^Oxford University Press (June 21, 2012). Benezit Dictionary of British Graphic Artists and Illustrators. OUP USA. p. 249. ISBN 978-0-19-992305-2.
^J. David Archibald (December 15, 2018). Charles Darwin: A Reference Guide to His Life and Works. Rowman & Littlefield. p. 118. ISBN 978-1-5381-1164-2.