otherm of 50 deg. for the warmest month forest trees and
cereals do not grow. In the northern hemisphere this line is well north
of the Arctic circle in the continental climate of Asia, and north of it
also in north-western North America and in northern Scandinavia, but
falls well south in eastern British America, Labrador and Greenland, and
also in the North Pacific Ocean. In the southern hemisphere this
isotherm crosses the southern extremity of South America, and runs
fairly east and west around the globe there. The conditions of life are
necessarily very specialized for the peculiar climatic features which
are met with in these zones. There is a minimum of life, but more in the
north polar than the south polar zone. Plants are few and lowly. Land
animals which depend upon plant food must therefore likewise be few in
number. Farming and cattle-raising cease. Population is small and
scattered. There are no permanent settlements at all within the
Antarctic circle. Life is a constant struggle for existence. Man seeks
his food by the chase on land, but chiefly in the sea. He lives along,
or near, the sea-coast. The interior lands, away from the sea, are
deserted. Gales and snow and cold cause many deaths on land, and,
especially during fishing expeditions, at sea. Under such hard
conditions of securing food, famine is a likely occurrence.

In the arctic climate vegetation must make rapid growth in the short,
cool summer. In the highest latitudes the summer temperatures are not
high enough to melt snow on a level. Exposure is therefore of the
greatest importance. Arctic plants grow and blossom with great rapidity
and luxuriance where the exposure is favourable, and where the water
from the melting snow can run off. The soil then dries quickly, and can
be effectively warmed. Protection against cold winds is another
important factor in the growth of vegetation. Over great stretches of
the northern plains the surface only is thawed out in the warmer months,
and swamps, mosses and lichens are found above eternally frozen ground.
Direct insolation is very effective in high latitudes. Where the
exposure is favourable, snow melts in the sun when the temperature of
the air in the shade is far below freezing.

Arctic and antarctic zones differ a good deal in the distribution and
arrangement of land and water around and in them. The southern zone is
surrounded by a wide belt of open sea; the northern, by land areas. The
northern i