[Sidenote:  Temperature and zonal location.]

Temperature, the other important element of climate, depends primarily upon zonal location, which has far different historical results from central and peripheral location, continental and insular.  It determines the amount of heat received from the sun, though air and ocean currents may redistribute that heat within certain limits, and humidity 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.]