Wallon has observed that Rome, “in the early times of the Republic, was chiefly preoccupied with having a numerous and strong population of freemen. Under the Empire she had but one anxiety--taxes.”[60] To speak with more precision, force changed the channel through which it operated. Native farmers and native soldiers were needless when such material could be bought cheaper in the North or East. With money the cohorts could be filled with Germans; with money, slaves and serfs could be settled upon the Italian fields; and for the last century, before the great inroads began, one chief problem of the imperial administration was the regulation of the inflow of new blood from without, lacking which the social system must have collapsed. The later campaigns on the Rhine and the Danube were really slave-hunts on a gigantic scale. Probus brought back sixteen thousand men from Germany, “the bravest and most robust of their youth,” and distributed them in knots of fifty or sixty among the legions. “Their aid was now become necessary.... 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.”[61] His importations of agricultural labour were much more considerable. In a single settlement in Thrace, Probus established one hundred thousand Bastarnæ; Constantius Chlorus is said to have made Gaul flourish by the herds of slaves he distributed among the landlords; in 370, large numbers of Alemanni were planted in the valley of the Po, and on the vast spaces of the public domain there were barbarian villages where the native language and customs were preserved. Probably none of these Germans came as freemen. Many, of course, were captives sold as slaves, but perhaps the majority were serfs. Frequently a tribe, hard pressed by enemies, asked leave to pass the frontier, and settle as tributaries, that is to say as _coloni_. On one such occasion Constantius II. was nearly murdered. A body of Limigantes, who had made a raid, surrendered, and petitioned to be given lands at any distance, provided they might have protection. The emperor was delighted at the prospect of such a harvest of labourers, to say nothing of recruits, and went among them to receive their submission. Seeing him alone, the barbarians attacked him, and he escaped with difficulty. His troops slaughtered the Germans to the last man.