# A Treatise on Geology/Chapter 5

CHAP V.
PERHAPS geology might never have escaped from the domain of empiricism and conjecture but for the innumerable testimonies of elapsed periods and perished creations, which the stratified rocks of the globe present in the remains of ancient plants and animals. So many important questions concerning their nature, circumstances of existence, and mode of inhumation in the rocks, have been suggested by the examination of these interesting reliquiæ, and the natural sciences have in consequence received so powerful an impulse, and been directed with such great success to the solution of problems concerning the past history of the earth, that we scarcely feel disposed to dissent from the opinion of Cuvier, "that without (fossil) zoology, there was no true geology."
The stratified crust of the globe may, without exaggeration, be said to be full of these monuments of the vanished forms of life: they are of extremely various kinds; lie in many different states of preservation; occur very unequally in rocks of different sorts and ages; and thus present a large field of contemplation to the philosophic geologist.
Fossil Plants.
The organic remains both of plants and animals occur abundantly in the earth; the latter are most numerous. Of fossil plants, many are terrestrial, a few are fluviatile, others are marine. In the present system of nature terrestrial plants are, probably, ten times as numerous as the marine tribes, and it does not appear that the ratio of the fossil tribes is very different. ​But the total number, as far as yet known, is wonderfully disproportionate. For, if we estimate the recent species of plants at only 60,000, and the fossil races, yet clearly distinguished, at 600, numbers which are, perhaps, equally below the truth, the proportion is 100 to 1. To infer, from this fact, that the ancient globe nourished few species of plants compared to the present rich flora of different latitudes, would be unauthorised by the data, though from other phenomena such a conclusion might appear probable. We must recollect that the stratified rocks were formed chiefly on the bed of the sea, and therefore could not be expected to contain, except rarely, the remains of terrestrial plants; just as at this day, it is only under particular conditions of the surface drainage that vegetables are carried abundantly to the deep. And, since most of the marine plants are natant or confined to rocky shores, there would be little reason in expecting these to be common among the oceanic sediments.
We must further observe, that the cellular substance of the marine tribes of plants might cause many of them to perish under the slow accumulation of the strata: nothing is less common than to find the substance of marine vegetables preserved in the same manner as the ligneous parts of land plants; and, indeed, among land plants, the experiments of Dr. Lindley show that many of them perish by maceration in water, while ferns, cycadeæ, and other tribes, resist decomposition for a long time. Hence, it is no wonder that such races of plants are the most frequently met with in a fossil state.
The ligneous parts of plants are sometimes (in the blue clays of the oolitic formation especially) converted to jet: sometimes, only the external layers of coniferous wood are so converted, while the internal parts are changed to carbonate of lime. In the latter case, the structure of every cell and vessel is distinctly seen in thin slices. When woody plants lie in limestone rock which contains silica, or in calcareous sandstone (as in ​the coralline oolite and calcareous grit), they are often silicified: very frequently in clays pyrites aids the beauty, but diminishes the duration of the specimens. In the shales of a coal tract plants of all kinds are converted to coal of different qualities: the same effect happens in the fine grained sandstones of the coal tracts; but in millstone grit, and other coarse sandstones, the only reliques of the plants are the external impressions of them, and a brown carbonaceous or ochraceous powder. In the upper coal measures of Lancashire, and in the shales of the peculiar oolitic strata of Yorkshire, we have found thin leaves yet retaining their elasticity, and changed to a brown translucent pellicle, in which the impressions of the superficial respiratory pores might be clearly seen. In other cases the nervures and seed vessels of fern leaves are perfectly retained in shale, fine sandstone, and ironstone.
The distribution of fossil plants in the earth is remarkable on many accounts. Being for the most part of terrestrial races, it is not surprising that they should be found principally in the sedimentary strata of sandstone and clay, for it is always associated with such sediments that they pass at this day with the Mississippi and other rivers to the ocean. So strict, however, is this connection, that in a series of alternating limestones, sandstones, and shales, the two latter may be richly stored with land plants, and the former filled with marine shells; neither partaking in the treasures of the other. It must be considered much in favour of this view of the dispersion of fossil plants by rivers entering the sea, that the trees are usually in fragments, the branches and leaves scattered, and roots generally wanting altogether. One case, indeed, has been apparently established, of the trees being buried in the very spot where they grew, by submergence of the land, "the Dirt Bed" of the Isle of Portland: but this is certainly an exceptional case; the rule is undoubtedly contrary.
Those who expect, consistently with general ​probability, that the earliest indications of life on the globe should be of the vegetable kingdom, may be somewhat astonished to learn, that traces of plants are really not known in a distinct form in strata so ancient as those which contain the shells of Brachiopoda in the mountains of Wales, and that only fucoids are discovered in the silurian system. What is calculated to add to this feeling of surprise is the circumstance that in the next but one system which lies upon the silurian, two of the formations are the repository of most enormous accumulations of fossil plants; for in these rocks principally lie the coal beds of Europe and America, which are nothing else than a mass of chemically altered vegetables. How vast must have been the luxuriance of the vegetable world at that era in particular parts, appears from the thickness and continuity of the coal beds; for it is probable that the most dense forest of tropical America would, if buried under sediments, and subjected to the changes which yield coal, produce but a very thin bed of that substance. Yet, in the coal formation, beds of three, four, six, ten, and more feet are not uncommon, and the different layers yield as much as sixty feet of solid coal.
Whatever were the causes which permitted that prodigious growth and aggregation of trees and other plants during the era of the production of coal, it appears they were never repeated, for the few unimportant deposits of coal in the oolitic system of Sutherland, Yorkshire, Bornholm, and Westphalia, which are chiefly formed of cycadeæ and equiseta, hardly deserve mention in comparison.
The races of plants entombed in the earth at different periods of its formation, are by no means the same. M. Adolphe Brongniart, to whom we are indebted for almost the first philosophical view of the affinities of fossil plants, presents the following comparative table of the extinct and living classes of plants:— ​
| | First period | Second period | Third period | Fourth period | Living. |
| Agamia. | 4 | 5 | 18 | 13 | 7000 |
| Cryptogamia cellulosa | | | | 2 | 1500 |
| ────── vasculosa | 222 | | 31 | 6 | 1700 |
| Phanerogamia gymnospermia | | 5 | 35 | 20 | 150 |
| ────── monocotyledonia | 16 | 5 | 3 | 25 | 9,000 |
| ────── dicotyledonia | | | | 100 | 32,000 |
| indeterminate | 22 | | | | |
| | 264 | 23 | 87 | 103 | 50,350 |
| | $\underbrace {\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }$ | $\underbrace {\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }$ | $\underbrace {\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }$ | $\underbrace {\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }$ |
| | 540 | 540 | 540 | 540 |
The first period ends with the carboniferous system; the second includes the saliferous and magnesian systems; the third comprises the oolitic and chalk systems; the fourth is the tertiary period.
The numbers of species are now considerably augmented since the table was drawn up (1829), but the proportions are not materially affected. It is still true, that vascular cryptogamia abound in an extraordinary degree among the earlier rocks, where ferns, calamities, and what seem like gigantic lycopodiaceæ are very prevalent; that in the second and third periods cycadiform and coniferous plants (phanerogamia gymnospermia) become remarkable and frequent, though ferns and lycopodiaceæ still prevail; while it is principally in the fourth period that the usual forms of dicotyledonous plants, now so plentiful on the earth, appear at all common. Moreover, on a close examination, it appears that nearly every fossil plant is of an extinct species, and that the several periods distinguished by M. Brongniart had each its own peculiar vegetable creation, distinct from every other that preceded and succeeded it.
