Opinions on Isolation. 133 



not occurred through natural selection, is perhaps 

 due to the less severe struggle for existence, owing 

 to the smaller number of competing species 1 ." But 

 even with regard to molluscs alone, there is a greatly 

 larger number of species in Ireland than occurs in 

 any one valley of the Sandwich Islands ; while if we 

 have regard to all the other classes of animal life, 

 comparison entirely fails. 



Much more to the point are certain cases which 

 were adduced long ago by Weismann in his essay 

 previously considered. Nevertheless, although this 

 essay was published as far back as 1872, and, 

 although it expressly deals with the question of 

 divergence of character through the mere prevention of 

 intercrossing (Amixia), Mr. Wallace nowhere alludes 

 to these cases per contra, which are so much more 

 weighty than his own ' : test case" of Ireland. Of 

 such are four species of butterflies, belonging to three 

 genera 2 , which are identical in the polar regions and 

 in the Alps, notwithstanding that the sparse Alpine 

 populations have been presumably separated from 

 their parent stocks since the glacial period ; or of 

 certain species of fresh water crustaceans (Apus\ the 

 representatives of which are compelled habitually to 

 form small isolated colonies in widely separated 

 ponds, and nevertheless exhibit no divergence of 

 character, although apogamy has probably lasted for 

 centuries. These cases are unquestionably of a very 

 cogent nature, and appear of themselves to prove 

 that apogamy alone is not invariably capable of 



1 Let. dt., p. 151. 



Namely, l.ycaena donztlii, L. phtretu, Argyiutis pales, Ercbia 

 man to. 



