S4 GENERAL CHARACTERS OF MAMMALIA. 



can climb trees simply grappling them, although their fingers are 

 neither long, very flexible, nor opposable one to the other. Most 

 animals of this kind have a long tail which serves them as a 

 balance-pole, and sometimes this organ is so flexible as to b 

 twisted around branches, and take the place of a species of hand. 



15. Differences are also observed in the conformation of the 

 extremities, according as the animal is designed to run or leap ; 

 in the latter case, the posterior extremities are generally much 

 longer than the anterior for example, the rabbit and particularly 

 the kangaroo. (l j late 3,^. 1 1, and plate 4,yfy. 11.) 



16. When the paws are to be used by the animal for burrow- 

 ing in the earth, they are short, (which gives them greater 

 strength,) stout, of a particular form, and armed with strong 

 nails. Of all the mammalia, the thoracic extremities of the mole 

 possess the best conformation for this purpose. (Plate 9^ fig. 10.) 



17. In order that the extremities be formed in the most favor- 

 able manner for swimming, they must be short and broad so as 

 to strike the water with the greater force and act upon a large 

 surface. With those mammalia whose lives are entirely aquatic, 

 these organs are formed like great battledoors, closely resembling 

 the fins of fishes ; the arm and fore-arm become so short that the 

 paw seems to be attached immediately to the body ; and the 

 fingers are all concealed beneath a common skin. (Plate 6, fiy. 6.) 

 When an animal has to make use of these fins to drag itself on 

 shore, their conformation approaches nearer to that of the paw of 

 an ordinary quadruped, ( I'Litc 3, -fiy. 7,) and when the extremi- 

 ties are to be chiefly used in running, without ceasing to be 

 appropriate for swimming, the fingers (toes) are simply united 

 by a loose fold of skin, called web, which is stretched when they 

 are separated, and thus affords the requisite breadth to the paw. 



18. When the extremities of the mammalia are formed for 

 flying, they also present a peculiar disposition ; the thoracic ex- 

 tremities become very long, and the fingers especially are elon- 

 gated out of proportion, and sustain a fold of sldn from the sides, 

 as the whale-bones of an umbrella stretch the silk or cloth. 

 Bats afford us an example of this kind of organization ; their 

 hands are in a manner transformed into true wings, (I'late %,fig 1.) 



15. What is the chu.acter of the extremities of those animals that run or 

 leap? 



1 6. What is the conformation of the extremities of burrowing animals ? 

 What animal has extremities best adapted for burrowing ? 



17. What is the form of the extremities of swimming animals ? What \ 

 the form of the extremities of aquatic mammalia ? 



16. What is the form of the extremities of those ma nmalii that fly ? 



