310 UNGULATA. 



sion of the solid hoofs against the ground. Accordingly, 

 among the Ungulates, the plan has been to gradually 

 lengthen and strengthen the bones of one or more of the 

 original five toes, and at the same time to dispense more 

 or less completely with the others. 



"A complete transition has thus been traced from a 

 five-toed Ungulate, walking partly on the soles of its 

 feet, to one provided with but a single toe to each foot, 

 and walking entirely upon the very tip of that one toe, 

 by which means the full extent of the limb comes into 

 play as an aid to speed. Throughout this series it is the 

 third or middle toe which has undergone development at 

 the expense of the others; and since this toe is always 

 symmetrical in itself, the term Odd-Toed Ungulates is 

 applied to the members of the group thus characterized. 



"The resources of nature are, however, manifold, and 

 instead of this being the only line of evolution of the 

 Ungulates, nearly similar results have been reached by a 

 totally different series of modifications; in some cases 

 instead of the third toe remaining symmetrical in itself 

 and gradually increasing in size at the expense of the 

 others, the third and fourth toes become symmetrical to 

 a vertical line drawn between them. When this takes 

 place the first toe disappears, and the second and fifth 

 become diminished ' in size; an instance of this stage of 

 development being presented by the pig, where the two 

 large and medially-symmetrical toes represent the third 

 and fourth of the typical series, while the two small lat- 

 eral ones are the second and fifth. In the pigs all the 

 metacarpal bones' remain distinct and relatively short; 

 but in the water-chevrotain of Africa the third and fourth 

 metacarpals become much elongated and closely applied 

 to one another, while the second and fifth are reduced 

 to mere splints, and their toes so diminished as to become 

 practically functionless. Finally, in the deer, oxen, and 

 their allies, the third and fourth metacarpals in the fore- 

 limb, and the corresponding metatarsal bones in the hind- 

 limb, have become completely fused into a single rod-like 

 bone, corresponding in function with the cannon-bone of 

 the horse, and generally known by the same name. The 

 dual origin of this cannon-bone is, how r ever, proclaimed 



