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Of course, all plants and animals are not preservable alike, nor are the same organisms, though occurring in the same formation, always found in the same state of preservation. Plants and animals that have been exposed to atmospheric decay before entombment will be less perfect than those that have been suddenly and thoroughly imbedded. The harder, parts of plants—roots, stems, leathery leaves, and nut-like fruits—will run a better chance of preservation than the soft and succulent portions. Corals, sheik, crusts, bones, teeth, scutes, and scales, will be preserved when all the softer parts of the animals to which they belong have entirely disappeared. The harder and more massive portions of a skeleton will resist when the softer and more slender have fallen to decay. The dense and thoroughly ossified bones of an old animal will endure where the spongy and unanchylosed members of a young one fall asunder and perish. Ferns, mosses, and resinous pines will resist maceration when other plants will totally disappear. All things considered, aquatic animals run a better chance of preservation than terrestrial ones; and the bulkier land-mammals and amphibia than the birds and insects. Gregarious animals, too, are likely to be found in greater abundance than those living isolated and solitary— the catastrophe (earthquake, land-flood, or windstorm) which would destroy only a few of the latter, overwhelming the former by hundreds of thousands. In this way shoals of fishes may be suddenly suffocated by submarine exhalations, shell-beds buried beneath obnoxious sediments, herds of ruminants borne from the land by floods, and clouds of insects swept into the sea by wind-storms. The inconceivable numbers in which fossils are sometimes found crowded into very limited areas would seem to point to such accidents for their entombment—their perfection,
indiscriminate aggregation, and individual positions, all indicating some sudden death-catastrophe.
The nature of the sediments must also exercise a marked influence in the number and perfection of the imbedded fossils. Loose and porous sands will be less preservative than impervious clays and muds, and heterogeneous silts than calcareous sediments. In this way minute organisms may totally disappear, and larger ones be preserved in a mutilated and fragmentary form. Every one acquainted with the nature of our peat-mosses, with the sands, clays, and marls that fill up our ancient lakes, and with the sands, gravels, and silts now accumulating in our estuaries and seacreeks, must have witnessed the different preservative effects of these sediments; how solid and dense the bones are in one, and how spongy and rotten they appear in another; how hard and firm the shells are in one, and how soft and friable they occur in another; and how sharp and clear every external marking is retained in one matrix, and how wasted and obscure it becomes in another. As with these recent accumulations, so with the strata of the older formations; some are destitute of organic remains, which must at one time have been imbedded in them in abundance, while in others they are so imperfectly preserved as to be of little or no value to the palaeontologist. The various ways in which plants and animals may be imbedded and preserved in sediments being so obvious, the different preservative effects of different sediments being also apparent, the reason why some forms should occur more abundantly than others being generally discoverable, and the evidences which such plants and animals afford of the geographical conditions under which they flourished being admitted, let us now inquire into the processes by which they are lapidified, or converted into stony matter.
Generally speaking, in recent accumulations, such as sandsilt, peat-moss, and the like, the remains of plants and animals are found little altered. The more volatile matters are expelled from the plant, and the more perishable integuments and softer tissues of the animal have perished. Eoots, trunks, branches, and the harder fruits, if excluded from the action of the atmosphere, become darker, denser, and assume a peaty aspect. In like manner, bones, horns, teeth, shells, and crusts, lose a portion of their animal matter, and become denser and heavier through some slight absorption of mineral ingredients. But in all the older formations, the vegetable, if not converted into coal, is thoroughly lapidified—that is, changed by a slow chemical process into flint, ironstone, limestone, or sandstone, as the case may be—and merely retains its organic form; while animal remains undergo a similar conversion, and are recognisable only through their individual forms and textures, which continue unchanged and persistent. There is nothing more marvellous than this process of petrifaction: particle after particle as the organic matter disappears, so particle after particle the mineral matter takes its place, and this so delicately that scarcely a cell or fibre is ever ruptured or displaced! Of course, where the mineral solutions percolating the earth are so numerous there will be great variety of petrifactions, some being calcareous or limy, some silicious or flinty, some ferruginous or irony, and others bituminous or coaly. But in whatever state they may occur the process seems the same—namely, a gradual dissipation, through decay, of the organic atoms, and a gradual substitution, through permeation, of the mineral or inorganic. Great differences will also arise from the chemical nature of the organisms themselves—wood, bone, horn, teeth, shells, crusts, and corals, each having its
own composition, and possessing its own power of resisting decay. Not only so, but as the percolation of mineral solutions through the earth's crust is incessant, what is deposited at one time may be dissolved at another and a new substance substituted in its place, or no new substance may be substituted, and merely the hollow mould of the organism left to prove that it once was there. For instance, a shell or coral, which consists of animal-formed carbonate of lime, may be converted into sparry mineral carbonate; or this may be dissolved and carried away, leaving merely a hollow mould with every ridge and line and pore impressed on the containing matrix; or this mould may be refilled with silicious matter, and the shell or coral then present itself as a flint, with every pore and ridge and wrinkle as delicately perfect as on the original organism the day it was imbedded. This perfection of preservation is often, indeed, truly marvellous. We have seen the facetted eyes of trilobites as perfect in form as when they received the rays of light through Silurian waters; carboniferous univalves with their colour-bands still unobliterated; internal casts of productae with their muscular apparatus displayed in a style of legibility to which no anatomical preparation could approach; and ink-bags of cuttle-fishes so little changed as to furnish the pigments for their own portraiture.
Of course, the consideration of these percolations, dissolutions and substitutions, involves many intricate questions in chemistry; but enough has been stated to inform the general reader that fossils may occur in many different conditions —as stony conversions, as moulds, as casts, or as mere impressions on the matrix in which they are entombed. But in whatever state they may occur, there is usually sufficient left, either in general form, in external character, or in internal texture, to enable the palaeontologist to determine their places, or at all events to approximate to their places, in the vegetable and animal schemes. These determinations, indeed, constitute the main duty of paleontology; and when one considers how widely scattered fossils usually are, how sorely mutilated and fragmentary they often appear, and that they are largely the chance findings of quarrymen and miners, it is truly marvellous how much of reliable worldhistory the science, within little more than half a century, has been able to reveal. It is true there is still very much to be done, and perhaps more in fossil botany than in fossil zoology, inasmuch as vegetable organisms are less perfectly preserved than animal, and because the classifications of the botanist are mainly founded on the flowers and leaves— portions which of all others are the most evanescent and perishable. Notwithstanding these difficulties, the palaeontologist finds that all his fossils belong to the same great scheme of life with existing plants and animals, and he is therefore restricted to the classifications that have been devised by the botanist and zoologist. Species, and genera, and families, and even what are called orders, may have become extinct, and others in the course of creation may have taken their places; still the great Scheme of Vitality has ever been evolved according to a fixed and determinate plan, and in harmony with this plan, and to the best of their knowledge, botanist, zoologist, and palaeontologist must endeavour to conform their systematic arrangements.
In speaking of fossil plants, therefore, the palreophytologist adopts the usual classification of the botanist, placing where he can his fossils under their proper genera and orders, and where he cannot, assigning to them a provisional place next to the genus or order to which they bear the greatest resemblance. In this course he uses the same botanical terms, and employs the same botanical phraseology, and these may