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 initial correspondence with the psychiatrist James Crichton-Browne Darwin set aside his material concerning emotional expression in order to complete "The Descent of Man", which covered human ancestry and sexual selection. Darwin concluded work on "The Descent of Man" on 15 January 1871. Two days later, he started on "The Expression of the Emotions" and, on 22 August 1872, he finished work on the proofs. In this book, Darwin brings his evolutionary theory into close approximation with behavioural science, although many Darwin scholars have remarked on a kind of spectral Lamarckism haunting the text of the "Emotions". Universal Nature of Expression: