Backward still, and the bones we find in the Miocene strata of the West, belonging to those ancestors of the horse to which the name of Mesohippus has been given because they are midway in time and structure between the horse of the past and present, tell us that then all horses were small and that all had three toes on a foot, while the fore feet bore even the suggestion of a fourth toe. From this to our Eocene Hyracothere with four toes is only another long-time step. We may go even beyond this in time and structure, and carry back the line of the horse to animals which only remotely resembled him and had five good toes to a foot; but while these contained the possibility of a horse, they made no show of it. [Illustration: Fig. 34.--The Development of the Horse.] Increase in size and decrease in number of the toes were not the only changes that were required to transform the progeny of the Hyracothere into a horse. These are the most evident; but the increased complexity in the structure of the teeth was quite as important. The teeth of gnawing animals have often been compared to a chisel which is made of a steel plate with soft iron backing, and the teeth of a horse, or of other grass-eating animals, are simply an elaboration of this idea. The hard enamel, which represents the steel, is set in soft dentine, which represents the iron, and in use the dentine wears away the faster of the two, so that the enamel stands up in ridges, each tooth becoming, as it is correctly termed, "a grinder." In a horse the plates of enamel form curved, complex, irregular patterns; but as we go back in time, the patterns become less and less elaborate, until in the Hyracothere, standing at the foot of the family tree, the teeth are very simple in structure. Moreover, his teeth were of limited growth, while those of the horse grow for a considerable time, thus compensating for the wear to which they are subjected. We have, then, this direct evidence as to the genealogy of the horse, that between the little Eocene Hyracothere and the modern horse we can place a series of animals by which we can pass by gradual stages from one to the other, and that as we come upward there is an increase in stature, in the complexity of the teeth, and in the size of the brain. At the same time, the number of toes decreases, which tells that the animals were developing more and more speed; for it is a rule that the fewer the toes the faster the animal: the fastest of birds, the ostrich, has but two toes, and one of these is mostly ornamental; and the fastest of mammals, the horse, has but one.