Beginning with the traits and qualities which distinguish the sexes, grouped as the secondary sex characters, he showed that they are correlated with the special sexual function of the species in which they occur. These traits appear only when the hormones occur which are present in one sex and that only when the gonads of that sex are mature. In some cases they appear only at the period of the year when reproduction takes place, disappearing again after the breeding season. Their presence makes certain cells develop in excessive numbers at a particular spot in the organism (as in the growth of breasts from a few sweat glands) or causes them to specialize (to make hair on the face in man, or to grow antlers on the head of a stag). After castration, the hormones being absent, all these points of contrast between the sexes fail to appear. So by analogy we may explain all somatic and psychic differentiation as functions of the glands of internal secretion. Contemplated from the angle of the effect of environment upon the endocrines, and a reflected action upon the germ cells, we may outline a mechanism of the inheritance of acquired characters at certain times and consequent adaptation. The cycle of events would be as follows:
1. A state of lability of cells at a point because of increased or decreased use.
2. An increased or decreased appropriation by them of the hormone controlling their function.
3. A corresponding increase or decrease in function of the gland of internal secretion and so,
4. An increased or decreased representation of it in the reproductive sex cells in the gonads.
To take a classic illustration, the long neck of the giraffe. The neck of certain animals living in a district populated by trees with high branches would be in state of instability. If at the same time the pituitary, for some reason, was unstable and reacted with an extra supply of its secretion, it would stimulate the neck cells to reproduce themselves. In turn the pituitary would become stabilized in the direction of increased secretion, and hand on the component of increased secretion to the sex cells. That component, in conjunction with other factors, would therefore determine the emergence of a definite species character. In other words, the glands of internal secretion, as intermediaries between the environment and body, and between the body and the reproductive sex cells or germplasm, tender the clue to a phase of the puzzle of heredity, adaptation and evolution. It is only a dotted outline of an explanation to be sure, but one certainly capable of being filled in.
THE BEARING ON BREEDING
Since the endocrine glands are so subtly sensitive and responsive to environment, and are at the same time so intimately concerned in the process of inheritance—a law which sums up their influence upon resemblance and variation in animals—there is no need to stress their importance for the practical science and art of good breeding, eugenics. Another mode of approach to its problems is opened up, and fresh enthusiasm instilled into its hopes and aspirations. A method of analysis of the factors involved, together with rules for the prediction of the outcome of certain matings, when finally worked out, will elevate its procedure to the level of the more exact sciences.
A man's chief gift to his children is his internal secretion composition. The endocrines are truly the matter of breeding as they are of growth. They are the material carriers of the inherited physical and psychic dispositions, powers, abilities and disabilities from the soma to the germplasm and back from the germplasm to the soma. All kinds of questions arise as soon as one attempts to consider the bearing of this underlying principle upon concrete situations. What happens, say, when a pituitocentric mates with a thyrocentric? Or when a pituitocentric marries a pituitocentric? Is there a reinforcement or a cancellation of the dominant endocrine? Is there a quantitative addition of internal glandular tendencies in the germplasm, or a more complex rearrangement dependent upon reactions between all the internal secretions?
The term endocrine dominants brings up the inquiries of Mendelism, and the relation of Mendelian conceptions of dominant and recessive to the internal secretions. The Mendelians have emphasized the rôle of the unit factor in heredity, and the conservation of the unit factor as an entity through all the adventures of matings. Also, that when unit factors, say of the color of the eyes, come into conflict, brown or black being mixed with blue or grey, one, the recessive, is submerged and overlaid but not destroyed by the other, the dominant. So brown or black eyes, dark hair, curly hair, dark skin, and so on, are dominant, while blue or grey eyes, light or straight hair, light skin are recessives. A nervous temperament is dominant to the phlegmatic. A number of psychic qualities have been declared to be Mendelian unit factors: memory, mechanical instinct, mathematical ability, literary ability, musical ability, and even handwriting.