Analogy plays an important part in the arguments of Darwin andthe proponents of the wave theory of light. Darwin used the analogy between artificial and natural selection for heuristic purposes,but he also claimed the analogy as one of the grounds for belief inhis theory.23 Huygens, Young, and Fresnel each used the analogiesbetween the phenomena of sound and those of light to support thewave theory of light.24 However, at first sight analogy appears tohave little to do with explanation. Darwin's analogy between artificial and natural selection and Huygens' analogy between soundand light are intended to support the respective theories, but it isnot clear how this is accomplished. I shall argue that the analogiessupport the theories by improving the explanations that the theories are used to give.