290 ZOOLOGY. 



VERTEBRATA. 

 Class III. Avks. Birds. 



No class of the Animal Kingdom is more stongly marked or more 

 competely isolated from other classes than that of Birds. Having the body 

 covered with feathers, which are peculiar to this class, Avith the two anterior 

 extremities analogous to the fore legs in quadrupeds transformed into wings, 

 and an organization completely adapted to inhabiting the air, at various 

 altitudes, during the greater portion of their lives, they are capable of swiftness 

 of motion surpassing any other of the Vertebrata, and wander over the regions 

 of space with great ease and celerity. We wish to be understood, however, as 

 speaking of the general character of the class ; for nature, ever disposed to 

 vary her productions, has given to some species very limited poAvers of flight, 

 and totally denied them to others. 



For the purposes of flight, and especially for such long continued 

 exertion upon the wing as may be observed in many birds, an admirable 

 adaptation of anatomical organization subserves. It is obvious that a 

 great degree of muscular power is indispensable, and we accordingly find 

 it, and sustained too by a provision for producing the most energetic and 

 protracted respiration. This is the curious arrangement by which the air 

 Avhich enters the body is not restricted to a pair of lungs, but is transmitted 

 to various large air-cells in the abdomen and other parts of the body, and 

 even into the interior of the bones. By this means the respiratory surface 

 is incalculably extended, and the specific gravity of the body greatly 

 diminished. There are also other anatomical characters, from which it is 

 demonstrable that to be filled Avith air is the natural or healthy state of 

 these cavities, and that they are ahvays filled by active or unimpaired 

 inspiration. This extraordinary arrangement is highly developed in rapacious 

 and other birds Avhich habitually fly to great heights in the atmosphere ; but 

 it is also found in great perfection in singing birds, and has evidently an 

 additional use in enabling them by deep and less frequent inspirations to 

 sustain protracted song. 



For general eleorance of form no class of vertebrated animals can be 

 compared to birds, and they alone of the entire circle of animal life are gifted 

 Avith A^oice which can be considered as musical or even agreeable to the ear. 

 It is the latter character Avhich immediately recommends them to all nations 

 and classes of mankind, and establishes them as favorites. It has somcAvhat 

 an aspect of misfortune, though, so far as relates to many of the pre-eminently 

 superior songsters of Europe, the business of catching them and reducing them 

 to subjection has been long pursued and carried on to an extent Avhich would 

 be scai-cely credited in America. 



Singing birds are almost exclusively restricted to the Insessorial or 



perching birds. The song Avas formerly supposed to be peculiarly the 



expression of love during the breeding season, but this is not strictly the 



case, for many species sing in the autumn long after the labors of incubation 



494 



