Applying these results especially to England, we [p417] have once more to remember that there was settled agriculture in Belgic Britain before the Roman invasion: that the fact vouched for by Pliny, that _marl_ and _manure_ were ploughed into the fields, is proof that the simplest form of the open-field system--the Welsh co-aration of the waste, and the German shifting every year of the '_arva_'--had already given place to a more settled and organised system, in which the same land remained under tillage year after year. Pliny's description of the marling of the land, however, points rather to the _one-field system_ of Northern Germany than to the three-field system, as that under which the corn was grown which Cæsar found ripening on British fields when he first landed on the southern coast.[631] «Roman introduction of the three-course rotation of crops.» In the meantime Roman improvements in agriculture may well have included the introduction into the province of Britain of the three-course rotation of crops. The open fields round the villa of the Roman lord, cultivated by his slaves, 'coloni,' 'tributarii,' and 'liti,' may have been first arranged on the three-field system; and, once established, that system would spread and become general during those centuries of Roman occupation in which so much corn was produced and exported from the island. The Roman _annonæ_--founded, perhaps, on the earlier tribal food-rents--were, in Britain, as we know from the 'Agricola' of Tacitus, taken mostly in corn; [p418] and the _tributum_ was probably assessed during the later empire on that system of _jugation_ which was found to be so like to the _hidation_ which prevailed after the Saxon conquest. «Conquest the rule. The invaders become lords of _hams_ manors.» Putting aside as exceptional the probably peaceful but at best obscure settlements in tribal households, and regarding conquest as the rule, the economic evidence seems to supply no solid reason for supposing that the German conquerors acted in Britain in a way widely different from that which they followed on the conquest of Continental Roman provinces. The conquered territory here as elsewhere probably became at first _terra regis_ of the English, Saxon, or Jutish kings. And though there may have been more cases in England than elsewhere of extermination of the old inhabitants, the evidence of the English open-field system seems to show that, taking England as a whole, the continuity between the Roman and English system of land management was not really broken. The Roman provincial _villa_ still seems to have remained the typical form of estate; and the management of the public lands, now _terra regis_, seems to have preserved its manorial character. For whenever estates are granted to the Church or monasteries, or to thanes of the king, they seem to be handed over as already existing manors, with their own customs and services fixed by immemorial usage.