Fossil Zoophyta.
Zoophyta being in the present system of nature all aquatic, and mostly marine, they may be expected to occur abundantly in the marine strata of the earth. They are, indeed, very plentiful, and it is interesting to observe that all, or nearly all, the species are marine. It ​is further remarkable, that few traces occur of any other zoophytata an such as, like the lithophyta, secreted stony supports; or like spongiadæ, had an internal horny or spicular skeleton; or like echinida, were covered with a crustaceous skin: the soft medusidæ, holothuridæ, &c., are, perhaps, sometimes recognisable by faint impressions in the rocks, but their substance has wholly vanished. The soft parts of nearly all the zoophyta are absent from the fossil state.
The recent zoophyta are either free in the sea, or attached for life after a very early period of growth: instances of both divisions occur in the earth. The fossil corals do not, perhaps, in general appear in the very place where they grew, but rather seem to have suffered some displacement before being buried in the oceanic sediments. But exceptions occur; and some of the fossil radiaria which were attached by a pedicle (crinoidea) are found in several places (near Bradford in Wiltshire), yet rooted to the limestone rock. In such cases, how vain is the supposition that the deposition of the substance of the rocks was either rapid, confused, or violent. The limestones of the Devonian and Silurian systems are so very rich in corals as to suggest to good observers the notion that these concretionary and rather irregular rocks were ancient coral reefs.
Calcareous matter composes the greater part of the hard parts of zoophyta; in a few instances besides the family of spongiadae, siliceous spiculæ and fibres enter into the skeleton of the animal. In a fossil state corals, echinida, crinoidea, &c., are generally calcareous; rarely particular tribes of corals (as millepera, syringopora) are converted to siliceous matter: sponges are commonly siliceous, but sometimes calcareous. Occasionally nothing remains of the original body; its place in the rock is vacant, and there is left only the external impression or mould. These circumstances depend partly on the nature of the rock in which they are imbedded, and partly on the composition and texture of the original body. In limestone rocks the substance of coral is usually ​little changed, except by the introduction of calcareous or siliceous matter into the minutest interstices; but, in the same circumstances, the crusts of echinida and stellerida are converted to crystallised calcareous spar. Even in arenaceous and argillaceous strata, and amidst flint nodules where every sponge is silicified, the stems of crinoidea and spines of echinida are thus represented. A curious circumstance was noticed some time ago by the Rev. H. Jelly of Bath, concerning some lamelliferous corals of the oolite: the great mass of the coral was decomposed, and the cavity it once filled was partially occupied by pyramidal crystals of carbonate of lime, in whose transparent substance the radiating plates of the coral were clearly discernible; a fact in harmony with many other phenomena indicative of the power of crystalline attractions to overcome and involve arrangements of matter depending on other causes.
The laws of the distribution of fossil zoophyta so far agree with what has, been already inferred concerning plants, as to prove that in this class of beings likewise, many distinct systems or assemblages of forms have existed at different ancient periods, which are all now extinct. Yet it is certain that the differences are mostly only such as belong to species, genera, and families, those minor groups of orders and classes which most distinctly reveal differences of physical condition, while agreements of a very general kind permit nearly all fossil zoophyta to be ranked as analogous to known living tribes. Even for the crinoidea, the most considerable exception, at least one living type is known. There is, undoubtedly, to be noticed a great difference as to the groups of zoophyta which belong to the different periods of the formation of the stratified crust of the globe; and a considerable discordance between the forms of the oldest fossil races, and those now actually existing. Zoophyta were collected by the author (1836) among bivalve shells, in one of the oldest fossiliferous slaty rocks of Britain, on the summit of Snowdon; they abound to admiration in the bands of Wenlock and ​Aymestry, and in most of the limestones of the Devonian and carboniferous rocks. The magnesian limestone has a small number; certain oolites are full of them; the green sand and chalk yield great plenty of sponges; the calcareous and arenaceous tertiaries of France furnish many beautiful forms, of genera often the same as those now found in the sea. Undoubtedly, as a general rule, zoophyta occur more plentifully in calcareous rocks than in any others; they are probably more numerous in the older strata; and there are probably more fossil than recent species, if we exclude from the latter, those whose bodies are unconnected to stony or horny external or internal supports.
It was once imagined that the higher orders of zoophyta, those ranked by Lamarck in his group of echinodermata, were absent from the older formations; and certainly they are, at least, not common among the very oldest fossils. Crinoidea, however, occur in the silurian rocks, and they are more plentiful in the carboniferous limestone than in any older or more recent deposits. Echinida and Stellerida first appear in the lower silurian rocks (Malvern and Wales), but become far more numerous in the oolitic and chalk systems. Sponges are by far most numerous in the cretaceous rocks.
| Systems. | Spongiæ | Lamelliferæ | Crinoides. | Echinoida. | Stellerida. |
| Tertiary | * | * | * | * | * |
| Cretaceous | ✽ | * | * | ✽ | ✽ |
| Ooolite | * | * | * | ✽ | ✽ |
| Red sandstone | | | * | | |
| Carboniferous | ? | ✽ | ✽ | * | |
| Silurian | * | * | * | ✽ | * |
In the above table, the small stars indicate that some species of the groups of zoophyta whose names occur above are found in the system of strata on the line of which they are situated; the large stars are placed on the line of that system of strata in which the group of zoophyta is specially numerous. ​
Mollusca.
Recent mollusca are principally found at moderate depths in the sea, and respire the air contained in water; some particular tribes live in fresh water, and either breathe the air in water by branchiæ, or come to the surface to respire by lungs; others live on the land. In a very few cases certain stratified masses appear to have been accumulated either in limited areas of fresh water, or in estuaries so much under the influence of rivers and inundations as to contain land and fresh-water shells alone or mixed with the exuviæ of marine animals. But these few and exceptional cases yield, perhaps, altogether in England not one twentieth part of the number of fossil testaceous remains; on the continent of Europe the proportion is not very different. In the existing economy of nature, however, the land shells are so extremely numerous that, with the fresh-water tribes, they probably constitute one fourth of the total number of known species. We must not, however, conclude, from the comparative rarity of land and fresh-water shells in a fossil state, that the ancient land and fresh waters were but scantily supplied with mollusca; for, in the first place, their remains would seldom be transported to the ocean; and further, the presumed fresh-water shells are extremely plentiful in the coal tracts, weald of Sussex, and fresh water beds of the Isle of Wight. The total number of fossil marine mollusca already collected is about equal to that of the living races: what may be the proportions hereafter is difficult to estimate, for it is certain that great additions will be made to both the catalogues.
It is not entirely without reason that geologists have been long accustomed to look on the study of fossil shells as more instructive with regard to the physical conditions of the globe in ancient times than most other reliquiæ of animal life. They are of all fossils the most numerous, the most generally diffused through rocks of all ages, most perfectly preserved, and of such definable forms as to be easily described, figured, and ​recognised. The state of perfection in which many delicately ornamented shells occur, is such as to leave little doubt of their having been quietly entombed on or near the spots where they lived in the deep sea; while in other cases the disunion of valves and the fragmentary state, even of the most solid shells, recall to our memory the agitation of waves over the sands and pebbles of the shore.
