MAMMALIA. 345 



their slipping on the ice, while their white fur renders them 

 invisible at any great distance. 



Fit. 368. Polar bear and walrus, showing how the bear walks with the heel 

 Hat on the ground, and the walrus also. 



NOTE. Many animals that can not migrate and are deprived of 

 food by cold weather are enabled to enter into a state of torpor called 

 winter sleep or hibernation, and thus bridge over the foodless season. 

 In the extreme south certain animals during the dry period enter into 

 a summer sleep called estivation. In complete hibernation all the 

 functions of life are almost at a stand still ; the respiration is reduced 

 and irritability of muscular fibre increased. Hibernation is favored by 

 cold but not produced directly by it, and the hibernator is not insen- 

 sible to extremes. In entering the sleep the temperature of the body 

 sinks to nearly that of the surrounding atmosphere. If, now, the cold 

 is intense, they are awakened and then are frozen. According to Sem- 

 per, the zizel, or Spermophilus, attains the lowest temperature in this 

 condition of any known animal, namely, 2" (centigrade), the exact tem- 

 perature of the outside air in one experiment, so that the animal maybe 



