Race hygiene or eugenics, which has been mentioned as the third and most important branch of hygiene, aims to conserve the health of _future_ generations, through the action of those now living. Hygiene (individual and public) teaches us how to create for ourselves healthful conditions of living, but on every side we see evidences of the fact that we cannot entirely control conditions of health through hygiene only. Not all maladies by any means can be attributed to unnatural or unhygienic conditions of living. It is true that if followed out faithfully, the rules of hygiene will enable a man to live out his maximum natural life-span, with the maximum of well-being, and to run no risk of allowing any inherent weakness to be brought out. But some persons, even if they followed what is very nearly the normal code for the human being, would scarcely be able to avoid dire physical and mental fates. In short, we find that besides the hygienic factor in life which we may call environment, there is something else on which the health of the individual depends. This something else is heredity, or "the nature of the breed." Back of all the individual can do by hygiene lies his inheritance. To change this the individual can do nothing, but the parents of the individual can affect his inheritance, and we as parents can affect the inheritance of our offspring. [Sidenote: Trustees of the Racial Germ-plasm] First, we can carry through life uninjured the essential germ plasm which has been entrusted to our care. We should never forget that this germ plasm, which we receive and transmit, really belongs, not to us, but to the race; and that we have no right, through alcoholic or other unhygienic practises, to damage it; but that, on the contrary, we are under the most solemn obligation to keep it up to the highest level within our power. We are the trustees of the racial germ plasm that we carry. [Sidenote: Wise Combinations of Germinal Traits] Second, we can affect the life of our offspring by our choice in marriage. The basis of the development of desirable or undesirable tendencies or traits lies, of course, in the mating from which the individual springs. On the kind of combinations of germinal traits that are made by marriage depends whether or not undesirable traits shall reappear in the offspring. For instance, a man may inherit a defect from his father because his father married a certain type of woman. Had the father selected a different type, the children might not have inherited the father's defect. The importance of choice in marriage results from certain laws of inheritance, which make it clear that by proper combinations of individuals certain bad traits may be entirely "bred out." [Sidenote: Choice in Marriage]