XVII] THE COMPARISON OF RELATED FORMS 1081 



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, accordingly, 

 represent a somewhat divergent branch or offshoot of the Equidae*. 

 It .may be noticed, especially in the case of Protohippus (P), that 

 the configuration of the angle of the jaw does not tally quite so 

 accurately with that of our hypothetical diagrams as do other parts 

 of the skull. As a matter of fact, this region is somewhat variable, 

 in different species of a genus, and even in different individuals of 

 the same species; in the small figure (Pp) of Protohippus placidus 

 the correspondence is more exact. 



In considering this series of figures we cannot but be struck, 

 not only with the regularity of the succession of "transformations," 

 but also with the slight and inconsiderable differences which separate 

 each recorded stage from the next, and even the two extremes of 

 the whole series from one another. These differences are no greater 

 (save in regard to actual magnitude) than those between one human 

 skull and another, at least if we take into account the older or 

 remoter races ; and they are again no greater, but if anything less, 

 than the range of variation, racial and individual, in certain other 

 human bones, for instance the scapula f. 



The variability of this latter bone is great, but it is neither sur- 

 prising nor peculiar; for it is linked with all the considerations of 



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

 find that any mammalian series at all cojnplete, such as that of the horses, is 

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

 what few gaps remain. So closely do successive stages follow upon one another 

 that it is sometimes extremely difficult to arrange them all in order, and to 

 distinguish clearly those members which belong in the main line of descent, and 

 those which represent incipient branches." Some phylogenies actually suffer from 

 an embarrassment of riches." 



t Cf. T. Dwight, The range of variation of the human scapula, Amer. Nat. 

 XXI, pp. 627-638, 1887. Cf. also Turner, Challenger Rep. xlvii, on Human Skele- 

 tons, p. 86, 1886: "I gather both from my own measurements, and those of other 

 observers, that the range of variation in the relative length and breadth of the 

 scapula is very considerable in the same race, so that it needs a large number of 

 bones to enable one to obtain an accurate idea of the mean of the race." 



