216 Darwin, and after Darwin. 



create his species 1 . But now we see that he must 

 be held to have neglected this inscrutable reason 

 (whatever it was) when he passed beyond the range 

 of genera and this always in proportion to the re- 

 moteness of systematic affinity on the part of the 

 species concerned. 



I cannot well conceive a rednctio ad absurdum 

 more complete than this. But, having now presented 

 these most general facts of geographical distribution 

 in their relation to the issue before us, we may next 

 proceed to consider a few illustrations of them in 

 detail, for in this way I think that their overwhelming 

 weight may become yet more abundantly apparent. 



It will assist us in dealing with these detailed illus- 

 trations if we begin by considering the means of 

 dispersal of organisms from one place to another. 

 Of course the most ordinary means is that of con- 

 tinuous wandering, or emigration ; but where geo- 

 graphical barriers of any kind have to be surmounted, 

 organisms may only be able to pass them by more 

 exceptional and accidental means. The principal 

 barriers of a geographical kind are oceans, rivers, 

 mountain-chains, and desert-tracts, in the case of 



1 I say " large areas " for the sake of argument ; but the same cor- 

 relation between distribution and affinity extends likewise to small 

 areas where only small differences of affinity are concerned. Thus, 

 for instance, speaking of smaller areas, Moritz Wagner says : " The 

 broader and more rapid the river, the higher and more regular the 

 mountain-chain, the calmer and more extensive the sea, the more 

 considerable, as a general rule, will be the taxonomic separation be- 

 tween the populations " ; and he shows that, in correlation with such 

 differences in the degrees of separation, are the degrees of diversification 

 i.e., the numbers of species, and even of varieties, which these topo- 

 graphical barriers determine. 



