The Making of Species 



discriminate isolation as a factor in evolution. 

 On this there can be no room for disagreement 

 among biologists. It is when we come to the 

 subject of indiscriminate isolation that we enter 

 a region of zoological strife. 



Is indiscriminate isolation per se a factor of 

 evolution ? Romanes, Gulick, and Wagner 

 assert that it is, Wallace and his adherents 

 assert that it is not. 



As the burden of proof is on the former, they 

 are entitled to the first hearing. 



" We may well be disposed, at first sight," 

 writes Romanes (Darwin and after Darwin, 

 p. 10), " to conclude that this kind of isolation 

 can count for nothing in the process of evolution. 

 For if the fundamental importance of isolation in 

 the production of organic forms be due to its 

 segregation of like with like, does it not follow 

 that any form of isolation which is indiscriminate 

 must fail to supply the very condition on which 

 all the forms of discriminate isolation depend for 

 their efficacy in the causing of organic evolution? 

 Or, to return to one's concrete example, is it not 

 self-evident that the farmer who separated his 

 flock into two or more parts indiscriminately, 

 would not effect any more change in his stock 

 than if he had left them all to breed together ? 

 Well, although at first sight this seems self- 

 evident, it is, in fact, untrue. For, unless the 

 individuals which are indiscriminately isolated 



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