climate, which have served to augment its population
both by natural increase and steady immigration (one-fourth of its
population is foreign), has expanded across the Rio Negro over the
grasslands of the Patagonian plain, and thereby enlarged its area by
259,620 square miles since 1881. The statesman of the plains is a
nature-made imperialist; he nurses wide territorial policies and draws
his frontiers for the future. To him a "far-flung battle line" is
significant only as a means to secure a far-flung boundary line.

[Sidenote: Arid plains.]

From these low, accessible plains of adequate rainfall, which at first
encourage primitive nomadism but finally make it yield to sedentary life
and to dense populations spreading their farms and cities farther and
farther over the unresisting surface of the land, we turn to those
boundless arid steppes and deserts which Nature has made forever the
homes of restless, rootless peoples. Here quiescence is impossible, the
_Voelkerwanderung_ is habitual, migration is permanent. The only change
is this eternal restlessness. While the people move, progress stands
still. Everywhere the sun-scorched grasslands and waterless waste have
drawn the dead-line to the advance of indigenous civilization. They
permit no accumulation of productive wealth beyond increasing flocks and
herds, and limit even their growth by the food supply of scanty,
scattered pasturage. The meager rainfall eliminates forests and
therewith a barrier to migrations; it also restricts vegetation to
grasses, sedges and those forms which can survive a prolonged summer
drought and require a short period of growth.

[Illustration: ANNUAL RAINFALL OF THE WORLD.]

[Sidenote: Distribution and extent of arid plains.]

The union of arid plains and steppe vegetation is based upon climate,
and is therefore a widely distributed phenomenon. These plains, whether
high or low, are found in their greatest extent in the dry trade-wind
belts, as in the deserts and steppes of Arabia, Persia, Sudan, the
Sahara, South Africa and Central Australia; and in vast continental
interiors, where the winds arrive robbed of their moisture in passing
intervening highlands, as in the grasslands of our western plains, the
llanos and pampas of South America, and the steppes of Central Asia. But
wherever they occur, whether in Argentina or Russian Turkestan or the
higher plains of Mongolia and Tibet, they present the same general
characteristics of la