In a volume by W. Canton entitled A Child's book of Saints a story is told in which the presence in Ireland of Mediterranean species receives a more picturesque explanation. The Monk Bresal was sent to teach the brethren in a Spanish monastery the music of Irish choirs. In later years Bresal longed for a sight of his native land, though he loved his home and 'every rock, tree, and flower' in his adopted country. After returning to Ireland, his thoughts reverted to Spain; 'it appeared to him as though he was once again in a granite nook among the rocks beside the Priory'; he saw the ice-plant with its little stars of white flowers sprinkled with red (the London Pride) and a small evergreen tree from which he had often gathered the orange-scarlet berries (Arbutus). The Prior of the Spanish monastery 'with heavenly vision saw Bresal gazing at the evergreen tree and the ice-plant, and turning to the trees blessed them and commended them to go and make real his dream. As Bresal brushed away his tears he saw with amazement at his feet the ice-plant and hard by the evergreen tree.'

The plant represented in (Fig. 2) is another British species which tasks the ingenuity of students of plant-geography. This is the Pipe Wort (Eriocaulon septangulare), the sole representative in Europe of a certain family of Monocotyledons: it flourishes in the west of Ireland and in the western islands of Scotland but nowhere else in Europe; it is native on the other side of the Atlantic in Canada and the northern United States of America. Mr Praeger in describing the striking mixture of species in the west of Ireland writes, 'The pool from which we gather the American Pipe Wort is fringed with Pyrenean Heathers. The cracks which are filled with the delicate green foam of the maiden hair are set in Bearberry and Spring Gentian; Habenaria intacta, far from its Mediterranean home, sends up its flower-spikes through carpets of mountain Avens; and St Dabeoc's Heath and the dwarf Juniper straggle together over the rocky knolls'(25).

The presence of Eriocaulon on the western edge of Europe may be attributed to migration in pre-Glacial days from North America by way of a land-connexion, of which Greenland and Iceland represent surviving portions. The opinion held by Forbes, and advocated by some later naturalists, that the southern companions of Eriocaulon in the west of Ireland are survivors from a Tertiary flora which have lived through the Ice Age, is consistently extended to the Pipe Wort. On the other hand, before yielding to the temptation to regard these American and Mediterranean species as links with the Tertiary period, we must be convinced that the possibilities of post-Glacial introduction, even without the aid of land-bridges, have been exhausted. The Pipe Wort is a botanical puzzle which affords a good example of the accession of interest to field-botany effected by a knowledge of the distribution of the component members of the British flora. The problem of its past history suggests an experimental enquiry into the adaptability of its seeds to dispersal, and emphasises the importance of the co-operation of botanists and geologists in a common endeavour to trace the origin of British plants.

In addition to the Pipe Wort, mention may be made of three other American flowering plants recognised in the Irish flora. Sisyrinchium angustifolium recorded from the west of Ireland is native in temperate North America; the orchid, Spiranthes romanzoffiana, met with in the south and north of Ireland, is widely spread in Canada and the northern States, while Sisyrinchium californicum, a native of California and Oregon, was discovered by Mr Marshall in marshy meadow-land near Wexford(26). In the case of the more recently discovered American immigrants, the possibility of human introduction must be borne in mind, though there are no special reasons for doubting that some, as in the case of Eriocaulon, reached the Irish coast by natural agencies.

CHAPTER III

THE GEOLOGICAL RECORD

'All the Epochs of the Past are only a few of the front carriages, and probably the least wonderful, in the van of an interminable procession.'   J. B. Bury (The Science of History).

The portion of the earth's surface accessible to investigation is made up in part of accumulations of old sediments, some indistinguishable from the shingle, sand, and mud now in process of formation by the ceaseless action of denudation; others have been hardened, gently folded or violently contorted and so far altered by crust-movements as to render their sedimentary origin well nigh unrecognisable. It is these sediments of former ages, the dust of lost continents, in which are preserved the majority of the fragmentary remains of plants and animals, the flotsam and jetsam of successive phases of evolution.

