The Story of the Rocks 



143 



North America has been connected with Asia by a mass of 

 land or "bridg:e" across Behrin<if Sea. It apjjcars likely 

 that there was a similar "bridge" joining America and Eu- 

 rope at this time (Lower Eocene), for many mammals were 

 common to both continents. 



The canse of their migration is likewise nncertain, but nat- 

 ural increase and competition for food may have been one 

 of the compelling causes tlien as they are now in determin- 

 ing animal movements. We need oidy recall the move- 

 ments of the herds of bison, which formerly roamed across 



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Ciit'm^'': 



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fiw-K,V 



EoHipPus, THE ' ' Dawn Horse ' ' 



From a restoration by Chas. K. Knight. 



Courtesy of the Amerwan Museum of Natural History. 



our western prairies in search of food, the plague of locusts 

 which overwhelmed the early settlers in Kansas, or the spread 

 of the English sparrow from east to west to find an explana- 

 tion for animal movements in the past. But possibly an- 

 other, and even more potent factor was the gradually in- 

 creasing refrigeration of the polar region, which occurred 

 subsequent to the Eocene, and which culminated in the gla- 

 eiation of the "great ice age" of the Pleistocene epoch. In 

 the latter there is abundant evidence of the movement of 

 northern mammals before the advancing ice, for we "find re- 

 mains of walruses in New Jersey, of reindeer in southern 

 France and of the musk ox in Kentucky. 



