74 PHILOSOPHY OF ZOOLOGY. 



These are the only instances with which we are acquaint- 

 ed of actual torpidity having occurred among the feathered 

 tribes. They seem calculated to remove all doubt as to 

 the fact, while they point out to us the numerous resources 

 of nature in extreme cases to preserve existence. Thus, 

 when birds, from disease or weakness, or youth, are inca- 

 pable of performing the ordinary migrations of their tribes, 

 they become dormant during the winter months, until the 

 heat of spring restores to them a supply of food and an 

 agreeable temperature. But it is only in extreme cases, 

 which happen seldom. The occurrence of torpid swallows 

 is very rare, and the examples quoted of the cuckoo and 

 land-rail are solitary instances, although the last mention- 

 ed bird is as abundant in Orkney, as partridges are in the 

 Lothians. 



Hitherto we have been considering the torpidity which 

 warm-blooded animals experience. Several cold-blooded 

 animals observe a similar mode of hybernation. 



The period of the year at which Reptiles prepare for 

 this state of lethargy, varies in the different species. In 

 general, when the temperature of the air sinks below the 

 50th degree of Fahrenheit, these animals begin their winter 

 slumbers. They adopt similar precautions as the mamma- 

 lia, in selecting proper places of retreat, to protect them 

 from their enemies, and to preserve them from sudden al- 

 ternations of temperature. Those which inhabit the wa- 

 ters sink into the soft mud, while those which live on the 

 land enter the holes and crevices of rocks, or other places 

 where the heat is but little affected by changes in the tem- 

 perature of the atmosphere. Thus provided, they obey 

 the impulse, and become torpid. 



As the temperature of these animals depends, in a consi- 

 derable degree, on the surrounding medium, they do not 

 exhibit any remarkable variations. When the air is under 



