Isolation. 17 



essentially the same — no one who examines this 

 collection can wonder that Mr. Gulick attributes the 

 results which he has observed to the influence of 

 apogamy alone, without any reference to utility or 

 natural selection. 



To this solid array of remarkable facts Mr. Wallace 

 has nothing further to oppose than his customary 

 appeal to the argument from ignorance, grounded on 

 the usual assumption that no principle other than 

 natural selection can be responsible for even the 

 minutest changes of form or colour. For my own 

 part, I must confess that I have never been so deeply 

 impressed by the dominating influence of the a priori 

 method as I was on reading Mr. Wallace's criticism 

 of Mr. Gulick's paper, after having seen the material 

 on which this paper is founded. To argue that every 

 one of some twenty contiguous valleys in the area of 

 the same small island must necessarily present such 

 differences of environment that all the shells in each 

 are difTerently modified thereby, while in no one out 

 of the hundreds of cases of modification in minute 

 respects of form and colour can any human being 

 suggest an adaptive reason therefor — to argue thus 

 is merely to affirm an intrinsically improbable dogma 

 in the presence of a great and consistent array of 

 opposing facts. 



I have laid special stress on this particular case of 

 the Sandwich Islands' moUusca, because the fifteen 

 years of labour which Mr. Gulick has devoted to their 

 exhaustive working out have yielded results more 

 complete and suggestive than any which so far have 

 been forthcoming with regard to the effects of isolation 

 in divergent evolution. But, if space permitted, it 



III. .=i C 



