Critiques and Addresses

by Thomas Henry Huxley

[Footnote 1: Hermann von Meyer gave the name of Anchitherium to A. Ezguerrae; and in his paper on the subject he takes great pains to distinguish the latter as the type of a new genus, from Cuvier's Palaeotherium d'Orléans. But it is precisely the Palaeotherium d'Orléans which is the type of Christol's genus Hipparitherium; and thus, though Hipparitherium is of later date than Anchitherium, it seemed to me to have a sort of equitable right to recognition when this address was written. On the whole, however, it seems most convenient to adopt Anchitherium.]

Again, the skeleton of Anchitherium is extremely equine. M. Christol goes so far as to say that the description of the bones of the horse, or the ass, current in veterinary works, would fit those of Anchitherium. And, in a general way, this may be true enough; but there are some most important differences, which, indeed, are justly indicated by the same careful observer. Thus the ulna is complete throughout, and its shaft is not a mere rudiment, fused into one bone with the radius. There are three toes, one large in the middle and one small on each side. The femur is quite like that of a horse, and has the characteristic fossa above the external condyle. In the British Museum there is a most instructive specimen of the leg-bones, showing that the fibula was represented by the external malleolus and by a flat tongue of bone, which extends up from it on the outer side of the tibia, and is closely ankylosed with the latter bone.[1] The hind toes are three, like those of the fore leg; and the middle metatarsal bone is much less compressed from side to side than that of the horse.

[Footnote 1: I am indebted to M. Gervais for a specimen which indicates that the fibula was complete, at any rate, in some cases; and for a very interesting ramus of a mandible, which shows that, as in the Palaeotheria, the hindermost milk-molar of the lower jaw was devoid of the posterior lobe which exists in the hindermost true molar.]

In the Hipparion the teeth nearly resemble those of the Horses, though the crowns of the grinders are not so long; like those of the Horses, they are abundantly coated with cement. The shaft of the ulna is reduced to a mere style ankylosed throughout nearly its whole length with the radius, and appearing to be little more than a ridge on the surface of the latter bone until it is carefully examined. The front toes are still three, but the outer ones are more slender than in Anchitherium, and their hoofs smaller in proportion to that of the middle toe: they are, in fact, reduced to mere dew-claws, and do not touch the ground. In the leg, the distal end of the fibula is so completely united with the tibia that it appears to be a mere process of the latter bone, as in the Horses.

In Equus, finally, the crowns of the grinding-teeth become longer, and their patterns are slightly modified; the middle of the shaft of the ulna usually vanishes, and its proximal and distal ends ankylose with the radius. The phalanges of the two outer toes in each foot disappear, their metacarpal and metatarsal bones being left as the "splints."

The Hipparion has large depressions on the face in front of the orbits, like those for the "larmiers" of many ruminants; but traces of these are to be seen in some of the fossil horses from the Sewalik Hills; and, as Leidy's recent researches show, they are preserved in Anchitherium.

When we consider these facts, and the further circumstance that the Hipparions, the remains of which have been collected in immense numbers, were subject, as M. Gaudry and others have pointed out, to a great range of variation, it appears to me impossible to resist the conclusion that the types of the Anchitherium, of the Hipparion, and of the ancient Horses constitute the lineage of the modern Horses, the Hipparion being the intermediate stage between the other two, and answering; to B in my former illustration.

The process by which the Anchitherium has been converted into Equus is one of specialization, or of more and more complete deviation from what might be called the average form of an ungulate mammal. In the Horses, the reduction of some parts of the limbs, together with the special modification of those which are left, is carried to a greater extent than in any other hoofed mammals. The reduction is less and the specialization is less in the Hipparion, and still less in the Anchitherium; but yet, as compared with other mammals, the reduction and specialization of parts in the Anchitherium remain great.

Is it not probable then, that, just as in the Miocene epoch, we find an ancestral equine form less modified than Equus, so, if we go back to the Eocene epoch, we shall find some quadruped related to the Anchitherium, as Hipparion is related to Equus, and consequently departing less from the average form?

I think that this desideratum is very nearly, if not quite, supplied by Plagiolophus, remains of which occur abundantly in some parts of the Upper and Middle Eocene formations. The patterns of the grinding-teeth of Plagiolophus are similar to those of Anchitherium, and their crowns are as thinly covered with cement; but the grinders diminish in size forwards, and the last lower molar has a large hind lobe, convex outwards and concave inwards, as in Palceotherium. The ulna is complete and much larger than in any of the Equidae, while it is more slender than in most of the true Palaeotheria; it is fixedly united, but not ankylosed, with the radius. There are three toes in the fore limb, the outer ones being slender, but less attenuated than in the Equidae. The femur is more like that of the Palaeotheria than that of the horse, and has only a small depression above its outer condyle in the place of the great fossa which is so obvious in the Equidae. The fibula is distinct, but very slender, and its distal end is ankylosed with the tibia. There are three toes on the hind foot having similar proportions to those on the fore foot. The principal metacarpal and metatarsal bones are flatter than they are in any of the Equidae; and the metacarpal bones are longer than the metatarsals, as in the Palaeotheria.