several
species of them, but they are all small, and have no chance to make
headway against the older masters of the earth.

The Jurassic or first part of the reptilian time shades insensibly
into the second part, called the Cretaceous, which immediately follows
it. During this period the lands were undergoing perpetual changes;
rather deep seas came to cover much of the land surfaces, and there is
some reason to believe that the climate of the earth became much
colder than it had been, at least in those regions where the great
reptiles had flourished. It may be that it is due to a colder climate
that we owe the rapid passing away of this gigantic reptilian life of
the previous age. The reptiles, being cold-blooded, cannot stand even
a moderate winter cold, save when they are so small 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 mode