PROGRESSION OF ANIMALS, x.-xi. 



their feet in place of a tail and use their legs instead 

 of a tail to direct their flight. The flight of flying 

 insects is slow and weak, because the growth of their 

 wings is not in proportion to the weight of their body ; 

 for their weight is considerable, while their wings 

 are small and weak ; so they use their power of flight 

 like a merchant-ship attempting to travel by means 

 of oars. The weakness also of the wings themselves 

 and of their manner of growth contributes to some 

 extent to the result which we have described. Among 

 birds, the peacock's tail is at one season of no service 

 because of its size, at another useless because the 

 bird moults. But birds are the exact opposite of 

 winged insects in the nature of their ^^ings, especially 

 the swiftest flyers among them, namely, those with 

 curved talons ; for their swiftness of flight is useful 

 in enabling them to gain their livelihood. The other 

 parts of their body, too, seem to be similarly adapted 

 for their particular movement, the head being always 

 small and the neck not thick and the breast strong and 

 sharp — sharp so as to be compact like the prow of a 

 light-built ship, and strong owing to the way the flesh 

 grows — so as to thrust aside the air which meets it, 

 and that easily and without effort ; but the hinder 

 parts are light and contract again to a narrow point, 

 in order that they may follow the forward parts with- 

 out sweeping the air by their breadth. 



XL So much for the discussion of these topics. 

 The reason why an animal which is to walk erect 

 must both be a biped and also have the upper part 

 of its body lighter and the parts situated beneath 

 these heavier is obvious ; for only if it were so 



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