Modus Darwin redux

Abstract

How should we evaluate Darwin and Wallace's arguments for common ancestry over separate ancestry? Elliott Sober defends a likelihood reconstruction of Darwin's reasoning that he dubs modus Darwin: similarity, therefore common ancestry. One assumption of Sober's approach is that separate ancestors have traits that are probabilistically independent. I motivate an objection to this assumption by appeal to 19th century naturalist alternatives such as those of Geoffroy and Owen. On Geoffroy and Owen's separate ancestry models, the ancestors can have traits that are probabilistically dependent. I then prove a generalization of Sober's approach that allows for similarity matching among traits to favour common ancestry over separate ancestry even when the traits of the separate ancestors are probabilistically dependent. I consider Helgeson's recent criticisms of Sober's approach and his alternative interpretation of Darwin's reasoning: more similar, hence, more recent common ancestry. I defend Sober's approach against Helgeson's objections.

Keywords: Biogeography; Common ancestry; Darwin; Law of likelihood; Sober; Wallace.