MIGRATION. 45 



true, in such numbers as when they left us ; but, amidst all 

 the dangers of their voyage, the race is preserved. 



We thus see, that animals possess various resources, to 

 enable them to accommodate themselves to the variations of 

 temperature corresponding with the seasons. But these 

 appear in some species to be inadequate for their protection, 

 and another is provided for their safety. 



4. Torpidity.- THIS is one of the most curious sub- 

 jects in zoology, and has long occupied the attention 

 of the natural historian and the physiologist. All ani- 

 mals we know require stated intervals of repose to 

 recruit exhausted nature, and prepare for farther exer- 

 tion, a condition which is termed Sleep. But there 

 are a few animals, which, besides this daily repose, ap- 

 pear to require annually some months of continued in- 

 activity, to enable them to undergo the common fatigues 

 of life during the remaining part of the year. These ani- 

 mals exhibit, therefore, two kinds of sleep, that which 

 they enjoy daily during the season of their activity, and 

 that which they experience during their brumal retirement. 

 This last kind of sleep is generally denominated torpidity, 

 and is also known by the term Jiybernation^ as it is evi- 

 dently designed to afford protection against the cold of 

 winter. 



As the phenomena which torpid animals exhibit are some- 

 what different, according to the classes to which they be- 

 long, it will be more convenient for us to treat of the ani- 

 mals of each class separately, beginning with QUADRU- 

 PEDS. 



The quadrupeds which are known to become torpid, 

 belong exclusively to the unguiculated division. Some spe- 

 cies are found among the fera^ as the different kinds of 

 bats ; the hedgehog and the tanric ; while among the glirea 



