112 THE THEOKY OF EVOLUTION 



great majority of the extinct species, the peculiarity 

 above mentioned may be observed. The upper jaw 

 has but one incisor on each side and no canine; the 

 remaining incisor is chisel-shaped, with a sharp 

 cutting edge; in the lower jaw are two incisors on 

 each side, but no canine. The first lower incisor is 

 very small and can be of little or no real use, but the 

 second is a fairly large tusk, which points directly 

 forward and the Indian species, at least, uses them 

 effectively as weapons. All of the existing species, 

 as well as most of the fossil forms, are provided with 

 solid horns, which have no bony core; with an excep- 

 tion, to be noted below, the horns are not paired, but, 

 if more than one is present, they are placed one be- 

 hind the other in the middle line of the skull. The 

 skull is much modified to form a strong support for 

 the horns and its upper profile is convex in front, 

 concave behind. The neck is short, the body very 

 large, the legs and feet short and very heavy, with 

 three toes in each foot. It is quite impossible to take 

 up here the many series of the true rhinoceroses; all 

 that can be done is to indicate very briefly the 

 changes that have been noted in the group as a whole. 

 Going back at once to the most ancient known forms 

 which can be positively identified as true rhinoceroses, 

 we find in the lowest Oligocene of North America very 

 interesting species which help to connect the rhinoc- 

 eroses with other families of their order (Perisso- 

 dactyla). In these far-off animals the first upper 

 incisor had already assumed its trenchant shape, 



