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