Excerpts from Books and Wikipedia
"The longest established were the Celtic Britons, who inhabited the whole of Britain south of the Forth-Clyde isthmus, and the Picts, who lived in the highlands and islands to the north. The main difference between the two was that the Britons had become Romanized during the Roman occupation of Britain (43-410), whereas the unconquered Picts had not. After the end of Roman rule in 410 Britain was invaded by Anglo-Saxons from northern Germany and by the Irish, who at this time were known as Scots. The Anglo-Saxons occupied southeast Scotland as far north as Edinburgh, and the Scots occupied the west coast district of Argyll, founding the kingdom of Dalriada. After the Picts had been weakened by Viking attacks in the early ninth century, they were conquered in 843 by Kenneth MacAlpin, the king of Dalriada. . . . However, it was only in the eleventh century, with the conquest of Anglo-Saxon and British territory south of the Forth-Clyde isthmus, that Scotland achieved (approximately) its modern borders." [Furtado: 1001 Days]