termediate links for
which we vainly seek. But here there is no such gap in the evidence. In
the very same sheets of water, covering limited areas, we have the
successive series of deposits containing the remains of animals which
continue perfectly unchanged during long intervals, and then, with a more
or less violent shifting of the surface,[4] traceable by the consequent
discordance of the strata, is introduced an entirely new set of animals,
differing as much from those immediately preceding them as do those of the
present period from the old Creation, (our predecessors, but _not_ our
ancestors,) traced by Cuvier in the Tertiary deposits underlying those of
our own geological age. I subjoin here a tabular view giving the Epochs in
their relation to the Ages, and indicating, at least approximately, the
number of Periods contained in each Epoch.

[Footnote 4: I use surface often in its geological significance,
meaning earth-crust, and applied to sea-bottom as well as to dry
land.]

Age of Man Present.

Tertiary Age: { Pliocene }
Age of Mammalia { Miocene } with at least twelve Periods.
{ Eocene }

Secondary Age: { Cretaceous }
Age of Reptiles { Jurassic } with at least twenty Periods.
{
{ Triassic }
{ Permian } with eight or nine Periods.
{ Carboniferous }

Palaeozoic or Primary Age: { Devonian }
Age of Fishes { Silurian } with ten or twelve Periods.

It will be noticed by those who have any knowledge of geological
divisions, that in this diagram I consider the Carboniferous epoch as
forming a part of the Secondary age. Some geologists have been inclined,
from the marked and peculiar character of its vegetation, to set it apart
as forming in itself a distinct geological age, while others have united
it with the Palaeozoic age. For many years I myself adopted the latter of
these two views, and associated the Carboniferous epoch with the Palaeozoic
age. But it is the misfortune of progress that one is forced not only to
unlearn a great deal, but, if one has been in the habit of communicating
his ideas to others, to destroy much of his own work. I now find myself in
this predicament; and after teaching my students for years that the
Carboniferous epoch b