Various workers have even experimentally produced a state of hermaphroditism in mammals by simultaneous grafting of portions of testes and ovary: the behaviour here oscillates between male and female.[34]
It is quite clear from these and other facts that in higher vertebrates there are present in every individual of either sex the nervous connections which give the possibility of either male or female behaviour; but that normally only one of these two possibilities is realized, since for the potentiality of action given by the nervous connections to become actual as behaviour it is necessary for the nervous system to be activated by the secretion of one or other of the reproductive organs. Castrated animals fail to realize either possibility of normal sex-behaviour, although their nervous machinery is untouched.
There are, further, some facts of observation which, even if they have not yet been fully analysed by experiment, still throw light on the matter. Although many of the most familiar birds--fowls, pheasant, peacock, duck, finches, and so forth--have bright-coloured males and drab females, with marked difference of behaviour between the sexes, there are, as we have seen, many others, such as herons, divers, swans, grebes, moorhens, and auks, in which the sexes are alike in plumage and furthermore show what may be called a “mutual” courtship in which both male and female play similar rôles. In this latter class it seems clear that the secretions of the male and female reproductive organs must be more alike than in the markedly dimorphic species: and this is borne out by some strange facts regarding not merely the courtship but the actions concerned with pairing itself. In the crested grebe and the little grebe, for example, close observation has shown that either member of the pair may assume the passive “female” attitude or the active “male” attitude in pairing: and in the moorhen we meet with the still more extraordinary phenomenon of double pairing, in which an act of pairing with male and female in normal position is immediately followed by a second act in which the normal position is reversed.[35] It would appear in such cases that the similarity of male and female internal secretion is so great that quite slight changes in nervous or metabolic activity can cause the nervous centres for the opposite sex’s mode of behaviour to become activated.