MORPHOLOGY AND MATHEMATICS. 



891 



of rhinoceros. For we perceive, by fig. 60, that the horizontal co-ordinates, which 

 in these latter cases became transformed into curves with the concavity upwards, are 

 curved, in the case of the horse, in the opposite direction. And the vertical ordinates, 

 which are also curved, somewhat in the same fashion as in the tapir, are very nearly 

 equidistant, instead of being, as in that animal, crowded together anteriorly. Ordi- 

 nates and abscissae form an oblique system, as is shown in the figure. In this case 

 I have attempted to produce the network beyond the region which is actually re- 



Fig. 58. — Titanotherium robustum. 



Fig. 59.— Tapir's skull. 



quired to include the diagram of the horse's skull, in order to show better the form 

 of the general transformation, with a part only of which we have actually to deal. 



It is at first sight not a little surprising to find that we can pass, by a cognate and 

 even simpler transformation, from our Perissodactyle skulls to that of the Rabbit ; 

 but the fact that we can easily do so is a simple illustration of the undoubted affinity 

 which exists between the Rodentia, especially the family of the Leporidae, and the 

 more primitive Ungulates. For my part, I would go further ; for I think there is 

 strong reason to believe that the Perissodactyles are more closely related to the 

 Leporidae than the former are to the other Ungulates, or than the Leporidae are to the 

 rest of the Rodentia. Be that as it may, it is obvious from fig. 61 that the rabbit's 



