746 THE UNGULATES, OR HOOFED MAMMALS 



bones or clavicles in the adult condition, although traces of these may occur in 

 the fetal state. 



Having said thus much, it may be well to endeavor to briefly sum- 

 .. . marize the chief characteristics by which the existing members of the 

 Ungulate order may be distinguished collectively from those of the 

 other groups of Mammals. 



In the first place, all Ungulates are adapted for a life on land; while, with the 

 exception of some species of hyrax, none of them are arboreal. Then, whereas some 

 of the more generalized forms are omnivorous, all the more specialized kinds are 

 strictly vegetable feeders. In all cases the cheek-teeth have broad crowns, furnished 

 with columns or ridges of greater or less complexity; and there are never less than 

 three pairs of molar teeth in each jaw. Collar bones are invariably absent; and the 

 limbs are, as a rule, restricted entirely to a backward and forward motion, there be- 

 ing in no case any power of rotating the fore-foot or the fore-leg. The upper end 

 of the radius, or smaller bone of the fore-limb, instead of being rounded, is accord- 

 ingly elongated transversely in the typical Ungulates. The terminal joints of the toes 

 are generally invested in solid horny hoofs, although in some cases furnished with broad 

 and blunt nails, but never with claws. Moreover, the number of toes is but very rarely 

 five, and may be reduced to three, two, or one; while in a large number of instances, 

 where four toes are present, only a single pair are of any functional importance. 



When, however, we have to take fossil species into consideration, many of these 

 characteristic features will not hold good; certain extinct Mammals, which it is very 

 difficult to separate satisfactorily from the Ungulates, having either collar bones, or 

 claws, or perhaps both together. In others, again, the upper molar teeth, instead 

 of having square crowns, show the triangular shape found in many Carnivores. 

 Indeed, strange though it may seem, the connection between the early Carnivores 

 and the early Ungulates is so close that it is frequently a matter of some difficulty 

 to determine to which group an extinct form should be referred; and it is highly 

 probable that the Ungulates are really a side branch, descended from the same stock 

 which gave rise to the Carnivores. This difficulty, or rather impossibility, of defin- 

 ing groups of animals, when we have to take into consideration their extinct rela- 

 tives, is merely what must of necessity be the case if the doctrine of evolution be the 

 true explanation of their mutual relationship. 



As a rule, existing Ungulates are characterized by their relatively- 

 large size; and among the order are included the most bulky of all land 

 Mammals. There is, however, a great variation in point of size among the order; the 

 smallest forms being the pigmy hog, the royal antelope, the chevrotains, and the hyrax; 

 while the largest are the elephants, the hippopotamus, the rhinoceroses, and the giraffe. 

 A frequent, although by no means general, peculiarity of the Ungu- 

 lates is the tendency to the development of horns of some kind or other 

 on the head; the nature of these horns, as we shall show later on, varying greatly in 

 the different groups. 



The order is well represented on all the continents of the globe, with 



the exception of Australia, but at the present day it has a far larger 



number of species in the Old World than in the New ; many of those from the 



