The finest study of natural ability that has as yet been composed is Francis Galton's on Hereditary Genius. It also remains the best study of the natural conditions of success. He showed that of the type of man he classed as "illustrious" there occurred about one in a million, and of the type "eminent" about two hundred and fifty in a million. Of the qualities which determine natural ability of this kind, he selected inherent capacity, zeal, and perseverance as the three prerequisites. And he states that "If a man is gifted with vast intellectual ability, eagerness to work, and power of working, I cannot comprehend how such a man should be suppressed." "Such men (those who have gained great reputations) biographies show to be haunted and driven by an incessant, instinctive craving for intellectual work." "They … work … to satisfy a natural craving for brain work." "It is very unlikely that any conjunction of circumstances should supply a stimulus to brain work commensurate with what these men carry in their own constitutions."

What is this inherent craving for brain work? What is this zeal? And what is power of endurance and perseverance, the quality of stamina? How are they to be interpreted in terms of the internal secretions?

In view of what has been said of the ante-pituitary as the gland of intellectuality, studies of intellectually gifted people having shown well functioning large pituitaries, and of mental defectives in a certain number of cases a small limited pituitary, it is justifiable to regard the factor of inherent capacity as a function of the ante-pituitary. The factor of zeal or enthusiasm points to the thyroid. Markedly enthusiastic types are thyroid dominant types. Vigor as a third factor, the ability to stand stress and strain of continued effort is dependent upon good adrenal and interstitial cell function. So we may say that craving and capacity for brain work plus ardor plus perseverance in its pursuit, the triplicate of natural ability, are the reflections in conduct and character of balanced and sufficient ante-pituitary, thyroid, and adrenal-interstitial contributions in the chemical formula of the personality. In the chapter on historic personages analyzed from the endocrine viewpoint, we shall see that some of the most eminent and illustrious people of history have been pituitary-centered.

MENTAL DEFICIENCY

Natural ability grows in an endocrine soil of a particular kind, perhaps affected by the internal secretions much as natural soil is by fertilizers like phosphates or nitrates. Increased production follows increased fertilization. Natural disability must vary similarly with a perversion or improper mixture, deficiency or absence of the hormones that combine in natural ability.

It is assumed as a matter of course that the brain itself is there, which, to carry out our analogy, means that the crude soil or earth is there. Sufficient quantity and adequate quality of nerve tissue must be regarded as prerequisite. If the brain has been damaged in any way during development or birth, if it has been smashed up in any way, or if it has failed to evolve the minimum number of healthy nerve cells, the endocrine influence becomes negligible. It is like attempting to insert a key into a door which has no lock.

It is among the specimens of normality of the brain cells that we may look for our examples of endocrine mental deficiency. Included are all sorts of examples of feeble-mindedness varying from the moron to the imbecile and idiot, arrested brain life. The cretin is the classic type of mental deficiency due to endocrine insufficiency, curable or improvable by the proper handling.

Insanity, degeneration of the normal brain life, may be caused by an upset of the endocrine balance. Among the commonest manifestations of insanity are excitements and depressions, apathies and manias, hallucinations, delusions and obsessions, all of which are reproducible under known conditions of internal secretion excess or failure. Alternating states of mania and depression are caused in some instances by extreme hyperthyroidism. The critical periods of life, when a profound revolution is overturning the endocrine equilibrium, puberty, pregnancy, and the menopause, are the periods of most frequent occurrence of insanity, when mental instability reveals endocrine instability (Dementia praecox, pregnancy psychosis, menopause neurosis). Actual insanity need not be the only manifestation. By far the greater number of mental disturbances due to aberrations of the internal secretions never see an asylum or a doctor. They live more or less close to the borderline of insanity as persons who have spells, eccentricities and peculiarities, hysteria, tics or just "nervousness."

About two-thirds of mental deficiency is definitely inherited, about one-third acquired. It is the opinion of a number of psychologists that it is inherited as what the Mendelians call a recessive, that is as a trait which will be overshadowed, if there is admixture of normal mentality, but will crop up by breeding with another mental defective. What we know of the endocrine factors in heredity leads us to suppose that it is the mating of one marked endocrine insufficiency with another that is often responsible for the inherited tendency to feeble-mindedness and insanity. The effect of the hormone system upon the vegetative apparatus may create the more obscure insanities and quasi-insanities. The direct action of the internal secretions upon the brain cells, producing a sort of hair trigger situation within them, may cause the explosive discharges from them which appear as overpowering impulses or uncontrollable conduct. The waves of feeling which precede them are unquestionably endocrine determined. The wave of fear a cat experiences upon seeing a dog is accompanied and indeed preceded by an increase of the amount of adrenalin in the blood. The picture of fright, as observed in a so-called normal person, staring eyes, trembling hands, dry lips and mouth, corresponds to the portrait of the appearance in hyperthyroidism. In persons afflicted with uncontrollable impulses, the inhibiting hormones may not be present in sufficient quantity.

Feeble-mindedness, ranging from stupidity to imbecility, may also be a direct effect of insufficient endocrine supply to the brain cells. When there is not enough of the thyroid secretion in the blood, the tissue between the cells in the brain become clogged and thickened, so that a gross barrier to the passage of the nerve impulses is created. We have here an illustration of internal secretion lack actually producing gross changes in the brain. But without a doubt, most endocrine influences upon the brain, at work every minute and second of its life, are the subtle ones of molecular chemistry and atomic energetics. We know that such mental qualities as irritability and stupidity, fatigability, and the power to recover quickly or slowly from fatigue, sexual potency and impotence, apathy and enthusiasm are endocrine qualities. We know also that the thyroid dominant tends to be irritable and excitable, the pituitary deficient to be placid and gentle, the adrenal dominant to be assertive and pugnacious, the thymus-centered to be childish and easy-go-lucky and the gonad deficient to be secretive and shy. This brings us to the relation of the internal secretions to the type of personality as a whole.