THE GENESIS AND MIGRATIONS OF PLANTS.

If, for convenience of reference, we divide the whole history of the earth, from the time when a solid crust first formed on its surface and began to be ridged up into islands or mountains in the primeval ocean, into four great periods, we shall find that each can be characterized by some features in relation to the world of plants.

That Archean age, in which the oldest known beds of rocks were produced—rocks now greatly crumpled by the first movements of the thin crust, and hardened and altered by heat and pressure has, it is true, little to tell us. But, as elsewhere stated, even it has beds of Carbon in the form of Graphite—veritable altered coal seams—which the analogy of later formations would lead us to believe must have been accumulated by the growth of plants. This growth is indeed the only known cause capable of producing such effects. If we should ever be fortunate enough to find beds of the Laurentian series in an unaltered state, we may hope to know something of this old flora. Nor need we be surprised if it should prove of higher grade and more noble development than we should at first sight anticipate. If there ever was a time when vegetation alone possessed the earth, and when there were no animals to devour or destroy it, we might expect to find it in its first and best estate, perhaps not comparable in variety and complexity of parts with the flora of the modern world, but grand in its luxuriance and majesty. Of such discoveries, however, we have no certain indication at present.

If such a primeval flora as that above indicated ever existed, it must have perished utterly before the incoming of the next great age of the world—that known as the PalÆozoic, whose rocks are surpassingly rich in the remains of animals, especially those of the lower or invertebrate classes and those that inhabit the waters.

In the oldest PalÆozoic rocks we find no plants certainly terrestrial, but abundance of AlgÆ or seaweeds, and some gigantic members of the vegetable kingdom which seem to have been trees, with structures more akin to those of aquatic than to those of land plants.[82] At a somewhat early stage, however, in the rocks of this period, we discover a few undoubted land plants.[83] These seem to be allied to the modern Club mosses and to their humble relations, the pillworts[84] and other small plants of similar structure found in ponds and swamps. Some of them, indeed, appear to be intermediate between these groups. All these plants are Cryptogams, or destitute of true flowers, but do not belong to the lowest forms of that type. Thus, so far as we know, plant life on the land began possibly with certain large trees of algoid structures, and more certainly with the club mosses and pillworts and their allies, and these last in the form of species not tree-like in dimensions, but of very moderate size. The structures of these plants are already sufficiently well known to inform us that the plan and functions of the root, stem and leaf, and of spores and spore case were set up; and that the structures and functions of vegetable cells, fibres and some kinds of vessels were perfected, and all the apparatus introduced necessary for the fertilization and reproduction of plants of some degree of complexity. At the same time, the peculiar structures of the higher AlgÆ were brought to a pitch of perfection not surpassed if equalled in modern times, and which may have enabled plants so constructed to exist even on the land.

From these beginnings in the early PalÆozoic, the progress of the vegetable kingdom went on, until, in the later parts of that great period, the Devonian and Carboniferous eras, it culminated in those magnificent forests which have left so many interesting remains, and which accumulated the materials of our great beds of coal. In these the families of the Club mosses, the Ferns and the Mare's-tails attained to a perfection in structure and size altogether unexampled in the modern world, and may be said to have overspread the earth almost to the exclusion of other trees. Here, however, two new families come in of higher grade, and leading the way to the flowering plants. These are the Pines and their allies and the Cycads, and certain intermediate forms, neither Pines nor Cycads, but allied to both.[85] This wonderful flora, which we have now the materials to reproduce in imagination almost in its entirety, decays and passes away in the Permian system, the last portion of the PalÆozoic, and in entering into the third great period of the earth's history—the Mesozoic, we again find an almost entire change of vegetation. Here, however, we are able to understand something of the reasons of this. The PalÆozoic floras seem to have originated in the North, and propagated themselves southward till they replenished the earth, and they were favoured by the existence at that time of vast swampy flats extending over great areas of the yet imperfectly elaborated continents. The Mesozoic floras, on the other hand, seem to have been of Southern or equatorial origin, and to have followed up the older vegetation as it decayed and disappeared, or retreated in its old age to its northern home. There is, of course, much in all this that we do not understand, but the general fact seems certain.

