Charles Darwin, James T. Costa (2009). “The Annotated Origin: A Facsimile of the First Edition of On the Origin of Species”, p.489, Harvard University Press
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 (2010). “The Works of Charles Darwin, Volume 28: The Formation of Vegetable Mould Through the Action of Worms With Observations on Their Habits”, p.43, NYU Press
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