and the fern-forests of the Carboniferous period, Reptiles and Insects found their place; and only when the earth was more extensive, when marshes had become dry land, when islands had united to form continents, when mountain-chains had been thrown up to make the inequalities of the surface, were the larger quadrupeds introduced, to whose mode of existence all these circumstances are important accessories. But while all the types and most of the classes were introduced upon the earth simultaneously at the beginning, these types and classes have nevertheless been represented in every great geological period by different sets or species of animals. In this sense, then, there has been a gradation in time among animals, and every successive epoch of the world's physical history has had its characteristic population. We have found that there is a correspondence between the gradation of structural complication among adult animals as known to us to-day, which we may call the Series of Rank, and the gradation of embryological changes in the same animals, which we may call the Series of Growth; and there is also a correspondence between these two series and the order of succession in time, that establishes a certain gradation in the introduction of animals upon earth, and which we may call the Series of Time. Take as an illustration the class of Echinoderms. The first representatives of this class were a sort of Star-Fishes on stems; then were introduced animals of the same order without stems; in later periods come in the true Star-Fishes and Sea-Urchins; and the highest order of the class, the Holothurians, are introduced only in the present geological epoch. Compare now with this the ordinal division of the class as it exists today. The present representative of those earliest Echinoderms on stems is an animal that upon structural evidence stands lowest in the class; next above it are the Comatulæ, corresponding to the early Echinoderms without stems; next in our classification are the Star-Fishes and Sea-Urchins; and the Holothurians stand highest, on account of certain structural features that place them at the head of their class. The Series of Time and the Series of Rank, then, accord perfectly, and investigations of the embryological development of these animals have shown that the higher Echinoderms pass through changes in the egg that indicate the same kind of gradation, for the young in some of them have a stem which is gradually dropped, and their successive phases of development recall the adult forms of the lower orders. Take as another illustration the class of Polyps. First in time we find a kind of Polyp Coral, one among the early Reef-Builders, who built their myriad lives into the solid crust of our globe then as their successors do now. These old Corals have their representatives among the present Polyps, and from their structure they are placed lowest in their class,