OTHER PERIODICITIES IN ORGANISMS. 337 



source of energy. In this way the isolated part undergoes a process 

 of dedifferentiation and in so doing becomes physiologically younger 

 and approaches or attains the generalized or undifferentiated con- 

 dition of that particular protoplasm. If sufficient energy is avail- 

 able in the form of nutrition, either from its own substance or from 

 its environment, this period of dedifferentiation is followed by a 

 new period of differentiation and senescence. In other words, from 

 being a differentiated, specialized part it becomes an undift'erentiated 

 or generalized whole by the loss of its speciaHzed features and then 

 undergoes a new course of speciaHzation as its constitution de- 

 termines. 



In the evolutionary adjustment between the period of rejuvenes- 

 cence and environmental conditions natural selection has unques- 

 tionably played an important part, for it is evident that no species 

 can persist from generation to generation in which a source of nu- 

 trition is not available in time to save the parts concerned in repro- 

 duction from death by starvation. If this were not the case the 

 period of dedifferentiation and rejuvenescence would end in death, 

 as in the case of the starving planarian. Rejuvenescence may follow 

 senescence automatically, but for a new period of senescence fol- 

 lowing the rejuvenescence, nutrition from without is sooner or later 

 necessary. 



We must now inquire whether this age cycle is unique in organic 

 life or whether there are other periodicities which in any way re- 

 semble it. I believe there are many other periodicities of essentially 

 similar character. Fatigue, for example, as distinguished from ex- 

 haustion, results from the accumulation of substances which retard 

 metabolism and recovery, from the removal of these substances. 

 The chief difference between this periodicity and the age cycle is 

 that the substances which produce fatigue are products of catabolism, 

 essentially waste products and that they are soluble and readily re- 

 moved, while in age they are essential constituents of the differen- 

 tiating protoplasmic substratum. 



Again, the period of so-called loading of the gland cell is a 

 period of decreasing metabolism and accumulation of substance 

 with marked change of structures in the cell and resembles the 



