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- 



8 in Fig ' L ( AfterShu ' 



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 



