3G6 KELATIONS OF THE INIIABITAXTS OF Ciia.-. XII. 



doubt liad some advanta<Te over tlie indig-cnous species. From 

 these considerations I think wc need not g^reatl}- marvel at the 

 endemic and representative species, Avliicli inhabit the several 

 islands of tlic Galapag-os Archipelago, not Laving universally 

 spread from island to island, Preoccu])ation has also probably 

 played an important part in checking the connningling of the 

 species Avhich inhabit different districts with nearly the same 

 physical conditions on the same continent. Thus, the south- 

 cast and southwest 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. 



The same principle which governs the general character of 

 the inhal)itants of oceanic islands, namely, their relation to tho 

 source whence colonists could have been most easily derived, 

 together with their subsequent modification, is of the widest 

 apjilication throughout Nature. We see this on every moun- 

 tain-summit, in every lake and marsh. For Alpine species, 

 cxcci)ting in as far as the same species have become widely 

 spread during the recent Glacial epoch, are related to those of 

 the surrounding lowlands : thus we have, in South America, 

 Aljiine humming-birds, Alpine rodents, Alpine plants, etc., all 

 strictly belonging to American forms ; and it is obvious that 

 a mountain, as it liccamo slowly upheaved, Avould he colonized 

 from the surrounding lowlands. So it is with the inhabitants 

 of lakes and marshes, excepting in so far that great facility of 

 transport has allowed many of the same species to prevail 

 through large portions of the world. AVe see this 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 universally found 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 sjiecies ; and wherever many 

 closely-allied species occur, there Avill be found many fonns 

 wliich 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 l)etweeu the power and extent of migration in 

 certain species, either at the present or at some fomier jieriod, 

 and the existence at remote jioints of the world of allied spe- 

 cies, is shown in another and more general way. Mr. Gould 

 remarked to me long ago, that, in those genera of birds which 



