While the comparative anatomy of the cortex gives us a good idea of the gradual historical development of the mind in the higher classes of vertebrates, we get at the same time from their fossilized remains positive indications as to the period of time in which this phylogenesis has slowly taken place. The historical series in which the classes of vertebrates have succeeded each other in the great periods of the organic history of the earth is directly demonstrated by their fossil remains--the real commemorative medals of natural creation--and gives us a most valuable record of the ancestral history of our race and of the mind. The oldest strata that contain vertebrate remains form the huge Silurian System, which were, on the latest calculations, formed more than a hundred million years ago. They contain a few fossil fishes. In the succeeding Devonian System these are followed by the dipneusta, transitional forms between the fishes and the amphibia. The latter, the oldest four-footed and five-toed vertebrates, appear in the Carboniferous Period. They are succeeded in the Permian, the next system, by the oldest amniotes, the primitive reptiles (tocosauria). It is not until the next period (the Triassic) that the oldest mammals are found, small primitive monotremes (_pantotheria_), then marsupials in the Jurassic, and the first placentals in the Cretaceans. The great wealth of varied and highly organized forms which are contained in this third and last sub-class of the mammals appear only in the succeeding Tertiary Period. The numbers of well-preserved skulls which these placentals have left behind in fossil form are particularly important, because they give us an idea of the quantitative and qualitative formation of the brain within the various orders; thus, for instance, in the modern carnivora the brain is from two to four times, and in the modern ungulates from six to eight times, as large (in proportion to the size of the body) as in their earliest Tertiary ancestors. It is also found that the cortex (the real organ of mind) has developed in the Tertiary Period at the expense of the other parts of the brain. The duration of this Cænozoic Period has lately been calculated at three million years (according to other geologists twelve to fourteen or more million years). It was, at all events, sufficient to make possible the gradual development of the human mind from the lower intelligence of our ape-ancestors and the instincts of the older placentalia.