.
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 your daily companions in the
streets and in your home, and you can see your less familiar cousins
behind the bars of the zoological garden.

And now we come to the parting of the ways when man suddenly leaves the
endless procession of dumbly living and dying creatures and begins to
use his reason to shape the destiny of his race