Popular Science Monthly/Volume 77/October 1910/The Paleontologic Record V: The Relation of Paleobotany to Phylogeny
|THE PALEONTOLOGIC RECORD|
THE history of plant life has been the central idea in all botanical studies from the very earliest times, whether expressed in the imperfect methods of the early German and Dutch botanists who desired simply to establish natural affinities on the basis of external resemblances, or in the ambitions of Cæsalpino to arrive at a classification of plants which should satisfy the conditions of relationship through the structure of all parts, and especially of the reproductive organs. For nearly four hundred years the external organs have been employed as the chief basis of those numerous systems of classification which have appeared from time to time. The idea that the reproductive organs and the minute interior structure of plants were of primary importance as first advocated by Cæsalpino, was for a long time lost to view, although it reappeared now and then in the works of later writers. Eventually it gained recognition and became a factor of increasing importance, until the most advanced systems are now employed involve an acceptance of both the external parts and the internal anatomy as essential factors.
From the time of Malpighi and Grew, to Goeppert and Corda, our knowledge of the interior structure of plants made great and rapid progress, and was later applied successfully by various investigators in the direction of establishing relationships. To no one are we more fully indebted for an elaboration of this idea than Williamson, whose researches into the structure of fossil plants from the Coal Measures of Great Britain, during the latter part of the last century, laid the real foundation of modern paleobotany.
In so brief a treatment as that which is now employed, it is impossible to more than touch upon some of the salient features in the relations of paleobotany to the course of phylogeny, but it is, nevertheless, worth while to give special emphasis to the now well-recognized fact that a thorough knowledge of the interior structure of the plant, and especially of the stem, leads to a more comprehensive and exact acquaintance with relationships than that of any other part. This arises from the fact that the minute anatomical details have a greater degree of stability than any other portion of the body, doubtless due to the fact that in its adjustment to the land habit, the environmental influences present the least variable features in those factors which determine relations to mechanical stress and physiological needs.
External organs are notoriously subject to variation, even under slight alterations of surrounding conditions, within the limits of the species or even within various stages of development of the same individual. From this it is clear that organs such as leaves must be very unreliable for phylogenetic purposes. It is, unfortunately, true that much of the paleobotanical work based upon a study of such parts must be of inferior value, and the conclusions drawn will require extensive revision when the more rigid tests to be applied through a knowledge of the stem structure are brought to bear.
The value of paleobotanical evidence consists in its ultimate correlation with known types of plants, and it is obvious that all such studies should be prosecuted with direct reference to the broader requirements of plant biology. This involves a comprehensive knowledge of the history of plant life from its earliest development; that the data derived from a study of living species should be correlated with the evidence obtained from fossilized remains. Existing vegetation shows a very incomplete record of plant life as a whole. Its history as known until very recent times, and even now to a very large extent, is displayed only through the medium of detached groups, and relates chiefly to the most highly organized types. Through the perspective afforded by paleobotany, it becomes possible to not only supply missing facts, but to establish what theory has for so long a time required a satisfactory demonstration of—a more or less continuous series of phenomena from the rudimentary forms to the most advanced organisms.
Until a very recent date the Linnæan division of plant life into two great phyla, the cryptogams and the phanerogams, was the prevailing conception of the constitution of the plant kingdom. This division recognized no connection between the two great groups, but regarded them as wholly distinct in origin as in character. But the rapid advances in a knowledge of plant anatomy, developed toward the middle of the last century, and especially the remarkable and epoch-making observations of Hofmeister respecting the process of reproduction, enabled him to break down the old barriers erected by the doctrine of the constancy of.species, and prove a genetic connection between the primary divisions of Linnæus. "With this starting-point, the cryptogams and the phanerogams were subjected to a severe scrutiny from an entirely new point of view, with the result that each underwent a revision which led to such a rearrangement of subdivisions as to present an entirely fresh conception of their relations to one another. The logical result was finally expressed in the subdivision of the plant world into four great phyla, which, in their evolutional sequence, came to be known as I., Thallophyta; II., Bryophyta; III., Pteridophyta; IV., Spermatophyta.
