156 UNGULATES. 



Definition of Having said thus much, it may be well to endeavour to briefly 



Ungulates, summarise 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 generalised forms are omnivorous, all the more specialised 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 in- 

 variably absent ; and the limbs are, as a rule, restricted entirely to a backward- 

 and-forward motion, there being 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 accordingly 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. More- 

 over, 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 Carni- 

 vores. 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 defining groups of animals, when we have to take into con- 

 sideration their extinct relatives, 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 characterised 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 rhino- 

 ceroses, and the giraffe. 



A frequent, although by no means general peculiarity of the 



Ungulates 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 former area belonging to groups quite unknown in the latter. Although repre- 



