XVII] THE COMPARISON OF RELATED FORMS 765 



co-ordinates corresponding to the Cartesian co-ordinates in which 

 we have inscribed the skull of Hyrachyus, with the difference, 

 firstly, that the horizontal ordinates of the latter are transformed 

 into equidistant curved lines, approximately arcs of circles, with 

 their concavity directed downwards; and secondly, that the 

 vertical ordinates are transformed into a pencil of rays approxi- 

 mately orthogonal to the circular arcs. In short, the configuration 

 of the rabbit's skull is derived from that of our primitive rhinoceros 

 by the unexpectedly simple process of submitting the latter to a 



Fig. 401. A, outline diagram of the Cartesian co-ordinates of the skull of Hyra- 

 cotherium or Eohippus, as shevni in Fig. 402, A. H, outline of the corresponding 

 projection of the horse's skull. B-G, intermediate, or interpolated, outlines. 



strong and uniform flexure in the downward direction (cf. Fig. 358, 

 p. 731). In the case of the rabbit the configuration of the 

 individual bones does not conform quite so well to the general 

 transformation as it does when we are comparing the several 

 Perissodactyles one with another; and the chief departures 

 from conformity will be found in the size of the orbit and in the 

 outline of the immediately surrounding bones. The simple fact 

 is that the relatively enormous eye of the rabbit constitutes an 

 independent variation, which cannot be brought into the general 

 and fundamental transformation, but must be dealt with 



