XXXVI INTRODUCTION. 
Britain, the discrepancy between the pliocene extinct and 
the existing groups of Mammalia appears to be extreme. 
But if we regard Great Britain in connection with the 
rest of Europe, and if we extend our view of the 
geographical distribution of extinct Mammals beyond 
the limits of technical geography,—and it needs but a 
glance at the map to detect the artificial character of 
the line which divides Europe from Asia,—we shall then 
find a close and interesting correspondence between the 
extinct Europzo-Asiatic Mammalian Fauna of the plio- 
cene period, and that of the present day. The very fact 
of the pliocene Fossil Mammalia of England being almost 
as rich in generic and specific forms as those of Europe, 
leads, as already stated, to the inference that the inter- 
secting branch of the ocean which now divides this island 
from the continent did not then exist as a barrier to the 
migration of the Mastodons, Mammoths, Rhinoceroses, 
Hippopotamuses, Bisons, Oxen, Horses, Tigers, Hyzenas, 
Bears, &e., which have left such abundant traces of their 
former existence in the superficial deposits and caves 
of Great Britain.* Now, it is a most interesting fact, 
that, in the EHuropeo-Asiatic expanse of dry land, 
species continue to exist of nearly all those genera which 
are represented by pliocene and post-pliocene Mammalian 
fossils of the same natural continent and of the imme- 
diately adjacent island of Great Britain. The Bear has 
its haunts in both Europe and Asia; the Beaver of the 
Rhone and Danube represents the great Trogontherium ; 
the Lagomys and the Tiger exist on both sides of the 
Himalayan mountain chain; a Hyena ranges through 
Mr. Lyell infers the former existence of an isthmus between Dover and 
Calais on other grounds. See his Memoir’ on the relative ages of the “Crag” 
of Norfolk and Suffolk. Mag. Nat. Hist. 1839, p. 326. 
