Seite - 444 - in The Origin of Species

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-