Those influences which led to the _Tuath_ with its _Toisech_ passing over into the Thanage and the Thane in the eastern districts were less felt in the more mountainous regions of the north and west, where the power of the Crown was comparatively weak, and more nominal than real, and here the tribe went through a different process. While the large districts continued to be ruled by their _Mormaers_ and the _Mortuath_, and the Province existed intact, there was little of external influence to affect the social organisation of their Celtic population; but the same internal modification which led to the development of the sept or clan from the tribe was no doubt silently at work, and when the break-up of the great provinces and the alienation of the lands of the tribe to feudal lords removed the veil, the clan appears exhibiting in the main the characteristics of the Irish sept. The clan organisation was in the main limited to that part of modern Scotland known as the Highlands and Islands, where the mountainous and rugged character of the former and the comparative inaccessibility of the latter led to the preservation of a population of pure Gaelic lineage, speaking a Gaelic dialect. Here the introduction, by marriage or royal grant, of feudal overlords with apparently feudal holdings was purely nominal. It led to nothing like the Teutonic colonisation which characterised the Lowlands, and neither affected the Gaelic population nor the institution of clanship among them. [Sidenote: The Highland line.] The boundary line which separated the Highlands from the Lowlands, and known as the Highland Line, was in the main an imaginary line separating the Gaelic-speaking people from those using the Teutonic dialect, but it likewise coincides in part with the natural boundaries formed by those physical features of the country which have influenced the relative position of the Gaelic and Teutonic-speaking portion of the population respectively. The southern part of this boundary coincides with the great barrier formed by the mountain range of the Grampians, and where this range is intersected by rivers which take their rise in the interior of the highland region, and flow through this range to the eastern sea, in deep ravines or narrow glens, with high mountains on each side, were narrow passes which formed the entrances into the Highlands, and were easily defended, rendering the country almost inaccessible, while similar passes characterise the northern portion of the line where it crosses the great rivers.