100 GEOGRAPHICAL DISTRIBUTION. 



previously been attributed to the changes of 

 the temperature gradually produced by the 

 change of seasons, and the growing scarcity of 

 food resulting from it. But Mr. Jenner has ob- 

 served that these cannot be the causes that 

 occasion the migration of those birds that leave 

 us early in the year, as the cuckoo, 1 which dis- 

 appears in the beginning of July ; and the swift, 8 

 which takes its departure early in the following 

 month. At these times they can feel no cold 

 blast to benumb them, and the food that forms 

 their usual support is in the greatest abundance. 

 There seems to be some analogy between the 

 birds that migrate annually to warmer climates 

 to spend their winter, and those animals, which 

 remaining in a country, seek a subterranean, or 

 other close retreat, to shelter them from the 

 rigours of that season, and in which they con- 

 tinue in a torpid state, till spring revives them 

 and they issue from their hiding-places to fulfil 

 the first law of their Creator. Several instances 

 also are upon record, even with regard to 

 birds that usually migrate, of their having 

 been found torpid in the clefts and cavities of 

 trees ; and Spallanzani relates experiments which 

 prove that swallows can bear a certain degree of 

 cold when torpid. I do not recollect any obser- 

 vations which serve to prove that hybernating 



1 Cuculus canorus. e Cypselus apits. 



