||Fossil floras and mammalian faunas from the Great Plains indicate that as aridity increased during the Miocene and Pliocene, forests and woodlands were confined gradually to moister valleys as grassland spread on the interfluves which were covered earlier with park-like openings. The initial rise of extensive grasslands probably commenced in the Miocene-Pliocene transition (7-5 m.y. ago), the driest part of the Tertiary, which restricted forests and woodlands and encouraged the explosive evolution of grasses and forbs. Following the fluctuation of Pleistocene climatevegetation zones, warm, dry Altithermal climate restricted wooded tracts at the expense of spreading grasslands. The rise of the grassland biome was thus due to occasional periods of increased aridity that restricted forests and woodlands and favored grasses and forbs; to increasing drought west of the 100th meridian which created a flammable source (dry grass); to natural and man-made fires on the relatively flat plains over which fire could spread uninterruptedly; to fire that destroyed relict trees and groves on the flat grasslands, restricting them to rocky ridges removed from fire; and probably also to large browsing mammals (many now extinct) that may have destroyed scattered trees and shrubs on the interfluves during the Altithermal. Youthfulness of the grassland biome agrees with a) the occurrence of most of its species in the bordering forests and woodlands, b) the presence of few endemic plants in it, a relation shown also by insects and birds, c) the relict occurrence of diverse trees over the region, and d) the invasion of grassland by woody vegetation.