CHAPTER XI 



SENESCENCE IN THE HIGHER ANIMALS AND MAN 



The problem of senescence in man and the higher animals has 

 very naturally claimed the attention of the anatomist, the physiolo- 

 gist, the investigator along medical lines, and the zoologist, and for 

 the layman also it has always possessed a vital interest quite differ- 

 ent from that which attaches to many scientific problems. Man's 

 interest in the problem of his own senescence, old age, and death 

 undoubtedly dates from the time when he first began to think and 

 ask himself questions concerning himself. From ancient times to 

 the present the problem has been discussed again and again, and 

 from the most various points of view. It has always been an 

 attractive field for speculation, but a large volume of scientific 

 data bearing upon one aspect or another of it has accumulated. 

 A considerable portion of the literature of the subject deals with 

 the problem from the point of view of the physician and medical 

 investigator rather than that of the general zoologist or physiolo- 

 gist, and of course the data are very largely descriptive and 

 statistical, rather than experimental and analytic. 



It is neither possible nor necessary at this time to attempt any 

 extended review and critique of the literature. My purpose is 

 merely to analyze and interpret the more important facts from the 

 point of view attained through study of the lower animals, and to 

 show how the age cycle in man and the higher animals, so far as it 

 differs from that in the lower organisms, is the necessary and inevi- 

 table result of the course of evolution. 



INDIVIDUATION AND REPRODUCTION IN THE HIGHER FORMS IN 

 RELATION. TO THE AGE CYCLE 



The increase in the degree of individuation or physiological 

 integration of the individual, which is a conspicuous feature of 

 evolution, is evident in the higher animals and man in the increasing 

 co-ordination and interrelation of parts, both dynamically and 

 chemically, and in the greater structural and functional specializa- 

 tion and differentiation. The problem of the nature of this change 



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