Isolation. 3 



Now, the forms of isolation — or the conditions 

 which may lead to exclusive breeding — are manifold. 

 One of the most important, as well as the most obvious, 

 is geographical isolation ; and no one questions that 

 this has been an important factor in the process of 

 evolution, although opinions still vary greatly as to 

 the degree of its importance in this respect. At one 

 end of the series we may place the opinion of Mr. 

 Wallace, who denies that any of what may be termed 

 the evolutionary effect of geographical isolation is due 

 to " influence exerted by isolation per se." This effect, 

 he says, is to be ascribed exclusively to the fact that 

 a geographically isolated portion of a species must 

 always encounter a change of environment, and there- 

 fore a new set of conditions necessitating a new set of 

 adaptations at the hands of natural selection^ At 

 the other end of the series we must place the opinion 

 of Moritz Wagner, who many years ago published 

 a masterly essay ^, the object of which was to prove 

 that, in the absence of geographical isolation (including 

 migration), natural selection would be powerless to 

 effect any change of specific type. For, he argued, 

 the initial variations on which the action of this 

 principle depends would otherwise be inevitably 

 swamped by free intercrossing. Wagner adduced 

 a large number of interesting facts in support of this 

 opinion ; but although he thus succeeded in en- 

 forcing the truth that geographical isolation is an 

 important aid to organic evolution, he failed to establish 

 his conclusion that it is an indispensable condition. 



' Darwinism, p. 150. 



• The Darwinian Theory, and the Law of Migration (Eng. Trans., 

 Stanfoid, London, 1873). y 



B 2 



