This article is based on a detailed case study of aspects of Darwin's thinking in the course of his construction of the theory of natural selection in 1837 to 1839. The study not only highlights the extended process by which profound analogies may be discovered but also calls attention to the role of contrastive reasoning and bridging analogies in the process. There are some neglected features of Darwin's coming to see and use analogies, such as the way Darwin used the same domain for different purposes as a source of both analogies and contrasts, Darwin's use of sequenced multiple interacting analogies, and Darwin's transformation of an initial negative analogy into a powerful positive analogy. Two domains for comparison inevitably contain many features that can be seen as similarities or differences. What becomes prominent in an analogy and whether it is seen as a similarity or difference is not given a priori but depends on a third factor, such as the current problem under consideration or one's background theory. (Arthur Millman and Carol Smith)