EARLY 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 opossum family (Didelphyidae) and 
the rhinoceroses (Rhinocerotidae) now make their first entry 
* Suess, E., “ Antlitz der Erde,” Vol. III. 2 , pp. 764 — 765. 
f Osborn, II. F., “ Cenozoic Mammal Horizons,” pp. 58—59. 
