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 primaeval 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 for 

 the extraction. Palaeontology supplies us with the his- 

 tory and derivation ; and in derivation we find the 

 causes of geographical distribution sketched in vivid 

 outlines. 



