VISION. 21 



tance, they would have been in constant danger of 

 dashing against every intervening obstacle. " In- 

 deed/' says Buflbn, " we may consider the celerity 

 with which an animal moves as a just indication of 

 the perfection of its vision. A bird, for instance, that 

 shoots swiftly through the air, must undoubtedly see 

 better than one which slowly describes a tortuous 

 tract. Among quadrupeds, again, the sloths have a 

 very limited sight." It may accordingly be inferred, 

 that birds have more precise ideas than slow-moving 

 caterpillars, of motion and its accompanying circum- 

 stances, such as those of relative velocity, extent of 

 country, the proportional height of eminences, and 

 the various inequalities of hill and dale, mountain and 

 valley. Our bird's-eye views, of which the accurate 

 execution is so tedious and difficult, give but a very 

 imperfect picture of the relative inequality of the sur- 

 faces which they represent ; but birds can choose the 

 proper stations^ can successively traverse a field in 

 all directions, and with one glance comprehend the 

 whole. On the other hand, the quadruped knows 

 only the spot where it feeds, its valley, its moun- 

 tain, or its plain ; but it has no conception of expanse 

 of surface, no idea of immense distances, and no 

 desire to push forward its excursions *. 



The eye of birds, it is worthy of remark, besides 

 being peculiar in structure, is also greatly larger than 

 in most other animals in proportion to the bulk of 

 the head. According to M. Petit, the ball of the 

 eye in a female eagle was, at its greatest width, an 

 inch and a half in diameter ; that of the male was 

 three lines less ; that of an ibis, six lines ; of a stork, 

 four times larger; that of the cassowary was four 

 times larger than its cornea, being an inch and a half 

 in diameter, while the cornea was only three lines f. 



* Montbeillard, Oiseaux, Prelim. 

 f M6m. I'Hist, des Animaux, 1726-36, 



