166 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



ever, 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 of Fahrenheit. The course of this line presents 

 numerous undulations. In general, it may be said to coin- 

 cide with the appearance of trees, so that it passes 

 where forest vegetation succeeds the vast arid plains, the 

 barrens of North America, or the tundras of the Samoyedes. 

 The uniformity of these plains involves 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 

 Alashka it bends northwards towards the Mackenzie, then 

 descends again towards the Bear Lake, and comes down 

 to near the northern shore of Newfoundland. 



423. II. TEMPERATE FAUNAS. The faunas of the tem- 

 perate regions of the northern hemisphere are much more 

 varied than that of the arctic zone. Instead of consisting 

 mainly of aquatic tribes, we have a considerable number 

 of terrestrial animals of graceful form, animated appearance, 

 and varied colors, though less brilliant than those found in 

 tropical regions. Those parts of the country covered with 

 forests especially swarm with insects, worms, terrestrial and 

 fluviatile mollusks, which become the food of still other 

 animals. 



424. Still, the climate is not sufficiently warm over the 

 whole extent of this zone to allow the trees to retain their 

 foliage throughout the year. At its northern margin the 

 leaves, excepting those of the pines and spruces, fall, on 

 the approach of the cold season, and vegetation is ar- 

 rested for a longer or shorter period. Insects retire, 

 and the animals which live upon them no longer find 

 nourishment, and are obliged to migrate to warmer re- 

 gions, on the borders of the tropics, where, on the ever- 

 verdant vegetation, they find the means of subsistence. 



