198 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



421. This assemblage of animals is evidently inferior 1 ft 

 that of other faunas, especially to those of the tropics. Not 

 that there is a deficiency of animal life ; for if the species 

 are less numerous, there is a compensation in the multitude 

 of individuals, and, also, in this other very significant fact, 

 that the largest of all animals, the whales, belong to this 

 fauna. 



422. It has already been said, (400,) that the arctic fauna 

 of the three continents is the same ; its southern limit, how- 

 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 numer- 

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

 the northern limit of trees, so that it terminates 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 eastern shore than on the 

 western. From the peninsula of Alashka, it bends north- 

 wards towards the Mackenzie, then descends again towards 

 the Bear Lake, and comes down nearly to 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, which become the food 

 of other animals ; worms and terrestrial and fluviatile mol- 

 lusks are also abundant. 



