Each of the parental half-cells contained, marvellously, the potential moiety of a living personality. But either, alone, would have been but an incomplete and valueless thing, had it not become united with the complementary half-cell required to complete it structurally, and to engender and energise its potentialities. Nevertheless, throughout all the immature and the mature phases of life, from conception to birth, and from birth onward to death, the opposite sides of the body represent normally the opposite sex-inherences of their respective parents. They are, in humans, the Man and the Woman--two in one--that exist in every living man and woman. They represent contrary principles; they perform different functions; they engender and energise dissimilar processes. One is the centre of the Male characteristics, Dominant upon the material plane; the other, of the Female characteristics, Recessive thereon. Normality and health are the mean and balance, in the individual, of the complementary and supplementary functions and processes of the opposite sex-inherences of his, or her, body. Precisely as in the social economy the complementary and supplementary rôles of men and women counterpoise the aptitudes and determine the effectiveness of human life and action. The left, Female-half of the body, with its allied half-brain,[1] is inhibitive, and engenders the evolution and the preservation, physical and mental, of The Type; sustaining health and vital power by way of the female attributes of rest and conservation. The right, Male half, with its allied half-brain, is executive, and energises the development (Adaptation) of The Type in its relation to Environment, and, disbursing and applying the vital resources, generates and differentiates potential faculty in terms of living function. IV This hypothesis of the dual constitution and of dual functions of the two-sided body supplies an explanation, equally simple and inevitable, of the parental transmission of Sex. _Natura simplex est_, said Newton. And Du Prel, "Nature is much more simple than we have any conception of." Because, as Biology shows, not only does each of the two parents contribute to offspring, but there being both a right and a left reproductive gland in members of both sexes, the contribution either parent supplies must have been derived from one or other of these glands in them. And if the two sides of the body are of different sex-inherence, it is only logical to conclude that the contribution the gland of one side makes will be of different sex-inherence from that of the other. Since all forms of Energy have two modes, potential (or latent) and kinetic (or active), on the plane of physics, this must be true, of course, of Vital Energy.