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 corôÙreôÙsponôÙdence (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 ãfitã 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 {769} ( 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 corôÙreôÙsponôÙdence is more exact.
In considering this series of figures we cannot but be struck, not only with the regularity of the succession of ãtransôÙforôÙmaôÙtions,ã but also with the slight and inconsiderable differences which separate the known and recorded stages, and even the two extremes of the whole series. 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 scapula21.
The variability of this latter bone is great, but it is neither {770}