In addition to a frequently original study of the development of Darwin's psychological theories, the authors publish for the first time two extremely significant manuscript notebooks by Darwin written in 1837. These notebooks, which the authors supplement with notes and commentary, provide the earliest available insight into the origins of Darwin's views on human and comparative psychology. Some of the ideas they record were later developed in The descent of man and The expression of emotions in man and animals. Also the notebooks prove something that Darwin never publically admitted: the belief that man and the great apes descended from a common ancestor was the core of his theory of human evolution as early as 1837. Furthermore, the notebooks show that Darwin recognized the relationship between animal and human sexual behavior during the height of Victorian prudery. No wonder he never developed in print such theories as "Our descent, then is the origin of our evil passions!!− The Devil under form of Baboon is our grandfather!" (M. Notebook, p. 123).