In their ensemble, the glands of internal secretion wield a determining influence upon the development of the individual from his very inception. If his various powers may be conceived of as an orchestra, they may be said to conduct it from the very beginning of its movements, and to cease only with its termination. From the moment when the spermatozoon penetrates and fecundates the ovum, the fate of the future being is settled by their disposition. The seal of his destiny is soaked with their substance. POST-NATAL DEVELOPMENT Every particle of protoplasm, every granule of the impregnated ovum carries the representatives of the parental ductless glands. As a consequence, they transmit chemically, with no figure of speech involved, the peculiar familial, racial and national characters from progenitors to offspring. They confer upon the child a number of the properties commonly recognized as inherited. All those features which distinguish Caucasian from Mongolian, Scandinavian from Italian, Italian from Jew are determined by them. In short, at every step of his life, in every relation and association, in every expression of the inner forces that control his being, the normal individual is influenced by his internal secretions. Let us now see how. CHAPTER V HOW THE GLANDS INFLUENCE THE NORMAL BODY 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