A great step in advance is taken when we come to study the courtship and sexual relationships of birds and mammals. There are many examples, in particular among birds, of a beautiful and high standard of love-morality. To the physical fondness of the sexes for one another there is now added a wealth of what must be recognised as psychical attraction, which finds its expression in many diverse ways. We shall find all forms of sexual association, very similar to marriage in the human species. There are temporary unions formed for the purpose of procreation, after which the partners separate and cease to care for one another. Polygamy is frequent, polyandry also occurs, and there are many cases of absolute profligate promiscuity. We shall, indeed, find the suggestion of all the sexual sins of humanity, every form of coquetry, of love-battles, jealousy and the like. There are as well many examples of monogamic unions lasting for the lives of the partners. This is especially the case with birds. Among the higher mammals polygamy is most common, but permanent unions are formed, especially among the anthropoid apes. Thus strictly monogamous marriages are frequent among gorillas and orang-utans, the young sometimes remaining with their parents to the age of six years, while any approach to loose behaviour on the part of the wife is severely punished by the husband.[50] We find both the matriarchate and patriarchate family; and we may observe the greatest difference in the conduct of the parents in their care of offspring. Even a rapid examination of these customs is worth while, for they cast forward many suggestions on our sexual, domestic, and social relationships. Let us take first the phenomena of courtship. It is possible to give only the briefest outline of this fascinating subject. We will begin with the law-of-battle. Courtship without combat is rare among mammals; it is less common in many species of birds. Special offensive and defensive weapons for use in these love-fights are found; such are the larger canine teeth of many male mammals, the antlers of stags, the tusks of elephants, the horns of antelopes, goats, oxen and other animals, while among birds the spurs of the cock and allied species are examples of sexual weapons.[51]