The desultory, intermittent, extensive use of the land practised by hunters and nomads tends, under the growing pressure of population, to pass into the systematic, continuous, intensive use practised by the farmer, except where nature presents positive checks to the transition.  The most obvious check consists in adverse conditions of climate and soil.  Where agriculture meets insurmountable obstacles, like the intense cold of Arctic Siberia and Lapland, or the alkaline soils of Nevada and the Caspian Depression, or the inadequate rainfall of Mongolia and Central Arabia, the land can produce no higher economic and social groups than pastoral hordes.  Hence shepherd folk are found in their purest types in deserts and steppes, where conditions early crystallized the social form and checked development. [Rainfall map chap.  XIV.]

[Sidenote:  Native animal and plant life as factors.]

Adverse conditions of climate and soil are not the only factors in this retardation.  The very unequal native equipment of the several continents with plant and animal forms likely to accelerate the advance to nomadism and agriculture also enters into the equation.  In Australia, the lack of a single indigenous mammal fit for domestication and of all cereals blocked from the start the pastoral and agricultural development of the natives.  Hence at the arrival of the Europeans, Australia presented the unique spectacle of a whole continent with its population still held in the vise of nature.  The Americas had a limited variety of animals susceptible of domestication, but were more meagerly equipped than the Old World.  Yet the Eskimo failed to tame and herd the reindeer, though their precarious food-supply furnished a motive for the transition.  Moreover, an abundance of grass and reindeer moss (Cladonia rangiferina), and congenial climatic conditions favored it especially for the Alaskan Eskimo, who had, besides, the nearby example of the Siberian Chukches as reindeer herders.[117] The buffalo, whose domesticability has been proved, was never utilized in this way by the Indians, though the Spaniard Gomara writes of one tribe, living in the sixteenth century in the southwestern part of what is now United States territory, whose chief wealth consisted in herds of tame buffalo.[118] North America, at the time of the discovery, saw only the dog hanging about the lodges of the Indians; but in South America the llama and alpaca, confined to the higher levels of the Andes (10,000 to 15,000 feet elevation) were used in domestic herds only in the mountain-rimmed valleys of ancient Peru, where, owing to the restricted areas of these intermontane basins, stock-raising early became stationary,[119] as we find it in the Alps.  Moreover, the high ridges of the Andes supported a species of grass called ichu, growing up to the snowline from the equator to the southern extremity of Patagonia.  Its geographical distribution coincided with that of the llama and alpaca, whose chief pasturage it furnished.[120] In contrast, the absence of any wild fodder plants in Japan, and the exclusion of all foreign forms by the successful competition of the native bamboo grass have together eliminated pastoral life from the economic history of the island.