Darwin from Descent of Man to Emotions under Domestication" which went on sale on 30 January 1868. He now published his ideas on human evolution and on how beautiful but apparently impractical features could have evolved in "The Descent of Man, and Selection in Relation to Sex". After revising "The Origin of Species" as the definitive 6th edition, his major works on species culminated in "The Expression of the Emotions in Man and Animals". This period was followed by extensive work on insectivorous plants and research into worms. Despite Darwin's concerns that his children were weakened as his wife Emma Darwin was his cousin or had inherited
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