Agriculture is adopted at first on a small scale as an adjunct to the chase or herding. It tends therefore to partake of the same extensive and nomadic character[112] as these other methods of gaining subsistence, and only gradually becomes sedentary and intensive. Such was the superficial, migratory tillage of most American Indians, shifting with the village in the wake of the retreating game or in search of fresh unexhausted soil. Such is the agriculture of the primitive Korkus in the Mahadeo Hills in Central India. They clear a forested slope by burning; rake over the ashes in which they sow their grain, and reap a fairly good crop in the fertilized soil. The second year the clearing yields a reduced product and the third year is abandoned. When the hamlet of five or six families has exhausted all the land about it, it moves to a new spot to repeat the process.[113] The same superficial, extensive tillage, with abandonment of fields every few years, prevails in the Tartar districts of the Russian steppes, as it did among the cattle-raising Germans at the beginning of their history. Tacitus says of them, _Arva per annos mutant et superest ager_,[114] commenting at the same time upon their abundance of land and their reluctance to till. Where nomadism is made imperative by aridity, the agriculture which accompanies it tends to become fixed, owing to the few localities blessed with an irrigating stream to moisten the soil. These spots, generally selected for the winter residence, have their soil enriched, moreover, by the long stay of the herd and thus avoid exhaustion.[115] Often, however, in enclosed basins the salinity of the irrigating streams in their lower course ruins the fields after one or two crops, and necessitates a constant shifting of the cultivated patches; hence agriculture remains subsidiary to the yield of the pastures. This condition and effect is conspicuous along the termini of the streams draining the northern slope of the Kuen Lun into the Tarim basin.[116] [Sidenote: Geographic checks to progress.] 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.]