84 THE BOOK OF MIGRATORY BIRDS 



incapable of seeing more than a few inches' distance, they 

 would have been in constant danger of dashing against 

 every intervening obstacle. "Indeed," remarks the same 

 writer, "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 circumstances, 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 birds'-eye views, of which the accurate execution 

 is so tedious and difficult, give but a very imperfect picture 

 of the relative inequality of the surfaces which they repre- 

 sent, but birds can choose the proper stations, can succes- 

 sively 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 or 

 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 a very distinguished writer, 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 times 

 less ; that of an ibis, six times ; of a stork, four times larger. 

 That of a cassowary was four times larger than its cornea, 

 being an inch and a half in diameter, while the cornea 

 was only three lines. The woodcock has very large, 

 prominent eyes, but it cannot support a strong light, and 

 sees best during twilight; and as Colonel Montagu 



