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[N22] 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[N23] 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.
The relations, however, which are exemplified in the oldest fishes, in the ichthyosaurians, in the pterodactyls or in the megalosaurians, seem to me to be clearly of a different character, and to differ from simple progressive types, inasmuch as those which appear earlier, combine peculiarities which, at a later period, appear separately in distinct forms. For instance, the reptilian characters which we recognise in the sauroid fishes, are developed at a later period in animals no longer belonging to the class of fishes, but constituting by themselves new types, provided with additional peculiarities which separate them fully from the fishes in general, as well as from those fishes in which we recognise some relation to reptiles during a period when no reptile existed.
Again, the ichthyosaurians, though true reptiles appearing long after fishes had been called into existence, and during an early period of the history of the reptiles, still shew their relation to fishes by the character of their vertebral column, and foreshadow, as it were, in their form, the cetacea of later ages, as well as many forms of the gigantic saurians of the secondary period. The same may be said of the pterodactyls, which are also true reptiles, but, in which the anterior extremity foreshadow peculiarities characteristic of birds and bats. Such types I shall call Prophetic Types.
To an analytic mind the examination of the peculiarities of such animals may foretell[N24] a higher progress of development, carried out in real existence, only during a later period, even if he had never seen the later ones; for in such types the germs of a future development may be recognised, and upon close examination, truly referred to the peculiarities of other higher groups, even if the intermediate links remained unknown, which, however, as the matter now stands, can leave no doubt in our mind that these prophetic types really foreshadowed that diversity of forms which has been created since they have gone by. We may also say that these prophetic types lay before us the course of thoughts which has been carried out in the plan of creation by the Supreme intelligence, who called them into existence in rich order of succession, and in so diversified relations. The recognition of this prophetic character of certain types of extinct animals is not only important in a philosophical point of view; I have no doubt it will ultimately and rapidly lead to a better, fuller, higher, and deeper understanding of the various relations which exist between animals. Let me at once point to some of these relations which might never have been understood but for this appreciation.
Among Crinoids, we have not only progressive types, as I have already quoted, but we have also prophetic ones. The Cystidæ are truly prophetic of the Echini proper. I may only mention the genus Echinocrinus to shew the link.