1080 THE THEORY OF TRANSFORMATIONS [ch 



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 

 strong and uniform flexure in the downward direction (cf. Fig. 538, 

 p. 1074). 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 separately. The enlargement of the eye, like 

 the modification in form and number of the teeth, is a separate 

 phenomenon, which supplements but in no way contradicts our 

 general comparison of the skulls taken in their entirety. 



Before we leave the Perissodactyla and their allies, let us look 

 a little more closely into the case of the horse and its immediate 

 relations or ancestors, doing so with the help of a set of diagrams 

 which I again owe to Mr Gerard Heilmann*. Here we start afresh, 

 with the skull (Fig. 546, A) of Hyracotherium (or Eohijypus), inscribed 

 in a simple Cartesian network. At the other end of the series (H) 

 is a skull of Equus, in its own corresponding network; and the 

 intermediate stages (B-G) are all drawn by direct and simple inter- 

 polation, as in Mr Heilmann's former series of drawings of Archaeop- 

 teryx and Apatornis. In this present case, the relative magnitudes 

 are shewn,, as well as the forms, of the several skulls. Alongside 

 of these reconstructed diagrams are set figures of certain extinct 

 "horses" (Equidae or Palaeotheriidae), and in two cases, viz. Meso- 

 hippus and Protohippus (M, P), it will be seen that the actual 

 fossil skull coincides in the most perfect fashion with one of the 

 hypothetical forms or stages which our method shews to be implicitly 

 involved in the transition from Hyracotherium to Equus f. In a third 

 case, that of Parahippus (Pa), the correspondence (as Mr Heilmann 



* These and also other coordinate diagrams will be found in Mr G. Heilmann's 

 beautiful and original hook Fuglenes Afstamning, 398 pp., Copenhagen, 1916; see 

 especially pp. 368-380. 



t Cf. Zittel, Grundziige d. Palaeontologie, 1911, p. 463. 



