These facts may help us to understand the nature of death and dissolution of the body in higher animals. Death in these animals is due to cessation of oxidations, but the surprising fact is that if the oxidations have been interrupted but a few minutes life cannot be restored even by artificial respiration. This suggests that the respiratory ganglia in the medulla oblongata suffer an irreparable injury or an irreversible change (comparable to that just described in the cells of _Ctenolabrus_) even when deprived of oxygen for only a short time. As a consequence of the irreversible injury to the medulla the respirations cease permanently, the heart-beat must also cease, and gradually the different tissues must undergo the dissolution characteristic of death. While all the cells may be immortal they are only so in the presence of oxygen and the nutritive solution which the circulating blood furnishes. With the proper supply of oxygen cut off they can no longer live. 4. It is an unquestionable fact that each form has a quite definite duration of life. Unicellular organisms are immortal; but for the higher organisms with sexual reproduction the duration of life is almost as characteristic as any morphological peculiarity of a species. No species can exist unless the natural life of its individuals outlasts the period of sexual maturity; and unless the average duration of life is long enough to allow as many offspring to be brought into the world as will compensate for loss by death. The male bee dies before it is a year old, while the queen may live several years. In a certain species of butterflies, the Psychidæ, the parthenogenetic female lays its eggs while still in the cocoon and then dies without ever leaving the cocoon. The imago of the ephemera leaves the water in the evening, copulates, lets its eggs fall into the water, and is dead the next morning. The imperfect condition of their mandibles and alimentary canal makes them unfit for a long duration of life. The males of the rotifers which are devoid of organs of digestion live but a few days. In the Zoölogical Station at Naples in 1906, an actinian, _Actinia equina_, was alive after having been in captivity fifteen years, and another one, _Cerianthus_, had been observed for twenty-four years. Korschelt kept earthworms for as long as ten years. The fresh-water mussel may reach the age of sixty years or more and crayfish may live for over twenty years. The differences in the duration of life of mammals are too well known to need discussion. If the cells and tissues are immortal, how does it happen that the duration of life is so characteristic for each species?