336 BELATIONS OF THE INHABITANTS OF [CHAP. XIII 



The same principle which governs the general character of the 

 inhabitants of oceanic islands, namely, the relation to the source 

 whence colonists could have been most easily derived, together with 

 their subsequent modification, is of the widest application through- 

 out nature. We see this on every mountain-summit, in every lake 

 and marsh. For Alpine 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, &c., all strictly belonging to American forms ; and it is 

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

 colonised from the surrounding lowlands. So it is with the inhabi- 

 tants 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 this 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. Amongst mammals, we see it strikingly displayed in 

 Bats, and in a lesser degree in the Felidae 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 process of modification has gone ', 

 for instance, two varieties of the same species inhabit America 

 and Europe, and thus the species has an immense range ; but, if 

 variation were to be carried a little further, the two varieties 

 would be ranked as distinct species, and their range would be 

 greatly reduced. Still less is it meant, that species which have 



