of the Sloths to their peculiar Mode of Life. 19 



The extraordinary length of the arm and fore-arm, so inconvenient for 

 moving on the earth, are of essential and obvious utility to a creature 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- 

 metiy and beauty. 



Let us now endeavour to illustrate further some of Cuvier's descriptions of 

 the details of the skeleton of the A'i, 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 



D 2 



