H. H. Johnston, E. Torday, T. Athol Joyce, C. G. Seligmann, A Survey of the Ethnography of Africa: And the Former Racial and Tribal Migrations in That Continent., The Journal of the Royal Anthropological Institute of Great Britain and Ireland, Vol. 43 (Jul. - Dec., 1913), pp. 375-421
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Johnston et al. (1913) studied this question.