6 
INTRODUCTION. 
The rapid motions executed by birds, have also a reference to the 
perfection of their vision ; for, if nature, while she endowed them 
with great agility and vast muscular strength, had left them as short- 
sighted as ourselves, their latent powers would have availed them 
nothing ; and the dangers of a perpetually impeded progress would 
have repressed or extinguished their ardor. We may then, in gen- 
eral, consider the celerity with which an animal moves, as a just 
indication of the perfection of its vision. A bird, therefore, shooting 
swiftly through the air, must undoubtedly see better than one 
which slowly describes a waving tract. The weak-sighted Bat, 
flying carefully through bars of willow, even when the eyes were ex- 
tinguished, may seem to suggest an exception to this rule of relative 
\ r elocity and vision ; but in this case, as in that of some blind indi- 
viduals of the human species, the exquisite auditory apparatus seems 
capable of supplying the defect of sight. Nor are the Bickerings of 
the Bat, constantly performed in a narrow circuit, at all to be com- 
pared to the distant and lofty soarings, of the Eagle, or the wide 
wanderings of the smaller birds, who often annually pass and repass 
from the arctic circle to the equator. 
The idea of motion, and all the other ideas connected with it ? 
such as those of relative velocities, extent of country, the propor- 
tional height of eminences, and of the various inequalities that 
prevail on the surface, are therefore more precise in birds, and 
occupy a larger share of their conceptions, than in the grovelling 
quadrupeds. Nature would seem to have pointed out this superiority 
of vision, by the more conspicuous and elaborate structure of its 
organ ; for in birds the eye is larger in proportion to the bulk of the 
head than in quadrupeds ; it is also more delicate and finely fash- 
ioned, and the impressions it receives must consequently excite 
more vivid ideas. 
Another cause of difference in the instincts of birds and quad- 
rupeds, is the nature of the element in which they live. Birds 
know better than man, the degrees of resistance in the air, its tem- 
perature at different heights, its relative density, and many other 
particulars, probably, of which we can form no adequate conception. 
They foresee more than we, and indicate better than our weather- 
glasses, the changes which happen in that voluble fluid ; for often 
have they contended with the violence of the wind, and still oftener 
have they borrowed the advantage of its aid. The Eagle, soaring 
above the clouds, can at will escape the scene of the storm, and 
in the lofty region of calm, far within the aerial boundary of 
