I think it extremely probable that the scientific study of old age and of death, two branches of science that may be called _gerontology_ and _thanatology_, will bring about great modifications in the course of the last period of life. All that we know on these subjects confirms my view. But will it lead to the development of an instinct of death? That instinct lies deep in the roots of the human constitution? Will the means be found to bring it to the surface? Has not the enormous period during which it has remained latent led to its atrophy? The science of the future alone can answer that question. But the persistence of organs and structures that are extremely ancient, as for instance, the survival of the mammary glands in males and of the vermiform appendage in anthropoid apes and man, gives us the hope that the instinct of natural death may emerge from its latent condition when old age has become a normal process. The mammary glands of males are functionless rudiments. They must be interpreted as vestiges of organs that were more highly developed in remote ancestors among which both sexes gave milk to nourish the young. This function exists in a latent condition in the males of living mammals. Extremely rare cases have existed in which males possessed large glands secreting enough milk to feed the young. These males, it is true, had the genital organs either very badly developed or in a condition approaching hermaphroditism.[345] But in other authentic cases (perfectly developed) he-goats and rams have been known to provide milk in considerable quantities, whilst married men have suckled children with milk secreted by unusually developed glands. It is stated that the secretion of milk can be excited by stimulation of the nipples.[346] Such examples of the reappearance of a latent property that has been lost for untold ages are extremely important.