Charles Darwin (1861). “On the Origin of Species by Means of Natural Selection; Or, The Preservation of Favoured Races in the Struggle for Life”, p.167
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 (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.219, NYU Press
Charles Darwin, James T. Costa (2009). “The Annotated Origin: A Facsimile of the First Edition of On the Origin of Species”, p.62, Harvard University Press