It is at first sight not a little surprising to find that we can pass, by a cognate and even simpler transformation, from our Perissodactyle skulls to that of the rabbit; but the fact that we can easily do so is a simple illustration of the undoubted affinity which exists between the Rodentia, especially the family of the Leporidae, and the more primitive Ungulates. For my part, I would go further; for I think 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. [400] that the rabbit’s skull conforms to a system of {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 approximately 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 Hyracotherium or Eohippus, as shewn 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
