IN the evolution of societies geography is always a factor; but while the factor itself is always present, its effect varies in each phase of evolution. The assumption that the effect of geography on society is constant can lead to distorted historical interpretations. If, for instance, in the oasis environment of Central Asia we assume that the relationship between the oasis environment and the oasis society is constant, and if we find that in one period oasis cities flourished, surrounded by a prosperous agriculture, while in another period large tracts of the agriculture were abandoned and the cities withered, what is the explanation? Huntington,1 assuming that the ratio between environment and society is constant, is forced to the conclusion that such historical changes are to be explained by changes in climate, because only changes arising out of the environment itself could change the balance between environment and society. The explanation is not adequate. Undoubtedly climates and the other natural factors which make up an environment do change. Nature is not static. Nevertheless there are also other factors to be considered. In the Central Asian oases, and in similar oases in north-west China, agri? culture depends on irrigation. The water comes from melting ice and snow in the high mountains. In these oases, primitive societies developed into flourishing societies by evolving engineering techniques for the control of water. There is unmistakable evidence however that in this general region the effect of irrigation was not uniform. When irrigation was applied over long periods two problems emerged, one mechanical and one chemical. Silting was a mechanical problem which tended to get beyond the control of a society which had not yet evolved a machine technology. Silting is also a chronic problem of the lower Yellow river in China. The chemical problem was inherent in the use of drainage water which, flowing through mountains and the desert before being applied to the field, picked up chemicals in solution? chemicals not present in rain water. The repeated application and evaporation of such water, in arid areas where the chemical deposits could not be washed out of the soil and carried away, tended to a chemical accumulation which was eventually poisonous to crops. Yet another factor must be considered. In oases which were surrounded by steppe rather than by desert, the oasis society was in contact with a steppe society. The growth of the oasis population was limited by the water supply and by the degree of skill attained in extending the oasis by irrigation. The x Ellsworth Huntington, 'The pulse of Asia/ Boston, 1907. The materialistic view of the moulding of society by environment presented by Professor Huntington in this now famous book has since been developed in much greater detail in his many subsequent publications.