Darwin describes the Origin of Species as 'one long argument'. The exact structure of this argument has been the subject of controversy among philosophers of biology. I will propose a novel analysis that sheds new light on Darwin’s argument. The central claim will be that the evidence that supports the theory of common descent can only satisfactorily support it only after the theory of natural selection has been regarded as an in principle possibility. This account helps us understand some enigmatic features of the structure of the Origin and has consequences for evidentialism and Bayesianism as applied to Darwin’s argument.