130 Darwin, and after Darwin. 



isolation is not found to be associated with diverg- 

 ence of character. For, he says, " there is an entire 

 absence of change, where, if this were a vera causa, 

 we should expect to find it 1 ." But the only case 

 which he gives is that of Ireland. 



This, he says, furnishes " an excellent test case, for 

 we know that it [Ireland] has been separated from 

 Britain since the end of the glacial epoch: . . . yet 

 hardly one of its mammals, reptiles, or land molluscs 

 has undergone the slightest change 2 ." Here, how- 

 ever, Mr. Wallace shows that he has failed to under- 

 stand "the views of those who, like Mr. Gulick. 

 believe isolation itself to be a cause of modification 

 of species " ; for it belongs to the very essence of these 

 views that the efficiency of indiscriminate isolation as 

 a " vera causa " of organic evolution varies inversely 

 with the number of individuals (i. e. the size of the 

 species-section) exposed to its influence. Therefore, 

 far from being " an excellent test case," the case 

 of Ireland is unsatisfactory. If we are in search of 

 excellent test cases, in the sense intended by Mr. 

 Wallace, we ought not to choose a large island, 

 which from the time of its isolation must have con- 

 tained large bulks of each of the geographically 

 separated species concerned : we ought to choose 

 cases where as small a number as possible of the 

 representatives of each species were in the first 

 instance concerned. And, when we do this, the 

 answer yielded by any really " excellent test case " is 

 unequivocal. 



No better test case of this kind has ever been 

 furnished than that of Mr. Gulick's land -shells, 



1 Darwinism, p. 151. 2 Ibid. 



