INTRODUCTION. xxi 



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

 blind individuals of the human species, the exquisite auditory 

 apparatus seems capable of supplying the defect of sight. Nor 

 are the flickerings of the bat, constantly performed in a narrow 

 circuit, at all to be compared 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 

 proportional height of eminences, and of the various inequali- 

 ties 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 deHcate and finely fashioned, and the impressions it 

 receives must consequently excite more vivid ideas. 



Another cause of difference in the instincts of birds and 

 quadrupeds is the nature of the element in which they live. 

 Birds know better than man the degrees of resistance in the 

 air, its temperature 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 indi- 

 cate 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 eternal frost,^ enjoy a 



1 The mean heights of eternal frost under the equator and at the latitude of 

 30° and 60° are, respectively, 15,207, 11,484, and 3,818 feet. 



