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, &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 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
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, &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
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
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
sh
orter 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