But a kingdom may have too much hinterland. Scotland taxed for centuries the assimilative capacity of united England; it was too much for Northumbria to digest. Northumbria's supremacy was distinguished by the religious labours of Aidan and Cuthbert and Wilfrid in England, by the missions of Willibrord on the Continent, and by the revival of literature and learning under Caedmon and Bede; but it spent its substance in efforts to conquer Scotland, and then fell a victim to the barbaric strength of Mercia and to civil strife between its component parts, Bernicia and Deira. Mercia was even less homogeneous than Northumbria; it had no frontiers worth mention; and in spite of its military prowess it could not absorb a hinterland treble the size of the Wales which troubled Edward I. Wessex, with serviceable frontiers consisting of the Thames, the Cotswolds, the Severn, and the sea, and with a hinterland narrowing down to the Cornish peninsula, developed a slower but more lasting strength. Political organization seems to have been its forte, and it had set its own house in some sort of order before it was summoned by Ecgberht to assume the lead in English politics. From that day to this the sceptre has remained in his house without a permanent break. Some slight semblance of political unity was thus achieved, but it was already threatened by the Northmen and Danes, who were harrying England in much the same way as the English, three centuries earlier, had harried Britain. The invaders were invaded because they had forsaken the sea to fight one another on land; and then Christianity had come to tame their turbulent vigour. A wave of missionary zeal from Rome and a backwash from unconquered Ireland had met at the synod of Whitby in 664, and Roman priests recovered what Roman soldiers had lost. But the church had not yet armed itself with the weapons of the world, and Christian England was no more a match than Christian Britain had been for a heathen foe. Ecgberht's feeble successors in Wessex, and their feebler rivals in the subordinate kingdoms, gave way step by step before the Danes, until in 879 Ecgberht's grandson Alfred the Great was, like a second King Arthur, a fugitive lurking in the recesses of his disappearing realm.