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The reduction is less and the specialisation is less in the Hipparion, and still less in the Anchitherium; but yet, as compared with other mammals, the reduction and specialisation of parts in the Anchitherium remain great.
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.
And, hence, I have ever since held that these facts afford evidence of the occurrence of evolution, which, in the sense already defined, may be termed demonstrative. All who have occupied themselves with the structure of Anchitherium, from Cuvier onwards, have acknowledged its many points of likeness to a well-known genus of extinct Eocene mammals, Palæotherium.
That the Anchitherium type had become metamorphosed into the Hipparion type, and the latter into the Equine type, in the course of that period of time which is represented by the latter half of the Tertiary deposits, seemed to me to be the only explanation of the facts for which there was even a shadow of probability.
Indeed, as we have seen, Cuvier regarded his remains of Anchitherium as those of a species of Palæotherium. I think that this opinion was fully justifiable; but the progress of investigation has thrown an unexpected light on the question, and has brought us much nearer than could have been anticipated to a knowledge of the true series of the progenitors of the horse.
In its general characters, the skeleton of Anchitherium is very similar to that of the horse. In fact, Lartet and De Blainville called it Palæotherium equinum or hippoides; and De Christol, in 1847, said that it differed from Hipparion in little more than the characters of its teeth, and gave it the name of Hipparitherium.
But in the fact that there are only six full-sized grinders in the lower jaw, the first premolar being very small; that the anterior grinders are as large as, or rather larger than, the posterior ones; that the second premolar has an anterior prolongation; and that the posterior molar of the lower jaw has, as Cuvier pointed out, a posterior lobe of much smaller size and different form, the dentition of Anchitherium departs from the type of the Palaeotherium, and approaches that of the Horse.
The process by which the Anchitherium has been converted into Equus is one of specialisation, 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.
They have yielded fossils in an excellent state of conservation and in unexampled number and variety. The researches of Leidy and others have shown that forms allied to the Hipparion and the Anchitherium are to be found among these remains.
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.
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