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 amceboid 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 
