242 THE MEANING OF EVOLUTION 



American zoologist, David Starr Jordan, who be- 

 lieves that no two closely related species of animals 

 ever occupied the same geographical area. Both 

 Wagner and Jordan are ardent admirers of Darwin, 

 and his theory of natural selection, but both believe 

 that it is necessary to add the idea of isolation in 

 order to make natural selection effective. 



George John Romanes, a British naturalist, has 

 added to Wagner's idea of isolation, the expanded 

 conception that there may be isolations that are not 

 geographical. For this phase, Romanes has coined 

 the term physiological isolation. Something in the 

 structure or habit of the animals with the new vari- 

 ation prevents them from mating with the older type. 

 Occasionally it is a difference in the structure of the 

 reproductive organs themselves. This, however, is 

 not the only possible divergence. The mating sea- 

 son in one group may come earlier than that of the 

 other, or may come during the day, while the main 

 group is in the habit of mating at night. Anything 

 which keeps some members of a species separate in 

 their mating from the rest, will result in the course 

 of a longer or shorter time, says Romanes, in the 

 formation of a new species. 



A third great objection was raised against Dar- 

 winism. The theory said that only useful variations 

 were selected by nature. It was asserted by objectors 



