The elephants, or trunk-bearing animals, illustrate a very different kind of evolution. They seem to have reached their climax of development in the late Tertiary when they grew to be as much as 14 feet high, and were more abundant and widespread over the earth than at any other time. The modern elephant, like the horse, has been traced back through many intermediate forms to its primitive early Tertiary ancestry. Some of the most important evolutionary changes took place in the head portion. The trunk is a highly developed form of snout, the earliest form of which was much like that of the modern tapir. The tusks are highly specialized and elongated teeth. During the earlier history the chin was very long and supported short tusks, so that there were then four tusks. Carnivorous mammals, like tigers and wolves, and gnawers, like rats and squirrels, may also be traced back to generalized early Tertiary types. Another kind of evolution is well illustrated by certain mammals which, even in early Tertiary time, so thoroughly adapted themselves to a water environment as to become whales, porpoises, etc. [Illustration: Fig. 67.--Comparison of feet of monkeys and man.] The primates include the highest group of all Vertebrates, and therefore of all animals. Monkeys, apes, and man belong to the primates. There is no evidence whatever for the appearance of even the simplest and most primitive forms before the opening of the Cenozoic era, but even very early in Tertiary time, lemurs and primitive types of monkeys existed. Later in the Tertiary true monkeys and apes were common, and by the close of the period some apes were highly enough developed to strongly resemble certain of the oldest and most primitive types of man. We have, however, no positive knowledge of the existence of man in even the latest Tertiary. In the light of much evidence in regard to the antiquity of man, it seems improbable that true human fossils will ever be found in rocks older than the Quaternary, though if we are willing to descend (far enough in the human scale toward apes) it is not unlikely that man-apes may be discovered in very late Tertiary rocks. The difficulty comes in the classification. Where are we to draw the line between the higher apes and the lowest forms of man? But this very difficulty is one of the strongest arguments in favor of the organic evolution of man because practically all intermediate forms between true man and certain other high-grade primates are known from the strata. The following tabular summary of the geological history of man is based upon the work of most of the ablest students of the subject.