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.] Adverse conditions of climate and soil are not the only factors in this retardation.