2^6 THE DOCTRINE OF DESCENT. 



characteristics of the horse and rhinoceros with those 

 of the camel. How far the latter, as ruminants, are 

 directly connected with the Macrauchenidae, or whether 

 the form of their skull, approaching that of the horse, 

 points to actual homology, it is for the present impossible 

 to say. 



The Anoplotheridse are likewise distinguished by a 

 sort of undifferentiated dentition, from \vhich a number 

 of specific forms might deviate in different directions. 

 The Tragulidse are descended from them in a direct 

 line ; they form a small group not unlike the musk 

 animals, and are confined to South Africa and Southern 

 Asia. As chewing the cud, they are more nearly allied 

 *-o the other typical ruminants with which we are ac- 

 quainted; but, on the other hand, they occupy an inter- 

 mediate position towards the other non-ruminants of the 

 division, of which the whole was united in the pre-historic 

 world through the Anoplotheridse. The Suidae, or pig- 

 like animals, were very profusely represented in the 

 Eocene and Miocene periods. From a side branch of 

 their predecessors, reaching up to the Anoplotheridae, are 

 descended the river-horses, or hippopotami. The function 

 of ruminating is, as we know, correlated with a complex 

 structure of the stomach as well as a peculiar, mechanism 

 of the cesophagal groove. It is naturally impossible to 

 determine in which fossil animals these arrangements 

 originated ; yet it seems to have occurred at a very 

 early period. Perhaps the more highly integrated struc- 

 ture of some non-ruminating genera, such as the hippo- 

 potamus and the peccary, may have been transmitted 

 from the age of the Anoplotheridae, and the very con- 

 spicuous accordance of the ruminating Tragulidse with 



