"As for the other spring of Nature, Love, ... In the animal kingdom one can hardly speak of an active sexual selection on the part of the male, even among the highest birds and mammals; of a passive selection through the struggle of the males in which the strongest remains victor, only among a small part of the higher animals, or the rest, the sexual impulse is not individual, but is purely general. But now in the infinitely larger part of the animal kingdom there do not exist organs of sexual pleasure acting as stimulants to coition; without such accordingly coition is an office indifferent to the egoism of the individual which is carried on by the impelling constraint of instinct, as the spinning of the web of the spider or the building of the bird's nest for the eggs hereafter to be laid. To the absence of enjoyment in the office of fecundation in the case of most animals also the frequently indirect form of this function deviating from direct copulation points. When in the vertebrates a personal physical enjoyment appears to occur, it is at first certainly as flat and insignificant as possible; but soon there is also added the contest of the males for the female, which in many species of animals is waged with the greatest bitterness, and has for its consequence often painful injuries, not seldom also the killing of one of the rivals. Add to that, among those animals which at the time of rut form herds led by the victorious male, the involuntary continence of the younger members, whether they separate into smaller detachments or remain with the main herd, when an invasion of the rights of the head of the family is punished in the cruellest fashion. This involuntary continence of the largest part of the males, and the pains and vexation caused the defeated by the contests, seem to me a hundredfold to exceed the pleasures accruing to the prosperous males from the sexual pleasure. As for the females, in the first place, among most animals they far more rarely couple than the privileged males; and, secondly, the pains of child-bearing in their case far outweigh the pleasure derived from copulation."