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 332 important barrier, uniting the two mighty streams that protected the provinces of Europe, seemed to fill up the vacant space through which the barbarians, and particularly the Alemanni, could penetrate with the greatest facility into the heart of the empire. But the experience of the world, from China to Britain, has exposed the vain attempt of fortifying any extensive tract of country. An active enemy, who can select and vary his points of attack, must, in the end, discover some feeble spot or unguarded moment. The strength as well as the attention of the defenders is divided; and such are the blind effects of terror on the firmest troops, that a line broken in a single place is almost instantly deserted. The fate of the wall which Probus erected may confirm the general observation. Within a few years after his death, it was overthrown by the Alemanni. Its scattered ruins, universally ascribed to the power of the Dæmon, now serve only to excite the wonder of the Swabian peasant.

Among the useful conditions of peace, imposed by Probus on the vanquished nations of Germany, was the obligation of supplying the Roman army with sixteen thousand recruits, the bravest and most robust of their youth. The emperor dispersed them through all the provinces, and distributed this dangerous reinforcement in small bands, of fifty or sixty each, among the national troops; judiciously observing that the aid which the republic derived from the barbarians should be felt but not seen. Their aid was now become necessary. The feeble elegance of Italy and the internal provinces could no longer support the weight of arms. The hardy frontier of the Rhine and Danube still produced minds and bodies equal to the labours of the camp; but a perpetual series of wars had gradually diminished their numbers. The infrequency of marriage, and the ruin of agriculture, affected the principles of population, and not only destroyed the strength of the present, but intercepted the hope of future, generations. The wisdom of Probus embraced a great and beneficial plan of replenishing the exhausted frontiers, by new colonies of captive or fugitive barbarians, on whom he bestowed lands, cattle, instruments of husbandry, and every