Again, if the Brythons or Belgic Celts, armed with iron, were able to cross the channel and displace the western Celts in Britain, it would surely have been much easier for them to cross the Marne and the Seine and displace the western Celts in Gaul. The theory seems to presuppose that an invasion was necessary to bring the Iron Age into Britain, but the same theory would have it that the Iron Age found its way into Ireland without any invasion, for it leaves the Bronze Age Goidels of Ireland to learn the use of iron in some more pleasant way than by meeting iron-headed spears in the hands of Belgic conquerors. It is certain that after the withdrawal of the Roman legions from Britain and after the Anglo-Saxon invasions, there were Gaelic populations in various parts of western Britain, in Argyllshire, North and South Wales, and the Cornish peninsula. Rhys supposed these to be the remnants of the Gaelic population which, in his view, had occupied all England during the Bronze Age. There is sufficient evidence to show that they were fresh settlements made by the Irish of Ireland during and after the collapse of the Roman power in Britain. The "P and Q" element in the theory is equally unsound. It is certain that, where the Irish Celts retained the consonant Q in their language, the British Celts replaced it by P. But no such distinction has been shown to have existed between the language of the western Celts and the language of the Belgic Celts on the Continent. Such phonetic changes as the substitution of P for Q spread in an almost mysterious way through languages. Their spread may be arrested by a geographical barrier so considerable as the Irish Sea, but it was not at all likely to have been brought to a stand by the waters of the Seine and Marne. Nor can a phonetic change of the kind be taken as necessarily corresponding to any racial or political boundaries. In all the western dialects of Latin which grew into the Romance languages, the initial W of Germanic words was changed into GW, and this identical change also took place in the Welsh language, but not in Irish. It took place in Spanish, yet that does not appear to prove that the Welsh are more near akin to the Spaniards than they are to the Irish, nor, if history happened to be silent, would it prove that Britain after the Roman occupation was peopled by a Spanish invasion which did not extend to Ireland.