Charles Manning Child, professor of biology in the University of Chicago, has given the most comprehensive statement of his problem to date from the standpoint of his science, although, as we shall see, much has been done since.[174] His most interesting and important contribution for our purpose is his refutation of the older view that life is always a progressive process and that true rejuvenescence does not occur. Of course, in higher animals the progressive features are predominant and development ends in death. But the above generalization does not take due account of what occurs in lower organisms, while even in man and other mammals the different tissues do not undergo senescence either alike or synchronously. Some, for example, cells of the epidermis remain relatively young till and after the death of the individual. In other tissues such replacement of old, differentiated, or dead cells by younger ones occurs more or less extensively and tissue regeneration following injury occurs more or less in all tissues save only the nervous system. Such regeneration retards the aging of the tissue or organ as a whole. Minot thought that in such cases regeneration arises from cells or parts of cells that have never undergone differentiation, so that even in such cases development is progressive and not regressive. Even if he is right in maintaining that fibrillar substance cannot regenerate, it must be noted that new fibrillar substance does arise in continuity with the old, while isolated cells apparently do not produce it. Child maintains that there is differentiation in such cases and that these regenerating cells have returned to a kind of activity characteristic of the early stages of embryonic development; that is, that cells _can_ assume an activity characteristic of an earlier stage. “Even in the outgrowth of new nerve fibers from the central stump of a cut nerve there is return to a process of growth and development which is normally characteristic of an earlier stage of development.” Thus regression and differentiation do occur in most tissues of man and higher animals, although cells of one tissue can never produce those of another.