Bees and the stork and other birds of passage kill or neglect their old, while in ancient German myth old men often slew themselves for the good of the community. Certain species eat their young rather than die themselves and even this is for the interest of the species. A certain wasp lays its eggs in the female plant and its young, when covered with its pollen, fly to a male plant to fertilize it, although this means their death. Thus death is a normative factor of development and to this extent Weismann is correct. Life has three periods. In the first there is an excess of energy and growth; then comes middle age with an equilibrium, while in old age the relation of these energies is reversed, so that the first and third stages contradict each other. But everywhere life is dependent on nutrition and all death, in whatever form, is due to its lack. So we have a kind of biological circulation and a kilogram of body substance evolves a different sum of total energy in each of these periods of life. The author gives us an interesting cut of the contraction of the vertebrae and their padding, which often in the old means ankylosis in the lower part of the spinal column. He dissents from the Conklin theory of the abatement of metabolism as a cause of old age because he thinks this activity has a wide range of play at all stages of life, monthly and even daily. Nor does he accept without modification the _Schilddrüse_ (thyroid) theory, although admitting that the endocrine glands play a very important function. Nor does he accept Von Hansemann’s view of altruistic nutritive disturbance, namely, that single cell groups are more or less reciprocally dependent upon others for their activity so that they in a sense work for each other. The physiological (that is, without outer stimulation) collapse of groups of cells is especially connected with the generative cells and their departure reacts upon the nutrition of the whole body and in a sense weakens it (the atrophy of age). The loss of the generative cells is physiological because they, in departing from the body, represent the growth of a new organism. Hansemann gives countless examples but they affect only the fact and not the kind or method of growing old, which is a progressive process, and we still are unable to answer the question why the generative cells in the ordinary process of life are so soon destroyed or why so few of them are devoted to this purpose. The author sees much truth in Minot’s differentiation or cytomorphosis of cells, according to which differentiation, which causes growth, is also the cause of death. Minot himself says that the biologist can no more grasp the essence of death than he can that of life. Science does not know the difference between these two despite the impulsions of our causal instinct. Horsley thinks the condition for a green old age is the conservation of a sound thyroid. With its activity is, of course, connected that of other glands.