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 

 (metacarpals) of the second 

 and third are co-ossified; 

 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 



Fig. 4. Skeleton of left wing of 

 the American Raven. Let- 

 ters as in Fig. 1. (After Shu- 

 feldt.) 



