As the higher elements of the body in old people become weaker and are devoured by the macrophags, it seems probable that the destruction or deterioration of these voracious cells would tend to the prolongation of life. However, as the macrophags are indispensable in the struggle against the microbes of infectious diseases, and particularly of chronic disease, such as tuberculosis, it is necessary to preserve them. We must turn rather to the idea of a remedy which could strengthen the higher elements and make them a less ready prey to the macrophags. In the “Nature of Man” (Chap. III.) in discussing the simian origin of mankind, I touched on the existence of animal serums that have the power of dissolving the blood corpuscles of other species of animals. There is now, in biological science, a new chapter upon such serums, which have been called cytotoxic serums because they are able to poison the cells of organs. The blood and blood serum of some animals act as poisons when they are introduced into an organism. Eels and snakes, even non-poisonous snakes, are cases in point. A small quantity of the blood of a snake, an adder for instance, injected into a mammal (rabbit, guinea-pig, or mouse) soon brings about death. The blood of some mammals is poisonous to other mammals, although in a lesser degree than that of snakes. The dog is specially notable from the fact that its blood is poisonous to other mammals, whilst, on the other hand, the blood and blood serum of the sheep, goat, and horse have generally little effect on other animals and on man. It is for this reason that these animals, and particularly the horse, are used in the preparation of the serums employed in medicine.