The London Morning Post, 06 November 1876, Page 2.
THE EVOLUTION OF PLANT LIFE
On Friday evening, Mr. W. Carruthers, F.R.S. (the keeper of the Botanical Department of the British Museum) gave a presidential address before the Geologists' Association, his subject being a comparison of the history of plant life preserved in the rocks so far as we know it, with the theory of the evolution of plant life as put forward by Haeckel. The origin of the existing organic forms has always been a question of interest. Until a comparatively recent period little diversity of opinion prevailed in regard to this matter amongst students of science in Europe. The position stated by Moses in the opening sentences of the Old Testament, in which all matter, organic and inorganic alike, is traced to the operation of an external and supernatural Creator, was universally adopted, though it was often misunderstood and misinterpreted by its expounders.
In the beginning of this century Lamarck proposed his evolution theory, according to which all organisms are derived from some few simple original forms, which had come into existence by spontaneous generation out of inorganic nature. Although Lamarck's theory found few supporters it was comparatively neglected by men of science until Darwin, in 1859, published his "Origin of Species; by Means of Natural Selection," which, at least in this country, has wrought an almost complete change in opinion of our present life forms. His theory of development as applied to the vegetable kingdom may be thus briefly stated: - The characters of plants are transmitted to their descendents. New characters besides those inherited may arise in some of the descendents of a plant which were not possessed by the parent. When these new characters are transmitted to descendents, and are permanent, the plants possessing then become a variety.
Some plants have a special tendency toward variation; others are remarkably constant in their characters. No explanation has been given of the reason for these differences in the nature of plants or of the cause of the appearance of new characters. The differences are at first small. Their continuance depends on external causes. In course of time new characters appear, or the old become intensified, and in the struggle for existence the varieties only which possess the characters best fitted to resist the prejudicial influences that surround them are able to maintain their ground. The less fortunate varieties perish, and in this way of connecting links between the common descendents of the original stock are destroyed. These descendents becoming more pronounced in their characters are recognised as species. The only difference between a variety and a species is the amount of divergence and the constancy of the characters.
Further, this in a greater degree is the only difference between a species and a genus. It is then concluded that all the forms now observed in the vegetable kingdom are due to the continual accumulations of differences in the genetic evolution of the existing plants from the one or the few original forms. It is held that the natural system of plants is the external expression of this phylogenesis, or genetic relationship, that the development of a plant from the embryonal cell to the perfect individual is a short and quick repetition of the genetic development of the tribe to which it belongs, and that the rocks of the earth reveal, so far as the record of life is preserved, the various steps by which the phylogenesis actually was accomplished. The lecturer asked attention to this last aspect of the subject, which specifically affects geologists. It deserves careful investigation, for if the theory of evolution be true, then the fossils which have come to our knowledge represent the extinct progenitors of existing plants, and on this account possess a higher interest to us than their comparative anatomy or systematic position can give them.
Reference was made to the imperfection of the geological record, but it was urged it was right to compare our knowledge of it as far as it goes with the theory. Where, then, is the phylogeny of the vegetable kingdom? The most rudimentary plants are either fungi or algae. The elementary fungal forms are believed by some to be the original stock of the vegetable kingdom, while others hold the primitive forms were algae. Carruthers argues against fungi being the earliest, on the grounds that they need organised food to grow on, and admits the probability that algae were the earliest. What is the testimony of the rocks as to the plants existing during the long early periods of the history? Mr. Carruthers showed that the vegetable markings preserved in the earliest rocks, are, though indistinct, referred to 16 species of algae.
But the nature of the plants which could flourish in the conditions under which these deep sea deposits were found, and the changes that have taken place in the primal strata since their deposition, prevent us expecting any extensive representation of these early floras. So far as the plant remains go they meet the requirements of the evolutionist who looks upon the algae as the primeval plants. In the phylogenesis of the vegetable kingdom we next come to the evolution of the fungi, lichens, mosses, and hepaticae, all of which are cellular plants. They came into existence, it is supposed, with the Devonian period-the beginning of the newer paleozoic series. As, however, no trace of any of these groups, except the mycelium of one or two species of fungi, has been detected in any of the paleozoic rocks they supply no evidence for or against the hypothetical account of their evolution.
But the later paleozoic rocks abound in plant remains. The first evidence of land plants on the globe are met with, as far as our knowledge at present goes, in the Devonian rocks. Here the three principal groups of vascular cryptograms appear together in highly differentiated forms. All of them - ferns, equisetaceae, and lycopodiaceae - possessed the same essential structure as their living representatives, and in all the subordinate points in which they differ it is in the possession of characters indicative of higher organisation, whether in the vegetative or reproductive organs, they are found in existing forms. The three orders appear together in the later paleozoic rocks, and that not in simpler or more generalised types, but with more varied and more complex structures than are found in the living representations. Thus, among ferns there is lost a remarkable group with a fundamental different stem structure, which was contemporaneous in the paleozoic ages with the type of ferns that have been represented all through the epochs,and are now abundant on the globe.
