CHAPTER XVIII. 

 SENESCENCE, DECADENCE, AND DEATH. 



Ernest Thompson Seton has done much, through 

 his wild-animal stories, to acquaint his readers with the 

 tragic circumstances with which the lives of the wild 

 creatures usually terminate, and has thus brought them 

 to understand the "struggle for existence." In spite 

 of the appalling number of unfavorable conditions to be 

 overcome, enemies to be fought, parasites to be endured, 

 infections to be survived, enough living things man- 

 age to grow old to show us that for each kind there 

 seems to be a certain age limit beyond which survival 

 is impossible because of internal changes resulting from 

 the inevitable anatomico-physiological wear and tear. 

 These changes are best known in man and the domestic 

 animals, for among the wild creatures they subject the 

 individual to insuperable handicaps in the struggle for 

 existence. 



We are accustomed to think of living things as mortal, 

 and it is difficult to escape this conviction. Living 

 things as individuals are mortal, but the germ-plasm 

 is immortal and continuous. 



Unicellular organisms whose multiplication takes 

 place by fission, and whose individuals periodically 

 rejuvenate their substance by conjugation, escape old 

 age. There seems to be no reason apart from accident 

 why any of them should die. 



The same obtains among such multicellular organisms 

 as multiply by gemmation. It is the substance of the 

 parent of which the offspring is formed, and the ancestral 

 substance is present in every individual. The condition 

 is not materially altered when the sexual mode of repro- 

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