COMPARATIVE ANATOMY. 
133 
excluded singly, and at considerable intervals. Ten, fif¬ 
teen, or twenty young birds may be produced in one cletch 
or covey, yet the parent bird have never been encum¬ 
bered by the load of more than one full-grown egg at one 
time. +, ■ 
VIII. A principal topic of comparison between animals, 
is in their instruments of motion. These come before us 
under three divisions; feet, wings, and fins. I desire any 
man to say, which of the three is best fitted for its use; or 
whether the same consummate art be not conspicuous in 
them all. The constitution of the elements in which the 
motion is to be performed, is very different. The animal 
action must necessarily follow that constitution. The 
Creator, therefore, if we might so speak, had to prepare for 
different situations, for different difficulties; yet the purpose 
is accomplished not less successfully in one case than in the 
other; and, as between wings and the corresponding limbs 
of quadrupeds, it is accomplished without deserting the 
general idea. The idea is modified, not deserted. Strip 
a wing of its feathers, and it bears no obscure resemblance 
to the fore leg of a quadruped. The articulations at the 
shoulder and the cubitus are much alike; and, what is a 
closer circumstance, in both cases the upper part of the 
limb consists of a single bone, the lower part of two. 
But, fitted up with its furniture of feathers and quills, it 
becomes a wonderful instrument, more artificial than its 
first appearance indicates, though that be very striking: at 
least, the use which the bird makes of its wings in flying 
is more complicated, and more curious, than is generally 
known. One thing is certain, that if the flapping of the 
wings in flight were no more than the reciprocal motion of 
the same surface in opposite directions, either upwards and 
downwards, or estimated in any oblique line, the bird 
would lose as much by one motion as she gained by another. 
The skylark could never ascend by such an action as this; 
for, though the stroke upon the air by the underside of her 
wing would carry her up, the stroke from the upper side, 
when she raised her wing again, would bring her down. 
In order, therefore, to account for the advantage which the 
bird derives from her wing, it is necessary to suppose that 
the surface of the wing, measured upon the same plane, is 
contracted whilst the wing is drawn up; and let out to its 
full expansion, when it descends upon the air for the pur¬ 
pose of moving the body by the reaction of that element. 
Now, the form and structure of the wing, its external con¬ 
vexity, the disposition, and particularly the overlapping, 
M 
