CHAPTER 4. (41-48)
NATURAL SELECTION.
Natural Selection: its power compared with man's selection, its power on characters of trifling importance, its
power at all ages and on both sexes. Sexual Selection. On the generality of intercrosses between individuals of
the same species. Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing,
isolation, number of individuals. Slow action. Extinction caused by Natural Selection. Divergence of
Character, related to the diversity of inhabitants of any small area, and to naturalisation. Action of Natural
Selection, through Divergence of Character and Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.
How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation? Can
the principle of selection, which we have seen is so potent in the hands of man, apply in nature? I think we
shall see that it can act most effectually. Let it be borne in mind in what an endless number of strange
peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the
hereditary tendency is. Under domestication, it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions of life. Can it, then, be thought improbable,
seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to
each being in the great and complex battle of life, should sometimes occur in the course of thousands of
generations? If such do occur, can we doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however slight, over others, would have the best
chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in
the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the
rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be
affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called
polymorphic.
We shall best understand the probable course of natural selection by taking the case of a country undergoing
some physical change, for instance, of climate. The proportional numbers of its inhabitants would almost
immediately undergo a change, and some species might become extinct. We may conclude, from what we
have seen of the intimate and complex manner in which the inhabitants of each country are bound together,
that any change in the numerical proportions of some of the inhabitants, independently of the change of
climate itself, would most seriously affect many of the others. If the country were open on its borders, new
forms would certainly immigrate, and this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been
shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and
better adapted forms could not freely enter, we should then have places in the economy of nature which would
assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the
area been open to immigration, these same places would have been seized on by intruders. In such case, every
slight modification, which in the course of ages chanced to arise, and which in any way favoured the
individuals of any of the species, by better adapting them to their altered conditions, would tend to be
preserved; and natural selection would thus have free scope for the work of improvement.
We have reason to believe, as stated in the first chapter, that a change in the conditions of life, by specially
acting on the reproductive system, causes or increases variability; and in the foregoing case the conditions of
life are supposed to have undergone a change, and this would manifestly be favourable to natural selection, by
giving a better chance of profitable variations occurring; and unless profitable variations do occur, natural
selection can do nothing. Not that, as I believe, any extreme amount of variability is necessary; as man can
certainly produce great results by adding up in any given direction mere individual differences, so could
Nature, but far more easily, from having incomparably longer time at her disposal. Nor do I believe that any
great physical change, as of climate, or any unusual degree of isolation to check immigration, is actually
necessary to produce new and unoccupied places for natural selection to fill up by modifying and improving
some of the varying inhabitants. For as all the inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or habits of one inhabitant would often give it
an advantage over others; and still further modifications of the same kind would often still further increase the
advantage. No country can be named in which all the native inhabitants are now so perfectly adapted to each
other and to the physical conditions under which they live, that none of them could anyhow be improved; for
in all countries, the natives have been so far conquered by naturalised productions, that they have allowed
foreigners to take firm possession of the land. And as foreigners have thus everywhere beaten some of the
natives, we may safely conclude that the natives might have been modified with advantage, so as to have
better resisted such intruders.
As man can produce and certainly has produced a great result by his methodical and unconscious means of
selection, what may not nature effect? Man can act only on external and visible characters: nature cares
nothing for appearances, except in so far as they may be useful to any being. She can act on every internal
organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his
own good; Nature only for that of the being which she tends. Every selected character is fully exercised by
her; and the being is placed under well-suited conditions of life. Man keeps the natives of many climates in
the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a
long and a short beaked pigeon on the same food; he does not exercise a long-backed or long-legged
quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. He does
not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals,
but protects during each varying season, as far as lies in his power, all his productions. He often begins his
selection by some half-monstrous form; or at least by some modification prominent enough to catch his eye,
or to be plainly useful to him. Under nature, the slightest difference of structure or constitution may well turn
the nicely-balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will his products be, compared with those
accumulated by nature during whole geological periods. Can we wonder, then, that nature's productions
should be far "truer" in character than man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation,
even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and
insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until
the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological
ages, that we only see that the forms of life are now different from what they formerly were.
Although natural selection can act only through and for the good of each being, yet characters and structures,
which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour
of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these
birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would
increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by
eyesight to their prey,--so much so, that on parts of the Continent persons are warned not to keep white
pigeons, as being the most liable to destruction. Hence I can see no reason to doubt that natural selection
might be most effective in giving the proper colour to each kind of grouse, and in keeping that colour, when
once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any
particular colour would produce little effect: we should remember how essential it is in a flock of white sheep
to destroy every lamb with the faintest trace of black. In plants the down on the fruit and the colour of the
flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent
horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a
curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums;
whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh. If, with
all the aids of art, these slight differences make a great difference in cultivating the several varieties,
assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of
enemies, such differences would effectually settle which variety, whether a smooth or downy, a yellow or
purple fleshed fruit, should succeed.
In looking at many small points of difference between species, which, as far as our ignorance permits us to
judge, seem to be quite unimportant, we must not forget that climate, food, etc., probably produce some slight
and direct effect. It is, however, far more necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is modified through variation, and the
modifications are accumulated by natural selection for the good of the being, will cause other modifications,
often of the most unexpected nature.
