The variation in three species ofSelaginella that occur in areas south and north of glaciated and periglacial regions is analysed. In each species the southern populations are more variable than the northern ones. These geographic patterns of variation are related to post-Wisconsin climatic changes and to the northward migration of each species. The geographic differentiation and migration of the species are seen as related phenomena. The concept of evolutionary migra tion-the operation of natural selection and of migration in mutually supportive roles-is discussed and applied to the examples inSelaginella. The possible broad significance of the process of evolutionary migration in floras directly under the influence of Wisconsin and post-Wisconsin environments is indicated.