who dwell
in them.

In the torrid zone the temperature varies but little. During the season
of rains it rarely falls to 70 deg. F., and in the dry season it is seldom
higher than 95 deg. F. As a result, all sorts of plants that are sensitive
to low temperatures thrive in the torrid zone. It is not a climate
suitable for heat-producing food-plants, and they are not required.

The constant heat and excessive moisture of the atmosphere in the torrid
zone is apt to produce a feeling of lassitude among the dwellers in such
regions, moreover, and great bodily activity is out of question. These
conditions seriously affect the lives of the people, and, with few
exceptions, tropical peoples are rarely noted for energy or enterprise.
Great commercial enterprises are the exception rather than the rule, and
they are usually carried on by foreigners who must live a part of the
time in cooler localities.

[Illustration: THE EFFECTS OF HIGH LATITUDE--TOO COLD TO PRODUCE
BREAD-STUFFS]

Polar regions are deficient both in the heat and light necessary for
food-stuffs. Neither the grasses nor the grains fructify. As a result,
but few herbivora can live there, and these are practically restricted
to the musk-ox and the reindeer, which subsist on mosses and lichens.
The native people are stunted in growth; their food consists mainly of
raw blubber, and they are scarcely above savagery.

The temperate zones are the regions of the great industries and
activities of human life. The larger part of the land surface of the
earth is situated in these zones; moreover, the people who dominate the
world also live in them, and their supremacy is due largely to
conditions of climate. The alternation of summer and winter causes a
struggle for existence that develops the intellectual faculties and
results in industrial supremacy.

=Effects of Altitude.=--There is a decrease of temperature of 1 deg. F. for
about every three hundred feet of ascent. But few people live at an
altitude of more than six thousand feet above sea-level, and in many
cases they depend on other localities for the greater part of their
food-stuffs, because very few of such regions produce food-stuffs
abundantly.

The chief exceptions to this rule are found in tropical regions. The
highlands of Mexico, the plateau-regions of Bolivia and Ecuador, and the
highlands of southern Asia are habitable, but they are not densely
peopled. Because of their altitude they are relieved of