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.