S90 



D'ARCY WENTWORTH THOMPSON ON 



will perhaps be noticed that the correspondence is not always quite accurate in small 

 details. It could easily have been made much more accurate by giving a slightly 

 sinuous curvature to certain of the co-ordinates. But as they stand, the corre- 

 spondence indicated is very close, and the simplicity of the figures illustrates all the 

 better the general character of the transformation. 



By similar and not more violent changes we pass easily to such allied forms as 

 the Titanotheres (fig. 58) ; and the well-known series of species of Titanotherium, by 

 which Professor Osborn has illustrated the evolution of this genus, constitutes a 

 simple and suitable case for the application of our method. 



Fig. 57. -Occipital view of the skulls of various extinct rhinoceroses {Acrratheriurn spp. ). (After OsnoiiN.) 



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. 59), firstly, by converting 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 oidinates a curvature such as to bring about a certain longi- 

 tudinal 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 Peris 

 sodactyle (that is to say our early type of rhinoceros, Hyracliyus) in a direction 

 that is nearly the opposite of that taken by Titanotherium and by the recent species 



