Darwin and Modern Science (1909)
Edited by A.C. Seward
XII. THE PALAEONTOLOGICAL RECORD.
II. PLANTS.
By D.H. SCOTT, F.R.S.
President of the Linnean Society.
here are several points of view from which the subject of the present
essay may be regarded. We may consider the fossil record of plants in its
bearing: I. on the truth of the doctrine of Evolution; II. on Phylogeny,
or the course of Evolution; III. on the theory of Natural Selection. The
remarks which follow, illustrating certain aspects only of an extensive
subject, may conveniently be grouped under these three headings.
I. THE TRUTH OF EVOLUTION.
When "The Origin of Species"
was written, it was necessary to show that the
Geological Record was favourable to, or at least consistent with, the
Theory of Descent. The point is argued, closely and fully, in Chapter X.
"On the Imperfection of the Geological Record," and Chapter XI. "On the
Geological Succession of Organic Beings"; there is, however, little about
plants in these chapters. At the present time the truth of Evolution is no
longer seriously disputed, though there are writers, like Reinke, who
insist, and rightly so, that the doctrine is still only a belief, rather
than an established fact of science. (J. Reinke, "Kritische
Abstammungslehre", "Wiesner-Festschrift", page 11, Vienna, 1908.)
Evidently, then, however little the Theory of Descent may be questioned in
our own day, it is desirable to assure ourselves how the case stands, and
in particular how far the evidence from fossil plants has grown stronger
with time.
As regards direct evidence for the derivation of one species from another,
there has probably been little advance since Darwin wrote, at least so we
must infer from the emphasis laid on the discontinuity of successive fossil
species by great systematic authorities like Grand'Eury and Zeiller in
their most recent writings. We must either adopt the mutationist views of
those authors (referred to in the last section of this essay) or must still
rely on Darwin's explanation of the absence of numerous intermediate
varieties. The attempts which have been made to trace, in the Tertiary
rocks, the evolution of recent species, cannot, owing to the imperfect
character of the evidence, be regarded as wholly satisfactory.
When we come to groups of a somewhat higher order we have an interesting
history of the evolution of a recent family in the work, not yet completed,
of Kidston and Gwynne-Vaughan on the fossil Osmundaceae. ("Trans. Royal
Soc. Edinburgh", Vol. 45, Part III. 1907, Vol. 46, Part II. 1908, Vol. 46,
Part III. 1909.) The authors are able, mainly on anatomical evidence, to
trace back this now limited group of Ferns, through the Tertiary and
Mesozoic to the Permian, and to show, with great probability, how their
structure has been derived from that of early Palaeozoic types.
The history of the Ginkgoaceae, now represented only by the isolated
maidenhair tree, scarcely known in a wild state, offers another striking
example of a family which can be traced with certainty to the older
Mesozoic and perhaps further back still. (See Seward and Gowan, "The
Maidenhair Tree (Gingko biloba)", "Annals of Botany", Vol. XIV. 1900, page
109; also A. Sprecher "Le Ginkgo biloba", L., Geneva, 1907.)
On the wider question of the derivation of the great groups of plants, a
very considerable advance has been made, and, so far as the higher plants
are concerned, we are now able to form a far better conception than before
of the probable course of evolution. This is a matter of phylogeny, and
the facts will be considered under that head; our immediate point is that
the new knowledge of the relations between the classes of plants in
question materially strengthens the case for the theory of descent. The
discoveries of the last few years throw light especially on the relation of
the Angiosperms to the Gymnosperms, on that of the Seed-plants generally to
the Ferns, and on the interrelations between the various classes of the
higher Cryptogams.
That the fossil record has not done still more for Evolution is due to the
fact that it begins too late--a point on which Darwin laid stress ("Origin
of Species" (6th edition), page 286.) and which has more recently been
elaborated by Poulton. ("Essays on Evolution", pages 46 et seq., Oxford,
1908.) An immense proportion of the whole evolutionary history lies behind
the lowest fossiliferous rocks, and the case is worse for plants than for
animals, as the record for the former begins, for all practical purposes,
much higher up in the rocks.
It may be well here to call attention to a question, often overlooked,
which has lately been revived by Reinke. (Reinke, loc. cit. page 13.) As
all admit, we know nothing of the origin of life; consequently, for all we
can tell, it is as probable that life began, on this planet, with many
living things, as with one. If the first organic beings were many, they
may have been heterogeneous, or at least exposed to different conditions,
from their origin; in either case there would have been a number of
distinct series from the beginning, and if so we should not be justified in
assuming that all organisms are related to one another. There may
conceivably be several of the original lines of descent still surviving, or
represented among extinct forms--to reverse the remark of a distinguished
botanist, there may be several Vegetable Kingdoms! However improbable this
may sound, the possibility is one to be borne in mind.
That all VASCULAR plants really belong to one stock seems certain, and here
the palaeontological record has materially strengthened the case for a
monophyletic history. The Bryophyta are not likely to be absolutely
distinct, for their sexual organs, and the stomata of the Mosses strongly
suggest community of descent with the higher plants; if this be so it no
doubt establishes a certain presumption in favour of a common origin for
plants generally, for the gap between "Mosses and Ferns" has been regarded
as the widest in the Vegetable Kingdom. The direct evidence of
consanguinity is however much weaker when we come to the Algae, and it is
conceivable (even if improbable) that the higher plants may have had a
distinct ancestry (now wholly lost) from the beginning. The question had
been raised in Darwin's time, and he referred to it in these words: "No
doubt it is possible, as Mr G.H. Lewes has urged, that at the first
commencement of life many different forms were evolved; but if so, we may
conclude that only a very few have left modified descendants." ("Origin of
Species", page 425.) This question, though it deserves attention, does not
immediately affect the subject of the palaeontological record of plants,
for there can be no reasonable doubt as to the interrelationship of those
groups on which the record at present throws light.
The past history of plants by no means shows a regular progression from the
simple to the complex, but often the contrary. This apparent anomaly is
due to two causes.
1. The palaeobotanical record is essentially the story of the successive
ascendancy of a series of dominant families, each of which attained its
maximum, in organisation as well as in extent, and then sank into
comparative obscurity, giving place to other families, which under new
conditions were better able to take a leading place. As each family ran
its downward course, either its members underwent an actual reduction in
structure as they became relegated to herbaceous or perhaps aquatic life
(this may have happened with the Horsetails and with Isoetes if derived
from Lepidodendreae), or the higher branches of the family were crowded out
altogether and only the "poor relations" were able to maintain their
position by evading the competition of the ascendant races; this is also
illustrated by the history of the Lycopod phylum. In either case there
would result a lowering of the type of organisation within the group.
