THE HISTORY OF PLANTS 417 



Andes had been uplifted at the end of the Mesozoic and worn down to 

 level plains. During the Eocene and Oligocene epochs moist, mild, uniform 

 climates prevailed over the world, with redwoods, beech, chestnut and 

 elm occurring in Greenland and Siberia, cycads, magnolias and figs in 

 Alaska, and palms and alligators in the Dakotas. But in Miocene times 

 the continents began to rise in the first stages of the Alpine or Cascadian 

 revolution from which the world is just emerging. This revolution brought 

 with it great climatic changes and correspondingly great effects upon the 

 life of the lands. 



The spread of grasslands. Grasses and herbs appeared early in the 

 Cenozoic era, but were at first of little importance. When the lands began 

 to rise and mountains to be formed during the Miocene, however, large 

 parts of North America and Eurasia began to receive less rainfall. Climates 

 cooled and became more seasonal, and the vegetational belts shifted 

 toward the equator. Many regions became too arid to support tree 

 growth, and here the grasses and herbs found opportunity to spread. 

 With their large root systems, small exposed surface, rapid growth and 

 maturation, resistant seeds, and ability to grow in dense soil-protecting 

 stands, the grasses are the best adapted of all plants for life in semiarid 

 regions with strongly seasonal climates. By the time the Cascadian revolu- 

 tion reached its climax in the Pliocene, vast prairies and grassy plains 

 like those of today had come into existence, 1 and the forests had 

 become restricted to the moister parts of the continents. The whole 

 of western North America, except in the mountains and along the 

 Pacific slope, became grassland or desert, and similar changes occurred 

 in the other continents. In the eastern half of North America and in 

 eastern Asia and Europe, the forests continued little changed from 

 those of the Cretaceous. Today there is less difference between the floras 

 of England, Virginia, and China than between those of Virginia and 

 Wyoming. 



The development of the grasslands, though not comparable in impor- 

 tance to the rise of the angiosperms, was an event of major significance 

 for land vertebrates. The new food source furnished by the harsh grasses 

 and their seeds permitted the evolution of a host of grazing mammals, 

 their multiplication to hitherto unheard of abundance, and the attain- 

 ment of new levels of adaptation by both the herbivores and their preda- 

 tory enemies. 



1 It is interesting to note that an increase in the grasslands during the Miocene and 

 Pliocene was originally assumed largely from the grazing modifications shown by the 

 teeth of horses and other groups that occurred at this time. Only recently has it been 

 discovered that the middle Tertiary rocks of the plains regions are full of the fossilized 

 seeds of grasses. These seeds show that the grasses were then undergoing much more 

 rapid evolution than were the forest plants. 