The crust of the earth, as Darwin wrote, 'with its imbedded remains must not be looked at as a well filled museum, but as a poor collection made at hazard and at rare intervals'(19). It is from this imperfect record that we seek to discover the relative antiquity of the several groups or genera of living plants, and in the structure of extinct types we endeavour to discover connecting links between divisions of the plant kingdom which in the course of evolution have retained little or no signs of a common descent.

Sir Joseph Hooker in a letter to Darwin in 1859 speaks of his 'conviction that we have not in a fossilised condition a fraction of the plants that have existed, and that not a fraction of those we have are recognisable specifically'(12). Considering the nature of the palaeontological documents the wonder is how much they have taught us, and we may look with confidence to the results of future research in a field of which the importance has only recently been appreciated. With the strata of sedimentary origin are frequently associated igneous rocks, and in many continental regions, as in the majority of oceanic islands, the crust of the earth consists wholly of volcanic material or of rocks produced by the gradual solidification of molten magmas. Rocks composed mainly of carbonate of lime, such as limestones and chalk, bear witness to ocean beds or to sediments deposited on the floors of inland seas beyond the reach of land detritus where coral reefs were reared or the shells and other calcareous skeletons of animals supplied the material for future land. In such rocks the remains of calcareous seaweeds are frequently recognisable and occasionally, as in the English chalk, fragments of wood testify to transport from a distant land.

While there is little difficulty in explaining the nature of much of the earth's crust, in several parts of the world the strata are totally unfossiliferous and closely simulate crystalline rocks. In many cases it is believed that such strata represent ancient sediments which in the course of ages have been reduced by metamorphic agencies to a condition which has obscured or entirely obliterated all traces of their pristine state.

Since the pioneer work of William Smith, who in the early days of the nineteenth century first realised the importance of fossils as aids to the determination of relative age, geologists have devoted themselves to the task of correlating the sedimentary rocks of the world, using as criteria the order of superposition of the strata and the nature of their organic remains. The result has been to classify portions of the earth's crust into periods or chapters, which together constitute a record of geological evolution as complete as it is possible to obtain from the available data. The accompanying table shows the order of sequence of the epochs, which stand for terms of years of a magnitude beyond our powers to grasp.

The division of geological history into larger and smaller periods does not imply the recurrence of sudden revolutions; it is in some measure dictated by considerations of convenience, but more particularly by our ignorance of certain stages in the history of the world due to the imperfection of the record.

GEOLOGICAL TABLE.

Showing the position in the Geological Series of the strata referred to in this volume.

In certain parts of the world, as for example the north-west Highlands of Scotland, the Malvern Hills, Scandinavia, and in many other regions in Europe and North America, geologists have recognised what they believe to be the foundation stones of the world. These Archaean rocks, which underlie the oldest fossiliferous strata, belong to a period of geological evolution from which it appears to be hopeless to obtain any light as to the nature of the contemporary organic world. The earliest vestiges of life so far discovered exhibit a high degree of organisation, which unmistakably points to their being links in a chain extending far beyond the limits of the oldest Cambrian strata in which recognisable fossils first occur. The rocks of the Cambrian and Ordovician epochs, as represented by the grits, shales, slates and other sedimentary strata in Wales, Shropshire, the Lake district and elsewhere, though in places rich in the remains of animals, afford no information in regard to the land vegetation. From the succeeding Silurian epoch very little evidence has been gleaned as to the nature of the flora, and it is not until we come to the sedimentary rocks of the Devonian era that records of plant-life occur in any abundance. The almost complete lack of botanical data in the pre-Devonian formations is in part due to the fact that these older rocks consist to a large extent of marine deposits formed under conditions unfavourable to the preservation of plants. That the land-surfaces of the older Palaeozoic eras supported an abundant vegetation there can be little doubt. The relics of plant-life furnished by the Devonian and succeeding formations represent the upper branching-systems of a deeply rooted and spreading tree, the lowest portions of which have been destroyed or have left no sign of their existence.

In descending the Geological series, we begin with superficial deposits, such as peat and river-gravels found subsequently to the underlying boulder-clay of the Glacial period. The remains of forest trees preserved in the peat and in submerged forests round the coast connect the vegetation of the historic period with that of the Neolithic age. At the base of the Pleistocene series, the name given to the latest chapter of geological history, we find evidence of the prevalence of arctic conditions in the widely spread boulder-clays and other deposits of the Glacial period.

