POEM OF THE BIKD. 



standing variations which correspond with the different apti- 

 tudes for flight with which each species is endowed, we find 

 that it is about one- sixth of the whole weight in 

 number of birds. 



the greater 



In conclusion, each animal which sustains itself in the air 

 must develop work proportionate to its weight ; it ought, for 

 this purpose, to possess muscular mass in proportion to this 

 weight ; for, as we have already seen, if the actions performed 

 by the muscles of birds are always of the same nature, these 

 actions and the work which they perform will be in proportion 

 to the mass of the muscles. 

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