McWhorter (2009)

• McWhorter, J. H. 2009. 'What else happened to English? A brief for the Celtic hypothesis', English Language and Linguistics 13 (02), 163-191.

 Abstract:
 "This article argues that despite traditional skepticism among most 
 specialists on the history of English that Brythonic Celtic languages 
 could have had any significant structural impact on English's evolution, 
 the source of periphrastic do in Cornish's equivalent construction is 
 virtually impossible to deny on the basis of a wide range of evidence. 
 That Welsh and Cornish borrowed the construction from English is impossible 
 given its presence in Breton, whose speakers left Britain in the fifth 
 century. 
 The paucity of Celtic loanwords in English is paralleled by equivalent 
 paucity in undisputed contact cases such as Uralic's on Russian. 
 Traditional language-internal accounts suffer from a degree of ad hocness. 
 Finally, periphrastic do is much rarer cross-linguistically than typically 
 acknowledged, which lends further support to a contact account."