Philip Parker King, Sir Francis Darwin, Robert Fitzroy, Charles Darwin (1839). “Narrative of the Surveying Voyages of His Majesty's Ships Adventure and Beagle: Between the Years 1826 and 1836 ...”, p.514
Charles Darwin (2007). “On the Origin of Species: By Means of Natural Selection Or the Preservation of Favored Races in the Struggle for Life”, p.303, Cosimo, Inc.
"The Descent of Man". Book by Charles Darwin, volume I, chapter II: "Comparison of the Mental Powers of Man and the Lower Animals", p. 40, darwin-online.org.uk. 1871.
Charles Darwin (2010). “The Works of Charles Darwin, Volume 29: “Erasmus Darwin” by Ernest Krause, with a Preliminary Notice by Charles Darwin; “The Autobiography of Charles Darwin” Edited by Nora Barlow; and Consolidated Index”, p.77, NYU Press