Darwin invites controversy by bringing human beings into the classification system. When he compares the structure of hands to similar organs in bats, moles, and porpoises, he is essentially claiming that humans evolved from other species the same way animals did. He implies, for example, that the hands of human beings were a variation of the front legs of the mole and were “naturally selected” in the human population because they helped humans grasp rather than dig. This argument remains one of the most controversial aspects of Darwin’s theory, as well as of evolutionary theory in general. In addition, in his discussion of classification rules, Darwin uses the example of physical similarities of the Hottentot group (an ethnic group discovered in Africa in the seventeenth century) to the “Negro race” to explain how physical similarities relate to classification. It was widely assumed among scientists in nineteenth-century Europe that “Negroes” were ancestors of the Hottentot, and although Darwin does not specifically say that different races of human beings evolved from one another, his use of the Hottentot/Negro example implies that all races of human beings are evolutionarily linked. According to some readings of Darwin, the implication is one of evolutionary human hierarchy: The theory that some humans have progressed further evolutionarily than others. Social theorists would later construct human hierarchies based on Darwin’s theory of evolution to justify discrimination, repression, and even genocide.