404 
GEOGKAPHICAL DISTEIBUTION. 
Chap. XIL 
&C .5 all of strictly American forms, and it is obvious 
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 which 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 Felida© and Canidse. 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 
