 Most striking in comparing the Origin to the Descent was the strong emphasis on the workings of the secondary process of sexual selection in the animal kingdom (E. Richards 2017; R.A. Richards 2013). Sexual selection—the selection of females by males or vice versa for breeding purposes—had given a general statement of this principle in chapter four of the Origin, but this played a minor role in the original argument, and its importance was denied by contemporaries like A. R. Wallace. Darwin now developed this secondary form of selection in extensive detail as a factor in evolution that could even work against ordinary natural selection. Sexual selection could now be marshaled to explain both sexual dimorphism and also those character and properties of organisms—elaborate feeding organs, bright colors on fish and birds, and seemingly maladaptive structures such as the great horn on the Rhinoceros beetle—, that might appear to be anomalous outcomes of ordinary natural selection working to the optimal survival of organisms in nature. In a dramatic extension of this principle to human beings, the combination of natural and sexual selection is used to explain the origins of the human beings from simian ancestors. It also explains the sexual dimorphism displayed by human beings, and is the main factor accounting for the origin of human races.