422 RELATIONS OF THE INEABITANTS OF 



same physical conditions. Thus, the south-east and south- 

 west corners of Australia have nearly the same physical 

 conditions, and are united by continuous land, yet they are 

 inhabited by a vast number of distinct mammals, birds 

 and plants; so it is, according to Mr. Bates, with the 

 butterflies and other animals inhabiting the great, open, 

 and continuous valley of the Amazons. 



The same principle which governs the general clnaracter 

 of the inhabitants of oceanic islands, namely, the relation 

 to the source whence colonists could have been most 

 easily derived, together with their subsequent modifica- 

 tion, 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 through- 

 out large portions of the world. We see the same princi- 

 ple 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 uni- 

 versally 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 migra- 

 tion 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. 



