 Toulmin (1972) produced an evolutionary model of scientific development in terms of populations of concepts, a gradualist account of scientific change that he considered more historically accurate and philosophically defensible than Kuhn’s discontinuous model. Toulmin’s “concepts” are historically malleable, yet they are characterized by historicity. He quotes Kierkegaard: “Concepts, like individuals, have their histories, and are just as incapable of withstanding the ravages of time as are individuals” (1972: frontispiece). Toulmin held that biological, social, and conceptual evolution, including scientific development, are all instances of the same generalized variation-selection-transmission schema, albeit with quite different concrete implementations. For Toulmin, disciplines (specialties) are analogous to biological species. He touted his model as naturalistic, indeed ecological, but not in a way that excludes rationality. Rationality enters primarily at the selection level, determining which families of concepts (including methodological ones) get selected and reproduced. Rationality is not a matter of “logicality”, i.e., of sticking to a given logical or Kuhnian framework through thick and thin. Rather, it is a matter of adapting appropriately to changing circumstances. Like Newtonian force, rationality has to do with change, not maintenance of the same state. Thus no Kuhnian revolution is needed in order to break out of an old conceptual framework.