rROGRESSION IN OR THROUGH THE AIR. 



185 



the strangeness of the appliances confused the bird, I allowed 

 it to walk about and to rest without removing the reeds. I 

 repeated the experiment at intervals, but with no better 

 results. The same phenomena, I may remark, were witnessed in 

 the sparrow ; so that I think there can be no doubt that a cer- 

 tain degree of flexion in the wings is indispensable to the flight 

 of all birds — the amount varying according to the length and 

 form of the pinions, and being greatest in the short broad- 

 winged birds, as the partridge and kingfisher, less in those 

 whose wings are moderately long and narrow, as the gulls, and 

 many of the oceanic birds, and least in the heavy-bodied long 

 and narrow- winged sailing or gliding birds, the best example 

 of which is the albatross. The degree of flexion, moreover, 

 varies according as the bird is rising, falling, or progressing 

 in a horizontal direction, it being greatest in the two former, 

 and least in the latter. 



It is true that in insects, unless perhaps in those which 

 fold or close the wing during repose, no flexion of the pinion 

 takes place in flight ; but this is no argument against this 

 mode of diminishing the wing-area during the up stroke 

 w^here the joints exist ; and it is more than probable that when 

 joints are present they are added to augment the power of 

 the wing during its active state, i.e. during flight, quite as 

 much as to assist in arranging the pinion on the back or side of 

 the body when the wing is passive and the animal is reposing. 

 The flexion of the wing is most obvious when the bird is 

 exerting itself, and may be detected in birds which skim or 

 glide when they are rising, or when they are vigorously flap- 

 ping their wings to secure the impetus necessary to the gliding 

 movement. It is less marked at the elbow-joint than at the 

 wrist ; and it may be stated generally that, as flexion de- 

 creases, the twisting flail-like movement of the wing at the 

 shoulder mcr eases, ^'^nd vice versa, — the great difference between 

 sailing birds and those which do not sail amounting to this, 

 that in the sailing birds the wing is worked from the shoulder 

 by being alternately rolled on and off* the wind, as in insects ; 

 whereas, in birds which do not glide, the spiral movement 

 travels along the arm as in bats, and manifests itself during 

 flexion and extension in the bending of the joints and in the 