2. The course of real progress is often from the complex to the simple.
If, as we shall find some grounds for believing, the Angiosperms came from
a type with a flower resembling in its complexity that of Mesozoic
"Cycads," almost the whole evolution of the flower in the highest plants
has been a process of reduction. The stamen, in particular, has
undoubtedly become extremely simplified during evolution; in the most
primitive known seed-plants it was a highly compound leaf or pinna; its
reduction has gone on in the Conifers and modern Cycads, as well as in the
Angiosperms, though in different ways and to a varying extent.
The seed offers another striking example; the Palaeozoic seeds (if we leave
the seed-like organs of certain Lycopods out of consideration) were always,
so far as we know, highly complex structures, with an elaborate vascular
system, a pollen-chamber, and often a much-differentiated testa. In the
present day such seeds exist only in a few Gymnosperms which retain their
ancient characters--in all the higher Spermophytes the structure is very
much simplified, and this holds good even in the Coniferae, where there is
no countervailing complication of ovary and stigma.
Reduction, in fact, is not always, or even generally, the same thing as
degeneration. Simplification of parts is one of the most usual means of
advance for the organism as a whole. A large proportion of the higher
plants are microphyllous in comparison with the highly megaphyllous fern-
like forms from which they appear to have been derived.
Darwin treated the general question of advance in organisation with much
caution, saying: "The geological record...does not extend far enough back,
to show with unmistakeable clearness that within the known history of the
world organisation has largely advanced." ("Origin of Species", page 308.)
Further on (Ibid. page 309.) he gives two standards by which advance may be
measured: "We ought not solely to compare the highest members of a class
at any two periods...but we ought to compare all the members, high and low,
at the two periods." Judged by either standard the Horsetails and Club
Mosses of the Carboniferous were higher than those of our own day, and the
same is true of the Mesozoic Cycads. There is a general advance in the
succession of classes, but not within each class.
Darwin's argument that "the inhabitants of the world at each successive
period in its history have beaten their predecessors in the race for life,
and are, in so far, higher in the scale" ("Origin of Species", page 315.)
is unanswerable, but we must remember that "higher in the scale" only means
"better adapted to the existing conditions." Darwin points out (Ibid. page
279.) that species have remained unchanged for long periods, probably
longer than the periods of modification, and only underwent change when the
conditions of their life were altered. Higher organisation, judged by the
test of success, is thus purely relative to the changing conditions, a fact
of which we have a striking illustration in the sudden incoming of the
Angiosperms with all their wonderful floral adaptations to fertilisation by
the higher families of Insects.
II. PHYLOGENY.
The question of phylogeny is really inseparable from that of the truth of
the doctrine of evolution, for we cannot have historical evidence that
evolution has actually taken place without at the same time having evidence
of the course it has followed.
As already pointed out, the progress hitherto made has been rather in the
way of joining up the great classes of plants than in tracing the descent
of particular species or genera of the recent flora. There appears to be a
difference in this respect from the Animal record, which tells us so much
about the descent of living species, such as the elephant or the horse.
The reason for this difference is no doubt to be found in the fact that the
later part of the palaeontological record is the most satisfactory in the
case of animals and the least so in the case of plants. The Tertiary
plant-remains, in the great majority of instances, are impressions of
leaves, the conclusions to be drawn from which are highly precarious; until
the whole subject of Angiospermous palaeobotany has been reinvestigated, it
would be rash to venture on any statements as to the descent of the
families of Dicotyledons or Monocotyledons.
Our attention will be concentrated on the following questions, all relating
to the phylogeny of main groups of plants: i. The Origin of the
Angiosperms. ii. The Origin of the Seed-plants. iii. The Origin of the
different classes of the Higher Cryptogamia.
i. THE ORIGIN OF THE ANGIOSPERMS.
The first of these questions has long been the great crux of botanical
phylogeny, and until quite recently no light had been thrown upon the
difficulty. The Angiosperms are the Flowering Plants, par excellence, and
form, beyond comparison, the dominant sub-kingdom in the flora of our own
age, including, apart from a few Conifers and Ferns, all the most familiar
plants of our fields and gardens, and practically all plants of service to
man. All recent work has tended to separate the Angiosperms more widely
from the other seed-plants now living, the Gymnosperms. Vast as is the
range of organisation presented by the great modern sub-kingdom, embracing
forms adapted to every environment, there is yet a marked uniformity in
certain points of structure, as in the development of the embryo-sac and
its contents, the pollination through the intervention of a stigma, the
strange phenomenon of double fertilisation (One sperm fertilising the egg,
while the other unites with the embryo-sac nucleus, itself the product of a
nuclear fusion, to give rise to a nutritive tissue, the endosperm.), the
structure of the stamens, and the arrangement of the parts of the flower.
All these points are common to Monocotyledons and Dicotyledons, and
separate the Angiosperms collectively from all other plants.
In geological history the Angiosperms first appear in the Lower Cretaceous,
and by Upper Cretaceous times had already swamped all other vegetation and
seized the dominant position which they still hold. Thus they are isolated
structurally from the rest of the Vegetable Kingdom, while historically
they suddenly appear, almost in full force, and apparently without
intermediaries with other groups. To quote Darwin's vigorous words: "The
rapid development, as far as we can judge, of all the higher plants within
recent geological times is an abominable mystery." ("More Letters of
Charles Darwin", Vol. II. page 20, letter to J.D. Hooker, 1879.) A couple
of years later he made a bold suggestion (which he only called an "idle
thought") to meet this difficulty. He says: "I have been so astonished at
the apparently sudden coming in of the higher phanerogams, that I have
sometimes fancied that development might have slowly gone on for an immense
period in some isolated continent or large island, perhaps near the South
Pole." (Ibid, page 26, letter to Hooker, 1881.) This idea of an
Angiospermous invasion from some lost southern land has sometimes been
revived since, but has not, so far as the writer is aware, been supported
by evidence. Light on the problem has come from a different direction.
The immense development of plants with the habit of Cycads, during the
Mesozoic Period up to the Lower Cretaceous, has long been known. The
existing Order Cycadaceae is a small family, with 9 genera and perhaps 100
species, occurring in the tropical and sub-tropical zones of both the Old
and New World, but nowhere forming a dominant feature in the vegetation.
