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 (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 (1866). “On the origin of species ... A facsimile of the first edition, with an introduction by Ernst Mayr. With a portrait and a bibliography”, p.370