As to the second point, the easy access to the foreign port whither export was to be made, London was again especially favoured, in its point of growth. To-day it is no longer so, as we shall see in a moment. London is going forward with the momentum of its past. But until quite modern times London looked more favourably at the points to which England exported than did the other island ports, its rivals. It lay in the full focus of that long curve of the Norman, the Picard, the Flemish and Dutch coasts, which were the principal markets of the Middle Ages. Right opposite were the Flemish cities that bought English wool--the staple export--right opposite, also, the mouths of the Rhine for communication with the interior Germanies and, though a little more distant than the northern ports, agreeable to all that Baltic and North Sea trade which grew up with the close of the Middle Ages. The league of mercantile towns called Hanseatic had its depôt in London from the middle of the thirteenth century. The northern ports of England that lie a little nearer had no hinterland to feed their commerce, and London was, moreover, the best clearing-house and centre of general exchange for them in all the north and west. In this character it surpassed even Bruges. When all these conditions changed--when the discovery of the New World, and of the Cape route to the Indies, when the growth of north-country industries and one hundred other factors had taken away from the site of London its old topographical supremacy, the port was so firmly established that it provided its own sources of vitality. It is always so in the economic affairs of a town or of a nation. Material causes are discovered to be more powerful in the middle period of their growth; once the town or nation are rooted they nourish themselves. The River of London to-day is the capital example of this truth out of all Europe. A basin far too small for modern commerce, lying much too distantly up a river too narrow for modern commercial needs, dangerous of entry for ships of modern draft, and needing perpetual and enormous labour to keep it properly open, none the less preserves its place at the head of the ports of the world. As a depôt for commerce its wharves and docks extend to Tilbury, and that system of clearing which grew up naturally on the low gravel height above London Bridge, when the pool held all the shipping of the place and was a large and secure harbour, is still established on that same hill in the shape of the banks and the exchanges of the City of London, which cancel the exchanges, not of the pool below, but of I know not what fraction of the shipping of the whole world.