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 induce directly opposing movements. Finally, he discusses a "direct action of the nervous system", in which an overflow of emotion is widely discharged, producing more generally expressed emotions. This is followed by a section (three more chapters) on modes of emotional expression peculiar to particular species, including man. He then moves on to the main argument with his characteristic approach of astonishingly widespread and detailed observations. Chapter 7 discusses "low spirits", including anxiety, grief, dejection and despair; and the contrasting Chapter 8 "high spirits" with joy, love, tender feelings and devotion. Subsequent chapters include considerations of "reflection and meditation" (associated