The early Mesozoic is altogether peculiar. It shows a vast predominance of Cycads, Pines and Ferns, to the exclusion both of the gigantic Cryptogams of the PalÆozoic and of the ordinary exogenous trees of the modern time. It has a strange, weird aspect, and more resembles that of some warm islands of the southern hemisphere at present, than anything else known to us. It is as if the flora of some southern island had migrated and invaded all parts of the world. The geographical and climated conditions which permitted this must have been of a character different from those both of earlier and later times.

As we approach to the termination of the Mesozoic, which, in regard to animal life, is the age of reptiles, a new and strange development meets us. We find beds filled with leaves of broad-leaved plants similar to those of our modern woods, and in most cases apparently belonging to the same genera with plants now living, and this new type of vegetation persists to the present, though with marked differences of species in successive eras, as in the Middle and Upper Cretaceous, and the Lower, Middle and Upper Kainozoic, or Tertiary. It is noteworthy that while this new vegetation not only altogether supersedes the great Cryptogamous forests of the PalÆozoic, but replaces the Cycads of the immediately preceding eras, the Pines retain all their prominence and grandeur, and even seem to excel in number of species, in breadth of dispersion, and in magnitude of growth their successors in the present world.

While in the latter Cretaceous and Early Tertiary, the northern hemisphere at least seems to have enjoyed an exceptionally warm climate, the later Tertiary introduces that period of cold known as the Glacial age. While there is no doubt that the intensity of this glaciation has been greatly exaggerated by extreme glacialists, and while it is certain that some vegetation, and this not altogether of Arctic types, continued to exist throughout this period, even in the now temperate regions of our continents, it is evident that a great reduction of the exuberance of the flora occurred by the removal of many species, and that the present flora of the northern hemisphere is inferior in variety and magnificence to that of the Middle Tertiary, just as it is found that the Mammalian fauna of our continents has since that time been reduced both in the number and magnitude of its species.

If the reader has followed this general sketch, he will be prepared to appreciate some examples of a more detailed character relating to the floras of different periods, and some discussions of general points relating to the genesis and vicissitudes of the vegetable kingdom.

The origination of the more important floras which have occupied the northern hemisphere in geological times, not, as one might at first sight suppose, in the sunny climates of the South, but under the arctic skies, is a fact long known or suspected. It is proved by the occurrence of fossil plants in Greenland, in Spitzbergen, and in Grinnell Land, under circumstances which show that these were their primal homes. The fact bristles with physical difficulties, yet is fertile of the most interesting theoretical deductions, to reach which we may well be content to wade through some intricate questions. Though not at all a new fact, its full significance seems only recently to have dawned on the minds of geologists, and within recent years it has produced a number of memoirs and addresses to learned societies, besides many less formal notices.[86]

The earliest suggestion on this subject known to the writer is that of my old and dear friend, Professor Asa Gray, in 1867, with reference to the probable northern source of the related floras of North America and Eastern Asia. With the aid of new facts disclosed by Heer and Lesquereux, Gray returned to the subject in 1872, and more fully developed this conclusion with reference to the Tertiary floras,[87] and still later he further discussed these questions in an able lecture on "Forest Geography and ArchÆology."[88] In this he puts the case so well and tersely that I may quote the following sentences as a text for what follows:—

"I can only say, at large, that the same species (of Tertiary fossil plants) have been found all round the world; that the richest and most extensive finds are in Greenland; that they comprise most of the sorts which I have spoken of, as American trees which once lived in Europe—Magnolias, Sassafras, Hickories, Gum-trees, our identical Southern Cypress (for all we can see of difference), and especially Sequoias, not only the two which obviously answer to the two Big-trees now peculiar to California, but several others; that they equally comprise trees now peculiar to Japan and China—three kinds of Gingko-trees, for instance, one of them not evidently distinguishable from the Japan species which alone survives; that we have evidence, not merely of Pines and Maples, Poplars, Birches, Lindens, and whatever else characterize the temperate-zone forests of our era, but also of particular species of these, so like those of our own time and country, that we may fairly reckon them as the ancestors of several of ours. Long genealogies always deal more or less in conjecture; but we appear to be within the limits of scientific inference when we announce that our existing temperate trees came from the north, and within the bounds of high probability when we claim not a few of them as the originals of present species. Remains of the same plants have been found fossil in our temperate region, as well as in Europe."

