Generally the interests and necessities of the young states introduce a prohibitory system to exclude the manufactures of the old one; and it is this necessity which England is now experiencing, and vainly endeavours to obviate, by introducing a system of free trade. But in one memorable instance, and one only, the preponderance of a particular power rendered this impossible, and illustrated on a great scale, and over the whole civilized world, for a course of centuries, the effects of a perfect freedom of trade. The Roman empire, spreading as it did round the shores of the Mediterranean, afforded the utmost facilities for a great internal traffic; while the equal policy of the emperors, and indeed the necessity of their situation, introduced a perfect freedom in the interchange of commodities between every part of their vast dominions. And what was the result? Why, that the agriculture of Italy was destroyed--that 300,000 acres in the champaign of Naples alone reverted to a state of nature, and were tenanted only by wild-boars and buffaloes, before a single barbarian had crossed the Alps--that the Grecian cities were entirely maintained by grain from the plains of Podolia--and the mistress of the world, according to the plaintive expression of the Roman annalist, depended for her subsistence on the floods of the Nile.[20] Not the corruption of manners, not the tyranny of the Caesars, occasioned the ruin of the empire, for they affected only a limited class of the people; but the practical working of free trade, joined to domestic slavery, which destroyed the agricultural population of the heart of the empire, and left only effeminate urban multitudes to contend with the hardy barbarians of the north. [20] Tacitus, Vol. xiv. p. 21; Michelet's _Hist. de France,_ Vol. i. p. 217.