Fig. 112.—Bones of the manus A, of the Indian Elephant, Elephas indicus. × ⅛. B, of the Cape Hyrax, Hyrax capensis. × 1. c, Cuneiform; cc, centrale; l, lunar; m, magnum; p, pisiform; R, radius; td, trapezoid; tm, trapezium; s, scaphoid; u, unciform; U, ulna. (From Flower's Osteology.)
Fig. 113.—Bones of the manus A, of Rhinoceros, Rhinoceros sumatrensis. × 1⁄5. B, of Pig, Sus scrofa. × ⅓. Letters as in Fig. 112. (From Flower's Osteology.)
The gradual perfecting of the fore- and hind-limbs as running organs has been put down to the advent of the grasses, and the formation of large plains covered with this herbage. The same reason would also be in harmony with the equally gradual change in the shape of the molar teeth, from a tubercular form calculated for a mixed or even a carnivorous diet, to the flatter crushing surfaces exhibited by the lophodont teeth of later Ungulates. Strong
canines would in the same way cease to be useful, and even become encumbrances to such grazing creatures; and their disappearance is one of the salient features in the history of the Ungulata, that is of the modern representatives of the order. The extraordinary hypertrophy of these teeth in such a line as that of the Amblypoda, which has left no descendants, was one of the reasons perhaps for the decay of those great pachyderms of mid-Tertiary times; their excessive armature became an encumbrance, since it was not accompanied by improvements in other necessary directions. Some of the features of the Tertiary Ungulates have, however, been dealt with in our general sketch of the mammalian life during that epoch, and need not be again referred to here. Of existing Ungulates there are no clear indications of the descent of the Elephants or of the Hyracoidea. Their structure proclaims these two divisions to be of ancient descent, and not to be modern twigs of the Ungulate stem. As to the Perissodactyla and the Artiodactyla we cannot bring them together nearer than in quite early Tertiary times. The order Condylarthra seems to be the starting-point of both these sub-divisions. Euprotogonia has been considered to be an ancestor of the Perissodactyle branch, and Protogonodon or Protoselene of the Artiodactyla. If this be true,
the likenesses which Titanotherium shows to the Artiodactyla must be either purely superficial and secondary, or a cropping out of ancient characters which had been dormant for many generations.
Horns.—The Ungulata are the only order of mammals which possess horns; as they are on the whole a more defenceless group than the Carnivora, it may be that the horns are a counterpoise to the teeth and claws of the latter; need for defence and for armature in the combats with their own kind for the favours of the does has led to a different kind of protective and aggressive mechanism. Horns as weapons are, however, particularly effective in this group wherever they exist. A Ruminant is most frequently a large and heavy animal without the agility and litheness of the Carnivore. It is precisely to this sort of animal, where weight is an important consideration, that horns are the most suitable weapons. This is further shown by the fact that although the general term horn is used to describe the weapons of the Ungulate mammals, there is more than one kind of structure included under this general term; it is indeed probable that the extreme terms in the series of horns have been independently acquired by their possessors. There is but little in common between the horns of a Giraffe and of a Rhinoceros. In the Rhinoceros we have one or two horns, in the latter case one placed behind the other, which are purely epidermic growths; they may indeed be regarded as matted masses of hair, borne, it is true, upon a boss of bone, which however is not a separate structure. The Giraffe supplies us with the simplest term in that series of horns which are partly epidermal and partly bony. The paired horns of this animal have often been contrasted with those of the Deer, for example; but there is no fundamental difference between them. In the Giraffe a pair of bony outgrowths, originally separate from the skull which bears them, but ultimately ankylosed to it, are covered by a layer of entirely unmodified skin. A distinction of undoubtedly practical importance is usually drawn between the Hollow-horned Ruminants, i.e. Oxen, Goats and Antelopes, and the Deer tribe. There is nevertheless no fundamental distinction. In the Antelopes there is a core of bone, the "os cornu" as it has been termed, which is covered by a horny layer, the horn proper, variously modified in shape and size according to the genus or species. In the Deer there is the
same os cornu, which may however be branched, but which is in the same way covered by a layer of modified integument; this is known as the "velvet"; it only lasts for a certain period, and is then torn off by the exertions of the animal itself, leaving behind the bony core, which is popularly termed the horn. It will be clear that here is only a difference of comparative unimportance; the same essential features are present in both groups of animals, but the modification of the epidermis has progressed along different lines. Both can be referred back to the primitive conditions seen in the paired horns of the Giraffe. Even the difference, such as it is, is bridged over by the Antelope Antilocapra, where the os cornu is bifid and the horn is periodically shed, as is the velvet of the stag; but in the stag the bony part of the horn is also shed, a state of affairs which has no parallel in the Hollow-horned Ruminants. The great Sivatherium may conceivably be an annectant form between the two types of compound horns, i.e. those of the Antelope and those of the Deer. This creature had two pairs of horns, of which, naturally, only the bony cores remain; the hinder pair of these were branched. But although so far they resemble the Deer's horns rather than the Antelope's, Dr. Murie has thought that they were covered by a horny sheath and not by soft skin as in the Deer. In any case these horns were apparently never shed, which is a point of likeness with the Antelope and of difference from the Deer. Apart therefore from the nature of the covering of the bony cores, there are good grounds for looking upon them as intermediate between those of the Deer and those of the Antelopes.
The horns of the Ruminants are frequently a secondary sexual character; this is especially the case with the Deer. The Reindeer is, however, an exception, both the stags and the does having horns. That they are associated with the reproductive function is shown by their being shed after the period of rut, the destruction of the velvet at that period, and also by the effect upon the horns which any injury to the reproductive glands produces. Some useful facts upon this latter head have been amassed by Dr. G. H. Fowler,[[116]] who noticed in a series of stags, horns showing various degrees of degeneration in the antlers produced by varying degrees and periods of gelding. From the facts
here collected it is clear that a direct effect is produced. If we are to regard horns as secondary sexual appendages which have been subsequently handed on to the female by heredity, we should expect to meet with examples of animals now horned in both sexes, of which the earlier representatives had the horns confined to one sex. This is most interestingly shown by the extinct and Miocene Giraffe, Samotherium, of which the male alone had a pair of short horns, while the skull of the female was entirely hornless; the modern Giraffa, as is well known, has horns in both sexes.