A kind of convergency in all this can be distinctly traced in the superficies of the earth to the state which it has now assumed. A similar approximation in its living inhabitants, as will immediately appear, can as clearly be pointed out to its present occupants. Intelligent will and design are equally manifest in the arrangements; for, however great the amount of change, the restraining hand of foresight is visibly present in them all, and, in every successive advance to the present order of things, a purpose is discernible in making the more effectual provision for the permanent stability of the human system.
II. The Organic Remains of the tertiary deposits, if they possess not so much of antiquity as those which have already passed in review, are all the more interesting and worthy of attention, as they admit of a closer comparison with the established order of things, and the laws now regulating the distribution of animal and vegetable life. The locality most fertile in the organic remains of this period is the small island of Sheppey, situated near the mouth of the Thames, which is not more welcomely descried by the home-bound mariner as a Pharos of light and safety from the howling waste of waters, than it has proved to the palæontologist a repository and beacon-light for determining the most recondite mysteries connected with almost every living thing, in sea or land, during the Eocene age. It consists entirely of the London clay deposit, of an average thickness of five hundred feet, and displaying in the cliffs vertical faces two hundred feet high. The fossils in both localities are almost identical; in the isle of Sheppey they are more abundant, as well as accessible; and, in consequence, they have been more minutely and generally described.
1. The shells are very abundant. A few genera have survived the changes and disturbances succeeding the upheaval of the chalk, and a single species of Gasteropodes (Actæon elongatus), is common to both formations. The Belemnites and Ammonites, swarming in the seas of the secondary period, are now entirely withdrawn. The Nautilus is but sparingly represented. The new genus Cerithium is introduced, a long, tapering, spiral-formed shell, and apparently of strong predaceous habits. Lobsters resembling existing species are very abundant. The Nummulites, of which entire rocks were formed during the secondary age, still survive. And, as an index to the state of temperature, it requires to be mentioned, that many species, now found only in tropical seas, are mixed with the testaceous fossils of these localities.
2. The fishes are equally peculiar and characteristic of the era upon which we enter. Nothing can more strikingly show the violence and universality of the change that was cotemporaneous with the tertiary arrangements, than the total disappearance of the old tribes of fishes, and their replacement by entirely new specific, and a large infusion likewise of new generic, types. The change is no less remarkable when viewed in its relation to existing races, every one of which, with the solitary exception of the salmon family, have here their representatives. Perch, cod, herring, mackerel, eels, had all become occupants of the seas of this period, and their remains deposited in the clays of Sheppey are in the greatest profusion. “The number of fossil fish from the London clay,” says Agassiz, “amounts to ninety-two in the one single locality of Sheppey, without counting ten species to which I have not yet assigned names, not having hitherto been able to characterize them in a satisfactory manner.” The difficulty arises from two causes—the imperfect and fragmentary state of the fossils themselves, and the new principle adopted by him for their classification.
Most of the fishes belonging to the tertiary era are of the Cycloid and Ctenoid orders, with thin fragile scales, which, unlike the Ganoids whose cuirassed bodies were protected by a thick covering of plates, have been unable to preserve the integrity of their form and outline. The greater number of these interesting remains, accordingly, have rotted in the matrix, their bones separating, and the soft parts all replaced by clay. The scales are disaggregated (leur sécailles désagrégées), and the cranium alone of the osseous structure remaining entire, owing to the soldering of the pieces composing it, the ingenious naturalist has adopted this single organ as the basis of the new system. “The characteristic features of the skulls of the mammalia and reptilia are known; the variations which such a bone, such a crest, such a groove may undergo in such and such a family are understood; and already, at the first glance, it is possible to ascertain whether the animal under consideration is carnivorous, ruminant, or solipedal. But nothing is more variable than the forms of the cranium and of the heads of fish. The multitude of bones and of spines which serve for the attachment of the muscles, the infinite variety of forms in the families themselves, impart such a diversity to the crania of the fish, that the ichthyologist frequently despairs of being able to reduce them to their respective types, and in fact a comparative craniology of fish does not exist. There is no one, that I know, who can tell at first sight, whether such and such a cranium belongs to a percoid, to a sparoid, or to a chetodontal type.”[8]
Isolated crania and detached vertebræ are nearly all that remain of the Sheppey fossils, and the conclusions established from them by M. Agassiz, are as follows, throwing new and important light upon the two last great and approximating geological epochs.
