Wherever amongst the animal species supremacy in love is obtained by force, the male, nearly always the more ardent, has necessarily become, through the action of selection, larger, stronger, and better armed than the female. Such is in reality the case in regard to the greater number of vertebrates; certain exceptions, however, exist, and naturally these are chiefly found among birds, as they are more inclined than other types to put a certain delicacy in their sexual unions. With many species of birds, indeed, the female is larger and stronger than the male. It is well known to be the same with certain articulates, and these facts authorise us to admit that there is no necessary correlation between relative weakness and the female sex. Must we therefore conclude, with Darwin, that the females of certain birds owe their excess of size and height to the fact that they have formerly contested also for the possession of the males? We may be allowed to doubt it. Almost universally, whether she is large or small, the female is less ardent than the male, and in the amorous tragi-comedy she generally plays, from beginning to end, a passive _rôle_; in the animal kingdom, as well as with mankind, amazons are rare. Among birds and vertebrates generally the male is much more impetuous than the female, and therefore he has no difficulty in accepting for the moment any companion whatever.[18] This uncontrollable ardour sometimes even urges the males to commit actual attempts on the safety of the family. Thus it happens that the male canary (_Fringilla_ _canaria_) persecutes his female while she is sitting, tears her nest, throws out the eggs, and, in short, tries to excite his mate to become again a lover, forgetting that she is a mother. In the same way our domestic cock pursues the sitting hen when she leaves her eggs in order to feed.[19] With the cousins-german of man, the mammals, sexual psychology has a general resemblance to that of birds, but more often it is far less delicate. And besides this, the sexual customs are naturally less refined in proportion as the nervous centres of the species are less perfected. Thus the stupid tatoways meet by chance, smell each other, copulate and separate with the greatest indifference. Our domestic dog himself, although so civilised and affectionate, is generally as gross in his amours as the tatoway.