good with
varieties in a state of nature. I have met with striking instances of
the rule in the case of varieties intermediate between well-marked
varieties in the genus Balanus. And it would appear from information
given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that generally
when varieties intermediate between two other forms occur, they are
much rarer numerically than the forms which they connect. Now, if we
may trust these facts and inferences, and therefore conclude that
varieties linking two other varieties together have generally existed
in lesser numbers than the forms which they connect, then, I think, we
can understand why intermediate varieties should not endure for very
long periods;—why as a general rule they should be exterminated and
disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run
a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on
both sides of it. But a far more important consideration, as I believe,
is that, during the process of further modification, by which two
varieties are supposed on my theory to be converted and perfected into
two distinct species, the two which exist in larger numbers from
inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists
in smaller numbers in a narrow and intermediate zone. For forms
existing in larger numbers will always have a better chance, within any
given period, of presenting further favourable variations for natural
selection to seize on, than will the rarer forms which exist in lesser
numbers. Hence, the more common forms, in the race for life, will tend
to beat and supplant the less common forms, for these will be more
slowly modified and improved. It is the same principle which, as I
believe, accounts for the common species in each country, as shown in
the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what
I mean by supposing three varieties of sheep to be kept, one adapted to
an extensive mountainous region; a second to a comparatively narrow,
hilly tract; and a third to wide plains at the base; and that the
inhabitants are all trying with equal steadiness and skill to improve
their stocks by selection; the chances in this case will be strongly in
favour of the great holders on the mountains or on the plains improving
their breeds more quickly than the small holders on the intermediate
narrow, hilly tract; and consequently the improved mountain or plain
breed will soon take the place of the less improved hill breed; and
thus the two breeds, which originally existed in greater numbers, will
come into close contact with each other, without the interposition of
the supplanted, intermediate hill-variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are very
slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable variations chance to occur,
and until a place in the natural polity
of the country can be better filled by some modification of some one or
more of its inhabitants. And such new places will depend on slow
changes of climate, or on the occasional immigration of new
inhabitants, and, probably, in a still more important degree, on some
of the old inhabitants becoming slowly modified, with the new forms
thus produced and the old ones acting and reacting on each other. So
that, in any one region and at any one time, we ought only to see a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties between
the several representative species and their common parent, must
formerly have existed in each broken portion of the land, but these
links will have been supplanted and exterminated during the process of
natural selection, so that they will no longer exist in a living state.
Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will, it
is probable, at first have been formed in the intermediate zones, but
they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely from what we know
of the actual distribution of closely allied or representative species,
and likewise of acknowledged varieties), exist in the intermediate
zones in lesser numbers than the varieties which they tend to connect.
From this cause alone the intermediate
varieties will be liable to accidental extermination; and during the
process of further modification through natural selection, they will
almost certainly be beaten and supplanted by the forms which they
connect; for these from existing in greater numbers will, in the
aggregate, present more variation, and thus be further improved through
natural selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but
the very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent forms and the intermediate
links. Consequently evidence of their former existence could be found
only amongst fossil remains, which are preserved, as we shall in a
future chapter attempt to show, in an extremely imperfect and
intermittent record.
_On the origin and transitions of organic beings with peculiar habits
and structure_.—It has been asked by the opponents of such views as I
hold, how, for instance, a land carnivorous animal could have been
converted into one with aquatic habits; for how could the animal in its
transitional state have subsisted? It would be easy to show that within
the same group carnivorous animals exist having every intermediate
grade between truly aquatic and strictly terrestrial habits; and as
each exists by a struggle for life, it is clear that each is well
adapted in its habits to its place in nature. Look at the Mustela vison
of North America, which has webbed feet and which resembles an otter in
its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter
it leaves the frozen waters, and preys like other polecats on mice and
land animals. If a different case had been taken, and it had been asked
how an insectivorous quadruped could possibly have been converted into
a flying bat, the question would have been far more difficult, and I
could have given no answer. Yet I think such difficulties have very
little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for out
of the many striking cases which I have collected, I can give only one
or two instances of transitional habits and structures in closely
allied species of the same genus; and of diversified habits, either
constant or occasional, in the same species. And it seems to me that
nothing less than a long list of such cases is sufficient to lessen the
difficulty in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as
Sir J. Richardson has remarked, with the posterior part of their bodies
rather wide and with the skin on their flanks rather full, to the
so-called flying squirrels; and flying squirrels have their limbs and
even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to an
astonishing distance from tree to tree. We cannot doubt that each
structure is of use to each kind of squirrel in its own country, by
enabling it to escape birds or beasts of prey, or to collect food more
quickly, or, as there is reason to believe, by lessening the danger
from occasional falls. But it does not follow from this fact that the
structure of each squirrel is the best that it is possible to conceive
under all natural conditions. Let the climate and vegetation change,
let other competing rodents or new beasts of prey immigrate, or old
ones
become modified, and all analogy would lead us to believe that some at
least of the squirrels would decrease in numbers or become
exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no
difficulty, more especially under changing conditions of life, in the
continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propagated,
until by the accumulated effects of this process of natural selection,
a perfect so-called flying squirrel was produced.
Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank membrane is, also, furnished
with an extensor muscle. Although no graduated links of structure,
fitted for gliding through the air, now connect the Galeopithecus with
the other Lemuridæ, yet I can see no difficulty in supposing that such
links formerly existed, and that each had been formed by the same steps
as in the case of the less perfectly gliding squirrels; and that each
grade of structure had been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might be
greatly lengthened by natural selection; and this, as far as the organs
of flight are concerned, would convert it into a bat. In bats which
have the wing-membrane extended from the top of the shoulder to the
tail, including the hind-legs, we perhaps see traces of an apparatus
originally constructed for gliding through the air rather than for
flight.
If about a dozen genera of birds had become extinct or were unknown,
who would have ventured to have
surmised that birds might have existed which used their wings solely as
flappers, like the logger-headed duck (Micropterus of Eyton); as fins
in the water and front legs on the land, like the penguin; as sails,
like the ostrich; and functionally for no purpose, like the Apteryx.
Yet the structure of each of these birds is good for it, under the
conditions of life to which it is exposed, for each has to live by a
struggle; but it is not necessarily the best possible under all
possible conditions. It must not be inferred from these remarks that
any of the grades of wing-structure here alluded to, which perhaps may
all have resulted from disuse, indicate the natural steps by which
birds have acquired their perfect power of flight; but they serve, at
least, to show what diversified means of transition are possible.
Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land, and seeing that
we have flying birds and mammals, flying insects of the most
diversified types, and formerly had flying reptiles, it is conceivable
that flying-fish, which now glide far through the air, slightly rising
and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected, who
would have ever imagined that in an early transitional state they had
been inhabitants of the open ocean, and had used their incipient organs
of flight exclusively, as far as we know, to escape being devoured by
other fish?
When we see any structure highly perfected for any particular habit, as
the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom
continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural selection.
Furthermore, we may conclude that transitional
grades between structures fitted for very different habits of life will
rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary
illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many subordinate
forms, for taking prey of many kinds in many ways, on the land and in
the water, until their organs of flight had come to a high stage of
perfection, so as to have given them a decided advantage over other
animals in the battle for life. Hence the chance of discovering species
with transitional grades of structure in a fossil condition will always
be less, from their having existed in lesser numbers, than in the case
of species with fully developed structures.
I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case occurs,
it would be easy for natural selection to fit the animal, by some
modification of its structure, for its changed habits, or exclusively
for one of its several different habits. But it is difficult to tell,
and immaterial for us, whether habits generally change first and
structure afterwards; or whether slight modifications of structure lead
to changed habits; both probably often change almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of the
many British insects which now feed on exotic plants, or exclusively on
artificial substances. Of diversified habits innumerable instances
could be given: I have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot and then
proceeding to another, like a kestrel, and at other times standing
stationary on the margin of water, and then dashing like a kingfisher
at a fish. In our own country the larger titmouse (Parus major) may be
seen climbing branches, almost like a creeper; it often, like a shrike,
kills small birds by blows on the head; and I have many times seen and
heard it hammering the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear was seen by
Hearne swimming for hours with widely open mouth, thus catching, like a
whale, insects in the water. Even in so extreme a case as this, if the
supply of insects were constant, and if better adapted competitors did
not already exist in the country, I can see no difficulty in a race of
bears being rendered, by natural selection, more and more aquatic in
their structure and habits, with larger and larger mouths, till a
creature was produced as monstrous as a whale.
As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other species
of the same genus, we might expect, on my theory, that such individuals
would occasionally have given rise to new species, having anomalous
habits, and with their structure either slightly or considerably
modified from that of their proper type. And such instances do occur in
nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and for seizing insects in the
chinks of the bark? Yet in North America there are woodpeckers which
feed largely on fruit, and others with elongated wings which chase
insects on the wing; and on the plains of La Plata, where not a tree
grows, there is a woodpecker, which in every essential part of its
organisation, even in its colouring, in the harsh tone of its voice,
and undulatory flight, told me plainly of its close blood-relationship
to our common species; yet it is a woodpecker which never climbs a
tree!
Petrels are the most aërial and oceanic of birds, yet in the quiet
Sounds of Tierra del Fuego, the Puffinuria
berardi, in its general habits, in its astonishing power of diving, its
manner of swimming, and of flying when unwillingly it takes flight,
would be mistaken by any one for an auk or grebe; nevertheless, it is
essentially a petrel, but with many parts of its organisation
profoundly modified. On the other hand, the acutest observer by
examining the dead body of the water-ouzel would never have suspected
its sub-aquatic habits; yet this anomalous member of the strictly
terrestrial thrush family wholly subsists by diving,—grasping the
stones with its feet and using its wings under water.
He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not at all in agreement. What can be plainer than
that the webbed feet of ducks and geese are formed for swimming? yet
there are upland geese with webbed feet which rarely or never go near
the water; and no one except Audubon has seen the frigate-bird, which
has all its four toes webbed, alight on the surface of the sea. On the
other hand, grebes and coots are eminently aquatic, although their toes
are only bordered by membrane. What seems plainer than that the long
toes of grallatores are formed for walking over swamps and floating
plants, yet the water-hen is nearly as aquatic as the coot; and the
landrail nearly as terrestrial as the quail or partridge. In such
cases, and many others could be given, habits have changed without a
corresponding change of structure. The webbed feet of the upland goose
may be said to have become rudimentary in function, though not in
structure. In the frigate-bird, the deeply-scooped membrane between the
toes shows that structure has begun to change.
He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the
Creator to cause a being of one type to take the place of one of
another type; but this seems to me only restating the fact in dignified
language. He who believes in the struggle for existence and in the
principle of natural selection, will acknowledge that every organic
being is constantly endeavouring to increase in numbers; and that if
any one being vary ever so little, either in habits or structure, and
thus gain an advantage over some other inhabitant of the country, it
will seize on the place of that inhabitant, however different it may be
from its own place. Hence it will cause him no surprise that there
should be geese and frigate-birds with webbed feet, either living on
the dry land or most rarely alighting on the water; that there should
be long-toed corncrakes living in meadows instead of in swamps; that
there should be woodpeckers where not a tree grows; that there should
be diving thrushes, and petrels with the habits of auks.
_Organs of extreme perfection and complication_.—To suppose that the
eye, with all its inimitable contrivances for adjusting the focus to
different distances, for admitting different amounts of light, and for
the correction of spherical and chromatic aberration, could have been
formed by natural selection, seems, I freely confess, absurd in the
highest possible degree. Yet reason tells me, that if numerous
gradations from a perfect and complex eye to one very imperfect and
simple, each grade being useful to its possessor, can be shown to
exist; if further, the eye does vary ever so slightly, and the
variations be inherited, which is certainly the case; and if any
variation or modification in the organ be ever useful to an animal
under changing conditions of life, then the difficulty of believing
that a perfect and complex eye could be formed by natural
selection, though insuperable by our imagination, can hardly be
considered real. How a nerve comes to be sensitive to light, hardly
concerns us more than how life itself first originated; but I may
remark that several facts make me suspect that any sensitive nerve may
be rendered sensitive to light, and likewise to those coarser
vibrations of the air which produce sound.
In looking for the gradations by which an organ in any species has been
perfected, we ought to look exclusively to its lineal ancestors; but
this is scarcely ever possible, and we are forced in each case to look
to species of the same group, that is to the collateral descendants
from the same original parent-form, in order to see what gradations are
possible, and for the chance of some gradations having been transmitted
from the earlier stages of descent, in an unaltered or little altered
condition. Amongst existing Vertebrata, we find but a small amount of
gradation in the structure of the eye, and from fossil species we can
learn nothing on this head. In this great class we should probably have
to descend far beneath the lowest known fossiliferous stratum to
discover the earlier stages, by which the eye has been perfected.
In the Articulata we can commence a series with an optic nerve merely
coated with pigment, and without any other mechanism; and from this low
stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach a
moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into facets,
within each of which there is a lens-shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and
which properly act only by excluding lateral pencils of light, are
convex at their upper ends
and must act by convergence; and at their lower ends there seems to be
an imperfect vitreous substance. With these facts, here far too briefly
and imperfectly given, which show that there is much graduated
diversity in the eyes of living crustaceans, and bearing in mind how
small the number of living animals is in proportion to those which have
become extinct, I can see no very great difficulty (not more than in
the case of many other structures) in believing that natural selection
has converted the simple apparatus of an optic nerve merely coated with
pigment and invested by transparent membrane, into an optical
instrument as perfect as is possessed by any member of the great
Articulate class.
He who will go thus far, if he find on finishing this treatise that
large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit
that a structure even as perfect as the eye of an eagle might be formed
by natural selection, although in this case he does not know any of the
transitional grades. His reason ought to conquer his imagination;
though I have felt the difficulty far too keenly to be surprised at any
degree of hesitation in extending the principle of natural selection to
such startling lengths.
It is scarcely possible to avoid comparing the eye to a telescope. We
know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that
the eye has been formed by a somewhat analogous process. But may not
this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination to
take a thick layer of transparent tissue, with a nerve sensitive to
light beneath, and then suppose every
part of this layer to be continually changing slowly in density, so as
to separate into layers of different densities and thicknesses, placed
at different distances from each other, and with the surfaces of each
layer slowly changing in form. Further we must suppose that there is a
power always intently watching each slight accidental alteration in the
transparent layers; and carefully selecting each alteration which,
under varied circumstances, may in any way, or in any degree, tend to
produce a distincter image. We must suppose each new state of the
instrument to be multiplied by the million; and each to be preserved
till a better be produced, and then the old ones to be destroyed. In
living bodies, variation will cause the slight alterations, generation
will multiply them almost infinitely, and natural selection will pick
out with unerring skill each improvement. Let this process go on for
millions on millions of years; and during each year on millions of
individuals of many kinds; and may we not believe that a living optical
instrument might thus be formed as superior to one of glass, as the
works of the Creator are to those of man?
If it could be demonstrated that any complex organ existed, which could
not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to my theory, there has been much
extinction. Or again, if we look to an organ common to all the members
of a large class, for in this latter case the organ must have been
first formed at an extremely remote period, since which all the many
members of the class have been developed; and in order to discover the
early transitional grades through which the organ has
passed, we should have to look to very ancient ancestral forms, long
since become extinct.
We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous
cases could be given amongst the lower animals of the same organ
performing at the same time wholly distinct functions; thus the
alimentary canal respires, digests, and excretes in the larva of the
dragon-fly and in the fish Cobites. In the Hydra, the animal may be
turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might easily
specialise, if any advantage were thus gained, a part or organ, which
had performed two functions, for one function alone, and thus wholly
change its nature by insensible steps. Two distinct organs sometimes
perform simultaneously the same function in the same individual; to
give one instance, there are fish with gills or branchiæ that breathe
the air dissolved in the water, at the same time that they breathe free
air in their swimbladders, this latter organ having a ductus
pneumaticus for its supply, and being divided by highly vascular
partitions. In these cases, one of the two organs might with ease be
modified and perfected so as to perform all the work by itself, being
aided during the process of modification by the other organ; and then
this other organ might be modified for some other and quite distinct
purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a wholly different purpose, namely respiration. The swimbladder
has, also, been worked in as an accessory to the auditory organs of
certain fish, or, for I do not know which
view is now generally held, a part of the auditory apparatus has been
worked in as a complement to the swimbladder. All physiologists admit
that the swimbladder is homologous, or “ideally similar,” in position
and structure with the lungs of the higher vertebrate animals: hence
there seems to me to be no great difficulty in believing that natural
selection has actually converted a swimbladder into a lung, or organ
used exclusively for respiration.
I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype,
of which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen’s interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchiæ have wholly
disappeared—the slits on the sides of the neck and the loop-like course
of the arteries still marking in the embryo their former position. But
it is conceivable that the now utterly lost branchiæ might have been
gradually worked in by natural selection for some quite distinct
purpose: in the same manner as, on the view entertained by some
naturalists that the branchiæ and dorsal scales of Annelids are
homologous with the wings and wing-covers of insects, it is probable
that organs which at a very ancient period served for respiration have
been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that I
will give one more instance. Pedunculated cirripedes have two minute
folds of skin,
called by me the ovigerous frena, which serve, through the means of a
sticky secretion, to retain the eggs until they are hatched within the
sack. These cirripedes have no branchiæ, the whole surface of the body
and sack, including the small frena, serving for respiration. The
Balanidæ or sessile cirripedes, on the other hand, have no ovigerous
frena, the eggs lying loose at the bottom of the sack, in the
well-enclosed shell; but they have large folded branchiæ. Now I think
no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchiæ of the other family; indeed, they
graduate into each other. Therefore I do not doubt that little folds of
skin, which originally served as ovigerous frena, but which, likewise,
very slightly aided the act of respiration, have been gradually
converted by natural selection into branchiæ, simply through an
increase in their size and the obliteration of their adhesive glands.
If all pedunculated cirripedes had become extinct, and they have
already suffered far more extinction than have sessile cirripedes, who
would ever have imagined that the branchiæ in this latter family had
originally existed as organs for preventing the ova from being washed
out of the sack?
Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.
One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; it is impossible to
conceive by what steps these wondrous organs have been produced; but,
as Owen and others have remarked,
their intimate structure closely resembles that of common muscle; and
as it has lately been shown that Rays have an organ closely analogous
to the electric apparatus, and yet do not, as Matteuchi asserts,
discharge any electricity, we must own that we are far too ignorant to
argue that no transition of any kind is possible.
The electric organs offer another and even more serious difficulty; for
they occur in only about a dozen fishes, of which several are widely
remote in their affinities. Generally when the same organ appears in
several members of the same class, especially if in members having very
different habits of life, we may attribute its presence to inheritance
from a common ancestor; and its absence in some of the members to its
loss through disuse or natural selection. But if the electric organs
had been inherited from one ancient progenitor thus provided, we might
have expected that all electric fishes would have been specially
related to each other. Nor does geology at all lead to the belief that
formerly most fishes had electric organs, which most of their modified
descendants have lost. The presence of luminous organs in a few
insects, belonging to different families and orders, offers a parallel
case of difficulty. Other cases could be given; for instance in plants,
the very curious contrivance of a mass of pollen-grains, borne on a
foot-stalk with a sticky gland at the end, is the same in Orchis and
Asclepias,—genera almost as remote as possible amongst flowering
plants. In all these cases of two very distinct species furnished with
apparently the same anomalous organ, it should be observed that,
although the general appearance and function of the organ may be the
same, yet some fundamental difference can generally be detected. I am
inclined to believe that in nearly the same way as two men have
sometimes independently hit on
the very same invention, so natural selection, working for the good of
each being and taking advantage of analogous variations, has sometimes
modified in very nearly the same manner two parts in two organic
beings, which owe but little of their structure in common to
inheritance from the same ancestor.
Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering that the proportion of living and known forms to the
extinct and unknown is very small, I have been astonished how rarely an
organ can be named, towards which no transitional grade is known to
lead. The truth of this remark is indeed shown by that old canon in
natural history of “Natura non facit saltum.” We meet with this
admission in the writings of almost every experienced naturalist; or,
as Milne Edwards has well expressed it, nature is prodigal in variety,
but niggard in innovation. Why, on the theory of Creation, should this
be so? Why should all the parts and organs of many independent beings,
each supposed to have been separately created for its proper place in
nature, be so invariably linked together by graduated steps? Why should
not Nature have taken a leap from structure to structure? On the theory
of natural selection, we can clearly understand why she should not; for
natural selection can act only by taking advantage of slight successive
variations; she can never take a leap, but must advance by the shortest
and slowest steps.
_Organs of little apparent importance_.—As natural selection acts by
life and death,—by the preservation of individuals with any favourable
variation, and by the destruction of those with any unfavourable
deviation of structure,—I have sometimes felt much difficulty in
understanding the origin of simple parts, of which the importance does
not seem sufficient to cause the preservation of successively varying
individuals. I have sometimes felt as much difficulty, though of a very
different kind, on this head, as in the case of an organ as perfect and
complex as the eye.
In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications
would be of importance or not. In a former chapter I have given
instances of most trifling characters, such as the down on fruit and
the colour of the flesh, which, from determining the attacks of insects
or from being correlated with constitutional differences, might
assuredly be acted on by natural selection. The tail of the giraffe
looks like an artificially constructed fly-flapper; and it seems at
first incredible that this could have been adapted for its present
purpose by successive slight modifications, each better and better, for
so trifling an object as driving away flies; yet we should pause before
being too positive even in this case, for we know that the distribution
and existence of cattle and other animals in South America absolutely
depends on their power of resisting the attacks of insects: so that
individuals which could by any means defend themselves from these small
enemies, would be able to range into new pastures and thus gain a great
advantage. It is not that the larger quadrupeds are actually destroyed
(except in some rare cases) by the flies, but they are incessantly
harassed and their strength reduced, so that they are more subject to
disease, or not so well enabled in a coming dearth to search for food,
or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a
former period, have been transmitted in nearly the same state, although
now become of very slight use; and any actually injurious deviations in
their structure will always have been checked by natural selection.
Seeing how important an organ of locomotion the tail is in most aquatic
animals, its general presence and use for many purposes in so many land
animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed
tail having been formed in an aquatic animal, it might subsequently
come to be worked in for all sorts of purposes, as a fly-flapper, an
organ of prehension, or as an aid in turning, as with the dog, though
the aid must be slight, for the hare, with hardly any tail, can double
quickly enough.
In the second place, we may sometimes attribute importance to
characters which are really of very little importance, and which have
originated from quite secondary causes, independently of natural
selection. We should remember that climate, food, etc., probably have
some little direct influence on the organisation; that characters
reappear from the law of reversion; that correlation of growth will
have had a most important influence in modifying various structures;
and finally, that sexual selection will often have largely modified the
external characters of animals having a will, to give one male an
advantage in fighting with another or in charming the females. Moreover
when a modification of structure has primarily arisen from the above or
other unknown causes, it may at first have been of no advantage to the
species, but may subsequently have been taken advantage of by the
descendants of the species under new conditions of life and with newly
acquired habits.
To give a few instances to illustrate these latter
remarks. If green woodpeckers alone had existed, and we did not know
that there were many black and pied kinds, I dare say that we should
have thought that the green colour was a beautiful adaptation to hide
this tree-frequenting bird from its enemies; and consequently that it
was a character of importance and might have been acquired through
natural selection; as it is, I have no doubt that the colour is due to
some quite distinct cause, probably to sexual selection. A trailing
bamboo in the Malay Archipelago climbs the loftiest trees by the aid of
exquisitely constructed hooks clustered around the ends of the
branches, and this contrivance, no doubt, is of the highest service to
the plant; but as we see nearly similar hooks on many trees which are
not climbers, the hooks on the bamboo may have arisen from unknown laws
of growth, and have been subsequently taken advantage of by the plant
undergoing further modification and becoming a climber. The naked skin
on the head of a vulture is generally looked at as a direct adaptation
for wallowing in putridity; and so it may be, or it may possibly be due
to the direct action of putrid matter; but we should be very cautious
in drawing any such inference, when we see that the skin on the head of
the clean-feeding male turkey is likewise naked. The sutures in the
skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has
been taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are immediately
made conscious of this by reflecting on the differences in the breeds
of our domesticated animals in different countries,—more especially in
the less civilized countries where there has been but little artificial
selection. Careful observers are convinced that a damp climate affects
the growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs from
exercising them more, and possibly even the form of the pelvis; and
then by the law of homologous variation, the front limbs and even the
head would probably be affected. The shape, also, of the pelvis might
affect by pressure the shape of the head of the young in the womb. The
laborious breathing necessary in high regions would, we have some
reason to believe, increase the size of the chest; and again
correlation would come into play. Animals kept by savages in different
countries often have to struggle for their own subsistence, and would
be exposed to a certain extent to natural selection, and individuals
with slightly different constitutions would succeed best under
different climates; and there is reason to believe that constitution
and colour are correlated. A good observer, also, states that in cattle
susceptibility to the attacks of flies is correlated with colour, as is
the liability to be poisoned by certain plants; so that colour would be
thus subjected to the action of natural selection. But we are far too
ignorant to speculate on the relative importance of the several known
and unknown laws of variation; and I have here alluded to them only to
show that, if we are unable to account for the characteristic
differences of our domestic breeds, which nevertheless we generally
admit to have arisen through ordinary generation, we ought not to lay
too much stress on our
ignorance of the precise cause of the slight analogous differences
between species. I might have adduced for this same purpose the
differences between the races of man, which are so strongly marked; I
may add that some little light can apparently be thrown on the origin
of these differences, chiefly through sexual selection of a particular
kind, but without here entering on copious details my reasoning would
appear frivolous.
The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists, against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor.
They believe that very many structures have been created for beauty in
the eyes of man, or for mere variety. This doctrine, if true, would be
absolutely fatal to my theory. Yet I fully admit that many structures
are of no direct use to their possessors. Physical conditions probably
have had some little effect on structure, quite independently of any
good thus gained. Correlation of growth has no doubt played a most
important part, and a useful modification of one part will often have
entailed on other parts diversified changes of no direct use. So again
characters which formerly were useful, or which formerly had arisen
from correlation of growth, or from other unknown cause, may reappear
from the law of reversion, though now of no direct use. The effects of
sexual selection, when displayed in beauty to charm the females, can be
called useful only in rather a forced sense. But by far the most
important consideration is that the chief part of the organisation of
every being is simply due to inheritance; and consequently, though each
being assuredly is well fitted for its place in nature, many structures
now have no direct relation to the habits of life of each species.
Thus, we can hardly believe that the webbed feet of the upland
goose or of the frigate-bird are of special use to these birds; we
cannot believe that the same bones in the arm of the monkey, in the
fore leg of the horse, in the wing of the bat, and in the flipper of
the seal, are of special use to these animals. We may safely attribute
these structures to inheritance. But to the progenitor of the upland
goose and of the frigate-bird, webbed feet no doubt were as useful as
they now are to the most aquatic of existing birds. So we may believe
that the progenitor of the seal had not a flipper, but a foot with five
toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse, and
bat, which have been inherited from a common progenitor, were formerly
of more special use to that progenitor, or its progenitors, than they
now are to these animals having such widely diversified habits.
Therefore we may infer that these several bones might have been
acquired through natural selection, subjected formerly, as now, to the
several laws of inheritance, reversion, correlation of growth, etc.
Hence every detail of structure in every living creature (making some
little allowance for the direct action of physical conditions) may be
viewed, either as having been of special use to some ancestral form, or
as being now of special use to the descendants of this form—either
directly, or indirectly through the complex laws of growth.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the
fang of the adder, and in the ovipositor of the ichneumon, by which its
eggs are deposited
in the living bodies of other insects. If it could be proved that any
part of the structure of any one species had been formed for the
exclusive good of another species, it would annihilate my theory, for
such could not have been produced through natural selection. Although
many statements may be found in works on natural history to this
effect, I cannot find even one which seems to me of any weight. It is
admitted that the rattlesnake has a poison-fang for its own defence and
for the destruction of its prey; but some authors suppose that at the
same time this snake is furnished with a rattle for its own injury,
namely, to warn its prey to escape. I would almost as soon believe that
the cat curls the end of its tail when preparing to spring, in order to
warn the doomed mouse. But I have not space here to enter on this and
other such cases.
Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each.
No organ will be formed, as Paley has remarked, for the purpose of
causing pain or for doing an injury to its possessor. If a fair balance
be struck between the good and evil caused by each part, each will be
found on the whole advantageous. After the lapse of time, under
changing conditions of life, if any part comes to be injurious, it will
be modified; or if it be not so, the being will become extinct, as
myriads have become extinct.
Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than, the other inhabitants of the same
country with which it has to struggle for existence. And we see that
this is the degree of perfection attained under nature. The endemic
productions of New Zealand, for instance, are perfect one compared with
another; but they are now rapidly yielding before the advancing legions
of plants
and animals introduced from Europe. Natural selection will not produce
absolute perfection, nor do we always meet, as far as we can judge,
with this high standard under nature. The correction for the aberration
of light is said, on high authority, not to be perfect even in that
most perfect organ, the eye. If our reason leads us to admire with
enthusiasm a multitude of inimitable contrivances in nature, this same
reason tells us, though we may easily err on both sides, that some
other contrivances are less perfect. Can we consider the sting of the
wasp or of the bee as perfect, which, when used against many attacking
animals, cannot be withdrawn, owing to the backward serratures, and so
inevitably causes the death of the insect by tearing out its viscera?
If we look at the sting of the bee, as having originally existed in a
remote progenitor as a boring and serrated instrument, like that in so
many members of the same great order, and which has been modified but
not perfected for its present purpose, with the poison originally
adapted to cause galls subsequently intensified, we can perhaps
understand how it is that the use of the sting should so often cause
the insect’s own death: for if on the whole the power of stinging be
useful to the community, it will fulfil all the requirements of natural
selection, though it may cause the death of some few members. If we
admire the truly wonderful power of scent by which the males of many
insects find their females, can we admire the production for this
single purpose of thousands of drones, which are utterly useless to the
community for any other end, and which are ultimately slaughtered by
their industrious and sterile sisters? It may be difficult, but we
ought to admire the savage instinctive hatred of the queen-bee, which
urges her instantly to destroy the
young queens her daughters as soon as born, or to perish herself in the
combat; for undoubtedly this is for the good of the community; and
maternal love or maternal hatred, though the latter fortunately is most
rare, is all the same to the inexorable principle of natural selection.
If we admire the several ingenious contrivances, by which the flowers
of the orchis and of many other plants are fertilised through insect
agency, can we consider as equally perfect the elaboration by our
fir-trees of dense clouds of pollen, in order that a few granules may
be wafted by a chance breeze on to the ovules?
_Summary of Chapter_.—We have in this chapter discussed some of the
difficulties and objections which may be urged against my theory. Many
of them are very grave; but I think that in the discussion light has
been thrown on several facts, which on the theory of independent acts
of creation are utterly obscure. We have seen that species at any one
period are not indefinitely variable, and are not linked together by a
multitude of intermediate gradations, partly because the process of
natural selection will always be very slow, and will act, at any one
time, only on a very few forms; and partly because the very process of
natural selection almost implies the continual supplanting and
extinction of preceding and intermediate gradations. Closely allied
species, now living on a continuous area, must often have been formed
when the area was not continuous, and when the conditions of life did
not insensibly graduate away from one part to another. When two
varieties are formed in two districts of a continuous area, an
intermediate variety will often be formed, fitted for an intermediate
zone; but from reasons assigned, the intermediate variety will usually
exist in lesser numbers than
the two forms which it connects; consequently the two latter, during
the course of further modification, from existing in greater numbers,
will have a great advantage over the less numerous intermediate
variety, and will thus generally succeed in supplanting and
exterminating it.
We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first could only glide through the
air.
We have seen that a species may under new conditions of life change its
habits, or have diversified habits, with some habits very unlike those
of its nearest congeners. Hence we can understand, bearing in mind that
each organic being is trying to live wherever it can live, how it has
arisen that there are upland geese with webbed feet, ground
woodpeckers, diving thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one;
yet in the case of any organ, if we know of a long series of gradations
in complexity, each good for its possessor, then, under changing
conditions of life, there is no logical impossibility in the
acquirement of any conceivable degree of perfection through natural
selection. In the cases in which we know of no intermediate or
transitional states, we should be very cautious in concluding that none
could have existed, for the homologies of many organs and their
intermediate states show that wonderful metamorphoses in function are
at least possible. For instance, a swim-bladder has apparently been
converted into an air-breathing lung. The same organ having performed
simultaneously very different functions, and then having been
specialised for one function; and two very distinct organs having
performed at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely
facilitated transitions.
We are far too ignorant, in almost every case, to be enabled to assert
that any part or organ is so unimportant for the welfare of a species,
that modifications in its structure could not have been slowly
accumulated by means of natural selection. But we may confidently
believe that many modifications, wholly due to the laws of growth, and
at first in no way advantageous to a species, have been subsequently
taken advantage of by the still further modified descendants of this
species. We may, also, believe that a part formerly of high importance
has often been retained (as the tail of an aquatic animal by its
terrestrial descendants), though it has become of such small importance
that it could not, in its present state, have been acquired by natural
selection,—a power which acts solely by the preservation of profitable
variations in the struggle for life.
Natural selection will produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs,
and excretions highly useful or even indispensable, or highly injurious
to another species, but in all cases at the same time useful to the
owner. Natural selection in each well-stocked country, must act chiefly
through the competition of the inhabitants one with another, and
consequently will produce perfection, or strength in the battle for
life, only according to the standard of that country. Hence the
inhabitants of one country, generally the smaller one, will often
yield, as we see they do yield, to the inhabitants of another and
generally larger country. For in
the larger country there will have existed more individuals, and more
diversified forms, and the competition will have been severer, and thus
the standard of perfection will have been rendered higher. Natural
selection will not necessarily produce absolute perfection; nor, as far
as we can judge by our limited faculties, can absolute perfection be
everywhere found.
On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, “Natura non facit
saltum.” This canon, if we look only to the present inhabitants of the
world, is not strictly correct, but if we include all those of past
times, it must by my theory be strictly true.
It is generally acknowledged that all organic beings have been formed
on two great laws—Unity of Type, and the Conditions of Existence. By
unity of type is meant that fundamental agreement in structure, which
we see in organic beings of the same class, and which is quite
independent of their habits of life. On my theory, unity of type is
explained by unity of descent. The expression of conditions of
existence, so often insisted on by the illustrious Cuvier, is fully
embraced by the principle of natural selection. For natural selection
acts by either now adapting the varying parts of each being to its
organic and inorganic conditions of life; or by having adapted them
during long-past periods of time: the adaptations being aided in some
cases by use and disuse, being slightly affected by the direct action
of the external conditions of life, and being in all cases subjected to
the several laws of growth. Hence, in fact, the law of the Conditions
of Existence is the higher law; as it includes, through the inheritance
of former adaptations, that of Unity of Type.
CHAPTER VII.
INSTINCT.
Instincts comparable with habits, but different in their origin.
Instincts graduated. Aphides and ants. Instincts variable. Domestic
instincts, their origin. Natural instincts of the cuckoo, ostrich, and
parasitic bees. Slave-making ants. Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts.
Neuter or sterile insects. Summary.
The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat
the subject separately, especially as so wonderful an instinct as that
of the hive-bee making its cells will probably have occurred to many
readers, as a difficulty sufficient to overthrow my whole theory. I
must premise, that I have nothing to do with the origin of the primary
mental powers, any more than I have with that of life itself. We are
concerned only with the diversities of instinct and of the other mental
qualities of animals within the same class.
I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this
term; but every one understands what is meant, when it is said that
instinct impels the cuckoo to migrate and to lay her eggs in other
birds’ nests. An action, which we ourselves should require experience
to enable us to perform, when performed by an animal, more especially
by a very young one, without any experience, and when performed by many
individuals in the same way, without their knowing for what purpose it
is performed, is usually said to be instinctive.
But I could show that none of these characters of instinct are
universal. A little dose, as Pierre Huber expresses it, of judgment or
reason, often comes into play, even in animals very low in the scale of
nature.
Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably
accurate notion of the frame of mind under which an instinctive action
is performed, but not of its origin. How unconsciously many habitual
actions are performed, indeed not rarely in direct opposition to our
conscious will! yet they may be modified by the will or reason. Habits
easily become associated with other habits, and with certain periods of
time and states of the body. When once acquired, they often remain
constant throughout life. Several other points of resemblance between
instincts and habits could be pointed out. As in repeating a well-known
song, so in instincts, one action follows another by a sort of rhythm;
if a person be interrupted in a song, or in repeating anything by rote,
he is generally forced to go back to recover the habitual train of
thought: so P. Huber found it was with a caterpillar, which makes a
very complicated hammock; for if he took a caterpillar which had
completed its hammock up to, say, the sixth stage of construction, and
put it into a hammock completed up only to the third stage, the
caterpillar simply re-performed the fourth, fifth, and sixth stages of
construction. If, however, a caterpillar were taken out of a hammock
made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already
done for it, far from feeling the benefit of this, it was much
embarrassed, and, in order to complete its hammock, seemed forced to
start from the third stage, where it had left off, and thus tried to
complete the already finished work.
If we suppose any habitual action to become inherited—and I think it
can be shown that this does sometimes happen—then the resemblance
between what originally was a habit and an instinct becomes so close as
not to be distinguished. If Mozart, instead of playing the pianoforte
at three years old with wonderfully little practice, had played a tune
with no practice at all, he might truly be said to have done so
instinctively. But it would be the most serious error to suppose that
the greater number of instincts have been acquired by habit in one
generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been thus acquired.
It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that may
be profitable. It is thus, as I believe, that all the most complex and
wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are of quite
subordinate importance to the effects of the natural selection of what
may be called accidental variations of instincts;—that is of variations
produced by the same unknown causes which produce slight deviations of
bodily structure.
No complex instinct can possibly be produced through
natural selection, except by the slow and gradual accumulation of
numerous, slight, yet profitable, variations. Hence, as in the case of
corporeal structures, we ought to find in nature, not the actual
transitional gradations by which each complex instinct has been
acquired—for these could be found only in the lineal ancestors of each
species—but we ought to find in the collateral lines of descent some
evidence of such gradations; or we ought at least to be able to show
that gradations of some kind are possible; and this we certainly can
do. I have been surprised to find, making allowance for the instincts
of animals having been but little observed except in Europe and North
America, and for no instinct being known amongst extinct species, how
very generally gradations, leading to the most complex instincts, can
be discovered. The canon of “Natura non facit saltum” applies with
almost equal force to instincts as to bodily organs. Changes of
instinct may sometimes be facilitated by the same species having
different instincts at different periods of life, or at different
seasons of the year, or when placed under different circumstances,
etc.; in which case either one or the other instinct might be preserved
by natural selection. And such instances of diversity of instinct in
the same species can be shown to occur in nature.
Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others.
One of the strongest instances of an animal apparently performing an
action for the sole good of another, with which I am acquainted, is
that of aphides voluntarily yielding their sweet excretion to ants:
that they do so voluntarily, the following facts show. I removed all
the ants from a group of about a dozen aphides on a dock-plant,
and prevented their attendance during several hours. After this
interval, I felt sure that the aphides would want to excrete. I watched
them for some time through a lens, but not one excreted; I then tickled
and stroked them with a hair in the same manner, as well as I could, as
the ants do with their antennæ; but not one excreted. Afterwards I
allowed an ant to visit them, and it immediately seemed, by its eager
way of running about, to be well aware what a rich flock it had
discovered; it then began to play with its antennæ on the abdomen first
of one aphis and then of another; and each aphis, as soon as it felt
the antennæ, immediately lifted up its abdomen and excreted a limpid
drop of sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. But as the excretion is
extremely viscid, it is probably a convenience to the aphides to have
it removed; and therefore probably the aphides do not instinctively
excrete for the sole good of the ants. Although I do not believe that
any animal in the world performs an action for the exclusive good of
another of a distinct species, yet each species tries to take advantage
of the instincts of others, as each takes advantage of the weaker
bodily structure of others. So again, in some few cases, certain
instincts cannot be considered as absolutely perfect; but as details on
this and other such points are not indispensable, they may be here
passed over.
As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to have been
here given; but want of space prevents me. I can only assert, that
instincts certainly do vary—for instance,
the migratory instinct, both in extent and direction, and in its total
loss. So it is with the nests of birds, which vary partly in dependence
on the situations chosen, and on the nature and temperature of the
country inhabited, but often from causes wholly unknown to us: Audubon
has given several remarkable cases of differences in nests of the same
species in the northern and southern United States. Fear of any
particular enemy is certainly an instinctive quality, as may be seen in
nestling birds, though it is strengthened by experience, and by the
sight of fear of the same enemy in other animals. But fear of man is
slowly acquired, as I have elsewhere shown, by various animals
inhabiting desert islands; and we may see an instance of this, even in
England, in the greater wildness of all our large birds than of our
small birds; for the large birds have been most persecuted by man. We
may safely attribute the greater wildness of our large birds to this
cause; for in uninhabited islands large birds are not more fearful than
small; and the magpie, so wary in England, is tame in Norway, as is the
hooded crow in Egypt.
That the general disposition of individuals of the same species, born
in a state of nature, is extremely diversified, can be shown by a
multitude of facts. Several cases also, could be given, of occasional
and strange habits in certain species, which might, if advantageous to
the species, give rise, through natural selection, to quite new
instincts. But I am well aware that these general statements, without
facts given in detail, can produce but a feeble effect on the reader’s
mind. I can only repeat my assurance, that I do not speak without good
evidence.
The possibility, or even probability, of inherited variations of
instinct in a state of nature will be strengthened by briefly
considering a few cases under
domestication. We shall thus also be enabled to see the respective
parts which habit and the selection of so-called accidental variations
have played in modifying the mental qualities of our domestic animals.
A number of curious and authentic instances could be given of the
inheritance of all shades of disposition and tastes, and likewise of
the oddest tricks, associated with certain frames of mind or periods of
time. But let us look to the familiar case of the several breeds of
dogs: it cannot be doubted that young pointers (I have myself seen a
striking instance) will sometimes point and even back other dogs the
very first time that they are taken out; retrieving is certainly in
some degree inherited by retrievers; and a tendency to run round,
instead of at, a flock of sheep, by shepherd-dogs. I cannot see that
these actions, performed without experience by the young, and in nearly
the same manner by each individual, performed with eager delight by
each breed, and without the end being known,—for the young pointer can
no more know that he points to aid his master, than the white butterfly
knows why she lays her eggs on the leaf of the cabbage,—I cannot see
that these actions differ essentially from true instincts. If we were
to see one kind of wolf, when young and without any training, as soon
as it scented its prey, stand motionless like a statue, and then slowly
crawl forward with a peculiar gait; and another kind of wolf rushing
round, instead of at, a herd of deer, and driving them to a distant
point, we should assuredly call these actions instinctive. Domestic
instincts, as they may be called, are certainly far less fixed or
invariable than natural instincts; but they have been acted on by far
less rigorous selection, and have been transmitted for an incomparably
shorter period, under less fixed conditions of life.
How strongly these domestic instincts, habits, and dispositions
are inherited, and how curiously they become mingled, is well shown
when different breeds of dogs are crossed. Thus it is known that a
cross with a bull-dog has affected for many generations the courage and
obstinacy of greyhounds; and a cross with a greyhound has given to a
whole family of shepherd-dogs a tendency to hunt hares. These domestic
instincts, when thus tested by crossing, resemble natural instincts,
which in a like manner become curiously blended together, and for a
long period exhibit traces of the instincts of either parent: for
example, Le Roy describes a dog, whose great-grandfather was a wolf,
and this dog showed a trace of its wild parentage only in one way, by
not coming in a straight line to his master when called.
Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this, I
think, is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler-pigeon to tumble,—an action
which, as I have witnessed, is performed by young birds, that have
never seen a pigeon tumble. We may believe that some one pigeon showed
a slight tendency to this strange habit, and that the long-continued
selection of the best individuals in successive generations made
tumblers what they now are; and near Glasgow there are house-tumblers,
as I hear from Mr. Brent, which cannot fly eighteen inches high without
going head over heels. It may be doubted whether any one would have
thought of training a dog to point, had not some one dog naturally
shown a tendency in this line; and this is known occasionally to
happen, as I once saw in a pure terrier. When the first tendency was
once displayed, methodical selection and the inherited effects of
compulsory training in each successive generation would soon complete
the work; and unconscious
selection is still at work, as each man tries to procure, without
intending to improve the breed, dogs which will stand and hunt best. On
the other hand, habit alone in some cases has sufficed; no animal is
more difficult to tame than the young of the wild rabbit; scarcely any
animal is tamer than the young of the tame rabbit; but I do not suppose
that domestic rabbits have ever been selected for tameness; and I
presume that we must attribute the whole of the inherited change from
extreme wildness to extreme tameness, simply to habit and
long-continued close confinement.
Natural instincts are lost under domestication: a remarkable instance
of this is seen in those breeds of fowls which very rarely or never
become “broody,” that is, never wish to sit on their eggs. Familiarity
alone prevents our seeing how universally and largely the minds of our
domestic animals have been modified by domestication. It is scarcely
possible to doubt that the love of man has become instinctive in the
dog. All wolves, foxes, jackals, and species of the cat genus, when
kept tame, are most eager to attack poultry, sheep, and pigs; and this
tendency has been found incurable in dogs which have been brought home
as puppies from countries, such as Tierra del Fuego and Australia,
where the savages do not keep these domestic animals. How rarely, on
the other hand, do our civilised dogs, even when quite young, require
to be taught not to attack poultry, sheep, and pigs! No doubt they
occasionally do make an attack, and are then beaten; and if not cured,
they are destroyed; so that habit, with some degree of selection, has
probably concurred in civilising by inheritance our dogs. On the other
hand, young chickens have lost, wholly by habit, that fear of the dog
and cat which no doubt was originally instinctive in them, in the same
way as it is so plainly instinctive in
young pheasants, though reared under a hen. It is not that chickens
have lost all fear, but fear only of dogs and cats, for if the hen
gives the danger-chuckle, they will run (more especially young turkeys)
from under her, and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away. But
this instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse the power
of flight.
Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar
mental habits and actions, which at first appeared from what we must in
our ignorance call an accident. In some cases compulsory habit alone
has sufficed to produce such inherited mental changes; in other cases
compulsory habit has done nothing, and all has been the result of
selection, pursued both methodically and unconsciously; but in most
cases, probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection, by considering a few cases. I will
select only three, out of the several which I shall have to discuss in
my future work,—namely, the instinct which leads the cuckoo to lay her
eggs in other birds’ nests; the slave-making instinct of certain ants;
and the comb-making power of the hive-bee: these two latter instincts
have generally, and most justly, been ranked by naturalists as the most
wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of
the cuckoo’s instinct is, that
she lays her eggs, not daily, but at intervals of two or three days; so
that, if she were to make her own nest and sit on her own eggs, those
first laid would have to be left for some time unincubated, or there
would be eggs and young birds of different ages in the same nest. If
this were the case, the process of laying and hatching might be
inconveniently long, more especially as she has to migrate at a very
early period; and the first hatched young would probably have to be fed
by the male alone. But the American cuckoo is in this predicament; for
she makes her own nest and has eggs and young successively hatched, all
at the same time. It has been asserted that the American cuckoo
occasionally lays her eggs in other birds’ nests; but I hear on the
high authority of Dr. Brewer, that this is a mistake. Nevertheless, I
could give several instances of various birds which have been known
occasionally to lay their eggs in other birds’ nests. Now let us
suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo; but that occasionally she laid an egg in
another bird’s nest. If the old bird profited by this occasional habit,
or if the young were made more vigorous by advantage having been taken
of the mistaken maternal instinct of another bird, than by their own
mother’s care, encumbered as she can hardly fail to be by having eggs
and young of different ages at the same time; then the old birds or the
fostered young would gain an advantage. And analogy would lead me to
believe, that the young thus reared would be apt to follow by
inheritance the occasional and aberrant habit of their mother, and in
their turn would be apt to lay their eggs in other birds’ nests, and
thus be successful in rearing their young. By a continued process of
this nature, I believe that the strange instinct of our cuckoo could
be, and has been,
generated. I may add that, according to Dr. Gray and to some other
observers, the European cuckoo has not utterly lost all maternal love
and care for her own offspring.
The occasional habit of birds laying their eggs in other birds’ nests,
either of the same or of a distinct species, is not very uncommon with
the Gallinaceæ; and this perhaps explains the origin of a singular
instinct in the allied group of ostriches. For several hen ostriches,
at least in the case of the American species, unite and lay first a few
eggs in one nest and then in another; and these are hatched by the
males. This instinct may probably be accounted for by the fact of the
hens laying a large number of eggs; but, as in the case of the cuckoo,
at intervals of two or three days. This instinct, however, of the
American ostrich has not as yet been perfected; for a surprising number
of eggs lie strewed over the plains, so that in one day’s hunting I
picked up no less than twenty lost and wasted eggs.
Many bees are parasitic, and always lay their eggs in the nests of bees
of other kinds. This case is more remarkable than that of the cuckoo;
for these bees have not only their instincts but their structure
modified in accordance with their parasitic habits; for they do not
possess the pollen-collecting apparatus which would be necessary if
they had to store food for their own young. Some species, likewise, of
Sphegidæ (wasp-like insects) are parasitic on other species; and M.
Fabre has lately shown good reason for believing that although the
Tachytes nigra generally makes its own burrow and stores it with
paralysed prey for its own larvæ to feed on, yet that when this insect
finds a burrow already made and stored by another sphex, it takes
advantage of the prize, and becomes for the occasion parasitic. In this
case, as with the supposed case of the cuckoo, I can
see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose nest
and stored food are thus feloniously appropriated, be not thus
exterminated.
_Slave-making instinct_.—This remarkable instinct was first discovered
in the Formica (Polyerges) rufescens by Pierre Huber, a better observer
even than his celebrated father. This ant is absolutely dependent on
its slaves; without their aid, the species would certainly become
extinct in a single year. The males and fertile females do no work. The
workers or sterile females, though most energetic and courageous in
capturing slaves, do no other work. They are incapable of making their
own nests, or of feeding their own larvæ. When the old nest is found
inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their
jaws. So utterly helpless are the masters, that when Huber shut up
thirty of them without a slave, but with plenty of the food which they
like best, and with their larvæ and pupæ to stimulate them to work,
they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca),
and she instantly set to work, fed and saved the survivors; made some
cells and tended the larvæ, and put all to rights. What can be more
extraordinary than these well-ascertained facts? If we had not known of
any other slave-making ant, it would have been hopeless to have
speculated how so wonderful an instinct could have been perfected.
Formica sanguinea was likewise first discovered by P. Huber to be a
slave-making ant. This species is found in the southern parts of
England, and its habits have been attended to by Mr. F. Smith, of the
British
Museum, to whom I am much indebted for information on this and other
subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as
any one may well be excused for doubting the truth of so extraordinary
and odious an instinct as that of making slaves. Hence I will give the
observations which I have myself made, in some little detail. I opened
fourteen nests of F. sanguinea, and found a few slaves in all. Males
and fertile females of the slave-species are found only in their own
proper communities, and have never been observed in the nests of F.
sanguinea. The slaves are black and not above half the size of their
red masters, so that the contrast in their appearance is very great.
When the nest is slightly disturbed, the slaves occasionally come out,
and like their masters are much agitated and defend the nest: when the
nest is much disturbed and the larvæ and pupæ are exposed, the slaves
work energetically with their masters in carrying them away to a place
of safety. Hence, it is clear, that the slaves feel quite at home.
During the months of June and July, on three successive years, I have
watched for many hours several nests in Surrey and Sussex, and never
saw a slave either leave or enter a nest. As, during these months, the
slaves are very few in number, I thought that they might behave
differently when more numerous; but Mr. Smith informs me that he has
watched the nests at various hours during May, June and August, both in
Surrey and Hampshire, and has never seen the slaves, though present in
large numbers in August, either leave or enter the nest. Hence he
considers them as strictly household slaves. The masters, on the other
hand, may be constantly seen bringing in materials for the nest, and
food of all kinds. During the present year, however, in the month
of July, I came across a community with an unusually large stock of
slaves, and I observed a few slaves mingled with their masters leaving
the nest, and marching along the same road to a tall Scotch-fir-tree,
twenty-five yards distant, which they ascended together, probably in
search of aphides or cocci. According to Huber, who had ample
opportunities for observation, in Switzerland the slaves habitually
work with their masters in making the nest, and they alone open and
close the doors in the morning and evening; and, as Huber expressly
states, their principal office is to search for aphides. This
difference in the usual habits of the masters and slaves in the two
countries, probably depends merely on the slaves being captured in
greater numbers in Switzerland than in England.
One day I fortunately chanced to witness a migration from one nest to
another, and it was a most interesting spectacle to behold the masters
carefully carrying, as Huber has described, their slaves in their jaws.
Another day my attention was struck by about a score of the
slave-makers haunting the same spot, and evidently not in search of
food; they approached and were vigorously repulsed by an independent
community of the slave species (F. fusca); sometimes as many as three
of these ants clinging to the legs of the slave-making F. sanguinea.
The latter ruthlessly killed their small opponents, and carried their
dead bodies as food to their nest, twenty-nine yards distant; but they
were prevented from getting any pupæ to rear as slaves. I then dug up a
small parcel of the pupæ of F. fusca from another nest, and put them
down on a bare spot near the place of combat; they were eagerly seized,
and carried off by the tyrants, who perhaps fancied that, after all,
they had been victorious in their late combat.
At the same time I laid on the same place a small parcel of the pupæ of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of the nest. This species is sometimes,
though rarely, made into slaves, as has been described by Mr. Smith.
Although so small a species, it is very courageous, and I have seen it
ferociously attack other ants. In one instance I found to my surprise
an independent community of F. flava under a stone beneath a nest of
the slave-making F. sanguinea; and when I had accidentally disturbed
both nests, the little ants attacked their big neighbours with
surprising courage. Now I was curious to ascertain whether F. sanguinea
could distinguish the pupæ of F. fusca, which they habitually make into
slaves, from those of the little and furious F. flava, which they
rarely capture, and it was evident that they did at once distinguish
them: for we have seen that they eagerly and instantly seized the pupæ
of F. fusca, whereas they were much terrified when they came across the
pupæ, or even the earth from the nest of F. flava, and quickly ran
away; but in about a quarter of an hour, shortly after all the little
yellow ants had crawled away, they took heart and carried off the pupæ.
One evening I visited another community of F. sanguinea, and found a
number of these ants entering their nest, carrying the dead bodies of
F. fusca (showing that it was not a migration) and numerous pupæ. I
traced the returning file burthened with booty, for about forty yards,
to a very thick clump of heath, whence I saw the last individual of F.
sanguinea emerge, carrying a pupa; but I was not able to find the
desolated nest in the thick heath. The nest, however, must have been
close at hand, for two or three individuals of F. fusca were rushing
about in the greatest agitation, and one was
perched motionless with its own pupa in its mouth on the top of a spray
of heath over its ravaged home.
Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with
those of the F. rufescens. The latter does not build its own nest, does
not determine its own migrations, does not collect food for itself or
its young, and cannot even feed itself: it is absolutely dependent on
its numerous slaves. Formica sanguinea, on the other hand, possesses
much fewer slaves, and in the early part of the summer extremely few.
The masters determine when and where a new nest shall be formed, and
when they migrate, the masters carry the slaves. Both in Switzerland
and England the slaves seem to have the exclusive care of the larvæ,
and the masters alone go on slave-making expeditions. In Switzerland
the slaves and masters work together, making and bringing materials for
the nest: both, but chiefly the slaves, tend, and milk as it may be
called, their aphides; and thus both collect food for the community. In
England the masters alone usually leave the nest to collect building
materials and food for themselves, their slaves and larvæ. So that the
masters in this country receive much less service from their slaves
than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants, which are not slave-makers, will,
as I have seen, carry off pupæ of other species, if scattered near
their nests, it is possible that pupæ originally stored as food might
become developed; and the ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their
presence proved useful to the species which had seized them—if it were
more advantageous
to this species to capture workers than to procreate them—the habit of
collecting pupæ originally for food might by natural selection be
strengthened and rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if carried out to
a much less extent even than in our British F. sanguinea, which, as we
have seen, is less aided by its slaves than the same species in
Switzerland, I can see no difficulty in natural selection increasing
and modifying the instinct—always supposing each modification to be of
use to the species—until an ant was formed as abjectly dependent on its
slaves as is the Formica rufescens.
_Cell-making instinct of the Hive-Bee_.—I will not here enter on minute
details on this subject, but will merely give an outline of the
conclusions at which I have arrived. He must be a dull man who can
examine the exquisite structure of a comb, so beautifully adapted to
its end, without enthusiastic admiration. We hear from mathematicians
that bees have practically solved a recondite problem, and have made
their cells of the proper shape to hold the greatest possible amount of
honey, with the least possible consumption of precious wax in their
construction. It has been remarked that a skilful workman, with fitting
tools and measures, would find it very difficult to make cells of wax
of the true form, though this is perfectly effected by a crowd of bees
working in a dark hive. Grant whatever instincts you please, and it
seems at first quite inconceivable how they can make all the necessary
angles and planes, or even perceive when they are correctly made. But
the difficulty is not nearly so great as it at first appears: all this
beautiful work can be shown, I think, to follow from a few very simple
instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered
only as a modification of his theory. Let us look to the great
principle of gradation, and see whether Nature does not reveal to us
her method of work. At one end of a short series we have humble-bees,
which use their old cocoons to hold honey, sometimes adding to them
short tubes of wax, and likewise making separate and very irregular
rounded cells of wax. At the other end of the series we have the cells
of the hive-bee, placed in a double layer: each cell, as is well known,
is an hexagonal prism, with the basal edges of its six sides bevelled
so as to join on to a pyramid, formed of three rhombs. These rhombs
have certain angles, and the three which form the pyramidal base of a
single cell on one side of the comb, enter into the composition of the
bases of three adjoining cells on the opposite side. In the series
between the extreme perfection of the cells of the hive-bee and the
simplicity of those of the humble-bee, we have the cells of the Mexican
Melipona domestica, carefully described and figured by Pierre Huber.
The Melipona itself is intermediate in structure between the hive and
humble bee, but more nearly related to the latter: it forms a nearly
regular waxen comb of cylindrical cells, in which the young are
hatched, and, in addition, some large cells of wax for holding honey.
These latter cells are nearly spherical and of nearly equal sizes, and
are aggregated into an irregular mass. But the important point to
notice, is that these cells are always made at that degree of nearness
to each other, that they would have intersected or broken into each
other, if the spheres had been completed; but this is never permitted,
the bees building perfectly flat walls of wax between the spheres
which thus tend to intersect. Hence each cell consists of an outer
spherical portion and of two, three, or more perfectly flat surfaces,
according as the cell adjoins two, three or more other cells. When one
cell comes into contact with three other cells, which, from the spheres
being nearly of the same size, is very frequently and necessarily the
case, the three flat surfaces are united into a pyramid; and this
pyramid, as Huber has remarked, is manifestly a gross imitation of the
three-sided pyramidal basis of the cell of the hive-bee. As in the
cells of the hive-bee, so here, the three plane surfaces in any one
cell necessarily enter into the construction of three adjoining cells.
It is obvious that the Melipona saves wax by this manner of building;
for the flat walls between the adjoining cells are not double, but are
of the same thickness as the outer spherical portions, and yet each
flat portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had made
them of equal sizes and had arranged them symmetrically in a double
layer, the resulting structure would probably have been as perfect as
the comb of the hive-bee. Accordingly I wrote to Professor Miller, of
Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
strictly correct:—
If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of
radius x the square root of 2 or radius x 1.41421 (or at some lesser
distance), from the centres of the six surrounding spheres in the same
layer; and at the same distance from the centres of the adjoining
spheres in the other and parallel layer; then, if planes of
intersection between the several spheres in
both layers be formed, there will result a double layer of hexagonal
prisms united together by pyramidal bases formed of three rhombs; and
the rhombs and the sides of the hexagonal prisms will have every angle
identically the same with the best measurements which have been made of
the cells of the hive-bee.
Hence we may safely conclude that if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as
that of the hive-bee. We must suppose the Melipona to make her cells
truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows in wood many insects can
make, apparently by turning round on a fixed point. We must suppose the
Melipona to arrange her cells in level layers, as she already does her
cylindrical cells; and we must further suppose, and this is the
greatest difficulty, that she can somehow judge accurately at what
distance to stand from her fellow-labourers when several are making
their spheres; but she is already so far enabled to judge of distance,
that she always describes her spheres so as to intersect largely; and
then she unites the points of intersection by perfectly flat surfaces.
We have further to suppose, but this is no difficulty, that after
hexagonal prisms have been formed by the intersection of adjoining
spheres in the same layer, she can prolong the hexagon to any length
requisite to hold the stock of honey; in the same way as the rude
humble-bee adds cylinders of wax to the circular mouths of her old
cocoons. By such modifications of instincts in themselves not very
wonderful,—hardly more wonderful than those which guide a bird to make
its nest,—I believe that the hive-bee
has acquired, through natural selection, her inimitable architectural
powers.
But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, square strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made
them wider and wider until they were converted into shallow basins,
appearing to the eye perfectly true or parts of a sphere, and of about
the diameter of a cell. It was most interesting to me to observe that
wherever several bees had begun to excavate these basins near together,
they had begun their work at such a distance from each other, that by
the time the basins had acquired the above stated width (_i.e._ about
the width of an ordinary cell), and were in depth about one sixth of
the diameter of the sphere of which they formed a part, the rims of the
basins intersected or broke into each other. As soon as this occurred,
the bees ceased to excavate, and began to build up flat walls of wax on
the lines of intersection between the basins, so that each hexagonal
prism was built upon the festooned edge of a smooth basin, instead of
on the straight edges of a three-sided pyramid as in the case of
ordinary cells.
I then put into the hive, instead of a thick, square piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin,
that the bottoms of the basins, if they had been excavated to the same
depth as in the former experiment, would have broken into each other
from the opposite sides. The bees, however, did not suffer this to
happen, and they stopped their excavations in due
time; so that the basins, as soon as they had been a little deepened,
came to have flat bottoms; and these flat bottoms, formed by thin
little plates of the vermilion wax having been left ungnawed, were
situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite sides of the
ridge of wax. In parts, only little bits, in other parts, large
portions of a rhombic plate had been left between the opposed basins,
but the work, from the unnatural state of things, had not been neatly
performed. The bees must have worked at very nearly the same rate on
the opposite sides of the ridge of vermilion wax, as they circularly
gnawed away and deepened the basins on both sides, in order to have
succeeded in thus leaving flat plates between the basins, by stopping
work along the intermediate planes or planes of intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of
wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at
exactly the same rate from the opposite sides; for I have noticed
half-completed rhombs at the base of a just-commenced cell, which were
slightly concave on one side, where I suppose that the bees had
excavated too quickly, and convex on the opposed side, where the bees
had worked less quickly. In one well-marked instance, I put the comb
back into the hive, and allowed the bees to go on working for a short
time, and again examined the cell, and I found that the rhombic plate
had been completed, and had become _perfectly flat:_ it was absolutely
impossible, from the extreme thinness of the little rhombic plate, that
they could have effected
this by gnawing away the convex side; and I suspect that the bees in
such cases stand in the opposed cells and push and bend the ductile and
warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.
From the experiment of the ridge of vermilion wax, we can clearly see
that if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the
spheres to break into each other. Now bees, as may be clearly seen by
examining the edge of a growing comb, do make a rough, circumferential
wall or rim all round the comb; and they gnaw into this from the
opposite sides, always working circularly as they deepen each cell.
They do not make the whole three-sided pyramidal base of any one cell
at the same time, but only the one rhombic plate which stands on the
extreme growing margin, or the two plates, as the case may be; and they
never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from
those made by the justly celebrated elder Huber, but I am convinced of
their accuracy; and if I had space, I could show that they are
conformable with my theory.
Huber’s statement that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of
wax; but I will not here enter on these details. We see how important a
part excavation plays in the construction of the cells; but it would be
a great error to suppose that the bees cannot build up a rough wall of
wax in the proper
position—that is, along the plane of intersection between two adjoining
spheres. I have several specimens showing clearly that they can do
this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in
position to the planes of the rhombic basal plates of future cells. But
the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build
is curious; they always make the first rough wall from ten to twenty
times thicker than the excessively thin finished wall of the cell,
which will ultimately be left. We shall understand how they work, by
supposing masons first to pile up a broad ridge of cement, and then to
begin cutting it away equally on both sides near the ground, till a
smooth, very thin wall is left in the middle; the masons always piling
up the cut-away cement, and adding fresh cement, on the summit of the
ridge. We shall thus have a thin wall steadily growing upward; but
always crowned by a gigantic coping. From all the cells, both those
just commenced and those completed, being thus crowned by a strong
coping of wax, the bees can cluster and crawl over the comb without
injuring the delicate hexagonal walls, which are only about one
four-hundredth of an inch in thickness; the plates of the pyramidal
basis being about twice as thick. By this singular manner of building,
strength is continually given to the comb, with the utmost ultimate
economy of wax.
It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls
of a single cell, or the extreme margin of the circumferential rim of a
growing comb, with an extremely thin layer of melted vermilion wax; and
I invariably found that the colour was most delicately diffused by the
bees—as delicately as a painter could have done with his brush—by atoms
of the coloured wax having been taken from the spot on which it had
been placed, and worked into the growing edges of the cells all round.
The work of construction seems to be a sort of balance struck between
many bees, all instinctively standing at the same relative distance
from each other, all trying to sweep equal spheres, and then building
up, or leaving ungnawed, the planes of intersection between these
spheres. It was really curious to note in cases of difficulty, as when
two pieces of comb met at an angle, how often the bees would entirely
pull down and rebuild in different ways the same cell, sometimes
recurring to a shape which they had at first rejected.
When bees have a place on which they can stand in their proper
positions for working,—for instance, on a slip of wood, placed directly
under the middle of a comb growing downwards so that the comb has to be
built over one face of the slip—in this case the bees can lay the
foundations of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices that the bees
should be enabled to stand at their proper relative distances from each
other and from the walls of the last completed cells, and then, by
striking imaginary spheres, they can build up a wall intermediate
between two adjoining spheres; but, as far as I have seen, they never
gnaw away and finish off the angles of a cell till a large part both of
that cell and of the adjoining cells has been built. This capacity in
bees of laying down under certain circumstances a rough wall in its
proper place between two just-commenced
cells, is important, as it bears on a fact, which seems at first quite
subversive of the foregoing theory; namely, that the cells on the
extreme margin of wasp-combs are sometimes strictly hexagonal; but I
have not space here to enter on this subject. Nor does there seem to me
any great difficulty in a single insect (as in the case of a
queen-wasp) making hexagonal cells, if she work alternately on the
inside and outside of two or three cells commenced at the same time,
always standing at the proper relative distance from the parts of the
cells just begun, sweeping spheres or cylinders, and building up
intermediate planes. It is even conceivable that an insect might, by
fixing on a point at which to commence a cell, and then moving outside,
first to one point, and then to five other points, at the proper
relative distances from the central point and from each other, strike
the planes of intersection, and so make an isolated hexagon: but I am
not aware that any such case has been observed; nor would any good be
derived from a single hexagon being built, as in its construction more
materials would be required than for a cylinder.
As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked,
how a long and graduated succession of modified architectural
instincts, all tending towards the present perfect plan of
construction, could have profited the progenitors of the hive-bee? I
think the answer is not difficult: it is known that bees are often hard
pressed to get sufficient nectar; and I am informed by Mr. Tegetmeier
that it has been experimentally found that no less than from twelve to
fifteen pounds of dry sugar are consumed by a hive of bees for the
secretion of each pound of wax; so that a prodigious quantity of fluid
nectar must be collected and consumed by the bees in a hive for
the secretion of the wax necessary for the construction of their combs.
Moreover, many bees have to remain idle for many days during the
process of secretion. A large store of honey is indispensable to
support a large stock of bees during the winter; and the security of
the hive is known mainly to depend on a large number of bees being
supported. Hence the saving of wax by largely saving honey must be a
most important element of success in any family of bees. Of course the
success of any species of bee may be dependent on the number of its
parasites or other enemies, or on quite distinct causes, and so be
altogether independent of the quantity of honey which the bees could
collect. But let us suppose that this latter circumstance determined,
as it probably often does determine, the numbers of a humble-bee which
could exist in a country; and let us further suppose that the community
lived throughout the winter, and consequently required a store of
honey: there can in this case be no doubt that it would be an advantage
to our humble-bee, if a slight modification of her instinct led her to
make her waxen cells near together, so as to intersect a little; for a
wall in common even to two adjoining cells, would save some little wax.
Hence it would continually be more and more advantageous to our
humble-bee, if she were to make her cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the Melipona;
for in this case a large part of the bounding surface of each cell
would serve to bound other cells, and much wax would be saved. Again,
from the same cause, it would be advantageous to the Melipona, if she
were to make her cells closer together, and more regular in every way
than at present; for then, as we have seen, the spherical surfaces
would wholly disappear, and would all be replaced by plane surfaces;
and the Melipona
would make a comb as perfect as that of the hive-bee. Beyond this stage
of perfection in architecture, natural selection could not lead; for
the comb of the hive-bee, as far as we can see, is absolutely perfect
in economising wax.
Thus, as I believe, the most wonderful of all known instincts, that of
the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more
perfectly, led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax
along the planes of intersection. The bees, of course, no more knowing
that they swept their spheres at one particular distance from each
other, than they know what are the several angles of the hexagonal
prisms and of the basal rhombic plates. The motive power of the process
of natural selection having been economy of wax; that individual swarm
which wasted least honey in the secretion of wax, having succeeded
best, and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turn will have had
the best chance of succeeding in the struggle for existence.
No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,—cases, in which we cannot see how
an instinct could possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could hardly have been
acted on by natural selection; cases of instincts almost identically
the same in animals so remote in the scale of nature, that we cannot
account
for their similarity by inheritance from a common parent, and must
therefore believe that they have been acquired by independent acts of
natural selection. I will not here enter on these several cases, but
will confine myself to one special difficulty, which at first appeared
to me insuperable, and actually fatal to my whole theory. I allude to
the neuters or sterile females in insect-communities: for these neuters
often differ widely in instinct and in structure from both the males
and fertile females, and yet, from being sterile, they cannot propagate
their kind.
The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much
greater than that of any other striking modification of structure; for
it can be shown that some insects and other articulate animals in a
state of nature occasionally become sterile; and if such insects had
been social, and it had been profitable to the community that a number
should have been annually born capable of work, but incapable of
procreation, I can see no very great difficulty in this being effected
by natural selection. But I must pass over this preliminary difficulty.
The great difficulty lies in the working ants differing widely from
both the males and the fertile females in structure, as in the shape of
the thorax and in being destitute of wings and sometimes of eyes, and
in instinct. As far as instinct alone is concerned, the prodigious
difference in this respect between the workers and the perfect females,
would have been far better exemplified by the hive-bee. If a working
ant or other neuter insect had been an animal in the ordinary state, I
should have unhesitatingly assumed that all its characters had been
slowly acquired through natural selection; namely, by an individual
having been born with some slight profitable modification of structure,
this being inherited by its offspring, which again varied and were
again selected, and so onwards. But with the working ant we have an
insect differing greatly from its parents, yet absolutely sterile; so
that it could never have transmitted successively acquired
modifications of structure or instinct to its progeny. It may well be
asked how is it possible to reconcile this case with the theory of
natural selection?
First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all
sorts of differences of structure which have become correlated to
certain ages, and to either sex. We have differences correlated not
only to one sex, but to that short period alone when the reproductive
system is active, as in the nuptial plumage of many birds, and in the
hooked jaws of the male salmon. We have even slight differences in the
horns of different breeds of cattle in relation to an artificially
imperfect state of the male sex; for oxen of certain breeds have longer
horns than in other breeds, in comparison with the horns of the bulls
or cows of these same breeds. Hence I can see no real difficulty in any
character having become correlated with the sterile condition of
certain members of insect-communities: the difficulty lies in
understanding how such correlated modifications of structure could have
been slowly accumulated by natural selection.
This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus gain
the desired end. Thus, a well-flavoured vegetable is cooked, and the
individual is destroyed; but the horticulturist sows seeds of the same
stock, and confidently expects to
get nearly the same variety; breeders of cattle wish the flesh and fat
to be well marbled together; the animal has been slaughtered, but the
breeder goes with confidence to the same family. I have such faith in
the powers of selection, that I do not doubt that a breed of cattle,
always yielding oxen with extraordinarily long horns, could be slowly
formed by carefully watching which individual bulls and cows, when
matched, produced oxen with the longest horns; and yet no one ox could
ever have propagated its kind. Thus I believe it has been with social
insects: a slight modification of structure, or instinct, correlated
with the sterile condition of certain members of the community, has
been advantageous to the community: consequently the fertile males and
females of the same community flourished, and transmitted to their
fertile offspring a tendency to produce sterile members having the same
modification. And I believe that this process has been repeated, until
that prodigious amount of difference between the fertile and sterile
females of the same species has been produced, which we see in many
social insects.
But we have not as yet touched on the climax of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes.
The castes, moreover, do not generally graduate into each other, but
are perfectly well defined; being as distinct from each other, as are
any two species of the same genus, or rather as any two genera of the
same family. Thus in Eciton, there are working and soldier neuters,
with jaws and instincts extraordinarily different: in Cryptocerus, the
workers of one caste alone carry a wonderful sort of shield on their
heads, the use of which is quite unknown: in the Mexican Myrmecocystus,
the workers of one caste never leave the nest; they are fed by the
workers of another caste, and they have an enormously developed abdomen
which secretes a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called, which our
European ants guard or imprison.
It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females,—in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few
alone; and that by the long-continued selection of the fertile parents
which produced most neuters with the profitable modification, all the
neuters ultimately came to have the desired character. On this view we
ought occasionally to find neuter-insects of the same species, in the
same nest, presenting gradations of structure; and this we do find,
even often, considering how few neuter-insects out of Europe have been
carefully examined. Mr. F. Smith has shown how surprisingly the neuters
of several British ants differ from each other in size and sometimes in
colour; and that the extreme forms can sometimes be perfectly linked
together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often happens that the
larger or the smaller sized workers are the most numerous; or that both
large and small are numerous, with those of an intermediate size scanty
in numbers. Formica flava has larger and
smaller workers, with some of intermediate size; and, in this species,
as Mr. F. Smith has observed, the larger workers have simple eyes
(ocelli), which though small can be plainly distinguished, whereas the
smaller workers have their ocelli rudimentary. Having carefully
dissected several specimens of these workers, I can affirm that the
eyes are far more rudimentary in the smaller workers than can be
accounted for merely by their proportionally lesser size; and I fully
believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate
condition. So that we here have two bodies of sterile workers in the
same nest, differing not only in size, but in their organs of vision,
yet connected by some few members in an intermediate condition. I may
digress by adding, that if the smaller workers had been the most useful
to the community, and those males and females had been continually
selected, which produced more and more of the smaller workers, until
all the workers had come to be in this condition; we should then have
had a species of ant with neuters very nearly in the same condition
with those of Myrmica. For the workers of Myrmica have not even
rudiments of ocelli, though the male and female ants of this genus have
well-developed ocelli.
I may give one other case: so confidently did I expect to find
gradations in important points of structure between the different
castes of neuters in the same species, that I gladly availed myself of
Mr. F. Smith’s offer of numerous specimens from the same nest of the
driver ant (Anomma) of West Africa. The reader will perhaps best
appreciate the amount of difference in these workers, by my giving not
the actual measurements, but a strictly accurate illustration: the
difference was the same as if we were to see a set of workmen building
a house of whom many were five feet four inches high, and many sixteen
feet high; but we must suppose that the larger workmen had heads four
instead of three times as big as those of the smaller men, and jaws
nearly five times as big. The jaws, moreover, of the working ants of
the several sizes differed wonderfully in shape, and in the form and
number of the teeth. But the important fact for us is, that though the
workers can be grouped into castes of different sizes, yet they
graduate insensibly into each other, as does the widely-different
structure of their jaws. I speak confidently on this latter point, as
Mr. Lubbock made drawings for me with the camera lucida of the jaws
which I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting
on the fertile parents, could form a species which should regularly
produce neuters, either all of large size with one form of jaw, or all
of small size with jaws having a widely different structure; or lastly,
and this is our climax of difficulty, one set of workers of one size
and structure, and simultaneously another set of workers of a different
size and structure;—a graduated series having been first formed, as in
the case of the driver ant, and then the extreme forms, from being the
most useful to the community, having been produced in greater and
greater numbers through the natural selection of the parents which
generated them; until none with an intermediate structure were
produced.
Thus, as I believe, the wonderful fact of two distinctly defined castes
of sterile workers existing in the same nest, both widely different
from each other and from their parents, has originated. We can see how
useful their production may have been to a social community of insects,
on the same principle that the division of
labour is useful to civilised man. As ants work by inherited instincts
and by inherited tools or weapons, and not by acquired knowledge and
manufactured instruments, a perfect division of labour could be
effected with them only by the workers being sterile; for had they been
fertile, they would have intercrossed, and their instincts and
structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants,
by the means of natural selection. But I am bound to confess, that,
with all my faith in this principle, I should never have anticipated
that natural selection could have been efficient in so high a degree,
had not the case of these neuter insects convinced me of the fact. I
have, therefore, discussed this case, at some little but wholly
insufficient length, in order to show the power of natural selection,
and likewise because this is by far the most serious special
difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount
of modification in structure can be effected by the accumulation of
numerous, slight, and as we must call them accidental, variations,
which are in any manner profitable, without exercise or habit having
come into play. For no amount of exercise, or habit, or volition, in
the utterly sterile members of a community could possibly have affected
the structure or instincts of the fertile members, which alone leave
descendants. I am surprised that no one has advanced this demonstrative
case of neuter insects, against the well-known doctrine of Lamarck.
_Summary_.—I have endeavoured briefly in this chapter to show that the
mental qualities of our domestic animals vary, and that the variations
are inherited. Still more briefly I have attempted to show that
instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore I can see no
difficulty, under changing conditions of life, in natural selection
accumulating slight modifications of instinct to any extent, in any
useful direction. In some cases habit or use and disuse have probably
come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and are
liable to mistakes;—that no instinct has been produced for the
exclusive good of other animals, but that each animal takes advantage
of the instincts of others;—that the canon in natural history, of
“natura non facit saltum” is applicable to instincts as well as to
corporeal structure, and is plainly explicable on the foregoing views,
but is otherwise inexplicable,—all tend to corroborate the theory of
natural selection.
This theory is, also, strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and
living under considerably different conditions of life, yet often
retaining nearly the same instincts. For instance, we can understand on
the principle of inheritance, how it is that the thrush of South
America lines its nest with mud, in the same peculiar manner as does
our British thrush: how it is that the male wrens (Troglodytes) of
North America, build “cock-nests,” to roost in, like the males of our
distinct Kitty-wrens,—a habit wholly unlike that of any other known
bird. Finally, it may not be a logical deduction, but to my imagination
it is far more satisfactory to look at such instincts as the young
cuckoo ejecting its foster-brothers,—ants making slaves,—the larvæ of
ichneumonidæ feeding within the live bodies of caterpillars,—not as
specially endowed or created instincts, but as small consequences of
one general law, leading to the advancement of all organic beings,
namely, multiply, vary, let the strongest live and the weakest die.
CHAPTER VIII.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication. Laws governing the sterility
of hybrids. Sterility not a special endowment, but incidental on other
differences. Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing. Fertility of varieties when crossed and of their mongrel
offspring not universal. Hybrids and mongrels compared independently of
their fertility. Summary.
The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of
sterility, in order to prevent the confusion of all organic forms. This
view certainly seems at first probable, for species within the same
country could hardly have kept distinct had they been capable of
crossing freely. The importance of the fact that hybrids are very
generally sterile, has, I think, been much underrated by some late
writers. On the theory of natural selection the case is especially
important, inasmuch as the sterility of hybrids could not possibly be
of any advantage to them, and therefore could not have been acquired by
the continued preservation of successive profitable degrees of
sterility. I hope, however, to be able to show that sterility is not a
specially acquired or endowed quality, but is incidental on other
acquired differences.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together;
namely, the sterility of two
species when first crossed, and the sterility of the hybrids produced
from them.
Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs
functionally impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the organs themselves are
perfect in structure, as far as the microscope reveals. In the first
case the two sexual elements which go to form the embryo are perfect;
in the second case they are either not at all developed, or are
imperfectly developed. This distinction is important, when the cause of
the sterility, which is common to the two cases, has to be considered.
The distinction has probably been slurred over, owing to the sterility
in both cases being looked on as a special endowment, beyond the
province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to
have descended from common parents, when intercrossed, and likewise the
fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kölreuter and Gärtner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility.
Kölreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he
unhesitatingly ranks them as varieties. Gärtner, also, makes the rule
equally universal; and he disputes the entire fertility of Kölreuter’s
ten cases. But in these and in many other cases, Gärtner is obliged
carefully to count the seeds, in order to show that there is any degree
of sterility. He always compares the maximum number of seeds produced
by two species when crossed and by their hybrid offspring, with the
average number produced by both pure parent-species in a state of
nature. But a serious cause of error seems to me to be here introduced:
a plant to be hybridised must be castrated, and, what is often more
important, must be secluded in order to prevent pollen being brought to
it by insects from other plants. Nearly all the plants experimentised
on by Gärtner were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gärtner gives in his table about a score
of cases of plants which he castrated, and artificially fertilised with
their own pollen, and (excluding all cases such as the Leguminosæ, in
which there is an acknowledged difficulty in the manipulation) half of
these twenty plants had their fertility in some degree impaired.
Moreover, as Gärtner during several years repeatedly crossed the
primrose and cowslip, which we have such good reason to believe to be
varieties, and only once or twice succeeded in getting fertile seed; as
he found the common red and blue pimpernels (Anagallis arvensis and
coerulea), which the best botanists rank as varieties, absolutely
sterile together; and as he came to the same conclusion in several
other analogous cases; it seems to me that we may well be permitted to
doubt whether many other species are really so sterile, when
intercrossed, as Gärtner believes.
It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so
insensibly, and, on the other hand, that the fertility of pure species
is so easily affected by various circumstances, that for all practical
purposes it is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever lived,
namely, Kölreuter and Gärtner, should have arrived at diametrically
opposite conclusions in regard to the very same species. It is also
most instructive to compare—but I have not space here to enter on
details—the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same author, from experiments made during
different years. It can thus be shown that neither sterility nor
fertility affords any clear distinction between species and varieties;
but that the evidence from this source graduates away, and is doubtful
in the same degree as is the evidence derived from other constitutional
and structural differences.
In regard to the sterility of hybrids in successive generations; though
Gärtner was enabled to rear some hybrids, carefully guarding them from
a cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in
the first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing
that close interbreeding lessens fertility, and, on the other hand,
that an occasional cross with a distinct individual or variety
increases fertility, that I cannot doubt the correctness of this almost
universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of
insects must be carefully prevented during the flowering season: hence
hybrids will generally be fertilised during each generation by their
own individual pollen; and I am convinced that this would be injurious
to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Gärtner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from the
anthers of the flower itself which is to be fertilised; so that a cross
between two flowers, though probably on the same plant, would be thus
effected. Moreover, whenever complicated experiments are in progress,
so careful an observer as Gärtner would have castrated his hybrids, and
this would have insured in each generation a cross with the pollen from
a distinct flower, either from the same plant or from another plant of
the same hybrid nature. And thus, the strange fact of the increase of
fertility in the successive generations of _artificially fertilised_
hybrids may, I believe, be accounted for by close interbreeding having
been avoided.
Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Honourable and
Reverend W. Herbert. He is as emphatic in his conclusion that some
hybrids are perfectly fertile—as fertile as the pure parent-species—as
are Kölreuter and Gärtner that some degree of sterility between
distinct species is a universal law of nature. He experimentised on
some of the very same species as did Gärtner. The difference in their
results may, I think, be in part accounted for by Herbert’s great
horticultural skill, and by his having hothouses at his command. Of his
many important statements I will here give only a single one as an
example, namely, that “every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which (he says) I never
saw to occur in a case of its natural fecundation.” So that we here
have perfect, or even more than commonly perfect, fertility in a first
cross between two distinct species.
This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants, as with certain species of
Lobelia, and with all the species of the genus Hippeastrum, which can
be far more easily fertilised by the pollen of another and distinct
species, than by their own pollen. For these plants have been found to
yield seed to the pollen of a distinct species, though quite sterile
with their own pollen, notwithstanding that their own pollen was found
to be perfectly good, for it fertilised distinct species. So that
certain individual plants and all the individuals of certain species
can actually be hybridised much more readily than they can be
self-fertilised! For instance, a bulb of Hippeastrum aulicum produced
four flowers; three were fertilised by Herbert with their own pollen,
and the fourth was subsequently fertilised by the pollen of a compound
hybrid descended from three other and distinct species: the result was
that “the ovaries of the three first flowers soon ceased to grow, and
after a
few days perished entirely, whereas the pod impregnated by the pollen
of the hybrid made vigorous growth and rapid progress to maturity, and
bore good seed, which vegetated freely.” In a letter to me, in 1839,
Mr. Herbert told me that he had then tried the experiment during five
years, and he continued to try it during several subsequent years, and
always with the same result. This result has, also, been confirmed by
other observers in the case of Hippeastrum with its sub-genera, and in
the case of some other genera, as Lobelia, Passiflora and Verbascum.
Although the plants in these experiments appeared perfectly healthy,
and although both the ovules and pollen of the same flower were
perfectly good with respect to other species, yet as they were
functionally imperfect in their mutual self-action, we must infer that
the plants were in an unnatural state. Nevertheless these facts show on
what slight and mysterious causes the lesser or greater fertility of
species when crossed, in comparison with the same species when
self-fertilised, sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, “reproduced itself as perfectly as if it
had been a natural species from the mountains of Chile.” I have taken
some pains to ascertain the degree of fertility of some of the complex
crosses of Rhododendrons, and I am assured that many of them are
perfectly fertile. Mr. C. Noble, for instance, informs me that he
raises stocks for grafting from a hybrid
between Rhododendron Ponticum and Catawbiense, and that this hybrid
“seeds as freely as it is possible to imagine.” Had hybrids, when
fairly treated, gone on decreasing in fertility in each successive
generation, as Gärtner believes to be the case, the fact would have
been notorious to nurserymen. Horticulturists raise large beds of the
same hybrids, and such alone are fairly treated, for by insect agency
the several individuals of the same hybrid variety are allowed to
freely cross with each other, and the injurious influence of close
interbreeding is thus prevented. Any one may readily convince himself
of the efficiency of insect-agency by examining the flowers of the more
sterile kinds of hybrid rhododendrons, which produce no pollen, for he
will find on their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is if the genera of animals are as distinct from each other, as are the
genera of plants, then we may infer that animals more widely separated
in the scale of nature can be more easily crossed than in the case of
plants; but the hybrids themselves are, I think, more sterile. I doubt
whether any case of a perfectly fertile hybrid animal can be considered
as thoroughly well authenticated. It should, however, be borne in mind
that, owing to few animals breeding freely under confinement, few
experiments have been fairly tried: for instance, the canary-bird has
been crossed with nine other finches, but as not one of these nine
species breeds freely in confinement, we have no right to expect that
the first crosses between them and the canary, or that their hybrids,
should be perfectly fertile. Again, with respect to the fertility in
successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of
the same hybrid have been raised at the same time from different
parents, so as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed in each
successive generation, in opposition to the constantly repeated
admonition of every breeder. And in this case, it is not at all
surprising that the inherent sterility in the hybrids should have gone
on increasing. If we were to act thus, and pair brothers and sisters in
the case of any pure animal, which from any cause had the least
tendency to sterility, the breed would assuredly be lost in a very few
generations.
Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that
the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus and with P. versicolor are perfectly
fertile. The hybrids from the common and Chinese geese (A. cygnoides),
species which are so different that they are generally ranked in
distinct genera, have often bred in this country with either pure
parent, and in one single instance they have bred _inter se_. This was
effected by Mr. Eyton, who raised two hybrids from the same parents but
from different hatches; and from these two birds he raised no less than
eight hybrids (grandchildren of the pure geese) from one nest. In
India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely Mr. Blyth and Capt.
Hutton, that whole flocks of these crossed geese are kept in various
parts of the country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly fertile.
A doctrine which originated with Pallas, has been
largely accepted by modern naturalists; namely, that most of our
domestic animals have descended from two or more aboriginal species,
since commingled by intercrossing. On this view, the aboriginal species
must either at first have produced quite fertile hybrids, or the
hybrids must have become in subsequent generations quite fertile under
domestication. This latter alternative seems to me the most probable,
and I am inclined to believe in its truth, although it rests on no
direct evidence. I believe, for instance, that our dogs have descended
from several wild stocks; yet, with perhaps the exception of certain
indigenous domestic dogs of South America, all are quite fertile
together; and analogy makes me greatly doubt, whether the several
aboriginal species would at first have freely bred together and have
produced quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite fertile
together; but from facts communicated to me by Mr. Blyth, I think they
must be considered as distinct species. On this view of the origin of
many of our domestic animals, we must either give up the belief of the
almost universal sterility of distinct species of animals when crossed;
or we must look at sterility, not as an indelible characteristic, but
as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids,is an extremely general result;
but that it cannot, under our present state of knowledge, be considered
as absolutely universal.
_Laws governing the Sterility of first Crosses and of Hybrids_.—We will
now consider a little more in detail the
circumstances and rules governing the sterility of first crosses and of
hybrids. Our chief object will be to see whether or not the rules
indicate that species have specially been endowed with this quality, in
order to prevent their crossing and blending together in utter
confusion. The following rules and conclusions are chiefly drawn up
from Gärtner’s admirable work on the hybridisation of plants. I have
taken much pains to ascertain how far the rules apply to animals, and
considering how scanty our knowledge is in regard to hybrid animals, I
have been surprised to find how generally the same rules apply to both
kingdoms.
It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown
to exist; but only the barest outline of the facts can here be given.
When pollen from a plant of one family is placed on the stigma of a
plant of a distinct family, it exerts no more influence than so much
inorganic dust. From this absolute zero of fertility, the pollen of
different species of the same genus applied to the stigma of some one
species, yields a perfect gradation in the number of seeds produced, up
to nearly complete or even quite complete fertility; and, as we have
seen, in certain abnormal cases, even to an excess of fertility, beyond
that which the plant’s own pollen will produce. So in hybrids
themselves, there are some which never have produced, and probably
never would produce, even with the pollen of either pure parent, a
single fertile seed: but in some of these cases a first trace of
fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than
it otherwise would have done; and the early withering of the flower is
well known to be a sign
of incipient fertilisation. From this extreme degree of sterility we
have self-fertilised hybrids producing a greater and greater number of
seeds up to perfect fertility.
Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced—two classes of facts which are
generally confounded together—is by no means strict. There are many
cases, in which two pure species can be united with unusual facility,
and produce numerous hybrid-offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed very
rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same genus,
for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable; for
it is not always the same when the same two species are crossed under
the same circumstances, but depends in part upon the constitution of
the individuals which happen to have been chosen for the experiment. So
it is with hybrids, for their degree of fertility is often found to
differ greatly in the several individuals raised from seed out of the
same capsule and exposed to exactly the same conditions.
By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and which
differ little in the allied species. Now the fertility of first crosses
between species, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species
which will not unite, or only with extreme difficulty; and on the other
hand of very distinct species which unite with the utmost facility. In
the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene, in
which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of
almost any other genus; but Gärtner found that N. acuminata, which is
not a particularly distinct species, obstinately failed to fertilise,
or to be fertilised by, no less than eight other species of Nicotiana.
Very many analogous facts could be given.
No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences in
every part of the flower, even in the pollen, in the fruit, and in the
cotyledons, can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted for
extremely different climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass,
and then a male-ass with a mare: these two species may then be said to
have been reciprocally crossed. There is often the widest possible
difference in the facility of making reciprocal crosses. Such cases are
highly important, for they prove that the capacity in any two species
to cross is often completely independent of their systematic affinity,
or of any recognisable difference in their whole organisation. On the
other hand, these cases clearly show that the capacity for crossing is
connected with constitutional differences imperceptible by us, and
confined to the reproductive system. This difference in the result of
reciprocal crosses between the same two species was long ago observed
by Kölreuter. To give an instance: Mirabilis jalappa can easily be
fertilised by the pollen of M. longiflora, and the hybrids thus
produced are sufficiently fertile; but Kölreuter tried more than two
hundred times, during eight following years, to fertilise reciprocally
M. longiflora with the pollen of M. jalappa, and utterly failed.
Several other equally striking cases could be given. Thuret has
observed the same fact with certain sea-weeds or Fuci. Gärtner,
moreover, found that this difference of facility in making reciprocal
crosses is extremely common in a lesser degree. He has observed it even
between forms so closely related (as Matthiola annua and glabra) that
many botanists rank them only as varieties. It is also a remarkable
fact, that hybrids raised from reciprocal crosses, though of course
compounded of the very same two species, the one species having first
been used as the father and then as the mother, generally differ in
fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from
Gärtner: for instance, some species have a remarkable power of crossing
with other species; other species of the same genus have a remarkable
power of impressing their likeness on their hybrid offspring; but these
two powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate character
between their two parents, always closely resemble one of them; and
such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are born,
which closely resemble one of their pure parents; and these hybrids are
almost always utterly sterile, even when the other hybrids raised from
seed from the same capsule have a considerable degree of fertility.
