 Donald Campbell (1960, 1974) had previously defended the generalized variation plus selective retention schema, which he traced back to William James. Popper regarded his own evolutionary account of scientific development as similar to Campbell’s (1974). Ditto for David Hull (1988) with his more detailed evolutionary model. However, Hull rejected evolutionary epistemology, as such, and denied that he was doing epistemology at all. (Evolutionary epistemologies face the problem of why we should expect a contingent selectionist process to be truth-conducive: see the entry on  evolutionary epistemology.  Assuming that it is can also tempt one to fall into whiggism regarding the past in a social Darwinist sort of way.) Hull rejected Toulmin’s biological species analogy, as based only on feature-similarity rather than on the historical-causal continuity of genuine biological species. Hull’s book reflected his own deep involvement in the controversy between cladists, evolutionary systematicists, and pheneticists over biological classification. (He served terms as president of both the Society for Systematic Biology and the Philosophy of Science Association.) Hull generalized his important biological concepts of replicator (gene) and interactor (organism) to scientists and communities. His central unit of and for analysis was the deme, or research group, in its competition with others.