SENESCENCE, DECADENCE, AND DEATH 



445 



seem to live longest that are longest in arriving at ma- 

 turity, but there are such striking exceptions that one 

 hesitates in making this a rule. For example, certain 

 ants that reach maturity in a few weeks are known to 

 live several years, while a species of Cicada remains a 

 subterranean nymph for seventeen years and then 



FIG. 150. Hair about to become gray. Chromophages transporting the pigment 

 granules. (Metchnikoff.) 



emerges from the ground to enjoy but a few days of 

 adult life. 



Too little attention has- been paid to the phenomena 

 of senescence to give us any clear understanding of 

 them. We are even uncertain how many of the 

 changes found in aged human beings are purely senile 

 and not the results of antecedent ailments. If we view 

 the senile state from the point of view of wear and tear, 

 we are not infrequently confronted by the paradoxical 



