Modus Darwin reconsidered

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. In the present paper I review and critique Sober's analysis of Darwin's reasoning. I argue that modus Darwin has serious limitations that make the argument form unsuited for supporting Darwin's conclusions, and that Darwin did not reason in this way