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
Animals' Rights: Considered in Relation to Social Progress Salt is undoubtedly ahead of his age by many years." A new edition of the book was published in 1980 with a preface by the Australian philosopher Peter Singer, who is well known for his work on the ethics of treatment towards animals (specifically in the book "Animal Liberation"). The 1980 reissue prompted a review from Stephen Clark who praised Salt's book with some provisos. He states that Salt's attempt to blame the treatment of non-human animals on the theological doctrine of man having "dominion" over the natural world was mistaken. Animals' Rights: Considered in Relation to Social Progress Animals'