walruses.<17> There was a marvelous abundance of reptilian life.
At the present day, there are not more than six species of
reptiles in the whole world having a length of over fifteen
feet, and not more than eighteen species exceeding ten feet in
length. But from one limited locality, representing but one era
of this age in England, there have been discovered four or five
species of carnivorous reptiles twenty to fifty feet long, ten
or twelve species of crocodiles, lizards, and swimming reptiles
from ten to sixty feet long--besides multitudes of great flying
reptiles and turtles. Doubtless similar scenes of animal life
were everywhere represented.

Illustration of The Labyrinthodon.--------


Birds made their first appearance during the Mesozoic time, and
here we obtain a clear view of nature's methods of work.
There is no longer a doubt but that the first birds were simply
modified reptiles. The first bird had a long jointed tail, and a
bill well supplied with formidable teeth.<18> It was during this
period that the first representative of the class Mammalia, to
which man belongs, appears.<19> It is in the rocks of this era
that we meet with remains of marsupials, the order to which
opossums belong. This is the lowest of the Mammalian class.
To the class Mammalia belong the most highly organized animals.
They have been the ruling animals since the close of Mesozoic
time. We must now watch their development with especial care.
For this brief review, as far as it has gone, has shown a steady
and gradual progress in life forms, the lower invariably
preceding the higher. We therefore feel that it will be vain to