INTRODUCTORY 5 



reckoned the phenomena of convergence and parallel develop- 

 ment. 



In taking a bird's-eye group of the Class Aves one may dis- 

 tinguish a number of apparently well-defined groups, such as the 

 Ostrich tribe, Diving-birds, Penguins, Petrels, Steganopodous- 

 birds, e.g., Gannets and Cormorants, the Goose tribe, Storks, 

 Birds of Prey, " Game-birds," Cranes, and the Limicolae or 

 "Shore-birds": and against these may be set the Parrots, 

 Cuckoos, a host of forms commonly known as " Picarians," 

 including, for example, the gorgeously coloured Rollers, King- 

 fishers and Bee-eaters, and the bizarre Hornbills, the Owls and 

 Night-jars, the Swifts, and the jewel-like Humming-birds, the 

 gaudy Toucans, and the Woodpeckers. P'inally, we have the 

 most puzzling of all, the " Passerines," of which the crows may 

 be taken as typical examples. 



So far, any classification of this medley of forms, which 

 shall express the lines of descent, has proved beyond attain- 

 ment, though much towards this end has been done by the 

 labours of such men as Huxley, Garrod, Forbes, Fiirbringer, 

 Gadow, Beddard and Chalmers Mitchell. 



General Characters of Structure 

 The Feathers 



For the purposes of these pages a very general survey of 

 the structural characters of birds will suffice. And this survey 

 cannot well begin better than with a general account of the 

 feathers, since these, as we have already remarked, are unique 

 structures. 



In their nature they answer to the scales of Reptiles rather 

 than to the hairs of Mammals. In addition to those which 

 form the outer covering of the body, and hence are known as 

 the " contour " feathers, there are several other forms of 

 feathers. The most familiar of these are the "down feathers" 

 which play so important a part in commerce. These form, in 

 many birds, such as the Goose tribe. Gulls, and aquatic birds 

 generally, a dense underclothing answering to the under-fur of 

 mammals, such as, for instance, the " fur-seal," and after these 

 come the " filo-plumes " and " powder-down " feathers to be 

 described presently. 



