154 SECRETS OF ANIMAL LIFE 
carbonic acid (a common index of the rate of me- 
tabolism), as measured by Tashiro’s delicate “ bi- 
ometer,” is greater than usual; and the piece is less 
susceptible and more resistant to poisons like 
cyanides than when it was part of its parent. If 
these be qualities of youth, then this regrowing 
fragment is again young. Similarly, when a 
Planarian is starved, it can continue living on its 
own resources for several months. Its cells become 
smaller and they also become fewer, but life is not 
surrendered. This, again, is an old story, but the 
new fact is that the starveling becomes curiously 
young—a quaint biological justification of asceti- 
cism—it is almost born again. Such facts have led 
Professor Child to a survey of the animal king- 
dom, the result of which is to show that there is a 
much wider occurrence of rejuvenescence than 
has been hitherto realized. It occurs especially 
in connection with vegetative multiplication, but 
there are other occasions in which de-differentiation 
sets in, and the creature becomes younger in whole 
or in part by lying low for a season. Perhaps this 
may be part of the value of processes of dying-back 
and rearrangement which occur in winter in some 
animals and in many plants. 
Senescence is an all but universal retardation of 
the rate of life, a diminution of vigor and resisting 
power, and there can be little doubt that Professor 
Child is right in regarding it as the necessary conse- 
quence of accumulation, differentiation, and other 
stereotyping changes in the colloid substratum 
