CONVERGENT EVOLUTION 441 



and great size of the caeca the Owls agree with the insectivorous 

 Caprimulgi — Night-jars. Thus the gut has remained apparently- 

 unchanged by the flesh diet which the Owls have adopted, 

 while the skeleton has undergone a really profound change, 

 thus showing that while extensive structural alterations were 

 necessary for the capture of vertebrate terrestrial prey, practically 

 no changes were necessary for its digestion ! The same rule 

 does not apply to the Hawks, since the gut shows many evi- 

 dences of specialisation, though derived from a flesh-eating 

 stock. Though both Hawks and Owls have acquired the same 

 skeletal peculiarities in adaptation to the requirements of a 

 raptorial life, the assimilation of this prey has curiously enough 

 demanded but little in the way of adaptation. 



The muscular systems of the Owls and Hawks, like the 

 intestines, differ widely. The former have the type prevailing 

 among the Caprimulgi, the latter the type of the Storks. But 

 there is no need to go on piling up instances of this kind, enough 

 has been shown to prove the independence of the two groups 

 despite their similarity to-day. 



In one instance the Accipitrine birds have approached the 

 Owls. Perhaps the most striking of these is the case of the 

 Harrier which has developed the same peculiar disc-like arrange- 

 ment of the feathers of the face which give the Owls their 

 characteristic appearance. On this account the older naturalists 

 regarded the Harriers as the natural transition from the Hawks 

 to the Owls. The second instance is furnished by the outer 

 toe, which in the Owls is reversible, or capable of being turned 

 backwards. The Osprey, and some other Hawks, possess a 

 similar freedom of movement in this toe. 



To appreciate the extent of the modification which the 

 framework of the Owl has undergone, the skeleton of one of 

 these birds should be compared with that of one of the Capri- 

 mulgi ; for choice that of the "Frog-mouth" Podargus, inas- 

 much as the species of this genus have adopted a partial flesh 

 diet. Although many points of difference would at once become 

 obvious on such a comparison, the most striking would be the 

 change in the form of the hip girdle and the greatly increased 

 length and size of the legs and toes, which in the Owl have 

 become grasping organs of enormous power. 



One Caprimulgine character the Owl has over-exaggerated 



