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 
