RELATION OF PALAEOBOTANY TO BOTANY AND GEOLOGY.
“La recherche du plan de la création, voilà le but vers lequel nos
efforts peuvent tendre aujourd’hui.” GAUDRY, 1883.
Since the greater refinements and thoroughness of scientific methods
and the enormous and ever-increasing mass of literature have inevitably
led to extreme specialisation, it is more than ever important to look
beyond the immediate limits of one’s own subject, and to note its
points of contact with other lines of research. A palaeobotanist is
primarily concerned with the determination and description of fossil
plants, but he must at the same time constantly keep in view the
bearing of his work on wider questions of botanical or geological
importance. From the nature of the case, we have in due measure to
adapt the methods of work to the particular conditions before us. It is
impossible to follow in the case of all fossil species precisely the
same treatment as with the more complete and perfect recent plants;
but it is of the utmost importance for a student of palaeobotany, by
adhering to the methods of recent botany, to preserve as far as he is
able the continuity of the past and present floras. Palaeontological
work has often been undertaken by men who are pure geologists, and
whose knowledge of zoology or botany is of the most superficial
character, with the result that biologists have not been able to avail
themselves, to any considerable extent, of the records of extinct forms
of life. They find the literature is often characterised by a special
palaeontological phraseology, and by particular methods of treatment,
which are unknown to the student of living plants and animals. From
this and other causes a purely artificial division has been made
between the science of the organic world of to-day and that of the past.
[Sidenote: NEGLECT OF FOSSILS BY BOTANISTS.]
Fossils are naturally regarded by a stratigraphical geologist
as records which enable him to determine the relative age of
fossil-bearing rocks. For such a purpose it is superfluous to inquire
into the questions of biological interest which centre round the
relics of ancient floras. Primarily concerned, therefore, with fixing
the age of strata, it is easy to understand how geologists have been
content with a special kind of palaeontology which is out of touch
with the methods of systematic zoology or botany. On the other hand,
the botanist whose observations and researches have not extended
beyond the limits of existing plants, sees in the vast majority of
fossil forms merely imperfect specimens, which it is impossible to
determine with any degree of scientific accuracy. He prefers to wait
for perfect material; or in other words, he decides that fossils must
be regarded as outside the range of taxonomic botany. It would seem,
then, that the unsatisfactory treatment or comparative neglect of
fossil plants, has been in a large measure due to the narrowness of
view which too often characterises palaeobotanical literature. This
has at once repelled those who have made a slight effort to recognise
the subject, and has resulted in a one-sided and, from a biological
standpoint, unscientific treatment of this branch of science. It must
be admitted that palaeobotanists have frequently brought the subject
into disrepute by their over-anxiety to institute specific names for
fragments which it is quite impossible to identify. This over-eagerness
to determine imperfect specimens, and the practice of drawing
conclusions as to botanical affinity without any trustworthy evidence,
have naturally given rise to considerable scepticism as to the value
of palaeobotanical records. Another point, which will be dealt with
at greater length in a later chapter, is that geologists have usually
shown a distinct prejudice against fossil plants as indices of
geological age; this again, is no doubt to a large extent the result of
imperfect and inaccurate methods of description, and of the neglect of
and consequent imperfect acquaintance with fossil plants as compared
with fossil animals.
The student of fossil plants should endeavour to keep before him the
fact that the chief object of his work is to deal with the available
material in the most natural and scientific manner; and by adopting
the methods of modern botany, he should always aim to follow such
lines as may best preserve the continuity of past and present types
of plants. Descriptions of floras of past ages and lists of fossil
species, should be so compiled that they may serve the same purpose to
a stratigraphical geologist, who is practically a geographer of former
periods of the Earth’s history, as the accounts of existing floras
to students of present day physiography. The effect of carrying out
researches on some such lines as these, should be to render available
to both botanists and geologists the results of the specialist’s work.
