DEVELOPMENT OF WING 351 



There is a directly opposite variation between the hand 

 and the arm, while the length of the middle segment remains 

 constant throughout. The lengthened hand and shortened 

 arm is undoubtedly a parallelism with the swifts, though it 

 does not attain such extremes. The ratio of the wins ses- 

 ments of the giant swift {Chaetura zonaris alhicincta) of 

 the Guianas, commencing with the humerus, is 20-27-53, 

 while that of the martin is 24. 6-35. 8-40 per cent. There is a 

 vast difference, but the tendency is toward that of the swift. 



It would be hard to imagine the albatross with its tre- 

 mendously long humerus or, indeed, any other bird with a 

 long arm, dodging and twisting through the air with the 

 agility of a swift in pursuit of an insect. The laws of me- 

 chanics make it impossible for sucli a bird to move its wings 

 with the rapidity necessary for an operation of this kind. 

 On the other hand, a bird with a short humerus, because the 

 bone has a lesser arc to describe, can do this with less expense 

 of energy. Therefore the swift, from a need of having to 

 follow its rapid and elusive insect prey on the wing, has a 

 short arm, and to make up for tliis discrepancy, a pro^^ortion- 

 ally lengthened hand. Tlieir nesting habits are also condu- 

 cive to a shortened arm — nesting as they do, in caverns and 

 hollow trees or building great elongated tunnels that hang 

 from palm lea\'es and cliff walls ; but the habits are probably 

 a result of and not a cause for the shortening. 



The martin more or less parallels the swift in its general 

 feeding habits and nesting sites. Though it does not have 

 such supreme command over the air, still it makes a living 

 catching its food by aerial pursuit, ^\^hich necessitates a 

 good control of the wings and great dodging ability. Thus 

 the arm must have a tendency to shorten, and the hand to 

 lengthen. 



SUMMARY 



Throughout the development of all the birds above men- 

 tioned there seems to be a certain balance kept between the 



