1. DCP numbers are the references to letters in the Darwin Correspondence Project: http://www.darwinproject.ac.uk .
2. Barlow, Nora, ed. 1958. The autobiography of Charles Darwin 1809–1882. London: Collins.
3. Bowler, Peter J. 2003. Evolution, the History of an Idea, 3rd ed. Berkeley: University of California Press.
4. Black, Jeremy. 2008. What If? Counterfactualism and the Problem of History. London: The Social Affairs Unit.
5. Browne, Janet. 1995. Charles Darwin, Voyaging. Princeton: Princeton University Press.