No. 493] 



1SOLAT10X AX I) SELECT I OX 



selection can not take place" (p. 59). These and many 

 other passages of similar import indicate how fully ho 

 recognized the importance of isolation, and how at the 

 same time his theory was a denial of some of the facts in 

 the origin of species. The following are some of the facts 

 with which his general theory as well as his special state- 

 ments were in conflict : 



1. That, through change of climate and of other condi- 

 tions due to geological changes, the whole fauna and flora 

 of an island like Iceland might be subjected to new forms 

 of selection producing a complete change of many species 

 into new species, without any chance for migration. 

 After the publication of my article in Nature, July 18, 

 1872, in which I emphasized the importance of isolation, I 

 met Darwin at his home, and he called my attention to 

 Wagner's theory, and suggested that it did not correspond 

 with the facts of nature, especially on this point. 



2. That the influence of geographical isolation in pro- 

 ducing divergence, and in opening the way for selection 

 to cooperate in producing divergence, is wholly due to its 

 prevention of free-crossing, and that the same result may, 

 in various ways, be brought about by divergent habits or 

 instincts, or other incompatibilities, while the isolated 

 groups remain in the original habitat of the species. 



3. That divergence does not necessarily require the in- 

 fluence of different forms of selection. 



4. And that different forms of selection do not neces- 

 sarily depend on exposure to unlike environments; for 

 diversity in the forms of selection may be due to diversity 

 in the methods of using the same environment adopted by 

 isolated branches of the same species. 



In August, 1872, I read before the British Association 



