Sivatherium there are two pairs of such appendages, of which the
hinder are large and were probably covered during life either with
skin or thin horn. In the giraffes the separation of the horns from
the skull may be a degenerate character.
Fig. 1.—Head of Siamese Deer (Cervus schomburgkis), showing antlers.
II. In the Asiatic muntjac deer we find a pair of skin-covered horns, or “pedicles,” corresponding to the paired horns of the giraffe, although welded to the skull. From the summits of these pedicles arise secondary outgrowths, at first covered with skin, which (owing to the growth of a ring of bone at the base arresting the flow of blood eventually dries up and leaves bare bone incapable of further growth. In the muntjac the bare bony part, or “antler,” is small in proportion to the skin-covered pedicle, and simple in structure; but in the majority of deer the antler increases in size at the expense of the pedicle—which dwindles—and in some species, like the Siamese deer (fig. 1), the sambar and the red deer, becomes very large and more or less branched. Owing to liability to necrosis, the permanent retention of such a mass of dead bone would be dangerous; and the antlers are consequently shed annually (or every few years), to be renewed the following year, when, till the animal becomes past its prime, they are larger than their predecessors. The periodical shedding is also necessary in order to allow of this increase in size. With the exception of the reindeer, antlers are confined to the malcs.
III. The third type of horn is presented by the American prong buck, or pronghorn, in which bony processes, or “cores,” corresponding to the horns of the giraffe, have acquired a horny sheath, in place of skin; the sheath being in this instance forked, and annually shed and renewed, although the core is simple. The sheaths are akin to hair in structure, thus suggesting affinity with the hairs surmounting the giraffe's horns. Female prong buck may or may not have horns.
IV. In the great majority of “Hollow-horned Ruminants,” such as oxen, sheep, goats and antelopes (fig. 2), the horny sheath (or true “horn”) forms a simple unbranched cone, which may be compressed, spirally twisted, or curved in one or more directions, but is permanently retained and continues to grow throughout life from the base, while it becomes worn away at the tip. Rarely, as in the four-horned antelope, there are two pairs of horns. In many cases these horns are present in both sexes.
Dr H. Gadow is of opinion that the antlers of the deer, the horn-like protuberances on the skull of the giraffe, and the true horns of the prong buck and other hollow-horned ruminants (Bovidae) are all different stages of evolution from a single common type: the antlers of the deer being the most primitive, and the horns of the Bovidae the most specialized. From the fact that the bony horn-core of the hollow-horned ruminants first develops as a separate ossification, as do the horns of the giraffe, while the pedicle of the antlers of the deer grow direct from the frontal bone, it has been proposed to place the hollow-horned ruminants (inclusive of the prongbuck) and the giraffes in one group and the deer in another. This arrangement has the disadvantage of separating the deer from the giraffes, to which they are evidently nearly related; but Dr Gadow's work brings them more into line. Whether he is right in regarding the hollow-horned ruminants as derived from the primitive deer may, however, be a matter of opinion. One very important fact recorded by Dr Gadow is that calves and lambs shed their horns at an early age. The Bovidae are thus brought into nearer relationship with the American prongbuck (the only living ruminant which sheds its horn-cover in the adult condition) than has generally been supposed.
The above-mentioned four types of skull appendages are generally regarded as severally characteristic of as many family groups, namely the Giraffidae, Cervidae, Antilocapridae and Bovidae. The two last are, however, much more closely connected than are either of the others, and should perhaps be united.
Giraffidae.—In the Giraffidae, which include not only giraffes (Giraffa) but also the okapi (Ocapia) and a number of extinct species from the Lower Pliocene Tertiary deposits of southern Europe, Asia and North Africa, the appendages on the skull are of type No I., and may well be designated “antler-horns.” Another important feature is that the lower canine has a cleft or two-lobed crown, so that it is unlike the incisors to which it is approximated. There are no upper canines; and the cheek-teeth are short-crowned (brachyodont) with a peculiar grained enamel, resembling the skin of a slug in character. The feet have only two hoofs, all traces of the small lateral pair found in many other ruminants having disappeared.
The giraffes (Giraffa) are now an exclusively African genus, and have long legs and neck, and three horns—a single one in front and a pair behind—supplemented in some instances with a rudimentary pair on the occiput.
Fig. 2.—Head of Grant's Gazelle (Gazella granti), showing horns.
The okapi (Ocapia), which is also African but restricted to the tropical forest-region, in place of being an inhabitant of more or less open country, represents a second genus, characterized by the shorter neck and limbs, the totally different type of colouring, and the restriction of the horns to the male sex, in which they form a pair on the forehead; these horns being more compressed than the paired horns of the giraffe, and penetrating the skin at their summits (see Giraffe and Okapi). Remains of extinct species of giraffe occur in the Lower Pliocene formations of Greece, Hungary, Persia, Northern India and China. From deposits of the same age in Greece, Samos and elsewhere have been obtained skulls and other remains of Palaeotragus or Samotherium, a ruminant closely allied to Ocapia, the males of which were armed with a very similar pair of dagger-shaped horns. Helladotherium was a much larger animal, known by a single hornless skull from the Pliocene of Greece, which may be that of a female. In the equally large
