366 THE ORIGIN OF SPECIES 



throughout 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, etc., 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 through- 

 out 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 repre- 

 sentative 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 show- 

 ing 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 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 

 the capacity of crossing barriers and ranging widely, as in the 



