 In “The Road Since Structure,” his 1990 Presidential Address to the Philosophy of Science Association, Kuhn reported on a book in progress, a project that would remain unfinished at his death. (See also Kuhn 1993.) In this and other fragments of that work, he develops the biological metaphor broached at the end of Structure. He retains his old parallel to biological evolution, that science progresses or evolves away from its previous forms rather than toward a final truth about the world; but he now extends the biological analogy by regarding scientific specialties themselves as akin to biological species that carve out research and teaching niches for themselves. In the process, he significantly modifies his conception of scientific revolutions and attendant claims concerning crises and incommensurable breaks. No longer do we hear of revolutions as paradigm change, certainly not in the sense of large paradigms. In fact, Kuhn prefers to speak of “developmental episodes” instead of revolutions. However, he does retain something of his original idea of small paradigms, the concrete problem solutions that he had termed “exemplars” in the “Postscript-1969” to Structure. Most revolutions, he tells us, are not major discontinuities in which a successor theory overturns and replaces its predecessor. Rather, they are like (allopatric) biological speciation, in which a group of organisms becomes reproductively isolated from the main population.