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 {81}
(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 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 {82} 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 {83} 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 {84} 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 metaphorically 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 {85} 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 quite unimportant, we must
not forget that climate, food, &c., 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 {86} 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 variations
profitable at that age, and by their inheritance at a corresponding age. If
it profit a plant to have its seeds more and more widely disseminated by
the wind, I can see no greater difficulty in this being effected through
natural selection, than in the cotton-planter increasing and improving by
selection the down in the pods on his cotton-trees. Natural selection may
modify and adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect. These modifications
will no doubt affect, through the laws of correlation, the structure of the
adult; and probably in the case of those insects which live only for a few
hours, and which never feed, a large part of their structure is merely the
correlated result of successive changes in the structure of their larvæ.
So, conversely, modifications in the adult will probably often affect the
structure of the larva; but in all cases natural selection will ensure that
modifications consequent on other modifications at a different period of
life, shall not be in the least degree injurious: for if they became so,
they would cause the extinction of the species.
Natural selection will modify the structure of the {87} 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, 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 {88} 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 depends 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 the 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, {89} 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 {90} 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, &c.), 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 {91} 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 {92} effected by glands at the base of the stipules in some
Leguminosæ, and at the back of the leaf of the common laurel. This juice,
though small in quantity, is greedily sought by insects. Let us now suppose
a little sweet juice or nectar to be excreted by the inner bases of the
petals of a flower. In this case insects in seeking the nectar would get
dusted with pollen, and would certainly often transport the pollen from one
flower to the stigma of another flower. The flowers of two distinct
individuals of the same species would thus get crossed; and the act of
crossing, we have good reason to believe (as will hereafter be more fully
alluded to), would produce very vigorous seedlings, which consequently
would have the best chance of flourishing and surviving. Some of these
seedlings would probably inherit the nectar-excreting power. Those
individual flowers which had the largest glands or nectaries, and which
excreted most nectar, would be oftenest visited by insects, and would be
oftenest crossed; and so in the long-run would gain the upper hand. Those
flowers, also, which had their stamens and pistils placed, in relation to
the size and habits of the particular insects which visited them, so as to
favour in any degree the transportal of their pollen from flower to flower,
would likewise be favoured or selected. We might have taken the case of
insects visiting flowers for the sake of collecting pollen instead of
nectar; and as pollen is formed for the sole object of fertilisation, its
destruction appears a simple loss to the plant; yet if a little pollen were
carried, at first occasionally and then habitually, by the pollen-devouring
insects from flower to flower, and a cross thus effected, although
nine-tenths of the pollen were destroyed, it might still be a great gain to
the plant; and those individuals which produced more and more pollen, and
had larger and larger anthers, would be selected. {93}
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 a
rather 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 {94} 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, &c., 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
{95} 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 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 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
{96} 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 (with the exception of
the curious and not well-understood cases of parthenogenesis) 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 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 {97} 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 {98} one flower and then the
stigma of another with the same brush to ensure fertilisation; but it must
not be supposed that bees would thus produce a multitude of hybrids between
distinct species; for if you bring on the same brush a plant's own pollen
and pollen from another species, the former will have such a prepotent
effect, that it will invariably and completely destroy, as has been shown
by Gärtner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely
to ensure self-fertilisation; and no doubt it is useful for this end: but,
the agency of insects is often required to cause the stamens to spring
forward, as Kölreuter has shown to be the case with the barberry; and in
this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that if 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 {99} 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 {100} 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 {101} 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. {102}
_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 {103} 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 physical
conditions at least 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 {104} 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, &c.; 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 {105} 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 {106} there supported, but the conditions
of life are infinitely complex from the large number of already existing
species; and if some of these many species become modified and improved,
others will have to be improved in a corresponding degree or they will be
exterminated. Each new form, also, as soon as it has been much improved,
will be able to spread over the open and continuous area, and will thus
come into competition with many others. Hence more new places will be
formed, and the competition to fill them will be more severe, on a large
than on a small and isolated area. Moreover, great areas, though now
continuous, owing to oscillations of level, will often have recently
existed in a broken condition, so that the good effects of isolation will
generally, to a certain extent, have concurred. Finally, I conclude that,
although small isolated areas probably have been in some respects highly
favourable for the production of new species, yet that the course of
modification will generally have been more rapid on large areas; and what
is more important, that the new forms produced on large areas, which
already have been victorious over many competitors, will be those that will
spread most widely, will give rise to most new varieties and species, and
will thus play an important part in the changing history of the organic
world.
