Charles Darwin first expressed his ideas on sexual selection and mate choice in his book "The Descent of Man, and Selection in Relation to Sex" in 1871. He was perplexed by the elaborate ornamentation that males of some species have, because such features appeared to be detrimental to survival and to have negative consequences for reproductive success. Darwin proposed two explanations for the existence of such traits: these traits are useful in male-male combat or they are preferred by females. This article focuses on the latter. Darwin treated natural selection and sexual selection as two different topics, although in the 1930s biologists defined sexual selection as being a part of natural selection.