Page:Popular Science Monthly Volume 33.djvu/88

78, which, from their isolated habitat are undoubtedly of considerable antiquity as species. Our robin is the only representative of the thrushes with solid-colored breasts found in North America, but a host of them occur in South America and some in Africa.

This contrast between the northern and southern continents of the New World, or, to speak zoölogically, between the nearctic and neotropical regions, in the number of species of thrush, has its solution in the peculiarity and variety of physical conditions offered by South America. The ranges of the species inhabiting the great forests of the equatorial zone are in the majority of instances restricted to comparatively limited areas. The varied conditions offered by high mountain-ranges and deep, low-lying forests, tend to the creation of new varieties or local races, which are consequently limited to certain narrow areas, and a particular species is often thus represented by several extreme forms. In temperate North America, on the other hand, the fewer species are kept true by migration, which tends to equalize surrounding conditions.

In northern South America, the valley of the Amazon, and the forests of Brazil, three distinct varieties of the white-throated thrush occur. The common South American thrush, a comparatively widely ranging species, reaching southward to Chili, has an extreme form in the northwest. The Sabian thrush is another species which has split up into several forms under the influence of local conditions. No region in the world is so rich in the number and variety of its passerine birds. In the south temperate portion of the continent the species of thrush are more uniform, and very few varieties are found.

Africa has several well-defined representatives of the solid-colored-breasted thrushes. The Zanzibar thrush and the Abyssinian thrush are eastern forms; in the west, the Ethiopian and Senegambian thrushes are found; while to the southward the olive-thrush, Cabani's thrush, and the Kurichane thrush range throughout the Transvaal, Caffraria and Damara Land, the Bechuana country, and the Cape.

The facts, as they are presented to-day by this interesting group of birds, become very significant when viewed in the light of evolution. The world-wide distribution, large number of