338 CHILD— AGE CYCLES AND 



period of senescence except that it may take place in a few hours. 

 The loaded gland cell, like the egg, usually requires stimulation be- 

 fore it is able to initiate the process of discharge, but, once begun, 

 this process undergoes acceleration, the metabolic rate rises, the cell 

 loses its peculiar structure and becomes less specialized in appear- 

 ance. This is comparable to a period of rejuvenescence, and in the 

 presence of nutritive material this period is again followed by a 

 period of loading and decreasing metabolic rate. 



In the green parts of a plant in the presence of light and carbon 

 dioxide loading of the cells with starch and a progressive decrease 

 in activity occurs, while in darkness the starch may be transformed 

 to sugar and carried to other parts, and the cells regain their former 

 condition. 



Again in seasonal and other periodicities similar alternations 

 appear, a period of accumulation and decreasing dynamic activity 

 is followed by a period of quiescence in which recovery gradually 

 occurs or after which activity may rapidly increase when external 

 conditions such as temperature permit. Periods of encystment fol- 

 lowing growth and accumulatory periods in the lower organisms, 

 and periods of hibernation or estivation following periods of active 

 nutritive intake are to a greater or less degree essentially periods of 

 rejuvenescence. 



In many of these cases the beginning of the period correspond- 

 ing to rejuvenescence is determined by environmental rather than 

 internal factors but this is incidental. The progressive period in- 

 stead of proceeding to its natural termination is interrupted, and a 

 regressive period initiated by external conditions. The organism 

 in such cases merely responds to the rhythms in its environment, 

 but such responses are minor periodicities which show certain re- 

 semblances to the periodicity of the age cycle. The course of life 

 is then a complex periodic curve made up of many periodicities of 

 various lengths, the normal age cycle being the longest for the in- 

 dividual. 



But in conclusion, we may at least raise the question whether 

 there are not periodicities of this sort extending beyond the life of 

 the individual. Is there not evidence in favor of the view that evo- 



