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 Rocky 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 toa 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 
