134 
COMPARATIVE ANATOMY. 
of its larger feathers, the action of the muscles,* and joints 
of the pinions, are all adapted to this alternate adjustment 
of its shape and dimensions. Such a twist, for instance, 
or semirotatory motion, is given to the great feathers of the 
wing, that they strike the air with their flat side, but rise 
from the stroke slantwise. The turning of the oar in row¬ 
ing whilst the rower advances his hand for a new stroke, 
is a similar operation to that of the feather, and takes its 
name from the resemblance. I believe that this faculty is 
not found in the great feathers of the tail. This is the 
place also for observing, that the pinions are so set up¬ 
on the body, as to bring down.the wings, not vertically, 
but in a direction obliquely tending towards the tail; which 
motion, by virtue of the common resolution of forces, does 
two .things at the same time; supports the body in the air, 
and carries it forward. 
The steerage of a bird in its flight is effected partly 
by the wing, ^ut in a principal degree by the tail. And 
herein we meet with a circumstance not a little remarka¬ 
ble. Birds with long legs have short tails, and in their 
flight place their legs close to their bodies, at the same 
time stretching them out backwards as far as they can.' 
fn this position the legs extend beyond the rump, and be¬ 
come the rudder; supplying that steerage which the tail 
could not. 
From the ivings of birds, the transition is easy to the 
fins of fish. They are both, to their respective tribes, the 
instruments of their motion; but in the work which they 
have to do, there is a considerable difference, founded on 
this circumstance. Fish, unlike birds, have very nearly 
the same specific gravity with the element in which they 
move. In the case of fish, therefore, there is little or no 
weight to bear up; what is wanted, is only an impulse suffi¬ 
cient to carry the body through a resisting medium, or to 
maintain the posture, or to support or restore the balance 
of the body, which is always the most unsteady where 
there is no weight to sink it. For these offices the fins are 
as large as necessary, though much smaller than wings, 
* There are three powerful muscles (the fleshy part of the breast) 
called pectoral muscles, which, with other smaller on the bones of the 
wing which are analogous to the arm, press with vigor on the air, the 
elasticity of which gives support. “ And it is remarkable that the gene¬ 
ral resemblance which the best form of windmill sails bears to the fea¬ 
thers of the wings of birds is striking, and one of those beautiful instances 
of truly mathematical principles on which the \vqrks of creation are cou* 
structed.”— Paxton. 