These facts show how completely fertility in the hybrid is independent
of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross. That the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent. And lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or
degree of resemblance to each other. This latter statement is clearly
proved by reciprocal crosses between the same two species, for
according as the one species or the other is used as the father or the
mother, there is generally some difference, and occasionally the widest
possible difference, in the facility of effecting an union. The
hybrids, moreover, produced from reciprocal crosses often differ in
fertility.
Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending
together? Why should the degree of sterility be innately variable in
the individuals of the same species? Why should some species cross with
facility, and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids? Why
should there often be so great a difference in the result of a
reciprocal cross between the same two species? Why, it may even be
asked, has the production of hybrids been permitted? to grant to
species the special power of producing hybrids, and then to stop their
further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems
to be a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility both of first crosses and of hybrids is
simply incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature,
that, in reciprocal crosses between two species the male sexual element
of the one will often freely act on the female sexual element of the
other, but not in a reversed direction. It will be advisable to explain
a little more fully by an example what I mean by sterility being
incidental on other differences, and not a specially endowed quality.
As the capacity of one plant to be grafted or budded on another is so
entirely unimportant for its welfare in a state of nature, I presume
that no one will suppose that this capacity is a _specially_ endowed
quality, but will admit that it is incidental on differences in the
laws of growth of the two plants. We can sometimes see the reason why
one tree will not take on another, from differences in their rate of
growth, in the hardness of their wood, in the period of the flow or
nature of their sap, etc.; but in a multitude of cases we can assign no
reason whatever. Great diversity in the size of two plants, one being
woody and the other herbaceous, one being evergreen and the other
deciduous, and adaptation to widely different climates, does not always
prevent the two grafting together. As in hybridisation, so with
grafting, the capacity is limited by systematic affinity, for no one
has been able to graft trees together belonging to quite distinct
families; and, on the other hand, closely allied species, and varieties
of the same species, can usually, but not invariably, be grafted with
ease. But this capacity, as in hybridisation, is by no means absolutely
governed by systematic affinity. Although many distinct genera within
the same family have been grafted together, in other cases species of
the same genus will not take on each other. The pear can be grafted far
more readily on the quince, which is ranked as a distinct genus, than
on the apple, which is a member of the same genus. Even different
varieties of the pear take
with different degrees of facility on the quince; so do different
varieties of the apricot and peach on certain varieties of the plum.
As Gärtner found that there was sometimes an innate difference in
different _individuals_ of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same two
species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different case
from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty on
another species, when thus grafted were rendered barren. On the other
hand, certain species of Sorbus, when grafted on other species, yielded
twice as much fruit as when on their own roots. We are reminded by this
latter fact of the extraordinary case of Hippeastrum, Lobelia, etc.,
which seeded much more freely when fertilised with the pollen of
distinct species, than when self-fertilised with their own pollen.
We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and
of crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted on
each other as incidental on unknown differences in their vegetative
systems, so I believe that the still more complex laws governing the
facility of first crosses, are incidental on unknown differences,
chiefly in their reproductive systems. These differences, in both
cases, follow to a certain extent, as might have been expected,
systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to me to indicate that the greater or lesser
difficulty of either grafting or crossing together various species has
been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms, as in the case of grafting it is unimportant for their welfare.
_Causes of the Sterility of first Crosses and of Hybrids_.—We may now
look a little closer at the probable causes of the sterility of first
crosses and of hybrids. These two cases are fundamentally different,
for, as just remarked, in the union of two pure species the male and
female sexual elements are perfect, whereas in hybrids they are
imperfect. Even in first crosses, the greater or lesser difficulty in
effecting a union apparently depends on several distinct causes. There
must sometimes be a physical impossibility in the male element reaching
the ovule, as would be the case with a plant having a pistil too long
for the pollen-tubes to reach the ovarium. It has also been observed
that when pollen of one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the
male element may reach the female element, but be incapable of causing
an embryo to be developed, as seems to have been the case with some of
Thuret’s experiments on Fuci. No explanation can be given of these
facts, any more than why certain trees cannot be grafted on others.
Lastly, an embryo may be developed, and then perish at an early period.
This latter alternative has not been sufficiently attended to; but I
believe, from observations communicated to me by Mr. Hewitt, who has
had great experience in hybridising gallinaceous birds, that the early
death of the embryo is a very frequent cause of sterility in first
crosses. I was at first very unwilling to believe in this view; as
hybrids, when once born, are generally healthy and long-lived, as we
see in the case of the common mule. Hybrids, however, are differently
circumstanced before and after birth: when born and living in a country
where their two parents can live, they are generally placed under
suitable conditions of life. But a hybrid partakes of only half of the
nature and constitution of its mother, and therefore before birth, as
long as it is nourished within its mother’s womb or within the egg or
seed produced by the mother, it may be exposed to conditions in some
degree unsuitable, and consequently be liable to perish at an early
period; more especially as all very young beings seem eminently
sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than
once alluded to a large body of facts, which I have collected, showing
that when animals and plants are removed from their natural conditions,
they are extremely liable to have their reproductive systems seriously
affected. This, in fact, is
the great bar to the domestication of animals. Between the sterility
thus superinduced and that of hybrids, there are many points of
similarity. In both cases the sterility is independent of general
health, and is often accompanied by excess of size or great luxuriance.
In both cases, the sterility occurs in various degrees; in both, the
male element is the most liable to be affected; but sometimes the
female more than the male. In both, the tendency goes to a certain
extent with systematic affinity, for whole groups of animals and plants
are rendered impotent by the same unnatural conditions; and whole
groups of species tend to produce sterile hybrids. On the other hand,
one species in a group will sometimes resist great changes of
conditions with unimpaired fertility; and certain species in a group
will produce unusually fertile hybrids. No one can tell, till he tries,
whether any particular animal will breed under confinement or any plant
seed freely under culture; nor can he tell, till he tries, whether any
two species of a genus will produce more or less sterile hybrids.
Lastly, when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations
are eminently liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general
state of health, is affected by sterility in a very similar manner. In
the one case, the conditions of life have been disturbed, though often
in so slight a degree as to
be inappreciable by us; in the other case, or that of hybrids, the
external conditions have remained the same, but the organisation has
been disturbed by two different structures and constitutions having
been blended into one. For it is scarcely possible that two
organisations should be compounded into one, without some disturbance
occurring in the development, or periodical action, or mutual relation
of the different parts and organs one to another, or to the conditions
of life. When hybrids are able to breed _inter se_, they transmit to
their offspring from generation to generation the same compounded
organisation, and hence we need not be surprised that their sterility,
though in some degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand, excepting on
vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent. Nor
do I pretend that the foregoing remarks go to the root of the matter:
no explanation is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted to show, is
that in two cases, in some respects allied, sterility is the common
result,—in the one case from the conditions of life having been
disturbed, in the other case from the organisation having been
disturbed by two organisations having been compounded into one.
It may seem fanciful, but I suspect that a similar parallelism extends
to an allied yet very different class of facts. It is an old and almost
universal belief, founded, I think, on a considerable body of evidence,
that slight changes in the conditions of life are beneficial to all
living things. We see this acted on by
farmers and gardeners in their frequent exchanges of seed, tubers,
etc., from one soil or climate to another, and back again. During the
convalescence of animals, we plainly see that great benefit is derived
from almost any change in the habits of life. Again, both with plants
and animals, there is abundant evidence, that a cross between very
distinct individuals of the same species, that is between members of
different strains or sub-breeds, gives vigour and fertility to the
offspring. I believe, indeed, from the facts alluded to in our fourth
chapter, that a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness and
sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour
and fertility to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce
hybrids which are generally sterile in some degree. I cannot persuade
myself that this parallelism is an accident or an illusion. Both series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life.
_Fertility of Varieties when crossed, and of their Mongrel
offspring_.—It may be urged, as a most forcible argument,
that there must be some essential distinction between species and
varieties, and that there must be some error in all the foregoing
remarks, inasmuch as varieties, however much they may differ from each
other in external appearance, cross with perfect facility, and yield
perfectly fertile offspring. I fully admit that this is almost
invariably the case. But if we look to varieties produced under nature,
we are immediately involved in hopeless difficulties; for if two
hitherto reputed varieties be found in any degree sterile together,
they are at once ranked by most naturalists as species. For instance,
the blue and red pimpernel, the primrose and cowslip, which are
considered by many of our best botanists as varieties, are said by
Gärtner not to be quite fertile when crossed, and he consequently ranks
them as undoubted species. If we thus argue in a circle, the fertility
of all varieties produced under nature will assuredly have to be
granted.
If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily than
other dogs with foxes, or that certain South American indigenous
domestic dogs do not readily cross with European dogs, the explanation
which will occur to everyone, and probably the true one, is that these
dogs have descended from several aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic varieties,
differing widely from each other in appearance, for instance of the
pigeon or of the cabbage, is a remarkable fact; more especially when we
reflect how many species there are, which, though resembling each other
most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic varieties
less remarkable than
at first appears. It can, in the first place, be clearly shown that
mere external dissimilarity between two species does not determine
their greater or lesser degree of sterility when crossed; and we may
apply the same rule to domestic varieties. In the second place, some
eminent naturalists believe that a long course of domestication tends
to eliminate sterility in the successive generations of hybrids, which
were at first only slightly sterile; and if this be so, we surely ought
not to expect to find sterility both appearing and disappearing under
nearly the same conditions of life. Lastly, and this seems to me by far
the most important consideration, new races of animals and plants are
produced under domestication by man’s methodical and unconscious power
of selection, for his own use and pleasure: he neither wishes to
select, nor could select, slight differences in the reproductive
system, or other constitutional differences correlated with the
reproductive system. He supplies his several varieties with the same
food; treats them in nearly the same manner, and does not wish to alter
their general habits of life. Nature acts uniformly and slowly during
vast periods of time on the whole organisation, in any way which may be
for each creature’s own good; and thus she may, either directly, or
more probably indirectly, through correlation, modify the reproductive
system in the several descendants from any one species. Seeing this
difference in the process of selection, as carried on by man and
nature, we need not be surprised at some difference in the result.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence of a certain amount of sterility
in the few following cases, which I will briefly abstract. The evidence
is at least as good as that from which we believe
in the sterility of a multitude of species. The evidence is, also,
derived from hostile witnesses, who in all other cases consider
fertility and sterility as safe criterions of specific distinction.
Gärtner kept during several years a dwarf kind of maize with yellow
seeds, and a tall variety with red seeds, growing near each other in
his garden; and although these plants have separated sexes, they never
naturally crossed. He then fertilised thirteen flowers of the one with
the pollen of the other; but only a single head produced any seed, and
this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes. No
one, I believe, has suspected that these varieties of maize are
distinct species; and it is important to notice that the hybrid plants
thus raised were themselves _perfectly_ fertile; so that even Gärtner
did not venture to consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.
The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of
experiments made during many years on nine species of Verbascum, by so
good an observer and so hostile a witness, as Gärtner: namely, that
yellow and white varieties of the same species of Verbascum when
intercrossed produce less seed, than do either coloured varieties when
fertilised with pollen from their own coloured flowers. Moreover, he
asserts that when
yellow and white varieties of one species are crossed with yellow and
white varieties of a _distinct_ species, more seed is produced by the
crosses between the same coloured flowers, than between those which are
differently coloured. Yet these varieties of Verbascum present no other
difference besides the mere colour of the flower; and one variety can
sometimes be raised from the seed of the other.
From observations which I have made on certain varieties of hollyhock,
I am inclined to suspect that they present analogous facts.
Kölreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one variety of the
common tobacco is more fertile, when crossed with a widely distinct
species, than are the other varieties. He experimentised on five forms,
which are commonly reputed to be varieties, and which he tested by the
severest trial, namely, by reciprocal crosses, and he found their
mongrel offspring perfectly fertile. But one of these five varieties,
when used either as father or mother, and crossed with the Nicotiana
glutinosa, always yielded hybrids not so sterile as those which were
produced from the four other varieties when crossed with N. glutinosa.
Hence the reproductive system of this one variety must have been in
some manner and in some degree modified.
From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety
if infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most
distinct domestic varieties, and from not wishing or being able to
produce recondite and functional differences in the reproductive
system; from these several considerations and facts, I do not think
that the very general
fertility of varieties can be proved to be of universal occurrence, or
to form a fundamental distinction between varieties and species. The
general fertility of varieties does not seem to me sufficient to
overthrow the view which I have taken with respect to the very general,
but not invariable, sterility of first crosses and of hybrids, namely,
that it is not a special endowment, but is incidental on slowly
acquired modifications, more especially in the reproductive systems of
the forms which are crossed.
_Hybrids and Mongrels compared, independently of their
fertility_.—Independently of the question of fertility, the offspring
of species when crossed and of varieties when crossed may be compared
in several other respects. Gärtner, whose strong wish was to draw a
marked line of distinction between species and varieties, could find
very few and, as it seems to me, quite unimportant differences between
the so-called hybrid offspring of species, and the so-called mongrel
offspring of varieties. And, on the other hand, they agree most closely
in very many important respects.
I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gärtner admits that hybrids from
species which have long been cultivated are often variable in the first
generation; and I have myself seen striking instances of this fact.
Gärtner further admits that hybrids between very closely allied species
are more variable than those from very distinct species; and this shows
that the difference in the degree of variability graduates away. When
mongrels and the more fertile hybrids are propagated for several
generations an extreme amount of variability in their offspring is
notorious;
but some few cases both of hybrids and mongrels long retaining
uniformity of character could be given. The variability, however, in
the successive generations of mongrels is, perhaps, greater than in
hybrids.
This greater variability of mongrels than of hybrids does not seem to
me at all surprising. For the parents of mongrels are varieties, and
mostly domestic varieties (very few experiments having been tried on
natural varieties), and this implies in most cases that there has been
recent variability; and therefore we might expect that such variability
would often continue and be super-added to that arising from the mere
act of crossing. The slight degree of variability in hybrids from the
first cross or in the first generation, in contrast with their extreme
variability in the succeeding generations, is a curious fact and
deserves attention. For it bears on and corroborates the view which I
have taken on the cause of ordinary variability; namely, that it is due
to the reproductive system being eminently sensitive to any change in
the conditions of life, being thus often rendered either impotent or at
least incapable of its proper function of producing offspring identical
with the parent-form. Now hybrids in the first generation are descended
from species (excluding those long cultivated) which have not had their
reproductive systems in any way affected, and they are not variable;
but hybrids themselves have their reproductive systems seriously
affected, and their descendants are highly variable.
But to return to our comparison of mongrels and hybrids: Gärtner states
that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference in
degree. Gärtner further insists that when any two species, although
most closely allied to each other, are
crossed with a third species, the hybrids are widely different from
each other; whereas if two very distinct varieties of one species are
crossed with another species, the hybrids do not differ much. But this
conclusion, as far as I can make out, is founded on a single
experiment; and seems directly opposed to the results of several
experiments made by Kölreuter.
These alone are the unimportant differences, which Gärtner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species,
follows according to Gärtner the same laws. When two species are
crossed, one has sometimes a prepotent power of impressing its likeness
on the hybrid; and so I believe it to be with varieties of plants. With
animals one variety certainly often has this prepotent power over
another variety. Hybrid plants produced from a reciprocal cross,
generally resemble each other closely; and so it is with mongrels from
a reciprocal cross. Both hybrids and mongrels can be reduced to either
pure parent-form, by repeated crosses in successive generations with
either parent.
These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to prepotency
in transmitting likeness running more strongly in one sex than in the
other, both when one species is crossed with another, and when one
variety is crossed with another variety. For instance, I think those
authors are right, who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny more resemble the ass
than the horse; but that the prepotency runs more strongly in the
male-ass than in
the female, so that the mule, which is the offspring of the male-ass
and mare, is more like an ass, than is the hinny, which is the
offspring of the female-ass and stallion.
Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but
it can be shown that this does sometimes occur with hybrids; yet I
grant much less frequently with hybrids than with mongrels. Looking to
the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly
appeared—such as albinism, melanism, deficiency of tail or horns, or
additional fingers and toes; and do not relate to characters which have
been slowly acquired by selection. Consequently, sudden reversions to
the perfect character of either parent would be more likely to occur
with mongrels, which are descended from varieties often suddenly
produced and semi-monstrous in character, than with hybrids, which are
descended from species slowly and naturally produced. On the whole I
entirely agree with Dr. Prosper Lucas, who, after arranging an enormous
body of facts with respect to animals, comes to the conclusion, that
the laws of resemblance of the child to its parents are the same,
whether the two parents differ much or little from each other, namely
in the union of individuals of the same variety, or of different
varieties, or of distinct species.
Laying aside the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.
_Summary of Chapter_.—First crosses between forms sufficiently distinct
to be ranked as species, and their hybrids, are very generally, but not
universally, sterile. The sterility is of all degrees, and is often so
slight that the two most careful experimentalists who have ever lived,
have come to diametrically opposite conclusions in ranking forms by
this test. The sterility is innately variable in individuals of the
same species, and is eminently susceptible of favourable and
unfavourable conditions. The degree of sterility does not strictly
follow systematic affinity, but is governed by several curious and
complex laws. It is generally different, and sometimes widely
different, in reciprocal crosses between the same two species. It is
not always equal in degree in a first cross and in the hybrid produced
from this cross.
In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown
differences in their vegetative systems, so in crossing, the greater or
less facility of one species to unite with another, is incidental on
unknown differences in their reproductive systems. There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature,
than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together in
order to prevent them becoming inarched in our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems
to depend on several circumstances; in some cases largely on the early
death of the embryo. The sterility of hybrids, which have their
reproductive systems imperfect, and which have had this system and
their whole organisation disturbed by being compounded of two distinct
species, seems closely allied to that sterility which so frequently
affects pure species, when their natural conditions of life have been
disturbed. This view is supported by a parallelism of another
kind;—namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that
slight changes in the conditions of life are apparently favourable to
the vigour and fertility of all organic beings. It is not surprising
that the degree of difficulty in uniting two species, and the degree of
sterility of their hybrid-offspring should generally correspond, though
due to distinct causes; for both depend on the amount of difference of
some kind between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, the fertility of the
hybrids produced, and the capacity of being grafted together—though
this latter capacity evidently depends on widely different
circumstances—should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment; for
systematic affinity attempts to express all kinds of resemblance
between all species.
First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable
we are to argue in a circle with respect to varieties in a state of
nature; and when we remember that the greater number of varieties have
been produced under domestication
by the selection of mere external differences, and not of differences
in the reproductive system. In all other respects, excluding fertility,
there is a close general resemblance between hybrids and mongrels.
Finally, then, the facts briefly given in this chapter do not seem to
me opposed to, but even rather to support the view, that there is no
fundamental distinction between species and varieties.
CHAPTER IX.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day. On the
nature of extinct intermediate varieties; on their number. On the vast
lapse of time, as inferred from the rate of deposition and of
denudation. On the poorness of our palæontological collections. On the
intermittence of geological formations. On the absence of intermediate
varieties in any one formation. On the sudden appearance of groups of
species. On their sudden appearance in the lowest known fossiliferous
strata.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific
forms, and their not being blended together by innumerable transitional
links, is a very obvious difficulty. I assigned reasons why such links
do not commonly occur at the present day, under the circumstances
apparently most favourable for their presence, namely on an extensive
and continuous area with graduated physical conditions. I endeavoured
to show, that the life of each species depends in a more important
manner on the presence of other already defined organic forms, than on
climate; and, therefore, that the really governing conditions of life
do not graduate away quite insensibly like heat or moisture. I
endeavoured, also, to show that intermediate varieties, from existing
in lesser numbers than the forms which they connect, will generally be
beaten out and exterminated during the course of further modification
and improvement. The main cause, however, of innumerable intermediate
links not now occurring everywhere throughout nature depends
on the very process of natural selection, through which new varieties
continually take the places of and exterminate their parent-forms. But
just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which
have formerly existed on the earth, be truly enormous. Why then is not
every geological formation and every stratum full of such intermediate
links? Geology assuredly does not reveal any such finely graduated
organic chain; and this, perhaps, is the most obvious and gravest
objection which can be urged against my theory. The explanation lies,
as I believe, in the extreme imperfection of the geological record.
In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself, forms _directly_ intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons have both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified, that if we
had no historical or indirect evidence regarding their origin, it would
not have been possible to have
determined from a mere comparison of their structure with that of the
rock-pigeon, whether they had descended from this species or from some
other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links ever existed directly intermediate between them, but between each
and an unknown common parent. The common parent will have had in its
whole organisation much general resemblance to the tapir and to the
horse; but in some points of structure may have differed considerably
from both, even perhaps more than they differ from each other. Hence in
all such cases, we should be unable to recognise the parent-form of any
two or more species, even if we closely compared the structure of the
parent with that of its modified descendants, unless at the same time
we had a nearly perfect chain of the intermediate links.
It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case _direct_ intermediate links will have existed between
them. But such a case would imply that one form had remained for a very
long period unaltered, whilst its descendants had undergone a vast
amount of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare event;
for in all cases the new and improved forms of life will tend to
supplant the old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the varieties of the same species at the
present
day; and these parent-species, now generally extinct, have in their
turn been similarly connected with more ancient species; and so on
backwards, always converging to the common ancestor of each great
class. So that the number of intermediate and transitional links,
between all living and extinct species, must have been inconceivably
great. But assuredly, if this theory be true, such have lived upon this
earth.
_On the lapse of Time_.—Independently of our not finding fossil remains
of such infinitely numerous connecting links, it may be objected, that
time will not have sufficed for so great an amount of organic change,
all changes having been effected very slowly through natural selection.
It is hardly possible for me even to recall to the reader, who may not
be a practical geologist, the facts leading the mind feebly to
comprehend the lapse of time. He who can read Sir Charles Lyell’s grand
work on the Principles of Geology, which the future historian will
recognise as having produced a revolution in natural science, yet does
not admit how incomprehensibly vast have been the past periods of time,
may at once close this volume. Not that it suffices to study the
Principles of Geology, or to read special treatises by different
observers on separate formations, and to mark how each author attempts
to give an inadequate idea of the duration of each formation or even
each stratum. A man must for years examine for himself great piles of
superimposed strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend anything of
the lapse of time, the monuments of which we see around us.
It is good to wander along lines of sea-coast, when formed of
moderately hard rocks, and mark the
process of degradation. The tides in most cases reach the cliffs only
for a short time twice a day, and the waves eat into them only when
they are charged with sand or pebbles; for there is reason to believe
that pure water can effect little or nothing in wearing away rock. At
last the base of the cliff is undermined, huge fragments fall down, and
these remaining fixed, have to be worn away, atom by atom, until
reduced in size they can be rolled about by the waves, and then are
more quickly ground into pebbles, sand, or mud. But how often do we see
along the bases of retreating cliffs rounded boulders, all thickly
clothed by marine productions, showing how little they are abraded and
how seldom they are rolled about! Moreover, if we follow for a few
miles any line of rocky cliff, which is undergoing degradation, we find
that it is only here and there, along a short length or round a
promontory, that the cliffs are at the present time suffering. The
appearance of the surface and the vegetation show that elsewhere years
have elapsed since the waters washed their base.
He who most closely studies the action of the sea on our shores, will,
I believe, be most deeply impressed with the slowness with which rocky
coasts are worn away. The observations on this head by Hugh Miller, and
by that excellent observer Mr. Smith of Jordan Hill, are most
impressive. With the mind thus impressed, let any one examine beds of
conglomerate many thousand feet in thickness, which, though probably
formed at a quicker rate than many other deposits, yet, from being
formed of worn and rounded pebbles, each of which bears the stamp of
time, are good to show how slowly the mass has been accumulated. Let
him remember Lyell’s profound remark, that the thickness and extent of
sedimentary formations
are the result and measure of the degradation which the earth’s crust
has elsewhere suffered. And what an amount of degradation is implied by
the sedimentary deposits of many countries! Professor Ramsay has given
me the maximum thickness, in most cases from actual measurement, in a
few cases from estimate, of each formation in different parts of Great
Britain; and this is the result:—
Feet
Palæozoic strata (not including igneous beds)...57,154. Secondary
strata................................13,190. Tertiary
strata..................................2,240.
—making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness
on the Continent. Moreover, between each successive formation, we have,
in the opinion of most geologists, enormously long blank periods. So
that the lofty pile of sedimentary rocks in Britain, gives but an
inadequate idea of the time which has elapsed during their
accumulation; yet what time this must have consumed! Good observers
have estimated that sediment is deposited by the great Mississippi
river at the rate of only 600 feet in a hundred thousand years. This
estimate may be quite erroneous; yet, considering over what wide spaces
very fine sediment is transported by the currents of the sea, the
process of accumulation in any one area must be extremely slow.
But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember having been much struck with the evidence of denudation, when
viewing volcanic islands, which have been
worn by the waves and pared all round into perpendicular cliffs of one
or two thousand feet in height; for the gentle slope of the
lava-streams, due to their formerly liquid state, showed at a glance
how far the hard, rocky beds had once extended into the open ocean. The
same story is still more plainly told by faults,—those great cracks
along which the strata have been upheaved on one side, or thrown down
on the other, to the height or depth of thousands of feet; for since
the crust cracked, the surface of the land has been so completely
planed down by the action of the sea, that no trace of these vast
dislocations is externally visible.
The Craven fault, for instance, extends for upwards of 30 miles, and
along this line the vertical displacement of the strata has varied from
600 to 3000 feet. Professor Ramsay has published an account of a
downthrow in Anglesea of 2300 feet; and he informs me that he fully
believes there is one in Merionethshire of 12,000 feet; yet in these
cases there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having been
smoothly swept away. The consideration of these facts impresses my mind
almost in the same manner as does the vain endeavour to grapple with
the idea of eternity.
I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the denudation
of the Weald has been a mere trifle, in comparison with that which has
removed masses of our palæozoic strata, in parts ten thousand feet in
thickness, as shown in Professor Ramsay’s masterly memoir on this
subject. Yet it is an admirable lesson to stand on the North Downs and
to look at the distant South Downs; for, remembering that at no great
distance to the west the northern and southern escarpments meet and
close, one can safely picture to
oneself the great dome of rocks which must have covered up the Weald
within so limited a period as since the latter part of the Chalk
formation. The distance from the northern to the southern Downs is
about 22 miles, and the thickness of the several formations is on an
average about 1100 feet, as I am informed by Professor Ramsay. But if,
as some geologists suppose, a range of older rocks underlies the Weald,
on the flanks of which the overlying sedimentary deposits might have
accumulated in thinner masses than elsewhere, the above estimate would
be erroneous; but this source of doubt probably would not greatly
affect the estimate as applied to the western extremity of the
district. If, then, we knew the rate at which the sea commonly wears
away a line of cliff of any given height, we could measure the time
requisite to have denuded the Weald. This, of course, cannot be done;
but we may, in order to form some crude notion on the subject, assume
that the sea would eat into cliffs 500 feet in height at the rate of
one inch in a century. This will at first appear much too small an
allowance; but it is the same as if we were to assume a cliff one yard
in height to be eaten back along a whole line of coast at the rate of
one yard in nearly every twenty-two years. I doubt whether any rock,
even as soft as chalk, would yield at this rate excepting on the most
exposed coasts; though no doubt the degradation of a lofty cliff would
be more rapid from the breakage of the fallen fragments. On the other
hand, I do not believe that any line of coast, ten or twenty miles in
length, ever suffers degradation at the same time along its whole
indented length; and we must remember that almost all strata contain
harder layers or nodules, which from long resisting attrition form a
breakwater at the base. Hence, under ordinary circumstances, I conclude
that for a cliff 500 feet in height, a denudation
of one inch per century for the whole length would be an ample
allowance. At this rate, on the above data, the denudation of the Weald
must have required 306,662,400 years; or say three hundred million
years.
The action of fresh water on the gently inclined Wealden district, when
upraised, could hardly have been great, but it would somewhat reduce
the above estimate. On the other hand, during oscillations of level,
which we know this area has undergone, the surface may have existed for
millions of years as land, and thus have escaped the action of the sea:
when deeply submerged for perhaps equally long periods, it would,
likewise, have escaped the action of the coast-waves. So that in all
probability a far longer period than 300 million years has elapsed
since the latter part of the Secondary period.
I have made these few remarks because it is highly important for us to
gain some notion, however imperfect, of the lapse of years. During each
of these years, over the whole world, the land and the water has been
peopled by hosts of living forms. What an infinite number of
generations, which the mind cannot grasp, must have succeeded each
other in the long roll of years! Now turn to our richest geological
museums, and what a paltry display we behold!
_On the poorness of our Palæontological collections_.—That our
palæontological collections are very imperfect, is admitted by every
one. The remark of that admirable palæontologist, the late Edward
Forbes, should not be forgotten, namely, that numbers of our fossil
species are known and named from single and often broken specimens, or
from a few specimens collected on some one spot. Only a small portion
of the surface of the earth has been geologically explored, and no part
with
sufficient care, as the important discoveries made every year in Europe
prove. No organism wholly soft can be preserved. Shells and bones will
decay and disappear when left on the bottom of the sea, where sediment
is not accumulating. I believe we are continually taking a most
erroneous view, when we tacitly admit to ourselves that sediment is
being deposited over nearly the whole bed of the sea, at a rate
sufficiently quick to embed and preserve fossil remains. Throughout an
enormously large proportion of the ocean, the bright blue tint of the
water bespeaks its purity. The many cases on record of a formation
conformably covered, after an enormous interval of time, by another and
later formation, without the underlying bed having suffered in the
interval any wear and tear, seem explicable only on the view of the
bottom of the sea not rarely lying for ages in an unaltered condition.
The remains which do become embedded, if in sand or gravel, will when
the beds are upraised generally be dissolved by the percolation of
rain-water. I suspect that but few of the very many animals which live
on the beach between high and low watermark are preserved. For
instance, the several species of the Chthamalinæ (a sub-family of
sessile cirripedes) coat the rocks all over the world in infinite
numbers: they are all strictly littoral, with the exception of a single
Mediterranean species, which inhabits deep water and has been found
fossil in Sicily, whereas not one other species has hitherto been found
in any tertiary formation: yet it is now known that the genus
Chthamalus existed during the chalk period. The molluscan genus Chiton
offers a partially analogous case.
With respect to the terrestrial productions which lived during the
Secondary and Palæozoic periods, it is superfluous to state that our
evidence from fossil
remains is fragmentary in an extreme degree. For instance, not a land
shell is known belonging to either of these vast periods, with one
exception discovered by Sir C. Lyell in the carboniferous strata of
North America. In regard to mammiferous remains, a single glance at the
historical table published in the Supplement to Lyell’s Manual, will
bring home the truth, how accidental and rare is their preservation,
far better than pages of detail. Nor is their rarity surprising, when
we remember how large a proportion of the bones of tertiary mammals
have been discovered either in caves or in lacustrine deposits; and
that not a cave or true lacustrine bed is known belonging to the age of
our secondary or palæozoic formations.
But the imperfection in the geological record mainly results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time. When we see the formations tabulated in written
works, or when we follow them in nature, it is difficult to avoid
believing that they are closely consecutive. But we know, for instance,
from Sir R. Murchison’s great work on Russia, what wide gaps there are
in that country between the superimposed formations; so it is in North
America, and in many other parts of the world. The most skilful
geologist, if his attention had been exclusively confined to these
large territories, would never have suspected that during the periods
which were blank and barren in his own country, great piles of
sediment, charged with new and peculiar forms of life, had elsewhere
been accumulated. And if in each separate territory, hardly any idea
can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent
and great changes in the mineralogical composition of consecutive
formations, generally implying great changes in the geography of the
surrounding lands, whence the sediment has been derived, accords with
the belief of vast intervals of time having elapsed between each
formation.
But we can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each
other in close sequence. Scarcely any fact struck me more when
examining many hundred miles of the South American coasts, which have
been upraised several hundred feet within the recent period, than the
absence of any recent deposits sufficiently extensive to last for even
a short geological period. Along the whole west coast, which is
inhabited by a peculiar marine fauna, tertiary beds are so scantily
developed, that no record of several successive and peculiar marine
faunas will probably be preserved to a distant age. A little reflection
will explain why along the rising coast of the western side of South
America, no extensive formations with recent or tertiary remains can
anywhere be found, though the supply of sediment must for ages have
been great, from the enormous degradation of the coast-rocks and from
muddy streams entering the sea. The explanation, no doubt, is, that the
littoral and sub-littoral deposits are continually worn away, as soon
as they are brought up by the slow and gradual rising of the land
within the grinding action of the coast-waves.
We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during
subsequent oscillations of level. Such thick and extensive
accumulations of sediment may be formed in two ways; either,
in profound depths of the sea, in which case, judging from the
researches of E. Forbes, we may conclude that the bottom will be
inhabited by extremely few animals, and the mass when upraised will
give a most imperfect record of the forms of life which then existed;
or, sediment may be accumulated to any thickness and extent over a
shallow bottom, if it continue slowly to subside. In this latter case,
as long as the rate of subsidence and supply of sediment nearly balance
each other, the sea will remain shallow and favourable for life, and
thus a fossiliferous formation thick enough, when upraised, to resist
any amount of degradation, may be formed.
I am convinced that all our ancient formations, which are rich in
fossils, have thus been formed during subsidence. Since publishing my
views on this subject in 1845, I have watched the progress of Geology,
and have been surprised to note how author after author, in treating of
this or that great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only ancient
tertiary formation on the west coast of South America, which has been
bulky enough to resist such degradation as it has as yet suffered, but
which will hardly last to a distant geological age, was certainly
deposited during a downward oscillation of level, and thus gained
considerable thickness.
All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, may
have been formed over wide spaces during periods of subsidence, but
only where the supply of sediment was sufficient to keep the sea
shallow and to embed and
preserve the remains before they had time to decay. On the other hand,
as long as the bed of the sea remained stationary, _thick_ deposits
could not have been accumulated in the shallow parts, which are the
most favourable to life. Still less could this have happened during the
alternate periods of elevation; or, to speak more accurately, the beds
which were then accumulated will have been destroyed by being upraised
and brought within the limits of the coast-action.
Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for
they are in strict accordance with the general principles inculcated by
Sir C. Lyell; and E. Forbes independently arrived at a similar
conclusion.
One remark is here worth a passing notice. During periods of elevation
the area of the land and of the adjoining shoal parts of the sea will
be increased, and new stations will often be formed;—all circumstances
most favourable, as previously explained, for the formation of new
varieties and species; but during such periods there will generally be
a blank in the geological record. On the other hand, during subsidence,
the inhabited area and number of inhabitants will decrease (excepting
the productions on the shores of a continent when first broken up into
an archipelago), and consequently during subsidence, though there will
be much extinction, fewer new varieties or species will be formed; and
it is during these very periods of subsidence, that our great deposits
rich in fossils have been accumulated. Nature may almost be said to
have guarded against the frequent discovery of her transitional or
linking forms.
From the foregoing considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if we
confine our attention to any one formation, it becomes more difficult
to understand,
why we do not therein find closely graduated varieties between the
allied species which lived at its commencement and at its close. Some
cases are on record of the same species presenting distinct varieties
in the upper and lower parts of the same formation, but, as they are
rare, they may be here passed over. Although each formation has
indisputably required a vast number of years for its deposition, I can
see several reasons why each should not include a graduated series of
links between the species which then lived; but I can by no means
pretend to assign due proportional weight to the following
considerations.
Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite to change one
species into another. I am aware that two palæontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us coming to any
just conclusion on this head. When we see a species first appearing in
the middle of any formation, it would be rash in the extreme to infer
that it had not elsewhere previously existed. So again when we find a
species disappearing before the uppermost layers have been deposited,
it would be equally rash to suppose that it then became wholly extinct.
We forget how small the area of Europe is compared with the rest of the
world; nor have the several stages of the same formation throughout
Europe been correlated with perfect accuracy.
With marine animals of all kinds, we may safely infer a large amount of
migration during climatal and other changes; and when we see a species
first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for
instance, that several species appeared somewhat earlier in the
palæozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the
European seas. In examining the latest deposits of various quarters of
the world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now abundant
in the neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascertained amount
of migration of the inhabitants of Europe during the Glacial period,
which forms only a part of one whole geological period; and likewise to
reflect on the great changes of level, on the inordinately great change
of climate, on the prodigious lapse of time, all included within this
same glacial period. Yet it may be doubted whether in any quarter of
the world, sedimentary deposits, _including fossil remains_, have gone
on accumulating within the same area during the whole of this period.
It is not, for instance, probable that sediment was deposited during
the whole of the glacial period near the mouth of the Mississippi,
within that limit of depth at which marine animals can flourish; for we
know what vast geographical changes occurred in other parts of America
during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial
period shall have been upraised, organic remains will probably first
appear and disappear at different levels, owing to the migration of
species and to geographical changes. And in the distant future, a
geologist examining these beds, might be tempted to conclude that the
average duration of life
of the embedded fossils had been less than that of the glacial period,
instead of having been really far greater, that is extending from
before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will
generally have to be a very thick one; and the species undergoing
modification will have had to live on the same area throughout this
whole time. But we have seen that a thick fossiliferous formation can
only be accumulated during a period of subsidence; and to keep the
depth approximately the same, which is necessary in order to enable the
same species to live on the same space, the supply of sediment must
nearly have counterbalanced the amount of subsidence. But this same
movement of subsidence will often tend to sink the area whence the
sediment is derived, and thus diminish the supply whilst the downward
movement continues. In fact, this nearly exact balancing between the
supply of sediment and the amount of subsidence is probably a rare
contingency; for it has been observed by more than one palæontologist,
that very thick deposits are usually barren of organic remains, except
near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of different mineralogical composition, we may
reasonably suspect that the process of deposition has been much
interrupted, as a change in the currents of the sea and a supply of
sediment of a different nature will
generally have been due to geographical changes requiring much time.
Nor will the closest inspection of a formation give any idea of the
time which its deposition has consumed. Many instances could be given
of beds only a few feet in thickness, representing formations,
elsewhere thousands of feet in thickness, and which must have required
an enormous period for their accumulation; yet no one ignorant of this
fact would have suspected the vast lapse of time represented by the
thinner formation. Many cases could be given of the lower beds of a
formation having been upraised, denuded, submerged, and then re-covered
by the upper beds of the same formation,—facts, showing what wide, yet
easily overlooked, intervals have occurred in its accumulation. In
other cases we have the plainest evidence in great fossilised trees,
still standing upright as they grew, of many long intervals of time and
changes of level during the process of deposition, which would never
even have been suspected, had not the trees chanced to have been
preserved: thus, Messrs. Lyell and Dawson found carboniferous beds 1400
feet thick in Nova Scotia, with ancient root-bearing strata, one above
the other, at no less than sixty-eight different levels. Hence, when
the same species occur at the bottom, middle, and top of a formation,
the probability is that they have not lived on the same spot during the
whole period of deposition, but have disappeared and reappeared,
perhaps many times, during the same geological period. So that if such
species were to undergo a considerable amount of modification during
any one geological period, a section would not probably include all the
fine intermediate gradations which must on my theory have existed
between them, but abrupt, though perhaps very slight, changes of form.
It is all-important to remember that naturalists have
no golden rule by which to distinguish species and varieties; they
grant some little variability to each species, but when they meet with
a somewhat greater amount of difference between any two forms, they
rank both as species, unless they are enabled to connect them together
by close intermediate gradations. And this from the reasons just
assigned we can seldom hope to effect in any one geological section.
Supposing B and C to be two species, and a third, A, to be found in an
underlying bed; even if A were strictly intermediate between B and C,
it would simply be ranked as a third and distinct species, unless at
the same time it could be most closely connected with either one or
both forms by intermediate varieties. Nor should it be forgotten, as
before explained, that A might be the actual progenitor of B and C, and
yet might not at all necessarily be strictly intermediate between them
in all points of structure. So that we might obtain the parent-species
and its several modified descendants from the lower and upper beds of a
formation, and unless we obtained numerous transitional gradations, we
should not recognise their relationship, and should consequently be
compelled to rank them all as distinct species.
It is notorious on what excessively slight differences many
palæontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D’Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on my
theory we ought to find. Moreover, if we look to rather wider
intervals, namely, to distinct but consecutive stages of the same great
formation, we find that the embedded fossils, though almost universally
ranked as specifically different,
yet are far more closely allied to each other than are the species
found in more widely separated formations; but to this subject I shall
have to return in the following chapter.
One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, there is reason to
suspect, as we have formerly seen, that their varieties are generally
at first local; and that such local varieties do not spread widely and
supplant their parent-forms until they have been modified and perfected
in some considerable degree. According to this view, the chance of
discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes
are supposed to have been local or confined to some one spot. Most
marine animals have a wide range; and we have seen that with plants it
is those which have the widest range, that oftenest present varieties;
so that with shells and other marine animals, it is probably those
which have had the widest range, far exceeding the limits of the known
geological formations of Europe, which have oftenest given rise, first
to local varieties and ultimately to new species; and this again would
greatly lessen the chance of our being able to trace the stages of
transition in any one geological formation.
It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until
many specimens have been collected from many places; and in the case of
fossil species this could rarely be effected by palæontologists. We
shall, perhaps, best perceive the improbability of our being enabled to
connect species by numerous, fine, intermediate, fossil links, by
asking
ourselves whether, for instance, geologists at some future period will
be able to prove, that our different breeds of cattle, sheep, horses,
and dogs have descended from a single stock or from several aboriginal
stocks; or, again, whether certain sea-shells inhabiting the shores of
North America, which are ranked by some conchologists as distinct
species from their European representatives, and by other conchologists
as only varieties, are really varieties or are, as it is called,
specifically distinct. This could be effected only by the future
geologist discovering in a fossil state numerous intermediate
gradations; and such success seems to me improbable in the highest
degree.
Geological research, though it has added numerous species to existing
and extinct genera, and has made the intervals between some few groups
less wide than they otherwise would have been, yet has done scarcely
anything in breaking down the distinction between species, by
connecting them together by numerous, fine, intermediate varieties; and
this not having been effected, is probably the gravest and most obvious
of all the many objections which may be urged against my views. Hence
it will be worth while to sum up the foregoing remarks, under an
imaginary illustration. The Malay Archipelago is of about the size of
Europe from the North Cape to the Mediterranean, and from Britain to
Russia; and therefore equals all the geological formations which have
been examined with any accuracy, excepting those of the United States
of America. I fully agree with Mr. Godwin-Austen, that the present
condition of the Malay Archipelago, with its numerous large islands
separated by wide and shallow seas, probably represents the former
state of Europe, when most of our formations were accumulating. The
Malay Archipelago is one of the richest regions of the
whole world in organic beings; yet if all the species were to be
collected which have ever lived there, how imperfectly would they
represent the natural history of the world!
But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an excessively imperfect manner
in the formations which we suppose to be there accumulating. I suspect
that not many of the strictly littoral animals, or of those which lived
on naked submarine rocks, would be embedded; and those embedded in
gravel or sand, would not endure to a distant epoch. Wherever sediment
did not accumulate on the bed of the sea, or where it did not
accumulate at a sufficient rate to protect organic bodies from decay,
no remains could be preserved.
In our archipelago, I believe that fossiliferous formations could be
formed of sufficient thickness to last to an age, as distant in
futurity as the secondary formations lie in the past, only during
periods of subsidence. These periods of subsidence would be separated
from each other by enormous intervals, during which the area would be
either stationary or rising; whilst rising, each fossiliferous
formation would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America.
During the periods of subsidence there would probably be much
extinction of life; during the periods of elevation, there would be
much variation, but the geological record would then be least perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would _exceed_ the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not fully
preserved, transitional varieties would merely appear as so many
distinct species. It is, also, probable that each great period of
subsidence would be interrupted by oscillations of level, and that
slight climatal changes would intervene during such lengthy periods;
and in these cases the inhabitants of the archipelago would have to
migrate, and no closely consecutive record of their modifications could
be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to believe
that it would be chiefly these far-ranging species which would oftenest
produce new varieties; and the varieties would at first generally be
local or confined to one place, but if possessed of any decided
advantage, or when further modified and improved, they would slowly
spread and supplant their parent-forms. When such varieties returned to
their ancient homes, as they would differ from their former state, in a
nearly uniform, though perhaps extremely slight degree, they would,
according to the principles followed by many palæontologists, be ranked
as new and distinct species.
If then, there be some degree of truth in these remarks, we have no
right to expect to find in our geological formations, an infinite
number of those fine transitional forms, which on my theory assuredly
have connected all the past and present species of the same group into
one long and branching chain of life. We ought only to look for a few
links, some more closely, some more distantly related to each other;
and these links, let them be ever so close, if found in different
stages of the same formation, would, by most palæontologists,
be ranked as distinct species. But I do not pretend that I should ever
have suspected how poor a record of the mutations of life, the best
preserved geological section presented, had not the difficulty of our
not discovering innumerable transitional links between the species
which appeared at the commencement and close of each formation, pressed
so hardly on my theory.
_On the sudden appearance of whole groups of Allied Species_.—The
abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palæontologists, for
instance, by Agassiz, Pictet, and by none more forcibly than by
Professor Sedgwick, as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging to the same
genera or families, have really started into life all at once, the fact
would be fatal to the theory of descent with slow modification through
natural selection. For the development of a group of forms, all of
which have descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long ages
before their modified descendants. But we continually over-rate the
perfection of the geological record, and falsely infer, because certain
genera or families have not been found beneath a certain stage, that
they did not exist before that stage. We continually forget how large
the world is, compared with the area over which our geological
formations have been carefully examined; we forget that groups of
species may elsewhere have long existed and have slowly multiplied
before they invaded the ancient archipelagoes of Europe and of the
United States. We do not make due allowance for the enormous intervals
of time, which have
probably elapsed between our consecutive formations,—longer perhaps in
some cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication
of species from some one or some few parent-forms; and in the
succeeding formation such species will appear as if suddenly created.
I may here recall a remark formerly made, namely that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance to fly through the air; but that when this
had been effected, and a few species had thus acquired a great
advantage over other organisms, a comparatively short time would be
necessary to produce many divergent forms, which would be able to
spread rapidly and widely throughout the world.
