EAELY TERTIARY MAMMALS 229 



in his palaeontological maps, unites America in this manner 

 with Asia, and by way of Greenland with Europe. But Pro- 

 fessor Suess contends that, although these affinities in the 

 Eocene faunas of America and Europe imply united conti- 

 nents in the north, this land connection was probably not in 

 the vicinity of Bering Sea. On the contrary, he .rather favours 

 a more direct land bridge between North America and 

 Europe.* 



During the latter stages of the Eocene Period, while the 

 Wind River, Bridger and Uinta beds were being laid down, 

 the descendants of the archaic and of the modernised 

 mammals gradually evolved, and we may suppose that the 

 archaic mammals finally succumbed in the struggle for exis- 

 tence. At any rate, they slowly disappeared, and during 

 the process of their elimination, the fauna of America assumed 

 a more independent aspect, the affinities with Europe becom- 

 ing less pronounced. This need not necessarily imply a cessa- 

 tion of the intimate geographical relationship between the two 

 continents. The growth of an impenetrable forest, like that 

 in the interior of Brazil, the development of local desert 

 conditions, or the existence of temporary volcanic distur- 

 bances on the supposed trans-Atlantic land connection, would 

 have been sufficient to faunistically isolate the two continents 

 from one another. In the succeeding period, the Oligocene, 

 the faunal resemblance of western North America and 

 western Europe once more became conspicuous. The land 

 area available for the development of mammalian life cer- 

 tainly increased in America during early Tertiary times, while 

 a corresponding decrease may have taken place on the trans - 

 Atlantic land connection, thus bringing a renewed influx of 

 strange forms to the New World. Professor Osborn f tells us 

 that in the White River, John Day and other American Oligo- 

 cene formations, sixteen new families of mammals made their 

 appearance, most of them still existing, and that a very 

 similar modernisation occurred in western Europe. Six new 

 families appear, apparently simultaneously, in both areas. It 

 is worthy of note that the opossu'm family (Didelphyidae) and 

 the rhinoceroses (Rhinocerotidae) now make their first entry 



* Suess, E., " Antlitz der Erde," Vol. III. 2 , pp. 764765. 

 f Osborn, H. F., " Cenozoic Mammal Horizons," pp. 58 59. 



