98 STRUCTURE AND COMPARISON. 



Wliile most birds have claws tliat are flattened or hollowed 

 beneath, the claws of birds of prey are generally rounded. 

 Yet hawks' claws have a slight groove beneath, owls' claws 

 have a flange on one side, and the foot of the fish-hawk, or 

 osprey, has a rounded or terete claw without either groove or 

 flange. Why these differences ? Perhaps no one can explain 

 them any more than why the long-legged heron and the short- 

 legged night-hawk — birds of utterly dissimilar habit and form 

 — should each have a comb-like ridge along the inner edge of 

 the middle toe-nail. The fact that differences exist in birds 

 of similar habits and that likenesses are found in birds of dis- 

 similar habits shows how hard it is to make a theory that will 

 cover all cases. 