From deposits of post-Glacial date abundant plant remains have been obtained, but we cannot say with any degree of certainty what proportion of these plants remained in Britain during the Ice age, and whether the greater part of the vegetation, the relics of which have been discovered in pre-Glacial beds, was destroyed or driven south by the advancing ice. We may briefly consider some of the more interesting facts brought to light by the investigation of the fossil plants in the Lower Pleistocene and Upper Tertiary beds. It is mainly to the researches of Mr Clement Reid into the vegetation of Britain immediately preceding the Glacial period, that our knowledge of this phase of the history of the British flora is due.

On the coast of Norfolk in the neighbourhood of Cromer the sections of the cliffs reveal the existence of a succession of sands, clays, and gravels underlying Glacial deposits; this material was probably laid down near the mouth of the ancient Rhine, which in the latter part of the Tertiary period flowed across a low area, which is now occupied by the shallow southern half of the North Sea(27). The plant-fragments found in these river-sediments indicate a temperate climate. Among the plants of this pre-Glacial flora are many familiar British species, such as Caltha palustris (marsh marigold), species of butter-cup, Stellaria holostea (greater stichwort) (Fig. 3, C), Bidens tripartita (bur-marigold) (Fig. 3, A), maple, hawthorn, the alder, hazel, the yew, Scots pine and numerous others. If, as is not improbable, these pre-Glacial plants were swept away by the subsequent arctic conditions, the great majority of them returned to their old home when a warmer climate ensued. There are however some pre-Glacial plants, such as the spruce fir (Picea excelsa), a cone of which is shown in Fig. 3, B, the water chestnut, Trapa natans (Fig. 4), and a few other species no longer represented in the British flora. The genus Trapa is a striking example of a flowering plant which has disappeared since the Tertiary period from many parts of Europe, including England, most of Sweden, and from several regions in northern Europe. It still grows in a few localities in Switzerland and in some of the Italian lakes. In pre-Glacial times the water chestnut was widely spread from Portugal and England in the west to Siberia in the east, and its hard four-pronged nuts have been recorded from many post-Glacial peat-moors in the north of Europe.

It may be remarked in passing that no satisfactory evidence has been discovered in Britain of the existence of man in this part of Europe in pre-Glacial days: it is, however, believed that flints from Tertiary strata on the continent show traces of human workmanship. As Sir Edwin Ray Lankester said in 1905, 'It is not improbable that it was in the remote period known as the Lower Miocene—remote as compared with the gravels in which Eoliths [primitive stone implements] occur—that Natural Selection began to favour that increase in the size of the brain of a large and not very powerful semi-erect ape'(28).

Though comparatively recent in terms of geological chronology, the remoteness, according to ordinary conceptions of time, of the Tertiary period is brought home to us when we endeavour to grasp the fact that it was during this chapter in the earth's history that some of our highest mountain-ranges, such as the Alps, the Carpathians, and Himalayas were formed by the uplifting of piles of marine sediments. From Tertiary strata in the Isle of Wight, on the Hampshire coast, and in the London basin numerous fossil plants have been obtained, which afford convincing evidence of climatic conditions much more genial than those of the present day. The presence of palm leaves and of a wealth of other sub-tropical plants in Lower Tertiary beds in England reveals the existence of a flora differing considerably both from that in the uppermost Tertiary beds of Norfolk and from the modern British flora, but closely allied to the present Mediterranean flora.

