  Finally, Mach can be seen as the precursor of what is today one of the major epistemological metaphors of scientific change. Mach offers a selectionist account of group-level change consisting of selection upon naturally produced variation, passed on through processes of heredity. As with Darwinian evolution, scientific variation itself is not directed. Emphasizing the importance of natural variation, Mach had a Feyerabendian tolerance of nonscientific beliefs—the familiar point being that it is better to tolerate some error than risk losing variation which might yield fruit later. As an indication of variation’s importance, several chapters in his various books are directed almost entirely to this subject.  Variation is not something which we consciously produce, but rather something which happens by accident.