The picture is providing the evolution of the horse along 40 million years. The changement of the animal, focusing in the change of its foot structure and four differents types of hourses too. It is clear that 40 million years ago was four differnts parts and samller fingers; however, nowadays it is only one single foot with any fine.
The structure of the Eohippus was and its seem very weak and fine. 10 million later, two of the fingers had met each other to creat one main finger as the Merchippus. Altought, the modern house has one foot with three parts that has developed to become one piece.
In comparison, the evolution of the horses feet have a such difference. For example, the Eohippus foot seem to be weak and soft in the illustration in the other man the modern one is stronger and hard. The size of the animal also had a diiference that can lead to this new structural form because of the weight.
In conclusion, the structure of the horse foot since its roots could have change thought the time beacuse of the body of the animal.
