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Some Early Mammals and the Story of the Horse: Hipparion

FIG. 89.-Skulls and bones of fossil horses showing evolution of the horse, from casts in the Natural History Museum, London. 1. Protorohippus (= Hyracotherium.) 2. Mesohippus. 3. Protohippus. 4. Hipparion. 5. Equus caballus. a half hands) high at the withers. The next stage is seen in the Mesohippus (see Fig. 90). One finger has been dropped, and the skull is decidedly bigger. Here we see the middle or third digit getting larger. Unfortunately the skull of Protohippus, which is the next stage, is unknown; but we have the feet, of which a fore foot is shown in Fig. 89. Here the centre digit is much bigger, and this increase becomes still more striking in the Hipparion: until at last, in the modern horse, this one digit is supreme, and the others are only represented by the two feeble "splint bones" that were for so long a puzzle to anatomists. Now we see in them the vestiges of former digits no longer used for walking on. The outline restorations 1, 2, 3, 4, 5, 6, in Fig. 90, may be studied in connection with the bones in Fig. 89. Hipparion was of the size of a donkey.