e amphibians crowded their
tadpole stage further and further back, until it was completely
accomplished before their young left the egg. An examination of the
development of the reptile in the egg will show a stage very similar
to the fish and to the amphibians, but this is all experienced before
the reptile emerges from the egg. The reptilian egg, unlike that of
the frog, is covered with a shell, packed away under the surface of
the ground, and left to its own fate. If, as most geologists believe,
the climate of the Mesozoic was distinctly warm, this habit of the
parent of forsaking the egg was not a serious matter. However the
creatures arose, it is certain that in this Mesozoic age reptiles
roamed the forests, swam the seas, and even flew in the air. Probably
at no other time in the earth's history has any one class of animals
so completely dominated the situation as did the reptiles of this age.
They were not only abundant; they were frequently enormously large.
Their skeletons are among the most interesting that we find to-day.
Gigantic lizards, seventy feet long and eighteen feet high at the
shoulders, dragged their heavy bodies through the marshy edges of the
lakes. Out upon the land others, not quite so heavy nor so large,
roamed about, some of them feeding upon the soft vegetation, others
having teeth fitted to tear down their herbivorous cousins. In some of
them the hind legs and tail were very heavy and the front legs so
light that it is quite clear they must have hopped around as do the
kangaroos to-day. Others of these reptiles went back to the sea, lost
the leglike development of their limbs and regained the flipper form,
though the bones of the fingers and toes are singularly
distinguishable in the paddle.

Strangest of all, a considerable group of these wonderful reptiles
lengthened their little fingers, sometimes to three or four feet in
length, and had a skin stretched from these fingers over to the body
in such a fashion as to give them wings not unlike those of the bat.
In the wing of the bat, however, four of the fingers of the hand run
through the membrane and support it. In the pterodactyl, as these
flying reptiles are called, the middle finger supports the web, while
the remaining fingers can still be used to clasp objects or serve the
animal to lift himself, as the bat can do with his thumbs.

Meanwhile an entire change is coming over the plant world. The last
third of this age of reptiles is