In some cases, palaeobotanical investigations may be of the utmost
service to botanical science, and of little or no value to geology.
The discovery of a completely preserved gametophyte of _Lepidodendron_
or _Calamites_, or of a petrified Moss plant in Palaeozoic rocks would
appeal to most botanists as a matter of primary importance, but for the
stratigraphical geologist such discoveries would possess but little
value. On the other hand the discovery of some characteristic species
of Coal-Measure plants from a deep boring through Mesozoic or Tertiary
strata might be a matter of special geological importance, but to the
botanist it would be of no scientific value. In very many instances,
however, if the palaeobotanist follows such lines as have been briefly
suggested, the results of his labours should be at once useful and
readily accessible to botanists and geologists. As Humboldt has said
in speaking of Palaeontology, “the analytical study of primitive
animal and vegetable life has taken a double direction; the one is
purely morphological, and embraces especially the natural history
and physiology of organisms, filling up the chasms in the series of
still living species by the fossil structures of the primitive world.
The second is more specially geognostic, considering fossil remains
in their relations to the superposition and relative age of the
sedimentary formations[38].”
[Sidenote: FOSSIL PLANTS AND DISTRIBUTION.]
To turn for a moment to some of the most obvious connections between
palaeobotany and the wider sciences of botany and geology. The records
of fossil species must occupy a prominent position in the data by
which we may hope to solve some at least of the problems of plant
evolution. From the point of view of distribution, palaeobotany is of
considerable value, not only to the student of geographical botany, but
to the geologist, who endeavours to map out the positions of ancient
continents with the help of palaeontological evidence. The present
distribution of plants and animals represents but one chapter in the
history of life on the Earth; and to understand or appreciate the facts
which it records, we have to look back through such pages as have been
deciphered in the earlier chapters of the volume. The distribution of
fossil plants lies at the foundation of the principles of the present
grouping of floras on the Earth’s surface. Those who have confined
their study of distribution to the plant geography of the present
age, must supplement their investigations by reference to the work of
palaeobotanical writers. If the lists of plant species drawn up by
specialists in fossil botany, have been prepared with a due sense of
the important conclusions which botanists may draw from them from the
standpoint of distribution, they will be readily accepted as sound
links in the chain of evidence. Unfortunately, however, if many of
the lists of ancient floras were made use of in such investigations,
the conclusions arrived at would frequently be of little value on
account of the untrustworthy determinations of many of the species.
In the case of particular genera the study of the distribution of
the former species both in time and space, that is geologically and
geographically, points to rational explanations of, or gives added
significance to, the facts of present day distribution. That isolated
conifer, _Ginkgo biloba_ L. now restricted to Japan and China, was in
former times abundant in Europe and in other parts of the world. It is
clearly an exceedingly ancient type, isolated not only in geographical
distribution but in botanical affinities, which has reached the last
stage in its natural life. The Mammoth trees of California (_Sequoia
sempervirens_ Endl., and _S. gigantea_ Lindl. and Gord.) afford other
examples of a parallel case. The Tulip tree of North America and China
and other allied forms are fairly common in the Tertiary plant beds
of Europe, but the living representatives are now exclusively North
American. Such differences in distribution as are illustrated by these
dicotyledonous forest trees in Tertiary times and at the present day,
have been clearly explained with the help of the geological record.
Forbes, Darwin, Asa Gray[39] and others have been able to explain many
apparent anomalies in the distribution of existing plants, and to
reconcile the differences between the past and present distribution
of many genera by taking account of the effect on plant life of the
glacial period. As the ice gradually crept down from the polar regions
and spread over the northern parts of Europe, many plants were driven
further south in search of the necessary warmth. In the American
continent such migration was rendered possible by the southern land
extension; in Europe on the other hand the southerly retreat was cut
off by impassable barriers, and the extinction of several genera was
the natural result.
