dity or
aridity modify its effects. Still zonal distinctions remain. The great
climatic regions of the earth, like the hot, wet equatorial belt or the
warm, dry trade-wind belts or the cool, well-watered temperate zones,
constitute, through the medium of their economic products and their
climatically imposed methods of production, so many socio-political
areas, regardless of ethnic and political boundaries. The Berber nomads
of the northern Sahara live much as the Semitic Bedouins of the Syrian
desert or the Turkoman stock of arid Turkestan. They have the same
tribal government, the same scattered distribution in small groups, the
same economic basis of subsistence, though of different races and
dominated respectively by France, Turkey and Russia. The history of the
tropical Antilles has in both its economic and political features
paralleled that of the East Indies since the early 16th century.
Temperate South America promises to follow in the historical footsteps
of temperate North America, South Africa in those of Europe and
temperate Australia.

[Sidenote: Reactions of contrasted zones.]

While people of the same latitude live approximately under the same
temperature conditions, those of contrasted zones are subjected to
markedly different influences. They develop different degrees of
civilization, wealth, economic efficiency, and density of population;
hence they give rise to great historical movements in the form of
migration, conquest, colonization, and commerce, which, like convection
currents, seek to equalize the differences and reach an equilibrium.
Nature has fixed the mutual destiny of tropical and temperate zones, for
instance, as complementary trade regions. The hot belt produces numerous
things that can never grow in colder countries, while a much shorter
list of products, coupled, however, with greater industrial efficiency,
is restricted to the Temperate Zone. This explains the enormous
importance of the East Indian trade for Europe in ancient and medieval
times, the value of tropical possessions for commercial countries like
England and Holland. It throws light upon the persistence of the
tropical plantation system in the Dutch East Indies and republican
Mexico, as formerly in the sugar and cotton fields of the Southern
States, with its relentless grip upon the throat of national life in hot
lands.

[Sidenote: Temperate products from tropical highlands.]

Tropical regions, however, may profit by t