Indiscriminate Isolation 



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 

 parent 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 the initial differ- 

 ences 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.' ' 



The words of Mr Gulick require close scrutiny. 

 We may admit that " no two portions of a species 

 possess exactly the same average character," but 

 why should the two, if prevented from inter- 

 breeding yet subjected to similar climatic and 

 other conditions, present the phenomenon of 

 " increasing divergence ? " The reason assigned 

 by Romanes is the " Law " of Delbceuf, which 

 runs: U A constant cause of variation, however 

 insignificant it may be, changes the uniformity of 

 type little by little, and diversifies it ad in- 



373 



