442 A HISTORY OF BIRDS 



— that is, the peculiarly " felted " character of the feathers, and 

 especially of the wing feathers. The " quills " of the Owl are 

 of great breadth, and the barbs or rami thereof are of consider- 

 able length and elasticity, while the radii or barbules are pro- 

 vided with specially elongated " fila " on the upper surface 

 which serve to ensure a perfectly noiseless flight — a very 

 important thing in birds which hunt their prey by twilight. 



A naturalist shown an Owl and a Hawk for the first time 

 would assuredly regard them as closely allied. Supposing him 

 to have procured the specimens for himself in the field, he would, 

 from his observations on their habits, attribute the peculiar 

 softness of the Owl's plumage to the need of a silent flight, 

 enabling the bird to hunt near the earth during twilight. 

 Harsh plumage creating a whirring sound would infallibly give 

 warning to the victims before capture became possible. It 

 would only be after a careful comparison of the details of 

 structure in the two types that he would discover the fact that 

 the resemblance was due to adaptation, and it would not be till 

 he had gone still further into the matter that he could decide 

 whether this resemblance was due to parallel development or to 

 homoplasy. 



Adaptation to a raptorial life has similarly transformed the 

 South American Cariama,^ one of the Gruiformes. But the 

 resemblance here is not so much to the typical " bird of prey," 

 such as the Hawk or Eagle, as to an aberrant and primitive 

 member of the Accipitres — the Secretary-bird {Serpentarius 

 secretariiis) of South Africa. Long-legged, and long-necked, 

 there is little in this bird to suggest relationship with the true 

 Accipitres at first sight, save the form of the beak. The internal 

 anatomy of this bird, however, leaves no room for doubt as to its 

 relationship. Compared with the Cariama, one cannot help 

 remarking that a kind of reciprocal convergence has taken place 

 here, inasmuch as the long neck and legs of the Secretary- 

 bird may well be regarded as Crane-like. By this standard 

 the only Accipitrine character of the Cariama is the hooked 

 beak ! 



But the Secretary-bird is — in so far as external appearance 

 is concerned — as it were, linked to the main Accipitrine chain 



^ There are really two species of Cariama, or " Seriema," representing as 

 many genera. 