Admirable as this scheme is, and scientifically acceptable as it has proved to be, it nevertheless presents certain well-recognized defects with respect to the requirements of theory, although at the time of its formulation and as late as 1899 it represented the sum of available knowledge. It was just at this time that paleobotany became available as a means of meeting those deficiencies which a knowledge of living plants could not overcome. For a long time botanists have been familiar with certain Paleozoic remains having a fern-like aspect which were generally accepted as ferns; but because of their want of direct connection with stems or fruit, there remained a serious doubt as to their real character. In the same horizons, detached fragments of stems were also observed with increasing frequency. The study of their anatomy disclosed a structure which, in some respects, was curiously like that of ferns, while in other respects it approximated to the anatomy of the higher plants as presented in some of the gymnosperms. This combination of filicinean and cycadean characters was noted by Potonié, who succeeded in correlating them and expressing their phylogenetic position in the name of a new order which he called the Cycadofilices.
There yet remained to be considered certain remarkable fruits for which no relationship has as yet been determined until, through the work of Scott, Oliver, Kidston and others, it was shown that they were of the nature of seed-bearing organs which could be correlated with the Cycadofilices. It thus became evident that there was a hitherto unknown group of plants combining the characters of ferns in their foliage and stem structure with those of primitive gymnosperms as presented in their stems and fruits. On the whole, however, these plants approached most nearly to the pteridophytes in their external features. To this new phylum, of which the Cycadofilices formed the most conspicuous member, Scott and Oliver in 1904 assigned the most appropriate name, Pteridospermæ. This result was based entirely upon paleontological evidence through comparative anatomy, and it compels us to recognize the existence of five, instead of four great phyla. The farreaching significance of this achievement can not be overestimated. It is not only of the utmost importance as proving the general course of evolution and bringing into the realm of proved facts what had previously been a working hypothesis only, but it offers an entirely new point of departure for the botanist of the future. Attention may also be directed to one other effect. The tendency of this discovery is to coordinate, unify and strengthen all branches of botanical knowledge, bringing to us the conviction that the more extended and thorough our knowledge of the earlier forms of vegetation becomes, the more satisfactory will be our knowledge of the science as a whole; for while the example selected is probably the most important for our special purposes, the general utility of paleontological research in relation to the history of development is enforced upon our consideration in a great many subordinate ways.
Recognizable plant remains first occur in the Silurian in the form of certain highly organized algas, the ancestral forms of which are unknown. Nevertheless, the history of Nematophycus shows that in the Silurian and extending through the Devonian, members of the brown algae directly comparable with the modern kelps, both in general character and in detailed structure, had attained to a development unknown to any of the marine algas of to-day. Arborescent forms with stems two feet in diameter and a corresponding height lead to the inference that they not only represent the culmination of the phylum at that time, but that they must have been preceded by a long line of ancestral forms, extending far back into the earlier horizons, possibly into the Eozoic itself.
Parka decipiens from the old Bed Sandstone of Scotland affords striking illustration of the very early period at which heterospory was developed among vascular plants, which, according to the evidence now available, are comparable with the genus Marsilea among existing types. In these remains we meet with prostrate stems often one to two inches in diameter, from which slender, upright branches are produced, bearing in turn conceptacles containing both micro- and mega-sporangia. Some of these latter further contain prothalli in various stages of development.
The earliest form of gymnosperm is that which we recognize in the genus Cordaites from the Devonian. The highly developed and dicotyledonous character of the stem affords abundant evidence that the ancestral type must be looked for in some remote and earlier horizon, but, taken as an isolated case, it affords no clue whatever to the origin of that particular phylum, although the subsequent course of development may be traced with considerable certainty to comparatively recent times.
The obvious conclusion to be drawn from the geological relations presented by such illustrations as those recited, is, that the evolution of even very simple forms from the most primitive plants must have called for enormously lengthy periods of time. Even the most liberal application of the law of mutation would fail to adequately account for the extensive gaps which are recognized as occurring between the simpler types and those which lie in the same general line of succession, but with greatly advanced organization.
