Charles Darwin's views on women were based on his view of natural selection. Darwin believed that the difference between males and females were partly due to "sexual selection". Darwin's theory of sexual selection, which can be found in his book The Descent of Man and Selection in Relation to Sex, states that women, and some men, will choose to mate with someone that is most suitable to culture. This proposition of sexual selection readily tied into his theory of natural selection in the way that evolution will have different outcomes depending on the traits of the suitor the females chooses to reproduce with. This also supports his principle of "survival of the fittest" in the human species.