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 ex