6 DISTRIBUTION OF ANIMALS. [part I. 



gers and mice. In like manner parts of North America and 

 Europe are very similar in all essentials of soil climate and 

 vegetation, yet the former has racoons, opossums, and humming- 

 birds ; while the latter possesses moles, hedgehogs and true fly- 

 catchers. Equally striking are the facts presented by the 

 distribution of many large and important groups of animals. 

 Marsupials (opossums, phalangers &c.) are found from temperate 

 Van Diemen's land to the tropical islands of New Guinea and 

 Celebes, and in America from Chili to Virginia. No crows 

 exist in South America, while they inhabit every other part of 

 the world, not excepting Australia. Antelopes are found only 

 in Africa and Asia ; the sloths only in South America ; the true 

 lemurs are confined to Madagascar, and the birds-of-paradise to 

 New Guinea. 



If we examine more closely the distribution of animals in 

 any extensive region, we find that different, though closely allied 

 species, are often found on the opposite sides of any considerable 

 barrier to their migration. Thus, on the two sides of the Andes 

 and Eocky Mountains in America, almost all the mammalia, birds, 

 and insects are of distinct species. To a less extent, the Alps 

 and Pyrenees form a similar barrier, and even great rivers and 

 river plains, as those of the Amazon and Ganges, separate more 

 or less distinct groups of animals. Arms of the sea are still 

 more effective, if they are permanent ; a circumstance in some 

 measure indicated by their depth. Thus islands far away from 

 land almost always have very peculiar animals found nowhere 

 else ; as is strikingly the case in Madagascar and New Zealand, 

 and to a less degree in the West India islands. But shallow 

 straits, like the English Channel or the Straits of Malacca, are 

 not found to have the same effect, the animals being nearly or 

 quite identical on their opposite shores. A change of climate or 

 a change of vegetation may form an equally effective barrier to 

 migration. Many tropical and polar animals are pretty accu- 

 rately limited by certain isothermal lines ; and the limits of the 

 great forests in most parts of the world strictly determine the 

 ranges of many species. 



Naturalists have now arrived at the conclusion, that by some 



