OTHER PERIODICITIES IN ORGANISMS. 333 



the same physiological age during nearly four years, while other 

 members of the same original stock, fed in the usual way, passed 

 through some twenty generations. 



These experiments afford some insight into the nature of the 

 process of senescence and rejuvenescence. We see that when the 

 animal is adding to its protoplasmic substratum by growth and 

 transforming it by processes of differentiation it is growing old, 

 but when it is using up previously formed protoplasmic material, 

 as it does in reconstitution and starvation, it is growing young. In 

 the simple animals where the structural substratum of the body 

 built up under one sort of conditions is readily broken down under 

 other conditions in the absence of nutrition, or in reconstitution, 

 rejuvenescence occurs readily and may carry the animal back almost 

 to embryonic stages, but the evolution of the higher forms is very 

 evidently associated with a physiological stabilization of the struc- 

 tural substratum of the body and we find that in man and the 

 mammals bodily rejuvenescence is apparently limited. A large 

 part of the structural substratum is either not available for nutri- 

 tive' purposes or cannot be broken down rapidly enough to supply 

 the demands, and death occurs before any great degree of rejuve- 

 nescence has taken place in the body as a whole. But that some 

 slight degree of bodily rejuvenescence may occur even in man can- 

 not be doubted. 



It is impossible to consider here all the various lines of evidence, 

 but it may be said that the facts at present available indicate that 

 senescence consists in a decrease in metabolic rate determined by 

 the changes in, and the progressive accumulation of, the relatively 

 stable components of the protoplasmic substratum during growth, 

 development and differentiation. Rejuvenescence, on the other 

 hand, is an increase in metabolic rate determined by the; break- 

 down of previously accumulated structural substances in dediffer- 

 entiation, starvation and reproduction and the development of new 

 protoplasmic substance in place of that eliminated. We may say, 

 in short, that the organism grows old when the primitive embryonic 

 protoplasmic substratum is modified or added to by changes in the 

 colloids and accumulation of relatively stable components by growth 



