OTHER PERIODICITIES IN ORGANISMS. 339 



lution is a secular senescence of protoplasm, interrupted here and 

 there by periods of rejuvenescence? Such periods of evolutionary 

 rejuvenescence if they have occurred, have probably, like some of 

 the minor periods in the life of an individual, been determined by 

 environmental factors, for the natural, internally determined period 

 of secular protoplasmic senescence has not yet reached its termina- 

 tion. In any case the general similarity between evolutionary and 

 individual differentiation suggests that similar factors are at work 

 in both cases, and since the period of individual differentiation is in 

 general a period of senescence, the suggestion that something of 

 similar character is occurring in evolution is not wholly unwar- 

 ranted. 



University of Chicago, 

 April, 1916. 



