464 SENESCENCE AND REJUVENESCENCE 



all cases exactly the same condition is attained in each successive 

 regression and rejuvenescence. It is quite conceivable, indeed 

 probable, that, in spite of the successive regressive changes in each 

 generation, there may be some slight, more or less continuous, pro- 

 gressive change, which perhaps becomes appreciable only after 

 many generations. Have we, in fact, any right to assume that 

 the organism returns to exactly the same condition in each succes- 

 sive regression ? Is it not probable that a gradual, progressive 

 senescence of protoplasm has occurred in the course of evolution ? 

 These questions have already been touched upon in chap, viii, and 

 here it need only be said that the facts point very definitely in the 

 direction of an affirmative answer. 



If protoplasmic senescence is the essential factor in progressive 

 evolution, then evolution is, like individual development, to a 

 large extent internally, rather than externally, determined. We 

 can accelerate, retard, or alter the course of individual development 

 experimentally, but in spite of all such changes it retains a remark- 

 able constancy of character. Have we not in evolution a somewhat 

 similar process, a progressive change, a secular differentiation and 

 senescence of protoplasm along lines which are determined primarily 

 by the constitution of protoplasm rather than by external factors ? 

 In our attempts to modify experimentally the course of evolution 

 are we not merely bringing about minor changes in a process 

 which, like individual development, is internally determined, rather 

 than determining the essential factors in evolution ? Here again 

 the facts seem to suggest an affirmative answer. 



If evolution is in some degree a secular differentiation and 

 senescence of protoplasm, the possibility of evolutionary rejuvenes- 

 cence must not be overlooked. Perhaps the relatively rapid rise and 

 increase of certain forms here and there in the course of evolution 

 may be the expression of changes of this sort. Perhaps also those 

 forms which have been, so to speak, left behind as the lower organ- 

 isms in evolutionary progress are forms in which senescence 

 and rejuvenescence more nearly balance than in those that have 

 gone on. 



Even if evolution is a process of this kind we must believe that 

 environmental factors affect its course to a greater or less extent, 



