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Charles Darwin, R. C. Stauffer (1987). “Charles Darwin's Natural Selection: Being the Second Part of His Big Species Book Written from 1856 to 1858”, p.279, Cambridge University Press
Charles Darwin (2010). “The Works of Charles Darwin, Volume 7: The Geology of the Voyage of the H. M. S. Beagle, Part I: Structure and Distribution of Coral Reefs”, p.3, NYU Press
Charles Darwin, James T. Costa (2009). “The Annotated Origin: A Facsimile of the First Edition of On the Origin of Species”, p.6, Harvard University Press