78 THE POPULAR SCIENCE MONTHLY. 



tioned, which, from their isolated habitat are undoubtedly of 

 considerable antiquity as species. Our robin is the only repre- 

 sentative 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 zoologically, between the nearc- 

 tic and neotropical regions, in the number of species of thrush, 

 has its solution in the peculiarity and variety of physical con- 

 ditions 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 sev- 

 eral 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 compar- 

 atively widely ranging species, reaching southward to Chili, has 

 an extreme form in the northwest. The Sabian thrush is an- 

 other 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 va- 

 riety of its passerine birds. 

 In the south temperate por- 

 tion of the continent tlie 

 species of thrush are more 

 uniform, and very few va- 

 rieties 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 Kuri- 

 chane 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 



