It was observed a few pages back that the tendency of social life to fall into rhythms or cycles becomes synchronized with rhythms in the natural world such as those which give us the day, the year, and the 8-year and 33-year cycles of rainfall. The theory propounded by Jevons that commercial crises are due to sun-spots may not be so absurd as some economists have tried to make it appear. Ellsworth Huntington, an American geographer, has given the best part of his life to working out still longer cycles. One of his books has the suggestive title, The Pulse of Asia. This, like his other books, gives conclusive evidence that climate is not a fixed characteristic of a region but has its pulsations, chief of which is variation in the amount of moisture. Such variation of course has its effect on society, and Huntington tries to correlate the variations of climate, of which material evidences remain for the geographer to study to-day, with the literary record of disturbances in society which the historians have studied.

. . . The relapse of Europe in the Dark Ages . . . was due apparently to a rapid change of climate in Asia and probably all over the world, - a change which caused vast areas which were habitable at the time of Christ to become uninhabitable a few centuries later. The barbarian inhabitants were obliged to migrate, and their migrations were the dominant fact in the history of the known world for centuries. We of to-day shall do well to ascertain whether we too are not facing the problem which faced the Romans. Parts of China have been growing drier and less habitable during recent centuries, and if the process continues, we are in danger of being overrun by hungry Chinese in search of bread. . . .

. . . The data which I obtained in Central Asia . . . confirm the surmise of the historians. There is a strong reason to believe that during the last two thousand years there has been a widespread and pronounced tendency toward aridity. In drier regions the extent of land available for pasturage and cultivation has been seriously curtailed; and the habitability of the country has decreased. . . . After a period of rapidly decreasing rainfall and rising temperature during the early centuries of the Christian era, there is evidence of a slight reversal, and of a tendency toward more abundant rainfall and lower temperature during the Middle Ages.

In relatively dry regions increasing aridity is a dire calamity, giving rise to famine and distress. These in turn are fruitful causes of wars and migrations, which engender the fall of dynasties and empires, the rise of new nations, and the growth of new civilizations. . . .

The main outlines of the history of Central Asia agree with what would be expected from a knowledge of the changes of climate through which the country has passed. The favorable changes coincide with periods of prosperity and progress; the unfavorable with depression and depopulation. . . .

. . . Apparently the climate of the earth is subject to pulsations of very diverse degrees of intensity and of varying length. The Glacial Period as a whole represents the largest type of pulsation; upon it are superposed the great pulsations known as glacial epochs, each with a length measured probably in tens of thousands of years; their steady progress is in turn interrupted by smaller changes of climate, such as those of which we have found evidence during historic times in Central Asia; and finally, the climate of the world pulsates in cycles of thirty-six years, and even these are interrupted by seasonal changes and by storms. . . . It is probable, though it has not been demonstrated, that the larger are also due to the same cause. - Huntington, The Pulse of Asia, pp. 5, 6, 13, 14, 366, 367.

By purely mathematical methods, unaffected by any personal bias, it has been possible to obtain curves indicating the climatic pulsations of the last 3000 years. A comparison of the curves with the results obtained from other lines of evolution, both in America and Asia, shows that in spite of certain disagreements the general climatic history of both continents appears to have been characterized by similar pulsations having a periodicity of hundreds or thousands of years.

Apparently the Southwest has been first relatively inhabitable and then relatively uninhabitable during periods lasting hundreds of years. The dates of these periods are ascertainable from ancient trees. Each propitious period has probably been a time of expanding culture, and comparatively dense population, while the unpropitious periods have been times of invasion, disaster, and depopulation.

In regard to the greater climatic changes, it appears that the pulsations of the past 3000 years are too large to be due to fortuitous rearrangements of the earth's crust. Hence we are led to conclude that they, too, are due to variations in the sun. The same conclusion seems to apply to the glacial and inter-glacial epochs, since their characteristics appear to be identical in nature with those of the pulsations of historic times, although differing greatly in degree. - Huntington, The Climatic Factor, pp. 3-5.

Three eras make up the tale of history. Three great pulsations characterize the course of climate during the same period. The eras and the pulsations agree in time. The first era comprises the hazy past when Egpyt and Babylonia were at their greatest. It ends with the chaos of the Aramean migrations. The second spans the life of Israel and Palestine, the Greeks in their islands and peninsula, Italy in the most western of the great lands of antiquity, and Assyria and Persia far to the east. It also ends in chaos with the migrations of the Barbarians and Mohammedans. The last of these eras had seen the rise of great nations in lands still farther north. Already it has endured twelve eventful centuries. We dare not prophesy how long it yet may last. Perhaps it, too, may end in drought and mighty movements of the races, unless by growing knowledge we avert the ills that hitherto have been man's heritage. - Huntington, Palestine and Its Transformation, pp. 403, 404.