BIRDS. 



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sistance of the air causes it to be supported by 

 the wings. When the bird is resting upon its 

 legs, the centre of gravity must, in like manner, 



224 



w 



be brought immediately over the base of support 

 formed by the toes : it becomes necessary, there- 

 fore, to provide means for shifting the centre of 

 gravity from one place to another, according to 



