PECULIAR ORGANIZATIONS. 
139 
H. The oil with which birds prune their feathers, and 
the organ which supplies it, is a specific provision for the 
winged creation. On each side of the rump of birds is ob¬ 
served a small nipple, yielding upon pressure a butter-like 
substance, which the bird extracts by pinching the pap with 
its bill. With this oil, or ointment, thus procured, the bird 
dresses its coat; and repeats the action as often as its own 
sensations teach it that it is in any part wanted, or as the ex¬ 
cretion may be sufficient for the expense. The gland, the 
pap, the nature and quality of the excreted substance, the 
manner of obtaining' it from its lodgment in the body, the 
application of it when obtained, form, collectively, an evi¬ 
dence of intention which it is not easy to withstand. Noth¬ 
ing similar to it is found in unfeathered animals. What 
blind conatus of nature should produce it in birds ? should 
not produce it in beasts? 
III. The air-bladder also of a jish, [PI. XXIII. fig. 3,] af¬ 
fords a plain and direct instance, not only of contrivance, 
but strictly of that species of contrivance which we denom¬ 
inate mechanical. It is a philosophical apparatus in the body 
of an animal. The principle of the contrivance is clear; the 
application ofthe principle is also clear. The use of the or¬ 
gan to sustain, and, at will, also to elevate the body of the 
fish in the water, is proved by observing, what has been tried, 
that, when the bladder is burst, the fish grovels at the bot¬ 
tom; and also, that flounders, soles, skates, which are with¬ 
out the air-bladder, seldom rise in the water, and that, with 
effort. The manner in which the purpose is attained, and 
the suitableness of the means to the end, are not difficult 
to be apprehended. The rising and sinking of a fish in 
water, so far as it is independent of the stroke of the fins 
and tail, can only be regulated by the specific gravity of 
the body. When the bladder contained in the body of 
the fish, is contracted, which the fish probably possesses a 
muscular power of doing, the bulk of the fish is contracted 
along with it; whereby, since the absolute weight remains 
the same, the specific gravity, which is the sinking force, 
is increased, and the fish descends; on the contrary, when, 
in consequence of the relaxation of the muscles, the elas¬ 
ticity of the enclosed and now compressed air restores the 
dimensions of the bladder, the tendency downwards be¬ 
comes proportionably less than it was before, or is turned 
into a contrary tendency. These are known properties of 
bodies immersed in a fluid. The enamelled figures, or 
little glass bubbles, in a jar of water, are made to rise and 
fall by the same artifice. A diving machine might be 
