XVII] THE COMPARISON OF RELATED FORMS 1077 



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. 544 that the rabbit's skull 



Fig. 544. Rabbit's skuU. 



conforms to a system of coordinates corresponding to the Cartesian 

 coordinates in which we have inscribed the skull of Hyrachyus, with 

 the difference, firstly, that the horizontal ordinates of the latter are 



Fig. 545. A, outline diagram of the Cartesian coordinates of the skull of Hyra- 

 cotherium or Eohippus, as shewn in Fig. 546, A. H, outline of the corresponding 

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



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- 



