Death of cells apparently from old age occurs at every stage of development and many cells do not die when the individual does, for he does so only because some tissue or organ that is essential reaches the point of death. Some have thought glands are primarily responsible for it; but others, whose view Child adopts, hold that it is the nervous system, especially its cephalic part, that dies first in man. In various insects and, for example, the salamander, death occurs almost at once after the exclusion of the sexual products, but this is exhaustion. In most, the length of life of the individual is determined by that of the shortest-lived essential organ or of the tissue that is least capable of regression and rejuvenation and the development of which, therefore, remains most continuously progressive. In cold-blooded animals where the rate of metabolism is dependent on external temperature, senescence can be reduced by cold, and in certain lower invertebrates by the simple method of underfeeding. When cells lose the capacity to divide, they differentiate, grow old, and sooner or later die, although death everywhere is the result of final progressive development if this process goes far enough and is not interrupted by regression caused by the need of repair, reproduction, or lack of food. Death is due, thus, to increased physiological stability of the substratum of the organism or to an increasing degree of differentiation that this general stability makes possible. And as individuation increases, death becomes more and more inevitable. Rubner calculated the total energy requirements in calories for doubling the body weight after birth and the requirements per kilogram in body weight for the whole period of life, for a number of domestic animals. His totals for all, except man, showed close agreement, and hence he concludes that the amounts of energy required are the same in all species except for man, who has a far greater amount of energy, that is, a smaller percentage of the energy of food is consumed in growth and maintenance of body weight and more in activity than in other animals. Very likely domestic animals expend less energy than their wild congeners but it is certainly difficult to correlate these results with Minot’s criteria of age as measured by the decrease of growth.