ti2 Darwin, and after Darwin. 



graphical isolation is only one among a number of 

 other forms of isolation. From these two radical 

 oversights — which, however, were shared by all other 

 writers of the time, with the partial exception of 

 Darwin himself, as previously shown — there arose the 

 following and most lamentable errors. 



Over and over again Moritz Wagner insists, as con- 

 stituting the fundamental doctrine of his attempted 

 reform of Darwinism, that evolution by natural 

 selection is impossible, unless natural selection be 

 assisted by geographical isolation, in order to prevent 

 the swamping effects of intercrossing ^. Now, if instead 

 of " evolution " he had said " divergence of type," 

 and if instead of " geographical isolation " he had 

 said " prevention of intercrossing," he would have 

 enunciated the general doctrine which it has been the 

 joint endeavour of Mr. Gulick and myself to set forth. 

 But by not perceiving that " evolution " is of two 

 radically different kinds — polytypic and monotypic — 

 he entirely failed to perceive that, while for one of its 

 kinds the prevention of intercrossing is an absolute 

 necessity, for the other of its kinds the permission of 

 intercrossing is a necessity no less absolute. And, 

 again, in missing the fact that geographical isolation 



' For instance, speaking of common, or continuous areas, he says : — 

 "In this case a constant variety, or new species, cannot be produced, 

 because the free crossing of a new variety with the old unaltered stock 

 will always cause it to revert to the original type ; in other words, will 

 destroy the new form. The formation of a real variety, which Darwin, 

 as we know, regards as the commencement of a new species, will only 

 succeed when a few individuals, having crossed the barrier of their 

 habitat, are able to separate themselves for a long time from the old 

 stock." And the last sentence, given as a summary of his whole 

 doctrine, is — "The geographical isolation of the form, a necessary 

 consequence of migration, is the cause of its typical character." 



