138 BIRDS. 



can be drawn over the ball of the eye on occasion, while 

 the lids continue open. 



External ears would be rather an inconvenience to an 

 animal so constituted ; but, to supply the place, birds are 

 furnished with auditory ducts of extreme delicacy. This 

 sense of hearing, indeed, must be exquisite : for how else 

 could they learn musical notes by artificial instruction, 

 or pronounce words exactly in the tone of the human 

 voice ? 



Nor are the olfactory nerves of this class less wonder- 

 ful. Some birds are capable of smelling their favourite 

 food at an astonishing distance, and even perceive danger 

 from the approach of other creatures before it is near. 

 The persons who earn a subsistence by catching wild fowl, 

 are so well aware of this instinctive sense, that they some- 

 times keep a piece of burning coal close to their mouths, 

 lest the objects of their pursuit should smell their breath, 

 and thus escape their lures. 



The legs and feet of birds are lightly constructed, for 

 their easier transportation through the air. The toes of 

 those that are destined to swim are webbed ; in other 

 kinds disjointed, the better to enable them to retain what 

 they have seized, or to cling occasionally to the branches 

 of trees. Such birds as are furnished with long legs have 

 also long necks, as they would otherwise be incapable 

 of picking up their food ; swans and geese, however, 

 with necks extremely long, have very short legs ; and 

 for that reason are better formed for swimming than for 

 walking. 



Thus each of the external parts appears fitted to the 

 life and situation of the animal. Nor are the inward, 

 though not immediately appropriated to flight, less neces- 

 sary to safety. All the bones are extremely thin and 

 light ; and the muscles very slight and feeble, excepting 

 those which afford motion to the wings. 



I shall next advert to the general history of this class. 

 To expatiate on such a subject would be not a little 



