There is a common misunderstanding that Scotland is really not one nation, but two, divided into Highlanders and Lowlanders. Others think that there are Norse Scots in the north, Gaelic Scots in the west, and Germanic Scots in the south. To be sure, there are regional differences in Scotland, but they are primarily cultural. Despite the mutli-ethnic origins of Scots, who have roots in ancient Britain, Ireland, Scandinavia, the Low Countries, France, Germany and elsewhere, the Scots are a race defined by Websters Dictionary as 'a family, tribe, people or nation belonging to the same stock'. The proportions of these components may be different in the country's various regions, but basically, this diverse ancestry is shared by all Scots excepting those recently immigrated from other countries who have not yet "married in".
The merging of the seven founding peoples started early in Scottish history, long before 843 A.D. when King Kenneth was able to unite the country because of his Pictish blood and established his fellow Gaels all over Scotland. For some time, thereafter, Gaelic was spoken throughout the country. When the Germanic Angles, who had already absorbed a substantial British population, moved into southern Scotland, the original Scots were not driven out, so even in the Lowlands the Gaelic strain persisted. There are still Gaelic place names down to the Borders; from Cairncross and Auchencrow on the east to Lochenbeck and Auchencairn on the west.
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Many of the great families and clans were of thoroughly mixed blood long before the War of Independence. By that time the Frasers, Hamilitons, Murrays and Stewarts all held both Highland and Lowland lands, and the supposedly "Norman" Cummings, Grahams, Hays and Lindsays were all of largely Gaelic descent. The "British" Galbraiths were already married to Gaels, while Norse-originated families in the west, such as the MacLoeds, McCorquodales, Macdonalds and MacDougalls, had completely integrated into the Scottish population and had Gaelicized their names. The progenitor of the Sutherlands of the far north was a Flemish noble from the Lowlands.
For a while the various elements of this ethnic hodgepodge thought of themselves as different peoples, and as late as 1199 a charter is addressed 'Francis it Englis et Flamingis et Scotis'. But during the War of Independence, the Lowland hero Wallace appeared at Dundee wearing an 'Ersche mantill', that is, a tartan plaid, symbolizing the Gaelic inheritance of all Scotland. By 1320 the signatories to the Arbroath Declaration were all claiming to be, and no doubt were, of a common Scottish heritage despite the mute testimony of their surnames' disclosures of all the various strains for the Scottish nation; Pict and Briton, Gael and Angle, Viking, Norman, and Fleming.
The intermarriage of the founding groups of Scotland has continued up to the time of the Industrial Revolution and beyond, to the point where the Scots can be thought of as one great family. Today in modern Scotland, Lowland names are seen on storefronts throughout Highland towns, while page after page of Gaelic patronymics swell the telephone directories of Edinburgh and Glasgow.

Source: 'The Mark of the Scots' by Duncan A. Bruce