EVIDENCE FROM COMPARATIVE ANATOMY 49 



of the feathers renders unnecessary any stretching 

 between bony supports. These supports are there- 

 fore developed according to another plan. The 

 forearm bones are compar- 

 atively short and strong, 

 complete and separate from 

 each other; the structure 

 of the hand varies some- 

 what in the different groups 

 of birds and it will there- 

 fore be simpler to take as a 

 type a particular bird, the 

 common raven. Here, the 

 bones of the wrist are re- 

 duced to two, partly by sup- 

 pression and partly by co- 

 ossification of the originally 

 more numerous elements. 

 There are three digits, of 

 which the first, or thumb, 

 is free, while the long bones 



. FIG. 4. Skeleton of left wing of 

 (metacarpals) OI the Second the American Raven. Let- 



and third are co-ossified; Sjy faIli>L (AfterShu ' 

 the fingers are represented 



by one or two free joints in each digit. In many 

 groups of birds the first digit has a claw, as, for ex- 

 ample, the spur on a swan's wing, and several birds 

 have claws on both the first and second digits. 



In the flightless birds, which, there is every reason 

 to believe, are descended from flying ancestors, the 



