The Story of Mankind [5]
tides of the ocean covered them with their brine. For the rest
of the time, the plants made the best of their uncomfortable
situation and tried to survive in the thin air which surrounded
the surface of the planet. After centuries of training, they
learned how to live as comfortably in the air as they had done in
the water. They increased in size and became shrubs and trees
and at last they learned how to grow lovely flowers which
attracted the attention of the busy big bumble-bees and the
birds who carried the seeds far and wide until the whole earth
had become covered with green pastures, or lay dark under the
shadow of the big trees. But some of the fishes too
had begun to leave the sea, and they had learned how to breathe
with lungs as well as with gills. We call such creatures amphibious,
which means that they are able to live with equal ease on the land
and in the water. The first frog who crosses your path can tell you
all about the pleasures of the double existence of the amphibian.
Once outside of the water, these animals gradually adapted
themselves more and more to life on land. Some became reptiles
(creatures who crawl like lizards) and they shared the
silence of the forests with the insects. That they might move
faster through the soft soil, they improved upon their legs
and their size increased until the world was populated with
gigantic forms (which the hand-books of biology list under
the names of Ichthyosaurus and Megalosaurus and Brontosaurus)
who grew to be thirty to forty feet long and who could have
played with elephants as a full grown cat plays with her kittens.
Some of the members of this reptilian family began to live in
the tops of the trees, which were then often more than a hundred
feet high. They no longer needed their legs for the purpose
of walking, but it was necessary for them to move quickly from
branch to branch. And so they changed a part of their skin
into a sort of parachute, which stretched between the sides of
their bodies and the small toes of their fore-feet, and gradually
they covered this skinny parachute with feathers and made
their tails into a steering gear and flew from tree to tree and
developed into true birds.
Then a strange thing happened. All the gigantic reptiles
died within a short time. We do not know the reason. Perhaps
it was due to a sudden change in climate. Perhaps they
had grown so large that they could neither swim nor walk nor
crawl, and they starved to death within sight but not within
reach of the big ferns and trees. Whatever the cause, the
million year old world-empire of the big reptiles was over.
The world now began to be occupied by very different
creatures. They were the descendants of the reptiles but they
were quite unlike these because they fed their young from the
``mammae'' or the breasts of the mother. Wherefore modern
science calls these animals ``mammals.'' They had shed the
scales of the fish. They did not adopt the feathers of the bird,
but they covered their bodies with hair. The mammals however
developed other habits which gave their race a great advantage
over the other animals. The female of the species
carried the eggs of the young inside her body until they were
hatched and while all other living beings, up to that time, had
left their children exposed to the dangers of cold and heat,
and the attacks of wild beasts, the mammals kept their young
with them for a long time and sheltered them while they were
still too weak to fight their enemies. In this way the young
mammals were given a much better chance to survive, because
they learned many things from their mothers, as you will know
if you have ever watched a cat teaching her kittens to take
care of themselves and how to wash their faces and how to
catch mice.
But of these mammals I need not tell you much for you
know them well. They surround you on all sides. They are