The warm-blooded birds and mammals followed the reptiles. This does not mean that all reptiles died, after having ruled the earth for thousands of years. It means that changes in climate and other life conditions were unfavourable to the giants of the cold-blooded races, and gradually they passed away. They are represented now on the earth by lesser reptiles, which live comfortably with the wild creatures of other tribes, but which in no sense rule in the brute creation. They live rather a lurking, cautious life, and have to hide from enemies, except a few more able kinds, provided with means of defense.

There were mammals on the earth in the days of reptilian supremacy, but they were small in size and numbers, and had to avoid any open conflict with the giant reptiles, or be worsted in a fight. Now the time came when the ruling power changed hands. The mammals had their turn at ruling the lower animals. It was the beginning of things as they are to-day, for mammals still rule. But many millions of years have probably stood between the age when this group of animals first began to swarm over the earth, and the time when Man came to be ruler over all created things.

Among the reptiles of the period when the sea, the land, and the air were swarming with these great creatures were certain kinds that had traits of mammals. Others were bird-like. From these reptilian ancestors birds and mammals have sprung. No one doubts this. The fossils prove it, step by step.

Yet the rocks surprise the geologist with the suddenness with which many new kinds of mammals appeared on the earth. Possibly the rocks containing the bones of so many kinds were fortunately located. The spots may have been morasses where migrating mammals were overwhelmed while passing. Possibly conditions favored the rapid development of new kinds, and the multiplication of their numbers. Warm, moist climate furnished abundant succulent plant food for the herbivors, and these in turn furnished prey for the carnivors.

The coal formed during the Tertiary Period gives added proof that the plant life was luxuriant. The kinds of trees that grew far north of our present warm zones have left in the rocks evidence in the form of perfect leaves and cones and other fruits. For instance, magnolias grew in Greenland, and palm trees in Dakota. The temperature of Greenland was thirty degrees warmer than it is now. Our Northern States lie in a belt that must have had a climate much like that of Florida now. Europe was correspondingly mild.

A special chapter tells of the gradual development of the horse. One hundred different kinds of mammals have been found in the Eocene rocks, many of which have representative species at the same time in Europe and America. The rocks of Asia probably have similar records.

The Eocene rocks, lowest of the Tertiary strata, contain remains of animals the families of which are now extinct. Next overlying the Eocene, the Miocene rocks have fossils of animals belonging to modern families—rhinoceroses, camels, deer, dogs, cats, horses—but the genera of which are now extinct. The Pliocene strata (above the Miocene) contains fossils of animals so closely related to the wild animals now on the earth as to belong to the same genera. They differ from modern kinds only in the species, as the red squirrel is a different species from the gray.

So the record in the rocks shows a gradual approach of the mammals to the kinds we know, a gradual passing of the mighty forms that ruled by size and strength, and the coming of forms with greater intelligence, adapted to the change to a colder climate.

It sometimes happens that a farmer, digging a well on the prairie, strikes the skeleton of a monster mammal, called the mastodon. This very thing happened on a neighbour's farm when I was a girl, in Iowa. Everybody was excited. The owner of the land dug out every bone, careful that the whole skeleton be found. As he expected, the director of a museum was glad to pay a high price for the bones.

The mastodon was about the size of an elephant, with massive limbs, and large, heavy head that bore two stout, up-curved tusks of ivory. The creature moved in herds like the buffalo from swamp to swamp; and old age coming on, the individual, unable to keep up with the herd, sank to his death in the boggy ground. The peat accumulated over his bones, undisturbed until thousands of years elapse, and the chance digging of a well discovers his skeleton.

Frozen in the ice of northern Siberia, near the mouths of rivers, a number of mammoths have been found. These are creatures of the elephant family, and belonging to the extinct race that lived in the Quaternary Period, just succeeding the Tertiary. The ice overtook the specimens, and they have been in cold storage ever since. For this reason, both flesh and bones are preserved, a rare thing to happen, and rarer still to be seen by a scientist.

The ignorant natives made a business of watching the ice masses at the river mouth for dark spots that showed where a mammoth was encased in the ice. If an iceberg broke off near such a place, the sun might thaw the ice front of the glacier, until the hairy monster could at length be reached. His long hair served for many uses, and the wool that grew under the hair was used as a protection from the Arctic winter. The frozen flesh was eaten; the bones carved into useful tools; but the chief value of the find was in the great tusks of ivory, that curved forward and pointed over the huge shoulders. It was worth a fortune to get a pair and sell them to a buyer from St. Petersburg.

One of the finest museum specimens of the mammoth was secured by buying the tusks of the dealer, and by his aid tracing the location of the carcass, which was found still intact, except that dogs had eaten away part of one foreleg, bone and all. From this carefully preserved specimen, models have been made, exactly copying the shape and the size of the animal, its skin, hair, and other details.

The sabre-toothed tiger, the sharp tusks of which, six to eight inches long, made it a far more ferocious beast than any modern tiger of tropical jungles, was a Quaternary inhabitant of Europe and America. So was a smaller tiger, and a lion. The Irish elk, which stood eleven feet high, with antlers that spread ten feet apart at the tips, was monarch in the deer family, which had several different species on both continents. Wild horses and wild cattle, one or two of great size, roamed the woods, while rhinos and the hippopotamus kept near the water-courses. Hyenas skulked in the shadows, and acted as scavengers where the great beasts of prey had feasted. Sloths and cuirassed animals, like giant armadillos, lived in America. Among bears was one, the cave bear, larger than the grizzly. True monkeys climbed the trees. Flamingo, parrots, and tall secretary birds followed the giant gastornis, the ancestor of wading birds and ostriches, which stood ten feet high, but had wings as small and useless as the auk of later times.

With the entrance of the modern types of trees, came other flowering plants, and with them the insects that live on the nectar of flowers. Through a long line of primitive forms, now extinct, flowering plants and their insect friends conform to modern types. The record is written in the great stone book.

The Age of Mammals in America and Europe ended with the gradual rise of the continental areas, and a fall of temperature that ushered in the Ice Age. With the death of tropical vegetation, the giant mammals passed away.