290 ZOOLOGY. 
VERTEBRATA. 
Crass II]. Aves. Birps. 
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, with 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 powers 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 1s 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 
which 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 incaleulably 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 with air is the natural or healthy state of 
these cavities, and that they are always filled by active or unimpaired 
inspiration. This extraordinary arrangement is highly developed in rapacious 
and other birds which 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 elegance of form no class of vertebrated animals can be 
compared to birds, and they alone of the entire circle of animal life are gifted 
with voice which can be considered as musical or even agreeable to the ear. 
It is the latter character which immediately recommends them to all nations 
and classes of mankind, and establishes them as favorites. It has somewhat 
an aspect of misfortune, though, so far as relates to many of the pre-eminently 
superior songsters of Kurope, the business of catching them and reducing them 
to subjection has been long pursued and carried on to an extent which would 
be scarcely credited in America. 
Singing birds are almost exclusively restricted to the Insessorial or 
perching birds. The song was 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 
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