A man’s nature is essentially his endocrine nature.  Primarily, when he is born, he represents a particular inherited combination of different glands of internal secretion.  They, constituting the inventory of his vital stock in trade, start him in life.  Afterwards, food, the routine of his existence, the accidents of experience, education, disease and misfortune, in short, environment, modify him because they modify his ductless glands and his vegetative apparatus, as well as his brain, depressing some parts, and stimulating others, and so rearranging the system.  In particular will he be transformed as the gland is affected which is the centre of the system to which the others adapt and accommodate themselves.  The inertia of the system is very great, almost absolute, and always tends to return.  If he has children, he hands on his constellation of endocrines, in spite of mishaps, not at all or only slightly transformed.  Sometimes, however, the experiential transformation has been sufficiently deep, and shaken the very constitution of his germ-plasm.  So family dispositions and traits, national and racial temperaments, are propagated, maintained and varied.

THE SEX INSTINCTS

Hormone reactions, as we have seen, initiate the complicated forces, processes and expressions of sex.  The dictum of the founder of modern pathology, Virchow, that Woman was in effect an appendix to the ovaries, has long been taken to apply to her psychic traits as well as somatic.  Her mind, like her skin, her hair and her pelvis, is a product of the ovarian endocrines.  But these determinations are by no means her monopoly.  Man is likewise a creation of the chemical wheels within wheels and springs within springs that are his glands of internal secretion.  That he is not so obviously an appendix to his testes is due to two reasons.  First, the male sex hormones have not the instability nor cyclic rhythmicity of the female.  Secondly, and perhaps consequently, his sex instincts have become overlayered with other more labile instincts, with habits and customs and necessities that appear to oust the sex instinct into an altogether decentralized position.  Moreover, it is the function of the female to be the excitor in the sex process:  her subconscious, thoroughly aware of the fact, sees to it that the sex instinct stands starkly central and dominating in her life.

The moods of love, like the more stereotyped manifestations of sex, are dependent upon a proper supply to the blood of the internal secretions of the reproductive organs, the gonadal endocrines.  If the testes are removed from frogs, it is found that the clasp-reflex, symptom of sex desire, is abolished.  If, after an interval of several days, the testes’ extract is injected into the frog, the reflex reappears for a few days.  The hormone provoking this sex reflex is present in the testes only during the breeding season.  In birds, the seasonal nesting and migrating