CALORIC. 445 



depressing the temperature of the eastern side of the con- 

 tinent. Whatever may be the cause, whether from topo- 

 graphic agency in modifying atmospheric movements, or 

 deeper geological and magnetic forces, or the "permanent 

 and drifting ice of the arctic region," etc., the great fact is 

 universally acceded. 



Speaking of the arctic fauna, Agassiz observes : 



"It has already been said that the arctic fauna of the three con- 

 tinents is the same ; its southern limit, however, is not a regular line. 

 It does not correspond precisely with the polar circle, but rather to 

 the isothermal zero; that is, the line where the average temperature 

 of the year is at 32 Fahrenheit. The course of this line presents 

 numerous undulations. In general, it may be said to coincide with 

 the northern limits of trees, so that it terminates where forest vege- 

 tation succeeds the vast arid plains; the barrens of North America 

 are the tundras of the Samoyides. The uniformity of these planes in- 

 volves a corresponding uniformity of plants and animals. On the North 

 American continent it extends much farther southward on the east- 

 ern shore than on the western. From the peninsula of Alaska, it 

 bends northward toward the Mackenzie, then descends again to- 

 ward the Bear Lake, and comes down nearly to the northern shore 

 of Newfoundland." 



On this subject Butler makes also the following state- 

 ment : 



" Take the isothermal line of or zero, that is, the line where the 

 mean or average height of the thermometer for the year is at zero. 

 At Behring's Straits this line is a little below the arctic circle, or the 

 parallel of 66 30 X north latitude. Passing east over North America, 

 it descends into Canada, almost to Lake Superior, and to about the 

 fiftieth parallel; that is to say, it is on an average during the year 

 as cold as our continent at the fiftieth parallel as it is at Behring's 

 Straits at the sixty-fifth parallel. Passing east, the line of zero rises 

 again over the Atlantic Ocean until, in the meridian of Spitsbergen, 

 it reaches, within the arctic circle, up almost to the seventy-fifth 

 parallel. So, too, the isothermal of 5 below zero, which is below 

 the sixtieth parallel in Siberia, rises in the North Sea, above Beh- 

 ring's Straits, to the parallel of 75, descending on the continent in 

 North America to the fifty-fifth parallel, and rising again almost to 

 the pole at Spitzbergen, to descend again in Siberia, while the iso- 

 thermals of 10 and 15 below zero, which in North America are but 



38 