Between 1860 and 1870 the writer was engaged in working out all that could be learned of the Devonian plants of Eastern America, the oldest known flora of any richness, and which consists almost exclusively of gigantic, and to us grotesque, representatives of the Club mosses, Ferns, and Mare's-tails, with some trees allied to the Cycads and Pines. In this pursuit nearly all the more important localities were visited, and access was had to the large collections of Professor Hall and Professor Newberry in New York and Ohio, as well as to those of the Geological Survey of Canada, and to those made in the remarkable plant-bearing beds of St. John, New Brunswick, by Messrs. Matthew and Hartt. In the progress of these researches, which developed an unexpectedly rich assemblage of species, the northern origin of this old flora seemed to be established by its earlier culmination in the north-east, in connection with the growth of the American land to the southward, which took place after the great Upper Silurian subsidence, by elevations which began in the north, while those portions of the continent to the south-west still remained under the sea.

When, in 1870, the labours of those ten years were brought before the Royal Society of London, in the Bakerian Lecture of that year, and in a memoir illustrating no less than one hundred and twenty-five species of plants older than the great Carboniferous system, these deductions were stated in connection with the conclusions of Hall, Logan, and Dana, as to the distributions of sediment along the north-east side of the American continent, and the anticipation was hazarded that the oldest PalÆozoic floras would be discovered to the north of Newfoundland. Mention was also made of the apparent earlier and more copious birth of the Devonian flora in America than in Europe, a fact which is itself connected with the greater northward extension of this continent.

Unfortunately the memoir containing these results was not published by the Royal Society, and its publication was secured in a less perfect form only in the reports of the Geological Survey of Canada. The part of the memoir relating to Canadian fossil plants, with a portion of the theoretical deductions, was published in a report issued in 1871.[89] In this report the following language was used:—

"In Eastern America, from the Carboniferous period onward, the centre of plant distribution has been the Appalachian chain. From this the plants and sediments extended westward in times of elevation, and to this they receded in times of depression. But this centre was non-existent before the Devonian period, and the centre of this must have been to the north-east, whence the great mass of older Appalachian sediment was derived. In the Carboniferous period there was also an eastward distribution from the Appalachians, and links of connection in the Atlantic bed between the floras of Europe and America. In the Devonian such connection can have been only far to the north-east. It is therefore in Newfoundland, Labrador, and Greenland that we are to look for the oldest American flora, and in like manner on the border of the old Scandinavian nucleus for that of Europe."

"Again, it must have been the wide extension of the sea of the Corniferous limestone that gave the last blow to the remaining flora of the Lower Devonian: and the re-elevation in the middle of that epoch brought in the Appalachian ridges as a new centre, and established a connection with Europe which introduced the Upper Devonian and Carboniferous floras. Lastly, from the comparative richness of the later Erian[90] flora in Eastern America, especially in the St. John beds, it might be a fair inference that the north-eastern end of the Appalachian ridge was the original birthplace or centre of creation of what we may call the later PalÆozoic flora, or a large part of that flora."

When my paper was written I had not seen the account published by the able Swiss palÆobotanist Heer, of the remarkable Devonian flora of Bear Island, near Spitzbergen.[91] From want of acquaintance with the older floras of America and Western Europe, Heer fell into the unfortunate error of regarding the Bear Island plants as Lower Carboniferous, a mistake which his great authority has tended to perpetuate, and which has even led to the still graver error of some European geologists, who do not hesitate to regard as Carboniferous the fossil plants of the American deposits from the Hamilton to the Chemung groups inclusive, though these belong to formations underlying the oldest Carboniferous, and characterized by animal remains of unquestioned Devonian age. In 1872 I addressed a note to the Geological Society of London on the subject of the so-called "Ursa stage" of Heer, showing that though it contained some forms not known at so early a date in temperate Europe, it was clearly Devonian when tested by North American standards; but that in this high latitude, in which, for reasons stated in the report above referred to, I believed the Devonian plants to have originated, there might be an intermixture of the two floras. But such a mixed group should in that latitude be referred to a lower horizon than if found in temperate regions.

