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A HISTORY OF BIRDS 



tion to similar mechanical requirements in the different kinds 

 of flight. That is to say, strenuous flight, where rapidity of 

 turning and great force in striking are required, is met by the 

 concentration of the breast muscles far forwards. Here the 

 necessary support is afforded either by lengthening the cora- 

 coids and strengthening the attachments of the furcula, either 

 by articular surfaces or by fusion, or by short coracoids widely 

 spread and of great breadth. These several modifications to 

 attain the same end have arisen as a result of compromise : 

 that is to say, of the transformation of the particular structural 



III. 41.— Shoulder-girdle and Sternum of Boatswain-bird 



peculiarities which had already been acquired by the species 

 before the adaptation in the new direction arose. 



The results of a concentration of development in the direc- 

 tion of flight have affected other parts of the skeleton, but in a 

 negative way. Thus, in birds which procure all or nearly all 

 their food on the wing the hind-limbs become reduced by dis- 

 use, or rather by the suspension of selection, to the smallest 

 possible limits compatible with functional existence. This is 

 well seen in the case of the Frigate-birds, Swifts and Humming- 

 birds, and the Swallow tribe, for example. 



Adaptations to the requirements of swimming have produced 