The basaltic columns of the Giants' Causeway and of the Staffa Cave, and the terraced rocks which form so characteristic a feature in the contours of the Inner Hebrides, are portions of lava-flows, which in the early days of the Tertiary period were poured out over a wide area of land stretching from the north-east of Ireland, through the Western isles of Scotland, the Faroë islands, to Iceland and Greenland. While in this northern region volcanic activity was being manifested on a stupendous scale, a shallow sea extended over part of what is now the south-east of England in which was deposited a considerable thickness of sedimentary material derived from the neighbouring land. In this upraised sea-floor, known as the London clay, which is exposed in the Isle of Sheppey and in many other localities, numerous fossil fruits and fragments of wood occur in association with marine shells. The fact that many of the fruits were ripe at the time of their entombment led some eighteenth century writers to assign an autumn date to the universal deluge. One of the Sheppey fruits may be mentioned as an especially interesting sample of the early Tertiary flora, namely the genus Nipadites, so named from the very close resemblance of the fossils to the fruits of the existing tropical plant Nipa. Nipa fruticans, sometimes described as a stemless palm because of the absence of the erect stem which is usually a characteristic feature of palms, grows in brackish estuaries of many tropical countries (Fig. 5, A): it has long leaves not unlike those of the date-palm and bears clusters of fruits as large as a man's head; a single fruit is two or three inches long and its hard fibrous shell is characterised by four or five longitudinal ribs (Fig. 5, B). The fruits of Nipa, which may be carried a considerable distance by ocean-currents without losing the power of germination, are constantly found with other vegetable drift on the beaches of tropical islands. The discovery of fruits of Nipa (or Nipadites), hardly distinguishable from those of the existing species, in Tertiary beds in England, Belgium, in the Paris basin, and in Egypt affords a striking instance of changes in the geographical distribution of an ancient plant now restricted to warmer regions.

While the higher members of the Cretaceous system, as seen in the chalk cliffs and downs, represent the upraised calcareous accumulations on the floor of a fairly deep and clear sea, the lower members testify to shallower water within reach of river-borne sand and mud. 'During the Chalk period,' as Huxley wrote, 'not one of the present great physical features of the globe was in existence. Our great mountain ranges, Pyrenees, Alps, Himalayas, Andes, have all been upheaved since the chalk was deposited, and the Cretaceous sea flowed over the sites of Sinai and Ararat'(29).

The Wealden strata, at the base of the Cretaceous system, as seen on the Sussex coast, in parts of the Isle of Wight, in the Weald district of Kent and neighbouring counties, point to the existence of a lake over a portion of the south of England and of the English Channel. The remains of a rich Wealden flora have been collected from these Wealden sediments, notably from the plant-beds of Ecclesbourne near Hastings, in which, so far as we know, flowering plants played no part or at most occupied a very subordinate position. A few fossil leaves have been described from rocks assigned to a Wealden age,—and from the older Stonesfield Slate, of Jurassic age, a single leaf is recorded,—which seem to be those of Dicotyledons; but it is certain that even in the early days of the Cretaceous period the present dominant group in the plant kingdom was in its infancy and in many regions probably unrepresented. When we glance at the geological table and consider that in all the floras from the Wealden down to the Devonian period, flowering plants played no part, we are able to appreciate the fact of their rapid development, referred to in a previous chapter, when once this highest type had become established.

The rocks comprised in the Jurassic system extend from East Yorkshire to the coast of Dorsetshire; they consist of a succession of limestones, clays, sandstones, and a few thin beds of impure coal. Sediments of this age also occur, though to a much less extent, on the north-east coast of Scotland and in a few places in the Inner Hebrides. Many of the Jurassic strata contain only marine shells, and corals are occasionally abundant, though in the lower members of the system in the cliffs near Lyme Regis and at Whitby fossil plants are fairly common. It is, however, from the middle Jurassic beds, in the cliffs between Whitby and Scarborough, and in some inland quarries in East Yorkshire, that we have obtained the richest Jurassic flora. Rivers from a northern land laden with sediment and carrying drift-wood, leaves and other plant fragments, deposited their burden in an estuary which occupied the eastern edge of Yorkshire. Sedimentary rocks laid down towards the close of the Jurassic period in the island of Portland in the south and on the Sutherland coast in the north have furnished valuable records of plant-life.

