NATURAL HISTORY OF BIRDS. 



CLASS IX. AVES. 



A BIRD is known by its feathers. Indeed, so distinctive is this sentence that it 

 does not admit of a single exception, for no bird is without feathers, and no animal is 

 invested with feathers except the birds. And so singularly adapted is this covering 

 to the aerial habits of most of the members of the bird class, that its structure is nearly 

 the same in all flying birds, while the only aberrant types of feathers are found in the 

 ostriches, the kiwis, and the penguins, all of which are deprived of the power of flight. 

 In the two first-mentioned groups the feathers resemble hairs more or less superficially, 

 and the representatives of the last order present a plumage somewhat suggestive of 

 scales, but both the hair-like and the scale-like appendages are in every respect true 

 feathers. Not less remarkable as indicative of the perfection of the feather is the 

 fact that the feathers of the oldest bird known, the fossil Archceopteryx from Solen- 

 hofen, were essentially like those of the majority of existing birds, and that nature 

 has not been able to improve much upon that admirable combination of lightness and 

 firmness since the Jurassic period. 



But the feather is not the only characteristic attribute of the birds, although it is 

 the only one which at once distinguishes them from all other living beings. From the 

 reptiles the feathered tribes differ, among other things, in possessing a complete double 

 circulation of the blood, which is warm, while the absence of milk glands separates 

 them widely from the mammals. Further characters which separate the birds from 

 the mammals are the single condyle of the occiput, and the articulation of the lower 

 jaw with a separate bone, the os qiiadratum, which again articulates with the skull. 

 The absence of a diaphragm may also be quoted in this connection. In these and 

 several other particulars the birds show a near relationship to the reptiles, so close, 

 indeed, that they have been included with them in a separate group, Sauropsida ; at any 

 rate, the birds are more nearly related to the reptiles than they are to the mammals, not- 

 withstanding the beak of the duck-mole and the recent re-discovery of the fact that 

 the Echidna lays eggs, and whatever was the origin of the mammals, so much is cer- 

 tain, that they sprang from an ancestral stock with which the birds are only remotely 

 connected. Their position between the reptiles and the mammals in our linear system 

 does not indicate any intermediate position in nature, but is simply due to our inability 

 of expressing exact relationships on a flat sheet of paper. 



There are other features which frequently are attributed to the bird class as diag- 

 nostic, but which really arc of but little account ; for instance, the modification of the 

 jaws into a beak sheathed with horn and destitute of teeth, for not only have the 

 VOL. iv. 1 



