There are several interpretations of the argument structure of Darwin's Origin of Species, representing Covering-Law, Inference-to-the-Best-Explanation, and (more recently) Semantic models. I argue that while all three types of interpretation enjoy some textual support, none succeeds in capturing the overall strategy of the Origin, consistent with Darwin's claim that it is 'one long argument'. I provide detailed criticisms of all three current models, and then offer an alternative interpretation based on the view that there are three main argument strategies in (...) the Origin, all supporting the 'causal efficacy' of Darwin's theory. This interpretation provides both a more unified treatment of the text, and some important implications concerning the relation between general philosophical models of scientific theory support and specific historical cases. (shrink)