The geographical distribution of animals, with a study of the relations ...

6 DISTRIBUTION OF ANIMALS. [part I.gers and mice. In like manner parts of North America andEurope are very similar in all essentials of soil climate andvegetation, yet the former has racoons, opossums, and hummingbirds; while the latter possesses moles, hedgehogs and true flycatchers.Equally striking are the facts presented by thedistribution of many large and important groups of animals.Marsupials (opossums, phalangers &c.) are found from temperateVan Diemen's land to the tropical islands of New Guinea andCelebes, and in America from Chili to Virginia. No crowsexist inSouth America, while they inhabit every other part ofthe world, not excepting Australia. Antelopes are found onlyin Africa and Asia ; the sloths only in South America ; the truelemurs are confined to Madagascar, and the birds-of-paradise toNew Guinea.If we examine more closely the distribution of animals inany extensive region, we find that different, though closely alliedspecies, are often found on the opposite sides of any considerablebarrier to their migration.Thus, on the two sides of the Andesand Eocky Mountains in America, almost all the mammalia, birds,and insects are of distinct species. To a less extent, the Alpsand Pyrenees forma similar barrier, and even great rivers andriver plains, as those of the Amazon and Ganges, separate moreor less distinct groups of animals. Arms of the sea are stillmore effective, if they are permanent ; a circumstance in somemeasure indicated by their depth. Thus islands far away fromland almost alwayselse ;have very peculiar animals found nowhereas is strikingly the case in Madagascar and New Zealand,and to a less degree in the West India islands. But shallowstraits, like the English Channel or the Straits of Malacca, arenot found to have the same effect, the animals being nearly orquite identical on their opposite shores. A change of climate ora change of vegetation may form an equally effectivebarrier tomigration. Many tropical and polar animals are pretty accuratelylimited by certain isothermal lines ;and the limits of thegreat forests in most parts of the world strictly determine theranges of many species.Naturalists have now arrived at theconclusion, that by some