174 THE GEOGRAPHY OF MAMMALS 



A great advance is found in the Fauna of the White- 

 river beds of Miocene age. In this case the mammals can 

 almost be referred to existing orders, but comparatively 

 few of the genera are common to the Old and New- 

 Worlds ; and it appears that, whilst during the older 

 Eocene there was a considerable emigration of New World 

 forms into Europe, in Miocene times the stream was re- 

 versed, and North America received the greater number 

 of its immigrants from this side of the Atlantic. This 

 immigration continued during the Middle Miocene epoch, 

 the Fauna of which has been well preserved in the John- 

 Day beds of the extreme west. At the same time many 

 endemic Families and forms are also met with, especially as 

 regards the early predecessors of the Camel Family, which 

 apparently had its origin and early development in the 

 Nearctic Region, though now entirely absent from it. In 

 the John-Day beds, we also find, for the first time, remains 

 of the modern genera, Rhinoceros, Sciurus, Hesperomys, 

 and Lepus. 



The succeeding "Loup-Fork beds" contain additional 

 recent genera, some of which, such as Equus and 

 Camelus, are now no longer found in the Nearctic 

 Region, while others, such as Canis, Mustela, and Lutra, 

 still remain there. On the whole, however, the Fauna 

 of this epoch is still further removed from that of the 

 corresponding period of the Old World than that of the 

 preceding. 



A little later, in the so-called " Equus-beds " of the 

 Western States, and in the contemporaneous " Megalonyx- 

 beds " of the Eastern, we first find a number of Neotropical 

 forms, such as Mylodon (a gigantic Sloth), Glyptodon 

 (a gigantic Armadillo), Hydrochozrus (the Capybara), and 



