132 F. E. Clements 



in the region of the present desert. This assumption is confirmed 

 by the prevalence of Merychippus, and it seems fair to surmise 

 that the precipitation amounted to about 20 inches and supported 

 a grassland similar to but richer than that of central Nebraska 

 today. This region is characterized along the valleys by relict 

 forest, composed almost wholly of flood-plain species, such as 

 willow, Cottonwood, box elder, elm, ash, hackberry, and walnut, 

 though bur oak and linden also reach it, and the more meso- 

 phytic oaks and hickories are not far to the east. 



In attempting to explain the wide distribution of grassland 

 and the associated ungulates in Miocene time, it is necessary to 

 invoke land connections and to consider the great succession that 

 followed the withdrawal of the Cretaceous Sea in the Laramie 

 or Paleocene. The most satisfactory hypothesis is that the result- 

 ing marshes of vast extent were long peopled with reed-grass 

 and sedge communities, as are the great tule swamps of Cali- 

 fornia today, and that these for the most part yielded to tall- 

 grass meadows in the Oligocene. With the broad connection be- 

 tween the two northern continents at this time, a wide pathway 

 came into existence for the mutual exchange that took place in 

 the Middle Miocene. Had this been limited to Holarctic forms, 

 a mere change of climate might be sufficient explanation, but the 

 movements of certain groups in opposite directions, such as 

 those of the dogs, true cats, catde, and elephants from Asia to 

 North America and that of camels and horses from the latter to 

 the former, presupposes an adequate connection at the north 

 (Osborn, 1909; Matthew, 1915, 1930). The assumption of such 

 an intercontinental grassland highway gains support from the 

 great migration and expansion of Stipa, its associates, and the 

 accompanying horses, tapirs, camels, and other forms in South 

 America in the Pliocene and Pleistocene. 



