Modus Darwin is the name given by Elliott Sober to a form of argument that Sober attributes to Darwin in the Origin of Species, and to subsequent evolutionary biologists who have reasoned in the same way. In short, the argument form goes: ‘Similarity, ergo common ancestry' (i.e., species X and Y are similar, therefore they evolved from a common ancestor). In the present paper I review and critique Sober's analysis of Darwin's reasoning. Sober's project is part exegesis, part epistemology: How did Darwin argue?, and Was it a good argument? In this paper I bracket the exegesis and focus on the epistemology. I argue that modus Darwin (as Sober understands it) has serious limitations that make the argument form unsuited for supporting Darwin's conclusions. In short, my criticism is that in rigorously spelling out `similarity', Sober employs a system of character correspondences that, at worst (1) begs the question by presupposing common ancestry, and at best (2) registers ‘similarity' in a way that illegitimately biases the subsequent inference in favor of shared ancestry. Thus, either Darwin argued badly (he gave bad reasons for a true conclusion), or he didn't use modus Darwin.