Ere long history will have to be re-written on new lines, so as to take into account these two currents of human life and to appreciate the part played by each of them in evolution. But in the meantime we may avail ourselves of the immense preparatory work recently done towards restoring the leading features of the second current, so much neglected. From the better-known periods of history we may take some illustrations of the life of the masses, in order to indicate the part played by mutual support during those periods; and, in so doing, we may dispense (for the sake of brevity) from going as far back as the Egyptian, or even the Greek and Roman antiquity. For, in fact, the evolution of mankind has not had the character of one unbroken series. Several times civilization came to an end in one given region, with one given race, and began anew elsewhere, among other races. But at each fresh start it began again with the same clan institutions which we have seen among the savages. So that if we take the last start of our own civilization, when it began afresh in the first centuries of our era, among those whom the Romans called the "barbarians," we shall have the whole scale of evolution, beginning with the gentes and ending in the institutions of our own time. To these illustrations the following pages will be devoted. Men of science have not yet settled upon the causes which some two thousand years ago drove whole nations from Asia into Europe and resulted in the great migrations of barbarians which put an end to the West Roman Empire. One cause, however, is naturally suggested to the geographer as he contemplates the ruins of populous cities in the deserts of Central Asia, or follows the old beds of rivers now disappeared and the wide outlines of lakes now reduced to the size of mere ponds. It is desiccation: a quite recent desiccation, continued still at a speed which we formerly were not prepared to admit.(1) Against it man was powerless. When the inhabitants of North-West Mongolia and East Turkestan saw that water was abandoning them, they had no course open to them but to move down the broad valleys leading to the lowlands, and to thrust westwards the inhabitants of the plains.(2) Stems after stems were thus thrown into Europe, compelling other stems to move and to remove for centuries in succession, westwards and eastwards, in search of new and more or less permanent abodes. Races were mixing with races during those migrations, aborigines with immigrants, Aryans with Ural-Altayans; and it would have been no wonder if the social institutions which had kept them together in their mother countries had been totally wrecked during the stratification of races which took place in Europe and Asia. But they were not wrecked; they simply underwent the modification which was required by the new conditions of life.