CHAPTER XIX 



AVES : THE BIRDS 



BIRDS are distinguished from all other animals by their clothing 

 of feathers. They differ conspicuously in many other ways from 

 the majority of vertebrates, but these differences are not altogether 

 peculiar to them. Thus, we may describe a bird as a vertebrate 

 creature capable of flight, but there are certain mammals, bats, 

 and in a modified degree flying squirrels, which also possess this 

 power ; while some species of bird, the apteryx, ostrich, cassowary, 

 penguin, cannot fly at all. Birds produce their young from eggs, 

 but this is also the case with fishes and many reptiles. The hard, 

 sheathed bill of a bird may be said to be a distinguishing feature, 

 but turtles also have beaks, and so has the duck-billed platypus, an 

 animal belonging to the mammals ; while the nest-building habits of 

 birds are shared by many small mammals such as mice and lemurs, 

 and several of the fishes. But in the possession of feathers the 

 class Aves stands alone ; and this characteristic is peculiar to 

 all birds, whether or not they possess the power of flight. 



To the casual observer no two creatures, perhaps, appear 

 more dissimilar than an agile, graceful bird and a sluggish, cold- 

 blooded reptile. Yet a bird is in reality only an extremely modified 

 reptile, and still plainly shows traces of its ancestry in the scales 

 of the foot, in the claw on the thumb of the wing, present in certain 

 species, and in the formation of the beak ; while in the skeleton 

 of a bird and of a reptile there are many points of resemblance. 



Both birds and reptiles produce the same type of egg, and 

 the development of the young within the shell is at first almost 

 identical. The bird-like character of a young chick is not apparent 

 until the sixth day, and the horny cap at the tip of the bill of an 

 unhatched chick, popularly called the " egg-tooth," which it uses to 

 break its way out of the shell, is also possessed by certain reptiles. 

 Indeed, birds and reptiles have so many points in common that 



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