768 THE THEOEY OF TRANSFOKMATIONS [ch. 



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. 402, A) of Hyracotherium (or Eohippus), 

 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 interpolation, as in Mr Heilmann's former series of drawings 

 of Archaeop'eryx 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. Mesohippus 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. In a third case, that of Parahippus 

 (Pa), the correspondence (as Mr Heilmann points out) is by no 

 means exact. The outline of this skull comes nearest to that of 

 the hypothetical transition stage D, but the "fit" is now a bad 

 one ; for the skull of Parahippus is evidently a longer, straighter 

 and narrower skull, and differs in other minor characters besides. 

 In short, though some writers have placed Parahippus in the 

 direct line of descent between Equus and. Eohippus, we see at 

 once that there is no place for it there, and that it must, accord- 

 ingly, represent a somewhat divergent branch or offshoot of the 

 Equidae J. It may be noticed, especially in the case of Pro ohippus 



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

 book Fughnes Afstamning, 398 pp., Copenhagen, 1916; see especially pp. 368-380. 



■j- Cf. Zittel, Grundziige d. Palaeontologie, p. 463, 1911. 



J Cf. W. B. Scott (Amer. Journ. of Science, XLvm, pp. 335-374, 1894), "We 

 find that any mammahan series at all complete, such as that of the horses, is 

 remarkably continuous, and that the progress of discovery is steadily filling up 



