Isolation. 15 



with Mr. Wallace wherever this is logically possible — 

 even Professor Ray Lankester observes : — 



Mr, Wallace does not, in my judgement, give sufficient 

 grounds for rejecting the proposition which he indicates as 

 the main point of Mr. Gulick's valuable essay on Divergent 

 Evolution through Cumulative Segregation. Mr. Gulick's 

 idea is that .... no two portions of a species possess exactly the 

 same average character, and the initial differences will, if the 

 individuals of the two groups are kept from intercrossing, 

 assert themselves continuously by heredity in such a way as 

 to ensure an increasing divergence of the forms belonging to 

 the two groups, amounting to what is recognized as specific 

 distinction. Mr. Gulick's idea is simply the recognition of 

 a permanence or persistency in heredity, which, caeteris 

 paribus, gives a twist or direction to the variations of the 

 descendants of one individual as compared with the descendants 

 of another^. 



Now we have seen that ' Mr. Gulick's idea," 

 although independently conceived by him, had been 

 several times propounded before ; and it is partly 

 implicated in more than one passage of the Origin 

 of Species., where free intercrossing, or the absence of 

 isolation, is alluded to as maintaining the constancy 

 of a specific type '. Moreover, it is still more fully 

 recognized in the last edition of the Variation of 

 Animals and Plants, where a paragraph is added for 

 the purpose of sanctioning the principle in the 

 imperfect form that it was stated by Weismann ^ 

 Nevertheless, to Mr. Gulick belongs the credit, not 

 only of having been the first to conceive (though the 

 last to publish) the " idea " in question, and of having 

 stated it with greater fullness than anybody else ; but 



' Nature, Oct. lo, 1889, p. 568. » e. g. p. 81. 



* See Chapter xxiii. vol. ii. p. 262. CEdition of 18SS.) 



