444 ORIGIN OF SPECIES 



marvel at the endemic species which inhabit the several 

 islands of the Galapagos Archipelago, not having all spread 

 from island to island. On the same continent, also, preoccu- 

 pation has probably played an important part in checking 

 the commingling of the species which inhabit different dis- 

 tricts with nearly the 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 anim^als inhabiting the great, 

 open, and continuous valley of the Amazons. 



The same principle which governs the general character 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 moun- 

 tain-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, Al- 

 pine 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 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, 

 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 migration in 

 certain species, either at the present or at some former pe- 



