Still later, forests came to be recognised as useful for fuel, and for carpentry and shipbuilding. Mineral deposits, usually found in hilly or mountainous districts, became pre-eminently important sources of wealth; and rivers were valued as highways of commerce and as sources of motive power by the force of their currents. To the Red Indians of the Ohio valley the places which were the most attractive camping-grounds were those whither the buffaloes came in vast herds to lick the rock salt exposed in the sides of the hills. It is now not the salt-licks, but the existence of immense deposits of coal and iron, that have determined the growth of huge communities in those regions whence the red man and the buffalo have both vanished. England was once, as New Zealand is now, a great wool-growing and wool-exporting country, whereas she is to-day a country which spins and weaves far more wool than she produces. [Sidenote: Ancient Harbours and Modern] So, too, the influence of the sea on man has changed. There was a time when towns were built upon heights some way off from the coast, because the sea was the broad high road of pirates who swooped down upon and pillaged the dwellings of those who lived near it. Now that the sea is safe, trading cities spring up upon its margin, and sandy tracts worthless for agriculture have gained an unexpected value as health resorts, or as places for playing games, places to which the inhabitants of inland districts flock in summer, as they do in England and Germany, or in winter, as they do on the Mediterranean coasts of France. The Greeks, when they began to compete with the Phœnicians in maritime commerce, sought for small and sheltered inlets in which their tiny vessels could lie safely--such inlets as Homer describes in the Odyssey, or as the Old Port of Marseilles, a city originally a colony from the Ionian Phocæa. Nowadays these pretty little rock harbours are useless for the large ships which carry our trade. The Old Port of Marseilles is abandoned to small coasters and fishing-boats, and the ocean steamers lie in a new harbour which is protected, partly by outlying islands, partly by artificial works. [Sidenote: The World-Importance of Medicine]