The pictures illustrate the evolution of the horse.

The pictures illustrate the evolution of the horse. Overall, the whole procedure consists of 4 stages, starting with Eohippus and finishing with the development of the modern horse. 40 million years ago, the first horse, Eohippus, was comparatively small with a short tail. Its defining characteristic. However, the feature was shown clearly about the structure of its foot, which had 4 digits. Particularly, each of the digits was relatively small. 10 million years later, it had evolved into the larger Mesohippus, with a longer tail and a foot structure of 3 digits. Then, in the next stage of development, Merychippus evolved 15 million years ago. It was a similar size to Mesohippus, and also had 3 digits on its feet, but now the digits on the feet were larger, divided into segments, and the middle digit was very prominent. Finally, the modern horse evolved. It is bigger than its predecessors, with a large mane and its foot structure is a single, extremely large digit.