4:1 ON THE STUDY OF ZOOLOGY 



internal mechanism and its movements, display the most 

 evident transitions and gradations of organization and eco- 

 nomy. We see classes and orders — as, for example, birds, 

 and the testudines (the turtle and tortoise kinds) — which, 

 by their external configuration, are quite insulated in the 

 creation, connected in the most natural manner with others 

 of quite different form, and united to them by the principle 

 of internal resemblance. 



The four component parts of the upper extremity, viz. the 

 shoulder, arm, fore-arm, and hand, can be clearly shewn 

 to exist in the anterior extremities of all mammalia ; how- 

 ever dissimilar they may appear on a superficial inspection, 

 and however widely they may seem to deviate from tlie 

 human structure. The wings of the bat, osteologically 

 considered, are hands ; the bony stretchers of the cutaneous 

 membrane being the digital phalanges extremely elongated. 

 The dolphin, porpoise, and all other whales, have a fin on 

 each side, just behind the head, consisting apparently of a 

 single piece. But we find, under the integuments of this 

 fin-like member, all the bones of an anterior extremity, 

 flattened indeed, and hardly susceptible of motion on each 

 other, but distinctly recognizable : there are, a scapula, 

 humerus, bones of the fore-arm, carpus, metacarpus, and 

 five fingers. The fore-feet of the sea-otter, seal, walrus, 

 and manati, form the connecting links between the anterior 

 extremities of other mammalia and the pectoral fins of the 

 whale kind : the bones are so covered and connected by 

 integuments, as to constitute a part adapted to swimming ; 

 but these are much more developed than in the latter 

 animal, and have free motion on each other. The bones 

 of the wing of birds have a great and unexpected resem- 

 blance to those of the fore-feet of the mammalia : and the 

 fin-like anterior member of the penguin, applicable only to 

 swimming, contains within the integuments the same bones 

 as the wings of other birds which execute the very different 

 office of flight. 



The same point is illustrated by another kind of cases 

 in comparative anatomy, viz. the existence of certain 