The hard calcareous coverings of mollusca are perfectly preserved in a fossil state; the semi-calcareous hinge ligament of bivalves is sometimes observed in cardia, veneridæ and unionidæ; very rarely the softer animal tissues.
Among recent shells the most contrasted appearances of structure are those presented by the oyster, which is lamellar, and the venus, which is, apparently, compact, and the internal plate of the cuttle, which is of a fibrous nature. All are full of carbonate of lime, as a hardening earth, and all mixed with membranous gelatine, which, by its different arrangements, determines the above and other interior structures. It is remarkable that oysters, and shells which like them are composed of distinct broad lamellæ of alternating membrane and carbonate of lime, have resisted in almost all rocks, argillaceous, calcareous, arenaceous, the chemical changes to which veneridæ, trigoniæ, and others of an apparently compact texture, have completely yielded. While the former retain their lamellæ and pearly surfaces, the latter have often been wholly dissolved in limestone rocks, and their places left vacant: while a cast of the inside of the shell, and an impression of the outside, disclose completely the history of the change. A further process is frequently superadded, by which the cavity is again partially or wholly filled with crystals of carbonate of lime, which has been introduced by filtration through the surrounding rock. In other cases siliceous matter, pyrites, and other substances, have passed by a similar process. The common fossil called belemnites, of the same group as the cuttle, ​is a remarkable instance of the force of original structure in controlling the effects of chemical agencies; for in clay, sands, chalk, flint, limestone, pyrites, this singular fossil generally retains its fibrous structure, colour, translucency, and chemical properties; while in the same masses echini are changed to calcareous spar, and sponges to flint, and many shells have totally vanished.
The conclusion which so strongly forces itself on the mind of an observer who considers the shelly treasures of the stratified rocks, that each of these was successively the bed of the sea, becomes of undoubted certainty, when the minuter circumstances of the distribution of molluscous exuviæ are known. In the present seas, some shells, like the oyster, are gregarious, and cover large surfaces, so as to constitute shelly banks in which but a few species live together; others are dredged promiscuously from a common feeding ground. There are fossil as well as recent beds of oysters, and they are in each case argillaceous beds; perhaps cardia are more plentiful in old sandy strata, as well as in modern sandy bays; terebratulæ and lingulæ are usually associated in nests or families; and it is certain that much curious information, as to the circumstances of their existence, may be gathered from studying the details of the distribution of fossil mollusca.
But on a great scale they present very important truths. From the ancient slates of Snowdon to the most modern deposits in Norfolk and Sicily, the stratified rocks abound with shells; and though it is certain that calcareous rocks, and the strata near to them, contain the greatest number, enough are found in the sandstones and clays to furnish the means of establishing some very important conclusions. The first which arrests our attention is the continual augmentation of the amount of marine life from the primary to the tertiary period. In the following table, drawn up by the author, the number of species known, and also the proportionate number to every 100 feet thickness of strata, were given for the successive systems (in 1836):— ​
| | No of Species. | Thickness of Strata. | No of Species to 100 feet thickness. |
| Living | 5000 | | |
| Tertiary | 272 | 2000 | 137 |
| Cretaceous | 500 | 1100 | 45.5 |
| Oolite | 771 | 2500 | 31 |
| Saliferous and Magnesian | 118 | 2000 | 6 |
| Carboniferous | 366 | 10,000 | 3.6 |
| Silurian, &c. | 349 | 20,000 | 1.7 |
The most predominant of the recent forms of mollusca are the classes of Conchifera, Gasteropoda, and Cephalopoda; these are also the most numerous in a fossil state, for of pteropodous mollusca, a few traces only occur in the tertiary strata. If the distinct species of shelly mollusca be supposed to amount to 5000 species, the numbers belonging to each of these great classes may be stated thus, in a recent state:—
| Conchifera | 1800 |
| Gasteropoda | 3100 |
| Cephalopoda | 100 |
The same classes, in a fossil state, contain in 5000:—
| Conchifera | 2086 |
| Gasteropoda | 2230 |
| Cephalopoda | 684 |
If we analyse the classes, greater discordances appear. Thus the existing conchifera, ranked in three groups, present the following proportions in 1000:—
| Conchifera plagimyona (Latreille) | 777 |
| —————— Mesomyona (Latr.) | 194 |
| —————— Brachiopoda | 29 |
but in a fossil state the proportions are,
| Conchifera plagimyona | 483 |
| —————— Mesomyona | 338 |
| —————— Brachiopoda | 179 |
In the same way it appears, that while in existing nature the shelly gasteropoda ranked in two great ​divisions, according to their principal food, give the following proportions in 1000:—
| Herbivorous gasteropoda | 451 |
| Zoophagous ────────── | 549 |
these divisions, in the fossil state, yield:—
| Herbivorous gasteropoda | 511 |
| Zoophagous ────────── | 489 |
It appears then, that the fossil world of mollusca differs remarkably from the actual creation in the greater proportionate abundance of cephalopoda, herbivorous gasteropoda and brachiopodous and mesomyonous conchifera. If the whole number of species of shelly mollusca of the three classes named, were supposed 1000 in the fossil and recent states, the proportions of the several groups would be nearly as under:—
| | Fossils | Recent. |
| Conchifera plagimyona | 205 | 280 |
| ───────── mesomyona | 142 | 70 |
| ───────── brachiopoda | 75 | 10 |
| Gasteropoda phytophaga | 225 | 280 |
| ─────────── zoophaga | 215 | 340 |
| Cephalopoda | 138 | 20 |
These differences, however, are by no means equal in all the several systems of strata: they are least in the newest, and greatest in the older classes of rocks. If the number of shelly mollusca in each of the three great periods be 1000, the proportionate number of the several classes may be seen in the following table, and compared with the recent creation.
| | First or palæzoic Period. | Second or Mesozoic Period. | Third or Cainozic Period. | Recent |
| Conchifera plagimyona | 150 | 228 | 268 | 280 |
| ──────── mesomyona | 102 | 202 | 70 | 70 |
| ──────── brachipoda | 320 | 105 | 8 | 10 |
| Gasteropoda phytophaga | 198 | 127 | 172 | 280 |
| ────────── zoophaga | 24 | 19 | 388 | 340 |
| Cephalopoda | 206 | 319 | 94 | 20 |
The analogy of the tertiary to the actual system of organic nature is very apparent in these numerical proportions, and the distinctness of both from the older types in the lower strata is one of the most remarkable and important generalisations in geology.
Nearly all the fossil mollusca, even in the tertiary system, belong to extinct species, a large proportion to extinct genera, particularly among the cephalopoda, brachiopoda, and mesomyona.
The following tables, will exhibit the numerical proportion of species of particular genera in the living and ancient systems of nature, and illustrate other important truths.
