The following section was omitted from the published paper.
This work was made possible by funding to the Darwin Correspondence Project from the Arts and Humanities Research Council through an international collaborative research grant with the National Science Foundation (NSF award no. SES-0957520). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. The author is grateful to Graham Richards for copies of two unpublished papers, and to Cambridge University Library and the Darwin family for access to the letters of Charles Darwin, his wife and children.
- © 2014 The Author(s) Published by the Royal Society.