462 
VERTEBRATA. 
THE UNAU. 
Both jaws are armed witTi molar and canine teeth ; the molars are four in the upper, and three in 
the lower jaw, and of a nearlj^ cylindrical form ; the canines are very small The tail is excess- 
ively short or entirely wanting. There are few animals which exhibit in a greater degree what 
appears to the careless observer to be deformity than the Sloth, and none that have on this account 
been more maligned by naturalists even of high standing. BnfFon, and many of the older zoologists, 
were eloquent upon the supposed defects of the unfortunate Sloth. We are gravely told by these 
writers, that when the Sloth ascends a tree for the purpose of feeding on its leaves, it is so lazy 
that it will not quit its station until every trace of verdure is devoured nay, some of them went 
so far as to assert that when this was the case, and the Sloth was compelled to look out for a 
fresh supply of sustenance, it would not take the trouble to descend the tree, but just allow itself 
to drop from a branch to the ground. Even Cuvier, who ought to have known better, echoes this 
tale, and insinuates that Nature, probably becoming weary of perfection, "wished to amuse her- 
self by producing something imperfect and grotesque" when the Sloths were formed ; and he 
proceeds with great gravity to show the "inconvenience of organization" which in his opinion, 
rendered the Sloths unfit for the enjoyment of life. 
It is perfectly true that on the ground these animals are about the most awkward creatures 
that can well be imagined, for their fore-legs are much longer than the hind ones ; all the toes 
are terminated by very long curved claws, and the general structure of the animals is such as en- 
tirely to preclude the possibility of their walking on all fours in the manner of an ordinary quad- 
ruped. In this, which is an unnatural situation, thoy certainly appear to be the most helpless of 
animals, and their only means of progression consists in hooking their claws to some inequality in 
the ground, and thus dragging their bodies painfully along. But i.n their natural home, among 
the branches of trees, all these seeming disadvantages vanish, nay, the very peculiarities of struc- 
ture which render the Sloths objects of pity on the ground, are found to adapt them the better 
for their true mode of existence. 
The structure of the anterior extremities agrees very closely with that of the same parts in 
man, and tliese members possess great freedom of motion. The feet are, however, very different 
in their construction from the human hand; the bones are firmly united together, and give sup- 
port to enormous claws, which are turned inward in repose, and rest against the palm. With 
these the Sloths cling firmly to the branches of the trees, from which they hang with their backs 
downward, and as it is in this position that most of their existence is passed, Ave can easily see that 
the mobility of the bones of the arms, coupled with a grasping arrangement, is peculiarly adapted 
to give them security and freedom of motion in their arboreal residence. Thus we perceive that 
