COMPENSATION. 
159 
fig. 6.] At the angle of its wing there is a bent claw, 
exactly in the form of a hook, by which the bat at¬ 
taches itself to the sides of rocks, caves, and buildings, 
laying hold of crevices, joinings, chinks, and roughnesses. 
It hooks itself by this claw; remains suspended by this 
hold; takes its flight from this position: which operations 
compensate for the decrepitude of its legs and feet. With¬ 
out her hook, the bat would be the most helpless of all 
animals. She can neither run upon her feet, nor raise 
herself from the ground. These inabilities are made up 
to her by the contrivance in her wing: and in placing 
a claw on that part, the Creator has deviated from the 
analogy observed in winged animals.—A singular defect 
required a singular substitute. 
III. The crane kind are to live and seek their food 
amongst the waters; yet, having no web-feet, are incapa¬ 
ble of swimming. To make up for this deficiency, they 
are furnished with long legs for wading, or long bills for 
groping; or usually with both. This is compensation. But 
I think the true reflection upon the present instance is, 
how every part of nature is tenanted by appropriate in¬ 
habitants. Not only is the surface of deep waters peopled 
by numerous tribes of birds that swim, but marshes and 
shallow pools are furnished with hardly less numerous tribes 
of birds that wade. 
IV. The common parrot has, in the structure of its 
beak, both an inconveniency, and a compensation for it. 
When I speak of an inconveniency, I have a view to a di¬ 
lemma which frequently occurs in the works of nature, viz. 
that the peculiarity of structure by which an organ is made 
to answer one purpose, necessarily unfits it for some other 
purpose. This is the case before us. The upper bill of a 
parrot is so much hooked, and so much overlaps the lower, 
that if, as in other birds, the lower chap alone had motion, 
the bird could scarcely gape wide enough to receive its 
food: yet this hook and overlapping of the bill could not 
be spared, for it forms the very instrument by which the 
bird climbs, to say nothing of the use which it makes of 
it in breaking nuts and the hard substances upon which it 
feeds. How, therefore, has nature provided for the open¬ 
ing of this occluded mouth? By making the upper chap 
movable, [PI. XXX. fig. 7,] as well as the lower. In 
most birds, the upper chap is connected, and makes but 
one piece with the skull; but in the parrot, the upper chap 
is joined to the bone of the head by a strong membrane. 
