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 piecemeal conceptual schemes from one research tradition to another. Popper bypassed the questions above because he relied very heavily on the selective power of criticism.In his Objective Knowledge (first published in 1972), Popper proposed that the growth of knowledge follows the same laws as the evolutionary development of organisms. By proposing this, he made the success of his theory of the growth of knowledge dependent on the success of Darwin's theory of evolution. According to the latter, species evolve by means of blind variation and selective retention.' Popper pushed this assertion further. He explained the actual growth of knowledge in terms of such mechanisms. Knowledge begins with problems, he says, and grows by means of attempts to solve these problems (blind variation), and elimination of the unfit solutions (selective retention), which leads to new problems.2 He presented