Child concludes that senescence is more continuous in man than in the lower forms. His long evolution has given a physiological stability to the protoplasmic substratum and a high degree of individuation results from this. But the central nervous system, being least capable of progressive change, always dies first, so that the length of man’s life is that of his nervous system and physiological death and senescence inhere in its fortunes. In the lower forms the death point may never be attained under normal conditions because of the low stability of the substratum and the consequent decrease of individuation that permits the frequent occurrence of a high degree of rejuvenation. But in the higher forms of life the capacity for the latter is limited by greater stability; and this, again, has been acquired through a process of evolution lasting through so many millennia that we must certainly “admit that this task [man’s rejuvenation] may prove to be one of considerable difficulty.” Thus, according to Child, whose views are the most philosophical and insightful in the field of biology up to date for our purposes, senescence and rejuvenescence are both going on all the time in all cells and organs and are not special processes. In most cells and in most lower organisms dedifferentiation and despecialization of structure and function, which we may term in general regressive tendencies, are always less pronounced than progressive impulsions, while the latter predominate still more in the higher forms of life. It is “quite impossible to account for the course of evolution and particularly for many so-called adaptations in organisms without the inheritance of such acquired characters, but since thousands or ten thousands of generations may be necessary in many cases for inheritance of this kind to become appreciable, it is not strange that experimental evidence upon this point is still conflicting” (p. 463). Germ plasm is not something apart from or uninfluenced by all that goes on in its immediate environment within the body. Regression and dedifferentiation involve reconstitution and always approximate reproduction. To state the matter roughly, all processes involved both in growing old and in growing young might conceivably be arranged on a kind of Porphery ladder with agamic forms of indefinite reproduction, as illustrated in unicellular organisms or in germ plasm at the lower or _summum gens_ end, and the most differentiated cells that have progressively lost the power of reproducing the whole organism, regenerating lost parts, power to grow, divide, and nourish themselves, at the top of the ladder, representing the _infima_ species. On such a ladder, development, differentiation, and individuation is progress up, and all rejuvenating activities are descent toward the most generalized function of perpetual self-reproduction. This conception is in very suggestive harmony with the analogous psychoanalytic law of restitution to mental