152 SECRETS OF ANIMAL LIFE 



dogs; fruit-trees smothered in lichen and rotting 

 in their hearts. Such senility marked by degenera- 

 tion or involution, by great liability to disease and 

 by an ungearing of important parts of the organism, 

 is practically unknown in wild Nature which has 

 not suffered from Man's interference. Even the 

 Sequoias with their two thousand years are not 

 senile, and the famous Edinburgh sea-anemone, 

 which lived longer than the average human span, 

 did not show its age. There are two main reasons 

 for this absence of senility in Nature; the first is 

 that the conditions of the struggle for existence 

 are such that senility is not tolerated; the second 

 is that the average duration of life seems to have 

 been punctuated in reference to wide issues, namely, 

 the welfare of the species. Creatures come to a 

 natural end when their processes of rejuvenescence 

 fail hopelessly to keep pace with their processes of 

 senescence. For to senescence as distinguished from 

 senility the great majority of organisms are liable. 

 To the question, " Why do we grow old ? " many 

 answers have been given. Metchnikoff suggested 

 that we are poisoned by the absorption of the 

 products of bacterial activity in the large intestine, 

 for this brings about hardening of the walls of the 

 arteries and also corrupts our bodyguard of wan- 

 dering amoeboid cells or phagocytes, so that they 

 become traitors, turning upon the cells of the cen- 

 tral nervous system. Others have suggested other 

 modes of auto-intoxication. To some it has seemed 

 enough to refer to the wear and tear of hard-worked 



