Human uses of mammals that "one is well advised to study the behaviour rather than attempting to get at any underlying emotion". Charles Darwin however had accepted the idea of emotion in animals, writing his 1872 book "The Expression of the Emotions in Man and Animals" on the subject. Darwin believed that mammals had social, mental and moral lives. In "The Descent of Man" (1871), he wrote: "There is no fundamental difference between man and the higher mammals in their mental faculties." Biologists remained wary of the idea, even for the great apes, but this led to serious difficulties, as Donald O. Hebb explained
The Expression of the Emotions in Man and Animals with "ill-temper", sulkiness and determination), Chapter 10 on hatred and anger, Chapter 11 on "disdain, contempt, disgust, guilt, pride, helplessness, patience and affirmation" and Chapter 12 on "surprise, astonishment, fear and horror". In his discussion of the emotion of disgust, Darwin notes its close links with the sense of smell, and conjectures an association with excretory products. In Chapter 13, Darwin discusses complex emotional states including self-attention, shame, shyness, modesty and blushing. Darwin describes blushing as "the most peculiar and most human of the expressions". Darwin closes the book with Chapter 14 in which he recapitulates his main argument: he