Table I.—Genera containing many Living Species. (gasteropoda.)
| | Cypræa | Conus | Voluta | Strombus | Murex | Fusus | Ceri- thium | Mitra | Pleu- rotoma |
| Living species | 135 | 181 | 66 | 45 | 75 | 67 | 87 | 112 | 71 |
| In Cainozoic strata | 19 | 49 | 32 | 9 | 89 | 111 | 220 | 66 | 156 |
| In Mesozoic strata | | | 2 | 1 | 3 | | 1 | | |
| In Palaeozoic strata | | | | | | | | | |
In this table the strong analogy of the tertiary and living forms of animals, and their distinctness from those of earlier date, are very decided. ​
Table II.—Genera Containing many Fossil Species.(conchifera.)
| | Producta | Spiri- fera | Tere- bratula | Trigonia | Phola- domya | Plagi- ostoma | Inoce- ramita | Gryphæa |
| Living species | | | 15 | 1 | 1 | | | 1 |
| In Cainozoic strata | | | 18 | | 1 | | | 1 |
| In Mesozoic strata | | 6 | 106 | 33 | 19 | 38 | 20 | 24 |
| In Palaeozoic strata | 64 | 90 | 65 | 10 | | | | |
The unequal periods of existence of different genera are here very apparent. Producta, after abounding in Devonian and carboniferous ages, perishes in the saliferous period. Spirifera passes through all these periods and ends in the oolitic; but terebratula occurs through all the strata, and still lives.
Table III.—Genera of Cephalopoda.
| | Belle- rophon | Ortho- ceras | Belem- nites | Nautilus | Ammo- nites | Hamites | Scaphites | Baculites | Nummu- lites |
| Living species | | | | 2 | | | | | |
| In Cainozoic strata | | | | 4 | ? | | | | 3 |
| In Mesozoic strata | | | 83 | 22 | 221 | 30 | 5 | 5 | |
| In Palaeozoic strata | 24 | 57 | | 31 | 53 | | | | |
Most of the fossil cephalopoda belong to extinct genera: of these, bellerophon and orthoceras are abundant in the lower and middle palaeozoic strata. Hamites, scaphites (almost peculiar to the cretaceous system, a few only in the oolites), and Belemnites, belong to the mesozoic series, and characterise the oolitic and chalk rocks exclusively. ​
Table IV.—Subgenera of Ammonites According to Von Buch and Munster.
| | Clyme- nia | Gonia- tites | Cera- tites | Arie- tes | Falici- feri | Amal- thel | Capri- corni | Planu- lati | Dor- sati | Coro- nati | Macro- cephali | Arm- ati | Den- tati | Ornati | Plexu- osi |
| Living species | | | | | | | | | | | | | | | |
| In tertiary strata | | | | | | | | | | | | | | | |
| In cretaceous system | | | | | 2 | 4 | | | | | 9 | 14 | 13 | 2 | 3 |
| In oolitic system | | | | 12 | 22 | 27 | 12 | 26 | 5 | 11 | 11 | 11 | 4 | 5 | 3 |
| In saliferous system | | | 3 | | | | | | | | | | | | |
| In carboniferous system | | 33 | | | | | | | | | | | | | |
| In Devonian system | 14 | 26 | | | | | | | | | | | | | |
These are all extinct forms, and while the greater number of species and sub genera abound in oolitic, and many in cretaceous rocks, none occur in tertiary rocks; one group occurs in saliferous, and different types in carboniferous and Devonian strata.
Thus general and particular results all agree in demonstrating that the physical conditions of the ancient ocean must have been very different in some respects from what obtain at present; and that these conditions were subject to great variation during the long periods which elapsed in the formation of the crust of the earth. In the course of these changes whole groups of animals perished; others were created, to perish in their turn; and these operations were many times repeated, not only before the present races of animals were formed, but even before the relative numbers in the leading groups approximated to the proportions which appear in the actual sea.
Articulated Animals.
The annulose animals form two great series; those without jointed feet, viz v vermes, annulosa, cirripeda; ​and those with jointed feet, viz., insecta, myriapoda, arachnid, Crustacea. Many of the vermes being wholly soft, and living as parasites; many of the true annulosa being also soft, their remains are rarely recognisable in the earth; while serpula, spirorbis, and other annulosa with tough or shelly tubes, are very numerous. The number of these curious fossils will undoubtedly be much enlarged by careful research in all the arenaceous groups of marine and estuary strata. Already we have them from the very oldest to the most recent of the fossiliferous strata. Cirripeda are not plentiful, and only found in the upper secondary and in tertiary deposits. Insects which, though not wholly terrestrial, are not found in the sea, numerous as they are in the air, the soil, and fresh water, are but locally met with in a fossil state. Arachnida and myriapoda, equally unknown in the sea, are as little common as fossil insects; but Crustacea, mostly a marine race, are not infrequent in all the series of the strata, though generally unlike existing tribes. The following table of some of the fossil genera of Crustacea may give a correct notion of their distribution in the earth.
| | Agno- stus | Ole- nus | Caly- mene | Asa- phus | Pali- nurus | Asta- cus | Pagu- rus | Can- cer |
| Living | | | | | * | * | * | * |
| Cainozoic | | | | | * | * | * | * |
| Mesozoic | | | | | * | * | * | |
| Palæozoic | * | * | * | * | | | | |
The whole great family of trilobites, including many other genera besides those named, is confined to the palæozoic and is especially abundant in the lower palæozoic strata. ​
Fishes.
The finny races of the sea and fresh waters amount to many thousand (perhaps 8000 or more) species; those yet recognised in a fossil state are about 800, or one tenth; but, since a few years ago the number known was very inconsiderable, and new forms are continually presented to M. Agassiz, the master of this department of fossil zoology, there is reason to suppose that the proportion of recent and fossil numbers will steadily change. One reason of the comparative paucity of fossil fishes may be their enormous destruction for food; thus they perish in greater proportion than the other inhabitants of the sea. In the present state of nature, we find very few fishes, or parts of fishes, in the mud of a drained pond, canal, or river; and it is only in particular parts of the sea that the sounding line brings up from the bottom sharks' teeth, hakes' teeth, &c. It is probable, therefore, that only a small proportion of the number of species of fishes, anciently existing, is now to be obtained from the rocks.
It is further to be observed, that the fleshy and ligamental substance of fishes decomposes more readily than the soft parts of many animals; their bones, teeth, scales, &c., are, for this reason, much scattered in certain rocks, which, like the sandstones of Sussex, and the forest marble of Wilts, appear to have undergone the littoral action of the sea. The circumstances under which the remains of fishes have been imbedded appear to have been various. In the upper part of the silurian system, a thin bed of fragmented fish bones occurs; a thicker bed of ichthyoid and sauroid bones has been long known in the lias of the Severn cliffs: considerable agitation accompanied the deposition of fish teeth in most of the oolites, wealden beds, greensand layers, &c. But in some parts of the old red sandstone, fishes lie in great perfection in Herefordshire and Brecon, as well as at Arbroath in Scotland; the amblypteri, ​holoptychi, &c. are very perfect in the coal measures of Newhaven, and Burdiehouse near Edinburgh, Bradford, Yorkshire, the Hundrück, &c. The marl slates of the magnesian limestone, the slaty lias clays of Lyme Regis, certain clays and limestones of the oolitic system, and the chalk of Lewes, have yielded abundance of beautiful marine and fluviatile fishes in an extraordinary state of perfection. Besides these, the deposits of Monte Bolca and many fresh water strata of later (tertiary) date, are stored with fishes, every part of whose structure remains uninjured.
