150 



ANIMAL LOCOMOTION. 



wards during the up stroke, to enable the air to pass through 

 it. The primary, secondary, and tertiary feathers have thus 

 a distinctly valvular action.^ The feathers rotate upon their 

 short axes (m n) during the descent and ascent of the wing, 

 the tip of the feathers rising slightly during the descent of 

 the pinion, and falling during its ascent. The same move- 

 ment virtually takes place in the posterior margin of the 

 wing of the insect and bat. 



The Wing vibrates unequally with reference to a given Line. — 

 The wing, during its vibration, descends further below the 

 body than it rises above it. This is necessary for elemting 

 purposes. In like manner the posterior margin of the wing 

 (whatever the position of the organ) descends further below 

 the anterior margin than it ascends above it. This is re- 

 quisite for elevating and propelling purposes ; the under surface 

 of the wing being always presented at a certain upward angle 

 to the horizon, and acting as a true kite (figs. 82 and 83, p. 

 158. Compare with fig. 116, p. 231). If the wing oscil- 

 lated equally above and beneath the body, and if the pos- 

 terior margin of the wing vibrated equally above and below 

 the line formed by the anterior margin, much of its elevating 

 and propelling power would be sacrificed. The tail of the 

 fish oscillates on either side of a given line, but it is other- 

 wise with the wing of a flying animal. The fish is of nearly 

 the same specific gravity as the water, so that the tail may 

 be said only to propel. The flying animal, on the other 

 hand, is very much heavier than the air, so that the Aving re- 

 quires both to propel and elevate. The wing, to be etFective as 

 an elevating organ, must consequently be vibrated rather below 

 than above the centre of gravity ; at all events, the intensity 

 of the vibration should occur rather below that point. In 

 making this statement, it is necessary to bear in mind that 

 the centre of gravity is ever varying^ the body rising and falling 

 in a series of curves as the wings ascend and descend. 



To elevate and propel, the posterior margin of the wing must 

 rotate round the anterior one ; the posterior margin being, as 

 a rule, always on a lower level than the anterior one. By 

 the oblique and more vigorous play of the wings under rather 

 than above the body, each wing expends its entire energy in 

 1 The degree of valvular action varies according to circunistanccs. 



