Fertility and Scientific Realism

It has been claimed that modern long-standing scientific theories are fertile, in the sense of having been progressively successfully modified to meet new experimental observations or theoretical developments in related areas, and that these modifications arise naturally from each preceding version of the theory. McMullin has advanced this form of fertility as a vindication of scientific realism, since if the theories did not approximate the real, the observation would be inexplicable. In response Nolan has denied the existence of fertility in this sense as an independent virtue. The present paper argues that the rebuttal is flawed.