i94 RELATIONS OF THE INHABITANTS OP [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 

 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 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, Al- 

 pine 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 surround- 

 ing 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 this same prin- 

 ciple in the character of most of the blind animals in- 

 habiting the eaves 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-al- 

 lied 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 mi- 

 gration 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 an- 



