Unlike Malus and Crataegus, Pyrus has no fossil representative in N. America, and its present geographic distribution also indicates a later origin than of the other 2 genera. Species diversity in pears occurs in 2 distinct areas, western Eurasia and eastern Asia. Because of the concentration there of Pomoideae, western China seems the most probable site of origin of Pyrus from a widespread Cretaceous progenitor. By following primitive features, the westward distribution of the genus can be shown to parallel the development of the great mountain chains. Xerophyti-zation and borealization have been the important processes for species differentiation.