A correlation exists between the genetic-physiologic differentiation of a species complex and the climatic zones it occupies. This has been established by expts. conducted in a transect across central California extending from near the seacoast to the high Sierra Nevada. Plants of several families have been employed, but the present paper discusses specifically regional differentiation in the Potentilla glandulosa and Achillea millefolium complexes. Races of spp. of distinct complexes from similar environments (the same life zones) have similar modifications and patterns of reaction when moved to new environments. Differentiation within the species complex is purely genetic, resulting in few major climatic races. This may be accomplished with or without a change in chromosome number. Fitness of plants to survive in their given environments appears to lie in their genic composition rather than in their chromosome number. Not the species itself, but the regional climatic race, or ecotype, is the important ecol. unit when fitness to the environment is considered, and its emergence through natural selection acting upon populations marks an important evolutionary step. A species may be composed of one or many ecotypes, and hence occupies a limited or wide range of habitats. The limits of the species depend upon the formation of isolating genetic barriers. A graph illustrates ecological and cytological differentiation of 11 species complexes into climatic races across the central Californian transect.