Isolation. ii 



the forms of discriminate isolation depend for their 

 efficacy in the causing of organic evolution ? Or, to 

 return to our concrete example, is it not self-evident 

 that the farmer who separated his stock 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 happen to be a very 

 large number, sooner or later their progeny will come 

 to differ from that of the parent type, or unisolated 

 portion of the previous stock. And, of course, as 

 soon as this change of type begins, the isolation | 

 ceases to be indiscriminate : the previous apogamy ^ 

 has been converted into homogamy, with the usual 

 result of causing a divergence of type. The reason 

 why progeny of an indiscriminately isolated section 

 of an originally uniform stock — e.g. of a species — will 

 eventually deviate from the original type is, to quote 

 Mr. Gulick, as follows : — " No two portions of a species 

 possess exactly the same average character, and, 

 therefore, the initial differences are for ever reacting 

 on the environment and on each other in such a way 

 as to ensure increasing divergence as long as the 

 individuals of the two groups are kept from inter- 

 generating i." Or, as I stated this principle in my 

 essay on Physiological Selection, published but a short 

 time before Mr. Gulick's invaluable contributions to 

 these topics : — 

 As a matter of fact, we find that no one individual " is like 



^ Divergent Evolution through Cumulative Segregation {Zool. Journal, 

 Linn. Soc, vol. xx. pp. 1 89-2 74). 



