Popper's thesis that the growth of knowledge lies in the emergence of problems out of criticism and takes place in an autonomous world of products of the human mind (his so-called world-3) raises two questions: (1) Why does criticism lead to new problems, and (2) Why can only a limited number of tentative solutions arise at a given time? I propose the following answer: Criticism entails an overlooked evolutionary world-3 mechanism, namely, the migration of piece meal conceptual schemes from one (...) research tradition to another. Popper by passed the questions above because he relied very heavily on the selective power of criticism. (shrink)