1. Bellanger, T. 1977. Bibliography Defined, in Book Collecting: A Modern Guide, ed. Jean Peters, 97–101. New York, NY: R. R. Bowker. Excerpted by The Bibliographical Society of America. https://bibsocamer.org/publications/bibliography-defined.
2. Bordalejo, B. 2009. Introduction to the Online Variorum of Darwin’s Origin of Species. John van Wyhe, ed. 2002–. The Complete Work of Charles Darwin Online. https://darwin-online.org.uk/Variorum/Introduction.html.
3. Browne, J. 2006. Darwin’s Origin of Species: A Biography. London: Atlantic Books.
4. Cannon, F.W. 1961. Two Letters from John Herschel to Charles Lyell, 1836–1837. Proceedings of the American Philosophical Society 105: 301–314.
5. Carpenter, W.B. 1860. Review V. British and Foreign Medico-chirurgical Review 25: 367–404. Internet Archive: https://archive.org/details/britishandforei50unkngoog/page/n460.