PREFACE 



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. 



About half the book is a presentation of results of my own 

 investigations and the larger part of these have not been published 

 elsewhere. Consequently the book stands as a record of research 

 as well as an attempt at a general survey. No attempt has been 

 made to present a complete bibliography of the subject of age. The 

 references are to a large extent intended to serve rather as guides 

 or aids in obtaining further knowledge of the literature than as an 



