There was a time when this traffic must have been systematically carried on; when the ancient miners of Lake Superior worked its rich copper veins with industrious zeal; and when, probably as part of the same aggressive energy, the valley of the Ohio was filling with a settled population; its great earthworks were in process of construction, and a native race entered on a course that gave promise of social progress. But, from whatever cause, the work of the old miners was abruptly terminated;[78] the race of the Mounds vanished from the scenes of their ingenious toil; and rudest barbarism resumed its sway over the whole northern continent. The same Aryan race that, before the dawn of history, before the Sanskrit-speaking people of India, or the Zends of Persia, entered on their southern homes, spoke in its own cradleland the mother tongue of Sanskrit, Greek, Celtic, and German, at length broke up and went forth on its long wanderings. Whatever peoples it found there; they were replaced by Celts, Romans, Greeks, Slavs, and Teutons, who broke in upon the barbarism of prehistoric Europe; displaced the older races, Allophylian, Neolithic, Iberian, Finnic, or by whatever other name we may find it convenient to designate them; but not without a considerable intermingling of the old blood with that of the intruders. The sparsely settled continent gradually filled up. Forests were cleared, swamps drained, rivers confined by artificial banks and levées to their channels; and there grew up in their new homes the Celtic, Classic, Slavic, and Teutonic tongues, with all the varied culture and civilisation which they represent. Agriculture, the special characteristic of the whole Aryan race, flourished. They brought with them the cereals; and, with plenty, the favoured race multiplied, till at length it has grown straitened within the bounds of the old continent which it had made its own.