Some few attain the stature of small trees, while in the majority the stem
is short, though often living to a great age. The large pinnate or rarely
bipinnate leaves give the Cycads a superficial resemblance in habit to
Palms. Recent Cycads are dioecious; throughout the family the male
fructification is in the form of a cone, each scale of the cone
representing a stamen, and bearing on its lower surface numerous pollen-
sacs, grouped in sori like the sporangia of Ferns. In all the genera,
except Cycas itself, the female fructifications are likewise cones, each
carpel bearing two ovules on its margin. In Cycas, however, no female cone
is produced, but the leaf-like carpels, bearing from two to six ovules
each, are borne directly on the main stem of the plant in rosettes
alternating with those of the ordinary leaves--the most primitive
arrangement known in any living seed-plant. The whole Order is relatively
primitive, as shown most strikingly in its cryptogamic mode of
fertilisation, by means of spermatozoids, which it shares with the
maidenhair tree alone, among recent seed-plants.
In all the older Mesozoic rocks, from the Trias to the Lower Cretaceous,
plants of the Cycad class (Cycadophyta, to use Nathorst's comprehensive
name) are extraordinarily abundant in all parts of the world; in fact they
were almost as prominent in the flora of those ages as the Dicotyledons are
in that of our own day. In habit and to a great extent in anatomy, the
Mesozoic Cycadophyta for the most part much resemble the recent Cycadaceae.
But, strange to say, it is only in the rarest cases that the fructification
has proved to be of the simple type characteristic of the recent family;
the vast majority of the abundant fertile specimens yielded by the Mesozoic
rocks possess a type of reproductive apparatus far more elaborate than
anything known in Cycadaceae or other Gymnosperms. The predominant
Mesozoic family, characterised by this advanced reproductive organisation,
is known as the Bennettiteae; in habit these plants resembled the more
stunted Cycads of the recent flora, but differed from them in the presence
of numerous lateral fructifications, like large buds, borne on the stem
among the crowded bases of the leaves. The organisation of these
fructifications was first worked out on European specimens by Carruthers,
Solms-Laubach, Lignier and others, but these observers had only more or
less ripe fruits to deal with; the complete structure of the flower has
only been elucidated within the last few years by the researches of Wieland
on the magnificent American material, derived from the Upper Jurassic and
Lower Cretaceous beds of Maryland, Dakota and Wyoming. (G.R. Wieland,
"American Fossil Cycads", Carnegie Institution, Washington, 1906.) The
word "flower" is used deliberately, for reasons which will be apparent from
the following brief description, based on Wieland's observations.
The fructification is attached to the stem by a thick stalk, which, in its
upper part, bears a large number of spirally arranged bracts, forming
collectively a kind of perianth and completely enclosing the essential
organs of reproduction. The latter consist of a whorl of stamens, of
extremely elaborate structure, surrounding a central cone or receptacle
bearing numerous ovules. The stamens resemble the fertile fronds of a
fern; they are of a compound, pinnate form, and bear very large numbers of
pollen-sacs, each of which is itself a compound structure consisting of a
number of compartments in which the pollen was formed. In their lower part
the stamens are fused together by their stalks, like the "monadelphous"
stamens of a mallow. The numerous ovules borne on the central receptacle
are stalked, and are intermixed with sterile scales; the latter are
expanded at their outer ends, which are united to form a kind of pericarp
or ovary-wall, only interrupted by the protruding micropyles of the ovules.
There is thus an approach to the closed pistil of an Angiosperm, but it is
evident that the ovules received the pollen directly. The whole
fructification is of large size; in the case of Cycadeoidea dacotensis, one
of the species investigated by Wieland, the total length, in the bud
condition, is about 12 cm., half of which belongs to the peduncle.
The general arrangement of the organs is manifestly the same as in a
typical Angiospermous flower, with a central pistil, a surrounding whorl of
stamens and an enveloping perianth; there is, as we have seen, some
approach to the closed ovary of an Angiosperm; another point, first
discovered nearly 20 years ago by Solms-Laubach in his investigation of a
British species, is that the seed was practically "exalbuminous," its
cavity being filled by the large, dicotyledonous embryo, whereas in all
known Gymnosperms a large part of the sac is occupied by a nutritive
tissue, the prothallus or endosperm; here also we have a condition only met
with elsewhere among the higher Flowering Plants.
Taking all the characters into account, the indications of affinity between
the Mesozoic Cycadophyta and the Angiosperms appear extremely significant,
as was recognised by Wieland when he first discovered the hermaphrodite
nature of the Bennettitean flower. The Angiosperm with which he specially
compared the fossil type was the Tulip tree (Liriodendron) and certainly
there is a remarkable analogy with the Magnoliaceous flowers, and with
those of related orders such as Ranunculaceae and the Water-lilies. It
cannot, of course, be maintained that the Bennettiteae, or any other
Mesozoic Cycadophyta at present known, were on the direct line of descent
of the Angiosperms, for there are some important points of difference, as,
for example, in the great complexity of the stamens, and in the fact that
the ovary-wall or pericarp was not formed by the carpels themselves, but by
the accompanying sterile scale-leaves. Botanists, since the discovery of
the bisexual flowers of the Bennettiteae, have expressed different views as
to the nearness of their relation to the higher Flowering Plants, but the
points of agreement are so many that it is difficult to resist the
conviction that a real relation exists, and that the ancestry of the
Angiosperms, so long shrouded in complete obscurity, is to be sought among
the great plexus of Cycad-like plants which dominated the flora of the
world in Mesozoic times. (On this subject see, in addition to Wieland's
great work above cited, F.W. Oliver, "Pteridosperms and Angiosperms", "New
Phytologist", Vol. V. 1906; D.H. Scott, "The Flowering Plants of the
Mesozoic Age in the Light of Recent Discoveries", "Journal R. Microscop.
Soc." 1907, and especially E.A.N. Arber and J. Parkin, "On the Origin of
Angiosperms", "Journal Linn. Soc." (Bot.) Vol. XXXVIII. page 29, 1907.)
The great complexity of the Bennettitean flower, the earliest known
fructification to which the word "flower" can be applied without forcing
the sense, renders it probable, as Wieland and others have pointed out,
that the evolution of the flower in Angiosperms has consisted essentially
in a process of reduction, and that the simplest forms of flower are not to
be regarded as the most primitive. The older morphologists generally took
the view that such simple flowers were to be explained as reductions from a
more perfect type, and this opinion, though abandoned by many later
writers, appears likely to be true when we consider the elaboration of
floral structure attained among the Mesozoic Cycadophyta, which preceded
the Angiosperms in evolution.
