CLASSIFICATION 87 



are known, do not fall into either of these groups, but are in some 

 respects annectant forms, we have placed them provisionally, at 

 least, in a third group by themselves, named Arclneoceti. There 

 is nothing known at present to connect the Cetacea with any 

 other order of Mammals ; but it is quite as likely that they are 

 offsets of a primitive Ungulate as of a Carnivorous type, or perhaps 

 of a still more generalised mammalian stock. 



The remaining Eutherian mammals are clearly united by the 

 characters of their teeth, being all heterodont and diphyodont, with 

 their dental system reducible to a common formula. 



Although older views of, the relationship of Ungulate mammals 

 expressed by the terms Pachydermaia, Ruminantia, and so forth, still 

 linger in some corners of zoological literature, no single point in 

 zoological classification can be considered so firmly established as the 

 distinction between the Perissodactyle and Artiodactyle Ungulates ; 

 both being in the existing fauna of the world perfectly natural 

 and distinctly circumscribed groups. The breaking-up of the latter 

 into four equivalent sections, the Pecora, Tylopoda, Tragulina, and 

 Suina, is equally in accordance with all known facts. Less certain, 

 however, is the association of the Proboscidea and the Hyracoidea 

 with the true Ungulates. By many zoologists they are each, 

 although containing so very few existing species, made into distinct 

 orders ; and much is to be said in favour of this view. The 

 discovery, however, of a vast number of extinct species of Ungu- 

 lates which cannot be brought under the definition of either Perisso- 

 dactyla or Artiodactyla, and yet are evidently allied to both, and 

 to a certain extent bridge over the interval between them and the 

 isolated groups just mentioned, make it necessary either to intro- 

 duce a number of new and ill-defined ordinal divisions, or so to 

 widen the scope of the original order as to embrace them all, 

 considering the Elephants and the Hyraces as representing sub- 

 orders equivalent to the great Perissodactyle and Artiodactyle groups. 

 It is the latter alternative that we have adopted. 



The Rodentia, although generally presenting a low grade of 

 development, are a very specialised and distinct group. The 

 position here assigned to them would accord with apparent relation- 

 ships with the Ungulates, through the Elephant on the one hand 

 and the extinct Tijpotherium on the other. 



In the present state of the fauna of the earth, the Carnivora 

 form a very distinct order, though naturally subdivided into two 

 groups, the members of the one being more typical, while those of 

 the other (the Pinnvpedia) are aberrant, having the whole of their 

 organisation specially modified for living habitually in the water. 



The Insectivora comprise various lowly organised and generalised 

 forms, exhibiting considerable divergence of character, and ap- 

 parently connected through transitional extinct species with the 



