IV. 12. 133 



they never settle upon rocks.^''' For the character of their claws 

 is ill-suited for either action. 



All this is the necessary consequence of the process of develop- 

 ment. For the earthy matter in the body issues from it and is 

 converted into some or other useful kind of weapon. That which 

 flows upwards gives hardness or size to the beak ; and, should 

 any flow downwards, it either forms spurs upon the legs or gives 

 size and strength to the talons. But it does not at one and the 

 same time produce both these results, one in the legs, the other 

 in the claws ; for such a dispersion of this residual matter would 

 destroy all its efiicacy. In other birds this earthy residue furnishes 

 the legs with the material for their elongation ; or sometimes,, in 

 place of this, fills up the interspaces between the toes. Thus it 

 is simply a matter of necessity,'® that such birds as swim shall 

 either be actually web-footed, or, if not so, shall at any rate have 

 a kind of broad blade-like margin running along the whole length 

 of each distinct toe.*^ The forms, then, of these feet are simply 

 the necessary results of the causes that have been mentioned. 

 Yet at the same time they are intended for the animal's advantage. 

 For they are in harmony with the mode of life of these birds, 

 who, living on the water, where their wings are useless, require 

 that their feet shall be such as to serve in swimming. For these 

 feet are so developed as to resemble the broad-bladed oars of 

 a boat, or the fins of a fish ; and the destruction of the foot-web 

 has precisely the same effect as the destruction of the fin ; that 

 is to say, it puts an end to all power of swimming.^** 



In some birds the legs are very long, the cause of this being 

 that they inhabit - marshes. I say the cause, because nature makes 

 the organs for the function, and not the function for the organs.^^ 

 It is, then, because these birds are not meant for swimming that 

 their feet are unwebbed, and it is because they live on ground that 

 gives way under the foot that their legs and toes are elongated, 

 and that these latter in most of them have an extra number of 

 joints.^ Again, seeing that all birds are made of the same 

 substance,^ and yet are not all constructed alike to fly, the 

 materials which in some are expended on organs of flight can 

 in others be diverted to different purposes ; and in these birds 

 the nutriment which would otherwise go to form the tail-feathers 

 is used in increasing the dimensions of the legs. This is the 

 reason why these birds when they fly make use of their legs 

 694b. 