Struck with the contrast offered by these layers of fishes in ancient marine sediments, with the few and scattered fragments which occur in modern deposits, M. Agassiz has conjectured that the rate of deposition of these ancient strata must have been almost inconceivably rapid. An examination of the lamination, frequent changes of composition, alternation of organic remains, and other marks indicating tranquil and slow deposition, which occur in nearly all the localities where the fossil fishes are found in this state of perfection, does not appear to countenance these views: but we must evidently ascribe the destruction of whole races of fishes at a certain exact date (as in the copper state of Thuringia) to some remarkable change of physical condition in the liquids.
The bones of fishes and other vertebrated animals differ from the internal and external shelly appendages of the lower tribes by the admixture of phosphate of lime. The state of conservation of bones differs much, therefore, from that of shells and corals; their substance, in almost every case, remains; the peculiar polish of the teeth and scales of many fishes causes their immediate detection; they are generally heavy, often dark in colour, very compact and brittle; the cells in bones are often filled with crystallised carbonate of lime, but sometimes remain open. It was therefore possible for naturalists profoundly versed in recent ichthyology, to determine the real analogies between ​the ancient and modern finny races of lakes, rivers, and the sea, and many attempts were made to ascertain these analogies. But until modern times, the knowledge of the structure and functions of fishes, their comparative osteology and lepidology (to coin a useful word) was of small value, and it was reserved to Cuvier and Agassiz to introduce precision and certainty where all before had been error and confusion.
To the latter of these eminent men M. Cuvier bequeathed his labours; and M. Agassiz, with a happy boldness, deviated from the ordinary modes of classification, and entered on a totally new contemplation of the subject. The dermal system, as a natural index of important structural and functional differences, has not, in general, been much attended to among vertebrated animals; though the hair of mammalia, the feathers of birds, the naked or plated skin of reptiles, the scales of fishes, might have allured inquiry into the variations which they undergo, and the uses they might furnish to systematists. M. Agassiz has seized this neglected thread of system, proved the importance of the indications afforded by the nature of the dermal covering, and applied it to the classification of fishes with peculiar success.
Instead of the divisions usually adopted from the nature of the skeleton,—cartilaginous and osseous—he distinguishes four great orders of fishes from the nature of their scales, and finds that with these differences of scales other great and important distinctions harmonize; but that the possession of a bony or cartilaginous skeleton is a question of comparative unimportance. The abundance and perfection of scales of fishes in a fossil state render this view, valuable as it is in recent zoology, absolutely essential to a study of the fossil kingdom; for thus a few scales remaining may lead to a knowledge of the species or genera belonging to each epoch; and as portions of fishes are found in every one system of strata, from the ancient silurian to the most recent of lacustrine deposits, we are presented ​with a second scale of organisation nearly as complete, and as distinctly related to time, higher in the ranks of creation, and therefore more sensibly dependent on physical conditions than the well known and justly valued series of remains of mollusca.
The orders of fishes, according to their scaly coverings, are four; viz.
1st. SCALES ENAMELLED.
M. Agassiz appears to have ascertained that the strata below the cretaceous rocks contain very few, if any, other fishes than such as are included in these orders.
2d. SCALES NOT ENAMELLED.
To the last two orders with unenamelled scales belongs by far the greater proportion of existing species ​of fishes, which, according to Cuvier, exceeded 5000; but are stated by M. Agassiz to amount to 8000. On the contrary, the greater number of fossil fishes belong to the two orders with enamelled scales. In the following table the geological distribution of these orders is sketched.
| | Placoid | Ganoid | Ctenoid | Cycloid |
| Living | ✽ | * | ✽ | ✽ |
| Tertiary | ✽ | * | * | * |
| Cretaceous | ✽ | * | | |
| Oolitic | ✽ | ✽ | | |
| Saliferous | ✽ | * | | |
| Carboniferous | ✽ | * | | |
| Silurian | ✽ | * | | |
Among existing fishes it is frequently found that the caudal tail fin divides into two equal branches; sometimes it is single and rounded, but in the case of some placoid and ganoid fishes (e. g. squalus and lepidosteus) the tail fin is double, the dorsal portion being prolonged to a considerable length, and the ventral portion much shorter. These three forms are seen in figs. 27, 28, 29., which represent the trout, the
wrasse, and the shark. Now it is a remarkable ​circumstance observed by M. Agassiz, that all, or nearly all, the fossil fishes found in strata in and below the magnesian limestone, are heterocercal, or have their tails unequally bilobate, like the shark, sturgeon, lepidosteus, &c. (fig. 29.); but this form of tail is rarely found in the oolitic and superior systems of strata.
What is the general determining cause or function of this remarkable heterocercal structure in fishes is at present matter of conjecture. In the shark and sturgeon it is accompanied with a remarkable position of the mouth; but as this is not the case in the recent lepidosteus, or the fossil palæoniscus, it is an unsafe basis of reasoning. Perhaps the true solution may be found in the analogy which placoid fishes in general, and certain ganoid fishes, present to the class of reptiles; an analogy perceived by Linnæus, and strongly corroborated by the recent researches of Agassiz, as to the structure of the teeth, cranial sutures, air-bladder, &c. That the upper lobe of the heterocercal tail may really be viewed as the analogue of the real tail of reptiles, appears from this, that the vertebral column is continued into it. We, therefore, view this remarkable structure as a character of the organisation of certain ancient geological periods, and refer to the scaly surface of the upper caudal lobe of tetragonolepis, and other oolitic genera, as indications of its gradual change to the truly double or homocercous tail fin (figs. 27, 28.), which is one of the characteristics of the existing period.
All the fishes of the silurian, carboniferous, saliferous, and oolitic systems, and two thirds of those in the cretaceous system, are stated by Agassiz to belong to extinct genera.
Reptiles.
Of the existing four orders of reptiles,—batrachida, chelonida, ophidia, saurida,—the two former are partly aquatic, partly terrestrial; the two latter principally tenants of the land. Agreeably to the general rule, the ​terrestrial families of reptiles, and especially ophidia, are the least common in a fossil state. The greater number of reptiles found in the various formations belong to tribes which now contain many aquatic species. But they were not all destined to live in water. There are fresh water batrachians in the brown coal deposits on the Rhine, and the quarries of Oeningen; land batrachians (labyrinthodon) in the new red sandstones, and aquatic batrachians (?) in the old red sandstone of Elgin (Mantell); land tortoises have been traced in the new red sandstone of Scotland, and the oolite of Stonesfield; fresh water turtles in the London clay of Sheppey, the Purbeck beds, Kimmeridge clay, and new red sandstone; marine turtles in the London clay, chalk, greensands, and oolites, perhaps even in the Silurians of Canada; land lizards in the wealden and oolites; aquatic lizards, and crocodilians in many tertiary and secondary strata; and, finally, the bones of flying lizards have been detected in the lias, oolite, chalk, and London clay.
In his instructive Reports on British Fossil Reptiles, professor Owen has divided the most numerous group of fossil reptiles—the Saurians,—into five sub-orders, viz. enaliosauria, or sea lizards; crocodilia, analogous to gavials, crocodiles, and alligators; dinosauria, or monstrous land lizards; lacertilia, analogous to the smaller lacertæ; and pterosauria, winged lizards.
