A beginning of this study of endocrine inheritance, on the pathologic side, has been made. Some of these have been along Mendelian lines. Following up abnormal growth (making giants and dwarfs) and abnormal metabolism (goitre, diabetes, and so on), it has been stated that it would seem that abnormal growth is dominant in the male, and recessive in the female, while abnormal metabolism is dominant in the female and recessive in the male. If an endocrine abnormality like a goitre, or cretinism, or a dwarf or giant appear in a family as a sign of endocrine instability, other members of that family will very likely show internal secretion abnormalities.
If one gland of internal secretion acts as the centre of the system and the others as satellites, we should be able to trace what happens to it in the different generations. Does it maintain its supremacy? Or will it be ousted by another member of the group? The time will come when we shall thus be able to advise prospective parents of the consequences of procreation and to forecast the meaning for the race of a particular marriage. Internal glandular analysis may become legally compulsory for those about to mate before the end of the present century.
What are desirable and undesirable matings? The general law followed by nature in her helterskelter way seems to be the production of the greatest number of hybrids and variations possible, whether for good or evil does not matter. Certain endocrine types appear to be specially attracted to others belonging to the same group. Thus thymus-centered types frequently marry. The ante-pituitary type of male, the strongly masculine, mates often with the post-pituitary type of female, the markedly feminine. The children exhibit the lineaments of the pituitary-centered type. The general trend seems to be the establishment of a better balanced, equilibrated type. Yet the children often are apt to segregate into pituitary dominants or pituitary deficients. Happiness and unhappiness in marriage should be examined from the standpoint of endocrine compatibility or incompatibility. Likewise those divorced or about to be divorced.
The correction of endocrine defects, disturbances, imbalances and instabilities, before mating, presents another field. It remains to be seen whether we shall thereby, in one generation, be able to affect at all the germplasm, hitherto revered by all pious biologists as an environment-proof holy of holies. No one can deny, in the face of the multitude of evidence available, that internal secretion disturbances occur in the mother, which, when grave, offer in the infant gross proof of their significance, and therefore when slight must more subtly work upon it. Endocrine disturbances in infancy have been traced to endocrine disturbances in the mother during pregnancy. Pregnant animals fed on thyroid give birth to young with large thymus glands. The diet of the mother has been proved conclusively to influence the development and constitution of the child. As the internal secretions influence the history of the food in the body, they affect development in the womb indirectly as well as directly. Certainly, whether or no we learn how to change the nature of germplasm within a short time, we have in the endocrines the means at hand for affecting the whole individual that is born and sees the light of day.
THE CONTROL OF MUTATIONS
The true physical and intellectual evolution of man depends upon the production of mutations of a desirable kind that can survive. The information furnished by the study of the endocrines concerning the genesis of personality provides the foundations for a positive eugenics, a eugenics of the encouragement of desirable matings, with the proper legal and social procedures. Selective breeding for the production of the best endocrine types should become practicable.
But the biologist should be able to go farther. If the eugenist is to limit himself to the method of the animal breeder he will have to rest satisfied with the characters or hereditary factors given, that turn up spontaneously in an individual. But with the internal secretions as the controllable controllers of mutations, the outlook changes. It should become possible to produce new mutations, good and bad, to speed up their production at any rate. The feeding of thyroid to a gifted father before procreation might enhance immeasurably the chances of transmission of his gift as well as of its intensification in his offspring. A field of investigation is opened that would embrace in due time the deliberate control of human evolution.
All the physical traits, stature, color, muscle function, and so on, offer themselves for improvement, as well as brain size, and the intellectual and emotional factors which have dominated man's social evolution. The general prevalence of nervous disorders in civilized countries, visible even in the nervous infants the specialist in children's diseases is called upon to treat, shows that the nervous system of the better part of mankind is in a state of unstable equilibrium. It may be another example of the curious coincidences that have been called the Fitness of the Environment that the investigation of the endocrines promises to put into our hands the instruments of the control of the future of the nervous system. In general, meanwhile, the eugenist should strive for raising the level of the endocrine potential, and discourage its lowering. That means the encouragement of matings in which all the internal secretion activities are reinforced. On the other hand, those internal secretion combinations, generally leading to a deficiency of all of them which produce types of mental defectives, delinquency and crime should not be allowed to occur.
THE INFLUENCE OF ENVIRONMENT
What suggestions now are there for the euthenist who would control the influence of environment upon child culture. There are certain pertinent facts and leads that are worth considering.