If, as now seems probable, the Angiosperms were derived from ancestors
allied to the Cycads, it would naturally follow that the Dicotyledons were
first evolved, for their structure has most in common with that of the
Cycadophyta. We should then have to regard the Monocotyledons as a side-
line, diverging probably at a very early stage from the main dicotyledonous
stock, a view which many botanists have maintained, of late, on other
grounds. (See especially Ethel Sargant, "The Reconstruction of a Race of
Primitive Angiosperms", "Annals of Botany", Vol. XXII. page 121, 1908.) So
far, however, as the palaeontological record shows, the Monocotyledons were
little if at all later in their appearance than the Dicotyledons, though
always subordinate in numbers. The typical and beautifully preserved Palm-
wood from Cretaceous rocks is striking evidence of the early evolution of a
characteristic monocotyledonous family. It must be admitted that the whole
question of the evolution of Monocotyledons remains to be solved.
Accepting, provisionally, the theory of the cycadophytic origin of
Angiosperms, it is interesting to see to what further conclusions we are
led. The Bennettiteae, at any rate, were still at the gymnospermous level
as regards their pollination, for the exposed micropyles of the ovules were
in a position to receive the pollen directly, without the intervention of a
stigma. It is thus indicated that the Angiosperms sprang from a
gymnospermous source, and that the two great phyla of Seed-plants have not
been distinct from the first, though no doubt the great majority of known
Gymnosperms, especially the Coniferae, represent branch-lines of their own.
The stamens of the Bennettiteae are arranged precisely as in an
angiospermous flower, but in form and structure they are like the fertile
fronds of a Fern, in fact the compound pollen-sacs, or synangia as they are
technically called, almost exactly agree with the spore-sacs of a
particular family of Ferns--the Marattiaceae, a limited group, now mainly
tropical, which was probably more prominent in the later Palaeozoic times
than at present. The scaly hairs, or ramenta, which clothe every part of
the plant, are also like those of Ferns.
It is not likely that the characters in which the Bennettiteae resemble the
Ferns came to them directly from ancestors belonging to that class; an
extensive group of Seed-plants, the Pteridospermeae, existed in Palaeozoic
times and bear evident marks of affinity with the Fern phylum. The fern-
like characters so remarkably persistent in the highly organised
Cycadophyta of the Mesozoic were in all likelihood derived through the
Pteridosperms, plants which show an unmistakable approach to the
cycadophytic type.
The family Bennettiteae thus presents an extraordinary association of
characters, exhibiting, side by side, features which belong to the
Angiosperms, the Gymnosperms and the Ferns.
ii. ORIGIN OF SEED-PLANTS.
The general relation of the gymnospermous Seed-plants to the Higher
Cryptogamia was cleared up, independently of fossil evidence, by the
brilliant researches of Hofmeister, dating from the middle of the past
century. (W. Hofmeister, "On the Germination, Development and
Fructification of the Higher Cryptogamia", Ray Society, London, 1862. The
original German treatise appeared in 1851.) He showed that "the embryo-sac
of the Coniferae may be looked upon as a spore remaining enclosed in its
sporangium; the prothallium which it forms does not come to the light."
(Ibid. page 438.) He thus determined the homologies on the female side.
Recognising, as some previous observers had already done, that the
microspores of those Cryptogams in which two kinds of spore are developed,
are equivalent to the pollen-grains of the higher plants, he further
pointed out that fertilisation "in the Rhizocarpeae and Selaginellae takes
place by free spermatozoa, and in the Coniferae by a pollen-tube, in the
interior of which spermatozoa are probably formed"--a remarkable instance
of prescience, for though spermatozoids have not been found in the Conifers
proper, they were demonstrated in the allied groups Cycadaceae and Ginkgo,
in 1896, by the Japanese botanists Ikeno and Hirase. A new link was thus
established between the Gymnosperms and the Cryptogams.
It remained uncertain, however, from which line of Cryptogams the
gymnospermous Seed-plants had sprung. The great point of morphological
comparison was the presence of two kinds of spore, and this was known to
occur in the recent Lycopods and Water-ferns (Rhizocarpeae) and was also
found in fossil representatives of the third phylum, that of the
Horsetails. As a matter of fact all the three great Cryptogamic classes
have found champions to maintain their claim to the ancestry of the Seed-
plants, and in every case fossil evidence was called in. For a long time
the Lycopods were the favourites, while the Ferns found the least support.
The writer remembers, however, in the year 1881, hearing the late Prof.
Sachs maintain, in a lecture to his class, that the descent of the Cycads
could be traced, not merely from Ferns, but from a definite family of
Ferns, the Marattiaceae, a view which, though in a somewhat crude form,
anticipated more modern ideas.
Williamson appears to have been the first to recognise the presence, in the
Carboniferous flora, of plants combining the characters of Ferns and
Cycads. (See especially his "Organisation of the Fossil Plants of the
Coal-Measures", Part XIII. "Phil. Trans. Royal Soc." 1887 B. page 299.)
This conclusion was first reached in the case of the genera Heterangium and
Lyginodendron, plants, which with a wholly fern-like habit, were found to
unite an anatomical structure holding the balance between that of Ferns and
Cycads, Heterangium inclining more to the former and Lyginodendron to the
latter. Later researches placed Williamson's original suggestion on a
firmer basis, and clearly proved the intermediate nature of these genera,
and of a number of others, so far as their vegetative organs were
concerned. This stage in our knowledge was marked by the institution of
the class Cycadofilices by Potonie in 1897.
Nothing, however, was known of the organs of reproduction of the
Cycadofilices, until F.W. Oliver, in 1903, identified a fossil seed,
Lagenostoma Lomaxi, as belonging to Lyginodendron, the identification
depending, in the first instance, on the recognition of an identical form
of gland, of very characteristic structure, on the vegetative organs of
Lyginodendron and on the cupule enveloping the seed. This evidence was
supported by the discovery of a close anatomical agreement in other
respects, as well as by constant association between the seed and the
plant. (F.W. Oliver and D.H. Scott, "On the Structure of the Palaeozoic
Seed, Lagenostoma Lomaxi, etc." "Phil. Trans. Royal Soc." Vol. 197 B.
1904.) The structure of the seed of Lyginodendron, proved to be of the
same general type as that of the Cycads, as shown especially by the
presence of a pollen-chamber or special cavity for the reception of the
pollen-grains, an organ only known in the Cycads and Ginkgo among recent
plants.
