b 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 tjirpugh bars of willow, even when the eyes were ex- 

 tinguished, may seem to suggest an exception to this rule of relative 

 velocity and vision; but in this case, as in that of some blind indi- 

 viduals of tlie human species, the exquisite auditory apparatus seems 

 capable of supplying the defect of sight. Nor are the flickering-s of 

 the Bat, constantly performed in a narrow circuit, at all to be com- 

 pared to the distant and lofly soaring's of the Ea^le, 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 volocities, extent of country, the propor- 

 tional heigiit 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 groveUing 

 quadrupeds. Nature would seem to have pointed out tliis superiority 

 of vision, by the more conspicuous and elaborate structure of its 

 organ ; for in birds the eye is lai-ger 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 natm-e 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 wliich 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 oflener 

 have they borrowed tire advantage of its aid. The Eagle, soaring 

 above the clouds, can at will escape the scene of the storm, and 

 in the lofly region of calm, far within the aerial boundary of 



