The Expression of the Emotions in Man and Animals public by arguing that humans had "animal traits", and instead charmed them by telling stories of "human traits in animals", thus avoiding too much explicit talk of natural selection at work. Darwin preferred to leave the evolutionary implications hanging. Korn points out that the book has never been out of print since 1872, calling into question Ekman's talk of "Darwin's lost masterpiece". The "Editor's notes" at the "Mead Project source page" on the book comment that Darwin concluded work on the book with a sense of relief. The proofs, tackled by his daughter Henrietta ("Ettie") and son Leo, required a
The Expression of the Emotions in Man and Animals with engravings of the Darwin family's domestic pets by the zoological illustrator T. W. Wood as well as work by the artists Briton Rivière, Joseph Wolf and A.D. May. It also included portraits by the Swedish photographer Oscar Rejlander (1813–1875), anatomical diagrams by Sir Charles Bell (1774–1842) and Friedrich Henle (1809–1885), as well as illustrational quotations from the "Mécanisme de la physionomie humaine" (1862) by the French neurologist Duchenne de Boulogne (1806–1875). As a result of his domestic psychology experiments, Darwin reduced the number of commonly observed emotions from Duchenne's calculation of more than sixty facial expressions, to just six