In addition to integration, integrative promise also invokes a number of other explanatory virtues. In particular, because integrative knowledge in biology is extremely difficult to come by, promise involves a trade-off between what I will call stringency and opportunism. Stringency refers to the idea that integrative explanations need to be put to severe tests, given the challenges that we face in attaining integration. But at the same time, opportunism describes the tendency of biologists to rely on explanations that use readily available materials, model systems, bodies of theory, and so forth, as a way to make integrative research pragmatically more feasible. In this chapter, I explore this trade-off by looking at the late works of Darwin, who spent much of his career attempting to offer evolutionary explanations with integrative promise using organisms like orchids or earthworms, and struggled mightily with the balance between stringency and opportunism.