CHAPTER XVI 

 SOME THEORIES OF SENESCENCE AND REJUVENESCENCE 



The present chapter makes no attempt at a complete historical 

 review of the various ideas and theories concerning the nature 

 of the age process: it is merely a brief critical consideration, in 

 the light of the preceding experimental data, of some of the more 

 recent theories and suggestions. 



SENESCENCE AS A SPECIAL OR INCIDENTAL FEATURE OF LIFE 



The popular belief, which is of course based on the phenomena 

 of old age and death in man and the higher animals, is that the 

 process of aging is a wearing out and death a final breakdown of the 

 organic mechanism, or some essential part of it. This idea has 

 from time to time found scientific support, chiefly among those 

 who have considered the problem of senescence primarily in rela- 

 tion to man. Among the earlier authorities of the modern era in 

 science Lotze ('51, '84) is one who holds this view, and recently 

 Magnus-Levy ('07) has expressed the same opinion. While the 

 phenomena of senile atrophy in extreme old age in man and the 

 higher animals may perhaps be interpreted as in some sense a 

 wearing out (see pp. 288-89), they represent only the final stages 

 of senescence and are the result of what has happened during the 

 earlier life of the organism. Both man and animals grow old 

 throughout the course of progressive development, as the decrease 

 in rate of metabolism indicates. 



Speculative attempts have been made to show that age and 

 death are associated in some way with the reproductive function. 

 Weismann regards the limitation of life as an adaptation which has 

 arisen by the action of natural selection, because continued life of 

 the individual after the reproductive period is a " senseless luxury' 

 for the species. Weismann's views are discussed in another 

 chapter (see pp. 304-5). In opposition to this hypothesis Goette 

 ('83) maintains that reproduction is the real cause of age and death 

 of the parent individual and at the same time brings about rejuve- 

 nescence in the offspring. The foundation of Goette's hypothesis 



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