Sexual Selection

Peculiarities often appear under domestication in one sex and become hereditarily attached to that sex.

• The same fact probably occurs in the wild.

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

This is sometimes the case with insects.

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
• strength to the wing to strike in the spurred leg.

The brutal cock-fighter knows that he can improve his breed by careful selection of the best cocks.

I do not know how low in the scale of nature this law of battle descends.

• Male alligators fight, bellow, and whirl round, like Indians in a war-dance, for the possession of the females.
• Male salmons fight all day long
• Male stag-beetles often bear wounds from the huge mandibles of other males.

The war is severest between the males of polygamous animals.

• These seem oftenest provided with special weapons.

The males of carnivorous animals are already well armed.

• The lion has the mane
• The boar has the shoulder-pad
• The male salmon has the hooked jaw

Amongst birds, the contest is more peaceful.

• Their severest male rivalry of many species is to attract by singing the females.
• Males display their gorgeous plumage and perform strange antics before the females.

Bird in cages often take individual preferences and dislikes.

• Sir R. Heron has described how one pied peacock was eminently attractive to all his hen birds.

It is childish to attribute any effect to such weak means.

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 suspect that some well-known laws with respect to the plumage of male and female birds, compared 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 are inherited at corresponding ages or seasons, either by the males alone, or by the males and females;

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;

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.

Our domestic animals have peculiarities arising and becoming attached to the male sex. Examples are:

• the wattle in male carriers
• horn-like protuberances in the cocks of certain fowls, &c.

These cannot be either useful to the males in battle, or attractive to the females.

We see analogous cases in the wild such as the tuft of hair on the breast of the turkey-cock.

• This can hardly be useful or ornamental to this bird.
• Had the tuft appeared under domestication, it would have been called a monstrosity.