he 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 rea