GEOGRAPHICAL DISTRIBUTION OF ANIMALS
America, about to the 49th parallel of latitude, belongs to a vast region (what is called the Holarctic region), which includes Europe, northern Africa, and extratropical Asia. Most of the remainder of North America, including the Mexican plateau, makes up a separate region—the Sonoran. The hot lowlands of Mexico, Central America, and the southern tips of Florida and Lower California are included in the same region as South America (the Neotropical region), as are also the West Indian islands. The regions thus designated are determined primarily by the distribution of mammals, the warm-blooded quadrupeds, for these animals are better known than almost any other group except the birds, and their geological history has been much more fully and minutely deciphered than that of any other of the higher groups. These zoölogical regions are really an inevitable result of the many changes, climatic and geographic, through which the earth has passed while mammals were abundant and diversified.
It is practically certain that no group of mammals arose twice independently in unconnected areas, and it is this fact that enables us to trace, by the aid of fossils, the migration of mammals from continent to continent. If a group of mammals could arise independently and more than once the presence of a given group in North America and in Asia would be no indication that those continents were once connected, but if each group arose but once and spread as far as geographic and climatic conditions permitted, then its presence in two areas now disconnected indicates the former connection, direct or indirect, of those areas. The outlines of the zoölogical regions and their geographic relations afford a key to their history and to the manner in which they received their faunas. The complete zoölogical difference of Australia, for example, from any other continent
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