xii THE ORDERS PECORA AND BELLU^ 165 



group. Judging by the popular works on Zoology, it is still 

 as difficult to apprehend that a chevrotain is not a musk deer, 

 as it is that a manatee is not a cetacean, both errors of the 

 same kind, if not quite so gross, as that of regarding a whale 

 as a fish, or a bat as a bird. The Linnean genus Cervus 

 contains six species of true deer, including the moose, rein- 

 deer, red deer, fallow, and roe, associated with the giraffe. 



The twenty -one species of the great group of hollow- horned 

 Euminants, at that time recognised, are distributed quite 

 arbitrarily in three genera, Capra, Ovis, and Bos. Though 

 subsequent investigations have greatly increased the number 

 of species known, we are still in much uncertainty about their 

 mutual affinities and generic distinctions. Being a group of 

 comparatively modern origin, and only just attaining its 

 complete development, variation has chiefly affected the less 

 essential and superficial organs. Moreover, the process of 

 extinction of intermediate forms has not operated sufficiently 

 long to break it up into distinctly -separated natural minor 

 groups, as is the case with many of the older families, which 

 lend themselves, therefore, far more readily to systematic classi- 

 fication, especially as long as the extinct forms are unknown 

 or ignored. 



The sixth order of land mammals, BELLU^E, corresponding 

 to the Pacliydermata of Cuvier, contains what is now known 

 to be a heterogeneous collection, viz. the horses, the hippo- 

 potamus, the pigs, rhinoceroses, and the rodent capybara. The 

 abolition of these two last orders and the entire rearrangement 

 of the ungulate mammals into two different natural groups, 

 now called Artiodactyla and Perissodactyla, first indicated by 

 Cuvier in the " Ossemens fossiles," from the structure of the 

 limbs alone, and afterwards confirmed by Owen from comparison 

 of every part of the organisation, has been one of the most 

 solid advances made in our knowledge of the classification of 

 the Mammalia during the present century. 



The past history of this, as of so many other groups of 

 vertebrated animals, has been brought to light in an unexpected 

 manner by the wonderful discoveries of fossil remains made 

 during the last ten years in the Eocky Mountains of 

 America, discoveries, the importance of which will only be 