The passage from the Jurassic to the underlying Triassic system is formed by some shales and limestones in South Wales containing remains of fish and other marine organisms. These so-called Rhaetic beds are poorly represented in the British area, but on the continent of Europe and in other regions the sediments of this age bulk much more largely and have yielded a rich collection of plants. The rocks of the upper division of the Triassic system, as seen in the Midlands, point to the prevalence of desert conditions; and in the grooved sand-polished surfaces of granite in Charnwood forest we have a glimpse of a Triassic landscape. The salt-bearing strata of this period in Cheshire and Worcestershire suggest conditions paralleled at the present day in the Caspian and Dead-Sea regions. The vegetation of Britain, and indeed of the world as a whole, seems to have undergone but little change during the enormous lapse of time represented by the sediments comprised between the Wealden and Triassic periods. The Lower Triassic flora affords evidence of a change in the facies of the vegetation and prepares us for the still greater differences revealed by a study of the Permian and Carboniferous floras. To the student of evolution these Palaeozoic floras are of special interest on account of the facts they have contributed in regard to the descent and inter-relationship of different branches of the vegetable kingdom.

It is by a patient study of the waifs and strays of the vegetation of successive phases of the world's history preserved in sedimentary strata, that it has been possible to follow the history of many existing plants and to establish links between the present and the past.

CHAPTER IV

PRESERVATION OF PLANTS AS FOSSILS

'Some whim of Nature locked them fast in stone for us afterthoughts of Creation.'   Lowell.

The failure of the earlier naturalists to grasp the true significance of fossils or even to appreciate their nature is an extraordinary fact when we consider the pioneer work which they accomplished in biological and geological science. The following extract from the writings of so enlightened a man as John Ray serves to illustrate an almost incredible disinclination to admit what seems to us the obvious. He wrote:—'Yet I must not dissemble that there is a Phenomenon in Nature, which doth somewhat puzzle us to reconcile with the prudence observable in all its work, and seems strongly to prove, that Nature doth sometimes ludere, and delineates figures, for no other end, but for the ornament of some stone, and to entertain or gratify our curiosity, and exercise our wits. This is, those elegant impressions of leaves and plants upon cole-slate, the knowledge of which, I must confess myself to leave to my learned and ingenious friend Mr Edward Lhwyd of Oxford.... He told me that Mr Woodward, a Londoner, shewed him very good draughts of the common female fern, naturally formed in cole.... But these figures are more diligently to be observed and considered... Dr Woodward will have them to be the impressions of the leaves of plants which were there lodged at the time of the Deluge'(31).

The Mr Woodward alluded to by Ray thus expressed his views on fossils in an Essay towards the Natural History of the Earth:—'The whole terrestrial globe was taken all to pieces and dissolved at the Deluge, the particles of stone, marble, and all solid fossils dissevered, taken up into the water, and then sustained together with sea shells and other animal and vegetable bodies; the present earth consists and was formed out of that promiscuous mass of sand, earth, shells, and the rest falling down again, and subsiding from the water'(32).

In the later part of the seventeenth-century Steno, a Dane by birth and Professor of Anatomy at Padua, by his recognition of the identity of the teeth in a shark's head, which he had dissected, with some fossils from Malta known as Glossopetrae, established the true nature of fossils. He also recognised a certain orderly sequence in fossiliferous strata, and in the opinion of Professor Sollas he is entitled to be considered the 'Father and Founder' of Geology(33).

It was by slow degrees that the early observers freed themselves from the obsession that the remains of animals and plants in the earth's crust bear witness to a Universal Deluge and are all identical with existing species. The possibility that some of the fossil plants in English strata might be more clearly related to forms now met with in warmer regions was gradually realised. The publication of the Origin of Species stimulated palaeontological research, and botanists as well as zoologists turned to the investigation of extinct genera in search of proofs of the doctrine of evolution.

The common occurrence of petrified wood in rocks of different ages is well known. Fossil stems are occasionally found in their natural position of growth, the structural details being rendered permanent by the deposition of siliceous or calcareous material from water drawn by capillarity into the dead but still sound tissues. Petrified wood from Upper Jurassic beds is abundant in the Island of Purbeck; an unusually long piece of stem may be seen in the small town of Portland fixed to the wall of a house. Some of these stems have been referred by an American author to the Araucarian family of Conifers, but the structure is as a rule hardly well enough preserved to afford satisfactory evidence for identification. In his Testimony of the Rocks, Hugh Miller speaks of fossil wood from the upper beds of the Jurassic system in sufficient abundance on the beach at Helmsdale in Sutherlandshire to be collected in cart-loads; it is still easy to pick up good specimens on the shingle beach a short distance north of Helmsdale, and a recent microscopical examination showed that some specimens are pieces of an Araucarian tree.