Within a few months after the discovery of the seed of Lyginodendron,
Kidston found the large, nut-like seed of a Neuropteris, another fern-like
Carboniferous plant, in actual connection with the pinnules of the frond,
and since then seeds have been observed on the frond in species of
Aneimites and Pecopteris, and a vast body of evidence, direct or indirect,
has accumulated, showing that a large proportion of the Palaeozoic plants
formerly classed as Ferns were in reality reproduced by seeds of the same
type as those of recent Cycadaceae. (A summary of the evidence will be
found in the writer's article "On the present position of Palaeozoic
Botany", "Progressus Rei Botanicae", 1907, page 139, and "Studies in Fossil
Botany", Vol. II. (2nd edition) London, 1909.) At the same time, the
anatomical structure, where it is open to investigation, confirms the
suggestion given by the habit, and shows that these early seed-bearing
plants had a real affinity with Ferns. This conclusion received strong
corroboration when Kidston, in 1905, discovered the male organs of
Lyginodendron, and showed that they were identical with a fructification of
the genus Crossotheca, hitherto regarded as belonging to Marattiaceous
Ferns. (Kidston, "On the Microsporangia of the Pteridospermeae, etc."
"Phil. Trans. Royal Soc." Vol. 198, B. 1906.)
The general conclusion which follows from the various observations alluded
to, is that in Palaeozoic times there was a great body of plants
(including, as it appears, a large majority of the fossils previously
regarded as Ferns) which had attained the rank of Spermophyta, bearing
seeds of a Cycadean type on fronds scarcely differing from the vegetative
foliage, and in other respects, namely anatomy, habit and the structure of
the pollen-bearing organs, retaining many of the characters of Ferns. From
this extensive class of plants, to which the name Pteridospermeae has been
given, it can scarcely be doubted that the abundant Cycadophyta, of the
succeeding Mesozoic period, were derived. This conclusion is of far-
reaching significance, for we have already found reason to think that the
Angiosperms themselves sprang, in later times, from the Cycadophytic stock;
it thus appears that the Fern-phylum, taken in a broad sense, ultimately
represents the source from which the main line of descent of the
Phanerogams took its rise.
It must further be borne in mind that in the Palaeozoic period there
existed another group of seed-bearing plants, the Cordaiteae, far more
advanced than the Pteridospermeae, and in many respects approaching the
Coniferae, which themselves begin to appear in the latest Palaeozoic rocks.
The Cordaiteae, while wholly different in habit from the contemporary fern-
like Seed-plants, show unmistakable signs of a common origin with them.
Not only is there a whole series of forms connecting the anatomical
structure of the Cordaiteae with that of the Lyginodendreae among
Pteridosperms, but a still more important point is that the seeds of the
Cordaiteae, which have long been known, are of the same Cycadean type as
those of the Pteridosperms, so that it is not always possible, as yet, to
discriminate between the seeds of the two groups. These facts indicate
that the same fern-like stock which gave rise to the Cycadophyta and
through them, as appears probable, to the Angiosperms, was also the source
of the Cordaiteae, which in their turn show manifest affinity with some at
least of the Coniferae. Unless the latter are an artificial group, a view
which does not commend itself to the writer, it would appear probable that
the Gymnosperms generally, as well as the Angiosperms, were derived from an
ancient race of Cryptogams, most nearly related to the Ferns. (Some
botanists, however, believe that the Coniferae, or some of them, are
probably more nearly related to the Lycopods. See Seward and Ford, "The
Araucarieae, Recent and Extinct", "Phil. Trans. Royal Soc." Vol. 198 B.
1906.)
It may be mentioned here that the small gymnospermous group Gnetales
(including the extraordinary West African plant Welwitschia) which were
formerly regarded by some authorities as akin to the Equisetales, have
recently been referred, on better grounds, to a common origin with the
Angiosperms, from the Mesozoic Cycadophyta.
The tendency, therefore, of modern work on the palaeontological record of
the Seed-plants has been to exalt the importance of the Fern-phylum, which,
on present evidence, appears to be that from which the great majority,
possibly the whole, of the Spermophyta have been derived.
One word of caution, however, is necessary. The Seed-plants are of
enormous antiquity; both the Pteridosperms and the more highly organised
family Cordaiteae, go back as far in geological history (namely to the
Devonian) as the Ferns themselves or any other Vascular Cryptogams. It
must therefore be understood that in speaking of the derivation of the
Spermophyta from the Fern-phylum, we refer to that phylum at a very early
stage, probably earlier than the most ancient period to which our record of
land-plants extends. The affinity between the oldest Seed-plants and the
Ferns, in the widest sense, seems established, but the common stock from
which they actually arose is still unknown; though no doubt nearer to the
Ferns than to any other group, it must have differed widely from the Ferns
as we now know them, or perhaps even from any which the fossil record has
yet revealed to us.
iii. THE ORIGIN OF THE HIGHER CRYPTOGAMIA.
The Sub-kingdom of the higher Spore-plants, the Cryptogamia possessing a
vascular system, was more prominent in early geological periods than at
present. It is true that the dominance of the Pteridophyta in Palaeozoic
times has been much exaggerated owing to the assumption that everything
which looked like a Fern really was a Fern. But, allowing for the fact,
now established, that most of the Palaeozoic fern-like plants were already
Spermophyta, there remains a vast mass of Cryptogamic forms of that period,
and the familiar statement that they formed the main constituent of the
Coal-forests still holds good. The three classes, Ferns (Filicales),
Horsetails (Equisetales) and Club-mosses (Lycopodiales), under which we now
group the Vascular Cryptogams, all extend back in geological history as far
as we have any record of the flora of the land; in the Palaeozoic, however,
a fourth class, the Sphenophyllales, was present.
As regards the early history of the Ferns, which are of special interest
from their relation to the Seed-plants, it is impossible to speak quite
positively, owing to the difficulty of discriminating between true fossil
Ferns and the Pteridosperms which so closely simulated them. The
difficulty especially affects the question of the position of Marattiaceous
Ferns in the Palaeozoic Floras. This family, now so restricted, was until
recently believed to have been one of the most important groups of
Palaeozoic plants, especially during later Carboniferous and Permian times.
Evidence both from anatomy and from sporangial characters appeared to
establish this conclusion. Of late, however, doubts have arisen, owing to
the discovery that some supposed members of the Marattiaceae bore seeds,
and that a form of fructification previously referred to that family
(Crossotheca) was really the pollen-bearing apparatus of a Pteridosperm
(Lyginodendron). The question presents much difficulty; though it seems
certain that our ideas of the extent of the family in Palaeozoic times will
have to be restricted, there is still a decided balance of evidence in
favour of the view that a considerable body of Marattiaceous Ferns actually
existed. The plants in question were of large size (often arborescent) and
highly organised--they represent, in fact, one of the highest developments
of the Fern-stock, rather than a primitive type of the class.
