I)K\I',I.()IVM1A'1' Ol- WINC 



.•{5' 



The grey-breasted martin has a greater variation than 

 most, but that it is (hie to tlie eH'eets of hiter speeiah'zation 

 is very well showji. In the vei-y young nestling the difl'er- 

 enee between the earpus and digitus is well mai-ked, hut as 

 it grows older these proportions a])proaeh each other in 

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

 evidently a shadow of what the bird was in more aneient 

 times. 



PnKjnc rJi(iI//J)ca ((xmel.) 



Car])us Digitus 



3 day nestling 18..") mm. 13. .3 mm. 



7 "* " 17.() " 14.4 " 



14 " " 17.4 " 14..J " 



Just able to fiv 16.5 " 15.4 " 



Adult * 10. " 16. " 



The inerease 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 inerease 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 

 dodsinff after insects. This, together with the increased use 



OCT f^ 



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



