Tropical regions, however, may profit by the fact that their mountains and plateaus permit the cultivation of temperate crops.  India during the last century has introduced tea culture extensively on the Assam and Nilgiri Hills, and in the Himalayan valleys up to an altitude of 7000 feet.[1423] Besides this temperate product, it has put large areas into cotton, chiefly in the peninsular plateau of the Deccan, and by means of these two crops has caused a considerable readjustment in world commerce.[1424] Nevertheless, here the infringement of the principle of tropical production in the torrid zone is after all slight.  In tropical America, on the other hand, the case is quite different; this region presents an interesting paradox in relation to its foreign commerce.  Here the highlands are the chief seats of population.  They contain, moreover, the most industrious and intelligent native stock, due to geographical and historical causes running back into the ancient civilizations, as well as the largest proportions of immigrant Europeans.  This is true not only of the Cordilleran states from northern Mexico to the borders of Chile, but also of Brazil, whose center of population falls on the plateau behind Rio de Janeiro and Santos.  The isolation of these high plateaus excludes them to a serious extent from foreign trade, while their great altitude permits only temperate products, with the exception of sub-tropical coffee, which is their only crop meeting a great demand.  The world wants, on the other hand, the long list of lowland tropical exports which torrid America furnishes as yet in inadequate amounts, owing to the lack of an industrious and abundant lowland population.  Commerce will eventually experience a readjustment in these localities to the natural basis of tropical production; but how soon or how effectively this change will take place depends upon the question of immigration of foreign tropical peoples, or the more difficult problem of white acclimatization.[1425]

[Sidenote:  Isothermal lines in anthropo-geography.]

Despite some purely climatological objections, anthropo-geography finds the division of climatic zones according to certain isothermal lines of mean annual temperature the most expedient one for its purpose.  The hot zone may be taken as the belt north and south of the equator enclosed between the annual isotherms of 20 deg.  C. (68 deg.  F.) These hold a course generally far outside the two tropics, and in the northern continents frequently reach the thirty-fifth parallel.  The temperate climatic zones extend from the annual isotherm of 20 deg.  C. to that of 0 deg.  C. (32 deg.  F.), which bears little relation to the polar circles forming the limits of the solar Temperate Zone.  The north temperate climatic zone has been further sub-divided along the annual isotherm of 5 deg.  C. (41 deg.  F.), distinguishing thus the warmer southern belt, which forms preeminently the zone of greatest historical intensity.  The areas beyond the annual isotherms of 0 deg.C. belong to the barren cold zones. [See map page 612.]