ious 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 h