Between 1870 and 1873 my attention was turned to the two sub-floras intermediate between those of the Devonian and the coal formation, the floras of the Lower Carboniferous (Sub-carboniferous of some American geologists) and the Millstone Grit, and in a report upon these[92] similar deductions were expressed. It was stated that in Newfoundland and Northern Cape Breton the coal formation species come in at an early part of that period, and as we proceed southward they belong to progressively newer portions of the Carboniferous system. The same fact is observed in the coal beds of Scotland, as compared with those of England, and it indicates that the coal formation flora, like that of the Devonian, spread itself from the north, and this accords with the somewhat extensive occurrence of Lower Carboniferous rocks and fossils in the Parry Islands and elsewhere in the Arctic regions.[93]

Passing over the comparatively poor flora of the earlier Mesozoic, consisting largely of cycads, pines, and ferns, which, as we have seen, is probably of southern origin, and is as yet little known in the arctic, though represented, according to Heer, by the supposed Jurassic flora of Cape Boheman, we find, especially at KomÉ and AtanÉ in Greenland, an interesting occurrence of those earliest precursors of the truly modern forms of plants which appear in the Cretaceous, the period of the English chalk, and of the New Jersey greensands. There are two plant groups of this age in Greenland, one, that of KomÉ consists almost entirely of ferns, cycads, and pines, and is of decidedly Mesozoic aspect. This was regarded by Heer as Lower Cretaceous. The other, that of AtanÉ, holds remains of many modern temperate genera, as Populus, Myrica, Ficus, Sassafras, and Magnolia. This he regards as Middle Cretaceous. Above this is the Patoot series, with many exogenous trees of modern genera, and representing the Upper Cretaceous. Resting upon these Upper Cretaceous beds, without the intervention of any other formation,[94] are beds rich in plants of much more modern appearance, and referred by Heer to the Miocene period, a reference which appeared at the time to be warranted by comparison with the Tertiary plants of Europe, but, as we shall see, not with those of America. Still farther north this so-called Miocene assemblage of plants appears in Spitzbergen and Grinnell Land; but there, owing to the predominance of trees allied to the spruces, it has a decidedly more boreal character than in Greenland, as might be anticipated from its nearer approach to the pole.[95]

If now we turn to the Cretaceous and Tertiary floras of Western America, as described by Lesquereux, Newberry, and Ward, we find in the lowest Cretaceous rocks known there until very recently—those of the Dakota group, which may be in the lower part of the Middle Cretaceous—a series of plants[96] essentially similar to those of the Middle Cretaceous of Greenland. To these I have been able to add, through the researches of Mr. Richardson and Dr. G. M. Dawson, a still earlier flora, that of the Kootanie and Queen Charlotte Island formations, as old as the Gault and Wealden. It wants the broad-leaved plants of the Dakota, and consists mainly of pines, cycads, and ferns; and only in its upper part contains a few forerunners of the exogens.[97] These plants occur in beds indicating shallow sea conditions as prevalent in the interior of America, causing, no doubt, a warm climate in the north. Overlying this plant-bearing formation we have an oceanic limestone (the Niobrara), corresponding in many respects to the European chalk, and containing similar microscopic organisms. This extends far north into the British territory,[98] indicating farther subsidence and the prevalence of a vast Mediterranean Sea, filled with warm water from the equatorial currents, and not invaded by cold waters from the north. This is succeeded by Upper Cretaceous deposits of clay and sandstone, with marine remains, though very sparsely distributed; and these show that further subsidence or denudation in the north had opened a way for the arctic currents, producing a fall of temperature at the close of the Cretaceous, and partially filling up the Mediterranean of that period.

Of the flora of the Middle and Upper Cretaceous periods, which must have been very long, we know something in the interior regions through the plants of Dunvegan and Peace River;[99] and on the coast of British Columbia we have the remarkable Cretaceous coal field of Vancouver's Island, which holds the remains of plants of modern genera, including species of fan palm, ginkgo, evergreen oak, tulip tree, and other forms proper to a warm temperature or subtropical climate. They probably indicate a warmer climate as then prevalent on the Pacific coast than in the interior, and in this respect correspond with a meagre transition flora, intermediate between the Cretaceous and Eocene or earliest Tertiary of the interior regions, and named by Lesquereux the Lower Lignitic.

