It is often said that the overflow of Europe over the world has been a sort of renewal of the folk-wandering of primitive ages. That is a misleading view: the movement has been far more deliberate and organised, and far less due to the pressure of external circumstances, than the early movements of peoples in the Old World. Not until the nineteenth century, when the industrial transformation of Europe brought about a really acute pressure of population, can it be said that the mere pressure of need, and the shortage of sustenance in their older homes, has sent large bodies of settlers into the new lands. Until that period the imperial movement has been due to voluntary and purposive action in a far higher degree than any of the blind early wanderings of peoples. The will-to-dominion of virile nations exulting in their nationhood; the desire to obtain a more abundant supply of luxuries than had earlier been available, and to make profits therefrom; the zeal of peoples to impose their mode of civilisation upon as large a part of the world as possible; the existence in the Western world of many elements of restlessness and dissatisfaction, adventurers, portionless younger sons, or religious enthusiasts: these have been the main operative causes of this huge movement during the greater part of the four centuries over which it has extended. And as it has sprung from such diverse and conflicting causes, it has assumed an infinite variety of forms; and both deserves and demands a more respectful study as a whole than has generally been given to it. II THE ERA OF IBERIAN MONOPOLY During the Middle Ages the contact of Europe with the rest of the world was but slight. It was shut off by the great barrier of the Islamic Empire, upon which the Crusades made no permanent impression; and although the goods of the East came by caravan to the Black Sea ports, to Constantinople, to the ports of Syria, and to Egypt, where they were picked up by the Italian traders, these traders had no direct knowledge of the countries which were the sources of their wealth. The threat of the Empire of Genghis Khan in the thirteenth century aroused the interest of Europe, and the bold friars, Carpini and Rubruquis, made their way to the centres of that barbaric sovereign's power in the remote East, and brought back stories of what they had seen; later the Poli, especially the great Marco, undertook still more daring and long-continued journeys, which made India and Cathay less unreal to Europeans, and stimulated the desire for further knowledge. The later mediaeval maps of the world, like that of Fra Mauro (1459),[3] which incorporate this knowledge, are less wildly imaginative than their predecessors, and show a vague notion of the general configuration of the main land-masses in the Old World. But beyond the fringes of the Mediterranean the world was still in the main unknown to, and unaffected by, European civilisation down to the middle of the fifteenth century.