The three marriages were intended to secure to Malcolm the south, the middle, and the north of Pictland through the fathers of Duncan, Macbeth, and Thorfinn respectively; and we may note that from Thorfinn are descended all subsequent Jarls and Earls of Orkney and Shetland and Caithness of the so-called Norse line.

Duncan I, Macbeth, and Thorfinn Sigurd’s son were thus first cousins, and, in spite of the fiction of Holinshed, Boece, and William Shakespeare, they were all about the same age, being born within seven years of each other; and none of them lived to old age.

By the victory of Carham in 1018 Malcolm II secured for ever the line of the Tweed as Scotland’s southern frontier; and this success in the south, one of the most important events in Scottish history, left him free to extend his kingdom and sovereignty towards the north, his object being to unite into one realm the whole mainland at least of Scotland.  To accomplish this, he would have to bring under the supremacy of the Scottish crown in addition to the Picts of Atholl, whom the Scots had absorbed, the Gallgaels of Argyll, the Picts of Moray and of Ross within and beyond the Grampians, and those of the province of Cat, with the Norsemen there as well.  He could thus ultimately hope to oust Somarled, Brusi and Einar, Jarl Sigurd’s sons by his first wife, and their overlords, the Norse kings, from Orkney and Shetland, and to add those islands to his dominions.  Meantime, Somarled, Brusi and Einar took no share in Cat.  Thorfinn had Cat, all for himself, as a fief of the Scottish king.

Although the history of the time of Thorfinn Sigurdson, the first Scottish Earl of Caithness and Sutherland,[4] would have been of great interest to inhabitants of those counties, the Orkneyinga Saga contains but little information about his doings in them, because he bent all his efforts towards extending his dominion over the islands which formed his father Sigurd’s jarldom, his policy, in his youth at least, being directed to this object by his grandfather, Malcolm II.  Indeed during the life of that king, Thorfinn appears to have established himself at Duncansby in Caithness, on the shore of the Pentland Firth, and to have occupied himself in endeavouring to induce his three surviving half-brothers, Somarled, Brusi, and Einar, to part with as large a share as possible of Orkney and Shetland, and cede it to himself.  In this he had much assistance from King Malcolm.  Thorfinn, whose mother probably died in his infancy if we are to credit his father’s matrimonial stipulations as regards an Irish wife in 1014, succeeded to the earldom and lands in that year, as a boy of about six years of age, and was early in coming to his full growth, the “tallest and strongest of men; his hair was black, his features sharp, his brows scowling, and, as soon as he grew up, it was easy to see that he was forward and grasping.”  From the description given in the Saga at Chapter 22, he was no more a Norseman in appearance than he was by blood.  He was, in fact, by race and descent, almost a pure Gael, and at Malcolm’s court must have spoken only Gaelic.