GENERAL LAWS OF ANIMAL STRUCTURE. 



253 



(see figure of horse, Fig. 160, E). All the gradations are 

 easily found. 



4. Wrist Joint. — In man (when he comes down on all 

 fours), in monkeys, in bears, and some other mammals, 



A B CD 



Fig. 162. — A, fore limb of turtle ; B, mole ; C, whale ; D, fish. 



and in all reptiles, the wrist joint comes down to the 

 ground — the tread is on the whole hand. This was the 

 original tread of early land animals; but in all the 

 more specialized mammals, and especially in hoofed ani- 

 mals, the tread is on the toes, and the wrist is elevated 

 far above the ground, and in hoofed animals, as the 

 horse and the cow, is usually, but erroneously, called 

 the knee. 



5. In man, in many mammals, and in all reptiles, 

 there are five palm bones (metacarpals) and five fingers, 

 and this was undoubtedly the original and typical num- 

 ber ; but in all the more specialized mammals the num- 

 ber of toes is reduced in some, as the carnivores, to 

 four, in some hoofed animals to three (rhinoceros), in 

 some to two, as in ruminants, and in the horse to one. 

 The palm bones in ruminants are reduced to one (canon 



