ten by comparative anatomy, wherever the facts enable a comparison to
be made.

But the extinct animals of the third and fourth ages are more interesting
to us, because there are more of them and because they are more like the
well-known organisms of our present era. These two ages are called the
Mesozoic or Secondary, and the Cenozoic or Tertiary. The former is so
named because it was a transitional age of animals that are intermediate
in a general way between the primitive forms of the preceding age and
those of the next period; the latter name means the "recent-animal" age,
when evolution produced not only the larger groups of our present animal
series, but also many of the smaller branches of the genealogical tree
like orders and families to which the species of to-day belong.

Confining our attention to the large vertebrate classes, the testimony of
the rocks proves, as we have said, that fishes appeared first in what are
called the Silurian and Devonian epochs, where they developed into a rich
and varied array of types unequaled in modern times. At that period, they
were the highest existing animals--the "lords of creation," as it were. To
change the figure, their branch constituted the top of the animal tree of
the time, but as other branches grew upwards to bear their twigs and
leaves, as the counterparts of species, the species of the branch of
fishes decreased in number and variety, as do the leaves of a lower part
of a tree when higher limbs grow to overshadow them.

Following the fishes, the amphibia arose during the coal age or
Carboniferous, usurping the proud position of the lower vertebrate class.
The reptiles then appeared and gained ascendancy over the amphibia, to
become in the Mesozoic age the highest and most varied of the existing
vertebrates. At that time there were the great land dinosaurs with a
length of 80 feet, like _Brontosaurus_; aquatic forms like _Ichthyosaurus_
and _Plesiosaurus_, whose mode of evolution from terrestrial to swimming
habits was like that of seals and penguins of far later eras. Flying
reptiles also evolved, to set an example for the bats of the mammalian
class, for both kinds of flying organisms converted their anterior limbs
into wings, although in different ways.

During the Triassic and Jurassic periods of the Mesozoic age, the first
birds and mammals appeared to follow out their diverging and independent
lines of descent. Palaeontology makes it possible to trace the