194 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 



The same principle which governs the general char- 

 acter of the inhabitants of oceanic islands, namely, the 

 relation to the source whence colonists could have been 

 mostly 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 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 inhabitants of lakes and 

 marshes, excepting in so far as gTcat facility of trans- 

 port has allowed the same forms to prevail throughout 

 large portions of the world. We see tliis same prin- 

 ciple 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 

 mioration 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 



