222 THE MEANING OF EVOLUTION 



Tertiary Age. Its close, occupying only the last few 

 hundred thousand years, is known as the Age of Man, 

 the Quaternary. Through perhaps three or four mil- 

 lions of these years stretches the known pedigree of 

 the horse. 



When we go back to the early Tertiary we find a 

 forest, with trees that shed their leaves, interspersed 

 with glades, in which already the grasses were begin- 

 ning to be developed. This state of affairs had 

 existed but for a comparatively short time, geologi- 

 cally speaking. It had come only in the latter part 

 of the preceding era. Lake and swamp, meadow and 

 forest intermingled to make a rich and varied scene. 

 Slowly the land toward the western side of North 

 America lifted itself into plateau and mountain range. 

 Slowly the westerly winds began to be cut off by 

 the barriers thus raised across their path. As they 

 swept over the plateau and down into the eastern 

 plain their moisture came to be diminished. Grad- 

 ually a very different state of affairs set in. The 

 ground became harder, the forest became sparser, the 

 plants became higher and firmer, the grasses tougher 

 and more wiry, and, by the time the Quaternary ar- 

 rived, a condition probably even drier than that of to- 

 day existed over our western highlands. Throughout 

 this long change, spread over millions of years, a crea- 

 ture which has become our horse steadily persisted 



