Jack London, Dale L. Walker, Jeanne Campbell Reesman (1999). “No Mentor But Myself: Jack London on Writers and Writing”, p.57, Stanford University Press
Charles Darwin (1864). “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”, p.299