of the Sloths to their peculiar Mode of Life. | c - 
The extraordinary length of the arm and fore-arm, so inconvenient for 
moving on the earth, are of essential and obvious utility to a greature whose 
body is of too great weight to allow it to crawl to the extremity of the branches 
to collect the extreme buds and youngest leaves, which form its food: these 
long arms in fact perform the office of the instrument called * lazy tongs,’ 
whereby the creature brings food to the mouth from a distant point without 
any movement of the trunk. The structure of the arm, fixed to the shoulder 
by an universal joint admitting of rotation, and having at the elbow two 
kinds of articulation, which allow pronation and supination, gives to the hand 
a power of moving in every possible direction. The breadth of the pelvis and 
outward position of the thigh-bones, which are also broad and flat, the distance 
of the knees from one another, and curvature of the bones of the leg, admirably 
adapt these extremities of the animal to the purpose of clasping, and, as it were, 
riding upon the trunks and branches of trees: a peculiar condition of life was 
to be provided for, viz. that of a quadruped which was to feed, to sleep, and, 
in short, to dwell ntirely upon trees; for the succulent nature of its food 
renders it unnecessary to descend to drink; and if we look at the anomalous 
extremities of this animal with a view to their use as instruments of continual 
suspension upon trunks and branches, the hind-legs performing the double 
office of adhesion and progression, and the fore-legs the quadruple function 
of adhesion, progression, prehension and defence, we shall find each article of 
deviation from ordinary structure adapted to some useful function in its pecu- 
liar economy; we shall find a new system of machinery, contrived, and set 
together, as it were, on a new plan from old materials, (as machines of differ- 
ent functions may be compounded from similar wheels, every motion having 
relation to some well-defined and useful end,) and the result of these devia- 
tions presenting an animal structure not less perfect in reference to its state, 
than those slender and graceful forms of light and active quadrupeds, with 
which we usually, and perhaps more justly associate our ideas of perfect sym- 
metry and beauty. 
Let us now endeavour to illustrate further some of Cuvier’s descriptions of 
the details of the skeleton of the 47, by considering the adaptation of each part 
to the habits of an animal living exclusively upon trees, and we shall not only 
discern no defect or imperfection, but shall find a probable final cause for each 
D2 