Among enaliosauria, we have sixteen species of plesiosaurus, ten of ichthyosaurus, and two of pliosaurus, distributed through the lias and oolitic formations, and ceasing in the cretaceous deposits. Crocodilia are represented by eight genera,—viz. crocodiles, suchosaurus, goniopholis, teleosaurus, steneosaurus, poikilopleuron, streptospondylus, and cetiosaurus, which include fourteen species found in the lias, oolites, wealden and tertiary beds.
The dinosauria are formed by three genera, , ​megalosaurus, hylseosaurus, and iguanodon. They occur in the oolitic and wealden deposits. Of the lacertilia we have seven genera,—viz. mososaurus, leiodon, raphiosaurus, lacerta; rhynchosaurus, thecodontosaurus, palæosaurus, and cladyodon. Of the ten species of these genera, five have been discovered in the new red sandstone series, one in the oolite, three in the chalk, and one in the crag. Of the pterosauria three species are known in the lias, oolite, and chalk.
Finally. Polyptychodon from the lower greensand, and rysosteus from the bone bed of Aust, are not yet referred to their proper sub-order.
Among the singularities revealed by these investigations, we may notice in the ichthyosaurus the curious and beautiful combination of the swimming form and retral nostrils of the dolphin; the teeth of the gavial, or crocodile; paddles somewhat like those of the turtle; vertebrae like those of a fish; and eyes furnished with sclerotic bones like those of birds and certain lizards.
The iguanodon and megalosaurus have the "immania membra " requisite to sustain their vast bulk on land; but pterodactylus, an almost fabulous creation, unites the wings of a bat with the skeleton of a lizard; its long neck being formed of only seven vertebrae; while the snake-like neck of plesiosaurus includes from thirty to forty.
In magnitude some of these fossil reptiles surpass the largest crocodiles. The iguanodon—not the largest of the wealden saurians—may have measured forty or fifty feet in length: the batrachian of the new red must have been something like a toad three or four feet long, and the largest pterodactyl of the chalk may have extended sixteen feet and a half between the tips of the wings.
The existing crocodiles offer in the saurian group a particular and distinct type, which seems to unite, in some degree, the characters of the chelonida and true ​lizards: their life is spent, principally, in the waters of rivers which communicate with the sea (Nile, Ganges, Senegal, Mississippi); and they sometimes pass from the shore to prey in the salt waters. Three great divisions of crocodiles correspond to three distinct physical regions:—the alligators are wholly American; the true crocodiles belong entirely to Africa and the West Indian islands; the gavials are found only in India. All the fossil races of crocodiles which occur in the saliferous and oolitic systems are analogous to the long snouted Indian gavials; those above the chalk approach the broader beaked Nilotic crocodiles.
The vertebræ of palæosaurus and thecodontosaurus agree with those of ichthyosaurus and common fishes, in being deeply concave at each end—a structure evidently adapted for free motion in water. In a fossil crocodile from Sheppey they are concave anteriorly, and convex retrally. The former are really of ichthyoid, as distinguished from the latter, or truly crocodilian type; and, in a paper read to the Bristol meeting of the British Association, the discoverers of palæosaurus and thecodontosaurus (Riley and Stutchbury) proposed the speculation that the system of doubly concave vertebrae (fig. 30.) is more ancient than that of the concavo-convex (fig. 31.), and that the change from one to the
| Thecodontosaurus. | Crocodile. |
so far correct that it is only in and above the chalk, that the recent or true crocodilian type of vertebræ has been recognised. But there is a third and singular modification of fossil saurians; for in streptospondylus, which occurs in lias, oolite, and wealden, the vertebræ are, contrary to those of crocodile, convex before and concave behind. (Cuvier, Ossemens fossiles; Von Meyer, Palæologica; Conybeare, De la Beche, Riley, Stutchbury, in Geol. Transactions; Owen, Brit. Assoc. Reports, 1839, 1841.)
Birds.—The remains of birds are extremely uncommon, even among the comparatively recent alluvial lacustrine and cavern deposits, still less frequent among the tertiary strata, and almost unknown among the older strata. This is one of many instances which agree in proving that the occurrence of the exuviæ of land animals and land plants in the stratified rocks, which were formed chiefly in the sea, is the result of causes so local, limited, and rare, as to be, in fact, accidental, and therefore no sufficient basis of reasoning as to what was the state of the ancient land at particular geological periods. At the present day we could learn little concerning the vegetables and animals of the land, from the few traces which remain of them in the beds of lakes, rivers, and the sea.
Mammalia.—The argument just used may be applied with equal justice to the paucity of remains of land mammalia in the marine strata of all ages; for even in the tertiary rocks such remains are rare. But it is, perhaps, necessary to find other causes for the scarcity of marine mammalia in all except certain of the tertiary strata and superficial sediments. The opinion formerly favoured was, that during the whole of the primary and secondary periods, at least, the class of ​mammalia had no existence, and only came into being during the tertiary period. But this conclusion, founded upon the mere want of such remains, was easily seen to be insecure, and at length proved to be erroneous by the decision of Cuvier, that certain small jaw bones, with teeth, found in the oolitic system at Stonesfield, near Oxford, belonged to viviparous quadrupeds, and approximated to the genus Didelphys.
Five specimens of these remarkable jaw bones are known, two of which are in the hands of Dr. Buckland, one belongs to Mr. Broderip, one to Mr. Prevost, and the fifth was selected by the author of this volume from an ancient collection of fossils, the property of the Rev. C. Sykes, of Rooss, in Yorkshire, by whom it was presented to the museum of the Yorkshire Philosophical Society. These specimens are of inestimable value, for were they unknown, the whole of the positive testimony that the earth, during the secondary period of geology, nourished land mammalia would vanish, and the course of inferences as to the succession of organic life on the globe be greatly modified.
Further research has shown that among the few specimens which Stonesfield has yielded in the course of the last hundred years are two clearly distinct genera, one of them containing two species.
The designations under which they have passed are various. Compared in vain to an ichthyoid type by De Blainville, to amphibious mammals by Agassiz, one section of the fossils was brought back by Valenciennes, under the name of Thylacotherium, to the marsupial division of mammalia, to which the penetrating glance of Cuvier had united it. Professor Owen adopts for them the title of Amphitherium, assigned by De Blainville, but associates them with the insectivore. The two species are named after Broderip and Prevost.
The lower jaw contained 16 teeth on each side,—the ​five or six posterior true molars are quinquecuspidate; the anterior false molars are tricuspid and bicuspid, as in the opossums. Of the four remaining teeth, three in front are incisors, the fourth is a canine tooth. (See Buckland's Bridgwater Treatise, pl. 2. B.)
Another section, supposed by Owen to be more positively related to marsupialia, is named Phascolotherium. The only species, P. Bucklandi, is represented below (fig. 32.).
It has eleven teeth on each side of the lower jaw; three or four are true quinquecuspid molars, as many tricuspid false molars, three incisors and one canine.
Phascolotherium Bucklandi. (From Buckland's Bridgwater Treatise.)
Those persons who, confiding in what are somewhat hastily called general views, believe too strictly in the gradual change and sequence of organic life on the globe, and have pictured to themselves the early land and sea as tenanted only by the simpler (and, as they are erroneously termed, inferior or imperfect) forms of life, while in each succeeding period new, more complicated, and more exalted plants and animals were called into being, till man was at last awakened to the supremacy of creation, will find the fossil quadrupeds of Stonesfield a very puzzling anomaly. On the contrary, the geologist who, in the full spirit of Cuvier, regards the systems of life as definitely related now, and at all past periods, to the contemporaneous physical conditions of the globe, and uses the remains of plants and animals as ​monuments and guides to a right knowledge of these conditions, draws from this singular and extraordinary discovery the confirmation of a hope, that the state of the ancient land may not for ever be wholly concealed from patient inquiry.
