Fig. 402. A, skull of Hyracotherium, from the Eocene, after W. B. Scott; H, skull of horse, represented as a co-ordinate transformation of that of Hyracotherium, and to the same scale of magnitude; B–G, various artificial or imaginary types, reconstructed as intermediate stages between A and H; M, skull of Mesohippus, from the Oligocene, after Scott, for comparison with C; P, skull of Protohippus, from the Miocene, after Cope, for comparison with E; Pp, lower jaw of Protohippus placidus (after Matthew and Gidley), for comparison with F; Mi, Miohippus (after Osborn), Pa, Parahippus (after Peterson), shewing resemblance, but less perfect agreement, with C and D.
be dealt with {768} 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[660]. 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 Archaeopteryx 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, accordingly, represent a somewhat divergent branch or offshoot of the Equidae[661]. 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 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 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 scapula[662].
Fig. 403. Human scapulae (after Dwight). A, Caucasian; B, Negro; C, North American Indian (from Kentucky Mountains).
The variability of this latter bone is great, but it is neither {770} surprising nor peculiar; for it is linked with all the considerations of mechanical efficiency and functional modification which we dealt with in our last chapter. The scapula occupies, as it were, a focus in a very important field of force; and the lines of force converging on it will be very greatly modified by the varying development of the muscles over a large area of the body and of the uses to which they are habitually put.
Fig. 404. Human skull.