CHILD'S SENESCENCE AND REJUVENESCENCE 423 



except for the fact that in gametic reproduction specialization 

 of the reproductive cells has proceeded so far that the peculiar 

 conditions associated with fertilization are necessary for the 

 initiation of the process of regression and rejuvenescence. And 

 if we accept this theory of reproduction, the Weismannian con- 

 ception of germ plasm as a self -perpetuating entity, independent 

 of other parts of the organism except as regards nutrition — in 

 short, a sort of parasite upon the body — becomes not only un- 

 necessary but impossible. Germ plasm is any protoplasm 

 capable, under the proper conditions, of undergoing regression, 

 rejuvenescence and reconstitution into a new individual, organ- 

 ism, or part " (Senecsence, p. 461). 



5. It is suggested (Senescence, p. 463, Individuality, p. 205) 

 that throughout the history of organic evolution there has been 

 a progressive change in the organization of protoplasm of animal 

 phyla analogous with that which occurs in the individual life 

 cycle, from a relatively undifferentiated and mobile condition 

 in lowest organisms (and in young parts of more highly special- 

 ized types) to a condition characterized by the fixation in proto- 

 plasm of stable structural elements which give to each species 

 and organ its characteristic mode of behavior. This is the 

 physical basis of differentiation. On the anatomical side this 

 leads to the highly complex and diversified tissue types. On 

 the behavior side it leads to progressive complexity in the in- 

 herited types of activity, which culminates in the instinctive 

 reactions of higher animals. (Of course, in the higher organisms 

 there is in addition to this stable framework of protoplasm and 

 of behavior a more labile stuff which may be metabolically very 

 active — see 9 below.) The more diversified life of higher organ- 

 isms, therefore, rests upon a more diversified structural organiza- 

 tion. And the advance in this organization is progressive in 

 evolution. This is a non-metaphysical type of orthogenesis, 

 which may be expressed by the proposition that in evolution 

 the protoplasm of the race is progressively growing older. Par- 

 allel with this the behavior is becoming stabilized in more com- 

 plex patterns Of course, not all of the protoplasm thus grows 

 old, and the labile protoplasm of the " younger " type is the 

 basis of that plasticity which makes advancement in evolution 

 possible. 



6. From the observed differences in metabolic rate in different 



