To the English of Northumbria, the Saxons of Wessex were almost foreigners.
Even at the present day, when the existence of a recognised literary dialect has done so much to obliterate provincial varieties of speech in England, a Dorsetshire peasant, speaking in a slightly altered form the classical West Saxon of AElfred, has great difficulty in understanding a Yorkshire peasant, speaking in a slightly altered form the classical Northumbrian of Baeda.

But in the ninth century the differences between the two dialects were probably far greater.

On the other hand, though Danish and Anglian have widely separated at the present day, and were widely distinct even in the days of Cnut, it is probable that at this earlier period they were still, to some extent, mutually comprehensible.
Thus, the heathen Scandinavian may have seemed to the Northumbrian and the East Anglian almost like a fellow-countryman, while the West Saxon seemed in part like an enemy and an intruder.

At any rate, the similarity of blood and language enabled the two races rapidly to coalesce; and when the cloud rises again from the North half a century later, the distinction of Dane and Englishman has almost ceased in the conquered provinces.

It is worthy of note in this connection that the part of Mercia afterwards given over by AElfred to Guthrum, was the Anglian half, while the part retained by Wessex was mostly the Saxon half--the land conquered by Penda from the West Saxons two hundred years before.
Nor must we suppose that this first wave of Scandinavian conquest in any way swamped or destroyed the underlying English population of the North.
The conquerors came merely as a "host," or army of occupation, not as a body of rural colonists.