Germanic invaders of England came into contact with Celts, speakers of Brythonic, as of the mid 5c AD. As opposed to former views, one now assumes that the Celts were subjugated by the Germanic tribes rather than banished to unpopulated parts of the west of England and Scotland.

The quantitative relationship of Germanic settlers to British Celts in the 5th and 6th centuries is an important consideration. Assuming that the population of Britain in the mid-fifth century was about one million then the following pie charts would indicate the relationship of the new Gemanic settlers to the existing population (which would have included Romanised Celts from the first four centuries AD and other small groups such as the Picts in the far north, present-day Scotland).

By any interpretation, the Germanic settlers formed a minority (small to tiny) of the existing population. Onomastic evidence points to a gradual spread of the Germanic tribes from east to west, with some new settlements, e.g. indicated by placenames ending in -ham, but with many Celtic retentions, e.g. Kent, London, etc. Given the later confinements of Celtic speakers of Brythonic (the Celtic language of Britain which was the precursor to later Welsh and Cornish) to the area of Wales and Cornwall the assumption would appear to be valid that the Celts who lived in the central, eastern and southern areas occupied by the Germanic settlers shifted to their language (later Old English). This means that in later centuries, in the 7th, 8th and 9th centuries the great majority of speakers of Germanic dialects (forms of Old English) were language shifters or their descendants.

Later historians, of an ‘Anglo-Saxonist’ persuasion, favour a purely Germanic lineage for the English and did not entertain the possibility of ethnic and linguistic mixture during the Old English period.

Historians nowadays favour the ‘acculturation theory’ whereby there was a gradual assimilation of the Celts by the Germanic peoples and no major battles or acts of extermination appear to have taken place.

Confirmation of the assimilation view has been forthcoming from other quarters: the Oxford Genetic Atlas Project collected and analysed both matrilinear mithocondrial DNA and patrilinear Y-chromosome samples of over ten thousand subjects from all over Britain and Ireland.

The results provide strong evidence for the survival of the Celtic-speaking population in Britain and Ireland and for the fact that is there is no clear delimitation of a genetic group deriving from the Germanic settlers and one from the Celtic population of Britain. Ethnic, and hence linguistic, mixture in the early Old English period is the only conclusion from this.

It is important to note that the Old English lived together with the Celts. The children of both groups would have grown up together so that the Old English language learners would have been affected during unconscious first language acquisition by the speech of the Old English-speaking Celts surrounding them. That way features of pronunciation and grammar could have been transferred from the substrate Celts to the superstrate Old English, spoken by both descendants of the original settlers and those of Celts who shifted to Old English.

Contact as the sole source?

Feature 1: Internal possessor construction (use of possessive pronouns with parts of the body, i.e. with cases of inalienable possession).

The case of inalienable possession is probably the clearest example of contact-induced change in Old English deriving form Celtic. Consider that other Germanic languages do not demand the use of possessive pronouns before parts of the body, e.g. German Mir tut der Kopf weh ‘My head is sore’, lit.‘me.DAT does the head hurt’. However, in English and in Celtic possessive pronouns are obligatory here, i.e. the external possessor constructions (with a pronoun in the dative) disappeared entirely.

Feature 2: The use of the progressive for duration

The rise of the progressive, as in She is talking to her neighbour, is not such a simple matter. In Old English there was already a structure which involved a gerund (a nominal form of the verb) where one could say something like 'He was on reading'. It is only a small step from this to 'He was reading' in the modern progressive sense, so an internal explanation is possible.

What one must also entertain is the possibility of con-vergence, i.e. the interplay of both internal developments within Old English and transfer from Celtic in the rise of the progressive.

Feature 3: Identical forms for intensifiers and reflexives

In present-day English (and Celtic languages) intensifiers and reflexives have the same form, e.g. The mayor himself held the speech and He cut himself with the razor.

This is a typologically unusual feature, not present in other European languages. It is true that Old English lost the s-reflexives (like German sich) but that does not explain why intensifiers and reflexives ended up with the same formal expression.

It-clefts are a common feature of Modern English and would appear to have their origin in the Old English period if examples like the following (with clefting in a relative clause) can be taken as evidence

    Þa cwæð Iohannes to Petre þæt hit wære se hælend þe on ðam strande stod;
    ‘Then said Johannes to Peter that it was the Saviour who stood on that strand;’  
     (Ælfric’s Homilies)

Such clefting is also found in the Celtic languages and there may have been some influence of these on the rise of this feature in English. In addition those modern varieties of English in contact with Celtic languages (Welsh, Scottish and Irish English) do indeed show a wide range of clefting possibilities.

Clefting in main clauses

    It’s to Galway he is gone for the weekend.
    It’s early in the morning they gets up in the summer.
    It’s out of his mind the man is.

So not the phenomenon itself - it-clefting - may well be an internal development in English, but its range may be due to contact with Celtic speakers who had more clefting, or, importantly it may be due to shifters from Celtic to Old English who transferred their range of this phenomenon from Brythonic (the Celtic language in England before 1000 AD) to their English which they initially spoke as a second language.

Authors working on Celtic, like the Finnish scholars Markku Filppula and Juhani Klemola, have noted the similarity between English and the Celtic languages in their use of word-final prepositions (or verbal particles depending on interpretation) and postulate Celtic influence in English.

However, such elements occur in word-final position in languages not in contact with Celtic, e.g. Swedish and the other Scandinavian languages, so it may be a phenomenon which developed under the influence of Old Norse in England.

English	That’s something which I know about.
Celtic (Irish)	Sin rud nach bhfuil fios agam faoi.
English	That is something which we will look at.
Swedish	Det är något som vi ska titta på.