GEOGRAPHICAL DISTRIBUTION 208 



otlier animals inhabiting the great, open, and continuous 

 valley of the Amazons. 



The same principle which governs the general char- 

 acter of the inhabitants of oceanic islands, namely, the 

 relation to the source M^hence 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, 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 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, manj^ 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 migra- 

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



