194 SENESCENCE AND REJUVENESCENCE 



That a process similar to senescence has occurred in the evolu- 

 tion of the higher organisms from the lower is suggested by various 

 lines of evidence. The protoplasmic substratum of the higher forms 

 is certainly more stable and undergoes structural alteration less 

 readily and less extensively than in the lower. The higher forms 

 undergo a greater degree of differentiation during development 

 than the lower, and in the higher animals the capacity for agamic 

 and experimental reproduction is absent and growth is limited. 

 Moreover, the metabolic activity for each unit of weight is prob- 

 ably less under similar conditions of temperature, oxygen supply, 

 nutrition, etc., in the higher than in the lower forms, even in early 

 stages of development. In short, there are various resemblances 

 between the course of evolution and that of individual development, 

 and the latter is a period of senescence. And as in the individual 

 altered conditions may bring about rejuvenescence, so in the course 

 of evolution the occurrence of rejuvenescence is conceivable. If a 

 secular senescence of protoplasm has constituted a factor in evolu- 

 tion, the protoplasm of the higher forms must have undergone this 

 change more rapidly than that of those which remained as lower 

 forms. Moreover, such a senescence might proceed more or less 

 independently of the environment, though the course and rate of 

 the change would doubtless be influenced by environmental con- 

 ditions. In other words, protoplasmic senescence, if it plays any 

 part in evolution, is to some extent an internal factor, and evolution 

 itself is in some degree a progressive change from less to more 

 stable equilibrium, rather than in the opposite direction. 



The purpose of the present section is to suggest possibilities, 

 rather than to develop theories. Since there is continuity of pro- 

 toplasmic substance from generation to generation, there nlay be 

 internally determined progressive change in that substance similar 

 in some degree to the change during individual Hfe (see pp. 464-65). 



REFERENCES 

 Barcroft, J. 



1908. "Zur Lehre vom Blutgaswechsel in den verschiedenen Organen," 

 Ergebn. d. Physiol., VII. 

 Cholodkowsky, N. 



1882. "Tod und Unsterblichkeit in der Tierwelt," Zool. Anzeiger, V. 



