Page:Van Loon--The Story of Mankind.djvu/42

6 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.