I may mention as such, the _Ichthyosaur_, with their fish-like vertebræ, their dolphin or porpoise-like general form, and several special characters reminding us of their close relation to the Crocodilian reptiles; thus combining characters of different classes in the most extraordinary manner. Again, the _Pterodactyli_, in which reptilian characters are combined with peculiarities reminding us both of birds and bats. Again, the large carnivorous fishes of the coal period, combining peculiarities of the _Saurians_, with true fish characters; and so on. These relations are of an entirely different kind from those which I have pointed out between some of the older fossils and the early stage of growth of the living representatives of the same families. For instance, the fossil fishes with a heterocercal tail, found below the new red sandstone, down to the lowest deposits, reminds us of the peculiar termination of the vertebral column in all fish embryos of species living in the present period, to whatever family they may belong, indicating a similarity of structure in the oldest representative of this class, with the earliest condition of the germs of those animals in our days. Let us now examine whether we can properly understand the bearings of these relations, and the meaning of such differences. In the first place, I have mentioned the gradual progress, which is observed in the succession of the different classes of _Vertebrata_. This progress is exemplified by a series of types which differ from each other, but which shew, when arranged in a series, a gradation which agrees in general with the structural gradation, which we may establish upon anatomical evidence. For instance, the salamanders, with their various forms, rank below the tailless _Batrachians_. And where we have a succession of those animals in the tertiary deposits as they occur in various parts of Europe, we may fairly say that the fossils form, in their succession, a series of progressive types. Another example may perhaps illustrate the point more fully. The _orthocera_ of the oldest periods precede the curved lituites, which, in their turn, are followed by the circumvolute nautilus. Here, again, we have a natural gradation of a series of progressive types. Again, among _crinoids_, we find, in the older deposits, a variety of species resting upon a stem, while free crinoids begin to appear only during the secondary deposit and prevail, in the present creation, over those attached to the soil. Here, again, we have a series of progressive types developed successively, which are apparently independent of each other and seem to bear no other relation to each than that arising from the general character of the group to which they belong. Such types exemplify simply in the groups to which they belong, a real progress in the successive development of the peculiarities which characterise them as natural divisions among animals. Such forms I shall call _Progressive Types_.