INTRODUCTION.
CHAPTER 1. 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 palaeontological 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 1. 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 Muller 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 Canidae--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 vertebrae 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 scutellae 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 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 Columbidae 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 Columbidae, 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 encyclopaedia.
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 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.
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 larvae 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 Gartner, 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 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.
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. Linnaeus 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 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.
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 larvae. 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 Leguminosae, 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
Gartner, 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 Kolreuter 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 Europaeo-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 a1 and m1. 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 a1 is supposed in the
diagram to have produced variety a2, which will, owing to the
principle of divergence, differ more from (A) than did variety a1.
Variety m1 is supposed to have produced two varieties, namely m2 and
s2, 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 a1 to a10. 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, a10, f10, and m10, 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 a14 and m14, 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 (w10 and z10) 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 n14 to z14, 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 (a14 to
m14); and (I) will have been replaced by six (n14 to z14) 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 a14 and z14 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 a14, q14,
p14, will be nearly related from having recently branched off from
a10; b14 and f14, from having diverged at an earlier period from a5,
will be in some degree distinct from the three first-named species;
and lastly, o14, e14, and m14, 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 a14 to p14,
those marked b14 and f14, and those marked o14 to m14, 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 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 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.
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 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 Schiodte
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
Schiodte 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 antennae 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 Encyclopaedias
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 Compositae countenances this idea; but, in the
case of the corolla of the Umbelliferae, 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 Compositae 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 Umbelliferae 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 antennae. 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 vertebrae
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 Engidae, 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 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.
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 Lemuridae, 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 aerial 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 branchiae 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 branchiae 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 branchiae
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 branchiae 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 branchiae, the whole
surface of the body and sack, including the small frena, serving for
respiration. The Balanidae 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
branchiae. Now I think no one will dispute that the ovigerous frena in
the one family are strictly homologous with the branchiae 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
branchiae, 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
branchiae 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 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.
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 antennae; 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 antennae on the abdomen first of one aphis and then of another;
and each aphis, as soon as it felt the antennae, 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 Gallinaceae; 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 Sphegidae (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 larvae 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 larvae. 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 larvae and pupae 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 larvae, 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
larvae and pupae 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 pupae to rear as slaves. I then dug up
a small parcel of the pupae 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 pupae
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 pupae 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 pupae of F. fusca, whereas they were much
terrified when they came across the pupae, 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 pupae.
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 pupae. 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 larvae, 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 larvae. 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 pupae of other species, if scattered near
their nests, it is possible that pupae 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 pupae 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 larvae of ichneumonidae 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 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.
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, Kolreuter and
Gartner, who almost devoted their lives to this subject, without being
deeply impressed with the high generality of some degree of sterility.
Kolreuter 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. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these
and in many other cases, Gartner 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
Gartner 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 Gartner 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
Leguminosae, in which there is an acknowledged difficulty in the
manipulation) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gartner 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 Gartner 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, Kolreuter and Gartner, 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 Gartner 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 Gartner, 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 Gartner 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
Kolreuter and Gartner 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 Gartner. 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,