organization as it did 2,400 years ago; stamps the cavalry tribes of
Cossacks as it did the mounted Huns, energizes its sons by its dry
bracing air, toughens them by its harsh conditions of life, organizes
them into a mobilized army, always moving with its pastoral
commissariat. Then when population presses too hard upon the meager
sources of subsistence, when a summer drought burns the pastures and
dries up the water-holes, it sends them forth on a mission of conquest,
to seek abundance in the better watered lands of their agricultural
neighbors. Again and again the productive valleys of the Hoangho, Indus,
Ganges, Tigris and Euphrates, Nile, Volga, Dnieper and Danube have been
brought into subjection by the imperious nomads of arid Asia, just as
the "hoe-people" of the Niger and upper Nile have so often been
conquered by the herdsmen of the African grasslands. Thus, regardless of
race or epoch--Hyksos or Kaffir--history tends to repeat itself in these
rainless tracts, and involves the better watered districts along their
borders when the vast tribal movements extend into these peripheral
lands.

[Illustration: DENSITY OF POPULATION IN EASTERN HEMISPHERE]

[Illustration: DENSITY OF POPULATION IN WESTERN HEMISPHERE]

[Sidenote: Climatic influences.]

Climatic influences are persistent, often obdurate in their control.
Arid regions permit agriculture and sedentary life only through
irrigation. The economic prosperity of Egypt to-day depends as
completely upon the distribution of the Nile waters as in the days of
the Pharaohs. The mantle of the ancient Egyptian priest has fallen upon
the modern British engineer. Arctic explorers have succeeded only by