animal
in the egg, or, in the case of the mammals, before birth. It is an
interesting fact that when the lung begins to form in the embryo it
starts as a simple sac which is an offspring from the gullet, and
occupies the position of the swim-bladder of the fish. This sac later
divides into two, and develops into the lungs of the animal. This
assures the zooelogist that the origin of the lungs in the higher
animals is found in the swim-bladder of the so-called lungfish. In
this Silurian time certain of these lungfish were perhaps trapped in
the basin in the marsh by the uplifting of the border. The waters
becoming progressively shallower and more crowded, these fishes took
to the land, their fins developing into awkward limbs which slowly
became more perfect.

To state the fact in this simple fashion is to make it seem far less
probable than is really the case. The simple forms of the life of
lowly creatures, as well as the simple character of the legs and feet
in the salamander class, make the explanation not so unlikely as would
at first sight appear. Suffice it to say that the scientist now
believes that out of the lungfish of the Devonian came the amphibians
of the Carboniferous period.

At the end of the coal period came the greatest change the face of the
globe had seen for many millions of years. Slowly the continent rose
on both sides of the old interior sea. A great plateau formed in the
region of the Alleghenies and another in the western district, though
this latter uplift was to be completely washed away, and later to rise
again into the Rocky Mountains and the Sierras. With the uplift at the
edges of the continent came a steady rise of the internal marshes,
until what had previously been swamp land became progressively first
dry land and, in the western part, even desert, in that respect being
somewhat like what it is now.

The amphibians of to-day (animals like the salamander and frog) all
lay their eggs in the water and their young have a tadpole stage. This
doubtless was true of the amphibians of the coal period. With the
beginning of the Mesozoic, or "middle life" period, a change and a
progression comes over the animal world. The tadpole life of the frog
is a rather lengthened one, while the toad has learned to crowd its
tadpole life within a few weeks. It would seem as if, in the earlier
times of the Mesozoic, this same change of habit had been going on.
With the drying up of the swamp, some of th