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.
Charles Darwin, Sir Francis Darwin, Paul H. Barrett, R. B. Freeman (1990). “The Works of Charles Darwin, Volume 23: The Expression of the Emotions in Man and Animals”, p.275, NYU Press
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.174, NYU Press