SUCCESSIVE MAMMALIAN FAUNAS 267 



evolutionary point of view. The ancestral camel {]Protylopus) 

 was a little creature no bigger than a fox-terrier, though the 

 fhypertragulids {^Leptotragulus) were as large as \Lepto?neryx 

 and ^Hypertragulus of the White River. The most ancient 

 known members of the foreodonts {^Protoreodon) and the 

 t agriochoerids {'\ Protagriochoerus) are found in the Uinta. 



The middle Eocene fauna, Bridger stage, though it passed 

 upward very gradually into that of the Uinta, was yet, on the 

 whole, very different from the latter. It was exclusively indig- 

 enous and so radically distinct from the mammals of corre- 

 sponding date in Europe as to preclude the possibility of a land- 

 bridge with that continent. In the lower Eocene, as will be 

 shown in a subsequent page, the communication between the 

 two continents was broadly open and the faunas of the two 

 continents were much more closely similar than they have 

 ever been since. It is really remarkable to see with what com- 

 parative rapidity the two regions, when severed, developed 

 different mammals under the operation of divergent evolution. 

 Had the separation continued throughout the Tertiary and 

 Quaternary periods, North America would now have been as 

 peculiar zoologically as South America is, a result which has 

 been prevented by the repeated renewal of the connection. 



The characteristic features of the Bridger mammalian fauna 

 were chiefly due to the great expansion and diversification of 

 certain families, which began their career at an earlier stage, 

 and to the disappearance of many archaic groups which had 

 marked the more ancient faunas. Other archaic groups, 

 however, survived and even flourished in the Bridger, and of 

 these it is particularly difficult to convey a correct notion to 

 the reader, because they were so utterly unlike anything that 

 now lives. One of these orders, the fTseniodontia, which had 

 so many points of resemblance to the fground-sloths that 

 several writers have not hesitated to include them in the 

 Edentata, survived only into the older Bridger, but the equally 

 problematical tTillodontia then reached their culmination, 



