110 CLASS AVES. 



and perfection, that sense which should be their principal guide. 

 They are able to traverse a great space in a very little time : 

 they, therefore, must be enabled to discern its extent and 

 limits. Had nature, in bestowing on them such rapidity of 

 flight, rendered them at the same time short-sighted, these two 

 qualities would have been contrary, and the bird would not 

 have dared to make use of his lightness, nor attempted a rapid 

 flight : he would only have hovered slowly along under the 

 dread of unforeseen shocks and resistances. The swiftness 

 with which a bird can fly may indicate the extent of his 

 reach of vision ; not, however, absolutely, but relatively. A 

 bird whose flight is quick, direct and sustained, certainly sees 

 farther than another of the same form, which moves more 

 slowly and obliquely ; and, had nature ever produced birds 

 with short sight and rapid wing, such species must have speedily 

 perished from this contrariety of qualities, one of which not 

 only hinders the exercise of the other, but exposes the indivi- 

 dual to an infinite number of risks. From all this we may pre- 

 sume that the birds whose flight is shortest and slowest are also 

 those whose power of vision is the least extended. Just as 

 among the quadrupeds we find the unau and the ai, which 

 move but slowly, have the eyes almost hidden, and the sight 

 but faint." 



We have already spoken of the nictitating membrane, Birds 

 have also in their eyes a large quantity of aqueous humour, 

 especially birds of elevated flight, that the light may be so 

 much the more refracted as the air in which they rise becomes 

 more rarefied. The reverse is the case with the fishes, for the 

 light is sufficiently refracted through the watery medium in 

 which they are immersed, and which is so much denser than 

 the air. 



The power, which, however it may be explained, birds do 

 certainly possess of altering the convexity of the eye, of ren- 

 dering the sight more or less distant, according to the wants of 

 the animal, by correcting the divergence of the visual rays, is 



