of this period, it underwent its greatest expansion subsequently. A small hoglike creature also developed which became the ancestor of the many species of swine, peccaries, and hippopotamuses. Camels and llamas had their origin in North America about the middle of this period and overran the western plains. Later, the llamas migrated to South America, the camels to Europe, and soon both were extinct in North America, though a few camels survived up to the ice age.

About this time a notable thing occurred in western North America: The early ancestors of the ancient lemurs first made their appearance. While this family cannot be regarded as true lemurs, their coming marked the establishment of the line from which the true lemurs subsequently sprang.

Like the land serpents of a previous age which betook themselves to the seas, now a whole tribe of placental mammals deserted the land and took up their residence in the oceans. And they have ever since remained in the sea, yielding the modern whales, dolphins, porpoises, seals, and sea lions.

The bird life of the planet continued to develop, but with few important evolutionary changes. The majority of modern birds were existent, including gulls, herons, flamingoes, buzzards, falcons, eagles, owls, quails, and ostriches.

By the close of this Oligocene period, covering ten million years, the plant life, together with the marine life and the land animals, had very largely evolved and was present on earth much as today. Considerable specialization has subsequently appeared, but the ancestral forms of most living things were then alive.

Land elevation and sea segregation were slowly changing the world's weather, gradually cooling it, but the climate was still mild. Sequoias and magnolias grew in Greenland, but the subtropical plants were beginning to migrate southward. By the end of this period these warm-climate plants and trees had largely disappeared from the northern latitudes, their places being taken by more hardy plants and the deciduous trees.

There was a great increase in the varieties of grasses, and the teeth of many mammalian species gradually altered to conform to the present-day grazing type.

25,000,000 years ago there was a slight land submergence following the long epoch of land elevation. The Rocky Mountain region remained highly elevated so that the deposition of erosion material continued throughout the lowlands to the east. The Sierras were well re-elevated; in fact, they have been rising ever since. The great four-mile vertical fault in the California region dates from this time.

20,000,000 years ago was indeed the golden age of mammals. The Bering Strait land bridge was up, and many groups of animals migrated to North America from Asia, including the four-tusked mastodons, short-legged rhinoceroses, and many varieties of the cat family.

The first deer appeared, and North America was soon overrun by ruminants—deer, oxen, camels, bison, and several species of rhinoceroses—but the giant pigs, more than six feet tall, became extinct.