There was, however, in the Palaeozoic period, a considerable group of
comparatively simple Ferns (for which Arber has proposed the collective
name Primofilices); the best known of these are referred to the family
Botryopterideae, consisting of plants of small or moderate dimensions,
with, on the whole, a simple anatomical structure, in certain cases
actually simpler than that of any recent Ferns. On the other hand the
sporangia of these plants were usually borne on special fertile fronds, a
mark of rather high differentiation. This group goes back to the Devonian
and includes some of the earliest types of Fern with which we are
acquainted. It is probable that the Primofilices (though not the
particular family Botryopterideae) represent the stock from which the
various families of modern Ferns, already developed in the Mesozoic period,
may have sprung.
None of the early Ferns show any clear approach to other classes of
Vascular Cryptogams; so far as the fossil record affords any evidence,
Ferns have always been plants with relatively large and usually compound
leaves. There is no indication of their derivation from a microphyllous
ancestry, though, as we shall see, there is some slight evidence for the
converse hypothesis. Whatever the origin of the Ferns may have been it is
hidden in the older rocks.
It has, however, been held that certain other Cryptogamic phyla had a
common origin with the Ferns. The Equisetales are at present a well-
defined group; even in the rich Palaeozoic floras the habit, anatomy and
reproductive characters usually render the members of this class
unmistakable, in spite of the great development and stature which they then
attained. It is interesting, however, to find that in the oldest known
representatives of the Equisetales the leaves were highly developed and
dichotomously divided, thus differing greatly from the mere scale-leaves of
the recent Horsetails, or even from the simple linear leaves of the later
Calamites. The early members of the class, in their forked leaves, and in
anatomical characters, show an approximation to the Sphenophyllales, which
are chiefly represented by the large genus Sphenophyllum, ranging through
the Palaeozoic from the Middle Devonian onwards. These were plants with
rather slender, ribbed stems, bearing whorls of wedge-shaped or deeply
forked leaves, six being the typical number in each whorl. From their weak
habit it has been conjectured, with much probability, that they may have
been climbing plants, like the scrambling Bedstraws of our hedgerows. The
anatomy of the stem is simple and root-like; the cones are remarkable for
the fact that each scale or sporophyll is a double structure, consisting of
a lower, usually sterile lobe and one or more upper lobes bearing the
sporangia; in one species both parts of the sporophyll were fertile.
Sphenophyllum was evidently much specialised; the only other known genus is
based on an isolated cone, Cheirostrobus, of Lower Carboniferous age, with
an extraordinarily complex structure. In this genus especially, but also
in the entire group, there is an evident relation to the Equisetales; hence
it is of great interest that Nathorst has described, from the Devonian of
Bear Island in the Arctic regions, a new genus Pseudobornia, consisting of
large plants, remarkable for their highly compound leaves which, when found
detached, were taken for the fronds of a Fern. The whorled arrangement of
the leaves, and the habit of the plant, suggest affinities either with the
Equisetales or the Sphenophyllales; Nathorst makes the genus the type of a
new class, the Pseudoborniales. (A.G. Nathorst, "Zur Oberdevonischen Flora
der Baren-Insel", "Kongl. Svenska Vetenskaps-Akademiens Handlingar" Bd. 36,
No. 3, Stockholm, 1902.)
The available data, though still very fragmentary, certainly suggest that
both Equisetales and Sphenophyllales may have sprung from a common stock
having certain fern-like characters. On the other hand the Sphenophylls,
and especially the peculiar genus Cheirostrobus, have in their anatomy a
good deal in common with the Lycopods, and of late years they have been
regarded as the derivatives of a stock common to that class and the
Equisetales. At any rate the characters of the Sphenophyllales and of the
new group Pseudoborniales suggest the existence, at a very early period, of
a synthetic race of plants, combining the characters of various phyla of
the Vascular Cryptogams. It may further be mentioned that the Psilotaceae,
an isolated epiphytic family hitherto referred to the Lycopods, have been
regarded by several recent authors as the last survivors of the
Sphenophyllales, which they resemble both in their anatomy and in the
position of their sporangia.
The Lycopods, so far as their early history is known, are remarkable rather
for their high development in Palaeozoic times than for any indications of
a more primitive ancestry. In the recent Flora, two of the four living
genera (Excluding Psilotaceae.) (Selaginella and Isoetes) have spores of
two kinds, while the other two (Lycopodium and Phylloglossum) are
homosporous. Curiously enough, no certain instance of a homosporous
Palaeozoic Lycopod has yet been discovered, though well-preserved
fructifications are numerous. Wherever the facts have been definitely
ascertained, we find two kinds of spore, differentiated quite as sharply as
in any living members of the group. Some of the Palaeozoic Lycopods, in
fact, went further, and produced bodies of the nature of seeds, some of
which were actually regarded, for many years, as the seeds of Gymnosperms.
This specially advanced form of fructification goes back at least as far as
the Lower Carboniferous, while the oldest known genus of Lycopods,
Bothrodendron, which is found in the Devonian, though not seed-bearing, was
typically heterosporous, if we may judge from the Coal-measure species. No
doubt homosporous Lycopods existed, but the great prevalence of the higher
mode of reproduction in days which to us appear ancient, shows how long a
course of evolution must have already been passed through before the oldest
known members of the group came into being. The other characters of the
Palaeozoic Lycopods tell the same tale; most of them attained the stature
of trees, with a corresponding elaboration of anatomical structure, and
even the herbaceous forms show no special simplicity. It appears from
recent work that herbaceous Lycopods, indistinguishable from our recent
Selaginellas, already existed in the time of the Coal-measures, while one
herbaceous form (Miadesmia) is known to have borne seeds.
The utmost that can be said for primitiveness of character in Palaeozoic
Lycopods is that the anatomy of the stem, in its primary ground-plan, as
distinguished from its secondary growth, was simpler than that of most
Lycopodiums and Selaginellas at the present day. There are also some
peculiarities in the underground organs (Stigmaria) which suggest the
possibility of a somewhat imperfect differentiation between root and stem,
but precisely parallel difficulties are met with in the case of the living
Selaginellas, and in some degree in species of Lycopodium.
In spite of their high development in past ages the Lycopods, recent and
fossil, constitute, on the whole, a homogeneous group, and there is little
at present to connect them with other phyla. Anatomically some relation to
the Sphenophylls is indicated, and perhaps the recent Psilotaceae give some
support to this connection, for while their nearest alliance appears to be
with the Sphenophylls, they approach the Lycopods in anatomy, habit, and
mode of branching.
The typically microphyllous character of the Lycopods, and the simple
relation between sporangium and sporophyll which obtains throughout the
class, have led various botanists to regard them as the most primitive
phylum of the Vascular Cryptogams. There is nothing in the fossil record
to disprove this view, but neither is there anything to support it, for
this class so far as we know is no more ancient than the megaphyllous
Cryptogams, and its earliest representatives show no special simplicity.