As we see that those variations which under domestication appear at any particular period of life, tend to
reappear in the offspring at the same period;--for instance, in the seeds of the many varieties of our culinary
and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of
poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly
adult;--so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age,
by the accumulation of profitable variations at that age, and by their inheritance at a corresponding age. If it
profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty
in this being effected through natural selection, than in the cotton-planter increasing and improving by
selection the down in the pods on his cotton-trees. Natural selection may modify and adapt the larva of an
insect to a score of contingencies, wholly different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation, the structure of the adult; and probably in
the case of those insects which live only for a few hours, and which never feed, a large part of their structure
is merely the correlated result of successive changes in the structure of their larvae. So, conversely,
modifications in the adult will probably often affect the structure of the larva; but in all cases natural selection
will ensure that modifications consequent on other modifications at a different period of life, shall not be in
the least degree injurious: for if they became so, they would cause the extinction of the species.
Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation
to the young. In social animals it will adapt the structure of each individual for the benefit of the community;
if each in consequence profits by the selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the good of another species; and though
statements to this effect may be found in works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal's whole life, if of high importance to it, might be
modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, and used
exclusively for opening the cocoon--or the hard tip to the beak of nestling birds, used for breaking the egg. It
has been asserted, that of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out
of it; so that fanciers assist in the act of hatching. Now, if nature had to make the beak of a full-grown pigeon
very short for the bird's own advantage, the process of modification would be very slow, and there would be
simultaneously the most rigorous selection of the young birds within the egg, which had the most powerful
and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken
shells might be selected, the thickness of the shell being known to vary like every other structure.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to
that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex
in its functional relations to the other sex, or in relation to wholly different habits of life in the two sexes, as is
sometimes the case with insects. And this leads me to say a few words on what I call Sexual Selection. This
depends, not on a struggle for existence, but on a struggle between the males for possession of the females;
the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore,
less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their
places in nature, will leave most progeny. But in many cases, victory will depend not on general vigour, but
on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor
chance of leaving offspring. Sexual selection by always allowing the victor to breed might surely give
indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, as well as the
brutal cock-fighter, who knows well that he can improve his breed by careful selection of the best cocks. How
low in the scale of nature this law of battle descends, I know not; male alligators have been described as
fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male
salmons have been seen fighting all day long; male stag-beetles often bear wounds from the huge mandibles
of other males. The war is, perhaps, severest between the males of polygamous animals, and these seem
oftenest provided with special weapons. The males of carnivorous animals are already well armed; though to
them and to others, special means of defence may be given through means of sexual selection, as the mane to
the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as
important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character. All those who have attended to the subject,
believe that there is the severest rivalry between the males of many species to attract by singing the females.
The rock-thrush of Guiana, birds of Paradise, and some others, congregate; and successive males display their
gorgeous plumage and perform strange antics before the females, which standing by as spectators, at last
choose the most attractive partner. Those who have closely attended to birds in confinement well know that
they often take individual preferences and dislikes: thus Sir R. Heron has described how one pied peacock was
eminently attractive to all his hen birds. It may appear childish to attribute any effect to such apparently weak
means: I cannot here enter on the details necessary to support this view; but if man can in a short time give
elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to
doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males,
according to their standard of beauty, might produce a marked effect. I strongly suspect that some well-known
laws with respect to the plumage of male and female birds, in comparison with the plumage of the young, can
be explained on the view of plumage having been chiefly modified by sexual selection, acting when the birds
have come to the breeding age or during the breeding season; the modifications thus produced being inherited
at corresponding ages or seasons, either by the males alone, or by the males and females; but I have not space
here to enter on this subject.
Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but
differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is,
individual males have had, in successive generations, some slight advantage over other males, in their
weapons, means of defence, or charms; and have transmitted these advantages to their male offspring. Yet, I
would not wish to attribute all such sexual differences to this agency: for we see peculiarities arising and
becoming attached to the male sex in our domestic animals (as the wattle in male carriers, horn-like
protuberances in the cocks of certain fowls, etc.), which we cannot believe to be either useful to the males in
battle, or attractive to the females. We see analogous cases under nature, for instance, the tuft of hair on the
breast of the turkey-cock, which can hardly be either useful or ornamental to this bird;--indeed, had the tuft
appeared under domestication, it would have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.
In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two
imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft,
some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had
from any change in the country increased in numbers, or that other prey had decreased in numbers, during that
season of the year when the wolf is hardest pressed for food. I can under such circumstances see no reason to
doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or
selected,--provided always that they retained strength to master their prey at this or at some other period of the
year, when they might be compelled to prey on other animals. I can see no more reason to doubt this, than that
man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious
selection which results from each man trying to keep the best dogs without any thought of modifying the
breed.
Even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might
be born with an innate tendency to pursue certain kinds of prey. Nor can this be thought very improbable; for
we often observe great differences in the natural tendencies of our domestic animals; one cat, for instance,
taking to catch rats, another mice; one cat, according to Mr. St. John, bringing home winged game, another
hares or rabbits, and another hunting on marshy ground and almost nightly catching woodcocks or snipes. The
tendency to catch rats rather than mice is known to be inherited. Now, if any slight innate change of habit or
of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same habits or structure, and by the repetition of this process, a
new variety might be formed which would either supplant or coexist with the parent-form of wolf. Or, again,
the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to
hunt different prey; and from the continued preservation of the individuals best fitted for the two sites, two
varieties might slowly be formed. These varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according to Mr. Pierce, there are two varieties of
the wolf inhabiting the Catskill Mountains in the United States, one with a light greyhound-like form, which
pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks.