I will now give a few examples to illustrate these remarks; and to show
how liable we are to error in supposing that whole groups of species
have suddenly been produced. I may recall the well-known fact that in
geological treatises, published not many years ago, the great class of
mammals was always spoken of as having abruptly come in at the
commencement of the tertiary series. And now one of the richest known
accumulations of fossil mammals belongs to the middle of the secondary
series; and one true mammal has been discovered in the new red
sandstone at nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum; but now
extinct species have been discovered in India, South America, and in
Europe even as far back as the eocene stage. The most striking case,
however, is that of the Whale family; as these animals have huge bones,
are marine, and range over the world, the fact of not a single bone of
a whale having been discovered in
any secondary formation, seemed fully to justify the belief that this
great and distinct order had been suddenly produced in the interval
between the latest secondary and earliest tertiary formation. But now
we may read in the Supplement to Lyell’s ‘Manual,’ published in 1858,
clear evidence of the existence of whales in the upper greensand, some
time before the close of the secondary period.
I may give another instance, which from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I have
stated that, from the number of existing and extinct tertiary species;
from the extraordinary abundance of the individuals of many species all
over the world, from the Arctic regions to the equator, inhabiting
various zones of depths from the upper tidal limits to 50 fathoms; from
the perfect manner in which specimens are preserved in the oldest
tertiary beds; from the ease with which even a fragment of a valve can
be recognised; from all these circumstances, I inferred that had
sessile cirripedes existed during the secondary periods, they would
certainly have been preserved and discovered; and as not one species
had been discovered in beds of this age, I concluded that this great
group had been suddenly developed at the commencement of the tertiary
series. This was a sore trouble to me, adding as I thought one more
instance of the abrupt appearance of a great group of species. But my
work had hardly been published, when a skilful palæontologist, M.
Bosquet, sent me a drawing of a perfect specimen of an unmistakeable
sessile cirripede, which he had himself extracted from the chalk of
Belgium. And, as if to make the case as striking as possible, this
sessile cirripede was a Chthamalus, a very common, large, and
ubiquitous genus, of which not one specimen has as yet been found even
in any tertiary
stratum. Hence we now positively know that sessile cirripedes existed
during the secondary period; and these cirripedes might have been the
progenitors of our many tertiary and existing species.
The case most frequently insisted on by palæontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down in the Chalk period. This group
includes the large majority of existing species. Lately, Professor
Pictet has carried their existence one sub-stage further back; and some
palæontologists believe that certain much older fishes, of which the
affinities are as yet imperfectly known, are really teleostean.
Assuming, however, that the whole of them did appear, as Agassiz
believes, at the commencement of the chalk formation, the fact would
certainly be highly remarkable; but I cannot see that it would be an
insuperable difficulty on my theory, unless it could likewise be shown
that the species of this group appeared suddenly and simultaneously
throughout the world at this same period. It is almost superfluous to
remark that hardly any fossil-fish are known from south of the equator;
and by running through Pictet’s Palæontology it will be seen that very
few species are known from several formations in Europe. Some few
families of fish now have a confined range; the teleostean fish might
formerly have had a similarly confined range, and after having been
largely developed in some one sea, might have spread widely. Nor have
we any right to suppose that the seas of the world have always been so
freely open from south to north as they are at present. Even at this
day, if the Malay Archipelago were converted into land, the tropical
parts of the Indian Ocean would form a large and perfectly enclosed
basin, in which any great group of marine animals might be multiplied;
and
here they would remain confined, until some of the species became
adapted to a cooler climate, and were enabled to double the southern
capes of Africa or Australia, and thus reach other and distant seas.
From these and similar considerations, but chiefly from our ignorance
of the geology of other countries beyond the confines of Europe and the
United States; and from the revolution in our palæontological ideas on
many points, which the discoveries of even the last dozen years have
effected, it seems to me to be about as rash in us to dogmatize on the
succession of organic beings throughout the world, as it would be for a
naturalist to land for five minutes on some one barren point in
Australia, and then to discuss the number and range of its productions.
_On the sudden appearance of groups of Allied Species in the lowest
known fossiliferous strata_.—There is another and allied difficulty,
which is much graver. I allude to the manner in which numbers of
species of the same group, suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group have descended from one
progenitor, apply with nearly equal force to the earliest known
species. For instance, I cannot doubt that all the Silurian trilobites
have descended from some one crustacean, which must have lived long
before the Silurian age, and which probably differed greatly from any
known animal. Some of the most ancient Silurian animals, as the
Nautilus, Lingula, etc., do not differ much from living species; and it
cannot on my theory be supposed, that these old species were the
progenitors of all the species of the orders to which they belong, for
they do not present characters in any degree intermediate between them.
If, moreover, they had been the progenitors of these orders, they would
almost certainly have been long ago supplanted and exterminated by
their numerous and improved descendants.
Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long
as, or probably far longer than, the whole interval from the Silurian
age to the present day; and that during these vast, yet quite unknown,
periods of time, the world swarmed with living creatures.
To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir R. Murchison at their head, are convinced that we
see in the organic remains of the lowest Silurian stratum the dawn of
life on this planet. Other highly competent judges, as Lyell and the
late E. Forbes, dispute this conclusion. We should not forget that only
a small portion of the world is known with accuracy. M. Barrande has
lately added another and lower stage to the Silurian system, abounding
with new and peculiar species. Traces of life have been detected in the
Longmynd beds beneath Barrande’s so-called primordial zone. The
presence of phosphatic nodules and bituminous matter in some of the
lowest azoic rocks, probably indicates the former existence of life at
these periods. But the difficulty of understanding the absence of vast
piles of fossiliferous strata, which on my theory no doubt were
somewhere accumulated before the Silurian epoch, is very great. If
these most ancient beds had been wholly worn away by denudation, or
obliterated by metamorphic action, we ought to find only small remnants
of the formations next succeeding them in age, and these ought to be
very generally in
a metamorphosed condition. But the descriptions which we now possess of
the Silurian deposits over immense territories in Russia and in North
America, do not support the view, that the older a formation is, the
more it has suffered the extremity of denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urged as
a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains, which do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that from
first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the existing continents of
Europe and North America. But we do not know what was the state of
things in the intervals between the successive formations; whether
Europe and the United States during these intervals existed as dry
land, or as a submarine surface near land, on which sediment was not
deposited, or again as the bed of an open and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but not one oceanic island
is as yet known to afford even a remnant of any palæozoic or secondary
formation. Hence we may perhaps infer, that during the palæozoic and
secondary periods, neither continents nor continental islands existed
where our oceans now extend; for had they existed there, palæozoic and
secondary formations would in all probability have been accumulated
from sediment derived from their wear and
tear; and would have been at least partially upheaved by the
oscillations of level, which we may fairly conclude must have
intervened during these enormously long periods. If then we may infer
anything from these facts, we may infer that where our oceans now
extend, oceans have extended from the remotest period of which we have
any record; and on the other hand, that where continents now exist,
large tracts of land have existed, subjected no doubt to great
oscillations of level, since the earliest silurian period. The coloured
map appended to my volume on Coral Reefs, led me to conclude that the
great oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But have we any right to assume that things have
thus remained from eternity? Our continents seem to have been formed by
a preponderance, during many oscillations of level, of the force of
elevation; but may not the areas of preponderant movement have changed
in the lapse of ages? At a period immeasurably antecedent to the
silurian epoch, continents may have existed where oceans are now spread
out; and clear and open oceans may have existed where our continents
now stand. Nor should we be justified in assuming that if, for
instance, the bed of the Pacific Ocean were now converted into a
continent, we should there find formations older than the silurian
strata, supposing such to have been formerly deposited; for it might
well happen that strata which had subsided some miles nearer to the
centre of the earth, and which had been pressed on by an enormous
weight of superincumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to the
surface. The immense areas in some parts of the world, for instance in
South America, of bare metamorphic rocks, which
must have been heated under great pressure, have always seemed to me to
require some special explanation; and we may perhaps believe that we
see in these large areas, the many formations long anterior to the
silurian epoch in a completely metamorphosed condition.
The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between
the many species which now exist or have existed; the sudden manner in
which whole groups of species appear in our European formations; the
almost entire absence, as at present known, of fossiliferous formations
beneath the Silurian strata, are all undoubtedly of the gravest nature.
We see this in the plainest manner by the fact that all the most
eminent palæontologists, namely Cuvier, Owen, Agassiz, Barrande,
Falconer, E. Forbes, etc., and all our greatest geologists, as Lyell,
Murchison, Sedgwick, etc., have unanimously, often vehemently,
maintained the immutability of species. But I have reason to believe
that one great authority, Sir Charles Lyell, from further reflexion
entertains grave doubts on this subject. I feel how rash it is to
differ from these great authorities, to whom, with others, we owe all
our knowledge. Those who think the natural geological record in any
degree perfect, and who do not attach much weight to the facts and
arguments of other kinds given in this volume, will undoubtedly at once
reject my theory. For my part, following out Lyell’s metaphor, I look
at the natural geological record, as a history of the world imperfectly
kept, and written in a changing dialect; of this history we possess the
last volume alone, relating only to two or three countries. Of this
volume, only here and there a short chapter has
been preserved; and of each page, only here and there a few lines. Each
word of the slowly-changing language, in which the history is supposed
to be written, being more or less different in the interrupted
succession of chapters, may represent the apparently abruptly changed
forms of life, entombed in our consecutive, but widely separated
formations. On this view, the difficulties above discussed are greatly
diminished, or even disappear.
CHAPTER X.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species. On their
different rates of change. Species once lost do not reappear. Groups of
species follow the same general rules in their appearance and
disappearance as do single species. On Extinction. On simultaneous
changes in the forms of life throughout the world. On the affinities of
extinct species to each other and to living species. On the state of
development of ancient forms. On the succession of the same types
within the same areas. Summary of preceding and present chapters.
Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between them, and to
make the percentage system of lost and new forms more gradual. In some
of the most recent beds, though undoubtedly of high antiquity if
measured by years, only one or two species are lost forms, and only one
or two are new forms, having here appeared for the first time, either
locally, or, as far as we know, on the face of the earth. If we may
trust the observations of Philippi in Sicily, the successive changes in
the marine inhabitants of that island have been many and most gradual.
The secondary formations are more broken; but, as Bronn has remarked,
neither the appearance
nor disappearance of their many now extinct species has been
simultaneous in each separate formation.
Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, in an
existing crocodile associated with many strange and lost mammals and
reptiles in the sub-Himalayan deposits. The Silurian Lingula differs
but little from the living species of this genus; whereas most of the
other Silurian Molluscs and all the Crustaceans have changed greatly.
The productions of the land seem to change at a quicker rate than those
of the sea, of which a striking instance has lately been observed in
Switzerland. There is some reason to believe that organisms, considered
high in the scale of nature, change more quickly than those that are
low: though there are exceptions to this rule. The amount of organic
change, as Pictet has remarked, does not strictly correspond with the
succession of our geological formations; so that between each two
consecutive formations, the forms of life have seldom changed in
exactly the same degree. Yet if we compare any but the most closely
related formations, all the species will be found to have undergone
some change. When a species has once disappeared from the face of the
earth, we have reason to believe that the same identical form never
reappears. The strongest apparent exception to this latter rule, is
that of the so-called “colonies” of M. Barrande, which intrude for a
period in the midst of an older formation, and then allow the
pre-existing fauna to reappear; but Lyell’s explanation, namely, that
it is a case of temporary migration from a distinct geographical
province, seems to me satisfactory.
These several facts accord well with my theory. I believe in no fixed
law of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species is
quite independent of that of all others. Whether such variability be
taken advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or
lesser amount of modification in the varying species, depends on many
complex contingencies,—on the variability being of a beneficial nature,
on the power of intercrossing, on the rate of breeding, on the slowly
changing physical conditions of the country, and more especially on the
nature of the other inhabitants with which the varying species comes
into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably from
their nearest allies on the continent of Europe, whereas the marine
shells and birds have remained unaltered. We can perhaps understand the
apparently quicker rate of change in terrestrial and in more highly
organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When
many of the inhabitants of a country have become modified and improved,
we can understand, on the principle of competition, and on that of the
many all-important relations of organism to organism, that any form
which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species
in the same region do at last, if we look to wide enough intervals of
time, become modified; for those which do not change will become
extinct.
In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and
irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations is
not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at
hazard, in a slowly changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances) to
fill the exact place of another species in the economy of nature, and
thus supplant it; yet the two forms—the old and the new—would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors. For instance, it is just
possible, if our fantail-pigeons were all destroyed, that fanciers, by
striving during long ages for the same object, might make a new breed
hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason to
believe that the parent-form will generally be supplanted and
exterminated by its improved offspring, it is quite incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from the other well-established races
of the domestic pigeon, for the newly-formed fantail would be almost
sure to inherit from its new progenitor some slight characteristic
differences.
Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
species, changing more or less quickly, and in a greater or lesser
degree. A group does not reappear after it has once disappeared; or its
existence, as long as it lasts, is continuous. I am aware that there
are some apparent exceptions to this rule, but the exceptions are
surprisingly few, so few, that E. Forbes, Pictet, and Woodward (though
all strongly opposed to such views as I maintain) admit its truth; and
the rule strictly accords with my theory. For as all the species of the
same group have descended from some one species, it is clear that as
long as any species of the group have appeared in the long succession
of ages, so long must its members have continuously existed, in order
to have generated either new and modified or the same old and
unmodified forms. Species of the genus Lingula, for instance, must have
continuously existed by an unbroken succession of generations, from the
lowest Silurian stratum to the present day.
We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule being
a gradual increase in number, till the group reaches its maximum, and
then, sooner or later, it gradually decreases. If the
number of the species of a genus, or the number of the genera of a
family, be represented by a vertical line of varying thickness,
crossing the successive geological formations in which the species are
found, the line will sometimes falsely appear to begin at its lower
end, not in a sharp point, but abruptly; it then gradually thickens
upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the
species of a group is strictly conformable with my theory; as the
species of the same genus, and the genera of the same family, can
increase only slowly and progressively; for the process of modification
and the production of a number of allied forms must be slow and
gradual,—one species giving rise first to two or three varieties, these
being slowly converted into species, which in their turn produce by
equally slow steps other species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
_On Extinction_.—We have as yet spoken only incidentally of the
disappearance of species and of groups of species. On the theory of
natural selection the extinction of old forms and the production of new
and improved forms are intimately connected together. The old notion of
all the inhabitants of the earth having been swept away at successive
periods by catastrophes, is very generally given up, even by those
geologists, as Elie de Beaumont, Murchison, Barrande, etc., whose
general views would naturally lead them to this conclusion. On the
contrary, we have every reason to believe, from the study of the
tertiary formations, that species and groups of species gradually
disappear, one after another, first from one spot, then from another,
and
finally from the world. Both single species and whole groups of species
last for very unequal periods; some groups, as we have seen, having
endured from the earliest known dawn of life to the present day; some
having disappeared before the close of the palæozoic period. No fixed
law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that
the complete extinction of the species of a group is generally a slower
process than their production: if the appearance and disappearance of a
group of species be represented, as before, by a vertical line of
varying thickness, the line is found to taper more gradually at its
upper end, which marks the progress of extermination, than at its lower
end, which marks the first appearance and increase in numbers of the
species. In some cases, however, the extermination of whole groups of
beings, as of ammonites towards the close of the secondary period, has
been wonderfully sudden.
The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a
horse embedded with the remains of Mastodon, Megatherium, Toxodon, and
other extinct monsters, which all co-existed with still living shells
at a very late geological period, I was filled with astonishment; for
seeing that the horse, since its introduction by the Spaniards into
South America, has run wild over the whole country and has increased in
numbers at an unparalleled rate, I asked myself what could so recently
have exterminated the former horse under conditions of life apparently
so favourable. But
how utterly groundless was my astonishment! Professor Owen soon
perceived that the tooth, though so like that of the existing horse,
belonged to an extinct species. Had this horse been still living, but
in some degree rare, no naturalist would have felt the least surprise
at its rarity; for rarity is the attribute of a vast number of species
of all classes, in all countries. If we ask ourselves why this or that
species is rare, we answer that something is unfavourable in its
conditions of life; but what that something is, we can hardly ever
tell. On the supposition of the fossil horse still existing as a rare
species, we might have felt certain from the analogy of all other
mammals, even of the slow-breeding elephant, and from the history of
the naturalisation of the domestic horse in South America, that under
more favourable conditions it would in a very few years have stocked
the whole continent. But we could not have told what the unfavourable
conditions were which checked its increase, whether some one or several
contingencies, and at what period of the horse’s life, and in what
degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not have
perceived the fact, yet the fossil horse would certainly have become
rarer and rarer, and finally extinct;—its place being seized on by some
more successful competitor.
It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. We see in many cases in the
more recent tertiary formations, that rarity precedes extinction; and
we know that this has been the progress of events with those animals
which have
been exterminated, either locally or wholly, through man’s agency. I
may repeat what I published in 1845, namely, that to admit that species
generally become rare before they become extinct—to feel no surprise at
the rarity of a species, and yet to marvel greatly when it ceases to
exist, is much the same as to admit that sickness in the individual is
the forerunner of death—to feel no surprise at sickness, but when the
sick man dies, to wonder and to suspect that he died by some unknown
deed of violence.
The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition;
and the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the
less improved varieties in the same neighbourhood; when much improved
it is transported far and near, like our short-horn cattle, and takes
the place of other breeds in other countries. Thus the appearance of
new forms and the disappearance of old forms, both natural and
artificial, are bound together. In certain flourishing groups, the
number of new specific forms which have been produced within a given
time is probably greater than that of the old forms which have been
exterminated; but we know that the number of species has not gone on
indefinitely increasing, at least during the later geological periods,
so that looking to later times we may believe that the production of
new forms has caused the extinction of about the same number of old
forms.
The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects.
Hence the improved and modified descendants of a species will generally
cause the extermination of the parent-species; and if many new forms
have been developed from any one species, the nearest allies of that
species, _i.e._ the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a
new species belonging to some one group will have seized on the place
occupied by a species belonging to a distinct group, and thus caused
its extermination; and if many allied forms be developed from the
successful intruder, many will have to yield their places; and it will
generally be allied forms, which will suffer from some inherited
inferiority in common. But whether it be species belonging to the same
or to a distinct class, which yield their places to other species which
have been modified and improved, a few of the sufferers may often long
be preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a great
genus of shells in the secondary formations, survives in the Australian
seas; and a few members of the great and almost extinct group of Ganoid
fishes still inhabit our fresh waters. Therefore the utter extinction
of a group is generally, as we have seen, a slower process than its
production.
With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palæozoic period and of
Ammonites at the close of the secondary period, we must remember what
has been already said on the probable wide intervals of time
between our consecutive formations; and in these intervals there may
have been much slow extermination. Moreover, when by sudden immigration
or by unusually rapid development, many species of a new group have
taken possession of a new area, they will have exterminated in a
correspondingly rapid manner many of the old inhabitants; and the forms
which thus yield their places will commonly be allied, for they will
partake of some inferiority in common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of
natural selection. We need not marvel at extinction; if we must marvel,
let it be at our presumption in imagining for a moment that we
understand the many complex contingencies, on which the existence of
each species depends. If we forget for an instant, that each species
tends to increase inordinately, and that some check is always in
action, yet seldom perceived by us, the whole economy of nature will be
utterly obscured. Whenever we can precisely say why this species is
more abundant in individuals than that; why this species and not
another can be naturalised in a given country; then, and not till then,
we may justly feel surprise why we cannot account for the extinction of
this particular species or group of species.
_On the Forms of Life changing almost simultaneously throughout the
World_.—Scarcely any palæontological discovery is more striking than
the fact, that the forms of life change almost simultaneously
throughout the world. Thus our European Chalk formation can be
recognised in many distant parts of the world, under the most different
climates, where not a fragment of the mineral chalk itself can be
found; namely, in North
America, in equatorial South America, in Tierra del Fuego, at the Cape
of Good Hope, and in the peninsula of India. For at these distant
points, the organic remains in certain beds present an unmistakeable
degree of resemblance to those of the Chalk. It is not that the same
species are met with; for in some cases not one species is identically
the same, but they belong to the same families, genera, and sections of
genera, and sometimes are similarly characterised in such trifling
points as mere superficial sculpture. Moreover other forms, which are
not found in the Chalk of Europe, but which occur in the formations
either above or below, are similarly absent at these distant points of
the world. In the several successive palæozoic formations of Russia,
Western Europe and North America, a similar parallelism in the forms of
life has been observed by several authors: so it is, according to
Lyell, with the several European and North American tertiary deposits.
Even if the few fossil species which are common to the Old and New
Worlds be kept wholly out of view, the general parallelism in the
successive forms of life, in the stages of the widely separated
palæozoic and tertiary periods, would still be manifest, and the
several formations could be easily correlated.
These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge
whether the productions of the land and of fresh water change at
distant points in the same parallel manner. We may doubt whether they
have thus changed: if the Megatherium, Mylodon, Macrauchenia, and
Toxodon had been brought to Europe from La Plata, without any
information in regard to their geological position, no one would have
suspected that they had coexisted with still living sea-shells; but as
these anomalous monsters coexisted with the Mastodon
and Horse, it might at least have been inferred that they had lived
during one of the latter tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year,
or even that it has a very strict geological sense; for if all the
marine animals which live at the present day in Europe, and all those
that lived in Europe during the pleistocene period (an enormously
remote period as measured by years, including the whole glacial epoch),
were to be compared with those now living in South America or in
Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe resembled
most closely those of the southern hemisphere. So, again, several
highly competent observers believe that the existing productions of the
United States are more closely related to those which lived in Europe
during certain later tertiary stages, than to those which now live
here; and if this be so, it is evident that fossiliferous beds
deposited at the present day on the shores of North America would
hereafter be liable to be classed with somewhat older European beds.
Nevertheless, looking to a remotely future epoch, there can, I think,
be little doubt that all the more modern _marine_ formations, namely,
the upper pliocene, the pleistocene and strictly modern beds, of
Europe, North and South America, and Australia, from containing fossil
remains in some degree allied, and from not including those forms which
are only found in the older underlying deposits, would be correctly
ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously, in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM.
de Verneuil and d’Archiac. After referring to the parallelism of the
palæozoic forms of life in various parts of Europe, they add, “If
struck by this strange sequence, we turn our attention to North
America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their
extinction, and the introduction of new ones, cannot be owing to mere
changes in marine currents or other causes more or less local and
temporary, but depend on general laws which govern the whole animal
kingdom.” M. Barrande has made forcible remarks to precisely the same
effect. It is, indeed, quite futile to look to changes of currents,
climate, or other physical conditions, as the cause of these great
mutations in the forms of life throughout the world, under the most
different climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the
present distribution of organic beings, and find how slight is the
relation between the physical conditions of various countries, and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by new varieties arising, which have some
advantage over older forms; and those forms, which are already
dominant, or have some advantage over the other forms in their own
country, would naturally oftenest give rise to new varieties or
incipient species; for these latter must be victorious in a still
higher degree in order to be preserved and to survive. We have distinct
evidence on this head, in the plants which are dominant, that is, which
are commonest in their own homes, and are most widely diffused, having
produced the greatest number of new varieties. It is also natural that
the dominant,
varying, and far-spreading species, which already have invaded to a
certain extent the territories of other species, should be those which
would have the best chance of spreading still further, and of giving
rise in new countries to new varieties and species. The process of
diffusion may often be very slow, being dependent on climatal and
geographical changes, or on strange accidents, but in the long run the
dominant forms will generally succeed in spreading. The diffusion
would, it is probable, be slower with the terrestrial inhabitants of
distinct continents than with the marine inhabitants of the continuous
sea. We might therefore expect to find, as we apparently do find, a
less strict degree of parallel succession in the productions of the
land than of the sea.
Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of individuals,
from giving a better chance of the appearance of favourable variations,
and that severe competition with many already existing forms, would be
highly favourable, as would be the power of spreading into new
territories. A certain amount of isolation, recurring at long intervals
of time, would probably be also favourable, as before explained. One
quarter of the world may have been most favourable for the production
of new and dominant species on the land, and another for those in the
waters of the sea. If two great regions had been for a long period
favourably circumstanced in an equal degree, whenever their inhabitants
met, the battle would be prolonged and severe; and some from one
birthplace and some from the other might be victorious. But in the
course of time, the
forms dominant in the highest degree, wherever produced, would tend
everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these inferior forms
would be allied in groups by inheritance, whole groups would tend
slowly to disappear; though here and there a single member might long
be enabled to survive.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new species having been
formed by dominant species spreading widely and varying; the new
species thus produced being themselves dominant owing to inheritance,
and to having already had some advantage over their parents or over
other species; these again spreading, varying, and producing new
species. The forms which are beaten and which yield their places to the
new and victorious forms, will generally be allied in groups, from
inheriting some inferiority in common; and therefore as new and
improved groups spread throughout the world, old groups will disappear
from the world; and the succession of forms in both ways will
everywhere tend to correspond.
There is one other remark connected with this subject worth making. I
have given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of
the sea was either stationary or rising, and likewise when sediment was
not thrown down quickly enough to embed and preserve organic remains.
During these long and blank intervals I suppose that the inhabitants of
each region underwent a considerable amount of modification and
extinction, and that there was much migration from
other parts of the world. As we have reason to believe that large areas
are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over very wide
spaces in the same quarter of the world; but we are far from having any
right to conclude that this has invariably been the case, and that
large areas have invariably been affected by the same movements. When
two formations have been deposited in two regions during nearly, but
not exactly the same period, we should find in both, from the causes
explained in the foregoing paragraphs, the same general succession in
the forms of life; but the species would not exactly correspond; for
there will have been a little more time in the one region than in the
other for modification, extinction, and immigration.
I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England
and France, is able to draw a close general parallelism between the
successive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult to
account for, considering the proximity of the two areas,—unless,
indeed, it be assumed that an isthmus separated two seas inhabited by
distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
surprising amount of difference in the species. If the several
formations in these regions have not been deposited during the same
exact
periods,—a formation in one region often corresponding with a blank
interval in the other,—and if in both regions the species have gone on
slowly changing during the accumulation of the several formations and
during the long intervals of time between them; in this case, the
several formations in the two regions could be arranged in the same
order, in accordance with the general succession of the form of life,
and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.
_On the Affinities of extinct Species to each other, and to living
forms_.—Let us now look to the mutual affinities of extinct and living
species. They all fall into one grand natural system; and this fact is
at once explained on the principle of descent. The more ancient any
form is, the more, as a general rule, it differs from living forms.
But, as Buckland long ago remarked, all fossils can be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up the wide intervals between existing genera,
families, and orders, cannot be disputed. For if we confine our
attention either to the living or to the extinct alone, the series is
far less perfect than if we combine both into one general system. With
respect to the Vertebrata, whole pages could be filled with striking
illustrations from our great palæontologist, Owen, showing how extinct
animals fall in between existing groups. Cuvier ranked the Ruminants
and Pachyderms, as the two most distinct orders of mammals; but Owen
has discovered so many fossil links, that he has had to alter the whole
classification of these two orders; and has placed certain pachyderms
in the same sub-order with ruminants: for example, he dissolves by fine
gradations the apparently
wide difference between the pig and the camel. In regard to the
Invertebrata, Barrande, and a higher authority could not be named,
asserts that he is every day taught that palæozoic animals, though
belonging to the same orders, families, or genera with those living at
the present day, were not at this early epoch limited in such distinct
groups as they now are.
Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If
by this term it is meant that an extinct form is directly intermediate
in all its characters between two living forms, the objection is
probably valid. But I apprehend that in a perfectly natural
classification many fossil species would have to stand between living
species, and some extinct genera between living genera, even between
genera belonging to distinct families. The most common case, especially
with respect to very distinct groups, such as fish and reptiles, seems
to be, that supposing them to be distinguished at the present day from
each other by a dozen characters, the ancient members of the same two
groups would be distinguished by a somewhat lesser number of
characters, so that the two groups, though formerly quite distinct, at
that period made some small approach to each other.
It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted to
those groups which have undergone much change in the course of
geological ages; and it would be difficult to prove the truth of the
proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and
Batrachians, the older Fish, the older Cephalopods, and the eocene
Mammals, with the more recent members of the same classes, we must
admit that there is some truth in the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the fourth
chapter. We may suppose that the numbered letters represent genera, and
the dotted lines diverging from them the species in each genus. The
diagram is much too simple, too few genera and too few species being
given, but this is unimportant for us. The horizontal lines may
represent successive geological formations, and all the forms beneath
the uppermost line may be considered as extinct. The three existing
genera, _a_14, _q_14, _p_14, will form a small family; _b_14 and _f_14
a closely allied family or sub-family; and _o_14, _e_14, _m_14, a third
family. These three families, together with the many extinct genera on
the several lines of descent diverging from the parent-form A, will
form an order; for all will have inherited something in common from
their ancient and common progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by
this diagram, the more recent any form is, the more it will generally
differ from its ancient progenitor. Hence we can understand the rule
that the most ancient fossils differ most from existing forms. We must
not, however, assume that divergence of character is a necessary
contingency; it depends solely on the descendants from a species being
thus enabled to seize on many and different places in the economy of
nature. Therefore it is quite possible, as we have seen in the case of
some Silurian forms, that a species might go on being slightly
modified in relation to its slightly altered conditions of life, and
yet retain throughout a vast period the same general characteristics.
This is represented in the diagram by the letter F14.
All the many forms, extinct and recent, descended from A, make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present
day.
By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered
at several points low down in the series, the three existing families
on the uppermost line would be rendered less distinct from each other.
If, for instance, the genera _a_1, _a_5, _a_10, _f_8, _m_3, _m_6, _m_9
were disinterred, these three families would be so closely linked
together that they probably would have to be united into one great
family, in nearly the same manner as has occurred with ruminants and
pachyderms. Yet he who objected to call the extinct genera, which thus
linked the living genera of three families together, intermediate in
character, would be justified, as they are intermediate, not directly,
but only by a long and circuitous course through many widely different
forms. If many extinct forms were to be discovered above one of the
middle horizontal lines or geological formations—for instance, above
Number VI.—but none from beneath this line, then only the two families
on the left hand (namely, _a_14, etc., and _b_14, etc.) would have to
be united into one family; and the two other families (namely, _a_14 to
_f_14 now including five genera, and _o_14 to _m_14) would yet remain
distinct. These two families, however, would be less distinct from each
other than they were before the
discovery of the fossils. If, for instance, we suppose the existing
genera of the two families to differ from each other by a dozen
characters, in this case the genera, at the early period marked VI.,
would differ by a lesser number of characters; for at this early stage
of descent they have not diverged in character from the common
progenitor of the order, nearly so much as they subsequently diverged.
Thus it comes that ancient and extinct genera are often in some slight
degree intermediate in character between their modified descendants, or
between their collateral relations.
In nature the case will be far more complicated than is represented in
the diagram; for the groups will have been more numerous, they will
have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no
right to expect, except in very rare cases, to fill up wide intervals
in the natural system, and thus unite distinct families or orders. All
that we have a right to expect, is that those groups, which have within
known geological periods undergone much modification, should in the
older formations make some slight approach to each other; so that the
older members should differ less from each other in some of their
characters than do the existing members of the same groups; and this by
the concurrent evidence of our best palæontologists seems frequently to
be the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.
On this same theory, it is evident that the fauna of any great period
in the earth’s history will be intermediate
in general character between that which preceded and that which
succeeded it. Thus, the species which lived at the sixth great stage of
descent in the diagram are the modified offspring of those which lived
at the fifth stage, and are the parents of those which became still
more modified at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life above and
below. We must, however, allow for the entire extinction of some
preceding forms, and for the coming in of quite new forms by
immigration, and for a large amount of modification, during the long
and blank intervals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubtedly is
intermediate in character, between the preceding and succeeding faunas.
I need give only one instance, namely, the manner in which the fossils
of the Devonian system, when this system was first discovered, were at
once recognised by palæontologists as intermediate in character between
those of the overlying carboniferous, and underlying Silurian system.
But each fauna is not necessarily exactly intermediate, as unequal
intervals of time have elapsed between consecutive formations.
It is no real objection to the truth of the statement, that the fauna
of each period as a whole is nearly intermediate in character between
the preceding and succeeding faunas, that certain genera offer
exceptions to the rule. For instance, mastodons and elephants, when
arranged by Dr. Falconer in two series, first according to their mutual
affinities and then according to their periods of existence, do not
accord in arrangement. The species extreme in character are not the
oldest, or the most recent; nor are those which are intermediate in
character, intermediate in age. But
supposing for an instant, in this and other such cases, that the record
of the first appearance and disappearance of the species was perfect,
we have no reason to believe that forms successively produced
necessarily endure for corresponding lengths of time: a very ancient
form might occasionally last much longer than a form elsewhere
subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things
with great: if the principal living and extinct races of the domestic
pigeon were arranged as well as they could be in serial affinity, this
arrangement would not closely accord with the order in time of their
production, and still less with the order of their disappearance; for
the parent rock-pigeon now lives; and many varieties between the
rock-pigeon and the carrier have become extinct; and carriers which are
extreme in the important character of length of beak originated earlier
than short-beaked tumblers, which are at the opposite end of the series
in this same respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is
the fact, insisted on by all palæontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a
well-known instance, the general resemblance of the organic remains
from the several stages of the chalk formation, though the species are
distinct in each stage. This fact alone, from its generality, seems to
have shaken Professor Pictet in his firm belief in the immutability of
species. He who is acquainted with the distribution of existing species
over the globe, will not attempt to account for the close resemblance
of the distinct species in closely consecutive
formations, by the physical conditions of the ancient areas having
remained nearly the same. Let it be remembered that the forms of life,
at least those inhabiting the sea, have changed almost simultaneously
throughout the world, and therefore under the most different climates
and conditions. Consider the prodigious vicissitudes of climate during
the pleistocene period, which includes the whole glacial period, and
note how little the specific forms of the inhabitants of the sea have
been affected.
On the theory of descent, the full meaning of the fact of fossil
remains from closely consecutive formations, though ranked as distinct
species, being closely related, is obvious. As the accumulation of each
formation has often been interrupted, and as long blank intervals have
intervened between successive formations, we ought not to expect to
find, as I attempted to show in the last chapter, in any one or two
formations all the intermediate varieties between the species which
appeared at the commencement and close of these periods; but we ought
to find after intervals, very long as measured by years, but only
moderately long as measured geologically, closely allied forms, or, as
they have been called by some authors, representative species; and
these we assuredly do find. We find, in short, such evidence of the
slow and scarcely sensible mutation of specific forms, as we have a
just right to expect to find.
_On the state of Development of Ancient Forms_.—There has been much
discussion whether recent forms are more highly developed than ancient.
I will not here enter on this subject, for naturalists have not as yet
defined to each other’s satisfaction what is meant by high and low
forms. But in one particular sense the
more recent forms must, on my theory, be higher than the more ancient;
for each new species is formed by having had some advantage in the
struggle for life over other and preceding forms. If under a nearly
similar climate, the eocene inhabitants of one quarter of the world
were put into competition with the existing inhabitants of the same or
some other quarter, the eocene fauna or flora would certainly be beaten
and exterminated; as would a secondary fauna by an eocene, and a
palæozoic fauna by a secondary fauna. I do not doubt that this process
of improvement has affected in a marked and sensible manner the
organisation of the more recent and victorious forms of life, in
comparison with the ancient and beaten forms; but I can see no way of
testing this sort of progress. Crustaceans, for instance, not the
highest in their own class, may have beaten the highest molluscs. From
the extraordinary manner in which European productions have recently
spread over New Zealand, and have seized on places which must have been
previously occupied, we may believe, if all the animals and plants of
Great Britain were set free in New Zealand, that in the course of time
a multitude of British forms would become thoroughly naturalized there,
and would exterminate many of the natives. On the other hand, from what
we see now occurring in New Zealand, and from hardly a single
inhabitant of the southern hemisphere having become wild in any part of
Europe, we may doubt, if all the productions of New Zealand were set
free in Great Britain, whether any considerable number would be enabled
to seize on places now occupied by our native plants and animals. Under
this point of view, the productions of Great Britain may be said to be
higher than those of New Zealand. Yet the most skilful naturalist from
an examination of the species
of the two countries could not have foreseen this result.
Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and
Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it hereafter confirmed, at least in
regard to subordinate groups, which have branched off from each other
within comparatively recent times. For this doctrine of Agassiz accords
well with the theory of natural selection. In a future chapter I shall
attempt to show that the adult differs from its embryo, owing to
variations supervening at a not early age, and being inherited at a
corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.
Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal. This
view may be true, and yet it may never be capable of full proof.
Seeing, for instance, that the oldest known mammals, reptiles, and fish
strictly belong to their own proper classes, though some of these old
forms are in a slight degree less distinct from each other than are the
typical members of the same groups at the present day, it would be vain
to look for animals having the common embryological character of the
Vertebrata, until beds far beneath the lowest Silurian strata are
discovered—a discovery of which the chance is very small.
_On the Succession of the same Types within the same
areas, during the later tertiary periods_.—Mr. Clift many years ago
showed that the fossil mammals from the Australian caves were closely
allied to the living marsupials of that continent. In South America, a
similar relationship is manifest, even to an uneducated eye, in the
gigantic pieces of armour like those of the armadillo, found in several
parts of La Plata; and Professor Owen has shown in the most striking
manner that most of the fossil mammals, buried there in such numbers,
are related to South American types. This relationship is even more
clearly seen in the wonderful collection of fossil bones made by MM.
Lund and Clausen in the caves of Brazil. I was so much impressed with
these facts that I strongly insisted, in 1839 and 1845, on this “law of
the succession of types,”—on “this wonderful relationship in the same
continent between the dead and the living.” Professor Owen has
subsequently extended the same generalisation to the mammals of the Old
World. We see the same law in this author’s restorations of the extinct
and gigantic birds of New Zealand. We see it also in the birds of the
caves of Brazil. Mr. Woodward has shown that the same law holds good
with sea-shells, but from the wide distribution of most genera of
molluscs, it is not well displayed by them. Other cases could be added,
as the relation between the extinct and living land-shells of Madeira;
and between the extinct and living brackish-water shells of the
Aralo-Caspian Sea.
Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing
the present climate of Australia and of parts of South America under
the same latitude, would attempt to account, on the one hand, by
dissimilar physical conditions for the dissimilarity of the inhabitants
of these two continents,
and, on the other hand, by similarity of conditions, for the uniformity
of the same types in each during the later tertiary periods. Nor can it
be pretended that it is an immutable law that marsupials should have
been chiefly or solely produced in Australia; or that Edentata and
other American types should have been solely produced in South America.
For we know that Europe in ancient times was peopled by numerous
marsupials; and I have shown in the publications above alluded to, that
in America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly
of the present character of the southern half of the continent; and the
southern half was formerly more closely allied, than it is at present,
to the northern half. In a similar manner we know from Falconer and
Cautley’s discoveries, that northern India was formerly more closely
related in its mammals to Africa than it is at the present time.
Analogous facts could be given in relation to the distribution of
marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter
of the world will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though in some degree
modified descendants. If the inhabitants of one continent formerly
differed greatly from those of another continent, so will their
modified descendants still differ in nearly the same manner and degree.
But after very long intervals of time and after great geographical
changes, permitting much inter-migration, the feebler will yield to the
more dominant forms, and there will be nothing immutable in the laws of
past and present distribution.
It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. These huge animals have become
wholly extinct, and have left no progeny. But in the caves of Brazil,
there are many extinct species which are closely allied in size and in
other characters to the species still living in South America; and some
of these fossils may be the actual progenitors of living species. It
must not be forgotten that, on my theory, all the species of the same
genus have descended from some one species; so that if six genera, each
having eight species, be found in one geological formation, and in the
next succeeding formation there be six other allied or representative
genera with the same number of species, then we may conclude that only
one species of each of the six older genera has left modified
descendants, constituting the six new genera. The other seven species
of the old genera have all died out and have left no progeny. Or, which
would probably be a far commoner case, two or three species of two or
three alone of the six older genera will have been the parents of the
six new genera; the other old species and the other whole genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of
South America, still fewer genera and species will have left modified
blood-descendants.
_Summary of the preceding and present Chapters_.—I have attempted to
show that the geological record is extremely imperfect; that only a
small portion of the globe has been geologically explored with care;
that only
certain classes of organic beings have been largely preserved in a
fossil state; that the number both of specimens and of species,
preserved in our museums, is absolutely as nothing compared with the
incalculable number of generations which must have passed away even
during a single formation; that, owing to subsidence being necessary
for the accumulation of fossiliferous deposits thick enough to resist
future degradation, enormous intervals of time have elapsed between the
successive formations; that there has probably been more extinction
during the periods of subsidence, and more variation during the periods
of elevation, and during the latter the record will have been least
perfectly kept; that each single formation has not been continuously
deposited; that the duration of each formation is, perhaps, short
compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in
any one area and formation; that widely ranging species are those which
have varied most, and have oftenest given rise to new species; and that
varieties have at first often been local. All these causes taken
conjointly, must have tended to make the geological record extremely
imperfect, and will to a large extent explain why we do not find
interminable varieties, connecting together all the extinct and
existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record, will
rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the several stages
of the same great formation. He may disbelieve in the enormous
intervals of time which have elapsed between our consecutive
formations; he
may overlook how important a part migration must have played, when the
formations of any one great region alone, as that of Europe, are
considered; he may urge the apparent, but often falsely apparent,
sudden coming in of whole groups of species. He may ask where are the
remains of those infinitely numerous organisms which must have existed
long before the first bed of the Silurian system was deposited: I can
answer this latter question only hypothetically, by saying that as far
as we can see, where our oceans now extend they have for an enormous
period extended, and where our oscillating continents now stand they
have stood ever since the Silurian epoch; but that long before that
period, the world may have presented a wholly different aspect; and
that the older continents, formed of formations older than any known to
us, may now all be in a metamorphosed condition, or may lie buried
under the ocean.
Passing from these difficulties, all the other great leading facts in
palæontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it
is that new species come in slowly and successively; how species of
different classes do not necessarily change together, or at the same
rate, or in the same degree; yet in the long run that all undergo
modification to some extent. The extinction of old forms is the almost
inevitable consequence of the production of new forms. We can
understand why when a species has once disappeared it never reappears.
Groups of species increase in numbers slowly, and endure for unequal
periods of time; for the process of modification is necessarily slow,
and depends on many complex contingencies. The dominant species of the
larger dominant groups tend to leave many modified
descendants, and thus new sub-groups and groups are formed. As these
are formed, the species of the less vigorous groups, from their
inferiority inherited from a common progenitor, tend to become extinct
together, and to leave no modified offspring on the face of the earth.
But the utter extinction of a whole group of species may often be a
very slow process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has been
broken.
We can understand how the spreading of the dominant forms of life,
which are those that oftenest vary, will in the long run tend to people
the world with allied, but modified, descendants; and these will
generally succeed in taking the places of those groups of species which
are their inferiors in the struggle for existence. Hence, after long
intervals of time, the productions of the world will appear to have
changed simultaneously.
We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to
divergence of character, why the more ancient a form is, the more it
generally differs from those now living. Why ancient and extinct forms
often tend to fill up gaps between existing forms, sometimes blending
two groups previously classed as distinct into one; but more commonly
only bringing them a little closer together. The more ancient a form
is, the more often, apparently, it displays characters in some degree
intermediate between groups now distinct; for the more ancient a form
is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become
widely divergent. Extinct forms are seldom directly intermediate
between existing forms; but are intermediate only by a long and
circuitous course through many extinct and very different forms. We can
clearly see why the organic remains of closely consecutive formations
are more closely allied to each other, than are those of remote
formations; for the forms are more closely linked together by
generation: we can clearly see why the remains of an intermediate
formation are intermediate in character.
The inhabitants of each successive period in the world’s history have
beaten their predecessors in the race for life, and are, in so far,
higher in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palæontologists, that organisation
on the whole has progressed. If it should hereafter be proved that
ancient animals resemble to a certain extent the embryos of more recent
animals of the same class, the fact will be intelligible. The
succession of the same types of structure within the same areas during
the later geological periods ceases to be mysterious, and is simply
explained by inheritance.
If then the geological record be as imperfect as I believe it to be,
and it may at least be asserted that the record cannot be proved to be
much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand, all
the chief laws of palæontology plainly proclaim, as it seems to me,
that species have been produced by ordinary generation: old forms
having been supplanted by new and improved forms of life, produced by
the laws of variation still acting round us, and preserved by Natural
Selection.
CHAPTER XI.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions. Importance of barriers. Affinity of the productions of the
same continent. Centres of creation. Means of dispersal, by changes of
climate and of the level of the land, and by occasional means.
Dispersal during the Glacial period co-extensive with the world.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be accounted for by their climatal and other physical conditions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to
prove its truth: for if we exclude the northern parts where the
circumpolar land is almost continuous, all authors agree that one of
the most fundamental divisions in geographical distribution is that
between the New and Old Worlds; yet if we travel over the vast American
continent, from the central parts of the United States to its extreme
southern point, we meet with the most diversified conditions; the most
humid districts, arid deserts, lofty mountains, grassy plains, forests,
marshes, lakes, and great rivers, under almost every temperature. There
is hardly a climate or condition in the Old World which cannot be
paralleled in the New—at least as closely as the same species generally
require; for it is a most rare case to find a group of organisms
confined to any small spot, having conditions peculiar in only a slight
degree; for instance, small areas in the Old World could be pointed out
hotter than any in the New World, yet these are not inhabited by a
peculiar fauna or flora. Notwithstanding this parallelism in the
conditions of the Old and New Worlds, how widely different are their
living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25° and 35°, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the productions
of South America south of lat. 35° with those north of 25°, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to
the productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the
sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions
of various regions. We see this in the great difference of nearly all
the terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a
slightly different climate, there might have been free migration for
the northern temperate forms, as there now is for the strictly arctic
productions. We see the same fact in the great difference between the
inhabitants of Australia, Africa, and South America under the same
latitude: for these countries are almost as much isolated from each
other as is possible. On each continent, also, we see the same fact;
for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts, and
sometimes even of large rivers, we find different productions; though
as mountain chains, deserts, etc., are not as impassable, or likely to
have endured so long as the oceans separating continents, the
differences are very inferior in degree to those characteristic of
distinct continents.
Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of
the eastern and western shores of South and Central America; yet these
great faunas are separated only by the narrow, but impassable, isthmus
of Panama. Westward of the shores of America, a wide space of open
ocean extends, with not an island as a halting-place for emigrants;
here we have a barrier of another kind, and as soon as this is passed
we meet in the eastern islands of the Pacific, with another and totally
distinct fauna. So that here three marine faunas range far northward
and southward, in parallel lines not far from each other, under
corresponding climates; but from being separated from each other by
impassable barriers, either of land or open sea, they are wholly
distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places,
until after travelling over a hemisphere we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although hardly one shell, crab or fish is
common to the above-named three approximate faunas of Eastern and
Western America and the eastern Pacific islands, yet many fish range
from the Pacific into the Indian Ocean, and many shells are common to
the eastern islands of the Pacific
and the eastern shores of Africa, on almost exactly opposite meridians
of longitude.
