FERNS, FOSSILS AND FUEL 
plants of the angiosperm variety spread like wild-fire, 
forcing the cycads into the background, where they have 
remained until our own day. The angiosperms have con¬ 
tinued to be the dominant flora of the earth. 
Toward the middle of the Mesozoic era a new type of 
animal began to appear. These were the mammals, 
destined to be the outstanding type in the next great era, 
the Cenozoic. 
The Cenozoic era is divided into the Tertiary period, 
during which it was still warm, and the earth was covered 
with rich tropical and sub-tropical vegetation from pole 
to pole, and the Quaternary, including the Pleistocene or 
glacial age, during which it was extremely cold in the 
northern hemisphere. 
The Tertiary period shows another climax, this one 
of animal development. Gigantic mammals like the hairy 
elephant, the hippopotamus, the rhinoceros, and the giant 
sloth now gave life and character to the landscape. The 
reptiles degenerated, and the animals of our own day 
began to develop. The horse was rising from a four¬ 
toed animal no bigger than a dog to the large, single-toed 
animal we know today. The earth was covered from 
pole to pole with forests of palms, laurels, cinnamon trees 
and similar tropical and subtropical plants. 
Toward the end of the Tertiary period great mountain 
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