1076 THE THEORY OF TRANSFORMATIONS [ch. 



rectangular into a triangular ;ietwork, by which we represent the 

 depression of the anterior and the progressively increasing elevation 

 of the posterior part of the skull; and secondly, by giving to 

 the vertical ordinates a curvature such as to bring about a certain 

 longitudinal compression, or condensation, in the forepart of the 

 skull, especially in the nasal and orbital regions. 



The conformation of the horse's skull departs from that of our 

 primitive Perissodactyle (that is to say our early type of rhinoceros, 

 Hyrachyus) in a direction that is nearly the opposite of that taken 

 by Titanotherium and by the recent species of rhinoceros. For we 

 perceive, by Fig. 543, that the horizontal coordinates, which in these 

 latter cases become transformed into curves with the concavity 



Fig. 543. Horse's skull. 



upwards, are curved, in the case of the horse, in the opposite direc- 

 tion. 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. 

 Ordinates and abscissae fol-m an oblique system, as is shewn in the 

 figure. In this case I have attempted to produce the network 

 beyond the region which is actually required to include the diagram 

 of the horse's skull, in order to shew 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 