Impressive examples of petrified trees on a large scale are to be seen in the United States, in Arizona and the Yellowstone Park. (Frontispiece.) In the northern part of Arizona the country for over an area of 10 square miles is covered with tree trunks, some reaching 200 feet in length and a diameter of 10 feet. The nature of the mineralising substance has given rise to the name Chalcedony Park for this Triassic forest(34). A striking example of one of the Arizona trees is exhibited in the British Museum and in a neighbouring case is a splendid petrified stem, 9 ft. in height, of a conifer discovered in Tertiary lavas in Tasmania(35).

Figure 6 illustrates the preservation of a series of forests of Tertiary age in the mass of volcanic sediments, 2000 feet in thickness, known as Amethyst mountain, in the Yellowstone Park district. By the weathering away of the surrounding volcanic material the tall stems of the trees are exposed in places on the mountain sides like the 'columns of a ruined temple.' The height of the river at the foot of the cliff is 6700 ft. above sea-level and the mountain rises to a height of 9400 ft. above the sea. In the lower part of the section the volcanic strata are seen to rest on a foundation of older rocks A, and these in turn were laid down on the eroded surface of a still more ancient foundation, B(36).

The section as a whole affords a striking demonstration of the magnitude of earth-movements since the last of these forests was buried below the surface of a sea in which the volcanic material was deposited. The account of the Yellowstone Park section recalls Darwin's description in the Naturalist's Voyage(37) of snow-white columns projecting from a bare slope, at an altitude of 7000 ft. in the Cordillera.

The abundance of drift-wood on the coasts of some countries at the present day helps us to picture the conditions under which the remains of former forests have been preserved. In his Letters from High Latitudes, Lord Dufferin gives the following description of drift-wood on the shores of Spitzbergen:—'A little to the northward, I observed, lying on the sea-shore innumerable logs of drift-wood. This wood is floated all the way from America by the Gulf Stream, and as I walked from one huge bole to another, I could not help wondering in what primeval forest each had grown, what chance had originally cast them on the waters, and piloted them to this desert shore'(38). A photograph reproduced in Amundsen's book on The North West Passage shows the beach on the Alaskan coast strewn with drifted timber(39). For the accompanying photograph (Fig. 7) of the flood-plain of the Colorado River(40), I am indebted to Professor MacDougal of the Desert Research Laboratory at Tucson, Arizona, who in a recent letter writes, 'During times of high-water a thin sheet of flood covers the flat for many miles and bears drift-wood so thickly that it is difficult to push a boat through it.' The drift-wood consists of poplar, willow, pine, and juniper, 'the last two have been brought from the upper river, from as far away as a thousand miles.' A picture such as this affords an admirable example of the wealth of material available for preservation in a fossil state.

It is only in the minority of cases that the accidents of preservation of fragments of ancient floras have given us the means of investigating the internal structure of the plant organs. It is far more frequently the case that fossil plants are represented only by a carbonised film on the surface of a piece of shale or other rock: the actual substance of the plant has been converted into a thin layer of coal, and though the venation and other surface-features may be clearly revealed, the internal tissues have been destroyed. If a lump of clay containing a piece of fern frond is heated, the result is an impression of the leaf on the hardened matrix and a coaly substance in place of the plant substance. It is occasionally possible by detaching a piece of the black film from a fossil, and heating it with nitric acid and chlorate of potash and then dipping it in ammonia, to obtain a transparent preparation suitable for microscopical examination of the cell-outlines of the superficial layer of the leaf or other plant-fragment. This method of examination, used by several students of fossil plants and with conspicuous success by Professor Nathorst of Stockholm, often affords valuable aids to identification.

