DEVEl.OI'.Ml.N r Ol' WING 851 



Tlierc is a (lirt'ctly ()|)|)<)sitc variation hctvveen tlic liaiid 

 and the arm, wliik' the leni>tli of tlic nn'ddlc st'oinent remains 

 eonstant throui^liont. The len^tliened hand and sliortened 

 arm is undoubtedly a parallehsm with tlie swifts, th()u_i>li it 

 does not attain sueli extremes. The ratio of the wing seg- 

 ments of the giant swift {Chactura zonan'.s alhicincia) of 

 tlie Guianas, eommeneing with the liumei'us, is 'iO-'iT-.^S, 

 while that of tlie martin is "2 1..()-.'J.).8- H) per eent. There is a 

 vast diff'erenee, hut the tendency is toward that of the swift. 



It would be hai'd 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 foi- 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 w^th 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 this discrepancy, a proportion- 

 ally lengthened hand. Their 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 leaves 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, which necessitates a 

 good control of the Avings 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 



