r, 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.]

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
domestica