Let us now take a more complex case. Certain plants excrete a sweet juice, apparently for the sake of
eliminating something injurious from their sap: this is effected by glands at the base of the stipules in some
Leguminosae, and at the back of the leaf of the common laurel. This juice, though small in quantity, is
greedily sought by insects. Let us now suppose a little sweet juice or nectar to be excreted by the inner bases
of the petals of a flower. In this case insects in seeking the nectar would get dusted with pollen, and would
certainly often transport the pollen from one flower to the stigma of another flower. The flowers of two
distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce very vigorous seedlings, which
consequently would have the best chance of flourishing and surviving. Some of these seedlings would
probably inherit the nectar-excreting power. Those individual flowers which had the largest glands or
nectaries, and which excreted most nectar, would be oftenest visited by insects, and would be oftenest
crossed; and so in the long-run would gain the upper hand. Those flowers, also, which had their stamens and
pistils placed, in relation to the size and habits of the particular insects which visited them, so as to favour in
any degree the transportal of their pollen from flower to flower, would likewise be favoured or selected. We
might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole object of fertilisation, its destruction appears a simple loss to the plant; yet if a
little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower
to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed, it might still be a great
gain to the plant; and those individuals which produced more and more pollen, and had larger and larger
anthers, would be selected.
When our plant, by this process of the continued preservation or natural selection of more and more attractive
flowers, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly
carry pollen from flower to flower; and that they can most effectually do this, I could easily show by many
striking instances. I will give only one--not as a very striking case, but as likewise illustrating one step in the
separation of the sexes of plants, presently to be alluded to. Some holly-trees bear only male flowers, which
have four stamens producing rather a small quantity of pollen, and a rudimentary pistil; other holly-trees bear
only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a
grain of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the
stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception,
there were pollen-grains, and on some a profusion of pollen. As the wind had set for several days from the
female to the male tree, the pollen could not thus have been carried. The weather had been cold and
boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined had been
effectually fertilised by the bees, accidentally dusted with pollen, having flown from tree to tree in search of
nectar. But to return to our imaginary case: as soon as the plant had been rendered so highly attractive to
insects that pollen was regularly carried from flower to flower, another process might commence. No
naturalist doubts the advantage of what has been called the "physiological division of labour;" hence we may
believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant,
and pistils alone in another flower or on another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female organs become more or less
impotent; now if we suppose this to occur in ever so slight a degree under nature, then as pollen is already
carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be
advantageous on the principle of the division of labour, individuals with this tendency more and more
increased, would be continually favoured or selected, until at last a complete separation of the sexes would be
effected.
Let us now turn to the nectar-feeding insects in our imaginary case: we may suppose the plant of which we
have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many facts, showing how anxious bees are to save
time; for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which
they can, with a very little more trouble, enter by the mouth. Bearing such facts in mind, I can see no reason to
doubt that an accidental deviation in the size and form of the body, or in the curvature and length of the
proboscis, etc., far too slight to be appreciated by us, might profit a bee or other insect, so that an individual so
characterised would be able to obtain its food more quickly, and so have a better chance of living and leaving
descendants. Its descendants would probably inherit a tendency to a similar slight deviation of structure. The
tubes of the corollas of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a
hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover,
but not out of the common red clover, which is visited by humble-bees alone; so that whole fields of the red
clover offer in vain an abundant supply of precious nectar to the hive-bee. Thus it might be a great advantage
to the hive-bee to have a slightly longer or differently constructed proboscis. On the other hand, I have found
by experiment that the fertility of clover greatly depends on bees visiting and moving parts of the corolla, so
as to push the pollen on to the stigmatic surface. Hence, again, if humble-bees were to become rare in any
country, it might be a great advantage to the red clover to have a shorter or more deeply divided tube to its
corolla, so that the hive-bee could visit its flowers. Thus I can understand how a flower and a bee might
slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner
to each other, by the continued preservation of individuals presenting mutual and slightly favourable
deviations of structure.
I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open
to the same objections which were at first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now very seldom hear the action, for instance, of the
coast-waves, called a trifling and insignificant cause, when applied to the excavation of gigantic valleys or to
the formation of the longest lines of inland cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as
modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave,
so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course
obvious that two individuals must always unite for each birth; but in the case of hermaphrodites this is far
from obvious. Nevertheless I am strongly inclined to believe that with all hermaphrodites two individuals,
either occasionally or habitually, concur for the reproduction of their kind. This view, I may add, was first
suggested by Andrew Knight. We shall presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample discussion. All vertebrate animals, all
insects, and some other large groups of animals, pair for each birth. Modern research has much diminished the
number of supposed hermaphrodites, and of real hermaphrodites a large number pair; that is, two individuals
regularly unite for reproduction, which is all that concerns us. But still there are many hermaphrodite animals
which certainly do not habitually pair, and a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals ever concur in reproduction? As it is
impossible here to enter on details, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a cross between different varieties, or between individuals of
the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that
CLOSE interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a
general law of nature (utterly ignorant though we be of the meaning of the law) that no organic being
self-fertilises itself for an eternity of generations; but that a cross with another individual is
occasionally--perhaps at very long intervals--indispensable.
