that they can crawl deep into crevices in the rocks to sleep the winter away, guarded from the cold by the warmth of the earth.  At any rate these gigantic animals rapidly ceased to be, so that by the middle of the Cretaceous period they were almost all gone, except those that inhabited the sea; and at the end of this time they had shrunk to lizards in size.  The birds continue to increase and to become more like those of our day; their tails shrink away, their long bills lose their teeth; they are mostly water-birds of large size, and there are none of our songsters yet; still they are for the first time perfect birds, and no longer half-lizard in their nature.

The greatest change in the plants is found in the coming of the broad-leaved trees belonging to the families of our oaks, maples, etc.  Now for the first time our woods take on their aspect of to-day; pines and other cone-bearers mingle with the more varied foliage of nut-bearing or large-seeded trees.  Curiously enough, we lose sight of the little mammals of the earlier time.  This is probably because there is very little in the way of land animals of this period preserved to us.  There are hardly any mines or quarries in the beds of this age to bring these fossils to light.  In the most of the other rocks there is more to tempt man to explore them for coal ores or building stones.

In passing from the Cretaceous to the Tertiary, we enter upon the threshold of our modern world.  We leave behind all the great wonders of the old world, the gigantic reptiles, the forests of tree ferns, the seas full of ammonites and belemnites, and come among the no less wonderful but more familiar modern forms.  We come at once into lands and seas where the back-boned animals are the ruling beings.  The reptiles have shrunk to a few low forms,—­the small lizards, the crocodiles and alligators, the tortoises and turtles, and, as if to mark more clearly the banishment of this group from their old empire, the serpents, which are peculiarly degraded forms of reptiles which have lost the legs they once had, came to be the commonest reptiles of the earth.

The first mammals that have no pouches now appear.  In earlier times, the suck-giving animals all belonged to the group that contains our opossums, kangaroos, etc.  These creatures are much lower and feebler than the mammals that have no pouches.  Although they have probably been on the earth two or three times as long as the higher mammals, they have never attained any eminent success whatever; they cannot endure cold climates; none of them are fitted for swimming as are the seals and whales, or for flying as the bats, or for burrowing as the moles; they are dull, weak things, which are not able to contend with their stronger, better-organized, higher kindred.  They seem not only weak, but unable to fit themselves to many different kinds of existence.