The forms which have been assumed by the historical movement are varied, but all have contributed to the spread of man over the habitable globe. The yellow, white and red races have become adapted to every zone; the black race, whether in Africa, Australia or Melanesia, is confined chiefly to the Tropics. A like conservatism as to habitat tends to characterize all sub-races, peoples, and tribes of the human family. The fact which strikes one in studying the migrations of these smaller groups is their adherence each to a certain zone or heat belt defined by certain isothermal lines (see map chap. XVII.), their reluctance to protrude beyond its limits, and the restricted range and small numerical strength of such protrusions as occur. This seems to be the conservatism of the mature race type, which has lost some of its plasticity and shuns or succumbs to the ordeal of adaptation to contrasted climatic conditions, except when civilization enables it partially to neutralize their effects.
Primitive Indian Stocks Of South America (From Helmolt's History of the World. By permission of Dodd, Mead & Co.)
Migrations in relation to zones and heat belts.
In South America, Caribs and Arawaks showed a strictly tropical distribution from Hayti to the southern watershed of the Amazon. The Tupis, moving down the Parana-La Plata system, made a short excursion beyond the Tropic of Capricorn, though not beyond the hot belt, then turned equator-ward again along the coast.[195] In North America we find some exceptions to the rule. For instance, though the main area of the Athapascan stock is found in the frigid belt of Canada and Alaska, north of the annual isotherm of 0°C. (32°F.) small residual fragments of these people are scattered also along the Pacific coast of Oregon and California, marking the old line of march of a large group which drifted southward into Arizona, New Mexico, Texas and the northern part of Mexico. The Shoshone stock, which originally occupied the Great Basin and western intermontane plateau up to the borders of Canada, sent out offshoots which developed into the ancient civilized tribes of tropical Mexico and Central America. Both these emigrations to more southern zones were part of the great southward trend characterizing all movements on the Pacific side of the continent, probably from an original ethnic port of entry near Bering Strait; and part also of the general southward drift in search of more genial climate, which landed the van of northern Siouan, Algonquin and Iroquoian stocks in the present area of South Carolina, Georgia, Alabama, Mississippi and Louisiana, while the base of their territory stretched out to its greatest width in southern Canada and contiguous parts of the United States. [See map page 54.][196]
Ethnographical Map Of India From The Indian Census Of 1901.