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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