It is possible to be quite sure that some facts carefully observed about mammals in a biological laboratory apply to similar structures in man, also a mammal. Because of this relationship, the data from medicine and surgery are priceless. Thus we are enabled to check up our systematic experimental knowledge of animals by an ascertained fact here and there in the human material, and to get a fairly exact idea of how great the correspondence actually is. Gaps thus filled in are narrow enough, and our certainty of the ground on either side sufficiently great, to give a good deal of justifiable assurance. If we use our general biological evidence in this way, merely to help in clearing up points about _human_ biology, we need not be entirely limited to mammals. Some sex phenomena are quite general, and may be drawn from the sexual species most convenient to study and control in experiments. When we get away from mammalian forms, however, we must be very sure that the cases used for illustrations are of general application, are similar in respect to the points compared, or that any vital differences are understood and conscientiously pointed out. Too much stress cannot be laid upon the point that such animal data, carefully checked up with the human material, cannot safely be used for any other purpose than to discover what the facts are about the human body. When the discussion of human social institutions is taken up in Part II, the obvious assumption will always be that these rest upon human biology, and that we must not let our minds wander into vague analogies concerning birds, spiders or crustacea. BIBLIOGRAPHY FOR CHAPTER I 1. Loeb, Jacques. Artificial Parthenogenesis and Fertilization. Chicago, 1913. 2. Loeb, Jacques. The Organism as a Whole. N.Y., 1916, p. 125--brief summary of results of [1]. 3. Bower, Kerr & Agar. Sex and Heredity. N.Y., 1919, 119 pp. 4. Schäfer, E.A. Nature, Origin and Maintenance of Life. Science, n.s., Vol. 36, pp. 306 f., 1912. 5. Guyer, M.F. Being Well-Born. Indianapolis, 1916; p. 123. CHAPTER II SEX IN TERMS OF INTERNAL SECRETIONS Continuity of germplasm; The sex chromosome; The internal secretions and the sex complex; The male and the female type of body; How removal of sex glands affects body type; Sex determination; Share of egg and sperm in heredity; Nature of sex--sexual selection of little importance; The four main types of secretory systems; Sex and sex-instincts of rats modified by surgery; Dual basis for sex; Opposite-sex basis in every individual; The Free-Martin cattle; Partial reversal of sex in man. In Chapter I, the "immortality" of the protoplasm in the germ cells of higher animals, as well as in simpler forms without distinct bodies, was mentioned. In these higher animals this protoplasm is known as _germplasm_, that in body cells as _somatoplasm_.