When is conceptual change so significant that we should talk about a new theory, not a new version of the same theory? We address this problem here, starting from Gould’s discussion of the individuation of the Darwinian theory. He locates his position between two extremes: ‘minimalist’—a theory should be individuated merely by its insertion in a historical lineage—and ‘maximalist’—exhaustive lists of necessary and sufficient conditions are required for individuation. He imputes the minimalist position to Hull and attempts a reductio : (...) this position leads us to give the same ‘name’ to contradictory theories. Gould’s ‘structuralist’ position requires both ‘conceptual continuity’ and descent for individuation. Hull’s attempt to assimilate into his general selectionist framework Kuhn’s notion of ‘exemplar’ and the ‘semantic’ view of the structure of scientific theories can be used to counter Gould’s reductio , and also to integrate structuralist and population thinking about conceptual change. (shrink)