Arms and Legs 89 



(which include frogs, toads, newts, salamanders), the rep- 

 tiles, the birds, and the mammals. In the body of each of 

 these animals there is an internal skeleton or framework con- 

 sisting of a series of separate bones or vertebrae, forming the 

 " backbone " and carrying the brain box at one end, and, 

 typically, two pairs of appendages or limbs corresponding to 

 our arms and legs. 



The correspondence between our legs and the hind legs 

 of four-legged beasts is obvious. The correspondence be- 

 tween our arms and the wings of birds has also been com- 

 monly observed. Many people fail to see a similar corre- 

 spondence between the four limbs of quadrupeds or birds 

 and the two pairs of fins in fishes. For one thing, most of 

 the common fishes have several unpaired fins in addition to 

 the two pairs, so that the number of outgrowths is not typi- 

 cally four. For another thing, the fins themselves do not 

 in their appearance suggest arms or legs outwardly, so that 

 there is no suggestion of correspondence in their structure. 

 There are also numerous backboned animals that do not 

 have four limbs. The whale, for example, has only one pair 

 of flippers, and as everybody can see for himself, snakes have 

 no limbs at all. 



When we examine the arms and legs more closely, and 

 especially the skeletal parts, which have distinct structures 

 that can be most easily preserved, measured, and compared, 

 we discover some peculiar relationship (Fig. 22) . In the first 

 place, arms and legs are found to be constructed on pretty 

 miuch the same plan. This correspondence between the hind 

 leg and the front leg in backboned animals is followed, in 

 the second place, by what is perhaps a more striking cor- 

 respondence between the limbs of one class of vertebrates 

 and those of other classes (Fig. 23). The legs of a horse 

 and a crocodile, the legs of a salamander and an eagle show 

 fundamentally the same plan of structure. There is noth- 

 ing in the mode of life or in the outward appearance that 

 would lead us to expect such remarkably close correspond- 

 ence. We might indeed say that the leg is a leg, and be 



