The evolution of the horse family included, indeed, certain trends, but none of these was undeviating or orthogenetic. The uniform, continuous transformation of Hyracotherium into Equus, so dear to the hearts of generations of textbook writers, never happened in nature. Increases in size, for instance, did not occur at all during the first third of the whole history of the family. Then it occurred quite irregularly, at different rates and to different degrees in a number of different lines of descent. Even after a trend toward larger size had started it was reversed in several groups of horses which became smaller instead of larger. As already briefly noted, the famous “gradual reduction of the side toes” also is something that never happened. There was no reduction for the 15 or 20 million years of the history. There was relatively rapid reduction from four front toes to three (the hind foot already had only three toes). Many horses simply retained the new sort of foot without further change. In one group there was later another relatively rapid change of foot mechanism involving some reduction in size of the side toes, which, however, remained functional. Thereafter most horses retained this type of foot without essential change. In just one group, again, another relatively rapid change eliminated functional side toes, after which their descendants simply retained the new sort of foot. (Fig. 39)
In the history of the horse family there is no known trend that affected the whole family. Moreover, in any one of the numerous different lines of descent there is no known trend that continued uniformly in the same direction and at the same rate throughout. Trends do not really have to act that way: there are not really orthogenetic.
(The evolution of the horse family, Equidae, is now no better known than that of numerous other groups of organisms, but it is still a classic example of evolution in action, and a very instructive example when correctly presented…)