We can, perhaps, on these views, understand some facts which will be again
alluded to in our chapter on geographical distribution; for instance, that
the productions of the smaller continent of Australia have formerly
yielded, and apparently are now yielding, before those of the larger
Europæo-Asiatic area. Thus, also, it is that continental productions have
everywhere become so largely naturalised on islands. On a small island, the
race for life will have been less severe, and there will have been less
modification and less {107} 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,
is the most favourable for the production of many new forms of life, likely
to endure long and to spread widely. For the area first 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 {108} 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 always acts 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, {109} 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 ratio 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 {110} 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 {111} 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 {112} 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. {113}
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. {114}
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 {115} 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 may gain some crude idea in what manner some of the natives
would have to be modified, in order to gain an advantage over the other
natives; and we may at least safely infer that diversification of
structure, amounting to new generic differences, would be 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
{116} Edwards. No physiologist doubts that a stomach 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 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, {117}
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.
[Illustration]
The intervals between the horizontal lines in the diagram, may represent
each a thousand generations; but it would have been better if each had
represented ten thousand generations. After a thousand generations, species
(A) is supposed to have produced two fairly well-marked varieties, namely
a^1 and m^1. These two varieties will generally continue to be exposed to
the same conditions which made their parents variable, {118} 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 parent (A)
more numerous than most of the other inhabitants of the same country; they
will likewise partake of those more general advantages which made the genus
to which the parent-species belonged, a large genus in its own country. And
these circumstances we know to be favourable to the production of new
varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a^1 is supposed in the
diagram to have produced variety a^2, which will, owing to the principle of
divergence, differ more from (A) than did variety a^1. Variety m^1 is
supposed to have produced two varieties, namely m^2 and s^2, differing from
each other, and more considerably from their common parent (A). We may
continue the process by similar steps for any length of time; some of the
varieties, after each thousand generations, producing only a single
variety, but in a more and more modified condition, some producing two or
three varieties, and some failing to produce any. Thus the varieties or
modified descendants, proceeding from the common parent (A), will generally
go on increasing in number and diverging in character. In the diagram the
process is represented up to the ten-thousandth generation, and under a
condensed and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever goes on
so regularly as is represented in the diagram, though in itself made
somewhat irregular. {119} 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 {120}
of divergent modification may have been increased in the successive
generations. This case would be represented in the diagram, if all the
lines proceeding from (A) were removed, excepting that from a^1 to a^{10}.
In the same way, for instance, the English race-horse and English pointer
have apparently both gone on slowly diverging in character from their
original stocks, without either having given off any fresh branches or
races.
After ten thousand generations, species (A) is supposed to have produced
three forms, a^{10}, f^{10}, and m^{10}, which, from having diverged in
character during the successive generations, will have come to differ
largely, but perhaps unequally, from each other and from their common
parent. If we suppose the amount of change between each horizontal line in
our diagram to be excessively small, these three forms may still be only
well-marked varieties; or they may have arrived at the doubtful category of
sub-species; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into well-defined species: thus the diagram illustrates the
steps by which the small differences distinguishing varieties are increased
into the larger differences distinguishing species. By continuing the same
process for a greater number of generations (as shown in the diagram in a
condensed and simplified manner), we get eight species, marked by the
letters between a^{14} and m^{14}, all descended from (A). Thus, as I
believe, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary. In
the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w^{10} and z^{10}) or two species, according to the amount of
change supposed to be represented {121} between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n^{14} to z^{14}, are supposed to have been produced. In each genus, the
species, which are already extremely different in character, will generally
tend to produce the greatest number of modified descendants; for these will
have the best chance of filling new and widely different places in the
polity of nature: hence in the diagram I have chosen the extreme species
(A), and the nearly extreme species (I), as those which have largely
varied, and have given rise to new varieties and species. The other nine
species (marked by capital letters) of our original genus, may for a long
period continue to transmit 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 {122} modified offspring of a species get into some
distinct country, or become quickly adapted to some quite new station, in
which child and parent do not come into competition, both may continue to
exist.
