J. Wójcik of the Mammal Research Centre, Polish Academy of Sciences in Bialowieza, Poland, provided access to the specimens on which the P matrix was based. J. Head, J. Jernvall, S. Le Comber, N. MacLeod, and I. Salazar-Ciudad provided valuable comments and criticisms at various stages of development. This research was supported by grant GR3/12996 from the UK Natural Environment Research Council.