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Charles Darwin, James T. Costa (2009). “The Annotated Origin: A Facsimile of the First Edition of On the Origin of Species”, p.2, Harvard University Press
Charles Darwin (1869). “On the Origin of Species by Means of Natural Selection: Or, The Preservation of Favoured Races in the Struggle for Life”, p.578
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
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