xviil THE COMPARISON OF RELATED FORMS 



763 



illustrated the evolution of this genus, constitutes a simple and 

 suitable case for the application of our method. 



But our method enables us to pass over greater gaps than these, 

 and to discern the general, and to a very large extent even the 

 detailed, resemblances between the skull of the rhinoceros and 

 those of the tapir or the horse. From the Cartesian co-ordinates 

 in which we have begun by inscribing the skull of a primitive 

 rhinoceros, we pass to the tapir's skull (Fig. 398), firstly, by con- 

 verting the rectangular into a triangular network, 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. 



Fig. 397. Titanotherium robustum. 



Fig. 398. Tapir's skull. 



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. 399, 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. Ajid 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 form an oblique 



