COMPARATIVE ANATOMY. 
127 
wards, let the external color of the feathers be what it 
will, their Creator has universally given them a bed of 
black down next their bodies. Black, we know, is the 
warmest color; and the purpose here is, to keep in the 
heat, arising from the heart and circulation of the blood. 
It is farther likewise remarkable, that this is not found in 
larger birds; for which there is also a reason:—small birds 
are much more exposed to the cold than large ones; foras¬ 
much as they present, in proportion to their bulk, a much 
larger surface to the air. If a turkey were divided into a 
number of wrens, (supposing the shape of the turkey and 
the wren to be similar,) the surface of all the wrens would 
exceed the surface of the turkey, in the proportion of the 
length, breadth, (or, of any homologous line,) of a turkey 
to that of a wren; which would be, perhaps, a proportion 
of ten to one. It was necessary, therefore, that small 
birds should be more warmly clad than large ones: and this 
seems to be the expedient by which that exigency is pro¬ 
vided for. 
II. In comparing different animals, I know no part of 
their structure which exhibits greater variety, or, in that 
variety, a nicer accommodation to their respective conve- 
niency, than that which is seen in the different formations 
of their months. Whether the purpose be the reception of 
aliment merely, or the catching of prey, the picking up 
of seeds, the cropping of herbage, the extraction of juices, 
the suction of liquids, the breaking and grinding of food, 
the taste of that food, together with the respiration of air, 
and, in conjunction with it, the utterance of sound; these 
various offices are assigned to this one part, and, indiffer¬ 
ent species, provided for, as they are wanted, by its differ¬ 
ent constitution. In the human species, forasmuch as 
there are hands to convey the food to the mouth, the mouth 
is flat, and by reason of its flatness, fitted only for recep¬ 
tion; whereas the projecting jaws, the wide rictus, the 
pointed teeth of the dog and his affinities, enable them to 
apply their mouths to snatch and seize the objects of their 
pursuit. The full lips, the rough tongue, the corrugated 
cartilaginous palate, the broad cutting teeth of the ox, the 
deer, the horse, and the sheep, qualify this tribe for brows¬ 
ing upon their pasture; either gathering large mouthfuls 
at once, where the grass is long, which is the case with 
the ox in particular; or biting close, where it is short, 
which the horse and the sheep are able to do, in a degree 
that one could hardly expect. The retired under jaw of a 
swine works in the ground, after the protruding snout, likn 
