WHAT EVOLUTION IS 25 



web of the wing, the finger bones. In 

 the bird, unlike the bat, the expansion 

 of the wing is due to feathers but the 

 skeletal axis that supports the feath- 

 ers is formed from a set of bones 

 such as occur in the human arm, ex- 

 cept that the fingers are reduced in 

 number and bound together to serve 

 as a supporting axis for the larger 

 plumes. The flipper of a whale or 

 of a porpoise, superficially so unlike 

 the human arm, nevertheless shows 

 closely compacted within it the bone 

 of the upper arm, the two forearm 

 bones, wrist bones, and finger bones. 

 In the foreleg of the horse the bone 

 corresponding to that in the upper 

 arm of man is hidden in the flesh of 

 the animal. This bone is followed, 

 however, by the two bones of the 

 forearm, fused together, by the wrist 

 bones, which are situated at what is 

 popularly called the knee of the horse. 



