Among animal remains found in the fossil state, shells and zoophytes are the most abundant. They form the principal parts of rocks which often occupy considerable districts. They are most frequent in calcareous strata, from the transition limestones to the highest of the marles. A remarkable fact is observed in respect to these shells, and the other fossils which accompany them; those which are found in the oldest, or transition formations, are more different from those that now exist, than those in the more modern deposits. Thus the transition limestones and slates contain terrebratulites, with encrinites, pentacrinites, and trilobites; in those of the submedial and medial series we find belemnites and the cornu ammonis; many of which are extinct genera, and some of which are of families that are no longer found living on our globe, while even where the genus is now to be met with, the species at least has become extinct; while in the latest of the tertiary or superior formations, we find ostracites, pectinites, buccinites, chamites, and many other genera that are still abundant, and even types of living species.
By far the greater part of the animals whose remains are found in the older strata are aquatic, and the vast extents over which they are distributed, show, that the waters must at one time have covered a very great proportion of what is now dry land. Nor has this change been produced by any gradual subsidence, for we find no coincidence in the levels of those portions of the land that contain similar fossils; some for instance are still lower than the level of the present ocean; others, again, of similar character, rest upon the tops or sides of the highest mountains. In Europe, the tops of the highest of the Pyrenees, rising 11000 feet above the level of the sea, are of limestone, containing numerous fossil remains, while Humboldt found a rock, similarly characterized, among the Andes, at the height of 14000 feet.
The ancient philosophers, who, in other departments of physical science, were far behind the moderns, seem in this alone to have pursued a process of inductive reasoning, which led to results far more accurate than any attained by the moderns, until within a very few years. The dogmatism which determined to find in every fossil aquatic remain a proof of the particular Noachic deluge, and the timidity of those whose researches had made them better informed, left the world wholly in the dark as to the real inferences to be drawn from a study of the structure of the earth; but what modern geologist could better express what are now admitted opinions, than the words which the Roman poet puts in the mouth of Pythagoras.
"Vidi ego, quod quondam fuerat solidissima tellus,
Esse Fretum. Vidi factas ex æquore terras:
Et procul a pelago conchæ jacuere marinæ;
Et vetus inventa est in montibus anchora summis.
Quodque fuit campus, vallem decursus aquarum
Fecit: et eluvie mons est deductus in æquor:
Eque paludosa siccis humus aret arenis;
Quæque sitim tulerant, stagnata paludibus hument.
Hic fontes Natura novos emisit, at illie
Clausit: et antiquis concussa tremoribus orbis
Flumina prosiliunt; aut exæcata resident."
The order in which fossil remains are found to succeed each other in the successive formations that are to be traced from the oldest rocks to the diluvial deposit, are well illustrated in the words of a late distinguished philosopher, whom we shall quote.
"In those strata which are deepest, and which must consequently be supposed to be the earliest deposited, forms, even of vegetable life, are rare; shells and vegetable remains are found the next in order; the bones of fishes and oviparous reptiles exist in the following class; the remains of birds, with those of the same genera mentioned before, in the next order; those of quadrupeds of extinct species in a still more recent class; and it is only in the loose and slightly consolidated strata of gravel and sand, and which are usually called diluvial formations, that the remains of animals such as now people the globe are found, with others of extinct species. But in none of these formations, whether called secondary, tertiary, or diluvial, have the remains of man, or any of his works, been discovered: and whoever dwells upon this subject, must be convinced that the present order of things, and the comparatively recent existence of man as the master of the globe, are as certain as the destruction of a former and different order, and the extinction of a number of living forms, which have types in being. In the oldest secondary strata there are no remains of such animals as now belong to the surface; and in the rocks which may be regarded as most recently deposited, these remains occur but rarely, and with abundance of distinct species;—there seems, as it were, a gradual approach to the present system of things, and a succession of destructions and creations preparatory to the existence of man."
We have stated that the zoophytes and shell-fish have left the most numerous fossil remains. Those of other families are not however rare. Fish, for instance, are found in great abundance, near Glarus in Switzerland, in clay slate; in Germany, at Papenheim, in a slaty marle, in the cupriferous slate of Eisleben, in the fetid limestone of Oehningen. They are also found in Egypt, and we have specimens of the same sort from Lyria, in a limestone apparently belonging to the oolitic or Jura formation. China and the coast of Coromandel have also fossils of this sort, but by far the greatest quantity have been procured from Mount Bolea, near Verona. A splendid suite from the last locality are to be seen in the Gibbs' Cabinet at New-Haven. Besides the impressions of entire fish, separate portions are very abundant, and perhaps the most frequent of these are the teeth of sharks, which are sometimes of a magnitude vastly greater than those of any living species. Animals of the class of amphibia appear not to have existed until after the æra that gave birth to fish. The oldest are probably the tortoises, of which a specimen has been found in sandstone near Berlingen. They have also been found in England, in the Netherlands near Brussels, at Aix in Provence, and in the quarries near Paris. The most remarkable fossils of this class belong, however, to the lizard family. Of these the most remarkable are the plesiosaurus, the megalosaurus, the iguanodon, and the crocodile of Maestricht, all belonging to extinct species.
The marine animals that are met with in a fossil state, are in great part foreign to the climates in which they are found buried. It has been shown that the fish of Bolea have their nearest living prototypes in the seas of Otaheite. The perpites of Gothland have been supposed to be petrifactions of the medusæ of India. The madrepores, so abundant in Russia and in the frozen deserts of Siberia, only live now in seas within the tropics. Shells analogous to a great part of those found fossil in England, are only to be seen in the Atlantic, in a living state, on the coasts of Florida and Cuba. A shell-formed fossil at Havre is only to be met with recent at Amboyna.
Of the shells found in Italy, fossil in the sub Appenine hills, many are common to the Mediterranean and the Indian oceans. But while those in the fossil slate and the recent specimens from the tropics correspond in size, individuals of the same species from the Mediterranean are dwarfish and degenerate.
Thus then the remains of aquatic and amphibious animals appear to confirm the conclusion drawn from vegetable fossils, that a climate of temperature as elevated as that now found in the tropics, once extended into high northern latitudes. It has been seen that the fossil remains and impressions of shells have been found at great heights upon the sides, and even upon the tops of mountains; and that in the older of the strata no trace is to be found of any but aquatic animals. Thus before our existing mountains and the minerals they contain had arisen above the general surface; before diluvial and alluvial deposits, or even the great formations of sandstone and conglomerate had arisen from their disintegration, the globe was covered, in a great degree, and as it appears from considerations we have not space to enter into, by various successive eruptions, with waters, sometimes fresh, sometimes saline. These waters have, it could be readily made to appear, often rested long on the surface in a quiet state, after having been in violent agitation; and long ages of tranquillity have been succeeded and closed by convulsions of the most violent character.