396 RELATIONS OF THE INHABITANTS OF 



the source whence colonists could have been most easily 

 derived, together with their subsequent modification, is of 

 the widest application throughout nature. We see this on 

 every mountain-summit, in every lake and marsh. For 

 aicine species, excepting in as far as the same species have 

 become widely spread during the Glacial epoch, are related 

 to those of the surrounding lowlands ; thus we have in South 

 America, alpine humming-birds, alpine rodents, alpine plants, 

 3tc, all strictly belonging to American forms ; and it is 

 obvious that a mountain, as it became slowly upheaved, would 

 be colonized from the surrounding lowlands. So it is with 

 the inhabitants of lakes and marshes, excepting in so far as 

 great facility of transport has allowed the same forms to 

 prevail throughout large portions of the world. We see the 

 same principle in the character of most of the blind animals 

 inhabiting the caves of America and of Europe. Other 

 analogous facts could be given. It will, I believe, be found 

 universally true, that wherever in two regions, let them be 

 ever so distant, many closely allied or representative species 

 occur, there will likewise be found some identical species ; 

 and wherever many closely allied species occur, there will 

 be found many forms which some naturalists rank as distinct 

 species, and others as mere varieties ; these doubtful forms 

 showing us the steps in the progress of modification. 



The relation between the power and extent of migration 

 in certain species, either at the present or at some former 

 period, and the existence at remote points of the world of 

 closely allied species, 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 difficult of proof. Among 

 mammals, we see it strikingly displayed in bats, and in a 

 lesser degree in the Fehdae and Canidae. We see the same 

 rule in the distribution of butterflies and beetles. So it is 

 with most of the inhabitants of fresh water, for many of the 

 genera in the most distinct classes range over the world, and 

 many of the species have enormous ranges. It is not meant 

 that all, but that some of the species, have very wide ranges 

 in the genera which range very widely. Nor is it meant 

 that the species in such genera have, on an average, a very 

 wide range ; for this will largely depend on how far the pro- 

 cess of modification has gone ; for instance, two varieties of 

 fche same species inhabit America and Europe, and thus the 