The comparatively abundant information which we possess as to the
past vegetation of polar regions and the value of such knowledge to
geologists and botanists alike is in striking contrast to the absence
of similar data as regards Antarctic fossils. Darwin in an exceedingly
interesting letter to Hooker _à propos_ of a forthcoming British
Association address, referring to this subject writes as follows:—
“The extreme importance of the Arctic fossil plants is self-evident.
Take the opportunity of groaning over our ignorance of the Lignite
plants of Kerguelen Land, or any Antarctic land. It might do good[40].”
In working out any collection of fossil plants, it would be well,
therefore, to bear in mind that our aim should be rather to reproduce
an accurate fragment of botanical history, than to perform feats of
determination with hopelessly inadequate specimens. Had this principle
been generally followed, the number of fossil plant species would be
enormously reduced, but the value of the records would be considerably
raised.
[Sidenote: FOSSIL PLANTS AND CLIMATE.]
Our knowledge of plant anatomy, and of those laws of growth which
govern certain classes of plants to-day and in past time, has been very
materially widened and extended by the facts revealed to us by the
detailed study of Coal-Measure species. The modern science of Plant
Biology, refounded by Charles Darwin, has thrown considerable light
on the laws of plant life, and it enables us to correlate structural
characteristics with physiological conditions of growth. Applying the
knowledge gained from living plants to the study of such extinct types
as permit of close microscopic examination, we may obtain a glimpse
into the secrets of the botanical binomics of Palaeozoic times. The
wider questions of climatic conditions depend very largely upon the
evidence of fossil botany for a rational solution. As an instance
of the best authenticated and most striking alternation in climatic
conditions in comparatively recent times, we may cite the glacial
period or Ice-Age. The existence of Arctic conditions has been proved
by purely geological evidence, but it receives additional confirmation,
and derives a wider importance from the testimony of fossil plants. In
rocks deposited before the spread of ice from high northern latitudes,
we find indubitable proofs of a widely distributed subtropical flora in
Central and Northern Europe. Passing from these rocks to more recent
beds there are found indications of a fall in temperature, and such
northern plants as the dwarf Birch, the Arctic Willow and others reveal
the southern extension of Arctic cold to our own latitudes.
The distribution of plants in time, that is the range of classes,
families, genera and species of plants through the series of strata
which make up the crust of the earth, is a matter of primary importance
from a botanical as well as from a geological point of view.
Among the earlier writers, Brongniart recognised the marked differences
between the earlier and later floras, and attempted to correlate
the periods of maximum development of certain classes of plants
with definite epochs of geological history. He gives the following
classification in which are represented the general outlines of plant
development from Palaeozoic to Tertiary times[41].
I. Reign of Acrogens ╭ 1. Carboniferous epoch.
╰ 2. Permian epoch.
II. Reign of Gymnosperms ╭ 3. Triassic epoch.
╰ 4. Jurassic epoch (including the Wealden).
III. Reign of Angiosperms ╭ 5. Cretaceous epoch.
╰ 6. Tertiary epoch.
Since Brongniart’s time this method of classification has been
extended to many of the smaller subdivisions of the geological epochs,
and species of fossil plants are often of the greatest value in
questions of correlation. In recent years the systematic treatment
of Coal-Measure and other plants in the hands of various Continental
and English writers has clearly demonstrated their capabilities
for the purpose of subdividing a series of strata into stages and
zones[42]. The more complete becomes our knowledge of any flora, the
greater possibility there is of making use of the plants as indices of
geological age[43].
[Sidenote: FOSSIL PLANTS AND PHYLOGENY.]
