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Georg Schweinfurth, The Heart of Africa, Vol. II, chap. XI, 3rd edition, London.
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Sir T.H. Holdich, India, pp. 202-203, map. London, 1905.
Darwin, Origin of Species, Vol. II, chap. XII. New York, 1895.
Anatole Leroy-Beaulieu, The Empire of the Tsars, Vol. I, pp. 66-70, maps facing pp. 64 and 80. New York, 1893.