Since the Modern Synthesis, the view that species are cohesive entities held together by gene flow has moved from being a the oretical insight amongst systematists to common knowledge amongst biologists. The plant biologist Vern Grant provides a classic and succinct expression of this view, hereafter simply The View, in saying that" species populations are homogenized and integrated by gene flow." 1 As one of us has recently detailed, several biologists have chal lenged the empirical adequacy of The View over the past forty-odd years. 2 Nevertheless, most biologists, including many phylogeneticists, have thought that species are cohesive entities, and the idea that gene flow is the primary cause of this cohesion continues to hold sway. 3

* The authors would like to thank audiences at the 2006 meetings of the Western Canadian Philosophical Association and the Philosophy of Science Association, both in Vancouver, for their constructive feedback on earlier versions of this paper and re lated work, as well as the Social Sciences and Humanities Research Council of Canada for financial support. For important comments, the first author is grateful to John Basl, Malcolm Forster, Remy Petit, Alex Rueger, Larry Shapiro, Joel Velasco, and members of the DC History and Philosophy of Biology reading group. The second author would like to acknowledge the work of Allison Dawe as a research assistant in the mid-1990s, which first got him puzzled about the topic of the paper. Both authors thank Ingo Brigandt and Elliott Sober for written comments that have led to improvements. 1 Grant," Gene Flow and the Homogeneity of Species Populations," Biologisches Zentralblatt, xcix (1980): 157-69.