The analogy between artificial selection and natural selection forms a powerful rhetorical component of Darwin's argument in On the Origin of Species. But the analogy, which compares the evolution of wild animals to that of their domesticated relatives, is far from complete. The combination of similarity and difference that made the comparison of wild and domesticated animals both effective and problematic for Darwin was not new with him, and it has continued to the present day. Indeed, as the impact of human beings throughout the environment has become more pervasive, the reciprocal resonance of these categories has similarly increased. Harriet Ritvo's project explored the history of this binary since the eighteenth century, during which period the relationship between its terms has remained constant, while their denotations and connotations have shifted—especially as embodied in practices including breeding, acclimatization, species protection, and landscape preservation.