28. And this he holds, not only with regard to the geological periods which have preceded the existing condition of the earth, but also with regard to the transition from those previous periods to that in which we live. The present population of the earth is not one in which all previous forms are extinct. The past population of the earth was not one in which there are found no creatures still living. On the contrary, he finds that there exists a vast mass of strata, superior to the secondary strata, which are characterized by extinct forms, and are yet inferior to those deposits which are now going on by the agency of obvious causes. These masses of strata contain a population of creatures, partly extinct species, and partly such species as are still living on our land and in our waters. The proportion in which the old and the new species occur in such strata, is various; and the strata are so numerous, so rich in organic remains, so different from each other, and have been so well explored, that they have been classified and named according to the proportion of new and of old species which they contain. Those which contain the largest proportion of species still living, have been termed _Pliocene_, as containing a _greater_ number of _new_ or recent species. Below these, are strata which are termed _Miocene_, implying a _smaller_ number of _new_ species. Below these again, are others which have been termed _Eocene_, as containing few new species indeed, but yet enough to mark the _dawn_, the _Eos_, of the existing state of the organic world. These strata are, in many places, of very considerable thickness; and their number, their succession, and the great amount of extinct species which they contain, shows, in a manner which cannot be questioned, (if the evidence of geology is accepted at all,) in what a gradual manner, a portion at least, of the existing forms of organic life have taken the place of a different population previously existing on the surface of the globe.