a more thorough investigation of the different sets of strata, resulting in the establishment by D'Orbigny of a still greater number of periods, marked by the successive deposits of the Jurassic and Cretaceous seas, all of which contained different organic remains. The attention of geologists being once turned in this direction, the other epochs were studied with the same view, and all were found to be susceptible of division into a greater or less number of such periods. I have dwelt at greater length on the Jurassic and Cretaceous divisions, because I believe that we have in the relation of these two epochs, as well as in that of the Cretaceous epoch with the Tertiary immediately following it, facts which are very important in their bearing on certain questions, now loudly discussed, not only by scientific men, but by all who are interested in the mode of origin of animals. Certainly, in the inland seas of the Cretaceous and subsequent Tertiary times, where we can trace in the same sheet of water not only the different series of deposits belonging to two successive epochs in immediate juxtaposition, but those belonging to all the periods included within these epochs, with the organic remains contained in each,--there, if anywhere, we should be able to trace the transition-types by which one set of animals is said to have been developed out of the preceding. We hear a great deal of the interruption in geological deposits, of long intervals, the record of which has vanished, and which may contain those intermediate 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 }