 His history of The Great Trek (1934) was couched in the narrative of a romantic adventure and he later deemed it to be his best work.In 1930, Walker gave an influential lecture in Oxford, printed as "The frontier tradition in South African history" (Oxford University Press, London, 1930), in which he outlined his theory that the origins of the apartheid system in South Africa lay in conflict between blacks and whites on the frontier regions in the nineteenth century which was then imported into the interior where it was institutionalised in the constitutions of the Orange Free State and the South African Republic.Walker's theory owed much to Frederick Jackson Turner and "The Oxford History of Historical Writing" described him as "in some respects the George Stanley of South Africa".