As sheep probably came into America from Asia during the Pleistocene, at a time when Bering's Strait was closed by land, it might be expected that those now found here would show relationship to the Kamtschatkan species (Ovis nivicola); and such is indeed the case, while furthermore, in the small size of the suborbital gland and pit, and in comparative smoothness of the horns, both species approach the bharal of Thibet and India, which in these respects is goat-like.
When one considers the poverty of the new world in bovine ruminants, it seems strange that three such anomalous forms should have fallen to its share as the prong-horn, the white goat and the musk-ox, of none of which have we the complete history; two of the number being entirely isolated species, sometimes regarded as the types of separate families.
The prong-horn is a curious compound. It resembles sheep in the minute structure of its hair, in its hairy muzzle, and in having interdigital glands on all its feet. Like goats, it has no sub-orbital gland nor distinct pit. Like the chamois, it has a gland below and behind the ear, the secretion of which has a caprine odor. It has also glands on the rump. It is like the giraffe in total absence of the accessory hoofs, even to the metapodials which support them. It differs from all hollow horned ungulates in having deciduous horns with a fork or anterior branch. There is not the least similarity, however, between these horns and the bony deciduous antlers of deer, for, like those of all bovines, they are composed of agglutinated hairs, set on a bony core projecting from the frontal region of the skull.
It is well known that these horn sheaths are at times shed and reproduced, but the exact regularity with which the process takes place is by no means certain, although such direct evidence as there is goes to prove that it occurs annually in the autumn. Prong-bucks have shed on eight occasions in the Zoological Gardens at Philadelphia, five times by the same animal, which reached the gardens in October, 1899, and has shed each year early in November, the last time on October 22, 1903,[1] and the writer has seen one fine head killed about November 5 in a wild state, on which the horn-sheaths were loose and ready to drop off.
[Footnote 1: It is interesting to note that the first pair shed measured 7–¼ inches, on the anterior curve; the second pair 9–½, and the last three 11 inches each. The largest horns ever measured by the writer were those of a buck killed late in November, 1892, near Marathon, Texas, and were 15–¾ inches in vertical height and 21 along the curve.]
But few of these delicate animals have lived long enough in captivity to permit study of the same individual through a course of years, and the scarcity of observations made upon them in a wild state is remarkable. That irregularity in the process would not be without analogy, is shown by the case of the Indian sambur deer, of which there is evidence from such authority as that king of sportsmen, Sir Samuel Baker, and others, that the shedding does not always occur at the same season, nor is it always annual in the same buck; and by Pore David's deer, which has been known to shed twice in one year.
When resemblances such as those of the prong-horn are so promiscuously distributed, the task of fixing their values in estimating affinities is not a light one, and in fact the most rational conclusion which we may draw from them is that they point back to a distant and generalized ancestor, who possessed them all, but that in the distribution of his physical estate, so to speak, these heirlooms have not come down alike to all descendants. There is again a complicating possibility that some may be no more than adaptive or analogous characters, similarly produced under like conditions of life, but quite independent of a common origin, and it is seldom that we know enough of the history of development of any species to conclude with certainty whether or not this has been the case. At all events, the prong-buck is quite alone in the world at present, and we know no fossils which unmistakably point to it, although it has been supposed that some of the later Miocene species of Cosoryx—small deer-like animals with non-deciduous horns, probably covered with hair, and molars of somewhat bovine type—may have been ancestral to it, but this is little more than a speculation. What is certain is that Antilocapra is now a completely isolated form, fully entitled to rank as a family all by itself.
In the musk-ox (Ovibos moschatus), or "sheep-ox," as the generic name given by Blainville has it, we meet with another strange and lonely form which has contributed its full share to the problems of systematic zoology. Its remote and inaccessible range has greatly retarded knowledge of its structure, and it is only within the last three years that acquaintance has been made with its soft anatomy, and at the same time with a maze of resemblances and differences toward other ruminants, that perhaps more than equals the irregularities of the prong-buck. But unlike that species, there is in the musk-ox no extreme modification, such as a deciduous horn, to separate it distinctly from the rest of the family. A recapitulation of these differences would be too minutely technical for insertion here, and it must be enough to say that while it cannot be assigned to either group, yet in the distribution of hair on the muzzle, in the presence of a small suborbital gland, in shortness of tail and the light color of its horns, it is sheep-like; in the absence of interdigital glands, the shortness and stoutness of its cannon bones, and in the presence of a small accessory inner column on the upper molars, it is bovine. But in the coarse longitudinal striation of the bases of its horns it differs from both. The shape of the horns is also peculiar. Curving outward, downward and then sharply upward, with broad, flattened bases meeting in the middle line, their outlines are not unlike those of old bulls of the African buffalo.
At the present time the musk-ox inhabits only arctic America, from Greenland westward nearly to the Mackenzie River, but its range was formerly circumpolar, and in Pleistocene times it inhabited Europe as far south as Germany and France. The musk-ox of Greenland has lately been set aside as a distinct species. The most we can say is that Ovibos is a unique form, standing perhaps somewhere between oxen and sheep, and descended from an ancient ruminant type through an ancestry of which we know nothing, for the only fossil remains which are at all distinguishable from the existing genus, are yet closely similar to it, and are no older than the Pleistocene of the central United States; in earlier periods its history is a blank about which it is useless to speculate.
The last of our three anomalies, the white, or mountain goat (Oreamnos montanus), is not as completely orphaned as the other two, for it seems quite surely to be connected with a small and peculiar series consisting of the European chamois and several species of Nemorhaedus inhabiting eastern Asia and Sumatra. These are often called mountain antelopes, or goat antelopes. So little is yet known of the soft anatomy of the white goat that we are much in the dark as to its minute resemblances, but its glandular system is certainly suggestive of the chamois, and many of its attitudes are strikingly similar. In all the points in which it approaches goats it is like some, at least, among antelopes, while in the elongated spines of the anterior dorsal vertebrae, which support the hump, and in extreme shortness of the cannon bone, it is far from goat-like. The goat idea, indeed, has little more foundation than the suggestive resemblance of the profile with its caprine beard. It is truly no goat at all, and should more properly be regarded as an aberrant antelope, if anything could be justly termed "aberrant" in an aggregation of animals, hardly any two of which agree in all respects of structure. No American fossils seem to point to Oreamnos, and as Nemorhaedus extends to Japan and eastern Siberia, it is probable that it was an Asiatic immigrant, not earlier than the Pleistocene.
From this intricate genealogical tangle one turns with relief to the deer family, where the course of development lies reasonably plain. If the rank of animals in the aristocracy of nature were to be fixed by the remoteness of the period to which we know their ancestors, the deer would out-rank their bovine cousins by a full half of the Miocene period, and the study of fossils onward from this early beginning presents few clearer lines of evidence supporting modern theories respecting the development of species, than is shown in the increasing size and complexity of the antlers in succeeding geological ages, from the simple fork of the middle Miocene to those with three prongs of the late Miocene, the four-pronged