154 THE ORIGIN OF THE NERVOUS SYSTEM 



of the changes in other parts of the organisms. We see 

 clearly enough that the organism has evolved as a 

 whole, not as a mosaic of independent parts, but the 

 changes in the nervous system are among the most 

 fundamental and significant changes which have 

 occurred. As long as we regard the nervous system 

 as representing physiologically a new factor superadded 

 at some period in evolution to the primitive organismic 

 system, the phylogeny as well as the ontogeny of the 

 nervous system and its relation to other parts remains a 

 complete mystery, and we can merely record and not 

 interpret the facts. But phylogeny as well as ontogeny 

 must sooner or later be interpreted into terms of develop- 

 mental physiology and potentialities of the hereditary 

 mechanism of the various protoplasms. The conception 

 which I have attempted to develop in this book provides 

 at least a working hypothesis for further attack upon 

 the problems of developmental physiology, not only as 

 they concern the individual but also in their phylo- 

 genetic aspects. It must not be forgotten, however, 

 that this hypothesis is purely physiological and is based 

 primarily on physiological evidence. It concerns the 

 actual physiological conditions under which potentiali- 

 ties of the hereditary mechanism or action system are 

 realized. The problem of the hereditary mechanism 

 remains, although I believe that increasing knowledge 

 concerning the physiological conditions under which 

 hereditary potentialities are realized must teach us 

 something concerning the physiological basis of these 

 potentialities. 