The equisetaceae were represented by a larger number of generic groups. Their stems were arborescent, the leaves large, and their first cones protected by special scales, but the spores being similar in size and form to those in the humbler living species, and being furnished with the bygrometric (hygrometric) elaters. The lycopodiaceae were also huge trees, and represented by several generic groups. The stem structure, which fundamentally agreeing, like those of the arborescent equisetaceae, with the living structure of the stems of their living representatives was more complex, being suited to their arborescent habits. But the flora of these later paleozoic rocks include higher elements than vascular cryptograms, for in the Devonian series we have coniferous plants, increasing greatly in number and variety in the carboniferous period; and in the calciferous sandstone at the very base of the carboniferous measures, there have been found an undoubted agiospermous plant.
The step from the spore-producing cryptogram to the seed-bearing phanerogam is a very great one. No doubt there is a general external resemblance between a lycopod and a conifer, and many points of analogy between the development of the seed and the various stages through which lycopod passes from the germination of the spore to the growth of the fertilised archegonium. But like is here, as it is often elsewhere, an ill mark, for the resemblance is purely superficial. The minute tissues of the conifer, as well as the method in which they are arranged, differ entirely from anything either in the existing or extinct lycopods, which the production of a seed, even though it be without a protecting ovary or fruit, at once distinguishes the gymnosperm from the spore bearing cryptogram.
According to Haeckel the gymnosperm sprang out of the lycopodiaceae during the carboniferous, or possibly in the Devonin period. But undoubted coniferous wood was discovered by Hugh Miller in the lower Devonian rocks of Cromarty, and several anomalous woods have been described by Unger from the Thuringian rocks of the Devonian age which are referred by him to coniferae without any positive evidence except the absence from them of ductiferous tissue. Had these woods been of earlier age than Miller's Cromarty wood they might have been looked upon as one of those steps leading up to true coniferous structure, but they occur in beds of Upper Devonian age. The calciferous sandstones at the base of the carboniferous period contain numerous and fine specimens of coniferous wood. Some trees at this time had attained to an immense size. The first appearance of the gymnosperms is not in the form of a generalised type, but both the wood and the fruits present a remarkable variety of genera and species as highly differentiated as the existing forms.
In connection with this remarkable development of taxinere in the paleozoic rocks, it deserves to be noticed that this section of the coniferae are all dioecious, while in other groups the flowers are generally monoecious. The historyof monocotyledonous plants, as far as it is preserved in the rocks of the earth, is very curious. We have to take it up here, for the first true monocotylewdon is the stem and spike of an aroideous plant, of which one well-preserved specimen was discovered nearly 40 years ago by Dr. Paterson in the calciferous sandstones near Edinburgh. Four species monocotyledons have been found in the trias, seven in the lias, the same number in the oolite, 15 in the chalk, 97 in the eocene, 185 in the miocene, and two in the pliocene.
We find, then, that the three groups of vascular cryptograms, and the seed-bearing gymnosperms appear together in the Devonian rocks, and that monocotyledons appear in the lowest beds of the immediately succeeding carboniferous system. Further, that these earliest plants are not generalised forms of the various tribes to which they belong, but that they are as highly specialised as any subsequent representatives of the particular tribe, and that wherever they differ from later plants it is the possession of more perfect organisation. It would be contrary to the theory of evolution to suppose that the highly organised cryptograms, the gymnosperms, and the monocotyledons were each developed at one step from the cellular plants which formed the only vegetation of the pre-Devonian periods.
We find, then, that the three groups of vascular cryptograms, and the seed-bearing gymnosperms appear together in the Devonian rocks, and that monocotyledons appear in the lowest beds of the immediately succeeding carboniferous system. Further, that these earliest plants are not generalised forms of the various tribes to which they belong, but that they are as highly specialised as any subsequent representatives of the particular tribe, and that wherever they differ from later plants it is the possession of more perfect organisation. It would be contrary to the theory of evolution to suppose that the highly organised cryptograms, the gymnosperms, and the monocotyledons were each developed at one step from the cellular plants which formed the only vegetation of the pre-devonian periods. No doubt there is in the older paleozoic rocks a great absence of any records of land life.But the (...) of the vascular cryptograms and the (...) from the green seaweeds, through the (....) must have been carried on through a long succession of ages and by an innumerable series of gradual advancing steps; and yet we find not a single trace either of the early water forms or of their later and still more numerous dry land forms. The conditions that permitted the preservation of the fucoid in the Landivere rocks at Malvern, and of similar cellular organisms elsewhere, were at least favorable to the preservation of other forms. Mr. Carruthers dwelt at some length on the supposed evolution of the dicotyledons, and urged that there was no trace of steps leading from one form to another.
Mr. WOODWARD, F.R.S., proposed a vote of thanks and Professor MORRIS, in seconding it, referred to the amount of carbonaceous material in the Haronian and Lawrentian rocks, probably the result of decayed vegetable structures of which we have no traces preserved.
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