TAPIRS, RHINOCEROSES, AND HORSES 1043 



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vertical direction. In this respect, therefore, the Odd-Toed Ungulates have 

 developed in a manner exactly paralleled among the Even-Toed group, a similar 

 parallelism being also noticeable in respect to the reduction of the number of toes on 

 the feet. Moreover, as we find in the Even-Toed 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 elongation of the 

 single metacarpal and metatarsal (canon) bones in its one-toed representatives, 

 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 recognized, 

 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 ad- 

 vance 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. 897), 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 pro- 

 vided 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 g 

 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 comparatively- 

 small number of species, while, with the exception of the horses, these species are fai 

 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 rhinoc- 

 eroses 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 genera, 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 devine. 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 specialized branch which has been raised to a platform of evolution 



