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.
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