We are now led to ask, how far have paleontological studies carried us in our knowledge of plant life from the earliest times, that is, do they enable us to trace an unbroken series of steps from the first to the last? To this the answer must be that, while paleobotany has been of the greatest service in supplying missing data, in filling great gaps in a supposed sequence and in giving the fullest support to the law of , it is as yet by no means adequate with respect to meeting all that theory demands. For this there is an intelligible explanation based in part upon the fact that the necessary material is available only under conditions of great difficulty; and that the character of the remains upon which research is based is conditioned by the original nature of the structure and its ability to survive in an unaltered form, the remarkable conditions of decay, infiltration, compression, upheaval and often of volcanic influences to which it has been subjected. The earliest type of vegetation was that which we now find in hot springs, continued with the alga? found in cool or cold waters, all of which possessed a delicacy of structure which permitted speedy decay. The great abundance of such organisms probably afford an adequate explanation of the Laurentian and later forms of graphite which is regarded by many as the remains of former vegetation. While this hypothesis may be accepted provisionally, paleobotany is nevertheless wholly unable to furnish any clue to the life history of the individuals, or even to inform us as to the specific types. Such knowledge as we possess in this direction is the result of inference from parallel conditions and structures as now found.
It might be assumed that with an increasing perfection in the preservation of fossil remains, as found especially in the later formations, it should be possible to trace the course of descent with accuracy and completeness. This is, in a measure, true, but although the general requirements of theory may be verified, yet the haphazard conditions involved in the collection of plant remains make it a very difficult matter to secure a complete narrative, and there remain many gaps which it is difficult to fill. The evolutional position of the Bryophytes demands that the origin of these plants should lie somewhere in the early Silurian or even in the Eozoic age, but we have no certain knowledge of them until the middle Mesozoic, and their remains do not become familiar or abundant until the later Tertiary. So important a deviation from what theory demands should lead us to caution in drawing conclusions from the direct testimony which is thus presented. Unless otherwise disposed of through paleontological evidence, it would be more correct to infer that the delicacy of the plants, and the conditions of their fossilization, have not admitted of their preservation in a recognizable condition; while there is also the further probability that many of their remains have been overlooked through resemblance to certain Pteridophyta for which they might well be mistaken.
In spite of such apparent contradictions, the evidence everywhere points with great force to the idea that each of the lesser phyla had its origin in some ancestral form, followed by growth and culmination. This latter was, in some cases, abrupt, as in many of the Pteridophytes; in other instances there was a gradual decline, as in the lycopods or the horsetails, which attained their highest development in the later Paleozoic, but have since been in a state of degeneracy, their present representatives being few in number and of a depauperate character. The application of this law throughout the enormously lengthy periods required for the evolution of existing species, has led to the survival of some of the most ancient types until the present day; to the absolute obliteration of others which at one time gained great prominence; and to the gradual dying out of yet others, some of which are now found in the last stages of their existence. But through the entire course of change, the evolution of higher and yet higher forms has been the most conspicuous fact. Furthermore, it is undoubtedly true that the general course of evolution is in progress to-day as in the past, since all the potentialities of such evolution exist now as always, though conditioned by the fact that owing to continued changes in the physical character of the earth's atmosphere as well as of its crust, the possibilities of evolution are steadily diminishing and will eventually cease.
There is one direction in which paleobotany gives well-defined assurance that the evidence derived from existing species leads to correct conclusions. In tracing the succession of types, we are led to the belief that there is no direct sequence. Conterminous evolution is in accord with neither theory nor ascertained facts, and it is, therefore, impossible to conceive of a figure which shall in any way represent a single and unbroken line of succession. If paleontology teaches us anything, it is that each great phylum, as well as its various subdivisions, finally reaches its culmination in a terminal member from which no further evolution is possible. But that from some inferior member, possessing high potentialities, a side line of development arises. There is thus, in the early life of each member of the series, a certain recapitulation of ancestral characters. This conception of a continuance of the main line of descent through a succession of lateral members is both logical and fully in accord with the evidence derived from both recent and extinct forms of plant life, as well as with our present theory of evolution.