28 THE ARCHITECTURE OF BIRDS. 



young, while security from enemies and accidents 

 is no less necessary for successfully rearing the 

 brood. The various means resorted to for fulfilling 

 these conditions afford numerous and interesting il- 

 lustrations of this delightful part of natural history, 

 and have given rise to much curious discussion 

 among those philosophers who severally ascribe the 

 building operations of birds to foresight and reason; 

 to what is termed instinct, meaning mechanism 

 without intelligence in the agent ; or to immediate 

 impulses from the great creative mind of the uni- 

 verse. Without entering into the intricacies of these 

 discussions, we shall enumerate a series of facts 

 from which inferences may be drawn by the advo- 

 cates of the various systems which attempt to ex- 

 plain the more mysterious operations of the lower 

 animals. 



Though the ground is proverbially termed " cold," 

 it requires but slight observation to prove that the 

 popular notion is not strictly correct, and, conse- 

 quently, that the great number of birds which select 

 it for their nests are not so foolish as might at first 

 view appear. The researches of Saussure, and, more 

 recently, of M. Cordier, prove that at considerable 

 depths the earth does not vary much in tempera- 

 ture ; and, without having recourse to the tables 

 drawn up from thermometrical observations, there 

 are two circumstances well known to everybody, 

 which prove that the ground cannot, with strict jus- 

 tice, be termed cold. In a morning, when the fields 

 are covered with hoarfrost, it may be observed to 

 be much longer on some places than on others ; and 

 if the nature of the substances on which it remains 

 longest be examined, they will uniformly be found 

 to be such as are considered by chymists bad con- 

 ductors of heat, such as wood, cowdung, and hay; 

 while on the bare ground, particularly in pathways, 

 where it is hard and beaten, and, consequently, better 

 fitted to conduct heat, the hoarfrost is always first 



