Or at least a version of Brittonic Celtic, the language that was spoken, along with Latin, when the Romans left Britain in the early 400s. Within a relatively short time the whole of what became England, or at least its southern part, was speaking English. We know this because the number of Celtic placenames in southern England is insignificant, replaced almost entirely by English names.
Originally it was assumed this shift resulted from mass immigration and population replacement. Then archaeologists assured us that the number of settlers was very low. But if this were so we might have expected the native language to survive rather than disappear outside Wales and Cornwall. If population replacement was not the reason it must have resulted from a small elite imposing their language on the mass of the native population. This is supposed to have occurred in the main peacefully, despite the frequent references in the Anglo-Saxon Chronicle to warfare between English and Britons. English archaeologists, embarrassed by the possibility that their ancestors may have indulged in ethnic cleansing, were keen to embrace this model.
It was never accepted by placename specialists, who pointed out that the later elite takeover by the Norman-French had very little effect on English placenames. We could also look further afield. The elite British dominance of India after the 1760s didn’t lead to the disappearance of native languages either.
More recently, DNA evidence suggests that the English immigration could have been on a larger scale, amounting to around ten per cent of the native population. Given a reproductive advantage, this proportion could have imposed their DNA within 300 to 450 years, which is coincidentally very similar to the period of time over which the language shift took place.
It looks as if the English settlement of Britain was more like the European takeover of North America. Of course, this didn’t just involve warfare, conflict and the ethnic cleansing of the native Americans, but trading, cooperation at times and intermarriage. Nevertheless, it resulted in a similarly comprehensive takeover and the extermination of several native languages.
The English cultural takeover of Britain came to a shuddering halt when they reached the Tamar, when they appear to have been content to leave the natives using their own tongue in their reservation beyond the river. The second question therefore becomes why didn’t the Cornish of the time speak English (like the Britons in Devon and further east)? Wholesale language replacement ground to a halt in the ninth and tenth centuries and Cornish survived west of the Tamar for another 800 years or so.

Were we just lucky? Had the English lost their enthusiasm for linguistic colonisation by the time they reached the Tamar? Or were the Cornish in those centuries in a position to trade political submission for cultural autonomy? Did their possession of the valuable resource of tin give them a bargaining chip? Furthermore, this speculation suggests that the answer could be political. Maybe the Cornish were more organised than British communities to the east when the English rode over the horizon.
Interesting as ever! Trading is also a reason why some languages come to dominate. Kiswahili for example is widespread across Eastern Africa – the coast and into the interior. It spread primarily (though not only) due to trade and interestingly I was once in a border town (a real dive) where the borders of Uganda, Congo and Rwanda more or less meet, and everyone – despite and because of the multitude of mother tongues – connected through speaking Kiswahili. It wasn’t a mother tongue to anyone. It is likely one element of how English spread was its use for trade – it is a much simpler language than many and can easily be used in a fairly vestigal form to get a lot done.
You also mention ethnic mixing, interesting also. That can also be associated with trade to some degree.
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Another possibility is that English/Frisian was spoken in parts of Britain well before the Roman arrival.
See my paper for the DNA aspect, which suggests that something like 10% of the population in the West Country turned up from the (much extended) Frisian shores 150 BC to 450 AD. https://www.academia.edu/44010171/U106_in_West_Country_Britain_the_Frisian_Diaspora
The Cornish were fortunate – they were always able to cut a deal with potential invaders to supply them with metals. Even the Romans largely stayed out, apart from a few border trading posts.
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I will definitely read your paper. Sounds fascinating.
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A wodhes? Yth entris vy dhe dhiwotti yn Loundres, hag y treylsonz i oll dhe gewsel Sowsnek ha dalleth klappya Y’M KEVER VY!
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This is a very interesting article. An English takeover may explain why some of us with strong documented Cornish ancestry have mainly Scandinavian and North West European DNA.
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If for instance one can trace family to the Lizard peninsula this is where the Danes came to be based. If you go to Cadgwith they still pull there boats up on thick tree branches as was traditionaly done by the Danes
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Fascinating comment. Remember also that many Huguenots came to Cornwall and leave their traces in DNA and and surnames some of which have been anglicized. One vicar of Warleggan was called Daniel Baudris and was a Huguenot. He built the current rectory in 1705 and his nephew Matthew was buried as a Huguenot refugee in 1717. The tombs of both men are still very obvious in the graveyard as they are true tombs next to each other. Baudris also had a secret room built into the cellar.
Just adding to your comment. People who feel 100 percent Cornish now may have Dna from all over and that makes life intriguing.
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