limate upon relief.]

The very ground under man's feet, moreover, feels the molding hand of
climate. In one region a former age of excessive cold has glaciated the
surface and scoured off the fertile loam down to the underlying rock, or
left the land coated with barren glacial drift or more productive clays.
In another, the cold still persists and caps the land with ice and snow,
or, as in the tundra, underlays it with a stratum of frozen earth, which
keeps the surface wet and chilled even in the height of summer. In yet
other regions, abundant moisture combined with heat covers the ground
with a pad of fertile humus, while some hundred miles away drying trade
winds parch and crack the steppe vegetation, convert most of its organic
substance into gases, and leave only a small residue to enrich the soil.
Rain itself modifies the relief of the land, and therefore often decides
in a slow, cosmic way what shall be the ultimate destination of its
precious store of water. A heavy precipitation on the windward side of a
mountain range, by increasing the mechanical force of its drainage
streams, makes them bite their way back into the heart of the system and
decapitate the rivers on the leeward side, thus diminishing the volume
of water left to irrigate the rainless slope. Thus the hydra-headed
Amazon has been spreading and multiplying its sources among the Andean
valleys, to the detriment of agriculture on the dry Pacific slope; thus
the torrents of the Western Ghats, gorged by the monsoon rains from the
Indian Ocean, are slowly nipping off the streams of the ill-watered
Deccan, [See map page 484.] All these direct and indirect effects of
climate may combine to produce ultimate politico-geographical results
which manifest themselves in the expansion, power and permanence of
states.

[Sidenote: Climate limits the habitable area.]

Climatic conditions limit the habitable area of the earth. This is
their most important anthropo-geographic effect. At either pole lurks an
invincible foe, with whom expanding humanity must always reckon, and who
brooks little encroachment upon his territory. His weapon is the
restriction of organic life, without which man cannot exist. The
geographical boundaries of organic life, however, are wider than those
of human life. The consequence of this climatic control, therefore, is
not only a narrowed distribution of the human race, but a concentration
which intensifies the struggle for existence, for