xxii INTRODUCTION. 
serene sky and a bright sun, while the terrestrial animals re- 
main involved in darkness and exposed to all the fury of the 
tempest. In twenty-four hours it can change its climate, and 
sailing over different countries, it will form a picture exceeding 
the powers of the pencil or the imagination. The quadruped 
knows only the spot where it feeds, — its valley, mountain, or 
plain ; it has no conception of the expanse of surface or of 
remote distances, and generally no desire to push forward its 
excursions beyond the bounds of its immediate wants. Hence 
remote journeys and extensive migrations are as rare among 
quadrupeds as they are frequent among birds. It is this 
desire, founded on their acquaintance with foreign countries, 
on the consciousness of their expeditious course, and on their 
foresight of the changes that will happen in the atmosphere, 
and the revolutions of seasons, that prompts them to retire 
together at the powerful suggestions of an unerring instinct. 
When their food begins to fail, or the cold and heat to incom- 
mode them, their innate feelings and latent powers urge them 
to seek the necessary remedy for the evils that threaten their 
being. The inquietude of the old is communicated to the 
young; and collecting in troops by common consent, influ- 
enced by the same general wants, impressed with the approach- 
ing changes in the circumstances of their existence, they give 
way to the strong reveries of instinct, and wing their way over 
land and sea to some distant and better country. 
Comparing animals with each other, we soon perceive that 
smell, in general, is much more acute among the quadrupeds 
than the birds. Even the pretended scent of the Vulture is 
imaginary, as he does not perceive the tainted carrion, on 
which he feeds, through a wicker basket, though its odor is as 
potent as in the open air. This choice also of decaying flesh 
is probably regulated by his necessities and the deficiency of 
his muscular powers to attack a living, or even tear in pieces a 
recent, prey. The structure of the olfactory organ in birds is 
obviously inferior to that of quadrupeds ; the external nostrils 
are wanting, and those odors which might excite sensation 
have access only to the duct leading from the palate ; and even 
