142 THE PROBLEMS OF EVOLUTION 



and map a sound course for future procedure, but 

 the results establish no method of evolution to the 

 exclusion of all others. We can scarcely doubt that 

 the way of nature embraces many factors which 

 are embodied in the various theories, hence we 

 may add our remaining articles of faith: Given 

 variation, it is probable that selective and isolating 

 factors contribute to the breaking up of species 

 and the formation of geographic races which are 

 potential species. Hybridization may play a part 

 in this reassortment of existing characters also. 

 But when we attempt to account for the origin of 

 variations we can only look to mutations or indi- 

 vidual response to environment; here the dividing 

 line is by no means clear. We need more knowledge, 

 and we must turn to experiment for it. 



