PROGRESSION IN OR THROUGH THE AIR. 183 



however, the bat's wing, as has been already stated, is covered 

 by a continuous and more or less elastic membrane, it follows 

 that it cannot be opened up to admit of the air passing 

 through it during the up stroke. Flight in the bat is therefore 

 secured by alternately diminishing and increasing the area of 

 the wing during the up and down strokes — the wing rotating 

 upon its root and along its anterior margin, and presenting a 

 variety of kite-like surfaces, during its ascent and descent, pre- 

 cisely as in the bird (fig. 80, p. 149, and fig. 83, p. 158). 



of the wings {a b, e f; a' h\ e' f ) in the flight of the kingfisher. The body 

 and wmgs when taken together form a kite. Compare with fig. 59, p. 

 126, where the wings are fully extended. 



^ Flexion of the JFing necessary to the Flight of Birds. — Con- 

 siderable diversity of opinion exists as to whether birds do or 

 do not flex their wings in flight. The discrepancy is OAving 

 to the great difficulty experienced in analysing animal move- 

 ments, particularly when, as in the case of the wings, they are 

 consecutive and rapid. My own opinion is, that the wings 

 are flexed in flight, but that all wings are not flexed to the 

 same extent, and that what holds true of one wing does not 

 necessarily hold true of another. To see the flexing of the 

 wing properly, the observer should be either immediately 

 above the bird or directly beneath it. If the bird be con- 



