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. Ina 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 
