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Show 1887.] THE PYGMY HIPPOPOTAMUS. 613 and accordingly all the species of Hippopotamus may be referred to a single genus" \ The new specimen affords an interesting corroboration of this opinion. In the front of the lower jaw are the two usual incisors; but on the right side is an additional smaller tooth placed between the normal incisor (i. 1) and the canine, and which evidently corresponds with the smaller outer incisor of H. amphibius (i. 3, according to Lydekker's determination). This tooth is procumbent (though rather less so than the contiguous incisor). It has a cylindrical root, and (as in the opposing outer incisor of the upper jaw) a portion of the enamel-covered crown remains, the greater part being worn obliquely away. Its diameter is 7 millimetres, that of the first incisor being 12. There is no corresponding tooth on the left side. The remaining dentition of the Pygmy species is essentially that of Hippopotamus, although undoubtedly differences in detail can be pointed out. The most striking of these are the larger development and greater persistence of the first premolar, the smaller relative size of the fourth upper premolar, and the greater simplicity of the form of the crowns of the true molars. Whether these characters are of sufficient importance for generic distinction is a point to be decided according to the view taken of the advisability or otherwise of multiplying such distinctions. With regard to the cranial differences, so strongly insisted upon by Leidy, Milne-Edwards, and Gratiolet, striking as they are on superficial observation, they all depend upon one circumstance, the greater relative size of the brain-cavity and capsules of the sense-organs (orbits and auditory bulla?) in H. liberiensis, contrasted with the huge development of the masticating organs, and ridges for the attachment of muscles to move the jaws, in H. amphibius. Apart from this the crania are essentially similar, even the remarkable thin-walled capsule formed by the lachrymal bone in the floor of the orbit, well known in the common species, is present, though on a smaller scale, in the Liberian animal. Now it is rather remarkable that these differential characters have been pointed out with great emphasis by the three eminent anatomists mentioned above, without anv indication of the circumstances that they are just those characters by which, in any natural group, the small members differ from the large, and just those in which in any species the young differ from the adult. The universal law of the arrest of growth of the nervous system and sense-organs in the large members of homogeneous groups fully accounts for all the differences of the two skulls which have been pointed out with such minuteness. Exactly similar differences are found between the Tiger and the smaller species of Felis, the Gorilla and Baboons and the smaller allied Apes, the large and small members of the genus Otaria, and in fact wherever there is great diversity in size in closely related forms and even in individuals of the same species. A pygmy Hippopotamus which should present all the exact proportions of the large form as regards these parts of its organization, would be as great an anomaly as a dwarf 1 Palseontologia Indica, ser, 10, vol. iii. p. 47. |
