The islands which are now known as the kingdom of Great Britain and Ireland were, at the opening of the Christian era, occupied by several Celtic peoples of whose customs and religion we know almost nothing. Julius Cæsar commenced the conquest of the islands (55 B.C.); but the Romans never succeeded in establishing their power beyond the wall which they built, from the Clyde to the Firth of Forth, to keep out the wild Celtic tribes of the North. Even south of the wall the country was not completely Romanized, and the Celtic tongue has actually survived down to the present day in Wales. [Sidenote: Saxons and Angles conquer Britain.] At the opening of the fifth century the barbarian invasions forced Rome to withdraw its legions from Britain in order to protect its frontiers on the continent. The island was thus left to be gradually conquered by the Germans, mainly Saxons and Angles, who came across the North Sea from the region south of Denmark. Almost all record of what went on during the two centuries following the departure of the Romans has disappeared. No one knows the fate of the original Celtic inhabitants of England. It is unlikely that they were, as was formerly supposed, all killed or driven to the mountain districts of Wales. More probably they were gradually lost among the dominating Germans with whom they merged into one people. The Saxon and Angle chieftains established petty kingdoms, of which there were seven or eight at the time when Gregory the Great became pope. [Sidenote: Conversion of Britain.] Gregory, while still a simple monk, had been struck with the beauty of some Angles whom he saw one day in the slave market of Rome. When he learned who they were he was grieved that such handsome beings should still belong to the kingdom of the Prince of Darkness, and, had he been permitted, he himself would have gone as a missionary to their people. Upon becoming pope he sent forty monks to England from one of the monasteries that he had founded, placing a prior, Augustine, at their head and designating him in advance as Bishop of England. The heathen king of Kent, in whose territory the monks landed with fear and trembling (597), had a Christian wife, the daughter of a Frankish king. Through her influence the monks were kindly received and were assigned an ancient church at Canterbury, dating from the Roman occupation before the German invasions. Here they established a monastery, and from this center the conversion, first of Kent and then of the whole island, was gradually effected. Canterbury has always maintained its early preëminence and may still be considered the religious capital of England.[33] [Illustration: Ancient Church of St. Martin's, Canterbury] [Sidenote: The Irish monks.]