IV. 12. 131 



and carnivorous birds must of course get their food from the 

 bodies of other animals, and in most cases by violence. In such 

 birds, again, as live in marshes and are herbivorous the beak 

 is broad, this form being best suited for digging and cropping, 

 and for pulling up plants. In some of these marsh birds, 

 however, the beak is elongated, as also is the neck, the reason 

 for this being that the bird gets its food from some depth below 

 the surface. For most birds that have this conformation, and 

 most of those that are either actually web-footed or web-footed 

 in the partial way already mentioned, live by preying on some 

 of the small animals that are to be found in water. In these 

 the neck acts the part of a fishing-rod, the beak representing 

 the line and hook. 



The upper and under sides of the body, that is of what in 

 quadrupeds is called the trunk, present in birds one unbroken 

 surface. For there are no arms nor fore legs attached to it, but 

 in their stead wings, which are a distinctive peculiarity of birds; 

 and, as these wings are substitutes for arms, their terminal seg- 

 ments lie on the back in the place of a shoulder-blade.^ 



The legs are two in number, as in man ; not however, as in 

 man, bent outwards, but bent inwards like the hind legs of a 

 quadruped.^" The wings are bent like the fore legs of a quadruped, 

 having their convexity turned outwards. That the feet should be 

 two in number is a matter of necessity. For a bird is essentially 

 a sanguineous animal, and at the same time essentially a winged 

 animal ; and no sanguineous animal has more than four points 

 for motion." In birds, then, as in those other sanguineous animals 

 that live and move upon the ground,^^ the limbs attached to the 

 trunk are four in number. But, while in all the rest these four 

 limbs consist of a pair of arms and a pair of legs, or of four legs 

 as in quadrupeds, in birds alone the arms or fore legs are replaced 

 by a pair of wings, and this is their distinctive character. For 

 it is of the essence of a bird that it shall be able to fly ; and it 

 is by the extension of wings that this is made possible. Of all 

 arrangements, then, the only possible, and so the necessary, one 

 is that birds shall have two feet ; for this with the wings will 

 give them four points for motion. The breast in all birds is sharp- 

 edged, and fleshy.^3 -p^e sharp edge is of advantage in flight ; 

 for broad surfaces move with considerable difficulty, owing to 

 the large quantity of air which they have to displace. The fleshy 

 693 b. 