Immediately above these Upper Cretaceous beds we have the great Lignite Tertiary of the west—the Laramie group of recent American reports[100]—abounding in fossil plants, proper to a temperate climate, at one time regarded as Miocene, but now known to be Lower Eocene.[101] These beds, with their characteristic plants, have been traced into the British territory north of the forty-ninth parallel, and it has been shown that their fossils are identical with those of the McKenzie River Valley, described by Heer as Miocene, and probably also with those of Alaska, referred to the same age.[102] Now this truly Eocene flora of the temperate and northern parts of America has so many species in common with that called Miocene in Greenland, that its identity can scarcely be doubted. These facts have led me to doubt the Miocene age of the upper plant-bearing beds of Greenland, and more recently Mr. J. Starkie Gardner has shown from comparison with the Eocene flora of England and other considerations, that they are really of that earlier date.[103]

In looking at these details, we might perhaps suppose that no conditions of climate could permit the vegetation of the neighbourhood of Disco in Greenland to be identical with that of Colorado and Missouri, at a time when little difference of level existed in the two regions. Either the southern flora migrated north in consequence of a greater amelioration of climate, or the northern flora moved southward as the climate became colder. The same argument, as Gardner has ably shown, applies to the similarity of the Tertiary plants of temperate Europe to those of Greenland. If Greenland required a temperature of about 50°, as Heer calculates, to maintain its "Miocene" flora, the temperature of England must have been at least 70°, and that of the south-western States still warmer. It is to be observed, however, that the geographical arrangements of the American land in Cretaceous and early Eocene times, included the existence of a great inland sea of warm water extending at some periods as far north as the latitude of 55°, and that this must have tended to much equality of climatical conditions.

We cannot certainly affirm anything respecting the origin and migrations of these floras, but there are some probabilities which deserve attention. The ferns and cycads of the so-called Lower Cretaceous of Greenland are nothing but a continuation of the previous Jurassic flora. Now this was established at an equally early date in the Queen Charlotte Islands,[104] and still earlier in Virginia.[105] The presumption is, therefore, that it came from the south. It has indeed the facies of a southern hemisphere and insular flora; and probably spread itself northward as far as Greenland at a time when the American land was long, narrow, and warm, and when the ocean currents were carrying tepid water far toward the arctic regions. The flora which succeeds this in the sections at AtanÉ and Patoot has no special affinities with the southern hemisphere, and is of a warm temperate and continental character. It is very similar in its general aspect to that of the Dakota group farther to the south, and this is probably Middle Cretaceous. This flora must have originated either somewhere in temperate America, or within the arctic circle, and it must have replaced the older one by virtue of increasing subsidence and gradual change of climate. It must therefore have been connected with the depression of the land which took place in the course of the Cretaceous. During this movement it spread over all Western America, and as the land again arose from the sea of the Niobrara chalk, it assumed an aspect more suited to a cool climate, or moved southward, and finally abandoned the Arctic regions, perhaps continuing to exist on the Pacific coast, and in sheltered places in the north, till the warm inland seas of the Upper Cretaceous had given place to the wide plains and land-locked brackish seas or fresh-water lakes of the Laramie period (Eocene). Thus the true Upper Cretaceous marks in the interior a cooler period intervening between the Middle Cretaceous and the Lower Eocene floras of Greenland.

This latter established itself in Greenland, and probably all around the Arctic circle, in the mild period of the earliest Eocene, and as the climate of the northern hemisphere became gradually reduced from that time till the end of the Pliocene, it marched on over both continents to the southward, chased behind by the modern arctic flora, and eventually by the frost and snow of the Glacial age. This history may admit of correction in details; but, so far as present knowledge extends, it is in the main not far from the truth.

