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 vite 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 
