178 



THE AGE OF MAMMALS 



IV. LOWER OLIGOCENE, FOURTH FAUNAL PHASE— THE SECOND 

 MODERNIZATION. SUDDEN APPEARANCE IN EUROPE AND 



. NORTH AMERICA OF NUMEROUS EXISTING FAMILIES OF 

 MAMMALS. REUNION OF THE NEW AND OLD WORLD INTO 

 A SINGLE ZOOLOGICAL REGION, FOLLOWED BY ANOTHER 

 LONG PERIOD OF INDEPENDENT EVOLUTION AND PARTIAL 

 EXTINCTION. 



The first impression of this phase is our sudden introduction to a large 

 number of modernized types which had been slowly evolving elsewhere, 

 probably in the plains of America and Eurasia. In North America this 

 second modernization is shown to be still more remarkable than the first 



' Bridx^er and Uinlu 

 V nfossUiferoxjLS 



l.a^nbclof/ieriurn zone 





^^MERICAN 'MUSEWM, CAMP 



Fig. 74. — Lowtr Olitiocciir liurizoiis resting upon the Upper, Middle, and Lower Eocene. 

 Titanotherium Zone = Lower Oligocene. "Bridger" and "Uinta" = Middle and Upper 

 Eocene. Lambdotherium Zone = Lower Eocene. Escarpment of the Beaver Divide, near 

 Hailey, Wyoming. Photograph by American Museum of Natural History, expedition of 1909. 

 Compare Fig. 95. 



or Wasatch modernization, which was one of appearance of existing orders, 

 because this is one of existing families, not as yet recognized in the mountain 

 basins. The Oligocene fauna thus is far more familiar in aspect than the 

 known Eocene. This new list in America includes six existing families of 

 rodents, four existing families of carnivores, one existing family of peris- 



