Fig. 87.—Ideal section through all the above stages. (After Le Conte.)
Lastly, as regards the skull, casts of the interior show that all the earlier mammals had small brains with comparatively smooth or unconvoluted surfaces; and that as time went on the mammalian brain gradually advanced in size and complexity. Indeed so small were the cerebral hemispheres of the primitive mammals that they did not overlap the cerebellum, while their smoothness must have been such as in this respect to have resembled the brain of a bird or reptile. This, of course, is just as it ought to be, if the brain, which the skull has to accommodate, has been gradually evolved into larger and larger proportions in respect of its cerebral hemispheres, or the upper masses of it which constitute the seat of intelligence. Thus, if we look at the above series of wood-cuts, which represents the comparative structure of the brain in the existing classes of the Vertebrata, we can immediately understand why the fossil skulls of Mammalia should present a gradual increase in size and furrowing, so as to accommodate the general increase of the brain in both these respects between the level marked “maml” and that marked “man,” in the last of the diagrams. (Fig. 87.)
The tabular statement on the following diagram, which I borrow from Prof. Cope, will serve at a glance to reveal the combined significance of so many lines of evidence, united within the limits of the same group of animals.
To give only one special illustration of the principle of evolution as regards the skull, here is one of the most recent instances that has occurred of the discovery of a missing link, or connecting form (see Fig. 88). The fossil (B), which was found in New Jersey, stands in an intermediate position between the stag and the elk. In the stag (A) the skull is high, showing but little of that anterior attenuation which is such a distinctive feature of the skull of the elk (C). The nasal bones (N) of the former, again, are remarkably long when compared with the similar bones of the latter, and the premaxillaries (PMX), instead of being projected forward along the horizontal plane of the base of the skull, are deflected sharply downward. In all these points, it will be seen, the newly discovered form (Cervalces) holds an intermediate position (B). “The skull exhibits a partial attenuation anteriorly, the premaxillaries are directed about equally downward and forward, and the nasal bones are measurably contracted in size. The horns likewise furnish characters which further serve to establish this dual relationship[18].”
Fig. 88.—Skulls of—A, Canadian Stag; B, Cervalces Americanus; and C, Elk. (After Heilprin.)
| Formation. | No. of toes. | Feet. | Astragalus. | Carpus and tarsus. | Ulno-radius | Superior molars. | Zygapophyses. | Brain. |
| Pliocene. | 1-1 2-2 | Digitigrade. (Plantigrade.) | Grooved. (Flat.) | Interlocking. (Opposite.) | Faceted. | 4-tubercles, crested and cemented | Doubly involute. Singly involute. | Hemispheres larger, convoluted. |
| Upper Miocene. (Loup Fork.) | 3-3 4-4 (5-5) | |||||||
| Middle. (John Day.) | 2-2 3-3 4-4 | Digitigrade. | Grooved. | Interlocking. | Faceted. Smooth. | 4-tubercles, and crested. | Singly involute. Doubly involute. | Hemispheres larger, convoluted. |
| Lower (White River.) | 3-3 4-3 4-4 | Digitigrade. Plantigrade. | Grooved. | Interlocking. | Smooth. Faceted. | 4-tubercles, and crested. | ? Singly involute. | Hemispheres small, and larger. |
| Eocene. Upper (bridger.) | 3-3 4-3 4-5 5-5 | (Digitigrade.) Plantigrade. | Grooved. (Flat.) | Opposite. Interlocking. | Smooth. | 4-tubercles. 3-tubercles, and crested. | Singly involute. Plane. | Hemispheres small |
| Middle. (Wasatch.) | 4-3 4-5 5-5 | Plantigrade. (Digitigrade.) | Flat. (Grooved.) | Opposite. Interlocking. | Smooth. | 4-tubercles. 3-tubercles, a few crested. | Plane. Singly involute. | Hemispheres small; mesencehpalon sometimes exposed. |
| Lower (Puerco.) | 5-5 | Plantigrade. | Flat. | Opposite. | Smooth. | 3-tubercles. (4-tubercles), none crested. | Plane. | Mesencephalon exposed; hemisphere small and smoother. |
The evidence, then, which is furnished by all parts of the vertebral skeleton—whether we have regard to Fishes, Reptiles, Birds, or Mammals—is cumulative and consistent. Nowhere do we meet with any deviation or ambiguity, while everywhere we encounter similar proofs of continuous transformation—proofs which vary only with the varying amount of material which happens to be at our disposal, being most numerous and detailed in those cases where the greatest number of fossil forms has been preserved by the geological record. Here, therefore, we may leave the vertebral skeleton; and, having presented a sample of the evidence as yielded by horns and bones, I will conclude by glancing with similar brevity at the case of shells—which, as before remarked, constitute the only other sufficiently hard or permanent material to yield unbroken evidence touching the fossil ancestry of animals.