DEVELOPMENT OF WING 



357 



The grey-breasted martin has a greater variation than 

 most, but that it is due to the effects of later specialization 

 is very well shown. In the very young nestling the differ- 

 ence between the carpus and digitus is well marked, but as 

 it grows older these proportions approach each other in 

 length until at maturity they are equal. The young stage is 

 evidently a shadow of what the bird was in more ancient 

 times. 



3 day nestling 



7 " " 



14 " 



Just able to fly - 



Progne chalybea (Gmel.) 



Carpus 



18.5 mm. 



17.6 " 



17.4 " 



16.5 " 



Adult ... 16. 



Digitus 

 13.5 mm. 



14.4 " 



14.5 " 

 15.4 " 

 16. 



The increase of the digitus is undoubtedly due to spe- 

 cialization of flight as in the case of the swift. As has al- 

 ready been shown in the case of this bird, a decrease of the 

 arm is followed by an increase of the hand. The digitus 

 must, therefore, be strengthened to support the strain put 

 upon it by the rapid beat of wings necessary for swiftly 

 dodging after insects. This, together with the increased use 



FIG. 121. DIAGRAM OF HAND DEVELOPMENT OF THE GREY-BREASTED MARTIN 



