To attempt to do this we must return to the position we left at the end of the fifth chapter. We had then concluded from our examination of the sexual habits of insects, mammals, and birds that a marked differentiation between the female and the male existed already in the early stages of the development of species, and that such divergence, or sex-dimorphism, to use the biological term, becomes more and more frequent and conspicuous as we ascend to the higher types. The essential functions of females and males become more separate, their habits of life tend to diverge, and to the primary differences there are added all manner of secondary peculiarities. We found, however, especially in our study of the familial habits, that these supplementary differences could not be regarded as fixed and unalterable in either the female or the male organism; but rather that the secondary sexual characters must be considered as depending on environmental conditions, among which are included the occupational activities, the scarcity or abundance of the food supply, the relative numbers of the two sexes, and, in particular, the brain development and the strength of the parental emotions. We followed the development of the female element and the male element. The male at first an insignificant addendum to the female, but the long process of love's selection, carrying on the expansion and aggrandisement of the male, led to the reversal of the early superiority of the female, replacing it by the superiority of the male. The female led and the male followed in the evolution process. We saw that there are many curious alternations in the superiority of one sex over the other in size and also in power of function. Below the line, among backboneless animals, there is much greater constancy of superiority among the females, and this predominance persists in many higher types. Even among birds, who afford the most perfect examples of sexual development, the cases are not infrequent in which the female equals, and sometimes even exceeds, the male in size and strength and in beauty of plumage. The curious case of the Phalaropes furnished us with a remarkable example of a reversal of the role of the sexes. We found further that (1) an extravagant development of the secondary sexual characters was not really favourable to the reproductive process, the males thus differentiated belonging to a lower grade of sexual evolution, being bad fathers and unsocial in their conduct; (2) that the most oppressed females are as a rule very faithful wives, and (3) that the highest expression of love among the birds must be sought in the beautiful cases in which the sexes, though maintaining the essential constitutional distinctions, are, through the higher individuation of the females more alike, equal in capacity, and co-operate together in the race-work.