48 ii. 13 — ii. 14. 



to the nostrils, that the membrane comes. For it is better to 

 have one starting-point for nictitation than two ; and in these 

 birds this starting-point is the junction of eye and nostrils, 

 an anterior starting-point being preferable to a lateral one.'" 

 Oviparous quadrupeds do not blink in like manner as the birds ; 

 for, living as they do on the ground, they are free from the 

 necessity of having eyes of fluid consistency and of keen sight, 

 whereas these are essential requisites for such birds as have to 

 use their eyes at long distances. This too explains why birds 

 with talons, that have to search for prey by eye from aloft, and 

 therefore soar to greater heights than other birds, are sharp- 

 sighted ; while common fowls and the like, that live on the 

 ground and are not made for flight, have no such keenness of 

 vision. For there is nothing in their mode of life which 

 imperatively requires it. 



Fishes and Insects and the hard-skinned Crustacea present 

 certain differences in their eyes, but so far resemble each other as 

 that none of them have eyelids.'^ As for the hard-skinned Crus- 

 tacea it is utterly out of the question that they should have any ; 

 for an eyelid, to be of use, requires the action of the skin to be 

 rapid. These animals then have no eyelids and, in default of 

 this protection, their eyes are hard, just as though the lid were 

 attached to the surface of the eye, and the animal saw through 

 it. Inasmuch however as such hardness must necessarily blunt 

 the sharpness of vision, nature has endowed the eyes of Insects, 

 and still more those of Crustacea,'^ with great mobility (just as 

 she has given some quadrupeds very moveable ears), in order 

 that they may be able to turn to the light and catch its rays, and 

 so see more plainly. Fishes however have eyes of a fluid con- 

 sistency. For all animals that move much about have to use 

 their vision at considerable distances. If now they live on land, 

 the air in which they move is transparent enough. But the water 

 in which fishes live is a hindrance to sharp sight, though it has 

 this advantage over the air, that it does not contain so many 

 objects to knock against the eyes. The risk of collision being 

 thus small, nature, who makes nothing in vain, has given no eye- 

 lids to fishes, while to counterbalance the opacity of the water she 

 has made their eyes of fluid consistency. 



(Ch. 14.^ All animals that have hairs on the body have lashes 

 on the eyelids ; but birds and animals with scale-like plates, being 

 658 a 



