Analogy plays an important part in the arguments of Darwin and the 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 in his theory.Huygens, Young, and Fresnel each used the analogies between the phenomena of sound and those of light to support the wave theory of light. However, at first sight analogy appears to have little to do with explanation. Darwin's analogy between artificial and natural selection and Huygens' analogy between sound and light are intended to support the respective theories, but it is not clear how this is accomplished. I shall argue that the analogies support the theories by improving the explanations that the theories are used to give.