ZOOLOGY. 



to the vertebral column by a single occipital con- 

 dyle. The young are hatched and nurtured by 

 the parent. Their body is covered with feathers, 

 instead of hair or wool ; and, as a rule, the fore- 

 limbs are in the form of wings adapted for flight, 

 but they are never used for prehension. The 

 lungs are fixed in the chest, and the air-tubes 

 open on the surface of the lungs into air-cells, 

 scattered throughout the body. By this organisa- 

 tion, they can increase or diminish their density, so 

 as to range over the aerial regions with ease and 

 celerity. The bones of birds are singularly light, 

 and exhibit a greater degree of hardness than in 

 any other vertebrate. The lightness is due to the 

 marrow being replaced by air, and the hardness 

 to the presence of a large proportion of phosphate 

 of lime. The eye is generally so constructed that 

 they can see objects far and near with almost 

 equal clearness. Their posterior extremities serve 

 as the sole support of the body on the ground. 

 Most commonly the feet exhibit four toes, of 

 which one is directed behind, and three in front. 

 In the single posterior toe, the number of joints is 

 two ; in the external, five. The toes are terminated 

 by claws, which are used for different purposes. 

 Birds have no teeth, and therefore cannot masti- 

 cate their food, which is either torn by the beak 

 or swallowed whole, and is reduced to a soft state 

 in the stomach. The plan of the digestive system 

 most usual in this class is that which is exempli- 

 fied in the common fowl. The stomach consists 

 of three cavities : the first being formed by an 

 expansion of the gullet, which produces a bag or 

 chamber known as the crop. In this receptacle 

 the food is stored up, and transferred by degrees 

 to the second or membranous stomach, where it 

 is softened by the action of the gastric juice. It 

 is then conducted to the gizzard, or third cavity, 

 in which the process of digestion is completed. 

 This last stomach presents modifications varying 

 with the nature of the food upon which the bird 

 subsists. If it feeds on grain, the sides of this 

 stomach are of considerable thickness, and are 

 moved by powerful muscles, which act as a mill 

 in grinding down the food ; but in those species 

 which subsist on animal substances, or soft herb- 

 age, the muscles are reduced to extreme delicacy. 

 In many cases the process of digestion is pro- 

 moted by the swallowing of small pebbles, which, 

 being brought into contact with the food in the 

 gizzard by the muscular action of the stomach, 

 produce an effect similar to that of teeth, and in 

 some measure serve the purpose of these agents. 



The change of the plumage, termed moulting, 

 generally takes place annually ; while with some 

 species a partial casting of the feathers occurs 

 also at the breeding season. Many birds migrate 

 from one latitude to another, chiefly for the sake 

 of obtaining a better supply of food. The summer 

 immigrants that visit our island, as the swallow, 

 the rail, the cuckoo, are from tropical regions ; 

 while all winter visitants, as the Swan and Wild 

 Goose, come from the north. Under the direc- 

 tion of their highly developed instinctive powers, 

 the place for their nests appears to be selected, 

 their material collected, the nests themselves built, 

 the young reared in them, and their migrations 

 performed. 



Many birds are capable of domestication, and 

 in this we see an obvious approach towards that 

 higher form of attachment to man which is 



exhibited by many species among Mammalia. 

 Birds are of great utility to man, not only as an 

 article of food, -but also by keeping in check 

 noxious animals ; and in consuming carrion and 

 other refuse. A good classification of birds is 

 still a desideratum. Various classifications have 

 been proposed, but the one given here is that 

 generally employed. In the meantime, it must be 

 looked upon as merely provisional, since it is 

 founded upon habit, and not upon anatomical 

 characters or structure. All true classification 

 must be founded upon structure. 



Natatores. 



ORDER r. Natatores, or Swimmers, are adapted 

 to an aquatic life, and have the toes united by 

 a membrane or web, the legs short, and placed 

 behind the point of equilibrium. The body is 

 closely covered with feathers, and coated with a 

 thick down next the skin. In this order we find 

 the nearest approach to reptiles which is to be 

 found amongst birds. 



The Alcidce (Auk tribe) exhibit the most re- 

 markable adaptation of the structure of the bird 

 to an aquatic life, with which the entire order 

 presents us. This is best seen in the Penguins 

 \Aptenodytes), whose wings are very small, and 

 covered with mere vestiges of feathers, resem- 

 bling scales ; so that they serve as admirable fins 

 or paddles, but are totally useless for flight. The 

 feet are placed very far back, so that, when upon 

 land, the bird stands nearly erect. Having no 

 power of flight, and not being able to run, the 

 penguin may be overtaken with ease upon land ; 

 but once in the water, it distances its pursuers, 

 swimming with the ease and rapidity of a fish. 

 The Penguins are exclusively inhabitants of the 

 southern seas ; but the Puffins (Fratercula arcticd) 



Puffin (Fratercula arcticd). 



and Auks of the northern seas approach them in 

 their peculiar characters. The former, which 

 come to Britain, have short wings, capable of 

 sustaining them for a little while. Of the latter, 

 only one species has wings adapted for flight ; all 

 the other species are fitted for aquatic progression. 

 The Great Auk (A. impennis) is now entirely 

 extinct, having been destroyed by man himself. 

 The Puffin obtains its food by diving into the 

 water from a height, and often captures three or 



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