The origin of the remarkable differences between individuals that distinguish species, varieties and families, has long been one of the chief puzzles of biology. It may indeed be called the leading puzzle, which led Darwin on to the collection of the data that culminated in the "Origin of Species." The why of the Unique is the fundamental problem of those who would understand life.
An explanation is an attempt at a consistent and persistent, sometimes an obstinate clarity of mind. A vast number of observations gathered by laboratory experimentalists as well as by those naturalists of the abnormal, physicians in active practice, prove that the construction of the individual both during development before maturity, and maintenance during maturity, his constitution, in short, is directed by the endocrine glands. It is possible now to present an explanation of the individuality of the individual.
To assert that variation is responsible for the individual, that it is the mechanism which isolates him as a being like none other of his fellows, not even his parents, brothers, and sisters, is merely to beg the question. What is variation? The internal secretion theory of the process offers, for the first time, an explanation that is coherent and comprehensive, based upon concrete and detailed observations. It provides an adequate interpretation of the numberless hereditary gradations and transitions, blendings and mixtures. It suggests a control of heredity in the future.
THE PURE TYPES
In the pure types, only one gland, either by being present in great excess above the average, or by being pretty well below the average, comes to exercise the dominating influence upon the traits of the organism. As the strongest link in the chain, or as the weakest, it rules. The others must accommodate themselves to it. Among them as commanders of growth, development and normal function, it holds the balance of power. In every emergency it stands out by its strength or by its weakness. It thus creates its own type of man or woman, with attributes and characteristics peculiar to itself. These pure types, as we have seen, are mainly the thyroid, the pituitary, and the adrenal-centered.
Each with the signs peculiar to it can be identified among the faces that pass one in the street. And they differ so markedly among themselves that they provide a new and accurate means of classifying varieties among the races of the species: man. The thyroid type differs as much from the adrenal type as does a greyhound from a bull-dog. The greyhound has a certain size, form, character and capacity. The bull-dog has similar qualities which are yet quite different. Each is built for a particular career. Among human beings, the pure thyroid type is easily distinguished from the pure adrenal type, and both of these from the pure pituitary type. Each is stamped with a significant figure, height, skin, hair, temperament, ambition, social reactions and predisposition to certain diseases.
THE MIXED TYPES
Among the mixed types, the lines of distinction are less clear, and so they are more difficult to classify. The mixed types may be said to be hyphenated. In them, two or even three of the internal secretory glands conflict for predominance. The combined action makes for a resultant modification in the primary glandular markings and effects. A hyphenated classification thus becomes inevitable. Especially is this so if the two glands are mutually antagonistic and inhibitory. A compromise effect is then necessitated. Or an individual may be dominated by one gland at one period of his life and by another at a later period. One of the glands, the thyroid, for example, will show, by the traces it has left upon the earliest developing features, that it was in control at the very earliest dates of his history, while other signs will disclose the more recent influence of the adrenal or of the pituitary. The combination becomes classifiable as the thyroid-pituitary type, or as the thyroid-adrenal type.
That the external features as well as the chronic diseases of human beings are controlled by some common factor has long been suspected. Inquiries into morbid phenomena with a hereditary trend yielded information that has paved the way for the internal secretion theory. It has long been known that certain diseases effect only certain individuals of a definite constitution. Apoplexy, diabetes, arteriosclerosis, Bright's disease, are met with almost exclusively in what the older clinicians talked about as the apopleptic type. On the other hand, they said, anemias, tuberculosis, hemophilias, scrofulas occurred more among the lymphatic type. But they had no idea whatever of the true functional basis of the two different types. The truth as we of today view it is that these two types represent different textures of human beings, fabricated of different internal secretions. They are really two different breeds of the species Homo Sapiens. The materials being different, the color and feel of them is different, and the resistance to wear and tear is different.