404 GEOGRAPHICAL DISTRIBUTION. Chaf. XII. 



that a mountain, as it became slowly upheaved, would 

 naturally be colonised from the surrounding lowlands. 

 So it is with the inhabitants of lakes and marshes, 

 excepting in so far as great facility of transport has 

 given the same general forms to the whole world. We 

 see this same principle in the blind animals inhabiting 

 the caves of America and of Europe. Other analogous 

 facts could be given. And it will, I believe, be uni- 

 versally found to be true, that wherever in two regions, 

 let them be ever so distant, many closely allied or re- 

 presentative species occur, there will likewise be found 

 some identical species, showing, in accordance with the 

 foregoing view, that at some former period there has 

 been intercommunication or migration between the two 

 regions. And wherever many closely-allied species 

 occur, there will be found many forms winch some 

 naturalists rank as distinct species, and some as varie- 

 ties ; these doubtful forms showing us the steps in the 

 process of modification. 



This relation between the power and extent of migra- 

 tion of a species, either at the present time or at some 

 former period under different physical conditions, and 

 the existence at remote points of the world of other 

 species allied to it, is shown in another and more 

 general way. Mr. Gould remarked to me long ago, that 

 in those genera of birds which range over the world, 

 many of the species have very wide ranges. I can 

 hardly doubt that this rule is generally true, though it 

 would be difficult to prove it. Amongst mammals, we 

 see it strikingly displayed in Bats, and in a lesser degree 

 in the Felidae and Canidae. We see it, if we compare 

 the distribution of butterflies and beetles. So it is with 

 most fresh- water productions, in which so many genera 

 range over the world, and many individual species have 

 enormous ranges. It is not meant that in world- 