The English coasts, at present, are inhabited by one hundred and sixty-three species of fish, of which there are eighty-one genera, divided among six predominant families, while two or three are only occasionally domiciled. Sixty species belong to the order of Ctenoids, fifty to that of the Cycloids, and eleven to the Ganoids. The fossil distribution establishes the following results: of Ctenoids twelve species, eleven genera, and three families, of which the perch tribe is the most numerous; three genera of the Teuthiæ, a family essentially meridional, and occurring only in southern seas, a fact which shows a higher climatic condition of temperature than now exists in this latitude; thirty-two species of the Cycloid order, twenty-six genera, and eleven families—of these the cod and mackerel tribes are the most numerous. While no trace of the family Salmonidæ has been detected in the tertiary deposit, a family exclusively tropical, the Characidæ, is found to have had congeners of very considerable size in the more ancient epoch. The haddock, cod, and ling races are very abundant—a fact, says Agassiz, which proves that, notwithstanding the more meridional physiognomy of the Sheppey deposit as a whole, there is nevertheless already an approximation in the fish of this interesting locality toward the actual character of the ichthyological fauna of England.
The living representatives of most of these fossils are, if anywhere, to be looked for in southern and tropical latitudes; for, notwithstanding of an approximation, there is not much of real identity of type between the existing and extinct races of the British seas. In fact, there are but four genera, Megalops, Cybium, Tetrapterus, and Myripristis, whose families are still known in the current epoch; and but very few species, from the rich prolific beds of Sheppey, have been as yet rendered into living forms.—The fishes most nearly related to the present inhabitants of warmer climes are those which are obtained from the fossil tertiary deposits of Monte Bolca in northern Italy, and in the little explored region of Mount Lebanon. Much remains to be done, therefore, before wider generalizations can be fully established. The knowledge already acquired in this department of ichthyology confirms every previous inference relating to periodic physical changes of the globe, and their convergency to the order and arrangements of nature which now prevail over the earth.
3. The reptiles and semi-natants of the tertiary period lead to the same general conclusions. The intercourse now so closely established betwixt this country and Borneo throws new light, every day more and more, upon the ancient condition of our island. The resemblance, both in the fauna and flora of these remote places, is striking throughout; when, for space on the one hand we substitute time on the other, we have nearly a transcript of their respective conditions. The northern swarmed with the crocodiles of the southern hemisphere: the boa constrictor has his representatives in the serpents of the London clay; and turtles, both of marine and fresh water characters, are equally abundant. The Pythonic monster is also there, represented by reptilians which now only inhabit tropical countries, and prey on quadrupeds and birds, both of which became abundant during the tertiary age.
4. The mammalia consisted of large pachyderms or thick-skinned animals, now represented by the rhinoceros, tapir, and elephant. Wolves, foxes, and raccoons, mice, rats, rabbits, hogs, even monkeys, began also to flit over the stage of stirring life. The existence of the order Quadrumana and the ape genus Macacus, during the earlier tertiary period, was determined by the discovery of the fragment of a lower jaw, including the socket of the last molar tooth, in a stratum of blue clay in Suffolk, and described in the “Magazine of Natural History,” for 1839, by Professor Owen. Other remains have been detected of the same animal in France, the East Indies, and South America, establishing beyond a doubt the co-existence of four different genera of apes and monkeys with the extinct mammalians of the English tertiary deposits.—That these creatures were anterior to Man, in point of creation, is in accordance with all geological evidence regarding the animal kingdom. The progressive development theory avails itself of the fact, but can establish less upon it than if it took the example of the bat—which, in anatomical structure, resembles the human family scarcely less than the monkey! But geographically considered, it furnishes a striking instance of the wonderful revolution which this island has undergone since the comparatively recent epoch of the tertiary formation. Images and pictures of life are thus called up in the vista of the past, which at once transport the mind into the bosom of the wilderness or remote Afric forest; and long ere man had betaken himself to cities, or a stone of all that huge capital had been dug out of the earth, or a sail of all its vast commercial greatness had been wafted over the waters, the very spot on which he has developed the greatest resources of his power, enterprise, and genius, was tenanted by those tribes which approach him nearest in form, which philosophers have mistaken for his type, but in which the semblance of external figure is lamentably contrasted by the absence of all that moral framework, mind, and spirit, which pre-eminently distinguish and glorify the human race!