Francis Palgrave and the Celtic and Anglo-Saxon racial distribution in Britain: nineteenth century thought and (recent) DNA evidence and its significance

What are the assumptions which have been made about legal and constitutional systems in Britain based upon the racial composition of the nation(s)? How has race been seen to have organised legal and constitutional forms and thought? Up until comparatively recent times our ideas about racial distribution in Britain have been unequivocally controlled by the evidence available, namely the linguistic division between Celtic and Anglo-Saxon / Germanic languages. The starting position, with which we are all too familiar, can be very simply put: in those areas where English is the historically prevailing language the racial make-up of the populace is of Germanic derivation; and in those areas where Celtic languages prevailed, at least until some considerable time into the second millennium AD, and thereafter continuing to exist as diminishing but still viable tongues (that is, in Scotland and Wales, but possibly also Cornwall, at least to some degree), the essential racial composition is Celtic. Because of the absence of any other widespread evidence-base this reasoning was for many years completely plausible and in fact difficult to dispute. The languages, literally, spoke for themselves as racial markers.