456 



THE CHANGING GENERATIONS 



of the Cenozoic, birds had become completely modernized; teeth had 

 disappeared and the mouth had become a horny beak adapted to pecking 

 and biting. Most of the modern families of birds appeared early in 

 Cenozoic time, and the group has continued to expand up to the present. 

 Flight is the most characteristic attribute of birds; yet many times in 

 the history of the group ability to fly has been lost, generally among birds 

 which grew very large or adopted aquatic habits. Sometimes the reduced 

 wings took on new functions, as in the penguins (Fig. 28.20), where they 

 became swimming paddles, or the ostriches (Fig. 15.11), which use them 

 as sails to increase speed in running. In the early Cenozoic there was a 



considerable development of large, 

 flightless birds, similar in habits to 

 the ostriches, rheas, and moas. The 

 mammals were then still in the 

 earlier phases of their development, 

 and it appears that these early giant 

 birds were for a time serious com- 

 petitors with mammals for domin- 

 ance of the ground. Diatryma, a 

 wingless predatory bird from 

 Wyoming, was a contemporary of 

 the earliest horses. The horse, at 

 that time, was the size of a fox 

 terrier; this bird was 7 feet tall. 



THE HISTORY OF MAMMALS 



Fig. 28.21. Diatryma, a large Eocene flight- 

 less bird of prey from Wyoming, and 

 Eohippvis, a contemporary horse, drawn to 

 the same scale. The horse is about a foot 

 high at the shoulder. {Redrawn from Ray- 

 mond, Prehistoric Life, by permission 

 Harvard University Press.) 



Although the mammals arose 

 from synapsid reptiles far back in 

 Mesozoic time, they remained inconspicuous until the close of the age 

 of reptiles. Then they began their spectacular rise to dominance; the 

 Cenozoic era was the age of mammals. Geologically speaking the Cenozoic 

 was short, comprising but a single full period (the Tertiary) and enduring 

 a mere 70 million years ; but it was crowded with events, as was described 

 in Chap. XXVI. The climax of mammalian evolution was reached in the 

 Miocene and Pliocene epochs. The Pleistocene glaciations caused the 

 extinction of many lines, so that our modern mammal faunas are mere 

 remnants of the preglacial ones. 



Man is one of the mammals, and in taking up the history of this group 

 we return from our digression on reptiles and birds to a consideration of 

 the line of human ancestry. By way of introduction let us review the 

 distinctive features of mammals. 



Mammalian characteristics. Just as feathers make the bird, so does 

 hair make the mammal; but hairiness is only one of the more obvious 



