CAUSES OF GEOGRAPHICAL DISTRIBUTION. 247 
and in the originality of their appearance as we approach 
its point of derivation.” 
Hence, on both sides of the ocean, north of the very 
sinuous boundary of the antarctic or southern fauna, 
we find ourselves still in the midst of the diluvial animal 
world, which extended itself, by a bridge in the vicinity 
of the North Pole, from the old continents to the 
mainland of America, and there for a longer period 
retained its ancient appearance in the mastodons and 
horses. 
There, as well as here, the present order of things— 
the cantonment of animals—has been in many ways 
determined and modified by mighty glacifications and 
prolonged periods of refrigeration. Hence the accord- 
ance of so many plants of the extreme north with 
Alpine plants after the Eocene vegetation had made its 
entry from the east. Since that age, the reindeer has 
been forced back to the north, and the musk ox has 
been expelled and exterminated from the Old World. 
The elephants, fleeing before the ice, have not returned ; 
and the mammoth, immigrating with a rhinoceros from 
the north-east, has been destroyed with his associate. 
Others of his comrades, such as the primzeval ox, died 
out only a few centuries ago as wild cattle; others, 
like the buffalo and the beaver, are nearly extinct as 
denizens of Europe; and others again, the deer and 
roe-deer, will perish with the forests and the game-laws. 
But of almost all the species of which we search tor 
the extraction, Palzontology supplies us with the his- 
tory and derivation; and in derivation we find the 
causes of geographical distribution sketclied in vivid 
outlines. 
