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The following study of senescence and rejuvenescence is pri marily a register of progress along certain lines of a research program on which I have been engaged during the last fifteen years. This program began with the attempt to analyze experimentally the simpler reproductive processes, but it at once became evident that the whole problem of the organic individual, its origin, development, physiological character, and limiting factors, was involved. In the study of the organic individual the importance of the physio logical age changes soon became apparent and it was found neces sary to devote considerable time to their analysis, for the origin of new individuals by reproduction is in many cases very closely associated with physiological aging. And since the conclusions reached concerning the age cycle finally attained a definite, positive form, differing to some extent from commonly accepted views, but seeming to throw some light upon various other biological problems, it has seemed desirable to attempt a general considera tion and synthesis of the subject of age changes from the point of view which has grown out of the research program mentioned above. It will appear clearly in the following pages that the problems of individuation, reproduction, and age are all closely connected. For that reason it has been necessary to devote a chapter — chap. Ix — to the problem of individuation and reproduction. This chapter is merely a brief statement of some of the more important evidence and the conclusions reached concerning the nature of the organic individual, a full consideration of the subject being left to another time.