Not only is it possible to derive valuable aid in the correlation of
strata from the facts of plant distribution, but we may often follow
the various stages in the history of a particular genus as we trace
the records of its occurrence through the geologic series. In studying
the march of plant life through past ages, the botanist may sometimes
follow the progress of a genus from its first appearance, through the
time of maximum development, to its decline or extinction. In the
Palaeozoic forests there was perhaps no more conspicuous or common tree
than the genus long known under the name of _Calamites_. This plant
attained a height of fifty or a hundred feet, with a proportionate
girth, and increased in thickness in a manner precisely similar to
that in which our forest trees grow in diameter. The exceptionally
favourable conditions under which specimens of calamitean plants
have been preserved, have enabled us to become almost as familiar
with the minute structure of their stems and roots, as well as with
their spore-producing organs, as with those of a living species.
In short, it is thoroughly established that _Calamites_ agrees in
most essential respects with our well known _Equisetum_, and must
be included in the same order, or at least sub-class, as the recent
genus of _Equisetaceae_. As we ascend the geologic series from the
Coal-Measures, a marked numerical decline of _Calamites_ is obvious
in the Permian period, and in the red sandstones of the Vosges, which
belong to the same series of rocks as the Triassic strata of the
Cheshire plain, the true _Calamites_ is replaced by a large _Equisetum_
apparently identical in external appearance and habit of growth with
the species living to-day. In the more recent strata the Horse-tails
are still represented, but the size of the Tertiary species agrees
more closely with the comparatively small forms which have such a
wide geographical distribution at the present time. Thus we are able
to trace out the history of a recent genus of Vascular Cryptogams,
and to follow a particular type of organisation from the time of its
maximum development, through its gradual transition to those structural
characters which are represented in the living descendants of the
arborescent Calamites of the coal-period forests. The pages of such a
history are frequently imperfect and occasionally missing, but others,
again, are written in characters as clear as those which we decipher by
a microscopical examination of the tissues of a recent plant.
As one of the most striking instances in which the microscopic study
of fossil plants has shown the way to a satisfactory solution of
the problems of development, we may mention such extinct genera as
_Lyginodendron_, _Myeloxylon_ and others. Each of these genera will be
dealt with at some length in the systematic part of the book, and we
shall afterwards discuss the importance of such types, from the point
of view of plant evolution.
The botanist who would trace out the phylogeny of any existing class or
family, makes it his chief aim to discover points of contact between
the particular type of structure which he is investigating, and that of
other more or less closely related classes or families.
Confining himself to recent forms, he may discover, here and there,
certain anatomical or embryological facts, which suggest promising
lines of inquiry in the quest after such affinities as point to a
common descent. Without recourse to the evidence afforded by the plants
of past ages, we must always admit that our existing classification
of the vegetable kingdom is an expression of real gaps which separate
the several classes of plants from one another. On the other hand our
recently acquired and more accurate knowledge of such genera as have
been alluded to, has made us acquainted with types of plant structure
which enable us to fill in some of the lacunae in our existing
classification. In certain instances we find merged in a single species
morphological characteristics which, in the case of recent plants, are
regarded as distinctive features of different subdivisions. It has
been clearly demonstrated that in _Lyginodendron_, we have anatomical
peculiarities typical of recent cycads, combined with structural
characteristics always associated with existing ferns. In rare cases,
it happens that the remarkably perfect fossilisation of the tissues of
fossil plants, enables us not only to give a complete description of
the histology of extinct forms, but also to speak with confidence as to
some of those physiological processes which governed their life.
[Sidenote: GEOLOGICAL HISTORY.]
So far, palaeobotany has been considered in its bearings on the
study of recent plants. From a geological point of view the records
of ancient floras have scarcely less importance. In recent years,
facts have been brought to light, which show that plants have played
a more conspicuous part than has usually been supposed as agents of
rock-building. As tests of geologic age, there are good grounds for
believing that the inferiority of plants to animals is more apparent
than real. This question, however, must be discussed at greater length
in a later chapter.
Enough has been said to show the many-sided nature of the science of
Fossil Plants, and the wide range of the problems which the geologist
or botanist may reasonably expect to solve, by means of trustworthy
data afforded by scientific palaeobotanical methods.