No. 596] EVOLUTIONARY THEORY 



403 



is based on experience, that traits shall be found the 

 germinal bases of which are undergoing current evolu- 

 tionary changes through loss of genes or through frac- 

 tionation. And it is in accord with expectation that 

 mutations shall reveal themselves just at the extreme of 

 a series as Castle's plus mutation revealed itself. Others 

 are inclined to think that the varied color pattern of 

 Castle's rats is really determined by several factors (re- 

 strictors or extensors of the gene for "hoodedness") 

 which were all in his race of rats at the beginning of bis 

 experiment. Upon one point all geneticists are, how- 

 ever, agreed— that we must interpret all of our results in 

 terms of genes alone. 



One other bearing of the orthogenetic theory deserves 

 to be pointed out. If the germ plasm is capable of 

 undergoing a spontaneous mutation which is the main 

 source of evolutionary change, this fact would seem, at 

 first blush, to indicate the futility of trying to control 

 genetic change experimentally, except by the selection of 

 germ plasms. Naturally, under these circumstances our 

 effort would be limited to what nature affords. How- 

 ever, we do not yet know enough to put these limits on 

 1 ' experimental evolution." There is some evidence, 

 although not as critical as might be wished, that the 

 germ plasm is not beyond the reach of modifying agents. 

 At least we must continue experimental efforts in that 

 direction. 



IV. Summary 



A theory of evolution that assumes internal changes 

 chiefly independent of external conditions, %. e., spon- 

 taneously arising, and which proceeds chiefly by a split- 

 ting up of and loss of genes from a primitively complex 

 molecular condition of the germ plasm seems best to 

 meet the present state of our knowledge. 



Such a theory receives support from various fields. 



1. From ontogenv, where the differentiated end stage 

 is derived from a relatively undifferentiated, but prob- 

 ably molecularly complex egg. 



