“There is reason to believe that the processes which underlie all great work in art, literature, or science take place unconsciously, or at least unwittingly. It is an interesting question to ask whence comes the energy of which this work is the expression. There are two chief possibilities: one, that it is derived from the instinctive tendencies which, through the action of controlling forces, fail to find their natural outlet; the other, that the energy so arising is increased in amount through the conflict between controlled and controlling forces.”--W. H. RIVERS. The biology of sex is a vast subject. Not only are there questions of sex-determination, but the whole sexual selection problem has to be considered, together with the evolutionary function of sex, and its first origin. I can only attempt, in the short space at my disposal, to deal with one or two of the chief points, and only in so far as they bear on questions of human sex psychology. In the first place, then, we have to consider the evolutionary history of sex. Of its origin we can say only that it is veiled in complete obscurity. Once present, however, it appears to have a definite function by making possible, through sexual reproduction, all the various combinations of any heritable variations that may arise in different individuals of a species, and so conferring greater evolutionary plasticity on the species as a whole.[28] Primarily, sex implies only the fusion of nuclei from two separate individuals; there is no need for sex differences to exist at all. Sex differences, however, are almost universal in sexually-reproducing organisms, and represent a division of labour between the active male cell and the passive female cell, the former taking over the task of uniting the two, the latter storing up nutriment for the new individual that will result from that union. The subsequent history of sex is, roughly speaking, the history of its invasion of more and more of the organization of its possessors. First the male as a whole, and not merely its reproductive cells, tends to become organized for finding the female. The female’s whole type of metabolism is altered to produce the most efficient storage of reserve material in her ova, and later she almost invariably protects and nourishes the young during the first part of their development, either within or without her own body. Appropriate instincts are of course developed in both male and female.