If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a^{14} to m^{14}); and
(I) will have been replaced by six (n^{14} to z^{14}) new species.
But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D, than
to the other species; and species (I) more to G, H, K, L, than to the
others. These two species (A) and (I), were also supposed to be very common
and widely diffused species, so that they must originally have had some
advantage over most of the other species of the genus. Their modified
descendants, fourteen in number at the fourteen-thousandth generation, will
probably have inherited some of the same advantages: they have also been
modified and improved in a diversified manner at each stage of descent, so
as to have become adapted to many related places in the natural economy of
their country. It seems, therefore, to me extremely probable that they will
have taken the places of, and thus exterminated, not only their parents (A)
and (I), but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will have
transmitted offspring to the fourteen-thousandth generation. We may suppose
that only one (F), of the two species which were least closely related to
the other nine original species, has transmitted descendants to this late
stage of descent. {123}
The new species in our diagram descended from the original eleven species,
will now be fifteen in number. Owing to the divergent tendency of natural
selection, the extreme amount of difference in character between species
a^{14} and z^{14} will be much greater than that between the most different
of the original eleven species. The new species, moreover, will be allied
to each other in a widely different manner. Of the eight descendants from
(A) the three marked a^{14}, q^{14}, p^{14}, will be nearly related from
having recently branched off from a^{10}; b^{14} and f^{14}, from having
diverged at an earlier period from a^5, will be in some degree distinct
from the three first-named species; and lastly, o^{14}, e^{14} and m^{14},
will be nearly related one to the other, but from having diverged at the
first commencement of the process of modification, will be widely different
from the other five species, and may constitute a sub-genus or even a
distinct genus.
The six descendants from (I) will form two sub-genera or even genera. But
as the original species (I) differed largely from (A), standing nearly at
the extreme points of the original genus, the six descendants from (I)
will, owing to inheritance alone, 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 {124} 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 F^{14}, 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 (F^{14}) 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 {125} the extinct species lived at very ancient epochs when
the branching lines of descent had diverged less.
I see no reason to limit the process of modification, as now explained, to
the formation of genera alone. If, in our diagram, we suppose the amount of
change represented by each successive group of diverging dotted lines to be
very great, the forms marked a^{14} to p^{14}, those marked b^{14} and
f^{14}, and those marked o^{14} to m^{14}, will form three very distinct
genera. We shall also have two very distinct genera descended from (I); and
as these latter two genera, both from continued divergence of character and
from inheritance from a different parent, will differ widely from the three
genera descended from (A), the two little groups of genera will form two
distinct families, or even orders, according to the amount of divergent
modification supposed to be represented in the diagram. And the two new
families, or orders, will have descended from two species of the original
genus; and these two species are supposed to have descended from one
species of a still more ancient and unknown genus.
We have seen that in each country it is the species of the larger genera
which oftenest present varieties or incipient species. This, indeed, might
have been expected; for as natural selection acts through one form having
some advantage over other forms in the struggle for existence, it will
chiefly act on those which already have some advantage; and the largeness
of any group shows that its species have inherited from a common ancestor
some advantage in common. Hence, the struggle for the production of new and
modified descendants, will mainly lie between the larger groups, which are
all trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large {126} 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 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 {127} 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 ratio of increase of each species, a severe
struggle for life at some age, season, or year, 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
manner 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; and it leads to
the improvement of each creature in relation to its organic and inorganic
conditions of life.
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 {128} 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 to the inhabitants of any small spot or to
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 for
life. Thus the small differences distinguishing varieties of the same
species, 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 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 {129} 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 {130} 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.
* * * * *
{131}