 Eventually, polygenesis declined through the intellectual success of Charles Darwin’s theory of evolution (Brace 2005, 124). Darwin himself weighed in on this debate in the chapter “On the Races of Man” in his book The Descent of Man (1871), arguing that as the theory of evolution gains wider acceptance, “the dispute between the monogenists and the polygenists will die a silent and unobserved death” (Bernasconi and Lott 2000, 68), with the former winning out. The rest of the essay entertained both sides of the debate regarding whether or not different races constitute different species or sub-species of humans. Although Darwin did not explicitly take sides in this debate, the preponderance of his argument gives little support to the idea of races being different species. For instance, he noted that couples from different races produce fertile offspring, and that individuals from different races seem to share many mental similarities. That said, while Darwinian evolution may have killed off polygenesis and the related idea that the races constituted distinct species, it hardly killed off race itself. Darwin himself did not think natural selection would by itself generate racial distinctions, since the physical traits associated with racial differences did not seem sufficiently beneficial to favor their retention; he did, however, leave open a role for sexual selection in the creation of races, through repeated mating among individuals with similar traits (Bernasconi and Lott 2000, 77–78). Consequently, later race thinkers would replace polygenesis with natural selection and sexual selection as scientific mechanisms whereby racial differentiation could slowly, unintentionally, but nevertheless inevitably proceed (Hannaford 1996, 273).