SELECTION 157 



places in the polity of nature, unless there is some 

 means by which the different varieties become iso- 

 lated from each other, and thus escape from the 

 effects of intercrossing. 



Mr. Darwin tacitly acknowledges the failure of 

 natural selection to produce divergence in the case 

 of insects inhabiting islands. After pointing out the 

 dangers encountered by flying insects inhabiting 

 small islands, he says : " When a new insect first 

 arrived on the island, the tendency of natural selec- 

 tion to enlarge or to reduce the wings would depend 

 on whether a greater number of individuals were 

 saved by successfully battling with the winds, or by 

 giving up the attempt and rarely or never flying. "* L 

 Here he recognises that there would be no splitting- 

 up into two varieties, but that only the commoner 

 one would be preserved, although both were useful. 

 Nevertheless this very case is given by Dr. Wallace 

 as an example of divergence under the action of 

 natural selection. 12 



Some years later Mr. Darwin saw this difficulty 

 more clearly, for in a letter to Moritz Wagner, dated 

 13th October, 1876, he says : " I do not believe that 

 one species will give birth to two or more new 

 species so long as they are mingled together in the- 

 same district."* 3 And on the 26th November, 1878, 

 he thus wrote to Professor K. Semper : " There 

 are two different classes of cases, as it appears to me, 

 viz., those in which a species becomes slowly modi- 



41 " Origin of Species," 1st edition, p. 136. 



* 2 " Darwinism," 2nd edition, p. 105. 



i3 " Life and Letters," vol. iii., p. 159. 



