456 UNGULATES. 



an increased length in the metacarpal and metatarsal bones of those forms in which 

 but two functional bones remain, so in the present group there is a similar elonga- 

 tion of the single metacarpal and metatarsal (cannon) bones in its one-toed repre- 

 sentatives, namely, the horses. It is only of late years that the great importance 

 played by parallelism in the development of allied groups of animals has been 

 fully recognised, and fresh instances of it are being constantly discovered. In no 

 group are there better examples of this phenomenon than among the Ungulates, 

 where it is displayed among several groups, and affects totally different parts of 

 the skeleton. 



The lower cheek-teeth of the Odd-toed Ungulates very generally differ from 

 those of the other main group in that the last of the series resembles those in 

 advance of it in having two lobes, this feature being distinctive of the whole of the 

 existing members of the group. On the other hand, in all the living representatives 

 of the even-toed group, with the single exception of one small antelope {Neotragus, 

 p. 309), the corresponding tooth has three distinct lobes. Generally, the lower 

 cheek-teeth of the present group carry either two transverse ridges or a pair of 

 crescents, one in front of the other, on their crowns. It may be added that all the 

 Odd-toed Ungulates have simple stomachs, and that in all cases the liver is not 

 provided with a gall-bladder. 



The whole of the living Odd-toed Ungulates may be divided into three well- 

 marked family groups, which are commonly designated as tapirs, rhinoceroses, and 

 horses (the latter term including zebras, asses, etc.); and according to the classifica- 

 tion adopted in this work, each of these three families is now represented only by 

 a single genus. With the exception of the tapirs, which are common to the 

 Malayan region and Central and South America, all the existing Odd-toed Ungulates 

 are Old World animals. Moreover, all the three groups are represented by a com- 

 paratively small number of species, while, with the exception of the horses, these 

 species are far inferior in the number of individuals by which they are represented 

 to the majority of the Even-toed Ungulates. All these circumstances point to the 

 conclusion that, as a whole, the Odd-toed Ungulates are a waning group ; and this 

 conclusion is fully supported by the discoveries of palaeontology. Thus, in the 

 first place, both rhinoceroses and horses were abundantly represented during former 

 epochs in the New World ; while, in the second place, the rocks of both hemispheres 

 have yielded fossil remains of an enormous number of extinct generic, and even 

 family, types of Odd-toed Ungulates, several of which serve to connect very closely 

 together the three living groups. What may have been the reason of this gradual 

 waning of the Odd-toed Ungulates, and the enormous development of the Even-toed 

 group during the later geological epochs, it is not easy to divine. Perhaps, however, 

 it may be that the former group is one of a lower and less adaptive nature than the 

 latter. The horses are, however, an exception to the other members of the present 

 group, both as regards the number of species and individuals (irrespective of those 

 bred by man), and belong to a specialised branch which has been raised to a platform 

 of evolution as high as that occupied by the Ox family in the other group. Even here, 

 however, it is hard to understand why horses (until reintroduced by the Spaniards) 

 became extinct throughout the New World, unless indeed Mr. W. H. Hudson's 

 suggestion that they were exterminated by pumas should prove to be well founded 



