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 
