Add to this the necessary effect which the free circulation of grain throughout the whole Roman world had in depressing the agriculture of Italy, Gaul, and Greece. They were unable to withstand the competition of Egypt, Lybia, and Sicily--the storehouses of the world; where the benignity of the climate, and the riches of the soil, rewarded seventy or an hundred fold the labours of the husbandman. Gaul, where the increase was only seven-fold--Italy, where it seldom exceeded twelve--Spain, where it was never so high, were crushed in the struggle. The mistress of the world, as Tacitus bewails, had come to depend for her subsistence on the floods of the Nile. Unable to compete with the cheap grain raised in the more favoured regions of the south, the cultivators of Italy and Gaul gradually retired from the contest. They devoted their extensive estates to pasturage, because live cattle or dairy produce could not bear the expense of being shipped from Africa; and the race of agriculturists, the strength of the legions, disappeared in the fields, and was lost in the needy and indolent crowd of urban citizens, in part maintained by tributes in corn brought from Egypt and Lybia. This augmented the burdens upon those who remained in the rural districts; for, as the taxes of each municipality remained the same, every one that withdrew into the towns left an additional burden on the shoulders of his brethren who remained behind. So powerful was the operation of these two causes--the fixity in the state burdens payable by each municipality, and the constantly declining prices, owing to the vast import from agricultural regions more favoured by nature--that it fully equaled the effect of the ravages of the barbarians in the frontier provinces exposed to their incursions; and the depopulation of the rural districts was as complete in Italy and Gaul, before a barbarian had passed the Alps or set his foot across the Rhine, as in the plains between the Alps or the Adriatic and the Danube, which had for long been ravaged by their arms.