CHAPTER III 

 STRUCTURE AND CLASSIFICATION OF BIRDS 



Though Birds are familiar objects to all, and obviously 

 characterized by the possession of wings and feathers, these 

 are not the only essential features distinguishing them, and it 

 will clear the ground to briefly describe some well-known and 

 typical example of the class. This is the better worth doing since 

 the great bulk of existing birds resemble one another to a much 

 greater extent than is the case with mammals. 



As convenient a type as any, and one possessing special 

 interest from the fact that Darwin devoted a very large amount 

 of attention to it from the evolutionary stand-point, is the Pigeon 

 (Columba livid]. Of this there are a large number of domestic 

 varieties, some very peculiar in appearance, but for our purpose 

 the best is the one known to fanciers as the " Blue Rock ", which 

 best represents the central form from which the different varieties 

 have sprung, and which is found wild in Europe, North Africa, 

 and West Asia as far as India. 



The wild Blue Rock nests in caves or clefts of the rock, 

 and is of common occurrence round the northern coasts of Ireland 

 and Scotland. Its prevailing hue is grey, but the rump is white, 

 and the neck and upper part of the breast green and purple, with 

 a metallic sheen. There is a characteristic broad black bar 

 running across the end of the tail, while the wing is crossed by 

 two similar but narrower bands. 



External Characters (fig. 99). The outlines of a bird are 

 mainly determined by the feathers, as is strikingly seen on com- 

 paring a living bird with a plucked specimen, the latter presenting 

 a very comical appearance. The head is well rounded behind, 

 in correspondence with the presence of a large brain, while the 

 face is produced in front into a somewhat conical horny beak, 

 at the base of which are situated the two slit-like nostrils, over- 



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