Perhaps the first great question which it raises is that as to the causes of the alternations of warm and cold climates in the north, apparently demanded by the vicissitudes of the vegetable kingdom. Here we may set aside the idea that in former times plants were suited to endure greater cold than at present. It is true that some of the fossil Greenland plants are of unknown genera, and many are new species to us; but we are on the whole safe in affirming that they must have required conditions similar to those necessary to their modern representatives, except within such limits as we now find to hold in similar cases among existing plants. Still we know that at the present time many species found in the equable climate of England will not live in Canada, though species to all appearance similar in structure are natives of the latter. There is also some reason to suppose that species, when new, may have greater hardiness and adaptability than when in old age, and verging toward extinction. In any case, these facts can account for but a small part of the phenomena, which require to be explained by physical changes affecting the earth as a whole, or at least the northern hemisphere. Many theoretical views have been suggested on this subject, which will be found discussed elsewhere, and perhaps the most practical way to deal with them here will be to refer to the actual conditions known to have prevailed in connection with the introduction and distribution of the principal floras which have succeeded each other in geological history.

If we can assume that all the carbon now sealed up in limestones and in coal was originally floating in the atmosphere as carbon dioxide, then we would have a cause which might seriously have affected the earlier land floras—that, for instance, which may have existed in the Eozoic age, and those well known to us in the PalÆozoic. Such an excess of carbonic acid would have required some difference of constitution in the plants themselves; it would have afforded them a super-abundance of wood-forming nutriment, and it would have acted as an obstacle to the radiation of heat from the earth, almost equal to the glass roof of a greenhouse, thus constituting a great corrective of changes of temperature. Under such circumstances we might expect a peculiar and exuberant vegetation in the earlier geological ages, though this would not apply to the later in any appreciable degree. In addition to this we know that the geographical arrangements of our continents were suited to the production of a great uniformity of climate. Taking the American continent as the simpler, we know that in this period there existed in the interior plateau between the rudimentary eastern and western mountains a great inland sea, so sheltered from the north that its waters contained hundreds of species of corals, growing with a luxuriance unsurpassed in the modern tropics. On the shores and islands of such a sea we do not wonder that there should have been tree-ferns and gigantic lycopods. In the succeeding Carboniferous, vast areas, both on the margins and in the interior of the continent, were occupied with swampy flats and lagoons, the atmosphere of which must have been loaded with vapour, and rich in compounds of carbon, though the temperature may have been lower than in the Devonian. There still remained, however, more especially in the west, a remnant of the old inland sea, which must have greatly aided in carrying a warm temperature to the north.

If now we pass to the succeeding Jurassic age, we find a more meagre and less widely distributed flora, corresponding to less favourable geographical and climatal conditions, while in the Cretaceous and Eocene ages a return to the old condition of a warm Mediterranean in continuation of the Gulf of Mexico gave those facilities for vegetable growth, which carried plants of the temperate zone as far north as Greenland.

It thus appears that those changes of physical geography and of the ocean currents to which reference is so often made in these papers, apply to the question of the distribution of plants in geological time.

These same causes may help us to deal with the peculiarities of the great Glacial age, which may have been rendered exceptionally severe by the combination of several of the continental and oceanic causes of refrigeration. We must not imagine, however, that the views of those extreme glacialists, who suppose continental ice caps reaching half way to the equator, are borne out by facts. In truth, the ice accumulating round the pole must have been surrounded by water, and there must have been tree-clad islands in the midst of the icy seas, even in the time of greatest refrigeration. This is proved by the fact that in the lower Leda clay of Eastern Canada, which belongs to the time of greatest submergence, and whose fossil shells show sea water almost at the freezing point, there are leaves of poplars and other plants which must have been drifted from neighbouring shores. Similar remains occur in clays of similar origin in the basin of the great lakes and in the West, and are not Arctic plants, but members of the North Temperate flora.[106] These have been called "interglacial," but there is no evidence to prove that they are not truly glacial. Thus, while the arctic flora must have continued to exist within the Arctic circle in the Glacial age, we have evidence that those of the cold temperate and subarctic zones continued to exist pretty far north. At the same time the warm temperate flora would be driven to the south, except where sustained in insular spots warmed by the equatorial currents. It would return northward on the re-elevation of the land and the return of warmth.

If, however, our modern flora is thus one that has returned from the south, this would account for its poverty in species as compared with those of the early Tertiary. Groups of plants descending from the north have been rich and varied. Returning from the south they are like the shattered remains of a beaten army. This, at least, has been the case with such retreating floras as those of the Lower Carboniferous, the Permian, and the Jurassic, and possibly that of the Lower Eocene of Europe.

