No. 579] PROGRESSIVE EVOLUTION 



17.1 



a composite body deriving its blastogenie characters 

 from different sources; but this cannot affect its funda- 

 mental structure, for the two parents must have been 

 alike in all essential respects or they could not have in- 

 terbred, and any important differences in the germ- 

 plasm must be confined to the "factors" for the differen- 

 tiating characters. The fundamental structure still de- 

 velops epigenetically on the basis of an essentially simi- 

 lar germ-plasm and under essentially similar conditions 

 as in the ease of eaeli of the two parents, and there is no 

 reason to suppose that special "factors" have anything 

 to do with it. 



We thus see how new unit characters may be added 

 by mutation and interchanged by hybridization while the 

 fundamental constitution of the organism remains the 

 same and the epigenetic course of development is not 

 seriously affected. All characters that arise in this way 

 must be regarded, from the point of view of the or- 

 ganism, as chance characters due to chance modifications 

 of the germ-plasm, and they appear to have compara- 

 tively little influence upon the course of evolution. 



One of the most remarkable features of organic evo- 

 lution is that it results in the adaptation of the organism 

 to its environment, and for this adaptation mutation and 

 hybridization utterly fail to account. Of course the ar- 

 gument of natural selection is called in to get over this 

 difficulty. Those organisms which happen to exhibit 

 favorable mutations will survive and hand on their ad- 

 vantages to the next generation, and so on. It has fre- 

 quently been pointed out that this is not sufficient. Mu- 

 tations occur in all directions, and the chances of a favor- 

 able one arising are extremely remote. Something more 

 is wanted, and this something, it appears to me, is to be 

 found in the direct response of the organism to environ- 

 mental stimuli at all stages of development, whereby in- 

 dividual adaptation is secured, and this individual adap- 

 tation must arise again and again in each succeeding 



