As time went on, the labors of St. Patrick were lightened by the arrival of the bishops Secundinus, Auxilius, and Isserninus, whom he had sent either to France or Britain to receive consecration. Their coming enabled him to extend the sphere of his operations, and he undertook missionary tours in Meath, Leinster, Ossory, and Munster. These continued for several years, during which he was occupied in preaching the word, baptizing new converts, and erecting churches. Knowing well how much his own acquaintance with the native language had contributed to his success, he labored diligently to establish a native ministry wherever he went. Cautiously selecting from the higher classes those whose piety and intelligence seemed to fit them for the work of the ministry, he established seminaries and monastic schools, where they were trained and educated; and to these schools the young of both sexes flocked with extraordinary eagerness. While he was laboring in the southeastern part of Munster, a petty prince of Cardiganshire, named Coroticus, though apparently professing Christianity, set out from Wales, and descending on the Irish coast with a band of armed followers, murdered several of the people, and carried off a large number with the intention of disposing of them as slaves. This outrage, perpetrated in one of the districts where St. Patrick was baptizing, roused his keenest indignation, and he wrote a letter, which he sent by one of his companions, calling upon Coroticus to restore the captives, many of whom had been baptized. But his request being treated with contempt and scorn, he composed another circular epistle, in which he inveighed in the strongest terms against the cruelty of the marauding tribe and its chief. He contrasted his conduct with that of the Christians of the Continent, who were in the habit of sending large sums of money to ransom captives, and concluded by threatening him and his followers with excommunication, unless he desisted in future from his piratical habits. What was the result of the epistle is not known, but it is to be feared that the attempt to recover the captives was not successful. Slavery and the trade in slaves was almost more difficult to root out than paganism, and the inhuman traffic was in full activity as late as the tenth century between England and Ireland, and the port of Bristol was one of its principal centres.