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04 - Natural Selection 04-05 - Sexual Selection 10 Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, so no doubt it will be under nature.

Thus it is rendered possible for the two sexes to be modified through natural selection in relation to different habits of life, as is sometimes the case; or for one sex to be modified in relation to the other sex, as commonly occurs.

This leads me to say a few words on what I have called Sexual Selection. This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex.

The result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection.

Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny.

But in many cases, victory depends not so much on general vigor, as on having special weapons, confined to the male sex.

A hornless stag or spurless cock would have a poor chance of leaving numerous offspring.

stag
stag

cock
cock

spurs
spurs


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, in nearly the same manner as does the brutal cockfighter by the careful selection of his best cocks.

cockfight
cockfight


How low in the scale of nature the law of battle descends, I know not; male alligators have been described as fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females; male salmons have been observed fighting all day long; male stagbeetles sometimes bear wounds from the huge mandibles of other males; the males of certain hymenopterous insects have been frequently seen by that inimitable observer M. Fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror.

alligator
alligator

crocodile
crocodile

Stag Beetle
Stag Beetle

Indian
Indian


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 of the lion, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear.

mane
mane

salmon
salmon

sword
sword

spear
spear
04 - Natural Selection 04-06 - On the generality of Intercross Between Individuals of the Same Species 10 Illustrations of the Action of Natural Selection:
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04 - Natural Selection 04-07 - Illustrations of the Action of Natural Selection: 10 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 was hardest pressed for food. Under such circumstances 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 some other period of the year, when they were compelled to prey on other animals.

wolf
wolf

deer
deer


I can see no more reason to doubt that this would be the result, than that man should be able to improve the fleetness of his greyhounds by careful and methodical selection, or by that kind of unconscious selection which follows from each man trying to keep the best dogs without any thought of modifying the breed.

dog
dog
04 - Natural Selection 04-08 - On the Intercrossing of Individuals 10 I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course obvious that two individuals must always (with the exception of the curious and not well-understood cases of parthenogenesis) unite for each birth; but in the case of hermaphrodites this is far from obvious.

snail
snail


Nevertheless there is reason to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind.

This view was long ago doubtfully suggested by Sprengel, Knight and Kolreuter. 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.