Darwin and Modern Science Part 17

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 cla.s.s 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 cla.s.ses, but not within each cla.s.s.

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 ill.u.s.tration 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. .h.i.therto made has been rather in the way of joining up the great cla.s.ses 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 cla.s.ses 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 cla.s.s (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 Bennett.i.teae; 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 Jura.s.sic and Lower Cretaceous beds of Maryland, Dakota and Wyoming. (G.R. Wieland, "American Fossil Cycads", Carnegie Inst.i.tution, Was.h.i.+ngton, 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 "exalb.u.minous," 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 Bennett.i.tean flower. The Angiosperm with which he specially compared the fossil type was the Tulip tree (Liriodendron) and certainly there is a remarkable a.n.a.logy 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 Bennett.i.teae, 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 bis.e.xual flowers of the Bennett.i.teae, 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. x.x.xVIII. page 29, 1907.)

The great complexity of the Bennett.i.tean 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 Bennett.i.teae, 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 Bennett.i.teae 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 Bennett.i.teae resemble the Ferns came to them directly from ancestors belonging to that cla.s.s; 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 Bennett.i.teae thus presents an extraordinary a.s.sociation 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 h.o.m.ologies 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 j.a.panese 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 cla.s.ses 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 cla.s.s, 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, antic.i.p.ated 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 inst.i.tution of the cla.s.s 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 a.s.sociation 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 acc.u.mulated, showing that a large proportion of the Palaeozoic plants formerly cla.s.sed 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 cla.s.s 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 a.s.sumption 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 ma.s.s of Cryptogamic forms of that period, and the familiar statement that they formed the main const.i.tuent of the Coal-forests still holds good. The three cla.s.ses, 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 cla.s.s, 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 cla.s.s.

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 cla.s.ses 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 cla.s.s 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 cla.s.s, 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 cla.s.s, 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 cla.s.s 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 h.o.m.osporous. Curiously enough, no certain instance of a h.o.m.osporous 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 h.o.m.osporous 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 pa.s.sed 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.