A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is
a law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance,
from north to south never fails to be struck by the manner in which
successive groups of beings, specifically distinct, yet clearly
related, replace each other. He hears from closely allied, yet distinct
kinds of birds, notes nearly similar, and sees their nests similarly
constructed, but not quite alike, with eggs coloured in nearly the same
manner. The plains near the Straits of Magellan are inhabited by one
species of Rhea (American ostrich), and northward the plains of La
Plata by another species of the same genus; and not by a true ostrich
or emeu, like those found in Africa and Australia under the same
latitude. On these same plains of La Plata, we see the agouti and
bizcacha, animals having nearly the same habits as our hares and
rabbits and belonging to the same order of Rodents, but they plainly
display an American type of structure. We ascend the lofty peaks of the
Cordillera and we find an alpine species of bizcacha; we look to the
waters, and we do not find the beaver or musk-rat, but the coypu and
capybara, rodents of the American type. Innumerable other instances
could be given. If we look to the islands off the American shore,
however much they may differ in geological structure, the inhabitants,
though they may be all peculiar species, are essentially American. We
may look back to past ages, as shown in the last chapter, and we find
American types then prevalent on
the American continent and in the American seas. We see in these facts
some deep organic bond, prevailing throughout space and time, over the
same areas of land and water, and independent of their physical
conditions. The naturalist must feel little curiosity, who is not led
to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone,
as far as we positively know, produces organisms quite like, or, as we
see in the case of varieties nearly like each other. The dissimilarity
of the inhabitants of different regions may be attributed to
modification through natural selection, and in a quite subordinate
degree to the direct influence of different physical conditions. The
degree of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been
effected with more or less ease, at periods more or less remote;—on the
nature and number of the former immigrants;—and on their action and
reaction, in their mutual struggles for life;—the relation of organism
to organism being, as I have already often remarked, the most important
of all relations. Thus the high importance of barriers comes into play
by checking migration; as does time for the slow process of
modification through natural selection. Widely-ranging species,
abounding in individuals, which have already triumphed over many
competitors in their own widely-extended homes will have the best
chance of seizing on new places, when they spread into new countries.
In their new homes they will be exposed to new conditions, and will
frequently undergo further modification and improvement; and thus they
will become still further victorious, and will produce groups of
modified descendants. On this principle of inheritance with
modification, we can understand how it is that sections of genera,
whole genera,
and even families are confined to the same areas, as is so commonly and
notoriously the case.
I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so
far as it profits the individual in its complex struggle for life, so
the degree of modification in different species will be no uniform
quantity. If, for instance, a number of species, which stand in direct
competition with each other, migrate in a body into a new and
afterwards isolated country, they will be little liable to
modification; for neither migration nor isolation in themselves can do
anything. These principles come into play only by bringing organisms
into new relations with each other, and in a lesser degree with the
surrounding physical conditions. As we have seen in the last chapter
that some forms have retained nearly the same character from an
enormously remote geological period, so certain species have migrated
over vast spaces, and have not become greatly modified.
On these views, it is obvious, that the several species of the same
genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they have descended
from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification,
there is not much difficulty in believing that they may have migrated
from the same region; for during the vast geographical and climatal
changes which will have supervened since ancient times, almost any
amount of migration is possible. But in many other cases, in which we
have reason to believe that the species of a genus have been produced
within comparatively recent times, there is great difficulty on this
head. It
is also obvious that the individuals of the same species, though now
inhabiting distant and isolated regions, must have proceeded from one
spot, where their parents were first produced: for, as explained in the
last chapter, it is incredible that individuals identically the same
should ever have been produced through natural selection from parents
specifically distinct.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth’s surface. Undoubtedly there are very many cases of
extreme difficulty, in understanding how the same species could
possibly have migrated from some one point to the several distant and
isolated points, where now found. Nevertheless the simplicity of the
view that each species was first produced within a single region
captivates the mind. He who rejects it, rejects the _vera causa_ of
ordinary generation with subsequent migration, and calls in the agency
of a miracle. It is universally admitted, that in most cases the area
inhabited by a species is continuous; and when a plant or animal
inhabits two points so distant from each other, or with an interval of
such a nature, that the space could not be easily passed over by
migration, the fact is given as something remarkable and exceptional.
The capacity of migrating across the sea is more distinctly limited in
terrestrial mammals, than perhaps in any other organic beings; and,
accordingly, we find no inexplicable cases of the same mammal
inhabiting distant points of the world. No geologist will feel any
difficulty in such cases as Great Britain having been formerly united
to Europe, and consequently possessing the same quadrupeds. But if the
same species can be produced at two separate points, why do we not find
a single mammal common to Europe and Australia or South America? The
conditions of life are
nearly the same, so that a multitude of European animals and plants
have become naturalised in America and Australia; and some of the
aboriginal plants are identically the same at these distant points of
the northern and southern hemispheres? The answer, as I believe, is,
that mammals have not been able to migrate, whereas some plants, from
their varied means of dispersal, have migrated across the vast and
broken interspace. The great and striking influence which barriers of
every kind have had on distribution, is intelligible only on the view
that the great majority of species have been produced on one side
alone, and have not been able to migrate to the other side. Some few
families, many sub-families, very many genera, and a still greater
number of sections of genera are confined to a single region; and it
has been observed by several naturalists, that the most natural genera,
or those genera in which the species are most closely related to each
other, are generally local, or confined to one area. What a strange
anomaly it would be, if, when coming one step lower in the series, to
the individuals of the same species, a directly opposite rule
prevailed; and species were not local, but had been produced in two or
more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration
and subsistence under past and present conditions permitted, is the
most probable. Undoubtedly many cases occur, in which we cannot explain
how the same species could have passed from one point to the other. But
the geographical and climatal changes, which have certainly occurred
within recent geological times, must have interrupted or rendered
discontinuous the formerly continuous range of many species. So that we
are reduced to consider whether the exceptions to
continuity of range are so numerous and of so grave a nature, that we
ought to give up the belief, rendered probable by general
considerations, that each species has been produced within one area,
and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend
that any explanation could be offered of many such cases. But after
some preliminary remarks, I will discuss a few of the most striking
classes of facts; namely, the existence of the same species on the
summits of distant mountain-ranges, and at distant points in the arctic
and antarctic regions; and secondly (in the following chapter), the
wide distribution of freshwater productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the
earth’s surface, can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatal and geographical changes and
various occasional means of transport, the belief that this has been
the universal law, seems to me incomparably the safest.
In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from
a common progenitor, can have migrated (undergoing modification during
some part of their migration) from the area inhabited by their
progenitor. If it can be shown to be almost invariably the case, that a
region, of which most of its inhabitants are closely related to, or
belong to the same genera with the species of a second region,
has probably received at some former period immigrants from this other
region, my theory will be strengthened; for we can clearly understand,
on the principle of modification, why the inhabitants of a region
should be related to those of another region, whence it has been
stocked. A volcanic island, for instance, upheaved and formed at the
distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be plainly related by
inheritance to the inhabitants of the continent. Cases of this nature
are common, and are, as we shall hereafter more fully see, inexplicable
on the theory of independent creation. This view of the relation of
species in one region to those in another, does not differ much (by
substituting the word variety for species) from that lately advanced in
an ingenious paper by Mr. Wallace, in which he concludes, that “every
species has come into existence coincident both in space and time with
a pre-existing closely allied species.” And I now know from
correspondence, that this coincidence he attributes to generation with
modification.
The previous remarks on “single and multiple centres of creation” do
not directly bear on another allied question,—namely whether all the
individuals of the same species have descended from
a single pair, or single hermaphrodite, or whether, as some authors
suppose, from many individuals simultaneously created. With those
organic beings which never intercross (if such exist), the species, on
my theory, must have descended from a succession of improved varieties,
which will never have blended with other individuals or varieties, but
will have supplanted each other; so that, at each successive stage of
modification and improvement, all the individuals of each variety will
have descended from a single parent. But in the majority of cases,
namely, with all organisms which habitually unite for each birth, or
which often intercross, I believe that during the slow process of
modification the individuals of the species will have been kept nearly
uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not
have been due, at each stage, to descent from a single parent. To
illustrate what I mean: our English racehorses differ slightly from the
horses of every other breed; but they do not owe their difference and
superiority to descent from any single pair, but to continued care in
selecting and training many individuals during many generations.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of “single
centres of creation,” I must say a few words on the means of dispersal.
_Means of Dispersal_.—Sir C. Lyell and other authors have ably treated
this subject. I can give here only the briefest abstract of the more
important facts. Change of climate must have had a powerful influence
on migration: a region when its climate was different may have been a
high road for migration, but now be impassable; I shall, however,
presently have to discuss this branch of the subject in some detail.
Changes of level in the land must also have been highly influential: a
narrow isthmus now separates two marine faunas; submerge it, or let it
formerly have been submerged, and the two faunas will now blend or may
formerly have blended: where the sea now extends, land may at a former
period have connected islands or possibly even continents together, and
thus have allowed terrestrial productions to pass from one to the
other.
No geologist will dispute that great mutations of level have occurred
within the period of existing organisms. Edward Forbes insisted that
all the islands in the Atlantic must recently have been connected with
Europe or Africa, and Europe likewise with America. Other authors have
thus hypothetically bridged over every ocean, and have united almost
every island to some mainland. If indeed the arguments used by Forbes
are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty: but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we
have abundant evidence of great oscillations of level in our
continents; but not of such vast changes in their position and
extension, as to have united them within the recent period to each
other and to the several intervening oceanic islands. I freely admit
the former existence of many islands, now buried beneath the sea, which
may have served as halting places for plants and for many animals
during their migration. In the coral-producing oceans such sunken
islands are now marked, as I believe, by rings of coral or atolls
standing over them. Whenever it is fully admitted, as I believe it will
some day be, that each species has proceeded from a single birthplace,
and when in the course of time we know something definite about the
means of distribution, we shall be enabled to speculate with security
on the former extension of the land. But I do not believe that it will
ever be proved that within the recent period continents which are now
quite separate, have been continuously, or almost continuously, united
with each other, and with the many existing oceanic islands. Several
facts in distribution,—such as the great difference in the marine
faunas on the opposite sides of almost every continent,—the close
relation of the tertiary inhabitants of several lands and even seas to
their present inhabitants,—a certain degree of relation (as we shall
hereafter see) between the distribution of mammals and the depth of the
sea,—these and other such facts seem to me opposed to the admission of
such prodigious geographical revolutions within the recent period, as
are necessitated on the view advanced by Forbes and admitted by his
many followers. The nature and relative proportions of the inhabitants
of oceanic islands likewise seem to me opposed to the belief of their
former continuity with continents. Nor does their almost universally
volcanic composition favour the admission that they are the wrecks of
sunken continents;—if they had originally existed as mountain-ranges on
the land, some at least of the islands would have been formed, like
other mountain-summits, of granite, metamorphic schists, old
fossiliferous or other such rocks, instead of consisting of mere piles
of volcanic matter.
I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution. I
shall here confine myself to plants. In botanical works, this or that
plant is stated to be ill adapted for wide dissemination; but for
transport across the sea, the greater or less facilities may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley’s aid, a few
experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of 87
kinds, 64 germinated after an immersion of 28 days, and a few survived
an immersion of 137 days.
For convenience sake I chiefly tried small seeds, without the capsule
or fruit; and as all of these sank in a few days, they could not be
floated across wide spaces of the sea, whether or not they were injured
by the salt-water. Afterwards I tried some larger fruits, capsules,
etc., and some of these floated for a long time. It is well known what
a difference there is in the buoyancy of green and seasoned timber; and
it occurred to me that floods might wash down plants or branches, and
that these might be dried on the banks, and then by a fresh rise in the
stream be washed into the sea. Hence I was led to dry stems and
branches of 94 plants with ripe fruit, and to place them on sea water.
The majority sank quickly, but some which whilst green floated for a
very short time, when dried floated much longer; for instance, ripe
hazel-nuts sank immediately, but when dried, they floated for 90 days
and afterwards when planted they germinated; an asparagus plant with
ripe berries floated for 23 days, when dried it floated for 85 days,
and the seeds afterwards germinated: the ripe seeds of Helosciadium
sank in two days, when dried they floated for above 90 days, and
afterwards germinated. Altogether out of the 94 dried plants, 18
floated for above 28 days, and some of the 18 floated for a very much
longer period. So that as 64/87 seeds germinated after an immersion of
28 days; and as 18/94 plants with ripe fruit (but not all the same
species as in the foregoing experiment) floated, after being dried, for
above 28 days, as far as we may infer anything from these scanty facts,
we may conclude that the seeds of 14/100 plants of any country might be
floated by sea-currents during 28 days, and would retain their power of
germination. In Johnston’s Physical Atlas, the average rate of the
several Atlantic currents is 33 miles per diem (some currents running
at the rate of 60 miles
per diem); on this average, the seeds of 14/100 plants belonging to one
country might be floated across 924 miles of sea to another country;
and when stranded, if blown to a favourable spot by an inland gale,
they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea,
so that they were alternately wet and exposed to the air like really
floating plants. He tried 98 seeds, mostly different from mine; but he
chose many large fruits and likewise seeds from plants which live near
the sea; and this would have favoured the average length of their
flotation and of their resistance to the injurious action of the
salt-water. On the other hand he did not previously dry the plants or
branches with the fruit; and this, as we have seen, would have caused
some of them to have floated much longer. The result was that 18/98 of
his seeds floated for 42 days, and were then capable of germination.
But I do not doubt that plants exposed to the waves would float for a
less time than those protected from violent movement as in our
experiments. Therefore it would perhaps be safer to assume that the
seeds of about 10/100 plants of a flora, after having been dried, could
be floated across a space of sea 900 miles in width, and would then
germinate. The fact of the larger fruits often floating longer than the
small, is interesting; as plants with large seeds or fruit could hardly
be transported by any other means; and Alph. de Candolle has shown that
such plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral-islands in the Pacific,
procure
stones for their tools, solely from the roots of drifted trees, these
stones being a valuable royal tax. I find on examination, that when
irregularly shaped stones are embedded in the roots of trees, small
parcels of earth are very frequently enclosed in their interstices and
behind them,—so perfectly that not a particle could be washed away in
the longest transport: out of one small portion of earth thus
_completely_ enclosed by wood in an oak about 50 years old, three
dicotyledonous plants germinated: I am certain of the accuracy of this
observation. Again, I can show that the carcasses of birds, when
floating on the sea, sometimes escape being immediately devoured; and
seeds of many kinds in the crops of floating birds long retain their
vitality: peas and vetches, for instance, are killed by even a few
days’ immersion in sea-water; but some taken out of the crop of a
pigeon, which had floated on artificial salt-water for 30 days, to my
surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may I think safely assume that under such circumstances their
rate of flight would often be 35 miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of
nutritious seeds passing through the intestines of a bird; but hard
seeds of fruit will pass uninjured through even the digestive organs of
a turkey. In the course of two months, I picked up in my garden 12
kinds of seeds, out of the excrement of small birds, and these seemed
perfect, and some of them, which I tried, germinated. But the following
fact is more important: the crops of birds do not secrete gastric
juice, and do not in the
least injure, as I know by trial, the germination of seeds; now after a
bird has found and devoured a large supply of food, it is positively
asserted that all the grains do not pass into the gizzard for 12 or
even 18 hours. A bird in this interval might easily be blown to the
distance of 500 miles, and hawks are known to look out for tired birds,
and the contents of their torn crops might thus readily get scattered.
Mr. Brent informs me that a friend of his had to give up flying
carrier-pigeons from France to England, as the hawks on the English
coast destroyed so many on their arrival. Some hawks and owls bolt
their prey whole, and after an interval of from twelve to twenty hours,
disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds of
the oat, wheat, millet, canary, hemp, clover, and beet germinated after
having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been
thus retained for two days and fourteen hours. Freshwater fish, I find,
eat seeds of many land and water plants: fish are frequently devoured
by birds, and thus the seeds might be transported from place to place.
I forced many kinds of seeds into the stomachs of dead fish, and then
gave their bodies to fishing-eagles, storks, and pelicans; these birds
after an interval of many hours, either rejected the seeds in pellets
or passed them in their excrement; and several of these seeds retained
their power of germination. Certain seeds, however, were always killed
by this process.
Although the beaks and feet of birds are generally quite clean, I can
show that earth sometimes adheres to them: in one instance I removed
twenty-two grains of dry argillaceous earth from one foot of a
partridge, and in this earth there was a pebble quite as large as
the seed of a vetch. Thus seeds might occasionally be transported to
great distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions
of quails which annually cross the Mediterranean; and can we doubt that
the earth adhering to their feet would sometimes include a few minute
seeds? But I shall presently have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, I can
hardly doubt that they must occasionally have transported seeds from
one part to another of the arctic and antarctic regions, as suggested
by Lyell; and during the Glacial period from one part of the now
temperate regions to another. In the Azores, from the large number of
the species of plants common to Europe, in comparison with the plants
of other oceanic islands nearer to the mainland, and (as remarked by
Mr. H. C. Watson) from the somewhat northern character of the flora in
comparison with the latitude, I suspected that these islands had been
partly stocked by ice-borne seeds, during the Glacial epoch. At my
request Sir C. Lyell wrote to M. Hartung to inquire whether he had
observed erratic boulders on these islands, and he answered that he had
found large fragments of granite and other rocks, which do not occur in
the archipelago. Hence we may safely infer that icebergs formerly
landed their rocky burthens on the shores of these mid-ocean islands,
and it is at least possible that they may have brought thither the
seeds of northern plants.
Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in
action year after year, for centuries and tens of thousands of
years, it would I think be a marvellous fact if many plants had not
thus become widely transported. These means of transport are sometimes
called accidental, but this is not strictly correct: the currents of
the sea are not accidental, nor is the direction of prevalent gales of
wind. It should be observed that scarcely any means of transport would
carry seeds for very great distances; for seeds do not retain their
vitality when exposed for a great length of time to the action of
seawater; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to
island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would
not by such means become mingled in any great degree; but would remain
as distinct as we now see them to be. The currents, from their course,
would never bring seeds from North America to Britain, though they
might and do bring seeds from the West Indies to our western shores,
where, if not killed by so long an immersion in salt-water, they could
not endure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to the
western shores of Ireland and England; but seeds could be transported
by these wanderers only by one means, namely, in dirt sticking to their
feet, which is in itself a rare accident. Even in this case, how small
would the chance be of a seed falling on favourable soil, and coming to
maturity! But it would be a great error to argue that because a
well-stocked island, like Great Britain, has not, as far as is known
(and it would be very difficult to prove this), received within the
last few centuries, through occasional means
of transport, immigrants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from the mainland,
would not receive colonists by similar means. I do not doubt that out
of twenty seeds or animals transported to an island, even if far less
well-stocked than Britain, scarcely more than one would be so well
fitted to its new home, as to become naturalised. But this, as it seems
to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological
time, whilst an island was being upheaved and formed, and before it had
become fully stocked with inhabitants. On almost bare land, with few or
no destructive insects or birds living there, nearly every seed, which
chanced to arrive, would be sure to germinate and survive.
_Dispersal during the Glacial period_.—The identity of many plants and
animals, on mountain-summits, separated from each other by hundreds of
miles of lowlands, where the Alpine species could not possibly exist,
is one of the most striking cases known of the same species living at
distant points, without the apparent possibility of their having
migrated from one to the other. It is indeed a remarkable fact to see
so many of the same plants living on the snowy regions of the Alps or
Pyrenees, and in the extreme northern parts of Europe; but it is far
more remarkable, that the plants on the White Mountains, in the United
States of America, are all the same with those of Labrador, and nearly
all the same, as we hear from Asa Gray, with those on the loftiest
mountains of Europe. Even as long ago as 1747, such facts led Gmelin to
conclude that the same species must have been independently created at
several distinct points; and we might have remained
in this same belief, had not Agassiz and others called vivid attention
to the Glacial period, which, as we shall immediately see, affords a
simple explanation of these facts. We have evidence of almost every
conceivable kind, organic and inorganic, that within a very recent
geological period, central Europe and North America suffered under an
Arctic climate. The ruins of a house burnt by fire do not tell their
tale more plainly, than do the mountains of Scotland and Wales, with
their scored flanks, polished surfaces, and perched boulders, of the
icy streams with which their valleys were lately filled. So greatly has
the climate of Europe changed, that in Northern Italy, gigantic
moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic boulders, and
rocks scored by drifted icebergs and coast-ice, plainly reveal a former
cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes
more readily, by supposing a new glacial period to come slowly on, and
then pass away, as formerly occurred. As the cold came on, and as each
more southern zone became fitted for arctic beings and ill-fitted for
their former more temperate inhabitants, the latter would be supplanted
and arctic productions would take their places. The inhabitants of the
more temperate regions would at the same time travel southward, unless
they were stopped by barriers, in which case they would perish. The
mountains would become covered with snow and ice, and their former
Alpine inhabitants would descend to the plains. By the time that the
cold had reached its maximum, we should have a uniform arctic fauna and
flora, covering the central parts of Europe, as far
south as the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered by
arctic plants and animals, and these would be nearly the same with
those of Europe; for the present circumpolar inhabitants, which we
suppose to have everywhere travelled southward, are remarkably uniform
round the world. We may suppose that the Glacial period came on a
little earlier or later in North America than in Europe, so will the
southern migration there have been a little earlier or later; but this
will make no difference in the final result.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending higher and higher, as the warmth increased,
whilst their brethren were pursuing their northern journey. Hence, when
the warmth had fully returned, the same arctic species, which had
lately lived in a body together on the lowlands of the Old and New
Worlds, would be left isolated on distant mountain-summits (having been
exterminated on all lesser heights) and in the arctic regions of both
hemispheres.
Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain-range are more especially related to the arctic forms
living due north or nearly due north of them: for the migration as the
cold came on, and the re-migration on the returning warmth, will
generally have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Ramond, are more especially
allied to the plants of northern Scandinavia; those of the United
States to Labrador; those of the mountains of Siberia to the arctic
regions of that country. These views, grounded as they are on the
perfectly well-ascertained occurrence of a former Glacial period, seem
to me to explain in so satisfactory a manner the present distribution
of the Alpine and Arctic productions of Europe and America, that when
in other regions we find the same species on distant mountain-summits,
we may almost conclude without other evidence, that a colder climate
permitted their former migration across the low intervening tracts,
since become too warm for their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe
to have been the case, chiefly from the distribution of the fossil
Gnathodon), then the arctic and temperate productions will at a very
late period have marched a little further north, and subsequently have
retreated to their present homes; but I have met with no satisfactory
evidence with respect to this intercalated slightly warmer period,
since the Glacial period.
The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as
is especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed,
and, in accordance with the principles inculcated in this volume, they
will not have been liable to much modification. But with our Alpine
productions, left isolated from the moment of the returning warmth,
first at the bases and ultimately on the summits of the mountains, the
case will have been somewhat different;
for it is not likely that all the same arctic species will have been
left on mountain ranges distant from each other, and have survived
there ever since; they will, also, in all probability have become
mingled with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which
during its coldest period will have been temporarily driven down to the
plains; they will, also, have been exposed to somewhat different
climatal influences. Their mutual relations will thus have been in some
degree disturbed; consequently they will have been liable to
modification; and this we find has been the case; for if we compare the
present Alpine plants and animals of the several great European
mountain-ranges, though very many of the species are identically the
same, some present varieties, some are ranked as doubtful forms, and
some few are distinct yet closely allied or representative species.
In illustrating what, as I believe, actually took place during the
Glacial period, I assumed that at its commencement the arctic
productions were as uniform round the polar regions as they are at the
present day. But the foregoing remarks on distribution apply not only
to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower
mountains and on the plains of North America and Europe; and it may be
reasonably asked how I account for the necessary degree of uniformity
of the sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the Atlantic Ocean and by the extreme
northern part of the Pacific. During the Glacial period, when the
inhabitants
of the Old and New Worlds lived further southwards than at present,
they must have been still more completely separated by wider spaces of
ocean. I believe the above difficulty may be surmounted by looking to
still earlier changes of climate of an opposite nature. We have good
reason to believe that during the newer Pliocene period, before the
Glacial epoch, and whilst the majority of the inhabitants of the world
were specifically the same as now, the climate was warmer than at the
present day. Hence we may suppose that the organisms now living under
the climate of latitude 60°, during the Pliocene period lived further
north under the Polar Circle, in latitude 66°-67°; and that the
strictly arctic productions then lived on the broken land still nearer
to the pole. Now if we look at a globe, we shall see that under the
Polar Circle there is almost continuous land from western Europe,
through Siberia, to eastern America. And to this continuity of the
circumpolar land, and to the consequent freedom for intermigration
under a more favourable climate, I attribute the necessary amount of
uniformity in the sub-arctic and northern temperate productions of the
Old and New Worlds, at a period anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected to
large, but partial oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some earlier and still
warmer period, such as the older Pliocene period, a large number of the
same plants and animals inhabited the almost continuous circumpolar
land; and that these plants and animals, both in the Old and New
Worlds, began slowly to migrate southwards as the climate became less
warm, long before the commencement
of the Glacial period. We now see, as I believe, their descendants,
mostly in a modified condition, in the central parts of Europe and the
United States. On this view we can understand the relationship, with
very little identity, between the productions of North America and
Europe,—a relationship which is most remarkable, considering the
distance of the two areas, and their separation by the Atlantic Ocean.
We can further understand the singular fact remarked on by several
observers, that the productions of Europe and America during the later
tertiary stages were more closely related to each other than they are
at the present time; for during these warmer periods the northern parts
of the Old and New Worlds will have been almost continuously united by
land, serving as a bridge, since rendered impassable by cold, for the
inter-migration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they must have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great
region with the native American productions, and have had to compete
with them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much
modification,—for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain-ranges and on the arctic lands of the two Worlds.
Hence it has come, that when we compare the now living productions of
the temperate regions of the New and Old Worlds, we find very few
identical
species (though Asa Gray has lately shown that more plants are
identical than was formerly supposed), but we find in every great class
many forms, which some naturalists rank as geographical races, and
others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in areas completely sundered. Thus, I think, we
can understand the presence of many existing and tertiary
representative forms on the eastern and western shores of temperate
North America; and the still more striking case of many closely allied
crustaceans (as described in Dana’s admirable work), of some fish and
other marine animals, in the Mediterranean and in the seas of
Japan,—areas now separated by a continent and by nearly a hemisphere of
equatorial ocean.
These cases of relationship, without identity, of the inhabitants of
seas now disjoined, and likewise of the past and present inhabitants of
the temperate lands of North America and Europe, are inexplicable on
the theory of creation. We cannot say that they have been created
alike, in correspondence with the nearly similar physical conditions of
the areas; for if we compare, for instance, certain parts of South
America with the southern continents of the Old World, we see countries
closely corresponding in all their physical conditions, but with their
inhabitants utterly dissimilar.
But we must return to our more immediate subject, the Glacial period. I
am convinced that Forbes’s view
may be largely extended. In Europe we have the plainest evidence of the
cold period, from the western shores of Britain to the Oural range, and
southward to the Pyrenees. We may infer, from the frozen mammals and
nature of the mountain vegetation, that Siberia was similarly affected.
Along the Himalaya, at points 900 miles apart, glaciers have left the
marks of their former low descent; and in Sikkim, Dr. Hooker saw maize
growing on gigantic ancient moraines. South of the equator, we have
some direct evidence of former glacial action in New Zealand; and the
same plants, found on widely separated mountains in this island, tell
the same story. If one account which has been published can be trusted,
we have direct evidence of glacial action in the south-eastern corner
of Australia.
Looking to America; in the northern half, ice-borne fragments of rock
have been observed on the eastern side as far south as lat. 36°-37°,
and on the shores of the Pacific, where the climate is now so
different, as far south as lat. 46 deg; erratic boulders have, also,
been noticed on the Rocky Mountains. In the Cordillera of Equatorial
South America, glaciers once extended far below their present level. In
central Chile I was astonished at the structure of a vast mound of
detritus, about 800 feet in height, crossing a valley of the Andes; and
this I now feel convinced was a gigantic moraine, left far below any
existing glacier. Further south on both sides of the continent, from
lat. 41° to the southernmost extremity, we have the clearest evidence
of former glacial action, in huge boulders transported far from their
parent source.
We do not know that the Glacial epoch was strictly simultaneous at
these several far distant points on opposite sides of the world. But we
have good evidence in almost every case, that the epoch was included
within
the latest geological period. We have, also, excellent evidence, that
it endured for an enormous time, as measured by years, at each point.
The cold may have come on, or have ceased, earlier at one point of the
globe than at another, but seeing that it endured for long at each, and
that it was contemporaneous in a geological sense, it seems to me
probable that it was, during a part at least of the period, actually
simultaneous throughout the world. Without some distinct evidence to
the contrary, we may at least admit as probable that the glacial action
was simultaneous on the eastern and western sides of North America, in
the Cordillera under the equator and under the warmer temperate zones,
and on both sides of the southern extremity of the continent. If this
be admitted, it is difficult to avoid believing that the temperature of
the whole world was at this period simultaneously cooler. But it would
suffice for my purpose, if the temperature was at the same time lower
along certain broad belts of longitude.
On this view of the whole world, or at least of broad longitudinal
belts, having been simultaneously colder from pole to pole, much light
can be thrown on the present distribution of identical and allied
species. In America, Dr. Hooker has shown that between forty and fifty
of the flowering plants of Tierra del Fuego, forming no inconsiderable
part of its scanty flora, are common to Europe, enormously remote as
these two points are; and there are many closely allied species. On the
lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the highest mountains of Brazil,
some few European genera were found by Gardner, which do not exist in
the wide intervening hot countries. So on the Silla of Caraccas the
illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the
peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope
a very few European species, believed not to have been introduced by
man, and on the mountains, some few representative European forms are
found, which have not been discovered in the intertropical parts of
Africa. On the Himalaya, and on the isolated mountain-ranges of the
peninsula of India, on the heights of Ceylon, and on the volcanic cones
of Java, many plants occur, either identically the same or representing
each other, and at the same time representing plants of Europe, not
found in the intervening hot lowlands. A list of the genera collected
on the loftier peaks of Java raises a picture of a collection made on a
hill in Europe! Still more striking is the fact that southern
Australian forms are clearly represented by plants growing on the
summits of the mountains of Borneo. Some of these Australian forms, as
I hear from Dr. Hooker, extend along the heights of the peninsula of
Malacca, and are thinly scattered, on the one hand over India and on
the other as far north as Japan.
On the southern mountains of Australia, Dr. F. Müller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable ‘Introduction to the
Flora of New Zealand,’ by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence we see that
throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern hemispheres,
are sometimes
identically the same; but they are much oftener specifically distinct,
though related to each other in a most remarkable manner.
This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, Professor Dana, that “it is certainly
a wonderful fact that New Zealand should have a closer resemblance in
its crustacea to Great Britain, its antipode, than to any other part of
the world.” Sir J. Richardson, also, speaks of the reappearance on the
shores of New Zealand, Tasmania, etc., of northern forms of fish. Dr.
Hooker informs me that twenty-five species of Algæ are common to New
Zealand and to Europe, but have not been found in the intermediate
tropical seas.
It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges
of the intertropical regions, are not arctic, but belong to the
northern temperate zones. As Mr. H. C. Watson has recently remarked,
“In receding from polar towards equatorial latitudes, the Alpine or
mountain floras really become less and less arctic.” Many of the forms
living on the mountains of the warmer regions of the earth and in the
southern hemisphere are of doubtful value, being ranked by some
naturalists as specifically distinct, by others as varieties; but some
are certainly identical, and many, though closely related to northern
forms, must be ranked as distinct species.
Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that
the whole world, or a large part of it, was during the Glacial period
simultaneously much
colder than at present. The Glacial period, as measured by years, must
have been very long; and when we remember over what vast spaces some
naturalised plants and animals have spread within a few centuries, this
period will have been ample for any amount of migration. As the cold
came slowly on, all the tropical plants and other productions will have
retreated from both sides towards the equator, followed in the rear by
the temperate productions, and these by the arctic; but with the latter
we are not now concerned. The tropical plants probably suffered much
extinction; how much no one can say; perhaps formerly the tropics
supported as many species as we see at the present day crowded together
at the Cape of Good Hope, and in parts of temperate Australia. As we
know that many tropical plants and animals can withstand a considerable
amount of cold, many might have escaped extermination during a moderate
fall of temperature, more especially by escaping into the warmest
spots. But the great fact to bear in mind is, that all tropical
productions will have suffered to a certain extent. On the other hand,
the temperate productions, after migrating nearer to the equator,
though they will have been placed under somewhat new conditions, will
have suffered less. And it is certain that many temperate plants, if
protected from the inroads of competitors, can withstand a much warmer
climate than their own. Hence, it seems to me possible, bearing in mind
that the tropical productions were in a suffering state and could not
have presented a firm front against intruders, that a certain number of
the more vigorous and dominant temperate forms might have penetrated
the native ranks and have reached or even crossed the equator. The
invasion would, of course, have been greatly favoured by high land, and
perhaps
by a dry climate; for Dr. Falconer informs me that it is the damp with
the heat of the tropics which is so destructive to perennial plants
from a temperate climate. On the other hand, the most humid and hottest
districts will have afforded an asylum to the tropical natives. The
mountain-ranges north-west of the Himalaya, and the long line of the
Cordillera, seem to have afforded two great lines of invasion: and it
is a striking fact, lately communicated to me by Dr. Hooker, that all
the flowering plants, about forty-six in number, common to Tierra del
Fuego and to Europe still exist in North America, which must have lain
on the line of march. But I do not doubt that some temperate
productions entered and crossed even the _lowlands_ of the tropics at
the period when the cold was most intense,—when arctic forms had
migrated some twenty-five degrees of latitude from their native country
and covered the land at the foot of the Pyrenees. At this period of
extreme cold, I believe that the climate under the equator at the level
of the sea was about the same with that now felt there at the height of
six or seven thousand feet. During this the coldest period, I suppose
that large spaces of the tropical lowlands were clothed with a mingled
tropical and temperate vegetation, like that now growing with strange
luxuriance at the base of the Himalaya, as graphically described by
Hooker.
Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial
period from the northern and southern temperate zones into the
intertropical regions, and some even crossed the equator. As the warmth
returned, these temperate forms would naturally ascend the higher
mountains, being exterminated on the lowlands; those which had not
reached the equator, would re-migrate northward or southward towards
their former
homes; but the forms, chiefly northern, which had crossed the equator,
would travel still further from their homes into the more temperate
latitudes of the opposite hemisphere. Although we have reason to
believe from geological evidence that the whole body of arctic shells
underwent scarcely any modification during their long southern
migration and re-migration northward, the case may have been wholly
different with those intruding forms which settled themselves on the
intertropical mountains, and in the southern hemisphere. These being
surrounded by strangers will have had to compete with many new forms of
life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus many
of these wanderers, though still plainly related by inheritance to
their brethren of the northern or southern hemispheres, now exist in
their new homes as well-marked varieties or as distinct species.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from
the north to the south, than in a reversed direction. We see, however,
a few southern vegetable forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from north to
south is due to the greater extent of land in the north, and to the
northern forms having existed in their own homes in greater numbers,
and having consequently been advanced through natural selection and
competition to a higher stage of perfection or dominating power, than
the southern forms. And thus, when they became commingled during the
Glacial period, the northern forms were enabled to beat the less
powerful southern forms. Just in the same manner as we see at the
present day,
that very many European productions cover the ground in La Plata, and
in a lesser degree in Australia, and have to a certain extent beaten
the natives; whereas extremely few southern forms have become
naturalised in any part of Europe, though hides, wool, and other
objects likely to carry seeds have been largely imported into Europe
during the last two or three centuries from La Plata, and during the
last thirty or forty years from Australia. Something of the same kind
must have occurred on the intertropical mountains: no doubt before the
Glacial period they were stocked with endemic Alpine forms; but these
have almost everywhere largely yielded to the more dominant forms,
generated in the larger areas and more efficient workshops of the
north. In many islands the native productions are nearly equalled or
even outnumbered by the naturalised; and if the natives have not been
actually exterminated, their numbers have been greatly reduced, and
this is the first stage towards extinction. A mountain is an island on
the land; and the intertropical mountains before the Glacial period
must have been completely isolated; and I believe that the productions
of these islands on the land yielded to those produced within the
larger areas of the north, just in the same way as the productions of
real islands have everywhere lately yielded to continental forms,
naturalised by man’s agency.
I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means of
migration, or the reason why certain species and not others have
migrated;
why certain species have been modified and have given rise to new
groups of forms, and others have remained unaltered. We cannot hope to
explain such facts, until we can say why one species and not another
becomes naturalised by man’s agency in a foreign land; why one ranges
twice or thrice as far, and is twice or thrice as common, as another
species within their own homes.
I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species at points so enormously remote as Kerguelen Land, New
Zealand, and Fuegia, I believe that towards the close of the Glacial
period, icebergs, as suggested by Lyell, have been largely concerned in
their dispersal. But the existence of several quite distinct species,
belonging to genera exclusively confined to the south, at these and
other distant points of the southern hemisphere, is, on my theory of
descent with modification, a far more remarkable case of difficulty.
For some of these species are so distinct, that we cannot suppose that
there has been time since the commencement of the Glacial period for
their migration, and for their subsequent modification to the necessary
degree. The facts seem to me to indicate that peculiar and very
distinct species have migrated in radiating lines from some common
centre; and I am inclined to look in the southern, as in the northern
hemisphere, to a former and warmer period, before the commencement of
the Glacial period, when the antarctic lands, now covered with ice,
supported a highly peculiar and isolated flora. I suspect that before
this flora was exterminated by the Glacial epoch, a few forms were
widely dispersed to various points of the southern hemisphere by
occasional means of transport, and by the aid, as halting-places, of
existing and now sunken islands, and perhaps at the commencement of the
Glacial period, by icebergs. By these means, as I believe, the southern
shores of America, Australia, New Zealand have become slightly tinted
by the same peculiar forms of vegetable life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt
one of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the
present distribution both of the same and of allied forms of life can
be explained. The living waters may be said to have flowed during one
short period from the north and from the south, and to have crossed at
the equator; but to have flowed with greater force from the north so as
to have freely inundated the south. As the tide leaves its drift in
horizontal lines, though rising higher on the shores where the tide
rises highest, so have the living waters left their living drift on our
mountain-summits, in a line gently rising from the arctic lowlands to a
great height under the equator. The various beings thus left stranded
may be compared with savage races of man, driven up and surviving in
the mountain-fastnesses of almost every land, which serve as a record,
full of interest to us, of the former inhabitants of the surrounding
lowlands.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION—_continued_.
Distribution of fresh-water productions. On the inhabitants of oceanic
islands. Absence of Batrachians and of terrestrial Mammals. On the
relation of the inhabitants of islands to those of the nearest
mainland. On colonisation from the nearest source with subsequent
modification. Summary of the last and present chapters.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is
apparently a still more impassable barrier, that they never would have
extended to distant countries. But the case is exactly the reverse. Not
only have many fresh-water species, belonging to quite different
classes, an enormous range, but allied species prevail in a remarkable
manner throughout the world. I well remember, when first collecting in
the fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.
But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and
frequent migrations from pond to pond, or from stream to stream; and
liability to wide dispersal would follow from this capacity as an
almost necessary consequence. We can here consider only a few cases. In
regard to
fish, I believe that the same species never occur in the fresh waters
of distant continents. But on the same continent the species often
range widely and almost capriciously; for two river-systems will have
some fish in common and some different. A few facts seem to favour the
possibility of their occasional transport by accidental means; like
that of the live fish not rarely dropped by whirlwinds in India, and
the vitality of their ova when removed from the water. But I am
inclined to attribute the dispersal of fresh-water fish mainly to
slight changes within the recent period in the level of the land,
having caused rivers to flow into each other. Instances, also, could be
given of this having occurred during floods, without any change of
level. We have evidence in the loess of the Rhine of considerable
changes of level in the land within a very recent geological period,
and when the surface was peopled by existing land and fresh-water
shells. The wide difference of the fish on opposite sides of continuous
mountain-ranges, which from an early period must have parted
river-systems and completely prevented their inosculation, seems to
lead to this same conclusion. With respect to allied fresh-water fish
occurring at very distant points of the world, no doubt there are many
cases which cannot at present be explained: but some fresh-water fish
belong to very ancient forms, and in such cases there will have been
ample time for great geographical changes, and consequently time and
means for much migration. In the second place, salt-water fish can with
care be slowly accustomed to live in fresh water; and, according to
Valenciennes, there is hardly a single group of fishes confined
exclusively to fresh water, so that we may imagine that a marine member
of a fresh-water group might travel far along the shores of the sea,
and subsequently
become modified and adapted to the fresh waters of a distant land.
Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not
likely to be transported by birds, and they are immediately killed by
sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country.
But two facts, which I have observed—and no doubt many others remain to
be observed—throw some light on this subject. When a duck suddenly
emerges from a pond covered with duck-weed, I have twice seen these
little plants adhering to its back; and it has happened to me, in
removing a little duck-weed from one aquarium to another, that I have
quite unintentionally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I suspended a
duck’s feet, which might represent those of a bird sleeping in a
natural pond, in an aquarium, where many ova of fresh-water shells were
hatching; and I found that numbers of the extremely minute and just
hatched shells crawled on the feet, and clung to them so firmly that
when taken out of the water they could not be jarred off, though at a
somewhat more advanced age they would voluntarily drop off. These just
hatched molluscs, though aquatic in their nature, survived on the
duck’s feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and would be sure to alight on a pool or rivulet, if blown
across sea to an oceanic island or to any other distant point. Sir
Charles Lyell also
informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a
water-beetle of the same family, a Colymbetes, once flew on board the
‘Beagle,’ when forty-five miles distant from the nearest land: how much
farther it might have flown with a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges
many fresh-water and even marsh-species have, both over continents and
to the most remote oceanic islands. This is strikingly shown, as
remarked by Alph. de Candolle, in large groups of terrestrial plants,
which have only a very few aquatic members; for these latter seem
immediately to acquire, as if in consequence, a very wide range. I
think favourable means of dispersal explain this fact. I have before
mentioned that earth occasionally, though rarely, adheres in some
quantity to the feet and beaks of birds. Wading birds, which frequent
the muddy edges of ponds, if suddenly flushed, would be the most likely
to have muddy feet. Birds of this order I can show are the greatest
wanderers, and are occasionally found on the most remote and barren
islands in the open ocean; they would not be likely to alight on the
surface of the sea, so that the dirt would not be washed off their
feet; when making land, they would be sure to fly to their natural
fresh-water haunts. I do not believe that botanists are aware how
charged the mud of ponds is with seeds: I have tried several little
experiments, but will here give only the most striking case: I took in
February three table-spoonfuls of mud from three different points,
beneath water, on the edge of a little pond; this mud when dry weighed
only 6 3/4 ounces; I kept it covered up in my study for six months,
pulling up and counting each plant as it grew; the plants were
of many kinds, and were altogether 537 in number; and yet the viscid
mud was all contained in a breakfast cup! Considering these facts, I
think it would be an inexplicable circumstance if water-birds did not
transport the seeds of fresh-water plants to vast distances, and if
consequently the range of these plants was not very great. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.
Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have gone
on daily devouring fish; they then take flight and go to other waters,
or are blown across the sea; and we have seen that seeds retain their
power of germination, when rejected in pellets or in excrement, many
hours afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks
on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron’s stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and
getting a hearty meal of fish, would probably reject from its stomach a
pellet containing the seeds of the Nelumbium undigested; or the seeds
might be dropped by the bird whilst feeding its young, in the same way
as fish are known sometimes to be dropped.
In considering these several means of distribution,
it should be remembered that when a pond or stream is first formed, for
instance, on a rising islet, it will be unoccupied; and a single seed
or egg will have a good chance of succeeding. Although there will
always be a struggle for life between the individuals of the species,
however few, already occupying any pond, yet as the number of kinds is
small, compared with those on the land, the competition will probably
be less severe between aquatic than between terrestrial species;
consequently an intruder from the waters of a foreign country, would
have a better chance of seizing on a place, than in the case of
terrestrial colonists. We should, also, remember that some, perhaps
many, fresh-water productions are low in the scale of nature, and that
we have reason to believe that such low beings change or become
modified less quickly than the high; and this will give longer time
than the average for the migration of the same aquatic species. We
should not forget the probability of many species having formerly
ranged as continuously as fresh-water productions ever can range, over
immense areas, and having subsequently become extinct in intermediate
regions. But the wide distribution of fresh-water plants and of the
lower animals, whether retaining the same identical form or in some
degree modified, I believe mainly depends on the wide dispersal of
their seeds and eggs by animals, more especially by fresh-water birds,
which have large powers of flight, and naturally travel from one to
another and often distant piece of water. Nature, like a careful
gardener, thus takes her seeds from a bed of a particular nature, and
drops them in another equally well fitted for them.
_On the Inhabitants of Oceanic Islands_.—We now come to the last of the
three classes of facts, which I
have selected as presenting the greatest amount of difficulty, on the
view that all the individuals both of the same and of allied species
have descended from a single parent; and therefore have all proceeded
from a common birthplace, notwithstanding that in the course of time
they have come to inhabit distant points of the globe. I have already
stated that I cannot honestly admit Forbes’s view on continental
extensions, which, if legitimately followed out, would lead to the
belief that within the recent period all existing islands have been
nearly or quite joined to some continent. This view would remove many
difficulties, but it would not, I think, explain all the facts in
regard to insular productions. In the following remarks I shall not
confine myself to the mere question of dispersal; but shall consider
some other facts, which bear on the truth of the two theories of
independent creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and Wollaston for insects. If we look
to the large size and varied stations of New Zealand, extending over
780 miles of latitude, and compare its flowering plants, only 750 in
number, with those on an equal area at the Cape of Good Hope or in
Australia, we must, I think, admit that something quite independently
of any difference in physical conditions has caused so great a
difference in number. Even the uniform county of Cambridge has 847
plants, and the little island of Anglesea 764, but a few ferns and a
few introduced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that the
barren island of Ascension aboriginally possessed under half-a-dozen
flowering
plants; yet many have become naturalised on it, as they have on New
Zealand and on every other oceanic island which can be named. In St.