If the indications of affinity with Sphenophylls are of any value the
Lycopods are open to suspicion of reduction from a megaphyllous ancestry,
but there is no direct palaeontological evidence for such a history.
The general conclusions to which we are led by a consideration of the
fossil record of the Vascular Cryptogams are still very hypothetical, but
may be provisionally stated as follows:
The Ferns go back to the earliest known period. In Mesozoic times
practically all the existing families had appeared; in the Palaeozoic the
class was less extensive than formerly believed, a majority of the supposed
Ferns of that age having proved to be seed-bearing plants. The oldest
authentic representatives of the Ferns were megaphyllous plants, broadly
speaking, of the same type as those of later epochs, though differing much
in detail. As far back as the record extends they show no sign of becoming
merged with other phyla in any synthetic group.
The Equisetales likewise have a long history, and manifestly attained their
greatest development in Palaeozoic times. Their oldest forms show an
approach to the extinct class Sphenophyllales, which connects them to some
extent, by anatomical characters, with the Lycopods. At the same time the
oldest Equisetales show a somewhat megaphyllous character, which was more
marked in the Devonian Pseudoborniales. Some remote affinity with the
Ferns (which has also been upheld on other grounds) may thus be indicated.
It is possible that in the Sphenophyllales we may have the much-modified
representatives of a very ancient synthetic group.
The Lycopods likewise attained their maximum in the Palaeozoic, and show,
on the whole, a greater elaboration of structure in their early forms than
at any later period, while at the same time maintaining a considerable
degree of uniformity in morphological characters throughout their history.
The Sphenophyllales are the only other class with which they show any
relation; if such a connection existed, the common point of origin must lie
exceedingly far back.
The fossil record, as at present known, cannot, in the nature of things,
throw any direct light on what is perhaps the most disputed question in the
morphology of plants--the origin of the alternating generations of the
higher Cryptogams and the Spermophyta. At the earliest period to which
terrestrial plants have been traced back all the groups of Vascular
Cryptogams were in a highly advanced stage of evolution, while innumerable
Seed-plants--presumably the descendants of Cryptogamic ancestors--were
already flourishing. On the other hand we know practically nothing of
Palaeozoic Bryophyta, and the evidence even for their existence at that
period cannot be termed conclusive. While there are thus no
palaeontological grounds for the hypothesis that the Vascular plants came
of a Bryophytic stock, the question of their actual origin remains
unsolved.
III. NATURAL SELECTION.
Hitherto we have considered the palaeontological record of plants in
relation to Evolution. The question remains, whether the record throws any
light on the theory of which Darwin and Wallace were the authors--that of
Natural Selection. The subject is clearly one which must be investigated
by other methods than those of the palaeontologist; still there are certain
important points involved, on which the palaeontological record appears to
bear.
One of these points is the supposed distinction between morphological and
adaptive characters, on which Nageli, in particular, laid so much stress.
The question is a difficult one; it was discussed by Darwin ("Origin of
Species" (6th edition), pages 170-176.), who, while showing that the
apparent distinction is in part to be explained by our imperfect knowledge
of function, recognised the existence of important morphological characters
which are not adaptations. The following passage expresses his conclusion.
"Thus, as I am inclined to believe, morphological differences, which we
consider as important--such as the arrangement of the leaves, the divisions
of the flower or of the ovarium, the position of the ovules, etc.--first
appeared in many cases as fluctuating variations, which sooner or later
became constant through the nature of the organism and of the surrounding
conditions, as well as through the inter-crossing of distinct individuals,
but not through natural selection; for as these morphological characters do
not affect the welfare of the species, any slight deviations in them could
not have been governed or accumulated through this latter agency." (Ibid.
page 176.)
This is a sufficiently liberal concession; Nageli, however, went much
further when he said: "I do not know among plants a morphological
modification which can be explained on utilitarian principles." (See "More
Letters", Vol. II. page 375 (footnote).) If this were true the field of
Natural Selection would be so seriously restricted, as to leave the theory
only a very limited importance.
It can be shown, as the writer believes, that many typical "morphological
characters," on which the distinction between great classes of plants is
based, were adaptive in origin, and even that their constancy is due to
their functional importance. Only one or two cases will be mentioned,
where the fossil evidence affects the question.
The pollen-tube is one of the most important morphological characters of
the Spermophyta as now existing--in fact the name Siphonogama is used by
Engler in his classification, as expressing a peculiarly constant character
of the Seed-plants. Yet the pollen-tube is a manifest adaptation,
following on the adoption of the seed-habit, and serving first to bring the
spermatozoids with greater precision to their goal, and ultimately to
relieve them of the necessity for independent movement. The pollen-tube is
constant because it has proved to be indispensable.
In the Palaeozoic Seed-plants there are a number of instances in which the
pollen-grains, contained in the pollen-chamber of a seed, are so
beautifully preserved that the presence of a group of cells within the
grain can be demonstrated; sometimes we can even see how the cell-walls
broke down to emit the sperms, and quite lately it is said that the sperms
themselves have been recognised. (F.W. Oliver, "On Physostoma elegans, an
archaic type of seed from the Palaeozoic Rocks", "Annals of Botany",
January, 1909. See also the earlier papers there cited.) In no case,
however, is there as yet any satisfactory evidence for the formation of a
pollen-tube; it is probable that in these early Seed-plants the pollen-
grains remained at about the evolutionary level of the microspores in
Pilularia or Selaginella, and discharged their spermatozoids directly,
leaving them to find their own way to the female cells. It thus appears
that there were once Spermophyta without pollen-tubes. The pollen-tube
method ultimately prevailed, becoming a constant "morphological character,"
for no other reason than because, under the new conditions, it provided a
more perfect mechanism for the accomplishment of the act of fertilisation.
We have still, in the Cycads and Ginkgo, the transitional case, where the
tube remains short, serves mainly as an anchor and water-reservoir, but yet
is able, by its slight growth, to give the spermatozoids a "lift" in the
right direction. In other Seed-plants the sperms are mere passengers,
carried all the way by the pollen-tube; this fact has alone rendered the
Angiospermous method of fertilisation through a stigma possible.
We may next take the seed itself--the very type of a morphological
character. Our fossil record does not go far enough back to tell us the
origin of the seed in the Cycadophyta and Pteridosperms (the main line of
its development) but some interesting sidelights may be obtained from the
Lycopod phylum. In two Palaeozoic genera, as we have seen, seed-like
organs are known to have been developed, resembling true seeds in the
presence of an integument and of a single functional embryo-sac, as well as
in some other points. We will call these organs "seeds" for the sake of
shortness. In one genus (Lepidocarpon) the seeds were borne on a cone
indistinguishable from that of the ordinary cryptogamic Lepidodendreae, the
typical Lycopods of the period, while the seed itself retained much of the
detailed structure of the sporangium of that family. In the second genus,
Miadesmia, the seed-bearing plant was herbaceous, and much like a recent
Selaginella. (See Margaret Benson, "Miadesmia membranacea, a new
Palaeozoic Lycopod with a seed-like structure", "Phil. Trans. Royal Soc.
