The predominance of certain classes of plants in a particular bed may be due to purely mechanical causes and to differential sorting by water, or it may be that the district traversed by the stream which carried down the fragments was occupied almost exclusively by one set of plants. The trees from higher ground may be deposited in a different part of a river’s course to those growing in the plains or lowland marshes. It is obviously impossible to lay down any definite rules as to the reading of plant records, as aids to the elucidation of past physical and botanical conditions. Each case must be separately considered, and the various probabilities taken into account, judging by reference to the analogy of present day conditions.
Various attempts, more or less successful, have been made to imitate the natural processes of plant mineralisation[131]. By soaking sections of wood for some time in different solutions, and then exposing them to heat, the organic substance of the cell walls has been replaced by a deposit of oxide of iron and other substances. Fern leaves heated to redness between pieces of shale have been reduced to a condition very similar to that of fossil fronds. Pieces of wood left for centuries in disused mines have been found in a state closely resembling lignite[132]. Attempts have also been made to reproduce the conditions under which vegetable tissues were converted into coal, but as yet these have not yielded results of much scientific value. The Geysers of Yellowstone Park have thrown some light on the manner in which wood may be petrified by the percolation of siliceous solutions; and it has been suggested that the silicification of plants may have been effected by the waters of hot springs holding silica in solution. Examples of wood in process of petrifaction in the Geyser district of North America have been recorded by Kuntze[133], and discussed by Schweinfurth[134], Solms-Laubach[135] and others[136]. The latter expresses the opinion that by a long continuance of such action as may now be observed in the neighbourhood of hot springs, the organic substance of wood might be replaced by siliceous material. The exact manner of replacement needs more thorough investigation. Kuntze describes the appearance of forest trees which have been reached by the waters of neighbouring Geysers. The siliceous solution rises in the wood by capillarity; the leaves, branches and bark are gradually lost, and the outer tissues of the wood become hardened and petrified as the result of evaporation from the exposed surface of the stem. The products of decay going on in the plant tissues must be taken into account, and the double decomposition which might result. There is no apparent reason why experiments undertaken with pieces of recent wood exposed to permeation by various calcareous and siliceous solutions under different conditions should not furnish useful results.
CHAPTER V.
DIFFICULTIES AND SOURCES OF ERROR IN THE DETERMINATION OF FOSSIL PLANTS.
“Robinson Crusoe did not feel bound to conclude, from the single human footprint which he saw in the sand, that the maker of the impression had only one leg.”
Huxley’s Hume, p. 105, 1879.
The student of palaeobotany has perhaps to face more than his due share of difficulties and fruitful sources of error; but on the other hand there is the compensating advantage that trustworthy conclusions arrived at possess a special value. While always on the alert for rational explanations of obscure phenomena by means of the analogy supplied by existing causes, and ready to draw from a wide knowledge of recent botany, in the interpretation of problems furnished by fossil plants, the palaeobotanist must be constantly alive to the necessity for cautious statement. That there is the greatest need of moderation and safe reasoning in dealing with the botanical problems of past ages, will be apparent to anyone possessing but a superficial acquaintance with fossil plant literature. The necessity for a botanical and geological training has already been referred to in a previous chapter.
It would serve no useful purpose, and would occupy no inconsiderable space, to refer at length to the numerous mistakes which have been committed by experienced writers on the subject of fossil plants. Laymen might find in such a list of blunders a mere comedy of errors, but the palaeobotanist must see in them serious warnings against dogmatic conclusions or expressions of opinion on imperfect data and insufficient evidence. The description of a fragment of a handle of a Wedgwood teapot as a curious form of Calamite[137] and similar instances of unusual determinations need not detain us as examples of instructive errors. The late Prof. Williamson has on more than one occasion expressed himself in no undecided manner as to the futility of attempting to determine specific forms among fossil plants, without the aid of internal structure[138]; and even in the case of well-preserved petrifactions he always refused to commit himself to definite specific diagnoses. In his remarks in this connection, Williamson no doubt allowed himself to express a much needed warning in too sweeping language. It is one of the most serious drawbacks in palaeobotanical researches that in the majority of cases the specimens of plants are both fragmentary and without any trace of internal structure. Specimens in which the anatomical characters have been preserved necessarily possess far greater value from the botanist’s point of view than those in which no such petrifaction has occurred. On the other hand, however, it is perfectly possible with due care to obtain trustworthy and valuable results from the examination of structureless casts and impressions. In dealing with the less promising forms of plant fossils, there is in the first place the danger of trusting to superficial resemblance. Hundreds of fossil plants have been described under the names of existing genera on the strength of a supposed agreement in external form; but such determinations are very frequently not only valueless but dangerously misleading. Unless the evidence is of the best, it is a serious mistake to make use of recent generic designations. If we consider the difficulties which would attend an attempt to determine the leaves, fragments of stems and other detached portions of various recent genera, we can better appreciate the greater probability of error in the case of imperfectly preserved fossil fragments.
EXTERNAL RESEMBLANCE.
The portions of stems represented in figures 20 and 21, exhibit a fairly close resemblance to one another; in the absence of microscopical sections or of the reproductive organs it would be practically impossible to discriminate with any certainty between fossil specimens of the plants shown in the drawings. Examples such as these, and many others which might be cited, serve to illustrate the possibility of confusion not merely between different genera of the same family, but even between members of different classes or groups. The long slender branches of the Polygonum represented in (fig. 21) would naturally be referred to Equisetum in the absence of the flowers (fig. 20 B), or without a careful examination of the insignificant scaly leaves borne at the nodes. The resemblance between Casuarina and Ephedra and the British species of Equisetum, or such a tropical form as E. debile, speaks for itself.