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 Anoplotheridae are likewise distinguished by a 

 sort of undifferentiated dentition, from which a number 

 of specific forms might deviate in different directions. 

 The Tragulidae are descended from them in a direct 

 line; they form a small group not unlike the musk ani- 

 mals, and are confined to South Africa and Southern 

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

 to 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 Anoplotheridae. 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 func- 

 tion of ruminating is, as we know, correlated with a com- 

 plex structure of the stomach as well as a peculiar mech- 

 anism of the oesophagal groove. It is naturally im- 

 possible to determine in which fossil animals these 

 arrangements originated; yet it seems to have occurred 

 at a very early period. Perhaps the more highly in- 

 tegrated structure of some non-ruminating genera, such 

 as the hippopotamus and the peccary, may have been 

 transmitted from the age of the Anoplotheridae, and the 

 very conspicuous accordance of the ruminating Tragulidae 



