GENERAL CHARACTERS OF BIRDS. 51 



The lowest is in the aquatic species, whose general activity is much 

 less than that of the tribes, which spend most of their time in the air ; the 

 highest is among those distinguished for the rapidity and energy of their flight, 

 such as the Swallow. 



45. Birds have been denominated, and not inappropriately, the Insects of the 

 Vertebrated series. As in the animals of that class, we find the whole struc- 

 ture peculiarly adapted to motion, not in water, nor upon solid ground, but in 

 the elastic and yielding air. It is impossible to conceive any more beautiful 

 series of adaptions of structure to conditions of existence, than that which is 

 exhibited in the conformation of the Bird, with reference to its intended mode 

 of life. In order to adapt the Vertebrated animal to its aerial residence, its 

 body must be rendered of as low specific gravity as possible. It is further 

 necessary that the surface should be capable of being greatly extended ; and 

 this by some kind of appendage that should be extremely light, and at the 

 same time possessed of considerable resistance. The degree of muscular 



' power required for support and propulsion in the air, involves the necessity of 

 a very high amount of respiration ( 392), for which it has been seen that an 

 express provision exists in Insects ; and as the general activity of the vital 

 processes depends greatly upon the high temperature which this energetic 

 respiration keeps up, a provision is required for keeping in this heat, and not 

 allowing it to be carried away by the atmosphere through which the Bird is 

 rapidly flying. 



46. The first and third of these objects, the lightening of the body, and 

 the extension of the respiratory surface, are beautifully fulfilled in a mode, 

 which will be found to correspond with the plan adopted for the same purpose 

 in Insects. The air which enters the body, is not restricted to a single pair of 

 air-sacs or lungs placed near the throat ; but is transmitted from the true lungs, 

 to a series of large air-cells, disposed in the abdomen and in various other parts 

 of the body. Even the interior of the bones is made subservient to the same 

 purpose ; being hollow, and lined with a delicate membrane, over which the 

 blood-vessels are minutely distributed. In this manner, the respiratory surface 

 is greatly extended ; whilst, by the large quantity of air introduced into the 

 mass, its specific gravity is diminished. The subservience of the cavities in 

 the bones to the respiratory function, is curiously shown by the fact, which 

 has been ascertained both accidentally and by a designed experiment, that, 

 if the trachea of a Bird be tied, and an aperture be made in one of the long 

 bones, it will respire through this. 



47. The other two objects, the extension of the surface, and the retention 

 of the heat within the body, are also accomplished in combination, by a most 

 beautiful and refined contrivance, the covering of feathers. Like hair or scales, 

 feathers are to be regarded as appendages to the cutis ; the stem is formed 

 from it by an apparatus, which may be likened to a hair-bulb on a very large 

 scale ; but there are some additional parts for the production of the laminse, 

 which form the vane of the feather, and which are joined to the stem during 

 its development. These laminae, when perfectly formed, are connected by 

 minute barbs at their edges, which hook into one another, and thus give the 

 necessary means of resistance to the air. The substance of which feathers 

 consists, is a very bad conductor of heat ; and when they are lying one over 

 the other, small quantities of air are included, which still further obstruct its 

 transmission by their non-conducting power. Thus the two chief objects are 

 fulfilled ; power of resistance and slow conducting properties being obtained, 

 in combination with lightness and elasticity. At the two extremes of the class, 

 however, we meet with remarkable modifications in the typical structure^ of 

 feathers. In the Penguin, those which cover the surface of the wings have 

 a strong resemblance to scales ; and the wings are not employed to raise this 