On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the
following, which on any other view are inexplicable. Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable, the fullest freedom for the entrance of
pollen from another individual will explain this state of exposure, more especially as the plant's own anthers
and pistil generally stand so close together that self-fertilisation seems almost inevitable. Many flowers, on the
other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family;
but in several, perhaps in all, such flowers, there is a very curious adaptation between the structure of the
flower and the manner in which bees suck the nectar; for, in doing this, they either push the flower's own
pollen on the stigma, or bring pollen from another flower. So necessary are the visits of bees to papilionaceous
flowers, that I have found, by experiments published elsewhere, that their fertility is greatly diminished if
these visits be prevented. Now, it is scarcely possible that bees should fly from flower to flower, and not carry
pollen from one to the other, to the great good, as I believe, of the plant. Bees will act like a camel-hair pencil,
and it is quite sufficient just to touch the anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees would thus produce a multitude of hybrids
between distinct species; for if you bring on the same brush a plant's own pollen and pollen from another
species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has
been shown by Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards
it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but,
the agency of insects is often required to cause the stamens to spring forward, as Kolreuter has shown to be
the case with the barberry; and curiously in this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that if very closely-allied forms or varieties are planted near each other, it is
hardly possible to raise pure seedlings, so largely do they naturally cross. In many other cases, far from there
being any aids for self-fertilisation, there are special contrivances, as I could show from the writings of C. C.
Sprengel and from my own observations, which effectually prevent the stigma receiving pollen from its own
flower: for instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which every
one of the infinitely numerous pollen-granules are swept out of the conjoined anthers of each flower, before
the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my
garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, I
raised plenty of seedlings; and whilst another species of Lobelia growing close by, which is visited by bees,
seeds freely. In very many other cases, though there be no special mechanical contrivance to prevent the
stigma of a flower receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the
anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower
is ready, so that these plants have in fact separated sexes, and must habitually be crossed. How strange are
these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close
together, as if for the very purpose of self-fertilisation, should in so many cases be mutually useless to each
other! How simply are these facts explained on the view of an occasional cross with a distinct individual being
advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other,
a large majority, as I have found, of the seedlings thus raised will turn out mongrels: for instance, I raised 233
seedling cabbages from some plants of different varieties growing near each other, and of these only 78 were
true to their kind, and some even of these were not perfectly true. Yet the pistil of each cabbage-flower is
surrounded not only by its own six stamens, but by those of the many other flowers on the same plant. How,
then, comes it that such a vast number of the seedlings are mongrelized? I suspect that it must arise from the
pollen of a distinct VARIETY having a prepotent effect over a flower's own pollen; and that this is part of the
general law of good being derived from the intercrossing of distinct individuals of the same species. When
distinct SPECIES are crossed the case is directly the reverse, for a plant's own pollen is always prepotent over
foreign pollen; but to this subject we shall return in a future chapter.
In the case of a gigantic tree covered with innumerable flowers, it may be objected that pollen could seldom
be carried from tree to tree, and at most only from flower to flower on the same tree, and that flowers on the
same tree can be considered as distinct individuals only in a limited sense. I believe this objection to be valid,
but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and female flowers may be produced on the
same tree, we can see that pollen must be regularly carried from flower to flower; and this will give a better
chance of pollen being occasionally carried from tree to tree. That trees belonging to all Orders have their
sexes more often separated than other plants, I find to be the case in this country; and at my request Dr.
Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as
I anticipated. On the other hand, Dr. Hooker has recently informed me that he finds that the rule does not hold
in Australia; and I have made these few remarks on the sexes of trees simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some hermaphrodites, as land-mollusca and
earth-worms; but these all pair. As yet I have not found a single case of a terrestrial animal which fertilises
itself. We can understand this remarkable fact, which offers so strong a contrast with terrestrial plants, on the
view of an occasional cross being indispensable, by considering the medium in which terrestrial animals live,
and the nature of the fertilising element; for we know of no means, analogous to the action of insects and of
the wind in the case of plants, by which an occasional cross could be effected with terrestrial animals without
the concurrence of two individuals. Of aquatic animals, there are many self-fertilising hermaphrodites; but
here currents in the water offer an obvious means for an occasional cross. And, as in the case of flowers, I
have as yet failed, after consultation with one of the highest authorities, namely, Professor Huxley, to discover
a single case of an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the
body, that access from without and the occasional influence of a distinct individual can be shown to be
physically impossible. Cirripedes long appeared to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the case of both animals and plants, species
of the same family and even of the same genus, though agreeing closely with each other in almost their whole
organisation, yet are not rarely, some of them hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the difference between hermaphrodites and
unisexual species, as far as function is concerned, becomes very small.
From these several considerations and from the many special facts which I have collected, but which I am not
here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I am well aware that there are, on this view,
many cases of difficulty, some of which I am trying to investigate. Finally then, we may conclude that in
many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.
NATURAL SELECTION.
Natural Selection: its power compared with man's selection, its power on characters of trifling importance, its
power at all ages and on both sexes. Sexual Selection. On the generality of intercrosses between individuals of
the same species. Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing,
isolation, number of individuals. Slow action. Extinction caused by Natural Selection. Divergence of
Character, related to the diversity of inhabitants of any small area, and to naturalisation. Action of Natural
Selection, through Divergence of Character and Extinction, on the descendants from a common parent.
Explains the Grouping of all organic beings.
How will the struggle for existence, discussed too briefly in the last chapter, act in regard to variation? Can
the principle of selection, which we have seen is so potent in the hands of man, apply in nature? I think we
shall see that it can act most effectually. Let it be borne in mind in what an endless number of strange
peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the
hereditary tendency is. Under domestication, it may be truly said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions of life. Can it, then, be thought improbable,
seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to
each being in the great and complex battle of life, should sometimes occur in the course of thousands of
generations? If such do occur, can we doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however slight, over others, would have the best
chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in
the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the
rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be
affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called
polymorphic.
