[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 land surface, climate, flora and fauna, and the same nomadic populations of pastoral or hunting tribes.  In them the movement of peoples reaches its culminating point, permanent settlement its nil point.  Here the hunting savage makes the widest sweep in pursuit of buffalo or antelope, and pauses least to till a field; here the pastoral nomad follows his systematic wandering in search of pasturage and his hardly less systematic campaigns of conquest.  It is the vast area and wide distribution of these arid plains, combined with the mobility which they impose on native human life, that has lent them historical importance, and reproduced in all sections of the world that significant homologous relation of arid and well-watered districts.

[Sidenote:  Pastoral life.]

The grasslands of the old world developed historical importance only after the domestication of cattle, sheep, goats, asses, horses, camels and yaks.  This step in progress resulted in the evolution of peoples who renounced the precarious subsistence of the chase and escaped the drudgery of agriculture, to devote themselves to pastoral life.  It was possible only where domesticable animals were present, and where the intelligence of the native or the peculiar pressure exerted by environment suggested the change from a natural to an artificial basis of subsistence.  Australia lacked the type of animal.  Though North America had the reindeer and buffalo, and South America the guanaco, llama and alpaca, only the last two were domesticated in the Andean highlands; but as these were restricted to altitudes from 10,000 to 14,000 feet, where pasturage was limited, stock raising in primitive South America was merely an adjunct to the sedentary agriculture of the high intermontane valleys, and never became the basis for pastoral nomadism on the grassy plains.  However, when the Spaniards introduced horses and cattle into South America, the Indians and half-breeds of the llanos and pampas became regular pastoral nomads, known as llaneros and gauchos.  They are a race of horsemen, wielding javelin and lasso and bola, living on meat, often on horse-flesh like the ancient Huns, dwelling in leather tents made on a cane framework, like those of the modern Kirghis and medieval Tartars, dressed in cloaks of horsehide sewn together, and raiding the Argentinian frontier of white settlement for horses, sheep and cattle, with the true marauding instinct of all nomads.[1051]