892 



D'ARCY WENTWORTH THOMPSON ON 



skull conforms to a system of 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 approximately ortho- 

 gonal to the circular arcs. In short, the configuration of the rabbit's skull is derived 



Fig. 60.— Horse's skull. 



Fig. 61.— Rabbit's skull. 



from that of our primitive rhinoceros by the unexpectedly simple process of submitting 

 the latter to a strong and uniform flexure in the downward direction (cf. fig. 7, p. 864). 

 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 surround- 

 ing bones. The simple fact is that the relatively enormous eye of the rabbit consti- 

 tutes an independent variation, which cannot be brought into the general and 

 fundamental transformation, but must be dealt with separately. The enlargement 



