PROGRESSION OF ANIMALS, xv. 



similar. For in birds the wings serve instead of front 

 legs, and so they are bent in the same manner as the 

 front legs of quadrupeds, since in the movement 

 involved in progression the natural beginning of 

 the change is from the wings, for their particular 

 form of movement is flight. Hence, if the wings 

 were taken away, no bird could stand or progress 

 forward. 



Further, since the bird is a biped and not erect, and 

 the front parts of its body are lighter, it is either 

 necessary (or at any rate more desirable), in order to 

 enable it to stand, that the thigh should be placed, as 

 it actually is, underneath, by which I mean growing 

 towards the hinder part. But if the thigh is neces- 

 sarily in this position, the bending of the leg must be 

 in a concave direction, as in the back legs of quad- 

 rupeds, and for the same reason as we gave in dealing 

 wdth viviparous quadrupeds. 



Generally in birds and winged insects and creatures 

 that s^^'im in the water (all, that is to say, that progress 

 in the water by means of their instrumental parts), it 

 is not difficult to see that it is better that the attach- 

 ment of such parts should be oblique, as in fact 

 it seems actually to be in the birds and the flying 

 insects. The same is also true of the fishes ; for the 

 wings in birds, the fins in fishes, and the wings in 

 flying insects all grow obliquely. This enables them 

 to cleave the air or water with the greatest speed 

 and force, and so effect their movement ; for the 

 hinder parts, too, can thus follow in a forward direc- 

 tion, being carried along in the yielding water or air. 



The oviparous quadrupeds which live in holes, 



* Kal TO. d-madev fj.6pia Jaeger : Kal rb oirLo-d^u to. (rd om. YZ) 

 fidpia libri. 



533 