We shall best understand the probable course of natural selection by taking the case of a country undergoing
some physical change, for instance, of climate. The proportional numbers of its inhabitants would almost
immediately undergo a change, and some species might become extinct. We may conclude, from what we
have seen of the intimate and complex manner in which the inhabitants of each country are bound together,
that any change in the numerical proportions of some of the inhabitants, independently of the change of
climate itself, would most seriously affect many of the others. If the country were open on its borders, new
forms would certainly immigrate, and this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a single introduced tree or mammal has been
shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and
better adapted forms could not freely enter, we should then have places in the economy of nature which would
assuredly be better filled up, if some of the original inhabitants were in some manner modified; for, had the
area been open to immigration, these same places would have been seized on by intruders. In such case, every
slight modification, which in the course of ages chanced to arise, and which in any way favoured the
individuals of any of the species, by better adapting them to their altered conditions, would tend to be
preserved; and natural selection would thus have free scope for the work of improvement.
We have reason to believe, as stated in the first chapter, that a change in the conditions of life, by specially
acting on the reproductive system, causes or increases variability; and in the foregoing case the conditions of
life are supposed to have undergone a change, and this would manifestly be favourable to natural selection, by
giving a better chance of profitable variations occurring; and unless profitable variations do occur, natural
selection can do nothing. Not that, as I believe, any extreme amount of variability is necessary; as man can
certainly produce great results by adding up in any given direction mere individual differences, so could
Nature, but far more easily, from having incomparably longer time at her disposal. Nor do I believe that any
great physical change, as of climate, or any unusual degree of isolation to check immigration, is actually
necessary to produce new and unoccupied places for natural selection to fill up by modifying and improving
some of the varying inhabitants. For as all the inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or habits of one inhabitant would often give it
an advantage over others; and still further modifications of the same kind would often still further increase the
advantage. No country can be named in which all the native inhabitants are now so perfectly adapted to each
other and to the physical conditions under which they live, that none of them could anyhow be improved; for
in all countries, the natives have been so far conquered by naturalised productions, that they have allowed
foreigners to take firm possession of the land. And as foreigners have thus everywhere beaten some of the
natives, we may safely conclude that the natives might have been modified with advantage, so as to have
better resisted such intruders.
As man can produce and certainly has produced a great result by his methodical and unconscious means of
selection, what may not nature effect? Man can act only on external and visible characters: nature cares
nothing for appearances, except in so far as they may be useful to any being. She can act on every internal
organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his
own good; Nature only for that of the being which she tends. Every selected character is fully exercised by
her; and the being is placed under well-suited conditions of life. Man keeps the natives of many climates in
the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a
long and a short beaked pigeon on the same food; he does not exercise a long-backed or long-legged
quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. He does
not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals,
but protects during each varying season, as far as lies in his power, all his productions. He often begins his
selection by some half-monstrous form; or at least by some modification prominent enough to catch his eye,
or to be plainly useful to him. Under nature, the slightest difference of structure or constitution may well turn
the nicely-balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will his products be, compared with those
accumulated by nature during whole geological periods. Can we wonder, then, that nature's productions
should be far "truer" in character than man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation,
even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and
insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until
the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological
ages, that we only see that the forms of life are now different from what they formerly were.
Although natural selection can act only through and for the good of each being, yet characters and structures,
which we are apt to consider as of very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour
of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these
birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would
increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by
eyesight to their prey,--so much so, that on parts of the Continent persons are warned not to keep white
pigeons, as being the most liable to destruction. Hence I can see no reason to doubt that natural selection
might be most effective in giving the proper colour to each kind of grouse, and in keeping that colour, when
once acquired, true and constant. Nor ought we to think that the occasional destruction of an animal of any
particular colour would produce little effect: we should remember how essential it is in a flock of white sheep
to destroy every lamb with the faintest trace of black. In plants the down on the fruit and the colour of the
flesh are considered by botanists as characters of the most trifling importance: yet we hear from an excellent
horticulturist, Downing, that in the United States smooth-skinned fruits suffer far more from a beetle, a
curculio, than those with down; that purple plums suffer far more from a certain disease than yellow plums;
whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh. If, with
all the aids of art, these slight differences make a great difference in cultivating the several varieties,
assuredly, in a state of nature, where the trees would have to struggle with other trees and with a host of
enemies, such differences would effectually settle which variety, whether a smooth or downy, a yellow or
purple fleshed fruit, should succeed.
In looking at many small points of difference between species, which, as far as our ignorance permits us to
judge, seem to be quite unimportant, we must not forget that climate, food, etc., probably produce some slight
and direct effect. It is, however, far more necessary to bear in mind that there are many unknown laws of
correlation of growth, which, when one part of the organisation is modified through variation, and the
modifications are accumulated by natural selection for the good of the being, will cause other modifications,
often of the most unexpected nature.
As we see that those variations which under domestication appear at any particular period of life, tend to
reappear in the offspring at the same period;--for instance, in the seeds of the many varieties of our culinary
and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of
poultry, and in the colour of the down of their chickens; in the horns of our sheep and cattle when nearly
adult;--so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age,
by the accumulation of profitable variations at that age, and by their inheritance at a corresponding age. If it
profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty
in this being effected through natural selection, than in the cotton-planter increasing and improving by
selection the down in the pods on his cotton-trees. Natural selection may modify and adapt the larva of an
insect to a score of contingencies, wholly different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation, the structure of the adult; and probably in
the case of those insects which live only for a few hours, and which never feed, a large part of their structure
is merely the correlated result of successive changes in the structure of their larvae. So, conversely,
modifications in the adult will probably often affect the structure of the larva; but in all cases natural selection
will ensure that modifications consequent on other modifications at a different period of life, shall not be in
the least degree injurious: for if they became so, they would cause the extinction of the species.
Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation
to the young. In social animals it will adapt the structure of each individual for the benefit of the community;
if each in consequence profits by the selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the good of another species; and though
statements to this effect may be found in works of natural history, I cannot find one case which will bear
investigation. A structure used only once in an animal's whole life, if of high importance to it, might be
modified to any extent by natural selection; for instance, the great jaws possessed by certain insects, and used
exclusively for opening the cocoon--or the hard tip to the beak of nestling birds, used for breaking the egg. It
has been asserted, that of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out
of it; so that fanciers assist in the act of hatching. Now, if nature had to make the beak of a full-grown pigeon
very short for the bird's own advantage, the process of modification would be very slow, and there would be
simultaneously the most rigorous selection of the young birds within the egg, which had the most powerful
and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken
shells might be selected, the thickness of the shell being known to vary like every other structure.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to
that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex
in its functional relations to the other sex, or in relation to wholly different habits of life in the two sexes, as is
sometimes the case with insects. And this leads me to say a few words on what I call Sexual Selection. This
depends, not on a struggle for existence, but on a struggle between the males for possession of the females;
the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore,
less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their
places in nature, will leave most progeny. But in many cases, victory will depend not on general vigour, but
on having special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor
chance of leaving offspring. Sexual selection by always allowing the victor to breed might surely give
indomitable courage, length to the spur, and strength to the wing to strike in the spurred leg, as well as the
brutal cock-fighter, who knows well that he can improve his breed by careful selection of the best cocks. How
low in the scale of nature this law of battle descends, I know not; male alligators have been described as
fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male
salmons have been seen fighting all day long; male stag-beetles often bear wounds from the huge mandibles
of other males. The war is, perhaps, severest between the males of polygamous animals, and these seem
oftenest provided with special weapons. The males of carnivorous animals are already well armed; though to
them and to others, special means of defence may be given through means of sexual selection, as the mane to
the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as
important for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful character. All those who have attended to the subject,
believe that there is the severest rivalry between the males of many species to attract by singing the females.
The rock-thrush of Guiana, birds of Paradise, and some others, congregate; and successive males display their
gorgeous plumage and perform strange antics before the females, which standing by as spectators, at last
choose the most attractive partner. Those who have closely attended to birds in confinement well know that
they often take individual preferences and dislikes: thus Sir R. Heron has described how one pied peacock was
eminently attractive to all his hen birds. It may appear childish to attribute any effect to such apparently weak
means: I cannot here enter on the details necessary to support this view; but if man can in a short time give
elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to
doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males,
according to their standard of beauty, might produce a marked effect. I strongly suspect that some well-known
laws with respect to the plumage of male and female birds, in comparison with the plumage of the young, can
be explained on the view of plumage having been chiefly modified by sexual selection, acting when the birds
have come to the breeding age or during the breeding season; the modifications thus produced being inherited
at corresponding ages or seasons, either by the males alone, or by the males and females; but I have not space
here to enter on this subject.
Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but
differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is,
individual males have had, in successive generations, some slight advantage over other males, in their
weapons, means of defence, or charms; and have transmitted these advantages to their male offspring. Yet, I
would not wish to attribute all such sexual differences to this agency: for we see peculiarities arising and
becoming attached to the male sex in our domestic animals (as the wattle in male carriers, horn-like
protuberances in the cocks of certain fowls, etc.), which we cannot believe to be either useful to the males in
battle, or attractive to the females. We see analogous cases under nature, for instance, the tuft of hair on the
breast of the turkey-cock, which can hardly be either useful or ornamental to this bird;--indeed, had the tuft
appeared under domestication, it would have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.
In order to make it clear how, as I believe, natural selection acts, I must beg permission to give one or two
imaginary illustrations. Let us take the case of a wolf, which preys on various animals, securing some by craft,
some by strength, and some by fleetness; and let us suppose that the fleetest prey, a deer for instance, had
from any change in the country increased in numbers, or that other prey had decreased in numbers, during that
season of the year when the wolf is hardest pressed for food. I can under such circumstances see no reason to
doubt that the swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or
selected,--provided always that they retained strength to master their prey at this or at some other period of the
year, when they might be compelled to prey on other animals. I can see no more reason to doubt this, than that
man can improve the fleetness of his greyhounds by careful and methodical selection, or by that unconscious
selection which results from each man trying to keep the best dogs without any thought of modifying the
breed.
Even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might
be born with an innate tendency to pursue certain kinds of prey. Nor can this be thought very improbable; for
we often observe great differences in the natural tendencies of our domestic animals; one cat, for instance,
taking to catch rats, another mice; one cat, according to Mr. St. John, bringing home winged game, another
hares or rabbits, and another hunting on marshy ground and almost nightly catching woodcocks or snipes. The
tendency to catch rats rather than mice is known to be inherited. Now, if any slight innate change of habit or
of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring.
