(Here, Davies compares two former Roman
provinces – Britannia (present-day
[T]he net product in the former Britannia
was very different from that, for instance, in the former
The simplest answer is extermination. Celtophiles love to relate how the murderous Saxons massacred the defenceless Britons and supposedly wiped them out. The word genocide is used. It is completely out of place. Though atrocious massacres did occur, both of civilians and of churchmen, as at Anderida in Sussex in 491 or on the even of the Battle of Chester in 616, there is plentiful evidence that the bulk of the British population continued to live on under Germanic rule, and to speak their own language. The law code of Ine, King of Wessex, from the late seventh century, for instance, makes special provision for the British still living in his domain.
A second answer invokes numbers. The native Celts were supposedly swamped by the overwhelming tide of Germanics. This too, is unlikely. Though fifth-century boat convoys might bring in enough migrants to fill the initial colonies, it is unthinkable that they could have repopulated the entire country. Apart from that, there is every indication that repopulation did not take place. Modern generic research is showing quite convincingly that the Germanic invasions, like the Celtic invasions before them, were insufficient to transform the existing gene pool to any major degree.
A third answer concerns the prevailing linguistic patterns in late Roman Britannia. The Germanics were moving into the most heavily Romanised regions of the south-east where Latin, not Brythonic [a Celtic language], was the main language. Celtic survivals there would naturally be less marked than in other regions where Brythonic had not been so seriously undermined. This makes sense.
Two
further factors may have had some impact. The bubonic plague which devastated
(Norman
Davies, The Isles: A History [Oxford University Press, 2000], pp.
195–196)