Origin of Species Variorum

namely, their 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 recent Glacial epoch, are related to those of the surrounding lowlands;— thus we have in South America, Alpine 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 that great facility of transport has allowed many of the same species to prevail through 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 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 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 period, ... and the existence at remote points of the world of 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. I can hardly doubt that this rule is generally true, though