tainous country is full of barriers which obstruct the diffusion of life. Distinctness is in direct proportion to isolation. What is true in this regard of the fauna of any region is likewise true of its individual species. The degree of resemblance among individuals is in strict proportion to the freedom of their movements. Variation within the limits of a species is again proportionate to the barriers which prevent equal and free diffusion.

159. Realms of animal life.—The various divisions or realms into which the land surface of the earth may be divided on the basis of the character of animal life have their boundary in the obstacles offered to the spread of the average animal. In spite of great inequalities in this regard, we may yet roughly divide the land of the globe into seven principal realms or areas of distribution, each limited by barriers, of which the chief are the presence of the sea and the occurrence of frost. There are the Arctic, North Temperate, South American, Indo-African, Lemurian, Patagonian, and Australian realms. Of these the Australian realm alone is sharply defined. Most of the others are surrounded by a broad fringe of debatable ground that forms a transition to some other zone.

The Arctic realm includes all the land area north of the isotherm of 32°. Its southern boundary corresponds closely with the northern limit of trees. The fauna of this region is very homogeneous. It is not rich in species, most of the common types of life of warmer regions being excluded. Among the large animals are the polar bear, the walrus, and certain species of "ice-riding" seals. There are a few species of fishes, mostly trout and sculpins, and a few insects. Some of these, as the mosquito, are excessively numerous in individuals. Reptiles are absent from this region and many of its birds migrate southward in the winter, finding in the arctic only their breeding homes. When we consider the distribution of insects and other small animals of wide diffusion we must add to the arctic realm all high moun

tains of other realms whose summits rise above the timber line. The characteristic large animals of the arctic, as the polar bear or the musk-ox or the reindeer, are not found there, because barriers shut them off. But the flora of the mountain top, even under the equator, may be characteristically arctic, and with the flowers of the north may be found the northern insects on whose presence the flower depends for its fertilization. So far as climate is concerned high altitude is equivalent to high latitude. On certain mountains the different zones of altitude and the corresponding zones of plant and insect life are very sharply defined (Fig. 178).

The North Temperate realm comprises all the land between the northern limit of trees and the southern limit of frost. It includes, therefore, nearly the whole of Europe, most of Asia, and most of North America. While there are large differences between the fauna of North America and that of Europe and Asia, these differences are of minor importance and are scarcely greater in any case than the difference between the fauna of California and that of our Atlantic coast. The close union of Alaska with Siberia gives the arctic region an almost continuous land area from Greenland to the westward around to Norway. To the south everywhere in the temperate zone realm the species. increase in number and variety, and the differences between the fauna of North America and that of Europe are due in part to the northward extension into the one and the other of types originating in the tropics. Especially is this true of certain of the dominant types of singing birds. The group of wood-warblers, tanagers, American orioles, vireos, mocking-birds, with the fly-catchers and humming-birds so characteristic of our forests, are unrepresented in Europe. All of them are apparently immigrants from the neotropical realm where nearly all of them spend the winter. In the same way central Asia has many immigrants from the Indian realm to the southward. With all these variations there

FIG. 178.-Mt. Orizaba, a high mountain rising near the city of Orizaba, Mexico, and showing zones of climate and life from tropical (at base) to arctic (at summit).

is an essential unity of life over this vast area, and the recognition of North America as a separate (nearctic) realm, which some writers have attempted, seems hardly practicable.

The Neotropical or South American realm includes South America, the West Indies, the hot coast lands of Mexico, and those parts of Florida and Texas where frost does not occur. Its boundaries through Mexico are not sharply defined, and there is much overlapping of the north temperate realm along its northern limit. Its birds especially range widely through the United States in the summer migrations, and a large part of them find in the North their breeding home. Southward, the broad barrier of the two oceans keeps the South American fauna very distinct from that of Africa or Australia. The neotropical fauna is richest of all in species. The great forests of the Amazon are the dreams of the naturalists. Characteristic types among the larger animals are the snout or broad-nosed (platyrrhine) monkeys, which in many ways are very distinct from the monkeys and apes of the Old World. In many of them the tip of the tail is highly specialized and is used as a hand. The Edentates (armadillos, ant-eaters, etc.) are characteristically South American, and there are many peculiar types of birds, reptiles, fishes, and insects.

The Indo-African realm corresponds to the neotropical realm in position. It includes the greater part of Africa, merging gradually northward into the north temperate realm through the transition districts which border the Mediterranean. It includes also Arabia, India, and the neighboring islands, all that part of Asia south of the limit. of frost. In monkeys, carnivora, ungulates, and reptiles this region is wonderfully rich. In variety of birds, fishes, and insects the neotropical realm exceeds it. The monkeys of this district are all of the narrow-nosed (catarrhine) type, various forms being much more nearly related to man than is the case with the peculiar monkeys of South