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.