Mollusca of Sandwich Isles 



to the amount of change which can be effected 

 by the accumulation of fluctuating variations ; 

 but, as we have already seen (on p. 70), there 

 is a very definite limit and this limit is quickly 

 reached. 



Thus the arguments of Romanes and Gulick 

 are fundamentally unsound. 



But the fact remains, and has to be accounted 

 for, that, as a general rule, when two portions of 

 a species are separated, so that they are pre- 

 vented from interbreeding, they begin to diverge 

 in character, and the longer they remain thus 

 separated the greater becomes that divergence. 

 This is an observed fact which cannot be 

 gainsaid. 



It was the observance of this fact which led 

 Gulick to insist with such emphasis on the im- 

 portance of geographical isolation as a factor in 

 evolution. He discovered that the land mollusca 

 of the Sandwich Islands fall into a great number 

 of varieties. 



These islands are very hilly, and Gulick found 

 that each of the varieties is confined not merely 

 to one island, but to one valley. " More- 

 over," writes Romanes, on p. 16 of Darwin 

 and after Darwin, "on tracing this fauna from 

 valley to valley, it is apparent that a slight 

 variation in the occupants of valley 2, as com- 

 pared with those of the adjacent valley i, 

 becomes more pronounced in the next, valley 3, 



375 



