Dogs and bears.  The dog family diverged from the creodonts late in the Eocene, and divided into two branches, one of which evolved the wolves and the other the foxes.  An offshoot gave rise to the family of the bears, and so closely do these two families, now wide apart, approach as we trace them back in Tertiary times that the Amphicyon, a genus doglike in its teeth and bearlike in other structures, is referred by some to the dog and by others to the bear family.  The well-known plantigrade tread of bears is a primitive characteristic which has survived from their creodont ancestry.

Cats.  The family of the cats, the most highly specialized of all the carnivores, divided in the Tertiary into two main branches.  One, the saber-tooth tigers (Fig. 351), which takes its name from their long, saberlike, sharp-edged upper canine teeth, evolved a succession of genera and species, among them some of the most destructive beasts of prey which ever scourged the earth.  They were masters of the entire northern hemisphere during the middle Tertiary, but in Europe during the Pliocene they declined, from unknown causes, and gave place to the other branch of cats,—­which includes the lions, tigers, and leopards.  In the Americas the saber-tooth tigers long survived the epoch.

Marine mammals.  The carnivorous mammals of the sea—­whales, seals, walruses, etc.—­seem to have been derived from some of the creodonts of the early Tertiary by adaptation to aquatic life.  Whales evolved from some land ancestry at a very early date in the Tertiary; in the marine deposits of the Eocene are found the bones of the Zeuglodon, a whalelike creature seventy feet in length.

Primates.  This order, which includes lemurs, monkeys, apes, and man, seems to have sprung from a creodont or insectivorous ancestry in the lower Eocene.  Lemur-like types, with small, smooth brains, were abundant in the United States in the early Tertiary, but no primates have been found here in the middle Tertiary and later strata.  In Europe true monkeys were introduced in the Miocene, and were abundant until the close of the Tertiary, when they were driven from the continent by the increasing cold.

Advance of the Mammalia during the tertiary.  During the several millions of years comprised in Tertiary time the mammals evolved from the lowly, simple types which tenanted the earth at the beginning of the period, into the many kinds of highly specialized mammals of the Pleistocene and the present, each with the various structures of the body adapted to its own peculiar mode of life.  The swift feet of the horse, the horns of cattle and the antlers of the deer, the lion’s claws and teeth, the long incisors of the beaver, the proboscis of the elephant, were all developed in Tertiary times.  In especial the brain of the Tertiary mammals constantly grew larger relatively to the size of body, and the higher portion of the brain—­the cerebral lobes—­increased in size in comparison with the cerebellum.  Some of the hoofed mammals now have a brain eight or ten times the size of that of their early Tertiary predecessors of equal bulk.  Nor can we doubt that along with the increasing size of brain went a corresponding increase in the keenness of the senses, in activity and vigor, and in intelligence.