EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 



23 



cording to the same pattern. True, there are modifications. In the hog two 

 of the "fingers" are much larger than the other two, whereas in the sheep 

 only two are present, forming the support of the so-called "split hoof." The 

 two remaining fingers are the third and fourth (in numbering, the human 

 thumb is designated as I, the "index finger" as II, and so on). Digit III is 

 the only one remaining intact in the horse; its enlarged fingernail forms the 



scapulcn 



phoilanqes 



MAN DOG PIG SHEEP HORSE 



FIG. 3.2. Forelimbs of man and of several mammals variously adapted for v/alking and 

 running. Roman numerals identify the five digits. (After Le Conte.) 



solid hoof. In the horse the radius and ulna are fused together, and the 

 metacarpal of digit III is greatly enlarged and elongated, forming the so- 

 called cannon bone. Closely attached to the rear surface of the cannon 

 bone are two slender bones known as the splint bones; they represent re- 

 duced metacarpals of digits II and IV. 



Directing our attention to animals living in the water we note that 

 whales, seals, and sea lions have their forelimbs modified into paddle-like 

 flippers. Dissection of one of these flippers reveals that its skeleton is 

 composed of the same five segments we have noted in the arm of man and 