Vol. 199, B. 1908.) The seeds of the two genera are differently
constructed, and evidently had an independent origin. Here, then, we have
seeds arising casually, as it were, at different points among plants which
otherwise retain all the characters of their cryptogamic fellows; the seed
is not yet a morphological character of importance. To suppose that in
these isolated cases the seed sprang into being in obedience to a Law of
Advance ("Vervollkommungsprincip"), from which other contemporary Lycopods
were exempt, involves us in unnecessary mysticism. On the other hand it is
not difficult to see how these seeds may have arisen, as adaptive
structures, under the influence of Natural Selection. The seed-like
structure afforded protection to the prothallus, and may have enabled the
embryo to be launched on the world in greater security. There was further,
as we may suppose, a gain in certainty of fertilisation. As the writer has
pointed out elsewhere, the chances against the necessary association of the
small male with the large female spores must have been enormously great
when the cones were borne high up on tall trees. The same difficulty may
have existed in the case of the herbaceous Miadesmia, if, as Miss Benson
conjectures, it was an epiphyte. One way of solving the problem was for
pollination to take place while the megaspore was still on the parent
plant, and this is just what the formation of an ovule or seed was likely
to secure.
The seeds of the Pteridosperms, unlike those of the Lycopod stock, have not
yet been found in statu nascendi--in all known cases they were already
highly developed organs and far removed from the cryptogamic sporangium.
But in two respects we find that these seeds, or some of them, had not yet
realised their possibilities. In the seed of Lyginodendron and other cases
the micropyle, or orifice of the integument, was not the passage through
which the pollen entered; the open neck of the pollen-chamber protruded
through the micropyle and itself received the pollen. We have met with an
analogous case, at a more advanced stage of evolution, in the Bennettiteae,
where the wall of the gynaecium, though otherwise closed, did not provide a
stigma to catch the pollen, but allowed the micropyles of the ovules to
protrude and receive the pollen in the old gymnospermous fashion. The
integument in the one case and the pistil in the other had not yet assumed
all the functions to which the organ ultimately became adapted. Again, no
Palaeozoic seed has yet been found to contain an embryo, though the
preservation is often good enough for it to have been recognised if
present. It is probable that the nursing of the embryo had not yet come to
be one of the functions of the seed, and that the whole embryonic
development was relegated to the germination stage.
In these two points, the reception of the pollen by the micropyle and the
nursing of the embryo, it appears that many Palaeozoic seeds were
imperfect, as compared with the typical seeds of later times. As evolution
went on, one function was superadded on another, and it appears impossible
to resist the conclusion that the whole differentiation of the seed was a
process of adaptation, and consequently governed by Natural Selection, just
as much as the specialisation of the rostellum in an Orchid, or of the
pappus in a Composite.
Did space allow, other examples might be added. We may venture to maintain
that the glimpses which the fossil record allows us into early stages in
the evolution of organs now of high systematic importance, by no means
justify the belief in any essential distinction between morphological and
adaptive characters.
Another point, closely connected with Darwin's theory, on which the fossil
history of plants has been supposed to have some bearing, is the question
of Mutation, as opposed to indefinite variation. Arber and Parkin, in
their interesting memoir on the Origin of Angiosperms, have suggested
calling in Mutation to explain the apparently sudden transition from the
cycadean to the angiospermous type of foliage, in late Mesozoic times,
though they express themselves with much caution, and point out "a distinct
danger that Mutation may become the last resort of the phylogenetically
destitute"!
The distinguished French palaeobotanists, Grand'Eury (C. Grand'Eury, "Sur
les mutations de quelques Plantes fossiles du Terrain houiller". "Comptes
Rendus", CXLII. page 25, 1906.) and Zeiller (R. Zeiller "Les Vegetaux
fossiles et leurs Enchainements", "Revue du Mois", III. February, 1907.),
are of opinion, to quote the words of the latter writer, that the facts of
fossil Botany are in agreement with the sudden appearance of new forms,
differing by marked characters from those that have given them birth; he
adds that these results give more amplitude to this idea of Mutation,
extending it to groups of a higher order, and even revealing the existence
of discontinuous series between the successive terms of which we yet
recognise bonds of filiation. (Loc. cit. page 23.)
If Zeiller's opinion should be confirmed, it would no doubt be a serious
blow to the Darwinian theory. As Darwin said: "Under a scientific point
of view, and as leading to further investigation, but little advantage is
gained by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms, over the old
belief in the creation of species from the dust of the earth." ("Origin of
Species", page 424.)
It most however be pointed out, that such mutations as Zeiller, and to some
extent Arber and Parkin, appear to have in view, bridging the gulf between
different Orders and Classes, bear no relation to any mutations which have
been actually observed, such as the comparatively small changes, of sub-
specific value, described by De Vries in the type-case of Oenothera
Lamarckiana. The results of palaeobotanical research have undoubtedly
tended to fill up gaps in the Natural System of plants--that many such gaps
still persist is not surprising; their presence may well serve as an
incentive to further research but does not, as it seems to the writer,
justify the assumption of changes in the past, wholly without analogy among
living organisms.
As regards the succession of species, there are no greater authorities than
Grand'Eury and Zeiller, and great weight must be attached to their opinion
that the evidence from continuous deposits favours a somewhat sudden change
from one specific form to another. At the same time it will be well to
bear in mind that the subject of the "absence of numerous intermediate
varieties in any single formation" was fully discussed by Darwin. ("Origin
of Species", pages 275-282, and page 312.); the explanation which he gave
may go a long way to account for the facts which recent writers have
regarded as favouring the theory of saltatory mutation.
The rapid sketch given in the present essay can do no more than call
attention to a few salient points, in which the palaeontological records of
plants has an evident bearing on the Darwinian theory. At the present day
the whole subject of palaeobotany is a study in evolution, and derives its
chief inspiration from the ideas of Darwin and Wallace. In return it
contributes something to the verification of their teaching; the recent
progress of the subject, in spite of the immense difficulties which still
remain, has added fresh force to Darwin's statement that "the great leading
facts in palaeontology agree admirably with the theory of descent with
modification through variation and natural selection." (Ibid. page 313.)
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