128 E E. Clements 



a drying climate and give it distinct advantages over all its 

 competitors in the processes of migration and ecesis. The genus 

 is the final expression of the phyletic line of Muhlenbergia-Ory- 

 zopsis-Stipa, which is predominandy American and originally 

 North American, the first genus having been derived apparently 

 from Calamagrostis, primarily a meadow grass of high latitudes. 

 The mid-grasses were evidently in process of dispossessing the 

 tall-grasses after the deformation of the Upper Oligocene, as 

 shown by fossil Stipa in the Miocene, and probably had reached 

 the Southwest in Lower Miocene in time to play their part in 

 the evolution of the Miocene grazing horses, Merychippus and 

 Protohippus, 



Further direct evidence of the presence of Stipa and its asso- 

 ciates has been obtained by Elias from deposits of the Ogallala 

 in the Lower Pliocene of western Kansas and adjacent Colorado 

 (1932, 1934). In addition to several forms of Stipa, two other 

 grasses occur, Panicum and Berriochloa, and four forbs of the 

 borage family belonging to the genera Biorbia and Krynitz\ia. 

 It is an axiom that the evolution of a particular grassland must 

 have preceded considerably the adaptation of the graminivor- 

 ous animals to it, just as deformation and climatic change, or 

 sometimes the latter alone, must have antedated the develop- 

 ment or migration of a particular climax. 



The short-grasses are likewise of American origin, but their 

 homeland was the mountain plateaus of Mexico and Central 

 America. Such far-ranging species as Botiteloua gracilis and 

 racemosa and Buchloe dacty hides have pushed north to the Ca- 

 nadian border or beyond, but the majority are confined to the 

 region west and south of Texas. Aristida, though of somewhat 

 different form, is a regular associate of Bouteloua and of similar 

 southern origin and xeric nature. The two genera constitute the 



