On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, published on 11/24/1859, is at the foundation of evolutionary biology. For the 6th edition of 1872, the short title was changed to The Origin of Species. It introduced the scientific theory that populations evolve over the course of generations thru a process of natural selection. It presented a body of evidence that the diversity of life arose by common descent thru a branching pattern of evolution. Darwin included evidence he'd gathered during the 1830s Beagle expedition & his subsequent findings from research, correspondence & experimentation. The Descent of Man & Selection in Relation to Sex was 1st published in 1871. It was Darwin's 2nd book on evolutionary theory, following his 1859 Origin of Species. The Descent of Man applies evolutionary theory to human evolution, detailing a theory of sexual selection. It discusses many related issues, including evolutionary psychology, evolutionary ethics, racial differences, differences between sexes, the dominant role of women in choosing mating partners & the relevance of the evolutionary theory to society.