That these are really the jaws of mammalia, and insectivorous or marsupiferous, we may safely admit, on the competent anatomical authority of Cuvier and Owen, notwithstanding the easy conjecture, that they might belong to Pterodactylus, of which bones but not jaws occur at Stonesfield. When we regard the pointed lobes of the teeth, and consider the position of the incisors, and the shape of the condyles, there appears no reason to doubt that the animal was insectivorous. It is worth remarking that elytra of land beetles (Buprestis?) are found in the same deposit, with terrestrial plants and other indications that the laminated rock, in which the specimens lie, was formed near the sea shore. No other parts of the animal have yet been found than the lower jaw,—there is no ascertained or even very probable instance of the occurrence of land or marine mammalia in older rocks than the Stonesfield oolitic beds,—none have yet been discovered in any of the superior strata of the oolitic system,—it is merely a conjecture that some bones in the marls of the cretaceous system of New Jersey and Delaware may belong to Balæna. With the exception of Stonesfield, it is only in the tertiary strata and superficial deposits that we can positively admit the occurrence of fossil marine or land mammalia at all.
It is chiefly in anthracitic tertiaries, as near Zurich; in lacustrine sediments, as at Ground and Oeningen;—in gypseous deposits from fresh water, as at Montmartre;—in shelly marls, as at Market Weigh ton;—in diluvial clay or gravel, as at Harwich,—at Lawford, at Hessle;—or in more recent peat bogs, as in Ireland, the Isle of Man, Lancashire;—or in caves and fissures of the rocks, as at Kirkdale, and Gibraltar, that the bones of mammiferous quadrupeds occur. ​
Some of these ossiferous deposits are of historical date, the others of greater but various antiquity, so as to permit the construction of the following table of the successive races of mammalia.
| | Qua- drumana | Cheiro- ptera | Carni- vora | Rodentia | Eden- tata | Pachy- dermenta | Soli- peda | Rumin- tantia | Marsu- pialia | Cetacea |
| Historical or modern | | | * | * | ? | ? | * | * | | * |
| Diluvial | | * | ✽ | * | ✽ | ✽ | ✽ | * | * | * |
| Tertiary | * | * | * | * | * | ✽ | * | * | * | ? |
| Secondary | | | | | | | | | | |
| Primary | | | | | | | | | | |
The preceding table adds another to the proofs already given of the extreme analogy between the tertiary and modern periods of geology. We find in the tertiary formations remains of nearly all the great natural orders and groups into which systematists have divided mammalia: in most instances, however, the species, and often the genera, differ; yet it must be borne in mind that these differences are not greater than now obtain between the animals of the analogous climates of America, Africa, and India. Admitting for the moment, what must hereafter be discussed, the distinctness of the alluvial, diluvial, and tertiary deposits, we may observe that in the diluvial reliquiæ of mammalia, most of the genera, and some of the species, are the same or extremely like to living tribes: while in the modern accumulations it is rare to find an extinct species, though some specimens of the great Elk of Ireland are probably of this date.
But there is one remarkable exception to this analogy of the tertiary and diluvial fauna, with our present races of mammalia; no remains of Man have yet been found in any of these deposits—no trace of his works; and it is yet entirely doubtful whether the race of man existed at all during what are called the diluvial periods. The same exception almost extends to the order of quadrumana, which, in their animal nature and organisation, most ​nearly resemble ourselves; for these have rarely been recognised in a fossil state. Perhaps, however, we ought not to insist very strongly on either of these negations: for the quadrumana could not be expected to occur often in a fossil state far from the tropical forests which might shelter and feed them; and man only braves the cold of northern climates by his superior knowledge of nature, and inventions to meet its variations. These arts and that knowledge must be supposed of slow growth; and we may consistently believe that, though mankind at the diluvial era might not have extended to these far northern lands, where, only, the ossiferous caves and deposits have been adequately examined, human remains may yet be discovered in those warmer regions of the globe, which seem more congenial to the easy existence of our race, and have not yet been searched for the bones of our progenitors.
The supposed exceptions to this law of the absence of the remains of man from tertiary and diluvial accumulations (the bone caves of Bize, near Narbonne, the valley of the Elster, &c.) may be discussed hereafter: suffice it now to say that they are not thought sufficient to establish the affirmative of this important proposition. It appears, therefore, that we must look upon the existence of man and many races of animals which, more strictly than he, are appointed to live under particular physical conditions, as characteristic of the last of several great periods of geological time, each marked by the creation of peculiar races of plants on the land and animals in the sea.
From what we now see of the dependence of animal and vegetable life on climate, moisture, soil, and other characters of physical geography, there can be no doubt that to every system of organic life in the successive geological periods belonged certain combinations of physical conditions. These conditions were, indeed, not the cause of those systems of life; but both are to be looked upon as mutually adjusted phenomena, ​happening in a determined order as part of a general plan. Some changes in the constitution of the globe have brought in succession various combinations of the manifold influences of those chemical and mechanical agencies which govern inanimate nature; and such appears to be the law of God's providence, that to these combinations the forms of each newly created system of life should correspond. The several successive systems of organic life which have been discovered in the earth, were, therefore, really successive creations, and must be expected to differ in large and general characters.
Thus the species, genera, and families of fossil plants and animals vary from formation to formation, and system to system: yet as the constitution of different races, enjoying animal and vegetable life, is unequally adjusted to external circumstances, it does not follow that the creation of many new should be always accompanied by destruction of all the old forms. On the contrary, the extensive collections of fossils now made in England, prove this to be an erroneous notion: for many fossils, as Terebratulæ—Astacidæ—Modiolæ—Gervilliæ, generically, and certain species of them individually, existed during the deposition of great ranges of strata, and endured the changes, whatever they were, which brought into existence many new and remarkable forms. It seldom, however, happens that any one species occurs in more than one system of strata; and thus we may consistently speak of the oolitic fauna and flora, as distinguished from the whole series of plants and animals belonging to the cretaceous or saliferous period, satisfied from adequate inquiry that few species are common to any two systems.
Though at present geological investigations have not been prosecuted in all accessible parts of the land, so as every where to bring proof of the universality of these laws of successive systems of life, enough is known to assure us that in every country yet examined, the fossils of the tertiary, secondary, and primary strata differ essentially, and by large and general characters. ​Everywhere the tertiary fossils are closely analogous to existing types; but in all countries the fossils of the primary strata appear to belong to a very different series. Wherever the systems of European strata can be paralleled,—in North America—the Himalaya—Australia—so much of analogy is evident in the organic reliquiæ, as to prove that the successive changes of physical conditions, and the coincident changes of organic life, were operated over very large parts of the globe; and nothing, yet known, forbids us to believe that they were universally felt, though in unequal degrees, and under differences of circumstances.