Pieces of plants embedded in sandy sediment, if not preserved by petrifaction, that is by the introduction into the tissues of some siliceous or calcareous solution, gradually decay and their fragmentary remains may be washed away by percolating water, leaving a hollow mould in the gradually hardening sediment, which is afterwards filled with sand or other material. The plant itself is destroyed, but a cast is taken which in the case of fine-grained sediments reproduces the form and surface-pattern of the original specimen. The incrustation of plants by the falsely named petrifying springs of Knaresborough and other places illustrate another method of fossilisation.

Plants which owe their preservation to amber occur both as incrustations and petrifactions. This fossil resin occurs in Tertiary, Cretaceous, and Jurassic rocks; the amber found in abundance on the Baltic coast near Danzig and occasionally picked up on the beach in Norfolk and Suffolk comes from beds of Tertiary age. Pieces of Pine-wood have been described from the Baltic beds in which the tissues are perfectly preserved as the result of the conversion into amber of the resinous secretion which permeates their cells: in this case the amber is a petrifying agent. More frequently the preservation is due to incrustation; as resin trickled down the stems of the Tertiary pines from an open wound, flowers and leaves, blown by the wind on to the sticky surface, were eventually sealed up in a translucent case of amber. Though the actual substance may have gone, the mould which remains exhibits in wonderful perfection each separate organ of a flower or the delicate hair-clusters on the surface of a leaf. The flower represented in Fig. 8, a species of Cinnamon, is one of several specimens described by the authors of a monograph of Tertiary plants in the Baltic amber(41).

The fragments of plants preserved in nodules of calcareous rock occasionally met with in some of the Lancashire and Yorkshire coal-seams are perhaps the most striking examples of the possibilities of petrifaction. By cutting sections of these nodules and grinding them to a transparent thinness, the most delicate tissues of Carboniferous plants are rendered accessible to investigation under the high power of a microscope. As our attention is absorbed by the examination of the details of cell-structure it is easy to forget that the section has not been cut from a living plant, but from the twig of a tree which grew in the forests of the Coal age. The preservation is such as to enable us not only to describe the anatomy of these extinct types of vegetation, but, by the application of the knowledge of the relation between the structure of the plant-machine and its functions gained by a study of living species, it is possible in some degree to picture the plants of the Coal period as living organisms and to see in the structural framework a reflection of external environment. The recognition in the general architectural plan of the Palaeozoic plants, as in many of the finer anatomical features, of the closest resemblance to plants of the modern world produces an almost overwhelming sense of continuity between the past and the present.

The plants of the Palaeozoic period, though often differing considerably from those of the same class in the floras of to-day, exhibit a remarkably high type of organisation. Some of the most abundant trees in the forest of the Coal age are decidedly superior in the complexity of their structure, as also in size, to modern survivals of the same stock. On the other hand, it must be remembered that Monocotyledons and Dicotyledons which now occupy the highest place in the hierarchy of plants have left no sign of their existence in any of the Palaeozoic strata. The greater size of some of the Palaeozoic plants, and in some respects the more advanced stage of evolution which they represent as compared with their nearest relatives of the present era, must be considered in relation to their more important and relatively higher position in the plant-world than that which is now held by their diminutive descendants. It is, however, impossible to get away from the conclusion that the oldest Palaeozoic flora of which we have an intimate knowledge must be the product of development of an age which is represented by a chapter in the history of the plant kingdom at least as far removed from the beginning as it is separated from the chapter now being written. Examples might be quoted in illustration of the risks attending the determination of fossils by means of external features alone, but it may suffice to mention the case of a specimen originally described as a fragment of a Cretaceous Dinosaur under the name Aachenosaurus multidens. By the examination of thin sections this supposed bone was shown to be a piece of Dicotyledonous wood(42). The methods of preservation of plants as fossils are numerous and varied and the few examples selected give but an incomplete idea of the subject: for a fuller treatment of fossilisation the reader is referred to more technical treatises (48 vol. I.).

