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 livestock breeders and pigeon fanciers; on observations on his infant son William Erasmus Darwin ("A Biographical Sketch of an Infant" - published as a paper in the philosophical journal "Mind"), on his family's dogs and cats, and on the orangutans at London Zoo; on simple psychology experiments with members of his family concerning the recognition of emotional expression; on the neurological insights of Duchenne de Boulogne, a physician at the Salpêtrière asylum in Paris; on hundreds of photographs of actors, babies and children; and on descriptions of psychiatric patients in the West Riding Pauper Lunatic Asylum at Wakefield in