The Evolution of Organisms 167 



same species; often severe between species of the 

 same genus," we find only five illustrations, and 

 these are not altogether convincing. 



Isolation. Besides selection we can discern an- 

 other directive factor what we qall Isolation. 

 One of the early competent critics of Darwin's 

 theory of Natural Selection, Professor Fleming 

 Jenkins, emphasized the difficulty that variations 

 of small amount and sparse occurrence would tend 

 to be swamped out by intercrossing. In artificial 

 selection, the breeder takes measures to prevent 

 this by removing unsuitable forms and by delib- 

 erately pairing similar and suitable mates; but 

 what in Nature corresponds to the breeder ? There 

 are several ways of meeting this criticism, but the 

 one that concerns us at present is the theory of 

 isolation, worked out by the late Dr. Romanes, by 

 Mr. Gulick, and others. Attention is directed to 

 the great variety of ways in which, in the course of 

 nature, the range of intercrossing is restricted 

 for instance, by geographical barriers, by differ- 

 ences of habit, by likes and dislikes, which re- 

 sult in assortive mating, by reproductive vari- 

 ations which cause mutual sterility between two 

 sections of a species living on a common area, 

 and so on. According to Romanes, '' without 

 isolation, or the prevention of free intercrossing, 

 organic evolution is in no case possible." It has 

 to be confessed, however, that the body of facts 



