 Rejecting a teleological view of science progressing towards the truth, Kuhn favours an evolutionary view of scientific progress (1962/1970a, 170–3), discussed in detail by Wray (2011) (see also Bird 2000 and Renzi 2009). The evolutionary development of an organism might be seen as its response to a challenge set by its environment. But that does not imply that there is some ideal form of the organism that it is evolving towards. Analogously, science improves by allowing its theories to evolve in response to puzzles and progress is measured by its success in solving those puzzles; it is not measured by its progress towards to an ideal true theory.  While evolution does not lead towards ideal organisms, it does lead to greater diversity of kinds of organism.  As Wray explains, this is the basis of a Kuhnian account of specialization in science, an account that Kuhn was developing particularly in the latter part of his career.  According to this account, the revolutionary new theory that succeeds in replacing another that is subject to crisis, may fail to satisfy all the needs of those working with the earlier theory.  One response to this might be for the field to develop two theories, with domains restricted relative to the original theory (one might be the old theory or a version of it).  This formation of new specialties will also bring with it new taxonomic structures and so leads to incommensurability.