4 SENESCENCE AND REJUVENESCENCE 



been elaborated. Unfortunately by far the greater part of the 

 work along this line has dealt chiefly with the process of senescence 

 as it appears in man and the higher mammals. Only now and 

 then has an attempt been made even to formulate a general theory 

 of senescence, and analytic investigation of senescence in the 

 lower organisms has scarcely been attempted. This limitation in 

 the investigation of the problem of senescence is due to the fact 

 that in the past interest in the problem has been very largely con- 

 fined to the medical profession. 



It is of course true that we are most familiar with the phenomena 

 of senescence in man and other mammals, the most complex of all 

 organisms. But man is a member of the organic world and a prod- 

 uct of evolution, and, as we have traced the development of his 

 structure from lower forms, so we must look to the lower forms for 

 adequate knowledge of his physiological processes. Before we 

 can understand senescence in man we must determine what it is in 

 its simplest terms. 



The present book finds its chief reason for existence in the fact 

 that it has been possible with the aid of certain experimental 

 methods of investigation to obtain some definite knowledge con- 

 cerning the processes of senescence and rejuvenescence in the lower 

 animals. The facts discovered afford, as I believe, a basis for the 

 further investigation of senescence and rejuvenescence in general, 

 and for an analytic consideration and interpretation of various 

 phenomena in plants and animals which are more or less closely 

 associated with these processes. Since the most important result 

 of these investigations is, in my opinion, the demonstration of the 

 occurrence of rejuvenescence quite independently of sexual repro- 

 duction, the book differs to some extent from most previous 

 studies of senescence in that it attempts to show that in the 

 organic world in general rejuvenescence is just as fundamental 

 and important a process as senescence. In the higher forms 

 the possibilities of rejuvenescence are apparently very narrowly 

 limited, but in the simpler organisms it is a characteristic feature 

 of life, and the nature of the process here enables us to under- 

 stand more clearly certain changes which occur in the higher 

 forms. 



