Ekman, Paul. 1998. Introduction, afterword, and commentary to the third edition of The expression of the emotions in man and animals, by Charles Darwin. London: HarperCollins Publishers.
Prodger, Phillip. 2009. Darwin’s camera: art and photography in the theory of evolution. Oxford: Oxford University Press.
Snyder, Peter J., Rebecca Kaufman, John Harrison, Paul Maruff. 2010. Charles Darwin’s emotional expression “experiment” and his contribution to modern neuropathy.Journal of the the Neurosciences, 19: 158–70.
Darwin Correspondence Project
Cambridge University Library