Helena there is reason to believe that the naturalised plants and
animals have nearly or quite exterminated many native productions. He
who admits the doctrine of the creation of each separate species, will
have to admit, that a sufficient number of the best adapted plants and
animals have not been created on oceanic islands; for man has
unintentionally stocked them from various sources far more fully and
perfectly than has nature.
Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (_i.e._ those found nowhere
else in the world) is often extremely large. If we compare, for
instance, the number of the endemic land-shells in Madeira, or of the
endemic birds in the Galapagos Archipelago, with the number found on
any continent, and then compare the area of the islands with that of
the continent, we shall see that this is true. This fact might have
been expected on my theory, for, as already explained, species
occasionally arriving after long intervals in a new and isolated
district, and having to compete with new associates, will be eminently
liable to modification, and will often produce groups of modified
descendants. But it by no means follows, that, because in an island
nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend on the species which do not become modified
having immigrated with facility and in a body, so that their mutual
relations have not been much disturbed. Thus in the Galapagos Islands
nearly every land-bird, but only two out of the eleven marine birds,
are peculiar; and it is obvious that
marine birds could arrive at these islands more easily than land-birds.
Bermuda, on the other hand, which lies at about the same distance from
North America as the Galapagos Islands do from South America, and which
has a very peculiar soil, does not possess one endemic land bird; and
we know from Mr. J. M. Jones’s admirable account of Bermuda, that very
many North American birds, during their great annual migrations, visit
either periodically or occasionally this island. Madeira does not
possess one peculiar bird, and many European and African birds are
almost every year blown there, as I am informed by Mr. E. V. Harcourt.
So that these two islands of Bermuda and Madeira have been stocked by
birds, which for long ages have struggled together in their former
homes, and have become mutually adapted to each other; and when settled
in their new homes, each kind will have been kept by the others to
their proper places and habits, and will consequently have been little
liable to modification. Madeira, again, is inhabited by a wonderful
number of peculiar land-shells, whereas not one species of sea-shell is
confined to its shores: now, though we do not know how seashells are
dispersed, yet we can see that their eggs or larvæ, perhaps attached to
seaweed or floating timber, or to the feet of wading-birds, might be
transported far more easily than land-shells, across three or four
hundred miles of open sea. The different orders of insects in Madeira
apparently present analogous facts.
Oceanic islands are sometimes deficient in certain classes, and their
places are apparently occupied by the other inhabitants; in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take the place of mammals. In the plants of the Galapagos Islands, Dr.
Hooker has shown that the proportional numbers of the different orders
are very different from
what they are elsewhere. Such cases are generally accounted for by the
physical conditions of the islands; but this explanation seems to me
not a little doubtful. Facility of immigration, I believe, has been at
least as important as the nature of the conditions.
Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully hooked
seeds; yet few relations are more striking than the adaptation of
hooked seeds for transportal by the wool and fur of quadrupeds. This
case presents no difficulty on my view, for a hooked seed might be
transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds, would
form an endemic species, having as useless an appendage as any
rudimentary organ,—for instance, as the shrivelled wings under the
soldered elytra of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only
herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence trees
would be little likely to reach distant oceanic islands; and an
herbaceous plant, though it would have no chance of successfully
competing in stature with a fully developed tree, when established on
an island and having to compete with herbaceous plants alone, might
readily gain an advantage by growing taller and taller and overtopping
the other plants. If so, natural selection would often tend to add to
the stature of herbaceous plants when growing on an island, to whatever
order they belonged, and thus convert them first into bushes and
ultimately into trees.
With respect to the absence of whole orders on
oceanic islands, Bory St. Vincent long ago remarked that Batrachians
(frogs, toads, newts) have never been found on any of the many islands
with which the great oceans are studded. I have taken pains to verify
this assertion, and I have found it strictly true. I have, however,
been assured that a frog exists on the mountains of the great island of
New Zealand; but I suspect that this exception (if the information be
correct) may be explained through glacial agency. This general absence
of frogs, toads, and newts on so many oceanic islands cannot be
accounted for by their physical conditions; indeed it seems that
islands are peculiarly well fitted for these animals; for frogs have
been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their
spawn are known to be immediately killed by sea-water, on my view we
can see that there would be great difficulty in their transportal
across the sea, and therefore why they do not exist on any oceanic
island. But why, on the theory of creation, they should not have been
created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not
found a single instance, free from doubt, of a terrestrial mammal
(excluding domesticated animals kept by the natives) inhabiting an
island situated above 300 miles from a continent or great continental
island; and many islands situated at a much less distance are equally
barren. The Falkland Islands, which are inhabited by a wolf-like fox,
come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank connected with the mainland; moreover,
icebergs formerly brought boulders to its western shores, and they may
have formerly transported foxes, as so frequently now happens in the
arctic regions. Yet it cannot be said that small islands will not
support small mammals, for they occur in many parts of the world on
very small islands, if close to a continent; and hardly an island can
be named on which our smaller quadrupeds have not become naturalised
and greatly multiplied. It cannot be said, on the ordinary view of
creation, that there has not been time for the creation of mammals;
many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is thought that
mammals appear and disappear at a quicker rate than other and lower
animals. Though terrestrial mammals do not occur on oceanic islands,
ærial mammals do occur on almost every island. New Zealand possesses
two bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne
Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it
may be asked, has the supposed creative force produced bats and no
other mammals on remote islands? On my view this question can easily be
answered; for no terrestrial mammal can be transported across a wide
space of sea, but bats can fly across. Bats have been seen wandering by
day far over the Atlantic Ocean; and two North American species either
regularly or occasionally visit Bermuda, at the distance of 600 miles
from the mainland. I hear from Mr. Tomes, who has specially studied
this family, that many of the same species have enormous ranges, and
are found on continents and on far distant islands. Hence we have only
to suppose that such wandering species have been modified
through natural selection in their new homes in relation to their new
position, and we can understand the presence of endemic bats on
islands, with the absence of all terrestrial mammals.
Besides the absence of terrestrial mammals in relation to the
remoteness of islands from continents, there is also a relation, to a
certain extent independent of distance, between the depth of the sea
separating an island from the neighbouring mainland, and the presence
in both of the same mammiferous species or of allied species in a more
or less modified condition. Mr. Windsor Earl has made some striking
observations on this head in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean; and this
space separates two widely distinct mammalian faunas. On either side
the islands are situated on moderately deep submarine banks, and they
are inhabited by closely allied or identical quadrupeds. No doubt some
few anomalies occur in this great archipelago, and there is much
difficulty in forming a judgment in some cases owing to the probable
naturalisation of certain mammals through man’s agency; but we shall
soon have much light thrown on the natural history of this archipelago
by the admirable zeal and researches of Mr. Wallace. I have not as yet
had time to follow up this subject in all other quarters of the world;
but as far as I have gone, the relation generally holds good. We see
Britain separated by a shallow channel from Europe, and the mammals are
the same on both sides; we meet with analogous facts on many islands
separated by similar channels from Australia. The West Indian Islands
stand on a deeply submerged bank, nearly 1000 fathoms in depth, and
here we find American forms, but the species and even the genera are
distinct. As the amount of modification in all cases depends to
a certain degree on the lapse of time, and as during changes of level
it is obvious that islands separated by shallow channels are more
likely to have been continuously united within a recent period to the
mainland than islands separated by deeper channels, we can understand
the frequent relation between the depth of the sea and the degree of
affinity of the mammalian inhabitants of islands with those of a
neighbouring continent,—an inexplicable relation on the view of
independent acts of creation.
All the foregoing remarks on the inhabitants of oceanic
islands,—namely, the scarcity of kinds—the richness in endemic forms in
particular classes or sections of classes,—the absence of whole groups,
as of batrachians, and of terrestrial mammals notwithstanding the
presence of ærial bats,—the singular proportions of certain orders of
plants,—herbaceous forms having been developed into trees, etc.,—seem
to me to accord better with the view of occasional means of transport
having been largely efficient in the long course of time, than with the
view of all our oceanic islands having been formerly connected by
continuous land with the nearest continent; for on this latter view the
migration would probably have been more complete; and if modification
be admitted, all the forms of life would have been more equally
modified, in accordance with the paramount importance of the relation
of organism to organism.
I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote
islands, whether still retaining the same specific form or modified
since their arrival, could have reached their present homes. But the
probability of many islands having existed as halting-places, of which
not a wreck now remains, must not be overlooked.
I will here give a single instance of one of the cases of difficulty.
Almost all oceanic islands, even the most isolated and smallest, are
inhabited by land-shells, generally by endemic species, but sometimes
by species found elsewhere. Dr. Aug. A. Gould has given several
interesting cases in regard to the land-shells of the islands of the
Pacific. Now it is notorious that land-shells are very easily killed by
salt; their eggs, at least such as I have tried, sink in sea-water and
are killed by it. Yet there must be, on my view, some unknown, but
highly efficient means for their transportal. Would the just-hatched
young occasionally crawl on and adhere to the feet of birds roosting on
the ground, and thus get transported? It occurred to me that
land-shells, when hybernating and having a membranous diaphragm over
the mouth of the shell, might be floated in chinks of drifted timber
across moderately wide arms of the sea. And I found that several
species did in this state withstand uninjured an immersion in sea-water
during seven days: one of these shells was the Helix pomatia, and after
it had again hybernated I put it in sea-water for twenty days, and it
perfectly recovered. As this species has a thick calcareous operculum,
I removed it, and when it had formed a new membranous one, I immersed
it for fourteen days in sea-water, and it recovered and crawled away:
but more experiments are wanted on this head.
The most striking and important fact for us in regard to the
inhabitants of islands, is their affinity to those of the nearest
mainland, without being actually the same species. Numerous instances
could be given of this fact. I will give only one, that of the
Galapagos Archipelago, situated under the equator, between 500 and 600
miles from the shores of South America. Here
almost every product of the land and water bears the unmistakeable
stamp of the American continent. There are twenty-six land birds, and
twenty-five of these are ranked by Mr. Gould as distinct species,
supposed to have been created here; yet the close affinity of most of
these birds to American species in every character, in their habits,
gestures, and tones of voice, was manifest. So it is with the other
animals, and with nearly all the plants, as shown by Dr. Hooker in his
admirable memoir on the Flora of this archipelago. The naturalist,
looking at the inhabitants of these volcanic islands in the Pacific,
distant several hundred miles from the continent, yet feels that he is
standing on American land. Why should this be so? why should the
species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plain a stamp of affinity to
those created in America? There is nothing in the conditions of life,
in the geological nature of the islands, in their height or climate, or
in the proportions in which the several classes are associated
together, which resembles closely the conditions of the South American
coast: in fact there is a considerable dissimilarity in all these
respects. On the other hand, there is a considerable degree of
resemblance in the volcanic nature of the soil, in climate, height, and
size of the islands, between the Galapagos and Cape de Verde
Archipelagos: but what an entire and absolute difference in their
inhabitants! The inhabitants of the Cape de Verde Islands are related
to those of Africa, like those of the Galapagos to America. I believe
this grand fact can receive no sort of explanation on the ordinary view
of independent creation; whereas on the view here maintained, it is
obvious that the Galapagos Islands would be likely to receive
colonists, whether by occasional means of transport or
by formerly continuous land, from America; and the Cape de Verde
Islands from Africa; and that such colonists would be liable to
modification;—the principle of inheritance still betraying their
original birthplace.
Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of
the nearest continent, or of other near islands. The exceptions are
few, and most of them can be explained. Thus the plants of Kerguelen
Land, though standing nearer to Africa than to America, are related,
and that very closely, as we know from Dr. Hooker’s account, to those
of America: but on the view that this island has been mainly stocked by
seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its
endemic plants is much more closely related to Australia, the nearest
mainland, than to any other region: and this is what might have been
expected; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote, that the
fact becomes an anomaly. But this difficulty almost disappears on the
view that both New Zealand, South America, and other southern lands
were long ago partially stocked from a nearly intermediate though
distant point, namely from the antarctic islands, when they were
clothed with vegetation, before the commencement of the Glacial period.
The affinity, which, though feeble, I am assured by Dr. Hooker is real,
between the flora of the south-western corner of Australia and of the
Cape of Good Hope, is a far more remarkable case, and is at present
inexplicable: but this affinity is confined to the plants, and will, I
do not doubt, be some day explained.
The law which causes the inhabitants of an archipelago,
though specifically distinct, to be closely allied to those of the
nearest continent, we sometimes see displayed on a small scale, yet in
a most interesting manner, within the limits of the same archipelago.
Thus the several islands of the Galapagos Archipelago are tenanted, as
I have elsewhere shown, in a quite marvellous manner, by very closely
related species; so that the inhabitants of each separate island,
though mostly distinct, are related in an incomparably closer degree to
each other than to the inhabitants of any other part of the world. And
this is just what might have been expected on my view, for the islands
are situated so near each other that they would almost certainly
receive immigrants from the same original source, or from each other.
But this dissimilarity between the endemic inhabitants of the islands
may be used as an argument against my views; for it may be asked, how
has it happened in the several islands situated within sight of each
other, having the same geological nature, the same height, climate,
etc., that many of the immigrants should have been differently
modified, though only in a small degree. This long appeared to me a
great difficulty: but it arises in chief part from the deeply-seated
error of considering the physical conditions of a country as the most
important for its inhabitants; whereas it cannot, I think, be disputed
that the nature of the other inhabitants, with which each has to
compete, is at least as important, and generally a far more important
element of success. Now if we look to those inhabitants of the
Galapagos Archipelago which are found in other parts of the world
(laying on one side for the moment the endemic species, which cannot be
here fairly included, as we are considering how they have come to be
modified since their arrival), we find a considerable amount
of difference in the several islands. This difference might indeed have
been expected on the view of the islands having been stocked by
occasional means of transport—a seed, for instance, of one plant having
been brought to one island, and that of another plant to another
island. Hence when in former times an immigrant settled on any one or
more of the islands, or when it subsequently spread from one island to
another, it would undoubtedly be exposed to different conditions of
life in the different islands, for it would have to compete with
different sets of organisms: a plant, for instance, would find the
best-fitted ground more perfectly occupied by distinct plants in one
island than in another, and it would be exposed to the attacks of
somewhat different enemies. If then it varied, natural selection would
probably favour different varieties in the different islands. Some
species, however, might spread and yet retain the same character
throughout the group, just as we see on continents some species
spreading widely and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous instances, is that the new
species formed in the separate islands have not quickly spread to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the British
Channel, and there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are rapid and
sweep across the archipelago, and gales of wind are extraordinarily
rare; so that the islands are far more effectually separated from each
other than they appear to be on a map. Nevertheless a good many
species, both those found in other parts of the world and those
confined to the archipelago, are common to
the several islands, and we may infer from certain facts that these
have probably spread from some one island to the others. But we often
take, I think, an erroneous view of the probability of closely allied
species invading each other’s territory, when put into free
intercommunication. Undoubtedly if one species has any advantage
whatever over another, it will in a very brief time wholly or in part
supplant it; but if both are equally well fitted for their own places
in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man’s agency, have spread with astonishing
rapidity over new countries, we are apt to infer that most species
would thus spread; but we should remember that the forms which become
naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct species, belonging in a
large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, are distinct on each;
thus there are three closely-allied species of mocking-thrush, each
confined to its own island. Now let us suppose the mocking-thrush of
Chatham Island to be blown to Charles Island, which has its own
mocking-thrush: why should it succeed in establishing itself there? We
may safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid there than can possibly be
reared; and we may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the species
peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston have
communicated to me a remarkable fact bearing on this subject; namely,
that Madeira and the adjoining islet of
Porto Santo possess many distinct but representative land-shells, some
of which live in crevices of stone; and although large quantities of
stone are annually transported from Porto Santo to Madeira, yet this
latter island has not become colonised by the Porto Santo species:
nevertheless both islands have been colonised by some European
land-shells, which no doubt had some advantage over the indigenous
species. From these considerations I think we need not greatly marvel
at the endemic and representative species, which inhabit the several
islands of the Galapagos Archipelago, not having universally spread
from island to island. In many other instances, as in the several
districts of the same continent, pre-occupation has probably played an
important part in checking the commingling of species under the same
conditions of life. Thus, the south-east and south-west corners of
Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants.
The principle which determines the general character of the fauna and
flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of
that region whence colonists could most readily have been derived,—the
colonists having been subsequently modified and better fitted to their
new homes,—is of the widest application throughout nature. We see this
on every mountain, in every lake and marsh. For Alpine species,
excepting in so far as the same forms, chiefly of plants, have spread
widely throughout the world during the recent Glacial epoch, are
related to those of the surrounding lowlands;—thus we have in South
America, Alpine humming-birds, Alpine rodents, Alpine plants, etc., all
of strictly American forms, and it is obvious
that a mountain, as it became slowly upheaved, would naturally be
colonised from the surrounding lowlands. So it is with the inhabitants
of lakes and marshes, excepting in so far as great facility of
transport has given the same general forms to the whole world. We see
this same principle in the blind animals inhabiting the caves of
America and of Europe. Other analogous facts could be given. And it
will, I believe, be universally found to be true, that wherever in two
regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some
identical species, showing, in accordance with the foregoing view, that
at some former period there has been intercommunication or migration
between the two regions. And wherever many closely-allied species
occur, there will be found many forms which some naturalists rank as
distinct species, and some as varieties; these doubtful forms showing
us the steps in the process of modification.
This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way.
Mr. Gould remarked to me long ago, that in those genera of birds which
range over the world, many of the species have very wide ranges. I can
hardly doubt that this rule is generally true, though it would be
difficult to prove it. Amongst mammals, we see it strikingly displayed
in Bats, and in a lesser degree in the Felidæ and Canidæ. We see it, if
we compare the distribution of butterflies and beetles. So it is with
most fresh-water productions, in which so many genera range over the
world, and many individual species have enormous ranges. It is not
meant that in world-ranging
genera all the species have a wide range, or even that they have on an
_average_ a wide range; but only that some of the species range very
widely; for the facility with which widely-ranging species vary and
give rise to new forms will largely determine their average range. For
instance, two varieties of the same species inhabit America and Europe,
and the species thus has an immense range; but, if the variation had
been a little greater, the two varieties would have been ranked as
distinct species, and the common range would have been greatly reduced.
Still less is it meant, that a species which apparently has the
capacity of crossing barriers and ranging widely, as in the case of
certain powerfully-winged birds, will necessarily range widely; for we
should never forget that to range widely implies not only the power of
crossing barriers, but the more important power of being victorious in
distant lands in the struggle for life with foreign associates. But on
the view of all the species of a genus having descended from a single
parent, though now distributed to the most remote points of the world,
we ought to find, and I believe as a general rule we do find, that some
at least of the species range very widely; for it is necessary that the
unmodified parent should range widely, undergoing modification during
its diffusion, and should place itself under diverse conditions
favourable for the conversion of its offspring, firstly into new
varieties and ultimately into new species.
In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off
from a common parent at a remote epoch; so that in such cases there
will have been ample time for great climatal and geographical changes
and for accidents of transport; and consequently for the migration of
some of the species into all
quarters of the world, where they may have become slightly modified in
relation to their new conditions. There is, also, some reason to
believe from geological evidence that organisms low in the scale within
each great class, generally change at a slower rate than the higher
forms; and consequently the lower forms will have had a better chance
of ranging widely and of still retaining the same specific character.
This fact, together with the seeds and eggs of many low forms being
very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately
been admirably discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms is, the more widely it is
apt to range.
The relations just discussed,—namely, low and slowly-changing organisms
ranging more widely than the high,—some of the species of
widely-ranging genera themselves ranging widely,—such facts, as alpine,
lacustrine, and marsh productions being related (with the exceptions
before specified) to those on the surrounding low lands and dry lands,
though these stations are so different—the very close relation of the
distinct species which inhabit the islets of the same archipelago,—and
especially the striking relation of the inhabitants of each whole
archipelago or island to those of the nearest mainland,—are, I think,
utterly inexplicable on the ordinary view of the independent creation
of each species, but are explicable on the view of colonisation from
the nearest and readiest source, together with the subsequent
modification and better adaptation of the colonists to their new homes.
_Summary of last and present Chapters_.—In these chapters I have
endeavoured to show, that if we make due allowance for our ignorance of
the full effects of all
the changes of climate and of the level of the land, which have
certainly occurred within the recent period, and of other similar
changes which may have occurred within the same period; if we remember
how profoundly ignorant we are with respect to the many and curious
means of occasional transport,—a subject which has hardly ever been
properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become extinct
in the intermediate tracts, I think the difficulties in believing that
all the individuals of the same species, wherever located, have
descended from the same parents, are not insuperable. And we are led to
this conclusion, which has been arrived at by many naturalists under
the designation of single centres of creation, by some general
considerations, more especially from the importance of barriers and
from the analogical distribution of sub-genera, genera, and families.
With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life change most slowly, enormous periods of time being thus granted
for their migration, I do not think that the difficulties are
insuperable; though they often are in this case, and in that of the
individuals of the same species, extremely grave.
As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how
diversified are the means of occasional transport, I have discussed at
some little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long
course of time the individuals of the same species, and likewise of
allied species, have proceeded from some one source; then I think all
the grand leading facts of geographical distribution are explicable on
the theory of migration (generally of the more dominant forms of life),
together with subsequent modification and the multiplication of new
forms. We can thus understand the high importance of barriers, whether
of land or water, which separate our several zoological and botanical
provinces. We can thus understand the localisation of sub-genera,
genera, and families; and how it is that under different latitudes, for
instance in South America, the inhabitants of the plains and mountains,
of the forests, marshes, and deserts, are in so mysterious a manner
linked together by affinity, and are likewise linked to the extinct
beings which formerly inhabited the same continent. Bearing in mind
that the mutual relations of organism to organism are of the highest
importance, we can see why two areas having nearly the same physical
conditions should often be inhabited by very different forms of life;
for according to the length of time which has elapsed since new
inhabitants entered one region; according to the nature of the
communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each
other and with the aborigines; and according as the immigrants were
capable of varying more or less rapidly, there would ensue in different
regions, independently of their physical conditions, infinitely
diversified conditions of life,—there would be an almost endless amount
of organic action and reaction,—and we should find, as we do find, some
groups of beings greatly, and some only slightly modified,—some
developed
in great force, some existing in scanty numbers—in the different great
geographical provinces of the world.
On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings, even within the same class,
should have all its species endemic, and another group should have all
its species common to other quarters of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands possess
their own peculiar species of ærial mammals or bats. We can see why
there should be some relation between the presence of mammals, in a
more or less modified condition, and the depth of the sea between an
island and the mainland. We can clearly see why all the inhabitants of
an archipelago, though specifically distinct on the several islets,
should be closely related to each other, and likewise be related, but
less closely, to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two areas, however
distant from each other, there should be a correlation, in the presence
of identical species, of varieties, of doubtful species, and of
distinct but representative species.
As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule
are so few, that they may
fairly be attributed to our not having as yet discovered in an
intermediate deposit the forms which are therein absent, but which
occur above and below: so in space, it certainly is the general rule
that the area inhabited by a single species, or by a group of species,
is continuous; and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by migration at some former period
under different conditions or by occasional means of transport, and by
the species having become extinct in the intermediate tracts. Both in
time and space, species and groups of species have their points of
maximum development. Groups of species, belonging either to a certain
period of time, or to a certain area, are often characterised by
trifling characters in common, as of sculpture or colour. In looking to
the long succession of ages, as in now looking to distant provinces
throughout the world, we find that some organisms differ little, whilst
others belonging to a different class, or to a different order, or even
only to a different family of the same order, differ greatly. In both
time and space the lower members of each class generally change less
than the higher; but there are in both cases marked exceptions to the
rule. On my theory these several relations throughout time and space
are intelligible; for whether we look to the forms of life which have
changed during successive ages within the same quarter of the world, or
to those which have changed after having migrated into distant
quarters, in both cases the forms within each class have been connected
by the same bond of ordinary generation; and the more nearly any two
forms are related in blood, the nearer they will generally stand to
each other in time and space; in both cases the laws of variation have
been the same, and modifications have been accumulated by the same
power of natural selection.
CHAPTER XIII.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY:
RUDIMENTARY ORGANS.
CLASSIFICATION, groups subordinate to groups. Natural system. Rules and
difficulties in classification, explained on the theory of descent with
modification. Classification of varieties. Descent always used in
classification. Analogical or adaptive characters. Affinities, general,
complex and radiating. Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the
same individual. EMBRYOLOGY, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding
age. RUDIMENTARY ORGANS; their origin explained. Summary.
From the first dawn of life, all organic beings are found to resemble
each other in descending degrees, so that they can be classed in groups
under groups. This classification is evidently not arbitrary like the
grouping of the stars in constellations. The existence of groups would
have been of simple signification, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on
flesh, another on vegetable matter, and so on; but the case is widely
different in nature; for it is notorious how commonly members of even
the same subgroup have different habits. In our second and fourth
chapters, on Variation and on Natural Selection, I have attempted to
show that it is the widely ranging, the much diffused and common, that
is the dominant species belonging to the larger genera, which vary
most. The varieties, or incipient species, thus produced ultimately
become converted, as I believe, into new and distinct species; and
these, on the principle of inheritance, tend to produce other new and
dominant
species. Consequently the groups which are now large, and which
generally include many dominant species, tend to go on increasing
indefinitely in size. I further attempted to show that from the varying
descendants of each species trying to occupy as many and as different
places as possible in the economy of nature, there is a constant
tendency in their characters to diverge. This conclusion was supported
by looking at the great diversity of the forms of life which, in any
small area, come into the closest competition, and by looking to
certain facts in naturalisation.
I attempted also to show that there is a constant tendency in the forms
which are increasing in number and diverging in character, to supplant
and exterminate the less divergent, the less improved, and preceding
forms. I request the reader to turn to the diagram illustrating the
action, as formerly explained, of these several principles; and he will
see that the inevitable result is that the modified descendants
proceeding from one progenitor become broken up into groups subordinate
to groups. In the diagram each letter on the uppermost line may
represent a genus including several species; and all the genera on this
line form together one class, for all have descended from one ancient
but unseen parent, and, consequently, have inherited something in
common. But the three genera on the left hand have, on this same
principle, much in common, and form a sub-family, distinct from that
including the next two genera on the right hand, which diverged from a
common parent at the fifth stage of descent. These five genera have
also much, though less, in common; and they form a family distinct from
that including the three genera still further to the right hand, which
diverged at a still earlier period. And all these genera, descended
from (A), form an order distinct from the
genera descended from (I). So that we here have many species descended
from a single progenitor grouped into genera; and the genera are
included in, or subordinate to, sub-families, families, and orders, all
united into one class. Thus, the grand fact in natural history of the
subordination of group under group, which, from its familiarity, does
not always sufficiently strike us, is in my judgment fully explained.
Naturalists try to arrange the species, genera, and families in each
class, on what is called the Natural System. But what is meant by this
system? Some authors look at it merely as a scheme for arranging
together those living objects which are most alike, and for separating
those which are most unlike; or as an artificial means for enunciating,
as briefly as possible, general propositions,—that is, by one sentence
to give the characters common, for instance, to all mammals, by another
those common to all carnivora, by another those common to the
dog-genus, and then by adding a single sentence, a full description is
given of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant
by the Natural System; they believe that it reveals the plan of the
Creator; but unless it be specified whether order in time or space, or
what else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Such expressions as that famous
one of Linnæus, and which we often meet with in a more or less
concealed form, that the characters do not make the genus, but that the
genus gives the characters, seem to imply that something more is
included in our classification, than mere resemblance. I believe that
something more is included; and that propinquity of descent,—the only
known cause of the similarity of organic beings,—is the bond, hidden as
it is by various degrees of modification,
which is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits
of life, and the general place of each being in the economy of nature,
would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely “adaptive or analogical characters;” but to
the consideration of these resemblances we shall have to recur. It may
even be given as a general rule, that the less any part of the
organisation is concerned with special habits, the more important it
becomes for classification. As an instance: Owen, in speaking of the
dugong, says, “The generative organs being those which are most
remotely related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true affinities. We
are least likely in the modifications of these organs to mistake a
merely adaptive for an essential character.” So with plants, how
remarkable it is that the organs of vegetation, on which their whole
life depends, are of little signification, excepting in the first main
divisions; whereas the organs of reproduction, with their product the
seed, are of paramount importance!
We must not, therefore, in classifying, trust to resemblances in parts
of the organisation, however important
they may be for the welfare of the being in relation to the outer
world. Perhaps from this cause it has partly arisen, that almost all
naturalists lay the greatest stress on resemblances in organs of high
vital or physiological importance. No doubt this view of the
classificatory importance of organs which are important is generally,
but by no means always, true. But their importance for classification,
I believe, depends on their greater constancy throughout large groups
of species; and this constancy depends on such organs having generally
been subjected to less change in the adaptation of the species to their
conditions of life. That the mere physiological importance of an organ
does not determine its classificatory value, is almost shown by the one
fact, that in allied groups, in which the same organ, as we have every
reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have worked
at any group without being struck with this fact; and it has been most
fully acknowledged in the writings of almost every author. It will
suffice to quote the highest authority, Robert Brown, who in speaking
of certain organs in the Proteaceæ, says their generic importance,
“like that of all their parts, not only in this but, as I apprehend, in
every natural family, is very unequal, and in some cases seems to be
entirely lost.” Again in another work he says, the genera of the
Connaraceæ “differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular æstivation. Any one of
these characters singly is frequently of more than generic importance,
though here even when all taken together they appear insufficient to
separate Cnestis from Connarus.” To give an example amongst insects, in
one great division of the Hymenoptera, the antennæ, as Westwood has
remarked, are most constant in structure;
in another division they differ much, and the differences are of quite
subordinate value in classification; yet no one probably will say that
the antennæ in these two divisions of the same order are of unequal
physiological importance. Any number of instances could be given of the
varying importance for classification of the same important organ
within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of high value in classification. No one will
dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between Ruminants and
Pachyderms. Robert Brown has strongly insisted on the fact that the
rudimentary florets are of the highest importance in the classification
of the Grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance, but
which are universally admitted as highly serviceable in the definition
of whole groups. For instance, whether or not there is an open passage
from the nostrils to the mouth, the only character, according to Owen,
which absolutely distinguishes fishes and reptiles—the inflection of
the angle of the jaws in Marsupials—the manner in which the wings of
insects are folded—mere colour in certain Algæ—mere pubescence on parts
of the flower in grasses—the nature of the dermal covering, as hair or
feathers, in the Vertebrata. If the Ornithorhynchus had been covered
with feathers instead of hair, this external and trifling character
would, I think, have been considered by naturalists as important an aid
in determining the degree of affinity of this strange creature to
birds and reptiles, as an approach in structure in any one internal and
important organ.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with several other characters of more
or less importance. The value indeed of an aggregate of characters is
very evident in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters, both of high
physiological importance and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also, it has
been found, that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is universally constant. The importance of an aggregate of
characters, even when none are important, alone explains, I think, that
saying of Linnæus, that the characters do not give the genus, but the
genus gives the characters; for this saying seems founded on an
appreciation of many trifling points of resemblance, too slight to be
defined. Certain plants, belonging to the Malpighiaceæ, bear perfect
and degraded flowers; in the latter, as A. de Jussieu has remarked,
“the greater number of the characters proper to the species, to the
genus, to the family, to the class, disappear, and thus laugh at our
classification.” But when Aspicarpa produced in France, during several
years, only degraded flowers, departing so wonderfully in a number of
the most important points of structure from the proper type of the
order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained amongst the Malpighiaceæ. This case
seems to me well to illustrate the spirit with which our
classifications are sometimes necessarily founded.
Practically when naturalists are at work, they do
not trouble themselves about the physiological value of the characters
which they use in defining a group, or in allocating any particular
species. If they find a character nearly uniform, and common to a great
number of forms, and not common to others, they use it as one of high
value; if common to some lesser number, they use it as of subordinate
value. This principle has been broadly confessed by some naturalists to
be the true one; and by none more clearly than by that excellent
botanist, Aug. St. Hilaire. If certain characters are always found
correlated with others, though no apparent bond of connexion can be
discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for propelling the
blood, or for ærating it, or those for propagating the race, are found
nearly uniform, they are considered as highly serviceable in
classification; but in some groups of animals all these, the most
important vital organs, are found to offer characters of quite
subordinate value.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications
of course include all ages of each species. But it is by no means
obvious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which alone
plays its full part in the economy of nature. Yet it has been strongly
urged by those great naturalists, Milne Edwards and Agassiz, that
embryonic characters are the most important of any in the
classification of animals; and this doctrine has very generally been
admitted as true. The same fact holds good with flowering plants, of
which the two main divisions have been founded on characters derived
from the embryo,—on the number and position of the embryonic
leaves or cotyledons, and on the mode of development of the plumule and
radicle. In our discussion on embryology, we shall see why such
characters are so valuable, on the view of classification tacitly
including the idea of descent.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of characters
common to all birds; but in the case of crustaceans, such definition
has hitherto been found impossible. There are crustaceans at the
opposite ends of the series, which have hardly a character in common;
yet the species at both ends, from being plainly allied to others, and
these to others, and so onwards, can be recognised as unequivocally
belonging to this, and to no other class of the Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity
of this practice in certain groups of birds; and it has been followed
by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary. Several
of the best botanists, such as Mr. Bentham and others, have strongly
insisted on their arbitrary value. Instances could be given amongst
plants and insects, of a group of forms, first ranked by practised
naturalists as only a genus, and then raised to the rank of a
sub-family or family; and this has been done, not because further
research has detected important structural differences, at first
overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have
been unconsciously seeking, and not some unknown plan of creation, or
the enunciation of general propositions, and the mere putting together
and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the
_arrangement_ of the groups within each class, in due subordination and
relation to the other groups, must be strictly genealogical in order to
be natural; but that the _amount_ of difference in the several branches
or groups, though allied in the same degree in blood to their common
progenitor, may differ greatly, being due to the different degrees of
modification which they have undergone; and this is expressed by the
forms being ranked under different genera, families, sections, or
orders. The reader will best understand what is meant, if he will take
the trouble of referring to the diagram in the fourth chapter. We will
suppose the letters A to L to represent allied genera, which lived
during the Silurian epoch, and these have descended from a species
which existed at an unknown anterior period. Species of three of these
genera (A, F, and I) have transmitted modified descendants to the
present day, represented by the fifteen genera (_a_14 to _z_14) on the
uppermost horizontal line. Now all these modified descendants from a
single species, are represented as related in blood or descent to the
same
degree; they may metaphorically be called cousins to the same millionth
degree; yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two or three
families, constitute a distinct order from those descended from I, also
broken up into two families. Nor can the existing species, descended
from A, be ranked in the same genus with the parent A; or those from I,
with the parent I. But the existing genus F14 may be supposed to have
been but slightly modified; and it will then rank with the parent-genus
F; just as some few still living organic beings belong to Silurian
genera. So that the amount or value of the differences between organic
beings all related to each other in the same degree in blood, has come
to be widely different. Nevertheless their genealogical _arrangement_
remains strictly true, not only at the present time, but at each
successive period of descent. All the modified descendants from A will
have inherited something in common from their common parent, as will
all the descendants from I; so will it be with each subordinate branch
of descendants, at each successive period. If, however, we choose to
suppose that any of the descendants of A or of I have been so much
modified as to have more or less completely lost traces of their
parentage, in this case, their places in a natural classification will
have been more or less completely lost,—as sometimes seems to have
occurred with existing organisms. All the descendants of the genus F,
along its whole line of descent, are supposed to have been but little
modified, and they yet form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position; for F
originally was intermediate in character between A and I, and the
several genera descended from these two genera will
have inherited to a certain extent their characters. This natural
arrangement is shown, as far as is possible on paper, in the diagram,
but in much too simple a manner. If a branching diagram had not been
used, and only the names of the groups had been written in a linear
series, it would have been still less possible to have given a natural
arrangement; and it is notoriously not possible to represent in a
series, on a flat surface, the affinities which we discover in nature
amongst the beings of the same group. Thus, on the view which I hold,
the natural system is genealogical in its arrangement, like a pedigree;
but the degrees of modification which the different groups have
undergone, have to be expressed by ranking them under different
so-called genera, sub-families, families, sections, orders, and
classes.
It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken throughout
the world; and if all extinct languages, and all intermediate and
slowly changing dialects, had to be included, such an arrangement
would, I think, be the only possible one. Yet it might be that some
very ancient language had altered little, and had given rise to few new
languages, whilst others (owing to the spreading and subsequent
isolation and states of civilisation of the several races, descended
from a common race) had altered much, and had given rise to many new
languages and dialects. The various degrees of difference in the
languages from the same stock, would have to be expressed by groups
subordinate to groups; but the proper or even only possible arrangement
would still be genealogical; and this would be strictly natural, as
it would connect together all languages, extinct and modern, by the
closest affinities, and would give the filiation and origin of each
tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin of
the existence of groups subordinate to groups, is the same with
varieties as with species, namely, closeness of descent with various
degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the
necessity of classing varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the swedish
and common turnips together, though the esculent and thickened stems
are so similar. Whatever part is found to be most constant, is used in
classing varieties: thus the great agriculturist Marshall says the
horns are very useful for this purpose with cattle, because they are
less variable than the shape or colour of the body, etc.; whereas with
sheep the horns are much less serviceable, because less constant. In
classing varieties, I apprehend if we had a real pedigree, a
genealogical classification would be universally preferred; and it has
been attempted by some authors. For we might feel sure, whether there
had been more or less modification, the principle of inheritance would
keep the forms together which were allied in the greatest number of
points. In tumbler pigeons, though some sub-varieties differ from the
others
in the important character of having a longer beak, yet all are kept
together from having the common habit of tumbling; but the short-faced
breed has nearly or quite lost this habit; nevertheless, without any
reasoning or thinking on the subject, these tumblers are kept in the
same group, because allied in blood and alike in some other respects.
If it could be proved that the Hottentot had descended from the Negro,
I think he would be classed under the Negro group, however much he
might differ in colour and other important characters from negroes.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade,
or that of a species, the two sexes; and how enormously these sometimes
differ in the most important characters, is known to every naturalist:
scarcely a single fact can be predicated in common of the males and
hermaphrodites of certain cirripedes, when adult, and yet no one dreams
of separating them. The naturalist includes as one species the several
larval stages of the same individual, however much they may differ from
each other and from the adult; as he likewise includes the so-called
alternate generations of Steenstrup, which can only in a technical
sense be considered as the same individual. He includes monsters; he
includes varieties, not solely because they closely resemble the
parent-form, but because they are descended from it. He who believes
that the cowslip is descended from the primrose, or conversely, ranks
them together as a single species, and gives a single definition. As
soon as three Orchidean forms (Monochanthus, Myanthus, and Catasetum),
which had previously been ranked as three distinct genera, were known
to be sometimes produced on the same spike, they were immediately
included as a single species.
But it may be asked, what ought we to do, if it could be proved that
one species of kangaroo had been produced, by a long course of
modification, from a bear? Ought we to rank this one species with
bears, and what should we do with the other species? The supposition is
of course preposterous; and I might answer by the _argumentum ad
hominem_, and ask what should be done if a perfect kangaroo were seen
to come out of the womb of a bear? According to all analogy, it would
be ranked with bears; but then assuredly all the other species of the
kangaroo family would have to be classed under the bear genus. The
whole case is preposterous; for where there has been close descent in
common, there will certainly be close resemblance or affinity.
As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvæ
are sometimes extremely different; and as it has been used in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, though in these cases the modification has been greater
in degree, and has taken a longer time to complete? I believe it has
thus been unconsciously used; and only thus can I understand the
several rules and guides which have been followed by our best
systematists. We have no written pedigrees; we have to make out
community of descent by resemblances of any kind. Therefore we choose
those characters which, as far as we can judge, are the least likely to
have been modified in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view
are as good as, or even sometimes better than, other parts of the
organisation. We
care not how trifling a character may be—let it be the mere inflection
of the angle of the jaw, the manner in which an insect’s wing is
folded, whether the skin be covered by hair or feathers—if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for
its presence in so many forms with such different habits, only by its
inheritance from a common parent. We may err in this respect in regard
to single points of structure, but when several characters, let them be
ever so trifling, occur together throughout a large group of beings
having different habits, we may feel almost sure, on the theory of
descent, that these characters have been inherited from a common
ancestor. And we know that such correlated or aggregated characters
have especial value in classification.
We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, and yet
be safely classed with them. This may be safely done, and is often
done, as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet if these extreme forms
are connected together by a chain of intermediate groups, we may at
once infer their community of descent, and we put them all into the
same class. As we find organs of high physiological importance—those
which serve to preserve life under the most diverse conditions of
existence—are generally the most constant, we attach especial value to
them; but if these same organs, in another group or section of a group,
are found to differ much, we at once value them less in our
classification. We shall hereafter, I think, clearly see why
embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play
in classing large and widely-distributed genera, because all the
species of the same genus, inhabiting any distinct and isolated region,
have in all probability descended from the same parents.
We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and he has been
ably followed by Macleay and others. The resemblance, in the shape of
the body and in the fin-like anterior limbs, between the dugong, which
is a pachydermatous animal, and the whale, and between both these
mammals and fishes, is analogical. Amongst insects there are
innumerable instances: thus Linnæus, misled by external appearances,
actually classed an homopterous insect as a moth. We see something of
the same kind even in our domestic varieties, as in the thickened stems
of the common and swedish turnip. The resemblance of the greyhound and
racehorse is hardly more fanciful than the analogies which have been
drawn by some authors between very distinct animals. On my view of
characters being of real importance for classification, only in so far
as they reveal descent, we can clearly understand why analogical or
adaptive character, although of the utmost importance to the welfare of
the being, are almost valueless to the systematist. For animals,
belonging to two most distinct lines of descent, may readily become
adapted to similar conditions, and thus assume a close external
resemblance; but such resemblances will not reveal—will rather tend to
conceal their blood-relationship to their proper lines of descent. We
can also understand the apparent paradox, that the very same characters
are analogical when one class or order is compared with another, but
give true affinities when the members of
the same class or order are compared one with another: thus the shape
of the body and fin-like limbs are only analogical when whales are
compared with fishes, being adaptations in both classes for swimming
through the water; but the shape of the body and fin-like limbs serve
as characters exhibiting true affinity between the several members of
the whale family; for these cetaceans agree in so many characters,
great and small, that we cannot doubt that they have inherited their
general shape of body and structure of limbs from a common ancestor. So
it is with fishes.
As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,—to
inhabit for instance the three elements of land, air, and water,—we can
perhaps understand how it is that a numerical parallelism has sometimes
been observed between the sub-groups in distinct classes. A naturalist,
struck by a parallelism of this nature in any one class, by arbitrarily
raising or sinking the value of the groups in other classes (and all
our experience shows that this valuation has hitherto been arbitrary),
could easily extend the parallelism over a wide range; and thus the
septenary, quinary, quaternary, and ternary classifications have
probably arisen.
As the modified descendants of dominant species, belonging to the
larger genera, tend to inherit the advantages, which made the groups to
which they belong large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups thus tend to go
on increasing in size; and they consequently supplant many smaller and
feebler groups. Thus we can account for the fact that all organisms,
recent and extinct, are included under a few great
orders, under still fewer classes, and all in one great natural system.
As showing how few the higher groups are in number, and how widely
spread they are throughout the world, the fact is striking, that the
discovery of Australia has not added a single insect belonging to a new
order; and that in the vegetable kingdom, as I learn from Dr. Hooker,
it has added only two or three orders of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some slight
degree intermediate between existing groups. A few old and intermediate
parent-forms having occasionally transmitted to the present day
descendants but little modified, will give to us our so-called osculant
or aberrant groups. The more aberrant any form is, the greater must be
the number of connecting forms which on my theory have been
exterminated and utterly lost. And we have some evidence of aberrant
forms having suffered severely from extinction, for they are generally
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been represented by a dozen
species instead of by a single one; but such richness in species, as I
find after some investigation, does not commonly fall to the lot of
aberrant genera. We can, I think, account for this fact only by looking
at aberrant forms as failing groups conquered by more successful
competitors, with a few members preserved by some unusual coincidence
of favourable circumstances.
Mr. Waterhouse has remarked that, when a member
belonging to one group of animals exhibits an affinity to a quite
distinct group, this affinity in most cases is general and not special:
thus, according to Mr. Waterhouse, of all Rodents, the bizcacha is most
nearly related to Marsupials; but in the points in which it approaches
this order, its relations are general, and not to any one marsupial
species more than to another. As the points of affinity of the bizcacha
to Marsupials are believed to be real and not merely adaptive, they are
due on my theory to inheritance in common. Therefore we must suppose
either that all Rodents, including the bizcacha, branched off from some
very ancient Marsupial, which will have had a character in some degree
intermediate with respect to all existing Marsupials; or that both
Rodents and Marsupials branched off from a common progenitor, and that
both groups have since undergone much modification in divergent
directions. On either view we may suppose that the bizcacha has
retained, by inheritance, more of the character of its ancient
progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all
or nearly all Marsupials, from having partially retained the character
of their common progenitor, or of an early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected
that the resemblance is only analogical, owing to the phascolomys
having become adapted to habits like those of a Rodent. The elder De
Candolle has made nearly similar observations on the general nature of
the affinities of distinct orders of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from
a common parent, together with their retention by inheritance of some
characters in common, we can understand the excessively complex and
radiating affinities by which all the members of the same family or
higher group are connected together. For the common parent of a whole
family of species, now broken up by extinction into distinct groups and
sub-groups, will have transmitted some of its characters, modified in
various ways and degrees, to all; and the several species will
consequently be related to each other by circuitous lines of affinity
of various lengths (as may be seen in the diagram so often referred
to), mounting up through many predecessors. As it is difficult to show
the blood-relationship between the numerous kindred of any ancient and
noble family, even by the aid of a genealogical tree, and almost
impossible to do this without this aid, we can understand the
extraordinary difficulty which naturalists have experienced in
describing, without the aid of a diagram, the various affinities which
they perceive between the many living and extinct members of the same
great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the
distinctness of whole classes from each other—for instance, of birds
from all other vertebrate animals—by the belief that many ancient forms
of life have been utterly lost, through which the early progenitors of
birds were formerly connected with the early progenitors of the other
vertebrate classes. There has been less entire extinction of the forms
of life which once connected fishes with batrachians. There has been
still less in some other classes, as in that of the Crustacea, for here
the most wonderfully diverse forms are still tied
together by a long, but broken, chain of affinities. Extinction has
only separated groups: it has by no means made them; for if every form
which has ever lived on this earth were suddenly to reappear, though it
would be quite impossible to give definitions by which each group could
be distinguished from other groups, as all would blend together by
steps as fine as those between the finest existing varieties,
nevertheless a natural classification, or at least a natural
arrangement, would be possible. We shall see this by turning to the
diagram: the letters, A to L, may represent eleven Silurian genera,
some of which have produced large groups of modified descendants. Every
intermediate link between these eleven genera and their primordial
parent, and every intermediate link in each branch and sub-branch of
their descendants, may be supposed to be still alive; and the links to
be as fine as those between the finest varieties. In this case it would
be quite impossible to give any definition by which the several members
of the several groups could be distinguished from their more immediate
parents; or these parents from their ancient and unknown progenitor.
