XI PALEONTOLOGY AND EVOLUTION 363 



Hyracothcrium, and Pliolophus. Suppose for a 

 moment, for the sake of following out the 

 argument, that Pliolophus represents the primary 

 stock of the Perissodactyles, and Dickobunc that 

 of the Artiodactyles (though I am far from saying 

 that such is the case), then we find, in the earliest 

 fauna of the Eocene epoch to which our investiga 

 tions cany us, the two divisions of the Ungulata 

 completely differentiated, and no trace of any 

 common stock of both, or of five-toed predecessors 

 to either. With the case of the Horses before us, 

 justifying a belief in the production of new 

 animal forms by modification of old ones, I sec no 

 escape from the necessity of seeking for these 

 ancestors of the Ungulata beyond the limits of 

 the Tertiary formations. 



I could as soon admit special creation, at 

 once, as suppose that the Perissodactyles and 

 Artiodactyles had no five-toed ancestors. And 

 when we consider how large a portion of the 

 Tertiary period elapsed before Anckithcrhm was 

 converted into JSquus, it is difficult to escape the 

 conclusion that a large proportion of time anterior 

 to the Tertiary period must have been expended 

 in converting the common stock of the Ungulata 

 into Perissodactyles and Artiodactyles. 



The same moral is inculcated by the study 

 of every other order of Tertiary monodelphous 

 Mammalia. Each of these orders is represented 

 in the Miocene epoch : the Eocene formation, as 



