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Charles Darwin (1869). “On the Origin of Species by Means of Natural Selection: Or, The Preservation of Favoured Races in the Struggle for Life”, p.578
Charles Darwin (1860). “Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the command of Capt. Fitz Roy: Tenth thousand”, p.374