Yet the natural arrangement in the diagram would still hold good; and,
on the principle of inheritance, all the forms descended from A, or
from I, would have something in common. In a tree we can specify this
or that branch, though at the actual fork the two unite and blend
together. We could not, as I have said, define the several groups; but
we could pick out types, or forms, representing most of the characters
of each group, whether large or small, and thus give a general idea of
the value of the differences between them. This is what we should be
driven to, if we were ever to succeed in collecting all the forms in
any class which have lived throughout all time and space. We shall
certainly never succeed in making
so perfect a collection: nevertheless, in certain classes, we are
tending in this direction; and Milne Edwards has lately insisted, in an
able paper, on the high importance of looking to types, whether or not
we can separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction
and divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages, although having few characters in
common, under one species; we use descent in classing acknowledged
varieties, however different they may be from their parent; and I
believe this element of descent is the hidden bond of connexion which
naturalists have sought under the term of the Natural System. On this
idea of the natural system being, in so far as it has been perfected,
genealogical in its arrangement, with the grades of difference between
the descendants from a common parent, expressed by the terms genera,
families, orders, etc., we can understand the rules which we are
compelled to follow in our classification. We can understand why we
value certain resemblances far more than others; why we are permitted
to use rudimentary and useless organs, or others of trifling
physiological importance; why, in comparing one group with a distinct
group, we summarily reject analogical or adaptive characters, and yet
use these same characters within the limits of the same group. We can
clearly see how it is that all living and extinct forms can be grouped
together in one great system; and how the several members of each class
are connected together by the most complex and radiating
lines of affinities. We shall never, probably, disentangle the
inextricable web of affinities between the members of any one class;
but when we have a distinct object in view, and do not look to some
unknown plan of creation, we may hope to make sure but slow progress.
_Morphology_.—We have seen that the members of the same class,
independently of their habits of life, resemble each other in the
general plan of their organisation. This resemblance is often expressed
by the term “unity of type;” or by saying that the several parts and
organs in the different species of the class are homologous. The whole
subject is included under the general name of Morphology. This is the
most interesting department of natural history, and may be said to be
its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include the same bones, in
the same relative positions? Geoffroy St. Hilaire has insisted strongly
on the high importance of relative connexion in homologous organs: the
parts may change to almost any extent in form and size, and yet they
always remain connected together in the same order. We never find, for
instance, the bones of the arm and forearm, or of the thigh and leg,
transposed. Hence the same names can be given to the homologous bones
in widely different animals. We see the same great law in the
construction of the mouths of insects: what can be more different than
the immensely long spiral proboscis of a sphinx-moth, the curious
folded one of a bee or bug, and the great jaws of a beetle?—yet all
these organs, serving for such different
purposes, are formed by infinitely numerous modifications of an upper
lip, mandibles, and two pairs of maxillæ. Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine
of final causes. The hopelessness of the attempt has been expressly
admitted by Owen in his most interesting work on the ‘Nature of Limbs.’
On the ordinary view of the independent creation of each being, we can
only say that so it is;—that it has so pleased the Creator to construct
each animal and plant.
The explanation is manifest on the theory of the natural selection of
successive slight modifications,—each modification being profitable in
some way to the modified form, but often affecting by correlation of
growth other parts of the organisation. In changes of this nature,
there will be little or no tendency to modify the original pattern, or
to transpose parts. The bones of a limb might be shortened and widened
to any extent, and become gradually enveloped in thick membrane, so as
to serve as a fin; or a webbed foot might have all its bones, or
certain bones, lengthened to any extent, and the membrane connecting
them increased to any extent, so as to serve as a wing: yet in all this
great amount of modification there will be no tendency to alter the
framework of bones or the relative connexion of the several parts. If
we suppose that the ancient progenitor, the archetype as it may be
called, of all mammals, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can at once
perceive the plain signification of the homologous construction of the
limbs throughout the whole class. So with the mouths of insects, we
have only to
suppose that their common progenitor had an upper lip, mandibles, and
two pair of maxillæ, these parts being perhaps very simple in form; and
then natural selection will account for the infinite diversity in
structure and function of the mouths of insects. Nevertheless, it is
conceivable that the general pattern of an organ might become so much
obscured as to be finally lost, by the atrophy and ultimately by the
complete abortion of certain parts, by the soldering together of other
parts, and by the doubling or multiplication of others,—variations
which we know to be within the limits of possibility. In the paddles of
the extinct gigantic sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems to have been thus to a
certain extent obscured.
There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous
with—that is correspond in number and in relative connexion with—the
elemental parts of a certain number of vertebræ. The anterior and
posterior limbs in each member of the vertebrate and articulate classes
are plainly homologous. We see the same law in comparing the
wonderfully complex jaws and legs in crustaceans. It is familiar to
almost every one, that in a flower the relative position of the sepals,
petals, stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves,
arranged in a spire. In monstrous plants, we often get direct evidence
of the possibility of one organ being transformed into another; and we
can actually see in embryonic crustaceans and in many other animals,
and in flowers, that organs, which when mature
become extremely different, are at an early stage of growth exactly
alike.
How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and
such extraordinarily shaped pieces of bone? As Owen has remarked, the
benefit derived from the yielding of the separate pieces in the act of
parturition of mammals, will by no means explain the same construction
in the skulls of birds. Why should similar bones have been created in
the formation of the wing and leg of a bat, used as they are for such
totally different purposes? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths?
Why should the sepals, petals, stamens, and pistils in any individual
flower, though fitted for such widely different purposes, be all
constructed on the same pattern?
On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebræ
bearing certain processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external appendages;
and in flowering plants, we see a series of successive spiral whorls of
leaves. An indefinite repetition of the same part or organ is the
common characteristic (as Owen has observed) of all low or
little-modified forms; therefore we may readily believe that the
unknown progenitor of the vertebrata possessed many vertebræ; the
unknown progenitor of the articulata, many segments; and the unknown
progenitor of flowering plants, many spiral whorls of leaves. We have
formerly seen that parts many times repeated are eminently liable to
vary in number and structure; consequently it is quite probable that
natural selection, during a long-continued course of modification,
should have seized on a certain number of the primordially similar
elements, many times repeated, and have adapted them to the most
diverse purposes. And as the whole amount of modification will have
been effected by slight successive steps, we need not wonder at
discovering in such parts or organs, a certain degree of fundamental
resemblance, retained by the strong principle of inheritance.
In the great class of molluscs, though we can homologise the parts of
one species with those of another and distinct species, we can indicate
but few serial homologies; that is, we are seldom enabled to say that
one part or organ is homologous with another in the same individual.
And we can understand this fact; for in molluscs, even in the lowest
members of the class, we do not find nearly so much indefinite
repetition of any one part, as we find in the other great classes of
the animal and vegetable kingdoms.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebræ: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these cases
probably be more correct, as Professor Huxley has remarked, to speak of
both skull and vertebræ, both jaws and legs, etc.,—as having been
metamorphosed, not one from the other, but from some common element.
Naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent,
primordial organs of any kind—vertebræ in the one case and legs in the
other—have actually been modified into skulls or jaws. Yet so strong is
the appearance of a modification of this nature having occurred, that
naturalists can hardly avoid employing language having this plain
signification. On my view
these terms may be used literally; and the wonderful fact of the jaws,
for instance, of a crab retaining numerous characters, which they would
probably have retained through inheritance, if they had really been
metamorphosed during a long course of descent from true legs, or from
some simple appendage, is explained.
_Embryology_.—It has already been casually remarked that certain organs
in the individual, which when mature become widely different and serve
for different purposes, are in the embryo exactly alike. The embryos,
also, of distinct animals within the same class are often strikingly
similar: a better proof of this cannot be given, than a circumstance
mentioned by Agassiz, namely, that having forgotten to ticket the
embryo of some vertebrate animal, he cannot now tell whether it be that
of a mammal, bird, or reptile. The vermiform larvæ of moths, flies,
beetles, etc., resemble each other much more closely than do the mature
insects; but in the case of larvæ, the embryos are active, and have
been adapted for special lines of life. A trace of the law of embryonic
resemblance, sometimes lasts till a rather late age: thus birds of the
same genus, and of closely allied genera, often resemble each other in
their first and second plumage; as we see in the spotted feathers in
the thrush group. In the cat tribe, most of the species are striped or
spotted in lines; and stripes can be plainly distinguished in the whelp
of the lion. We occasionally though rarely see something of this kind
in plants: thus the embryonic leaves of the ulex or furze, and the
first leaves of the phyllodineous acaceas, are pinnate or divided like
the ordinary leaves of the leguminosæ.
The points of structure, in which the embryos of widely different
animals of the same class resemble each other, often have no direct
relation to their conditions
of existence. We cannot, for instance, suppose that in the embryos of
the vertebrata the peculiar loop-like course of the arteries near the
branchial slits are related to similar conditions,—in the young mammal
which is nourished in the womb of its mother, in the egg of the bird
which is hatched in a nest, and in the spawn of a frog under water. We
have no more reason to believe in such a relation, than we have to
believe that the same bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life. No one
will suppose that the stripes on the whelp of a lion, or the spots on
the young blackbird, are of any use to these animals, or are related to
the conditions to which they are exposed.
The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period
of activity may come on earlier or later in life; but whenever it comes
on, the adaptation of the larva to its conditions of life is just as
perfect and as beautiful as in the adult animal. From such special
adaptations, the similarity of the larvæ or active embryos of allied
animals is sometimes much obscured; and cases could be given of the
larvæ of two species, or of two groups of species, differing quite as
much, or even more, from each other than do their adult parents. In
most cases, however, the larvæ, though active, still obey more or less
closely the law of common embryonic resemblance. Cirripedes afford a
good instance of this: even the illustrious Cuvier did not perceive
that a barnacle was, as it certainly is, a crustacean; but a glance at
the larva shows this to be the case in an unmistakeable manner. So
again the two main divisions of cirripedes, the pedunculated and
sessile, which differ widely in external appearance, have larvæ in all
their several stages barely distinguishable.
The embryo in the course of development generally rises in
organisation: I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by the organisation
being higher or lower. But no one probably will dispute that the
butterfly is higher than the caterpillar. In some cases, however, the
mature animal is generally considered as lower in the scale than the
larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larvæ in the first stage have three pairs of legs, a
very simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in size. In the second stage,
answering to the chrysalis stage of butterflies, they have six pairs of
beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antennæ; but they have a closed and
imperfect mouth, and cannot feed: their function at this stage is, to
search by their well-developed organs of sense, and to reach by their
active powers of swimming, a proper place on which to become attached
and to undergo their final metamorphosis. When this is completed they
are fixed for life: their legs are now converted into prehensile
organs; they again obtain a well-constructed mouth; but they have no
antennæ, and their two eyes are now reconverted into a minute, single,
and very simple eye-spot. In this last and complete state, cirripedes
may be considered as either more highly or more lowly organised than
they were in the larval condition. But in some genera the larvæ become
developed either into hermaphrodites having the ordinary structure, or
into what I have called complemental males: and in the latter, the
development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time, and is destitute of mouth, stomach, or
other organ of importance, excepting for reproduction.
We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or
the fin of a porpoise, should not have been sketched out with all the
parts in proper proportion, as soon as any structure became visible in
the embryo. And in some whole groups of animals and in certain members
of other groups, the embryo does not at any period differ widely from
the adult: thus Owen has remarked in regard to cuttle-fish, “there is
no metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed;” and again in spiders, “there is
nothing worthy to be called a metamorphosis.” The larvæ of insects,
whether adapted to the most diverse and active habits, or quite
inactive, being fed by their parents or placed in the midst of proper
nutriment, yet nearly all pass through a similar worm-like stage of
development; but in some few cases, as in that of Aphis, if we look to
the admirable drawings by Professor Huxley of the development of this
insect, we see no trace of the vermiform stage.
How, then, can we explain these several facts in embryology,—namely the
very general, but not universal difference in structure between the
embryo and the adult;—of parts in the same individual embryo, which
ultimately become very unlike and serve for diverse purposes, being at
this early period of growth alike;—of embryos of different species
within the same class, generally, but not universally, resembling each
other;—of the structure of the embryo not being closely related to its
conditions of existence, except when the
embryo becomes at any period of life active and has to provide for
itself;—of the embryo apparently having sometimes a higher organisation
than the mature animal, into which it is developed. I believe that all
these facts can be explained, as follows, on the view of descent with
modification.
It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily
appear at an equally early period. But we have little evidence on this
head—indeed the evidence rather points the other way; for it is
notorious that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after the animal has been born,
what its merits or form will ultimately turn out. We see this plainly
in our own children; we cannot always tell whether the child will be
tall or short, or what its precise features will be. The question is
not, at what period of life any variation has been caused, but at what
period it is fully displayed. The cause may have acted, and I believe
generally has acted, even before the embryo is formed; and the
variation may be due to the male and female sexual elements having been
affected by the conditions to which either parent, or their ancestors,
have been exposed. Nevertheless an effect thus caused at a very early
period, even before the formation of the embryo, may appear late in
life; as when an hereditary disease, which appears in old age alone,
has been communicated to the offspring from the reproductive element of
one parent. Or again, as when the horns of cross-bred cattle have been
affected by the shape of the horns of either parent. For the welfare of
a very young animal, as long as it remains in its mother’s womb, or in
the egg, or as long as it is nourished and protected by its parent, it
must be quite unimportant whether most of its characters are fully
acquired a little earlier or later in life. It would not signify, for
instance, to a bird which obtained its food best by having a long beak,
whether or not it assumed a beak of this particular length, as long as
it was fed by its parents. Hence, I conclude, that it is quite
possible, that each of the many successive modifications, by which each
species has acquired its present structure, may have supervened at a
not very early period of life; and some direct evidence from our
domestic animals supports this view. But in other cases it is quite
possible that each successive modification, or most of them, may have
appeared at an extremely early period.
I have stated in the first chapter, that there is some evidence to
render it probable, that at whatever age any variation first appears in
the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages,
for instance, peculiarities in the caterpillar, cocoon, or imago states
of the silk-moth; or, again, in the horns of almost full-grown cattle.
But further than this, variations which, for all that we can see, might
have appeared earlier or later in life, tend to appear at a
corresponding age in the offspring and parent. I am far from meaning
that this is invariably the case; and I could give a good many cases of
variations (taking the word in the largest sense) which have supervened
at an earlier age in the child than in the parent.
These two principles, if their truth be admitted, will, I believe,
explain all the above specified leading facts in embryology. But first
let us look at a few analogous cases in domestic varieties. Some
authors who have written on Dogs, maintain that the greyhound and
bulldog, though appearing so different, are really varieties most
closely allied, and have probably descended from
the same wild stock; hence I was curious to see how far their puppies
differed from each other: I was told by breeders that they differed
just as much as their parents, and this, judging by the eye, seemed
almost to be the case; but on actually measuring the old dogs and their
six-days old puppies, I found that the puppies had not nearly acquired
their full amount of proportional difference. So, again, I was told
that the foals of cart and race-horses differed as much as the
full-grown animals; and this surprised me greatly, as I think it
probable that the difference between these two breeds has been wholly
caused by selection under domestication; but having had careful
measurements made of the dam and of a three-days old colt of a race and
heavy cart-horse, I find that the colts have by no means acquired their
full amount of proportional difference.
As the evidence appears to me conclusive, that the several domestic
breeds of Pigeon have descended from one wild species, I compared young
pigeons of various breeds, within twelve hours after being hatched; I
carefully measured the proportions (but will not here give details) of
the beak, width of mouth, length of nostril and of eyelid, size of feet
and length of leg, in the wild stock, in pouters, fantails, runts,
barbs, dragons, carriers, and tumblers. Now some of these birds, when
mature, differ so extraordinarily in length and form of beak, that they
would, I cannot doubt, be ranked in distinct genera, had they been
natural productions. But when the nestling birds of these several
breeds were placed in a row, though most of them could be distinguished
from each other, yet their proportional differences in the above
specified several points were incomparably less than in the full-grown
birds. Some characteristic points of difference—for instance, that of
the width of mouth—could hardly be detected in the young.
But there was one remarkable exception to this rule, for the young of
the short-faced tumbler differed from the young of the wild rock-pigeon
and of the other breeds, in all its proportions, almost exactly as much
as in the adult state.
The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties.
Fanciers select their horses, dogs, and pigeons, for breeding, when
they are nearly grown up: they are indifferent whether the desired
qualities and structures have been acquired earlier or later in life,
if the full-grown animal possesses them. And the cases just given, more
especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been
accumulated by man’s selection, have not generally first appeared at an
early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old had acquired its proper
proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been
inherited, not at the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles—which latter,
though not proved true, can be shown to be in some degree probable—to
species in a state of nature. Let us take a genus of birds, descended
on my theory from some one parent-species, and of which the several new
species have become modified through natural selection in accordance
with their diverse habits. Then, from the many slight successive steps
of variation having supervened at a rather late age, and having been
inherited at a corresponding
age, the young of the new species of our supposed genus will manifestly
tend to resemble each other much more closely than do the adults, just
as we have seen in the case of pigeons. We may extend this view to
whole families or even classes. The fore-limbs, for instance, which
served as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles—namely of
each successive modification supervening at a rather late age, and
being inherited at a corresponding late age—the fore-limbs in the
embryos of the several descendants of the parent-species will still
resemble each other closely, for they will not have been modified. But
in each individual new species, the embryonic fore-limbs will differ
greatly from the fore-limbs in the mature animal; the limbs in the
latter having undergone much modification at a rather late period of
life, and having thus been converted into hands, or paddles, or wings.
Whatever influence long-continued exercise or use on the one hand, and
disuse on the other, may have in modifying an organ, such influence
will mainly affect the mature animal, which has come to its full powers
of activity and has to gain its own living; and the effects thus
produced will be inherited at a corresponding mature age. Whereas the
young will remain unmodified, or be modified in a lesser degree, by the
effects of use and disuse.
In certain cases the successive steps of variation might supervene,
from causes of which we are wholly ignorant, at a very early period of
life, or each step might be inherited at an earlier period than that at
which it first appeared. In either case (as with the short-faced
tumbler) the young or embryo would closely
resemble the mature parent-form. We have seen that this is the rule of
development in certain whole groups of animals, as with cuttle-fish and
spiders, and with a few members of the great class of insects, as with
Aphis. With respect to the final cause of the young in these cases not
undergoing any metamorphosis, or closely resembling their parents from
their earliest age, we can see that this would result from the two
following contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for
their own wants at a very early stage of development, and secondly,
from their following exactly the same habits of life with their
parents; for in this case, it would be indispensable for the existence
of the species, that the child should be modified at a very early age
in the same manner with its parents, in accordance with their similar
habits. Some further explanation, however, of the embryo not undergoing
any metamorphosis is perhaps requisite. If, on the other hand, it
profited the young to follow habits of life in any degree different
from those of their parent, and consequently to be constructed in a
slightly different manner, then, on the principle of inheritance at
corresponding ages, the active young or larvæ might easily be rendered
by natural selection different to any conceivable extent from their
parents. Such differences might, also, become correlated with
successive stages of development; so that the larvæ, in the first
stage, might differ greatly from the larvæ in the second stage, as we
have seen to be the case with cirripedes. The adult might become fitted
for sites or habits, in which organs of locomotion or of the senses,
etc., would be useless; and in this case the final metamorphosis would
be said to be retrograde.
As all the organic beings, extinct and recent, which
have ever lived on this earth have to be classed together, and as all
have been connected by the finest gradations, the best, or indeed, if
our collections were nearly perfect, the only possible arrangement,
would be genealogical. Descent being on my view the hidden bond of
connexion which naturalists have been seeking under the term of the
natural system. On this view we can understand how it is that, in the
eyes of most naturalists, the structure of the embryo is even more
important for classification than that of the adult. For the embryo is
the animal in its less modified state; and in so far it reveals the
structure of its progenitor. In two groups of animal, however much they
may at present differ from each other in structure and habits, if they
pass through the same or similar embryonic stages, we may feel assured
that they have both descended from the same or nearly similar parents,
and are therefore in that degree closely related. Thus, community in
embryonic structure reveals community of descent. It will reveal this
community of descent, however much the structure of the adult may have
been modified and obscured; we have seen, for instance, that cirripedes
can at once be recognised by their larvæ as belonging to the great
class of crustaceans. As the embryonic state of each species and group
of species partially shows us the structure of their less modified
ancient progenitors, we can clearly see why ancient and extinct forms
of life should resemble the embryos of their descendants,—our existing
species. Agassiz believes this to be a law of nature; but I am bound to
confess that I only hope to see the law hereafter proved true. It can
be proved true in those cases alone in which the ancient state, now
supposed to be represented in many embryos, has not been obliterated,
either by the successive variations in a long course of modification
having supervened
at a very early age, or by the variations having been inherited at an
earlier period than that at which they first appeared. It should also
be borne in mind, that the supposed law of resemblance of ancient forms
of life to the embryonic stages of recent forms, may be true, but yet,
owing to the geological record not extending far enough back in time,
may remain for a long period, or for ever, incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many
descendants from some one ancient progenitor, at a very early period in
the life of each, though perhaps caused at the earliest, and being
inherited at a corresponding not early period. Embryology rises greatly
in interest, when we thus look at the embryo as a picture, more or less
obscured, of the common parent-form of each great class of animals.
_Rudimentary, atrophied, or aborted organs_.—Organs or parts in this
strange condition, bearing the stamp of inutility, are extremely common
throughout nature. For instance, rudimentary mammæ are very general in
the males of mammals: I presume that the “bastard-wing” in birds may be
safely considered as a digit in a rudimentary state: in very many
snakes one lobe of the lungs is rudimentary; in other snakes there are
rudiments of the pelvis and hind limbs. Some of the cases of
rudimentary organs are extremely curious; for instance, the presence of
teeth in foetal whales, which when grown up have not a tooth in their
heads; and the presence of teeth, which never cut through the gums, in
the upper jaws of our unborn calves. It has even been stated on good
authority that rudiments of teeth can be detected
in the beaks of certain embryonic birds. Nothing can be plainer than
that wings are formed for flight, yet in how many insects do we see
wings so reduced in size as to be utterly incapable of flight, and not
rarely lying under wing-cases, firmly soldered together!
The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same
species) resembling each other most closely in all respects, one of
which will have full-sized wings, and another mere rudiments of
membrane; and here it is impossible to doubt, that the rudiments
represent wings. Rudimentary organs sometimes retain their
potentiality, and are merely not developed: this seems to be the case
with the mammæ of male mammals, for many instances are on record of
these organs having become well developed in full-grown males, and
having secreted milk. So again there are normally four developed and
two rudimentary teats in the udders of the genus Bos, but in our
domestic cows the two sometimes become developed and give milk. In
individual plants of the same species the petals sometimes occur as
mere rudiments, and sometimes in a well-developed state. In plants with
separated sexes, the male flowers often have a rudiment of a pistil;
and Kölreuter found that by crossing such male plants with an
hermaphrodite species, the rudiment of the pistil in the hybrid
offspring was much increased in size; and this shows that the rudiment
and the perfect pistil are essentially alike in nature.
An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules protected in the ovarium at
its base. The pistil consists of a stigma
supported on the style; but in some Compositæ, the male florets, which
of course cannot be fecundated, have a pistil, which is in a
rudimentary state, for it is not crowned with a stigma; but the style
remains well developed, and is clothed with hairs as in other
compositæ, for the purpose of brushing the pollen out of the
surrounding anthers. Again, an organ may become rudimentary for its
proper purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or
lung. Other similar instances could be given.
Rudimentary organs in the individuals of the same species are very
liable to vary in degree of development and in other respects.
Moreover, in closely allied species, the degree to which the same organ
has been rendered rudimentary occasionally differs much. This latter
fact is well exemplified in the state of the wings of the female moths
in certain groups. Rudimentary organs may be utterly aborted; and this
implies, that we find in an animal or plant no trace of an organ, which
analogy would lead us to expect to find, and which is occasionally
found in monstrous individuals of the species. Thus in the snapdragon
(antirrhinum) we generally do not find a rudiment of a fifth stamen;
but this may sometimes be seen. In tracing the homologies of the same
part in different members of a class, nothing is more common, or more
necessary, than the use and discovery of rudiments. This is well shown
in the drawings given by Owen of the bones of the leg of the horse, ox,
and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a
universal
rule, that a rudimentary part or organ is of greater size relatively to
the adjoining parts in the embryo, than in the adult; so that the organ
at this early age is less rudimentary, or even cannot be said to be in
any degree rudimentary. Hence, also, a rudimentary organ in the adult,
is often said to have retained its embryonic condition.
I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment: for
the same reasoning power which tells us plainly that most parts and
organs are exquisitely adapted for certain purposes, tells us with
equal plainness that these rudimentary or atrophied organs, are
imperfect and useless. In works on natural history rudimentary organs
are generally said to have been created “for the sake of symmetry,” or
in order “to complete the scheme of nature;” but this seems to me no
explanation, merely a restatement of the fact. Would it be thought
sufficient to say that because planets revolve in elliptic courses
round the sun, satellites follow the same course round the planets, for
the sake of symmetry, and to complete the scheme of nature? An eminent
physiologist accounts for the presence of rudimentary organs, by
supposing that they serve to excrete matter in excess, or injurious to
the system; but can we suppose that the minute papilla, which often
represents the pistil in male flowers, and which is formed merely of
cellular tissue, can thus act? Can we suppose that the formation of
rudimentary teeth which are subsequently absorbed, can be of any
service to the rapidly growing embryonic calf by the excretion of
precious phosphate of lime? When a man’s fingers have been amputated,
imperfect nails sometimes appear on the stumps: I could as soon believe
that these vestiges of nails have appeared, not from unknown laws
of growth, but in order to excrete horny matter, as that the
rudimentary nails on the fin of the manatee were formed for this
purpose.
On my view of descent with modification, the origin of rudimentary
organs is simple. We have plenty of cases of rudimentary organs in our
domestic productions,—as the stump of a tail in tailless breeds,—the
vestige of an ear in earless breeds,—the reappearance of minute
dangling horns in hornless breeds of cattle, more especially, according
to Youatt, in young animals,—and the state of the whole flower in the
cauliflower. We often see rudiments of various parts in monsters. But I
doubt whether any of these cases throw light on the origin of
rudimentary organs in a state of nature, further than by showing that
rudiments can be produced; for I doubt whether species under nature
ever undergo abrupt changes. I believe that disuse has been the main
agency; that it has led in successive generations to the gradual
reduction of various organs, until they have become rudimentary,—as in
the case of the eyes of animals inhabiting dark caverns, and of the
wings of birds inhabiting oceanic islands, which have seldom been
forced to take flight, and have ultimately lost the power of flying.
Again, an organ useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small and exposed
islands; and in this case natural selection would continue slowly to
reduce the organ, until it was rendered harmless and rudimentary.
Any change in function, which can be effected by insensibly small
steps, is within the power of natural selection; so that an organ
rendered, during changed habits of life, useless or injurious for one
purpose, might easily be modified and used for another purpose. Or an
organ might be retained for one alone of its
former functions. An organ, when rendered useless, may well be
variable, for its variations cannot be checked by natural selection. At
whatever period of life disuse or selection reduces an organ, and this
will generally be when the being has come to maturity and to its full
powers of action, the principle of inheritance at corresponding ages
will reproduce the organ in its reduced state at the same age, and
consequently will seldom affect or reduce it in the embryo. Thus we can
understand the greater relative size of rudimentary organs in the
embryo, and their lesser relative size in the adult. But if each step
of the process of reduction were to be inherited, not at the
corresponding age, but at an extremely early period of life (as we have
good reason to believe to be possible) the rudimentary part would tend
to be wholly lost, and we should have a case of complete abortion. The
principle, also, of economy, explained in a former chapter, by which
the materials forming any part or structure, if not useful to the
possessor, will be saved as far as is possible, will probably often
come into play; and this will tend to cause the entire obliteration of
a rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency in
every part of the organisation, which has long existed, to be
inherited—we can understand, on the genealogical view of
classification, how it is that systematists have found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clue in seeking for its
derivation. On the view of descent with modification, we may conclude
that the existence of organs in a rudimentary, imperfect, and useless
condition, or quite aborted, far
from presenting a strange difficulty, as they assuredly do on the
ordinary doctrine of creation, might even have been anticipated, and
can be accounted for by the laws of inheritance.
_Summary_.—In this chapter I have attempted to show, that the
subordination of group to group in all organisms throughout all time;
that the nature of the relationship, by which all living and extinct
beings are united by complex, radiating, and circuitous lines of
affinities into one grand system; the rules followed and the
difficulties encountered by naturalists in their classifications; the
value set upon characters, if constant and prevalent, whether of high
vital importance, or of the most trifling importance, or, as in
rudimentary organs, of no importance; the wide opposition in value
between analogical or adaptive characters, and characters of true
affinity; and other such rules;—all naturally follow on the view of the
common parentage of those forms which are considered by naturalists as
allied, together with their modification through natural selection,
with its contingencies of extinction and divergence of character. In
considering this view of classification, it should be borne in mind
that the element of descent has been universally used in ranking
together the sexes, ages, and acknowledged varieties of the same
species, however different they may be in structure. If we extend the
use of this element of descent,—the only certainly known cause of
similarity in organic beings,—we shall understand what is meant by the
natural system: it is genealogical in its attempted arrangement, with
the grades of acquired difference marked by the terms varieties,
species, genera, families, orders, and classes.
On this same view of descent with modification, all the great facts in
Morphology become intelligible,—whether
we look to the same pattern displayed in the homologous organs, to
whatever purpose applied, of the different species of a class; or to
the homologous parts constructed on the same pattern in each individual
animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the great
leading facts in Embryology; namely, the resemblance in an individual
embryo of the homologous parts, which when matured will become widely
different from each other in structure and function; and the
resemblance in different species of a class of the homologous parts or
organs, though fitted in the adult members for purposes as different as
possible. Larvæ are active embryos, which have become specially
modified in relation to their habits of life, through the principle of
modifications being inherited at corresponding ages. On this same
principle—and bearing in mind, that when organs are reduced in size,
either from disuse or selection, it will generally be at that period of
life when the being has to provide for its own wants, and bearing in
mind how strong is the principle of inheritance—the occurrence of
rudimentary organs and their final abortion, present to us no
inexplicable difficulties; on the contrary, their presence might have
been even anticipated. The importance of embryological characters and
of rudimentary organs in classification is intelligible, on the view
that an arrangement is only so far natural as it is genealogical.
Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera, and families of organic beings, with which this world
is
peopled, have all descended, each within its own class or group, from
common parents, and have all been modified in the course of descent,
that I should without hesitation adopt this view, even if it were
unsupported by other facts or arguments.
CHAPTER XIV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species. How far
the theory of natural selection may be extended. Effects of its
adoption on the study of Natural history. Concluding remarks.
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I
have endeavoured to give to them their full force. Nothing at first can
appear more difficult to believe than that the more complex organs and
instincts should have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable
slight variations, each good for the individual possessor.
Nevertheless, this difficulty, though appearing to our imagination
insuperably great, cannot be considered real if we admit the following
propositions, namely,—that gradations in the perfection of any organ or
instinct, which we may consider, either do now exist or could have
existed, each good of its kind,—that all organs and instincts are, in
ever so slight a degree, variable,—and, lastly, that there is a
struggle for existence leading to the preservation of each profitable
deviation of structure or instinct. The truth of these propositions
cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially amongst
broken and failing groups of organic beings; but we see so many strange
gradations in nature, as is proclaimed by the canon, “Natura non facit
saltum,” that we ought to be extremely cautious in saying that any
organ or instinct, or any whole being, could not have arrived at its
present state by many graduated steps. There are, it must be admitted,
cases of special difficulty on the theory of natural selection; and one
of the most curious of these is the existence of two or three defined
castes of workers or sterile females in the same community of ants; but
I have attempted to show how this difficulty can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter,
which seem to me conclusively to show that this sterility is no more a
special endowment than is the incapacity of two trees to be grafted
together, but that it is incidental on constitutional differences in
the reproductive systems of the intercrossed species. We see the truth
of this conclusion in the vast difference in the result, when the same
two species are crossed reciprocally; that is, when one species is
first used as the father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either
their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the
varieties which have been experimentised on have been produced under
domestication; and as domestication apparently tends to eliminate
sterility, we ought not to expect it also to produce sterility.
The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds
are rendered in some degree sterile from their constitutions having
been disturbed by slightly different and new conditions of life, we
need not feel surprise at hybrids being in some degree sterile, for
their constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is supported
by another parallel, but directly opposite, class of facts; namely,
that the vigour and fertility of all organic beings are increased by
slight changes in their conditions of life, and that the offspring of
slightly modified forms or varieties acquire from being crossed
increased vigour and fertility. So that, on the one hand, considerable
changes in the conditions of life and crosses between greatly modified
forms, lessen fertility; and on the other hand, lesser changes in the
conditions of life and crosses between less modified forms, increase
fertility.
Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, must have descended from common parents; and
therefore, in however distant and isolated parts of the world they are
now found, they must in the course of successive generations have
passed from some one part to the others. We are often wholly unable
even to conjecture how this could have been effected. Yet, as we have
reason to believe that some species have retained the same specific
form for very long periods, enormously long as measured by years, too
much stress ought not to be laid on the occasional wide diffusion of
the same species; for during very long periods of time there will
always be a good chance for wide migration by many means. A broken or
interrupted range may often be accounted for by the extinction of the
species in the intermediate regions. It cannot be denied that we are as
yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern
periods; and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent has been
the influence of the Glacial period on the distribution both of the
same and of representative species throughout the world. We are as yet
profoundly ignorant of the many occasional means of transport. With
respect to distinct species of the same genus inhabiting very distant
and isolated regions, as the process of modification has necessarily
been slow, all the means of migration will have been possible during a
very long period; and consequently the difficulty of the wide diffusion
of species of the same genus is in some degree lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species
in each group by gradations as fine as our present varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover _directly_ connecting
links between them, but only between each and some extinct and
supplanted form. Even on a wide area, which has during a long period
remained continuous, and of which the climate and other conditions of
life change insensibly in going from a district occupied by one species
into another district occupied by a closely allied species, we have no
just right to expect often to find intermediate varieties in the
intermediate zone. For we have reason to believe that only a few
species are undergoing change at any one period; and all changes are
slowly effected. I have also shown that the intermediate varieties
which will at first probably exist in the intermediate zones, will be
liable to be supplanted by the allied forms on either hand; and the
latter, from existing in greater numbers, will generally be modified
and improved at a quicker rate than the intermediate varieties, which
exist in lesser numbers; so that the intermediate varieties will, in
the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why
is not every geological formation charged with such links? Why does not
every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? We meet with no such
evidence, and this is the most obvious and forcible of the many
objections which may be urged against my theory. Why, again, do whole
groups of allied species appear, though certainly they often falsely
appear, to have come in suddenly on the several geological stages? Why
do we not find great piles of strata beneath the Silurian system,
stored with the remains of the progenitors of the Silurian groups of
fossils? For certainly on my theory such
strata must somewhere have been deposited at these ancient and utterly
unknown epochs in the world’s history.
I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which
certainly have existed. We should not be able to recognise a species as
the parent of any one or more species if we were to examine them ever
so closely, unless we likewise possessed many of the intermediate links
between their past or parent and present states; and these many links
we could hardly ever expect to discover, owing to the imperfection of
the geological record. Numerous existing doubtful forms could be named
which are probably varieties; but who will pretend that in future ages
so many fossil links will be discovered, that naturalists will be able
to decide, on the common view, whether or not these doubtful forms are
varieties? As long as most of the links between any two species are
unknown, if any one link or intermediate variety be discovered, it will
simply be classed as another and distinct species. Only a small portion
of the world has been geologically explored. Only organic beings of
certain classes can be preserved in a fossil condition, at least in any
great number. Widely ranging species vary most, and varieties are often
at first local,—both causes rendering the discovery of intermediate
links less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved;
and when they do spread, if discovered in a geological formation, they
will appear as if suddenly created there, and will be simply classed as
new species. Most formations have been intermittent in their
accumulation; and their duration, I am inclined to believe, has been
shorter than the average duration of specific forms. Successive
formations are separated from each other by enormous blank intervals of
time; for fossiliferous formations, thick enough to resist future
degradation, can be accumulated only where much sediment is deposited
on the subsiding bed of the sea. During the alternate periods of
elevation and of stationary level the record will be blank. During
these latter periods there will probably be more variability in the
forms of life; during periods of subsidence, more extinction.
With respect to the absence of fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in the
ninth chapter. That the geological record is imperfect all will admit;
but that it is imperfect to the degree which I require, few will be
inclined to admit. If we look to long enough intervals of time, geology
plainly declares that all species have changed; and they have changed
in the manner which my theory requires, for they have changed slowly
and in a graduated manner. We clearly see this in the fossil remains
from consecutive formations invariably being much more closely related
to each other, than are the fossils from formations distant from each
other in time.
Such is the sum of the several chief objections and difficulties which
may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to
doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly
ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most
perfect organs; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that we know
how imperfect the Geological Record is. Grave as these several
difficulties are, in my judgment they do not overthrow the theory of
descent with modification.
Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the
reproductive system being eminently susceptible to changes in the
conditions of life; so that this system, when not rendered impotent,
fails to reproduce offspring exactly like the parent-form. Variability
is governed by many complex laws,—by correlation of growth, by use and
disuse, and by the direct action of the physical conditions of life.
There is much difficulty in ascertaining how much modification our
domestic productions have undergone; but we may safely infer that the
amount has been large, and that modifications can be inherited for long
periods. As long as the conditions of life remain the same, we have
reason to believe that a modification, which has already been inherited
for many generations, may continue to be inherited for an almost
infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not wholly cease;
for new varieties are still occasionally produced by our most anciently
domesticated productions.
Man does not actually produce variability; he only
unintentionally exposes organic beings to new conditions of life, and
then nature acts on the organisation, and causes variability. But man
can and does select the variations given to him by nature, and thus
accumulate them in any desired manner. He thus adapts animals and
plants for his own benefit or pleasure. He may do this methodically, or
he may do it unconsciously by preserving the individuals most useful to
him at the time, without any thought of altering the breed. It is
certain that he can largely influence the character of a breed by
selecting, in each successive generation, individual differences so
slight as to be quite inappreciable by an uneducated eye. This process
of selection has been the great agency in the production of the most
distinct and useful domestic breeds. That many of the breeds produced
by man have to a large extent the character of natural species, is
shown by the inextricable doubts whether very many of them are
varieties or aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is
common to all organic beings. This high rate of increase is proved by
calculation, by the effects of a succession of peculiar seasons, and by
the results of naturalisation, as explained in the third chapter. More
individuals are born than can possibly survive. A grain in the balance
will determine which individual shall live and which shall die,—which
variety or species shall increase in number, and which shall decrease,
or finally become extinct. As the individuals
of the same species come in all respects into the closest competition
with each other, the struggle will generally be most severe between
them; it will be almost equally severe between the varieties of the
same species, and next in severity between the species of the same
genus. But the struggle will often be very severe between beings most
remote in the scale of nature. The slightest advantage in one being, at
any age or during any season, over those with which it comes into
competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of
defence, or on the charms of the males; and the slightest advantage
will lead to victory.
As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would have
varied under nature, in the same way as they generally have varied
under the changed conditions of domestication. And if there be any
variability under nature, it would be an unaccountable fact if natural
selection had not come into play. It has often been asserted, but the
assertion is quite incapable of proof, that the amount of variation
under nature is a strictly limited quantity. Man, though acting on
external characters alone and often capriciously, can produce within a
short period a great result by adding up mere individual differences in
his domestic productions; and every one admits that there are at least
individual differences in species under nature. But, besides such
differences, all naturalists
have admitted the existence of varieties, which they think sufficiently
distinct to be worthy of record in systematic works. No one can draw
any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and sub-species,
and species. Let it be observed how naturalists differ in the rank
which they assign to the many representative forms in Europe and North
America.
If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any way
useful to beings, under their excessively complex relations of life,
would be preserved, accumulated, and inherited? Why, if man can by
patience select variations most useful to himself, should nature fail
in selecting variations useful, under changing conditions of life, to
her living products? What limit can be put to this power, acting during
long ages and rigidly scrutinising the whole constitution, structure,
and habits of each creature,—favouring the good and rejecting the bad?
I can see no limit to this power, in slowly and beautifully adapting
each form to the most complex relations of life. The theory of natural
selection, even if we looked no further than this, seems to me to be in
itself probable. I have already recapitulated, as fairly as I could,
the opposed difficulties and objections: now let us turn to the special
facts and arguments in favour of the theory.
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see
why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation,
and varieties which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that in each region
where many species of a genus have been produced, and where they now
flourish, these same species should present many varieties; for where
the manufactory of species has been active, we might expect, as a
general rule, to find it still in action; and this is the case if
varieties be incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of the
larger genera apparently have restricted ranges, and they are clustered
in little groups round other species—in which respects they resemble
varieties. These are strange relations on the view of each species
having been independently created, but are intelligible if all species
first existed as varieties.
As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by so much the more as they
become more diversified in habits and structure, so as to be enabled to
seize on many and widely different places in the economy of nature,
there will be a constant tendency in natural selection to preserve the
most divergent offspring of any one species. Hence during a
long-continued course of modification, the slight differences,
characteristic of varieties of the same species, tend to be augmented
into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and
thus species are rendered to a large extent defined and distinct
objects. Dominant species belonging to the larger groups tend to give
birth to new and dominant
forms; so that each large group tends to become still larger, and at
the same time more divergent in character. But as all groups cannot
thus succeed in increasing in size, for the world would not hold them,
the more dominant groups beat the less dominant. This tendency in the
large groups to go on increasing in size and diverging in character,
together with the almost inevitable contingency of much extinction,
explains the arrangement of all the forms of life, in groups
subordinate to groups, all within a few great classes, which we now see
everywhere around us, and which has prevailed throughout all time. This
grand fact of the grouping of all organic beings seems to me utterly
inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification;
it can act only by very short and slow steps. Hence the canon of
“Natura non facit saltum,” which every fresh addition to our knowledge
tends to make more strictly correct, is on this theory simply
intelligible. We can plainly see why nature is prodigal in variety,
though niggard in innovation. But why this should be a law of nature if
each species has been independently created, no man can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have
been created to prey on insects on the ground; that upland geese, which
never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic
insects; and that a petrel should have been created with habits and
structure fitting it for the life of an auk or grebe! and so on in
endless other cases. But on the view of each
species constantly trying to increase in number, with natural selection
always ready to adapt the slowly varying descendants of each to any
unoccupied or ill-occupied place in nature, these facts cease to be
strange, or perhaps might even have been anticipated.
As natural selection acts by competition, it adapts the inhabitants of
each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of any
one country, although on the ordinary view supposed to have been
specially created and adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as far as we can
judge, absolutely perfect; and if some of them be abhorrent to our
ideas of fitness. We need not marvel at the sting of the bee causing
the bee’s own death; at drones being produced in such vast numbers for
one single act, and being then slaughtered by their sterile sisters; at
the astonishing waste of pollen by our fir-trees; at the instinctive
hatred of the queen bee for her own fertile daughters; at ichneumonidæ
feeding within the live bodies of caterpillars; and at other such
cases. The wonder indeed is, on the theory of natural selection, that
more cases of the want of absolute perfection have not been observed.
The complex and little known laws governing variation are the same, as
far as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to
have produced but little direct effect; yet when varieties enter any
zone, they occasionally assume some of the characters of the species
proper to that zone. In both varieties and species, use and disuse seem
to have produced some effect; for it is difficult to resist this
conclusion
when we look, for instance, at the logger-headed duck, which has wings
incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, which is occasionally
blind, and then at certain moles, which are habitually blind and have
their eyes covered with skin; or when we look at the blind animals
inhabiting the dark caves of America and Europe. In both varieties and
species correlation of growth seems to have played a most important
part, so that when one part has been modified other parts are
necessarily modified. In both varieties and species reversions to
long-lost characters occur. How inexplicable on the theory of creation
is the occasional appearance of stripes on the shoulder and legs of the
several species of the horse-genus and in their hybrids! How simply is
this fact explained if we believe that these species have descended
from a striped progenitor, in the same manner as the several domestic
breeds of pigeon have descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently created,
why should the specific characters, or those by which the species of
the same genus differ from each other, be more variable than the
generic characters in which they all agree? Why, for instance, should
the colour of a flower be more likely to vary in any one species of a
genus, if the other species, supposed to have been created
independently, have differently coloured flowers, than if all the
species of the genus have the same coloured flowers? If species are
only well-marked varieties, of which the characters have become in a
high degree permanent, we can understand this fact; for they have
already varied since they branched off from a common progenitor in
certain characters, by which they have come to be specifically distinct
from each other;
and therefore these same characters would be more likely still to be
variable than the generic characters which have been inherited without
change for an enormous period. It is inexplicable on the theory of
creation why a part developed in a very unusual manner in any one
species of a genus, and therefore, as we may naturally infer, of great
importance to the species, should be eminently liable to variation;
but, on my view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount of variability
and modification, and therefore we might expect this part generally to
be still variable. But a part may be developed in the most unusual
manner, like the wing of a bat, and yet not be more variable than any
other structure, if the part be common to many subordinate forms, that
is, if it has been inherited for a very long period; for in this case
it will have been rendered constant by long-continued natural
selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation throws
on the admirable architectural powers of the hive-bee. Habit no doubt
sometimes comes into play in modifying instincts; but it certainly is
not indispensable, as we see, in the case of neuter insects, which
leave no progeny to inherit the effects of long-continued habit. On the
view of all the species of the same genus having descended from a
common parent, and having inherited much in common, we can understand
how it is that allied species, when placed under considerably different
conditions of life,
yet should follow nearly the same instincts; why the thrush of South
America, for instance, lines her nest with mud like our British
species. On the view of instincts having been slowly acquired through
natural selection we need not marvel at some instincts being apparently
not perfect and liable to mistakes, and at many instincts causing other
animals to suffer.
If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents,—in being
absorbed into each other by successive crosses, and in other such
points,—as do the crossed offspring of acknowledged varieties. On the
other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary
laws.
