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