In these pages an attempt has been made to show that this differentiation of sex has, throughout, been accompanied by, and largely moulded by, common instincts and behaviour, and this interpretation is only to be reached by a study of the phenomena in their simplest form among the lower grades of animal life. Colour and the various sexual differences in form have been allowed to dominate this investigation of the problem of sex, and have diverted attention from more profitable and fruitful channels. The lower we descend in the scale of animal life the less convincing becomes the argument that the colour, ornament or armature of the males is the result of sexual selection in the older, Darwinian sense. The argument of Geddes and Thomson and others that the males are more “katabolic,” the females more “anabolic,” seems no less unsatisfactory, for in many cases the female is just as highly ornamented as the male, and in others she is considerably large. Further, in their less specialized species the sexes are almost or quite indistinguishable externally, and are sombrely clad, just as at the opposite extreme we find them equally ornamented and equally active. We shall be nearer the truth if we regard these secondary sexual characters as expression points of germinal variations. Though we seem hopelessly ignorant as to the inciting cause of the variations, at least we seem to be able to lay a finger on the agents by which they are effected. And these are the hormones of the primary and secondary sexual glands, whose functions affect more than the merely sexual side of the organism. They profoundly affect the coloration of animals, giving rise on the one hand to purely ornamental “secondary sexual characters,” and on the other to changes of coloration which achieve the ends of protective resemblance colours, or of “warning coloration,” as circumstances may demand. There is nothing more remarkable in this than the control which the pituitary body exercises over stature, either when in a pathological condition, or when the controlling action of the other gland secretions is removed, as by castration. Hitherto much has been made of trophic nerves, which control growth; but it is probable we have overlooked the still more important action of “trophic” glands, such as the thyroid. This apparently controls growth in many directions. Adaptations to environment which are effected by changes in bodily shape-as in the transformation of land-dwelling mammals into Seals and Whales—are probably largely controlled by these glands. Their activity is as great as their manifestation is varied.