156 SECRETS OF ANIMAL LIFE 



nomena as the return to an almost embryonic state 

 that we are familiar with in the pupa-stage in the 

 life-history of flies and some other insects with 

 complete metamorphosis. But what of the dis- 

 tinctly higher animals and man himself, where there 

 has been an epoch-making evolutionary increase in 

 the physiological stability of the protoplasmic sub- 

 stratum and an associated heightening of the degree 

 of individuation? The central nervous system in 

 particular limits the capacity for rejuvenescence. 

 " For his high degree of individuation man pays the 

 penalty of individual death, and the conditions and 

 processes in the human organism which lead to 

 death in the end are the conditions and processes 

 which make man what he is." Professor Child 

 has an interesting speculation, in support of which 

 some experimental evidence is adduced, that the 

 very early pre-differentiating stages of embryonic 

 life in some complex organisms, where the indi- 

 vidual certainly fails to evade senescence, may afford 

 opportunities for rejuvenescence at the very start of 

 life for lessening the risk, that is to say, of heredi- 

 tary stereotyping or of being born old. We should 

 think that there were many opportunities for this 

 sort of reorganization at the beginning of every 

 new life that develops from a fertilized egg-cell. 

 But of an elixir vitae for the individual there seems 

 little prospect. " The advance of knowledge and of 

 experimental technique may make it possible at 

 some future time to bring about a greater degree of 

 rejuvenescence and retardation of senescence in 



