[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 belongs to the Palaeozoic or Primary age, I am convinced—­and this conviction grows upon me constantly as I free myself from old prepossessions and bias on the subject—­that with the Carboniferous epoch we have the opening of the Secondary age in the history of the world.  A more intimate acquaintance with organic remains has shown me that there is a closer relation between the character of the animal and vegetable world of the Carboniferous epoch, as compared with that of the Permian and Triassic epochs, than between that of the Carboniferous epoch and any preceding one.  Neither do I see any reason for separating it from the others as a distinct age.  The plants as well as the animals of the two subsequent epochs seem to me to show, on the contrary, the same pervading character, indicating that the Carboniferous epoch makes an integral part of that great division which I have characterized as the Secondary age.