"‘The temptation to read every cline on a map of genetic features as a migration and tie it to a putative linguistic movement has led to ostensibly circular reasoning. … [T]here is an assumed correlation between language and human physical type. … [But] there is no requirement whatsoever that the trail of language shift should also leave a clearly defined genetic trail as well. Nor for that matter can we assume that if we do find a genetic trail, this necessarily resulted in a language shift favourable for those carrying the gene rather than their absorption by local populations’"