The employment of fossil plants as 'Thermometers of the ages' is a branch of Palaeobotany to which a passing allusion may be permitted though it is only indirectly connected with the main question. As one of the most interesting examples of changed climatic conditions revealed by a study of fossil plants, reference may be made to the wealth of material collected within the Arctic circle. The problems suggested by the discovery of plants in rocks of various ages in North Siberia, Spitzbergen, Franz Josef Land, Bear Island, Greenland, and in many other localities in the far north are too difficult and far-reaching to be discussed in these pages. In the Cretaceous and Tertiary strata of the west coast of Greenland and Disco Island from 69° to 72° north latitude, to refer only to one case, a great number of plants have been obtained by several of the earlier Arctic explorers and more recently by members of one of the Peary Expeditions. At the present day on the fringe of land on the western edge of Greenland which is not permanently covered with ice, a considerable number of herbaceous plants are able to exist and to produce seed during their concentrated period of development; while trees are represented only by a few low-growing shrubs such as the dwarf Juniper. In places accessible to investigation beyond the ice-covered hills of northern Greenland the rocks have been shown to consist of Cretaceous and Tertiary sediments containing fossil plants associated with seams of coal. From these beds numerous Dicotyledons have been obtained, some of them almost identical with living species characteristic of sub-tropical or tropical countries. In the lowest of the Cretaceous series no Dicotyledons have been found, but flowering plants are abundant in the higher Cretaceous rocks. Allowing for the fact that closely allied species are often able to live under very different climatic conditions, there can be no doubt that the Cretaceous and Tertiary floras of Greenland indicate an average temperature considerably higher than that which now prevails in the warmest parts of the British Isles.

In the far south a fairly rich Jurassic flora has recently been discovered by the members of a Swedish Antarctic expedition in Graham's Land in latitude 63°·15 S. and longitude 57° W., which in its general facies bears a close resemblance to the Jurassic flora of Yorkshire.

Although the great majority of the records of ancient plants are difficult to interpret by reason of imperfect preservation and because of the frequent separation of leaves, stems, and reproductive organs, the student who tries to piece together the disjecta membra of the floras of the past shares the opinion expressed by the late Marquis of Saporta,—'Si l'on s'attache à les déchiffrer, on oublie bien vite la singularité des caractères, et le mauvais état des pages. La pensée se lève, les ideés se développent, le manuscrit se déroule; c'est la tombe qui parle et livre son secret.'

CHAPTER V

FERNS; THEIR DISTRIBUTION AND ANTIQUITY

'It has been shown that certain forms persist with very little change, from the oldest to the newest fossiliferous formations; and thus show that progressive development is a contingent, and not a necessary, result of the nature of living matter.'   Huxley.

The Ferns as a whole represent a section of the vegetable kingdom which traces its ancestry as far into the past as any group of plants. Impressions of leaves on the shales of the Coal-measures and on rocks of the earlier Devonian period are hardly distinguishable in form and in the venation and shape of the leaflets from the finely divided fronds of modern ferns. Until a few years ago these Palaeozoic fossils were generally regarded as true ferns, and it was believed that ferns played a conspicuous part in the vegetation of the earliest periods, of which we have any botanical knowledge. Conclusions based on external form must frequently be revised in the light of more trustworthy evidence. It was shown in the later part of the nineteenth century by the late Professor Williamson of Manchester, whose researches into the plants of the Coal age shed a flood of light on the ancestry and inter-relationship of many existing plants, that some of the fern-like leaves which have long been familiar to those who search among the shales of the refuse heaps of collieries, were borne on stems differing in anatomical features from those of any known fern. The investigation of the structure of the leaves and their supporting stems led to the recognition of certain extinct genera of Palaeozoic plants of exceptional interest, to which the term generalised type is aptly applied. Associated with anatomical and other characters such as we now regard as the attributes of ferns, these plants exhibit other features not met with in modern ferns but characteristic of a group of seed-bearing plants known as the Cycads. Recent research has revealed the existence of several such generalised types which, by their combination of characters now met with in distinct sub-divisions of the plant-kingdom, clearly indicate the derivation of Ferns, and Cycads as we know them to-day, from a common stock. It was in the first instance by means of anatomical evidence—obtained by the microscopical examination of sections of petrified fragments of stems and leaves—that the generalised nature of these Palaeozoic plants was recognised. Nothing was known as to the reproductive organs. Ferns as now represented in the floras of the world are essentially seed-less plants. As the author of Hudibras wrote: