The 4th Lord Dalkeith succeeded to his estates upon the resignation of his father c. 1457 and in 1458 was raised to the peerage as Earl of Morton, prior to his marriage to Joanna, the deaf and dumb daughter of King James I. Lord Dalkeith was then a subsidiary title held by the Earls of Morton, and used as a courtesy title for the eldest son and heir, until the title and estates of Dalkeith were sold to the Earl of Buccleuch by the 7th Earl in 1642. When in 1458, James Douglas, lord Dalkeith, was to receive the name 'Morton' for his intended earldom, a protest was presented against this creation, asserting correctly that the lands of Mortoun belonged heritably to his step-grandmother, Janet Borthwick, widow of Sir James Douglas, known as 1st Lord of Dalkeith, and to her son William Douglas (progenitor of the Whittingehame branch of the Douglases), to which the Chancellor answered that "Lord Dalkeith was not to receive his title in the Earldom for the lands of Mortoun lying in the Lordship of Niddisdale but for the lands of Mortoun in the territory of Caldercleir".