Could we suppose produced on the present globe some general change of conditions in the sea, on the land, and in the atmosphere—either simultaneously, or by communication from a central area of disturbance —the effects upon organic life might be everywhere manifested, though unequally and variously. The extinction of some tribes, the decrease or enlargement of others—the creation of new types to fill the void spaces of creation, and be adapted to the new conditions, might seem to us quite in harmony with the designs of providence, and fully in accordance with past geological effects. There would, however, be this difference in the cases:—the races of animals and plants of this modern period of the globe are more various in different countries than the fossils of any one older geological period appear to have been; there is now more of local diversity in organic life upon the globe, than formerly obtained; and from this we infer that the physical conditions of the globe in former periods were more general —more uniform over large areas than at present. This character of uniformity among the organic contents of a system of strata, augments continually from the modern period toward the older, and is greatest among the most ancient strata, whose organic contents, though less numerous, are more similar in all countries yet explored than those of later date.
Since it thus appears that general laws of variation ​connect the phenomena of all geological periods, from the most ancient to the most modern epoch, into one grand system of natural revolutions, it follows that we may look upon the present condition of our globe as one term of a magnificent series of appointed changes, to which others may from analogy be expected to follow, according to the same laws. The creation of intelligent man is indeed an event not in the calculation which man can make of the effects of such laws; nor, indeed, is it given to us creatures of a day exactly to know the laws of variation which bind all the phenomena of nature —past, present, and to come—into one great system of appointed effects, flowing from a predetermined cause,—much less to deduce these effects. Yet let not the search for these laws—which comprises the whole of geological theory—be censured as a chimerical inquiry. The augmentation of light that has already been poured on the dark pages of geology encourages perseverance; the extent of man's power to interpret the phenomena of nature may be vast, compared with his present knowledge; however small, compared to the amount of things unknown. In searching for general theory we shall at least find limited truth; and the experience of some thousand years has proved the labour, which seemed vainly tasked in abstract discovery, to be seldom unproductive of practical utility.
To understand rightly the daily accumulating stores of organic reliquiæ, requires more than a slight knowledge of existing nature,—more even than an acquaintance with animal and vegetable forms. The philosophy of their existence must be considered—the variations of their structure, with respiration in air or in water—life in fresh or salt water—in trees or on the ground—carnivorous or herbivorous food—their geographical distribution dependence—on climate and atmospheric conditions. Thus viewed, the present system of nature appears, when compared with the older periods, one in which local diversities of condition have gone to extreme—where all the peculiarities of climate and surface have ​given the fullest effect to the variety of nature, and yielded that astonishing complexity of dependent phenomena which incessantly engages the mind of reasoning man in an endless train of inquiry. These local diversities are so great, as to permit us to propose questions concerning the degree of resemblance which fossil remains may offer to the recent tribes of different climates and regions of the globe.
Where shall we look for the living analogues of the numerous fossil ferns, including arborescent species of great size, the sigillariæ, lepidodendra, and gigantic equisetaceæ, which fill the coal shales of England, the cycadeæ, coniferæ of the oolites, and the palms of the tertiary rocks of France?
In what climate grow the modern coral reefs comparable to the fossil zoophytic rocks? where live the parallels to thousands of echinida, crinoidea, trilobites, brachiopoda, cephalopoda, sauroid fishes, crocodiles, pachydermata, ruminantia, which characterise different geological periods?
It is difficult to answer this inquiry with precision; for, though upon a comprehensive review, the most prevalent analogies in modern nature point to a tropical climate; yet as the species always, and the genera and families frequently, differ, and as, besides, other causes than climate limit the distribution of life, it is not possible to found such a conclusion on individual instances. A prevalence of ferns to the extent which we observe among the plants of the coal formation, is only known among the islands and on the shores of warm tropical seas; but if these fossil plants had been much drifted or long immersed before inhumation, such a predominance of ferns, cycadeæ, &c., might be expected to happen, whatever was the original proportion; for Dr. Lindley's experiments on recent plants prove, that long immersion in water would destroy the greater number of plants, but leave the ferns, cycadeæ, coniferæ, &c. comparatively uninjured, as we find them in the earth. Compared as to form, the tree ferns, palms, ​cycadeæ, &c. indicate growth in a warm climate, as do also the gigantic lycopodiaceæ, sigillariæ, and calamities; but this is not the case with coniferæ. Zoophyta, hoth spongoid and stony, lead us to the same conclusion; for the greater part of horny sponges and stony corals belong to the regions within 33° of latitude from the equator (except the S. E. coast of Australia). As far as can be judged by comparing fossil and recent radiaria (echinida, crinoidea, stellerida), the same inference applies. Molluscous remains teach us little in this respect, except the cephalopoda, which, by their size and abundance, seem to indicate a warm climate for the cretaceous, oolitic, and older deposits. Enough, perhaps, is not yet known of the relations of fossil and recent fishes, to justify any general conclusion; but the great families of fossil saurian reptiles prove, by their magnitude, and analogy to crocodiles, iguanas, monitors, the decided influence of a warm climate during the oolitic and cretaceous periods; for nothing can be more clear than the dependence of the numerous tribes of living reptiles generally, and the sauroid families in particular, upon a warm climate. More than a thousand species live in the tropical regions of the new and old world; but only a few dwindled races visit the colder zones of Europe, and mostly enter the earth in winter, a provision whereby the animals which generate little heat in their bodies are preserved during the periods when the sources of external warmth are too feeble to sustain their functions in activity. With regard to the degree of analogy, which the productions of different regions may be found to present with fossil reliquiæ, we are not aware that any investigations are on record; and yet it is impossible to turn to Australia without a suspicion that the anomalous productions of that region have more than the average resemblance to the primeval fauna and flora. For here, and near it, tree ferns, cycadeæ, araucariæ, casuarinæ, grow upon the land; corals and sponges abound on the coast even of Van Diemen's Land,—while trigonia, cerithium, isocardia, a cardium ​like C. hillanum of the green sand, and quadrupeds of the peculiar marsupial races to which the Stonesfield animal is referred by Cuvier, seem to invite attention to the yet unexplored sea and land of this prolific region, as likely to yield still farther analogies to ancient animals and plants, and, by consequence, to furnish new and important grounds for determining the ancient physical conditions of the globe.
* ↑ See Mr. Gray on the Structure of Shells in Phil. Trans.
* ↑ Guide to Geology, 3d. edit. p. 68.
* ↑ The numbers in these columns might now receive considerable augmentation; but the proportions are not materially changed.
* ↑ Taken from the Guide to Geology, 3d edition.
* ↑ Buckland's Bridgewater Treatise, p. 70.
* ↑ Reports of the British Association, 1839, 1841.
* ↑ Mantell has recently added a new and still mightier genus—the Pelorosaurus, from the wealden formation.
* ↑ Cuvier, Ossemens fossiles.
* ↑ Birds' bones do, perhaps, occur among the pterodactyls of Stonesfield and Sussex, and their foot-prints appear on new red sandstones in New England.
* ↑ Since transferred to the British Museum.
* ↑ Quadrumana have been found by M. Lartet, In the lacustrine deposit of Sansan (Dep, de Gers.), by Capt. Cautley and Dr. Falconer, in the tertiary strata of the Sewalik Hills, Hindoostan, and by Mr. Colchester, in the red crag at Kyson in Suffolk.
* ↑ The view here presented (in 1836) has been confirmed by further researches, and illustrated by the remarks of professor Owen (Reports of British Association, 1842. p. 74).