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 



