From the INTRODUCTION. THE movements of peoples are determined by two main factors, which may be briefly described as the driving force and the control; or, in other words, the cause of a migration is due to one set of circumstances and its direction to another. When reduced to its simplest terms a migration is caused by an expulsion and an attraction, the former nearly always resulting from dearth of food or from over-population, which practically comes to the same thing. Sooner or later, a time comes when the increase of the population of a country exceeds its normal food-supply. Among hunting communities the game may be so reduced by over-hunting or by disease that it cannot support even a stationary or decreasing population. The chief danger to be feared by pastoral peoples is lack of water ; a succession of small droughts can make pasturing unprofitable, but when a whole country definitely becomes more arid, migrations on a large scale are inevitable. Evidence has now accumulated which proves that various regions of the earth have undergone slow climatic changes, and that a given area at one period of time may be more or less wooded, while at another, owing to a drier climate, steppes arise, or even desert conditions may supervene. Changes of this nature occurred in parts of Europe during the ages when Palaeolithic men hunted reindeer and chased bison and wild horses; and the desiccation of central Asia has had a profound effect upon human history in Europe as well as in Asia. Agriculturists are affected in the same way ; but to a certain extent, by means of irrigation in some cases, and by more intensive cultivation in others, the soil may be made to support an increased population; nevertheless, a limit is soon reached, unless the resources are supplemented by trade....