Some of its young would probably inherit the same habits or structure, and by the repetition of this process, a
new variety might be formed which would either supplant or coexist with the parent-form of wolf. Or, again,
the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to
hunt different prey; and from the continued preservation of the individuals best fitted for the two sites, two
varieties might slowly be formed. These varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according to Mr. Pierce, there are two varieties of
the wolf inhabiting the Catskill Mountains in the United States, one with a light greyhound-like form, which
pursues deer, and the other more bulky, with shorter legs, which more frequently attacks the shepherd's flocks.
Let us now take a more complex case. Certain plants excrete a sweet juice, apparently for the sake of
eliminating something injurious from their sap: this is effected by glands at the base of the stipules in some
Leguminosae, and at the back of the leaf of the common laurel. This juice, though small in quantity, is
greedily sought by insects. Let us now suppose a little sweet juice or nectar to be excreted by the inner bases
of the petals of a flower. In this case insects in seeking the nectar would get dusted with pollen, and would
certainly often transport the pollen from one flower to the stigma of another flower. The flowers of two
distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason
to believe (as will hereafter be more fully alluded to), would produce very vigorous seedlings, which
consequently would have the best chance of flourishing and surviving. Some of these seedlings would
probably inherit the nectar-excreting power. Those individual flowers which had the largest glands or
nectaries, and which excreted most nectar, would be oftenest visited by insects, and would be oftenest
crossed; and so in the long-run would gain the upper hand. Those flowers, also, which had their stamens and
pistils placed, in relation to the size and habits of the particular insects which visited them, so as to favour in
any degree the transportal of their pollen from flower to flower, would likewise be favoured or selected. We
might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole object of fertilisation, its destruction appears a simple loss to the plant; yet if a
little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower
to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed, it might still be a great
gain to the plant; and those individuals which produced more and more pollen, and had larger and larger
anthers, would be selected.
When our plant, by this process of the continued preservation or natural selection of more and more attractive
flowers, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly
carry pollen from flower to flower; and that they can most effectually do this, I could easily show by many
striking instances. I will give only one--not as a very striking case, but as likewise illustrating one step in the
separation of the sexes of plants, presently to be alluded to. Some holly-trees bear only male flowers, which
have four stamens producing rather a small quantity of pollen, and a rudimentary pistil; other holly-trees bear
only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a
grain of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the
stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception,
there were pollen-grains, and on some a profusion of pollen. As the wind had set for several days from the
female to the male tree, the pollen could not thus have been carried. The weather had been cold and
boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined had been
effectually fertilised by the bees, accidentally dusted with pollen, having flown from tree to tree in search of
nectar. But to return to our imaginary case: as soon as the plant had been rendered so highly attractive to
insects that pollen was regularly carried from flower to flower, another process might commence. No
naturalist doubts the advantage of what has been called the "physiological division of labour;" hence we may
believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant,
and pistils alone in another flower or on another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female organs become more or less
impotent; now if we suppose this to occur in ever so slight a degree under nature, then as pollen is already
carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be
advantageous on the principle of the division of labour, individuals with this tendency more and more
increased, would be continually favoured or selected, until at last a complete separation of the sexes would be
effected.
Let us now turn to the nectar-feeding insects in our imaginary case: we may suppose the plant of which we
have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many facts, showing how anxious bees are to save
time; for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which
they can, with a very little more trouble, enter by the mouth. Bearing such facts in mind, I can see no reason to
doubt that an accidental deviation in the size and form of the body, or in the curvature and length of the
proboscis, etc., far too slight to be appreciated by us, might profit a bee or other insect, so that an individual so
characterised would be able to obtain its food more quickly, and so have a better chance of living and leaving
descendants. Its descendants would probably inherit a tendency to a similar slight deviation of structure. The
tubes of the corollas of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a
hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover,
but not out of the common red clover, which is visited by humble-bees alone; so that whole fields of the red
clover offer in vain an abundant supply of precious nectar to the hive-bee. Thus it might be a great advantage
to the hive-bee to have a slightly longer or differently constructed proboscis. On the other hand, I have found
by experiment that the fertility of clover greatly depends on bees visiting and moving parts of the corolla, so
as to push the pollen on to the stigmatic surface. Hence, again, if humble-bees were to become rare in any
country, it might be a great advantage to the red clover to have a shorter or more deeply divided tube to its
corolla, so that the hive-bee could visit its flowers. Thus I can understand how a flower and a bee might
slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner
to each other, by the continued preservation of individuals presenting mutual and slightly favourable
deviations of structure.
I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open
to the same objections which were at first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now very seldom hear the action, for instance, of the
coast-waves, called a trifling and insignificant cause, when applied to the excavation of gigantic valleys or to
the formation of the longest lines of inland cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as
modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave,
so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course
obvious that two individuals must always unite for each birth; but in the case of hermaphrodites this is far
from obvious. Nevertheless I am strongly inclined to believe that with all hermaphrodites two individuals,
either occasionally or habitually, concur for the reproduction of their kind. This view, I may add, was first
suggested by Andrew Knight. We shall presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample discussion. All vertebrate animals, all
insects, and some other large groups of animals, pair for each birth. Modern research has much diminished the
number of supposed hermaphrodites, and of real hermaphrodites a large number pair; that is, two individuals
regularly unite for reproduction, which is all that concerns us. But still there are many hermaphrodite animals
which certainly do not habitually pair, and a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals ever concur in reproduction? As it is
impossible here to enter on details, I must trust to some general considerations alone.
