The diagrams illustrate the development of the horse over a period of 40 million years, from Eohippus to the modern horse, with particular emphasis on changes in body size and foot structure.
Overall, the horse underwent a gradual increase in body size, while its feet became progressively simpler. In particular, the number of toes decreased significantly, with the modern horse having a single enlarged central hoof.
Forty million years ago, Eohippus was relatively small and had a short body and several toes. Over the following 10 million years, Mesohippus became noticeably larger and developed longer legs. Its foot structure also changed, with the central toe becoming more prominent while the other toes were reduced.
By 15 million years ago, Merychippus had grown considerably larger and more closely resembled the modern horse in terms of its overall body shape. At this stage, the central toe was substantially enlarged, although two smaller toes were still visible on either side.
Eventually, the modern horse developed a much larger and more elongated body, supported by long legs. Its foot was further simplified, leaving only one large central hoof, which represents the final stage of this evolutionary development.