The Evolution of Cooperation insects, where the workers – which are sterile, and therefore incapable of passing on their genes – benefit the queen, who is essentially passing on copies of "their" genes. This is further elaborated in the "selfish gene" theory of Richard Dawkins, that the unit of evolution is not the individual organism, but the gene. (As stated by Wilson: "the organism is only DNA's way of making more DNA.") However, kinship selection works only where the individuals involved are closely related; it fails to explain the presence of altruism and cooperation between unrelated individuals, particularly across species. In a 1971 paper
The Evolution of Beauty preeminent. Prum proceeds to apply the principle of aesthetic evolution as an independent force in human evolution. Prum indicates that Darwin recognizing difficulties in the theory of natural selection advanced the view that sexual selection is an additional force in evolution and presented this argument in "The Descent of Man". Alfred Russel Wallace, St. George Mivart and other early evolutionists helped to elevate natural selection as the only force in evolution and delegated sexual selection to a subsidiary role. It was argued that sexual selection was based on recognition of markers of survival fitness, so that, for instance in birds,