An Analytical Study of the Geographic Distribution of Rana septentrionalis

Where moisture is a limiting factor, xerophytic plant communities possess fewer species than do the communities of mesophytic regions, but the number of communities per unit of area is likely to be larger than in regions of adequate moisture. This promotes geographic separation of species populations and races, and thus might be expected to speed up evolution. Evidence that certain mesophytic types are derived from xerophytic ancestors is presented for the genera Scorzonera and Tragopogon of the Compositae, tribe Cichorieae, and for certain genera of the subfamilies Mimosoideae and Caesalpinoideae of the family Leguminosae. The evidence in the latter family is derived chiefly from the development of both adult and seedling leaves.