If we admit that the geological record is imperfect in an extreme
degree, then such facts as the record gives, support the theory of
descent with modification. New species have come on the stage slowly
and at successive intervals; and the amount of change, after equal
intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world, almost
inevitably follows on the principle of natural selection; for old forms
will be supplanted by new and improved forms. Neither single species
nor groups of species reappear when the chain of ordinary generation
has once been broken. The gradual diffusion of dominant forms, with the
slow modification of their descendants, causes the forms of life, after
long intervals of time, to appear as if they had changed simultaneously
throughout the world. The fact of the fossil remains of each formation
being in some degree intermediate in character between the
fossils in the formations above and below, is simply explained by their
intermediate position in the chain of descent. The grand fact that all
extinct organic beings belong to the same system with recent beings,
falling either into the same or into intermediate groups, follows from
the living and the extinct being the offspring of common parents. As
the groups which have descended from an ancient progenitor have
generally diverged in character, the progenitor with its early
descendants will often be intermediate in character in comparison with
its later descendants; and thus we can see why the more ancient a
fossil is, the oftener it stands in some degree intermediate between
existing and allied groups. Recent forms are generally looked at as
being, in some vague sense, higher than ancient and extinct forms; and
they are in so far higher as the later and more improved forms have
conquered the older and less improved organic beings in the struggle
for life. Lastly, the law of the long endurance of allied forms on the
same continent,—of marsupials in Australia, of edentata in America, and
other such cases,—is intelligible, for within a confined country, the
recent and the extinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the
world to another, owing to former climatal and geographical changes and
to the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in Distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both
cases the beings have been connected by the bond of ordinary
generation, and the means of
modification have been the same. We see the full meaning of the
wonderful fact, which must have struck every traveller, namely, that on
the same continent, under the most diverse conditions, under heat and
cold, on mountain and lowland, on deserts and marshes, most of the
inhabitants within each great class are plainly related; for they will
generally be descendants of the same progenitors and early colonists.
On this same principle of former migration, combined in most cases with
modification, we can understand, by the aid of the Glacial period, the
identity of some few plants, and the close alliance of many others, on
the most distant mountains, under the most different climates; and
likewise the close alliance of some of the inhabitants of the sea in
the northern and southern temperate zones, though separated by the
whole intertropical ocean. Although two areas may present the same
physical conditions of life, we need feel no surprise at their
inhabitants being widely different, if they have been for a long period
completely separated from each other; for as the relation of organism
to organism is the most important of all relations, and as the two
areas will have received colonists from some third source or from each
other, at various periods and in different proportions, the course of
modification in the two areas will inevitably be different.
On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that
many should be peculiar. We can clearly see why those animals which
cannot cross wide spaces of ocean, as frogs and terrestrial mammals,
should not inhabit oceanic islands; and why, on the other hand, new and
peculiar species of bats, which can traverse the ocean, should so often
be found on islands far distant from any continent. Such facts
as the presence of peculiar species of bats, and the absence of all
other mammals, on oceanic islands, are utterly inexplicable on the
theory of independent acts of creation.
The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the
same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species common to both still exist. Wherever many closely
allied yet distinct species occur, many doubtful forms and varieties of
the same species likewise occur. It is a rule of high generality that
the inhabitants of each area are related to the inhabitants of the
nearest source whence immigrants might have been derived. We see this
in nearly all the plants and animals of the Galapagos archipelago, of
Juan Fernandez, and of the other American islands being related in the
most striking manner to the plants and animals of the neighbouring
American mainland; and those of the Cape de Verde archipelago and other
African islands to the African mainland. It must be admitted that these
facts receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group,
and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies
of extinction and divergence of character. On these same principles we
see how it is, that the mutual affinities of the species and genera
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification;—why
adaptive characters, though of paramount importance to the being, are
of hardly any
importance in classification; why characters derived from rudimentary
parts, though of no service to the being, are often of high
classificatory value; and why embryological characters are the most
valuable of all. The real affinities of all organic beings are due to
inheritance or community of descent. The natural system is a
genealogical arrangement, in which we have to discover the lines of
descent by the most permanent characters, however slight their vital
importance may be.
The framework of bones being the same in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse,—the same number of
vertebræ forming the neck of the giraffe and of the elephant,—and
innumerable other such facts, at once explain themselves on the theory
of descent with slow and slight successive modifications. The
similarity of pattern in the wing and leg of a bat, though used for
such different purpose,—in the jaws and legs of a crab,—in the petals,
stamens, and pistils of a flower, is likewise intelligible on the view
of the gradual modification of parts or organs, which were alike in the
early progenitor of each class. On the principle of successive
variations not always supervening at an early age, and being inherited
at a corresponding not early period of life, we can clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
and arteries running in loops, like those in a fish which has to
breathe the air dissolved in water, by the aid of well-developed
branchiæ.
Disuse, aided sometimes by natural selection, will often tend to reduce
an organ, when it has become useless by changed habits or under changed
conditions
of life; and we can clearly understand on this view the meaning of
rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in
the struggle for existence, and will thus have little power of acting
on an organ during early life; hence the organ will not be much reduced
or rendered rudimentary at this early age. The calf, for instance, has
inherited teeth, which never cut through the gums of the upper jaw,
from an early progenitor having well-developed teeth; and we may
believe, that the teeth in the mature animal were reduced, during
successive generations, by disuse or by the tongue and palate having
been fitted by natural selection to browse without their aid; whereas
in the calf, the teeth have been left untouched by selection or disuse,
and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each
organic being and each separate organ having been specially created,
how utterly inexplicable it is that parts, like the teeth in the
embryonic calf or like the shrivelled wings under the soldered
wing-covers of some beetles, should thus so frequently bear the plain
stamp of inutility! Nature may be said to have taken pains to reveal,
by rudimentary organs and by homologous structures, her scheme of
modification, which it seems that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability
of species? It cannot be
asserted that organic beings in a state of nature are subject to no
variation; it cannot be proved that the amount of variation in the
course of long ages is a limited quantity; no clear distinction has
been, or can be, drawn between species and well-marked varieties. It
cannot be maintained that species when intercrossed are invariably
sterile, and varieties invariably fertile; or that sterility is a
special endowment and sign of creation. The belief that species were
immutable productions was almost unavoidable as long as the history of
the world was thought to be of short duration; and now that we have
acquired some idea of the lapse of time, we are too apt to assume,
without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.
But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the
term of a hundred million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an almost
infinite number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy
to hide our ignorance under such expressions as the “plan of creation,”
“unity of design,” etc., and to think that we give an explanation when
we only restate a fact. Any one whose disposition leads him to attach
more weight to unexplained difficulties than to the explanation of a
certain number of facts will certainly reject my theory. A few
naturalists, endowed with much flexibility of mind, and who have
already begun to doubt on the immutability of species, may be
influenced by this volume; but I look with confidence to the future, to
young and rising naturalists, who will be able to view both sides of
the question with impartiality. Whoever is led to believe that species
are mutable will do good service by conscientiously expressing his
conviction; for only thus can the load of prejudice by which this
subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority
of naturalists, and which consequently have every external
characteristic feature of true species,—they admit that these have been
produced by variation, but they refuse to extend the same view to other
and very slightly different forms. Nevertheless they do not pretend
that they can define, or even conjecture, which are the created forms
of life, and which are those produced by secondary laws. They admit
variation as a _vera causa_ in one case, they arbitrarily reject it in
another, without assigning any distinction in the two cases. The day
will come when this will be given as a curious illustration of
the blindness of preconceived opinion. These authors seem no more
startled at a miraculous act of creation than at an ordinary birth. But
do they really believe that at innumerable periods in the earth’s
history certain elemental atoms have been commanded suddenly to flash
into living tissues? Do they believe that at each supposed act of
creation one individual or many were produced? Were all the infinitely
numerous kinds of animals and plants created as eggs or seed, or as
full grown? and in the case of mammals, were they created bearing the
false marks of nourishment from the mother’s womb? Although naturalists
very properly demand a full explanation of every difficulty from those
who believe in the mutability of species, on their own side they ignore
the whole subject of the first appearance of species in what they
consider reverent silence.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we may consider, by so much the arguments fall away
in force. But some arguments of the greatest weight extend very far.
All the members of whole classes can be connected together by chains of
affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very
wide intervals between existing orders. Organs in a rudimentary
condition plainly show that an early progenitor had the organ in a
fully developed state; and this in some instances necessarily implies
an enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at an
embryonic age the species closely resemble each other. Therefore I
cannot doubt that the theory of descent with modification
embraces all the members of the same class. I believe that animals have
descended from at most only four or five progenitors, and plants from
an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see
this even in so trifling a circumstance as that the same poison often
similarly affects plants and animals; or that the poison secreted by
the gall-fly produces monstrous growths on the wild rose or oak-tree.
Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed.
When the views entertained in this volume on the origin of species, or
when analogous views are generally admitted, we can dimly foresee that
there will be a considerable revolution in natural history.
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 in essence a 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 true species
will cease. Systematists will have only to decide (not that this will
be easy) whether any form be sufficiently constant and distinct from
other forms, to be capable of definition; and if definable, whether the
differences be sufficiently important to deserve a specific name. This
latter point will become a far more essential consideration
than it is at present; for differences, however slight, between any two
forms, if not blended by intermediate gradations, are looked at by most
naturalists as sufficient to raise both forms to the rank of species.
Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed, to be connected at the present day by
intermediate gradations, whereas species were formerly thus connected.
Hence, without quite rejecting the consideration of the present
existence of intermediate gradations between any two forms, we shall be
led to weigh more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names, as with the primrose and cowslip; and in this case
scientific and common language will come into accordance. In short, we
shall have to treat species in the same manner as those naturalists
treat genera, who admit that genera are merely artificial combinations
made for convenience. This may not be a cheering prospect; but we shall
at least be freed from the vain search for the undiscovered and
undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a history; when we contemplate every
complex structure
and instinct as the summing up of many contrivances, each useful to the
possessor, nearly in the same way as when we look at any great
mechanical invention as the summing up of the labour, the experience,
the reason, and even the blunders of numerous workmen; when we thus
view each organic being, how far more interesting, I speak from
experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects
of use and disuse, on the direct action of external conditions, and so
forth. The study of domestic productions will rise immensely in value.
A new variety raised by man will be a far more important and
interesting subject for study than one more species added to the
infinitude of already recorded species. Our classifications will come
to be, as far as they can be so made, genealogies; and will then truly
give what may be called the plan of creation. The rules for classifying
will no doubt become simpler when we have a definite object in view. We
possess no pedigrees or armorial bearings; and we have to discover and
trace the many diverging lines of descent in our natural genealogies,
by characters of any kind which have long been inherited. Rudimentary
organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species, which are called aberrant,
and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal
to us the structure, in some degree obscured, of the prototypes of each
great class.
When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not
very remote period descended
from one parent, and have migrated from some one birthplace; and when
we better know the many means of migration, then, by the light which
geology now throws, and will continue to throw, on former changes of
climate and of the level of the land, we shall surely be enabled to
trace in an admirable manner the former migrations of the inhabitants
of the whole world. Even at present, by comparing the differences of
the inhabitants of the sea on the opposite sides of a continent, and
the nature of the various inhabitants of that continent in relation to
their apparent means of immigration, some light can be thrown on
ancient geography.
The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must
not be looked at as a well-filled museum, but as a poor collection made
at hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between
the successive stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We must be
cautious in attempting to correlate as strictly contemporaneous two
formations, which include few identical species, by the general
succession of their forms of life. As species are produced and
exterminated by slowly acting and still existing causes, and not by
miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism,—the
improvement of one being entailing the improvement or the extermination
of
others; it follows, that the amount of organic change in the fossils of
consecutive formations probably serves as a fair measure of the lapse
of actual time. A number of species, however, keeping in a body might
remain for a long period unchanged, whilst within this same period,
several of these species, by migrating into new countries and coming
into competition with foreign associates, might become modified; so
that we must not overrate the accuracy of organic change as a measure
of time. During early periods of the earth’s history, when the forms of
life were probably fewer and simpler, the rate of change was probably
slower; and at the first dawn of life, when very few forms of the
simplest structure existed, the rate of change may have been slow in an
extreme degree. The whole history of the world, as at present known,
although of a length quite incomprehensible by us, will hereafter be
recognised as a mere fragment of time, compared with the ages which
have elapsed since the first creature, the progenitor of innumerable
extinct and living descendants, was created.
In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation, that of the
necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the
view that each species has been independently created. To my mind it
accords better with what we know of the laws impressed on matter by the
Creator, that the production and extinction of the past and present
inhabitants of the world should have been due to secondary causes, like
those determining the birth and death of the individual. When I view
all beings not as special creations, but as the lineal descendants of
some few beings which lived long before the
first bed of the Silurian system was deposited, they seem to me to
become ennobled. Judging from the past, we may safely infer that not
one living species will transmit its unaltered likeness to a distant
futurity. And of the species now living very few will transmit progeny
of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species of
each genus, and all the species of many genera, have left no
descendants, but have become utterly extinct. We can so far take a
prophetic glance into futurity as to foretel that it will be the common
and widely-spread species, belonging to the larger and dominant groups,
which will ultimately prevail and procreate new and dominant species.
As all the living forms of life are the lineal descendants of those
which lived long before the Silurian epoch, we may feel certain that
the ordinary succession by generation has never once been broken, and
that no cataclysm has desolated the whole world. Hence we may look with
some confidence to a secure future of equally inappreciable length. And
as natural selection works solely by and for the good of each being,
all corporeal and mental endowments will tend to progress towards
perfection.
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different
from each other, and dependent on each other in so complex a manner,
have all been produced by laws acting around us. These laws, taken in
the largest sense, being Growth with Reproduction; Inheritance which is
almost implied by reproduction; Variability from the indirect and
direct action of the external conditions
of life, and from use and disuse; a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less-improved
forms. Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur
in this view of life, with its several powers, having been originally
breathed into a few forms or into one; and that, whilst this planet has
gone cycling on according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful have been,
and are being, evolved.
INDEX.
Aberrant groups, 429.
Abyssinia, plants of, 375.
Acclimatisation, 139.
Affinities:
of extinct species, 329.
of organic beings, 411.
Agassiz:
on Amblyopsis, 139.
on groups of species suddenly appearing, 302, 305.
on embryological succession, 338.
on the glacial period, 366.
on embryological characters, 418.
on the embryos of vertebrata, 439.
on parallelism of embryological development and geological succession,
449.
Algæ of New Zealand, 376.
Alligators, males, fighting, 88.
Amblyopsis, blind fish, 139.
America, North:
productions allied to those of Europe, 371.
boulders and glaciers of, 373.
South, no modern formations on west coast, 290.
Ammonites, sudden extinction of, 321.
Anagallis, sterility of, 247.
Analogy of variations, 159.
Ancylus, 386.
Animals:
not domesticated from being variable, 17.
domestic, descended from several stocks, 19.
acclimatisation of, 141.
of Australia, 116.
with thicker fur in cold climates, 133.
blind, in caves, 137.
extinct, of Australia, 339.
Anomma, 240.
Antarctic islands, ancient flora of, 399.
Antirrhinum, 161.
Ants:
attending aphides, 211.
slave-making instinct, 219.
Ants, neuter, structure of, 236.
Aphides attended by ants, 211.
Aphis, development of, 442.
Apteryx, 182.
Arab horses, 35.
Aralo-Caspian Sea, 339.
Archiac, M. de, on the succession of species, 325.
Artichoke, Jerusalem, 142.
Ascension, plants of, 389.
Asclepias, pollen of, 193.
Asparagus, 359.
Aspicarpa, 417.
Asses, striped, 163.
Ateuchus, 135.
Audubon:
on habits of frigate-bird, 185.
on variation in birds’-nests, 212.
on heron eating seeds, 387.
Australia:
animals of, 116.
dogs of, 215.
extinct animals of, 339.
European plants in, 375.
Azara on flies destroying cattle, 72.
Azores, flora of, 363.
Babington, Mr., on British plants, 48.
Balancement of growth, 147.
Bamboo with hooks, 197.
Barberry, flowers of, 98.
Barrande, M.:
on Silurian colonies, 313.
on the succession of species, 325.
on parallelism of palæozoic formations, 328.
on affinities of ancient species, 330.
Barriers, importance of, 347.
Batrachians on islands, 393.
Bats:
how structure acquired, 180.
distribution of, 394.
Bear, catching water-insects, 184.
Bee:
sting of, 202.
queen, killing rivals, 202.
Bees fertilising flowers, 73.
Bees:
hive, not sucking the red clover, 95.
cell-making instinct, 224.
humble, cells of, 225.
parasitic, 218.
Beetles:
wingless, in Madeira, 135.
with deficient tarsi, 135.
Bentham, Mr.:
on British plants, 48.
on classification, 419.
Berkeley, Mr., on seeds in salt-water, 358.
Bermuda, birds of, 391.
Birds:
acquiring fear, 212.
annually cross the Atlantic, 364.
colour of, on continents, 132.
fossil, in caves of Brazil, 339.
of Madeira, Bermuda, and Galapagos, 390.
song of males, 89.
transporting seeds, 361.
waders, 386.
wingless, 134, 182.
with traces of embryonic teeth, 451.
Bizcacha, 349.
affinities of, 429.
Bladder for swimming in fish, 190.
Blindness of cave animals, 137,
Blyth, Mr.:
on distinctness of Indian cattle, 18.
on striped Hemionus, 163.
on crossed geese, 253.
Boar, shoulder-pad of, 88.
Borrow, Mr., on the Spanish pointer, 35.
Bory St. Vincent on Batrachians, 393.
Bosquet, M., on fossil Chthamalus, 304.
Boulders, erratic, on the Azores, 363.
Branchiæ, 190.
Brent, Mr.:
on house-tumblers, 214.
on hawks killing pigeons, 362.
Brewer, Dr., on American cuckoo, 217.
Britain, mammals of, 395.
Bronn on duration of specific forms, 293.
Brown, Robert, on classification, 414.
Buckman on variation in plants, 10.
Buzareingues on sterility of varieties, 270.
Cabbage, varieties of, crossed, 99.
Calceolaria, 251.
Canary-birds, sterility of hybrids, 252.
Cape de Verde islands, 398.
Cape of Good Hope, plants of, 110, 375.
Carrier-pigeons killed by hawks, 362.
Cassini on flowers of compositæ, 145.
Catasetum, 424.
Cats:
with blue eyes, deaf, 12.
variation in habits of, 91.
curling tail when going to spring, 201.
Cattle:
destroying fir-trees, 71.
destroyed by flies in La Plata, 72.
breeds of, locally extinct, 111.
fertility of Indian and European breeds, 254.
Cave, inhabitants of, blind, 137.
Centres of creation, 352.
Cephalopodæ, development of, 442.
Cervulus, 253.
Cetacea, teeth and hair, 144.
Ceylon, plants of, 375.
Chalk formation, 322.
Characters:
divergence of, 111.
sexual, variable, 156.
adaptive or analogical, 427.
Charlock, 76.
Checks:
to increase, 67.
mutual, 71.
Chickens, instinctive tameness of, 216.
Chthamalinæ, 288.
Chthamalus, cretacean species of, 304.
Circumstances favourable:
to selection of domestic products, 40.
to natural selection, 101.
Cirripedes:
capable of crossing, 101.
carapace aborted, 148.
their ovigerous frena, 192.
fossil, 304.
larvæ of, 440.
Classification, 413.
Clift, Mr., on the succession of types, 339.
Climate:
effects of, in checking increase of beings, 68.
adaptation of, to organisms, 139.
Cobites, intestine of, 190.
Cockroach, 76.
Collections, palæontological, poor, 287.
Colour:
influenced by climate, 132.
in relation to attacks by flies, 198.
Columba livia, parent of domestic pigeons, 23.
Colymbetes, 386.
Compensation of growth, 147.
Compositæ:
outer and inner florets of, 144.
male flowers of, 451.
Conclusion, general, 480.
Conditions, slight changes in, favourable to fertility, 267.
Coot, 185.
Coral:
islands, seeds drifted to, 360.
reefs, indicating movements of earth, 309.
Corn-crake, 185.
Correlation:
of growth in domestic productions, 11.
of growth, 143, 198.
Cowslip, 49.
Creation, single centres of, 352.
Crinum, 250.
Crosses, reciprocal, 258.
Crossing:
of domestic animals, importance in altering breeds, 20.
advantages of, 96.
unfavourable to selection, 102.
Crustacea of New Zealand, 376.
Crustacean, blind, 137.
Cryptocerus, 238.
Ctenomys, blind, 137.
Cuckoo, instinct of, 216.
Currants, grafts of, 262.
Currents of sea, rate of, 359.
Cuvier:
on conditions of existence, 206.
on fossil monkeys, 303.
Cuvier, Fred., on instinct, 208.
Dana, Professor:
on blind cave-animals, 139.
on relations of crustaceans of Japan, 372.
on crustaceans of New Zealand, 376.
De Candolle:
on struggle for existence, 62.
on umbelliferæ, 146.
on general affinities, 430.
De Candolle, Alph.:
on low plants, widely dispersed, 406.
on widely-ranging plants being variable, 53.
on naturalisation, 115.
on winged seeds, 146.
on Alpine species suddenly becoming rare, 175.
on distribution of plants with large seeds, 360.
on vegetation of Australia, 379.
on fresh-water plants, 386.
on insular plants, 389.
Degradation of coast-rocks, 282.
Denudation:
rate of, 285.
of oldest rocks, 308.
Development of ancient forms, 336.
Devonian system, 334.
Dianthus, fertility of crosses, 256.
Dirt on feet of birds, 362.
Dispersal:
means of, 356.
during glacial period, 365.
Distribution:
geographical, 346.
means of, 356.
Disuse, effects of, under nature, 134.
Divergence of character, 111.
Division, physiological, of labour, 115.
Dogs:
hairless, with imperfect teeth, 12.
descended from several wild stocks, 18.
domestic instincts of, 213.
inherited civilisation of, 215.
fertility of breeds together, 254.
of crosses, 268.
proportions of, when young, 444.
Domestication, variation under, 7.
Downing, Mr., on fruit-trees in America, 85.
Downs, North and South, 285.
Dragon-flies, intestines of, 190.
Drift-timber, 360.
Driver-ant, 240.
Drones killed by other bees, 202.
Duck:
domestic, wings of, reduced, 11.
logger-headed, 182.
Duckweed, 385.
Dugong, affinities of, 414.
Dung-beetles with deficient tarsi, 135.
Dyticus, 386.
Earl, Mr. W., on the Malay Archipelago, 395.
Ears:
drooping, in domestic animals, 11.
rudimentary, 454.
Earth, seeds in roots of trees, 361.
Eciton, 238.
Economy of organisation, 147.
Edentata:
teeth and hair, 144.
fossil species of, 339.
Edwards, Milne:
on physiological divisions of labour, 115.
on gradations of structure, 194.
on embryological characters, 418.
Eggs, young birds escaping from, 87.
Electric organs, 192.
Elephant:
rate of increase, 64.
of glacial period, 141.
Embryology, 439.
Existence:
struggle for, 60.
conditions of, 206.
Extinction:
as bearing on natural selection, 109.
of domestic varieties, 111.
317.
Eye:
structure of, 187.
correction for aberration, 202.
Eyes reduced in moles, 137.
Fabre, M., on parasitic sphex, 218.
Falconer, Dr.:
on naturalization of plants in India, 65.
on fossil crocodile, 313.
on elephants and mastodons, 334.
and Cautley on mammals of sub-Himalayan beds, 340.
Falkland Island, wolf of, 393.
Faults, 285.
Faunas, marine, 348.
Fear, instinctive, in birds, 212.
Feet of birds, young molluscs adhering to, 385.
Fertility:
of hybrids, 249.
from slight changes in conditions, 267.
of crossed varieties, 267.
Fir-trees:
destroyed by cattle, 71.
pollen of, 203.
Fish:
flying, 182.
teleostean, sudden appearance of, 305.
eating seeds, 362, 387.
fresh-water, distribution of, 384.
Fishes:
ganoid, now confined to fresh water, 107.
electric organs of, 192.
ganoid, living in fresh water, 321.
of southern hemisphere, 376.
Flight, powers of, how acquired, 182.
Flowers:
structure of, in relation to crossing, 97.
of compositæ and umbelliferæ, 144.
Forbes, E.:
on colours of shells, 132.
on abrupt range of shells in depth, 175.
on poorness of palæontological collections, 287.
on continuous succession of genera, 316.
on continental extensions, 357.
on distribution during glacial period, 366
on parallelism in time and space, 409.
Forests, changes in, in America, 74.
Formation, Devonian, 334.
Formations:
thickness of, in Britain, 284.
intermittent, 290.
Formica rufescens, 219.
Formica sanguinea, 219.
Formica flava, neuter of, 239.
Frena, ovigerous, of cirripedes, 192.
Fresh-water productions, dispersal of, 383.
Fries on species in large genera being closely allied to other species,
57.
Frigate-bird, 185.
Frogs on islands, 393.
Fruit-trees:
gradual improvement of, 37.
in United States, 85.
varieties of, acclimatised in United States, 142.
Fuci, crossed, 258.
Fur, thicker in cold climates, 133.
Furze, 439.
Galapagos Archipelago:
birds of, 390.
productions of, 398, 400.
Galeopithecus, 181.
Game, increase of, checked by vermin, 68.
Gärtner:
on sterility of hybrids, 247, 255.
on reciprocal crosses, 258.
on crossed maize and verbascum, 270.
on comparison of hybrids and mongrels, 272.
Geese:
fertility when crossed, 253.
upland, 185.
Genealogy important in classification, 425.
Geoffrey St. Hilaire:
on balancement, 147.
on homologous organs, 434.
Geoffrey St. Hilaire, Isidore:
on variability of repeated parts, 149.
on correlation in monstrosities, 11.
on correlation, 144.
on variable parts being often monstrous, 155.
Geographical distribution, 346.
Geography, ancient, 487.
Geology:
future progress of, 487.
imperfection of the record, 279.
Giraffe, tail of, 195.
Glacial period, 365.
Gmelin on distribution, 365.
Gnathodon, fossil, 368.
Godwin-Austen, Mr., on the Malay Archipelago, 299.
Goethe on compensation of growth, 147.
Gooseberry, grafts of, 262.
Gould, Dr. A., on land-shells, 397.
Gould, Mr.:
on colours of birds, 132.
on birds of the Galapagos, 398.
on distribution of genera of birds, 404.
Gourds, crossed, 270.
Grafts, capacity of, 261.
Grasses, varieties of, 113.
Gray, Dr. Asa:
on trees of United States, 100.
on naturalised plants in the United States, 115.
on rarity of intermediate varieties, 176.
on Alpine plants, 365.
Gray, Dr. J. E., on striped mule, 165.
Grebe, 185.
Groups, aberrant, 429.
Grouse:
colours of, 84.
red, a doubtful species, 49.
Growth:
compensation of, 147.
correlation of, in domestic products, 11.
correlation of, 143.
Habit:
effect of, under domestication, 11.
effect of, under nature, 134.
diversified, of same species, 183.
Hair and teeth, correlated, 144.
Harcourt, Mr. E. V., on the birds of Madeira, 391.
Hartung, M., on boulders in the Azores, 363.
Hazel-nuts, 359.
Hearne on habits of bears, 184.
Heath, changes in vegetation, 72,
Heer, O., on plants of Madeira, 107.
Helix pomatia, 397.
Helosciadium, 359.
Hemionus, striped, 163.
Herbert, W.:
on struggle for existence, 62.
on sterility of hybrids, 249.
Hermaphrodites crossing, 96.
Heron eating seed, 387.
Heron, Sir R., on peacocks, 89.
Heusinger on white animals not poisoned by certain plants, 12.
Hewitt, Mr., on sterility of first crosses, 264.
Himalaya:
glaciers of, 373.
plants of, 375.
Hippeastrum, 250.
Holly-trees, sexes of, 93.
Hollyhock, varieties of, crossed, 271.
Hooker, Dr., on trees of New Zealand, 100.
Hooker, Dr.:
on acclimatisation of Himalayan trees, 140.
on flowers of umbelliferæ, 145.
on glaciers of Himalaya, 373.
on algæ of New Zealand, 376.
on vegetation at the base of the Himalaya, 378.
on plants of Tierra del Fuego, 374, 378.
on Australian plants, 375, 399.
on relations of flora of South America, 379.
on flora of the Antarctic lands, 381, 399.
on the plants of the Galapagos, 391, 398.
Hooks:
on bamboos, 197.
to seeds on islands, 392.
Horner, Mr., on the antiquity of Egyptians, 18.
Horns, rudimentary, 454.
Horse, fossil, in La Plata, 318.
Horses:
destroyed by flies in La Plata, 72.
striped, 163.
proportions of, when young, 445.
Horticulturists, selection applied by, 32.
Huber on cells of bees, 230.
Huber, P.:
on reason blended with instinct, 208.
on habitual nature of instincts, 208.
on slave making ants, 219.
on Melipona domestica, 225.
Humble-bees, cells of, 225.
Hunter, J., on secondary sexual characters, 150.
Hutton, Captain, on crossed geese, 253.
Huxley, Professor:
on structure of hermaphrodites, 101.
on embryological succession, 338.
on homologous organs, 438.
on the development of aphis, 442.
Hybrids and mongrels compared, 272.
Hybridism, 245.
Hydra, structure of, 190.
Ibla, 148.
Icebergs transporting seeds, 363.
Increase, rate of, 63.
Individuals:
numbers favourable to selection, 102.
many, whether simultaneously created, 356.
Inheritance:
laws of, 12.
at corresponding ages, 14, 86.
Insects:
colour of, fitted for habitations, 84.
sea-side, colours of, 132.
blind, in caves, 138.
luminous, 193.
neuter, 236.
Instinct, 207.
Instincts, domestic, 213.
Intercrossing, advantages of, 96.
Islands, oceanic, 388.
Isolation favourable to selection, 104.
Japan, productions of, 372.
Java, plants of, 375.
Jones, Mr. J. M., on the birds of Bermuda, 391.
Jussieu on classification, 417.
Kentucky, caves of, 137.
Kerguelen-land, flora of, 381, 399.
Kidney-bean, acclimatisation of, 142.
Kidneys of birds, 144.
Kirby on tarsi deficient in beetles, 135.
Knight, Andrew, on cause of variation, 7.
Kölreuter:
on the barberry, 98.
on sterility of hybrids, 247.
on reciprocal crosses, 258.
on crossed varieties of nicotiana, 271.
on crossing male and hermaphrodite flowers, 451.
Lamarck on adaptive characters, 427.
Land-shells:
distribution of, 397.
of Madeira, naturalised, 402.
Languages, classification of, 422.
Lapse, great, of time, 282.
Larvæ, 440.
Laurel, nectar secreted by the leaves, 92.
Laws of variation, 131.
Leech, varieties of, 76.
Leguminosæ, nectar secreted by glands, 92.
Lepidosiren, 107, 330.
Life, struggle for, 60.
Lingula, Silurian, 306.
Linnæus, aphorism of, 413.
Lion:
mane of, 88.
young of, striped, 439.
Lobelia fulgens, 73, 98.
Lobelia, sterility of crosses, 250.
Loess of the Rhine, 384.
Lowness of structure connected with variability, 149.
Lowness, related to wide distribution, 406.
Lubbock, Mr., on the nerves of coccus, 46.
Lucas, Dr. P.:
on inheritance, 12.
on resemblance of child to parent, 275.
Lund and Clausen on fossils of Brazil, 339.
Lyell, Sir C.:
on the struggle for existence, 62.
on modern changes of the earth, 95.
on measure of denudation, 283.
on a carboniferous land-shell, 289.
on fossil whales, 303.
on strata beneath Silurian system, 307.
on the imperfection of the geological record, 310.
on the appearance of species, 312.
on Barrande’s colonies, 313.
on tertiary formations of Europe and North America, 323.
on parallelism of tertiary formations, 328.
on transport of seeds by icebergs, 363.
on great alternations of climate, 382.
on the distribution of fresh-water shells, 385.
on land-shells of Madeira, 402.
Lyell and Dawson on fossilized trees in Nova Scotia, 296.
Macleay on analogical characters, 427.
Madeira:
plants of, 107.
beetles of, wingless, 135.
fossil land-shells of, 339.
birds of, 390.
Magpie tame in Norway, 212.
Maize, crossed, 270.
Malay Archipelago:
compared with Europe, 299.
mammals of, 395.
Malpighiaceæ, 417.
Mammæ, rudimentary, 451.
Mammals:
fossil, in secondary formation, 303.
insular, 393.
Man, origin of races of, 199.
Manatee, rudimentary nails of, 454.
Marsupials:
of Australia, 116.
fossil species of, 339.
Martens, M., experiment on seeds, 360.
Martin, Mr. W. C., on striped mules, 165.
Matteuchi on the electric organs of rays, 193.
Matthiola, reciprocal crosses of, 258.
Means of dispersal, 356.
Melipona domestica, 225.
Metamorphism of oldest rocks, 308.
Mice:
destroying bees, 74.
acclimatisation of, 141.
Migration, bears on first appearance of fossils, 296.
Miller, Professor, on the cells of bees, 226.
Mirabilis, crosses of, 258.
Missel-thrush, 76.
Misseltoe, complex relations of, 3.
Mississippi, rate of deposition at mouth, 284.
Mocking-thrush of the Galapagos, 402.
Modification of species, how far applicable, 483.
Moles, blind, 137.
Mongrels:
fertility and sterility of, 267.
and hybrids compared, 272.
Monkeys, fossil, 303,
Monocanthus, 424.
Mons, Van, on the origin of fruit-trees, 29, 39.
Moquin-Tandon on sea-side plants, 132.
Morphology, 434.
Mozart, musical powers of, 209.
Mud, seeds in, 386.
Mules, striped, 165.
Müller, Dr. F., on Alpine Australian plants, 375.
Murchison, Sir R.:
on the formations of Russia, 289.
on azoic formations, 307.
on extinction, 317.
Mustela vison, 179.
Myanthus, 424.
Myrmecocystus, 238.
Myrmica, eyes of, 240.
Nails, rudimentary, 453.
Natural history:
future progress of, 484.
selection, 80.
system, 413.
Naturalisation:
of forms distinct from the indigenous species, 115.
in New Zealand, 201.
Nautilus, Silurian, 306.
Nectar of plants, 92.
Nectaries, how formed, 92.
Nelumbium luteum, 387.
Nests, variation in, 212.
Neuter insects, 236.
Newman, Mr., on humble-bees, 74.
New Zealand:
productions of, not perfect, 201.
naturalised products of, 337.
fossil birds of, 339.
glacial action in, 373.
crustaceans of, 376.
algæ of, 376.
number of plants of, 389.
flora of, 399.
Nicotiana:
crossed varieties of, 271.
certain species very sterile, 257.
Noble, Mr., on fertility of Rhododendron, 251.
Nodules, phosphatic, in azoic rocks, 307.
Oak, varieties of, 50.
Onites apelles, 135.
Orchis, pollen of, 193.
Organs:
of extreme perfection, 186.
electric, of fishes, 192.
of little importance, 194.
homologous, 434.
rudiments of, 450.
Ornithorhynchus, 107, 416.
Ostrich:
not capable of flight, 134.
habit of laying eggs together, 218.
American, two species of, 349.
Otter, habits of, how acquired, 179.
Ouzel, water, 185.
Owen, Professor:
on birds not flying, 134.
on vegetative repetition, 149.
on variable length of arms in ourang-outang, 150.
on the swim-bladder of fishes, 191.
on electric organs, 192.
on fossil horse of La Plata, 319.
on relations of ruminants and pachyderms, 329.
on fossil birds of New Zealand, 339.
on succession of types, 339.
on affinities of the dugong, 414.
on homologous organs, 435.
on the metamorphosis of cephalopods and spiders, 442.
Pacific Ocean, faunas of, 348.
Paley on no organ formed to give pain, 201.
Pallas on the fertility of the wild stocks of domestic animals, 253.
Paraguay, cattle destroyed by flies, 72.
Parasites, 217.
Partridge, dirt on feet, 362.
Parts:
greatly developed, variable, 150.
degrees of utility of, 201.
Parus major, 183.
Passiflora, 251.
Peaches in United States, 85.
Pear, grafts of, 261.
Pelargonium:
flowers of, 145.
sterility of, 251.
Pelvis of women, 144.
Peloria, 145.
Period, glacial, 365.
Petrels, habits of, 184.
Phasianus, fertility of hybrids, 253.
Pheasant, young, wild, 216.
Philippi on tertiary species in Sicily, 312.
Pictet, Professor:
on groups of species suddenly appearing, 302, 305.
on rate of organic change, 313.
on continuous succession of genera, 316.
on close alliance of fossils in consecutive formations, 335.
on embryological succession, 338.
Pierce, Mr., on varieties of wolves, 91.
Pigeons:
with feathered feet and skin between toes, 12.
breeds described, and origin of, 20.
breeds of, how produced, 39, 42.
tumbler, not being able to get out of egg, 87.
reverting to blue colour, 160.
instinct of tumbling, 214.
carriers, killed by hawks, 362.
young of, 445.
Pistil, rudimentary, 451.
Plants:
poisonous, not affecting certain coloured animals, 12.
selection applied to, 32.
gradual improvement of, 37.
not improved in barbarous countries, 38.
destroyed by insects, 67.
in midst of range, have to struggle with other plants, 77.
nectar of, 92.
fleshy, on sea-shores, 132.
fresh-water, distribution of, 386.
low in scale, widely distributed, 406.
Plumage, laws of change in sexes of birds, 89.
Plums in the United States, 85.
Pointer dog:
origin of, 35.
habits of, 213.
Poison not affecting certain coloured animals, 12.
Poison, similar effect of, on animals and plants, 484.
Pollen of fir-trees, 203,
Poole, Col., on striped hemionus, 163.
Potamogeton, 387.
Prestwich, Mr., on English and French eocene formations, 328.
Primrose, 49.
sterility of, 247.
Primula, varieties of, 49.
Proteolepas, 148.
Proteus, 139.
Psychology, future progress of, 488.
Quagga, striped, 165.
Quince, grafts of, 261.
Rabbit, disposition of young, 215.
Races, domestic, characters of, 16.
Race-horses:
Arab, 35.
English, 356.
Ramond on plants of Pyrenees, 368.
Ramsay, Professor:
on thickness of the British formations, 284.
on faults, 285.
Ratio of increase, 63.
Rats:
supplanting each other, 76.
acclimatisation of, 141.
blind in cave, 137.
Rattle-snake, 201.
Reason and instinct, 208.
Recapitulation, general, 459.
Reciprocity of crosses, 258.
Record, geological, imperfect, 279.
Rengger on flies destroying cattle, 72.
Reproduction, rate of, 63.
Resemblance to parents in mongrels and hybrids, 273.
Reversion:
law of inheritance, 14.
in pigeons to blue colour, 160.
Rhododendron, sterility of, 251.
Richard, Professor, on Aspicarpa, 417.
Richardson, Sir J.:
on structure of squirrels, 180.
on fishes of the southern hemisphere, 376.
Robinia, grafts of, 262.
Rodents, blind, 137.
Rudimentary organs, 450.
Rudiments important for classification, 416.
Sageret on grafts, 262.
Salmons, males fighting, and hooked jaws of, 88.
Salt-water, how far injurious to seeds, 358.
Saurophagus sulphuratus, 183.
Schiödte on blind insects, 138.
Schlegel on snakes, 144.
Sea-water, how far injurious to seeds, 358.
Sebright, Sir J.:
on crossed animals, 20.
on selection of pigeons, 31.
Sedgwick, Professor, on groups of species suddenly appearing, 302.
Seedlings destroyed by insects, 67.
Seeds:
nutriment in, 77.
winged, 146.
power of resisting salt-water, 358.
in crops and intestines of birds, 361.
eaten by fish, 362, 387.
in mud, 386.
hooked, on islands, 392.
Selection:
of domestic products, 29.
principle not of recent origin, 33.
unconscious, 34.
natural, 80.
sexual, 87.
natural, circumstances favourable to, 101.
Sexes, relations of, 87.
Sexual:
characters variable, 156.
selection, 87.
Sheep:
Merino, their selection, 31.
two sub-breeds unintentionally produced, 36.
mountain, varieties of, 76.
Shells:
colours of, 132.
littoral, seldom embedded, 288.
fresh-water, dispersal of, 385.
of Madeira, 391.
land, distribution of, 397.
Silene, fertility of crosses, 257.
Silliman, Professor, on blind rat, 137.
Skulls of young mammals, 197, 437.
Slave-making instinct, 219.
Smith, Col. Hamilton, on striped horses, 164.
Smith, Mr. Fred.:
on slave-making ants, 219.
on neuter ants, 239.
Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.
Snap-dragon, 161.
Somerville, Lord, on selection of sheep, 31.
Sorbus, grafts of, 262.
Spaniel, King Charles’s breed, 35.
Species:
polymorphic, 46.
common, variable, 53.
in large genera variable, 54.
groups of, suddenly appearing, 302, 306.
beneath Silurian formations, 306.
successively appearing, 312.
changing simultaneously throughout the world, 322.
Spencer, Lord, on increase in size of cattle, 35.
Sphex, parasitic, 218.
Spiders, development of, 442.
Spitz-dog crossed with fox, 268.
Sports in plants, 9.
Sprengel, C. C.:
on crossing, 98.
on ray-florets, 145.
Squirrels, gradations in structure, 180.
Staffordshire, heath, changes in, 72.
Stag-beetles, fighting, 88.
Sterility:
from changed conditions of life, 9.
of hybrids, 246.
laws of, 254.
causes of, 263.
from unfavourable conditions, 265.
of certain varieties, 269.
St. Helena, productions of, 389.
St. Hilaire, Aug., on classification, 418.
St. John, Mr., on habits of cats, 91.
Sting of bee, 202.
Stocks, aboriginal, of domestic animals, 18,
Strata, thickness of, in Britain, 284.
Stripes on horses, 163.
Structure, degrees of utility of, 201.
Struggle for existence, 60.
Succession, geological, 312.
Succession of types in same areas, 338.
Swallow, one species supplanting another, 76.
Swim-bladder, 190.
System, natural, 413.
Tail:
of giraffe, 195.
of aquatic animals, 196.
rudimentary, 454.
Tarsi deficient, 135.
Tausch on umbelliferous flowers, 146.
Teeth and hair:
correlated, 144.
embryonic, traces of, in birds, 451.
rudimentary, in embryonic calf, 450, 480.
Tegetmeier, Mr., on cells of bees, 228, 233.
Temminck on distribution aiding classification, 419.
Thouin on grafts, 262.
Thrush:
aquatic species of, 185.
mocking, of the Galapagos, 402.
young of, spotted, 439.
nest of, 243.
Thuret, >M., on crossed fuci, 258.
Thwaites, Mr., on acclimatisation, 140.
Tierra del Fuego:
dogs of, 215.
plants of, 374, 378.
Timber-drift, 360.
Time, lapse of, 282.
Titmouse, 183.
Toads on islands, 393.
Tobacco, crossed varieties of, 271.
Tomes, Mr., on the distribution of bats, 394.
Transitions in varieties rare, 172.
Trees:
on islands belong to peculiar orders, 392.
with separated sexes, 99.
Trifolium pratense, 73, 94.
Trifolium incarnatum, 94.
Trigonia, 321.
Trilobites, 306.
sudden extinction of, 321.
Troglodytes, 243.
Tucutucu, blind, 137.
Tumbler pigeons:
habits of, hereditary, 214.
young of, 446.
Turkey-cock, brush of hair on breast, 90.
Turkey:
naked skin on head, 197.
young, wild, 216.
Turnip and cabbage, analogous variations of, 159.
Type, unity of, 206.
Types, succession of, in same areas, 338.
Udders:
enlarged by use, 11.
rudimentary, 451.
Ulex, young leaves of, 439.
Umbelliferæ, outer and inner florets of, 144.
Unity of type, 206.
Use:
effects of, under domestication, 11.
effects of, in a state of nature, 134.
Utility, how far important in the construction of each part, 199.
Valenciennes on fresh-water fish, 384.
Variability of mongrels and hybrids, 274.
Variation:
under domestication, 7.
caused by reproductive system being affected by conditions of life, 8.
under nature, 44.
laws of, 131.
Variations:
appear at corresponding ages, 14, 86.
analogous in distinct species, 159.
Varieties:
natural, 44.
struggle between, 75.
domestic, extinction of, 111.
transitional, rarity of, 172.
when crossed, fertile, 267.
when crossed, sterile, 269.
classification of, 423.
Verbascum:
sterility of, 251.
varieties of, crossed, 270.
Verneuil, M. de, on the succession of species, 325.
Viola tricolor, 73.
Volcanic islands, denudation of, 284.
Vulture, naked skin on head, 197.
Wading-birds, 386.
Wallace, Mr.:
on origin of species, 2.
on law of geographical distribution, 355.
on the Malay Archipelago, 395.
Wasp, sting of, 202.
Water, fresh, productions of, 383.
Water-hen, 185.
Waterhouse, Mr.:
on Australian marsupials, 116.
on greatly developed parts being variable, 150.
on the cells of bees, 225.
on general affinities, 429.
Water-ouzel, 185.
Watson, Mr. H. C.:
on range of varieties of British plants, 58.
on acclimatisation, 140.
on flora of Azores, 363.
on Alpine plants, 367, 376.
on rarity of intermediate varieties, 176.
Weald, denudation of, 285.
Web of feet in water-birds, 185.
West Indian islands, mammals of, 395.
Westwood:
on species in large genera being closely allied to others, 57.
on the tarsi of Engidæ, 157.
on the antennæ of hymenopterous insects, 416.
Whales, fossil, 303.
Wheat, varieties of, 113.
White Mountains, flora of, 365.
Wings, reduction of size, 134.
Wings:
of insects homologous with branchiæ, 191.
rudimentary, in insects, 451.
Wolf:
crossed with dog, 214.
of Falkland Isles, 393.
Wollaston, Mr.:
on varieties of insects, 48.
on fossil varieties of land-shells in Madeira, 52.
on colours of insects on sea-shore, 132.
on wingless beetles, 135.
on rarity of intermediate varieties, 176.
on insular insects, 389.
on land-shells of Madeira, naturalised, 402.
Wolves, varieties of, 90.
Woodpecker:
habits of, 184.
green colour of, 197.
Woodward, Mr.:
on the duration of specific forms, 293.
on the continuous succession of genera, 316.
on the succession of types, 339.
World, species changing simultaneously throughout, 322.
Wrens, nest of, 243.
Youatt, Mr.:
on selection, 31.
on sub-breeds of sheep, 36.
on rudimentary horns in young cattle, 454.
Zebra, stripes on, 163.
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