1. For a topographically featureless area lacking easily detected geographical barriers Western Australia has an unexpectedly large number of species of frogs in the genera Crinia, Heleioporus and Neobatrachus. 2. The origin of these species is reconciled with current evolutinary theory relating to the origin of species by postulating: (a) A primary speciation pattern across the Great Dividing Range in eastern Australia. In this manner originated a series of species pairs one of which, the Bassian, was coastal, adapted to a uniform climate, and the other, the Eyrean, was central and adapted to a markedly seasonal climate. (b) That during Pleistocene time three pluvials, separated by interpluvials, allowed three migrations of the Bassian and Eyrean faunal elements into western Australia. (c) That during the seasonally arid inter-pluvials the earlier Bassian migrants were lost so that at the present time only the last migration is completely represented; however, each of the earlier Eyrian migratory elements has persisted.