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^." 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*." 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. GuHck's land-shells, 



' Darwinism, p. 151. ^ Ibid. 



