Opinions on Isolation. in 



of the principle in question, because, if the latter is 

 good for explaining the initial divergence of type as 

 between separated stocks of " domesticated animals," 

 much more must it be competent to explain the 

 further divergence of type which is " almost always " 

 observable in the case of " a long isolated form " 

 under nature. The very essence of the principle 

 being that, when divergence of type has once begun, 

 this divergence must ipso facto proceed at an ever-ac- 

 celerating pace, it is manifestly inconsistent to entertain 

 the principle as explaining the first commencement of 

 divergence, and then to ignore it as explaining the 

 further progress of divergence. Hence, I can only 

 conclude that Darwin had forgotten this principle 

 altogether when he wrote his letter to Semper in 1878 

 owing, no doubt, as he says in the sentence which 

 immediately follows, to his having "not attended 

 much of late years to such questions." 



So much, then, for Darwin's opinions. Next in 

 order of time we must consider Moritz Wagner's 

 essays on what he called the " Law of Migration J ." 

 The merit of these essays was, first, the firm ex- 

 pression of opinion upon the swamping effects of free 

 intercrossing ; and, second, the production of a large 

 body of facts showing the importance of geographical 

 isolation in the prevention of these effects, and in 

 the consequent differentiation of specific types. On 

 the other hand, the defect of these essays was, first, 

 not distinguishing between evolution as monotypic 

 and polytypic ; and, second, not perceiving that geo- 



1 Die Darwin sche Theorie und das Migrationsgesetz (1868) : Ueber 

 den Einfltiss der geographischen holirung, &c. (1870). 



