“Darwin spent the twenty years subsequent to the publication of the Origin of Species, as he had spent the preceding twenty years, in study and research, the results of which appeared in nine additional volumes. Three of these perhaps may be singled out as primarily an elaboration of the Origin: {{w|The Variation of Animals and Plants under Domestication}} (1868), The Descent of Man (1871), and {{w|The Expression of the Emotions in Man and Animals|The Expression of the Emotions}} (1872).”