In the first place, I have collected so large a body of facts, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a cross between different varieties, or between individuals of
the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that
CLOSE interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a
general law of nature (utterly ignorant though we be of the meaning of the law) that no organic being
self-fertilises itself for an eternity of generations; but that a cross with another individual is
occasionally--perhaps at very long intervals--indispensable.
On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the
following, which on any other view are inexplicable. Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully
exposed to the weather! but if an occasional cross be indispensable, the fullest freedom for the entrance of
pollen from another individual will explain this state of exposure, more especially as the plant's own anthers
and pistil generally stand so close together that self-fertilisation seems almost inevitable. Many flowers, on the
other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family;
but in several, perhaps in all, such flowers, there is a very curious adaptation between the structure of the
flower and the manner in which bees suck the nectar; for, in doing this, they either push the flower's own
pollen on the stigma, or bring pollen from another flower. So necessary are the visits of bees to papilionaceous
flowers, that I have found, by experiments published elsewhere, that their fertility is greatly diminished if
these visits be prevented. Now, it is scarcely possible that bees should fly from flower to flower, and not carry
pollen from one to the other, to the great good, as I believe, of the plant. Bees will act like a camel-hair pencil,
and it is quite sufficient just to touch the anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees would thus produce a multitude of hybrids
between distinct species; for if you bring on the same brush a plant's own pollen and pollen from another
species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has
been shown by Gartner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards
it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but,
the agency of insects is often required to cause the stamens to spring forward, as Kolreuter has shown to be
the case with the barberry; and curiously in this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that if very closely-allied forms or varieties are planted near each other, it is
hardly possible to raise pure seedlings, so largely do they naturally cross. In many other cases, far from there
being any aids for self-fertilisation, there are special contrivances, as I could show from the writings of C. C.
Sprengel and from my own observations, which effectually prevent the stigma receiving pollen from its own
flower: for instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which every
one of the infinitely numerous pollen-granules are swept out of the conjoined anthers of each flower, before
the stigma of that individual flower is ready to receive them; and as this flower is never visited, at least in my
garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, I
raised plenty of seedlings; and whilst another species of Lobelia growing close by, which is visited by bees,
seeds freely. In very many other cases, though there be no special mechanical contrivance to prevent the
stigma of a flower receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the
anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower
is ready, so that these plants have in fact separated sexes, and must habitually be crossed. How strange are
these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close
together, as if for the very purpose of self-fertilisation, should in so many cases be mutually useless to each
other! How simply are these facts explained on the view of an occasional cross with a distinct individual being
advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other plants, be allowed to seed near each other,
a large majority, as I have found, of the seedlings thus raised will turn out mongrels: for instance, I raised 233
seedling cabbages from some plants of different varieties growing near each other, and of these only 78 were
true to their kind, and some even of these were not perfectly true. Yet the pistil of each cabbage-flower is
surrounded not only by its own six stamens, but by those of the many other flowers on the same plant. How,
then, comes it that such a vast number of the seedlings are mongrelized? I suspect that it must arise from the
pollen of a distinct VARIETY having a prepotent effect over a flower's own pollen; and that this is part of the
general law of good being derived from the intercrossing of distinct individuals of the same species. When
distinct SPECIES are crossed the case is directly the reverse, for a plant's own pollen is always prepotent over
foreign pollen; but to this subject we shall return in a future chapter.
In the case of a gigantic tree covered with innumerable flowers, it may be objected that pollen could seldom
be carried from tree to tree, and at most only from flower to flower on the same tree, and that flowers on the
same tree can be considered as distinct individuals only in a limited sense. I believe this objection to be valid,
but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and female flowers may be produced on the
same tree, we can see that pollen must be regularly carried from flower to flower; and this will give a better
chance of pollen being occasionally carried from tree to tree. That trees belonging to all Orders have their
sexes more often separated than other plants, I find to be the case in this country; and at my request Dr.
Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States, and the result was as
I anticipated. On the other hand, Dr. Hooker has recently informed me that he finds that the rule does not hold
in Australia; and I have made these few remarks on the sexes of trees simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some hermaphrodites, as land-mollusca and
earth-worms; but these all pair. As yet I have not found a single case of a terrestrial animal which fertilises
itself. We can understand this remarkable fact, which offers so strong a contrast with terrestrial plants, on the
view of an occasional cross being indispensable, by considering the medium in which terrestrial animals live,
and the nature of the fertilising element; for we know of no means, analogous to the action of insects and of
the wind in the case of plants, by which an occasional cross could be effected with terrestrial animals without
the concurrence of two individuals. Of aquatic animals, there are many self-fertilising hermaphrodites; but
here currents in the water offer an obvious means for an occasional cross. And, as in the case of flowers, I
have as yet failed, after consultation with one of the highest authorities, namely, Professor Huxley, to discover
a single case of an hermaphrodite animal with the organs of reproduction so perfectly enclosed within the
body, that access from without and the occasional influence of a distinct individual can be shown to be
physically impossible. Cirripedes long appeared to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance, elsewhere to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, in the case of both animals and plants, species
of the same family and even of the same genus, though agreeing closely with each other in almost their whole
organisation, yet are not rarely, some of them hermaphrodites, and some of them unisexual. But if, in fact, all
hermaphrodites do occasionally intercross with other individuals, the difference between hermaphrodites and
unisexual species, as far as function is concerned, becomes very small.
From these several considerations and from the many special facts which I have collected, but which I am not
here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I am well aware that there are, on this view,
many cases of difficulty, some of which I am trying to investigate. Finally then, we may conclude that in
many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it
occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.
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