The question of the supply of light to an Arctic flora is much less difficult than some have imagined. The long summer day is in this respect a good substitute for a longer season of growth, while a copious covering of winter snow not only protects evergreen plants from those sudden alternations of temperature which are more destructive than intense frost, and prevents the frost from penetrating to their roots, but by the ammonia which it absorbs preserves their greenness. According to Dr. Brown, the Danish ladies of Disco long ago solved this problem.[107] He informs us that they cultivate in their houses most of our garden flowers, as roses, fuchsias, and geraniums, showing that it is merely warmth, and not light that is required to enable a subtropical flora to thrive in Greenland. Even in Canada, which has a flora richer in some respects than that of temperate Europe, growth is effectually arrested by cold for nearly six months, and though there is ample sunlight there is no vegetation. It is indeed not impossible that in the plans of the Creator the continuous summer sun of the Arctic regions may have been made the means for the introduction, or at least for the rapid growth and multiplication, of new and more varied types of plants. It is a matter of familiar observation in Canada that our hardy garden flowers attain to a greater luxuriance and intensity of colour in those more northern latitudes where they have the advantage of long and sunny summer days.

Much, of course, remains to be known of the history of the old floras whose fortunes I have endeavoured to sketch, and which seem to have been driven like shuttlecocks from north to south, and from south to north, especially on the American continent, whose meridional extension seems to have given a field specially suited for such operations.

This great stretch of the western continent from north to south is also connected with the interesting fact that, when new floras are entering from the Arctic regions, they appear earlier in America than in Europe; and that in times when the old floras are retreating from the south, old genera and species linger longer in America. Thus, in the Devonian and Cretaceous new forms of those periods appear in America long before they are recognised in Europe, and in the modern epoch forms that would be regarded in Europe as Miocene still exist. Much confusion in reasoning as to the geological ages of the fossil flora has arisen from want of attention to this circumstance.

What we have learned respecting this wonderful history has served strangely to change some of our preconceived ideas. We must now be prepared to admit that an Eden might exist even in Spitsbergen, that there are possibilities in this old earth of ours which its present condition does not reveal to us; that the present state of the world is by no means the best possible in relation to climate and vegetation; that there have been and might be again conditions which could convert the ice-clad Arctic regions into blooming paradises, and which, at the same time, would moderate the fervent heat of the tropics. We are accustomed to say that nothing is impossible with God; but how little have we known of the gigantic possibilities which lie hidden under some of the most common of His natural laws.

Yet these facts have been made the occasion of speculations as to the spontaneous development of plants without any direct creative intervention. It would, from this point of view, be a nice question to calculate how many revolutions of climate would suffice to evolve the first land plant; what are the chances that such plant would be so dealt with by physical changes as to be preserved and nursed into a meagre flora like that of the Upper Silurian or the Jurassic; how many transportations to Greenland would suffice to promote such meagre flora into the rich and abundant forests of the Upper Cretaceous, and to people the earth with the exuberant vegetation of the early Tertiary. Such problems we may never be able to solve. Probably they admit of no solution, unless we invoke the action of a creative mind, operating through long ages, and correlating with boundless power and wisdom all the energies inherent in inorganic and organic nature. Even then we shall perhaps be able to comprehend only the means by which, after specific types have been created, they may, by the culture of their Maker, be "sported" into new varieties or sub-species, and thus fitted to exist under different conditions, or to occupy higher places in the economy of nature.

Before venturing on such extreme speculations as some now current on questions of this kind, we would require to know the successive extinct floras as perfectly as those of the modern world, and to be able to ascertain to what extent each species can change, either spontaneously or under the influence of struggle for existence, or expansion under favourable conditions, and under Arctic semi-annual days and nights, or the shorter days of the tropics. Such knowledge, if ever acquired, it may take ages of investigation to accumulate. In any case the subject of this paper indicates one hopeful line of study with the object of arriving at some comprehension of the laws of creation.

While the facts above slightly sketched impress us with the grand progress of the vegetable kingdom in geological time, they equally show the persistence of vegetable forms as compared with that of the dead continental masses and the decay of some forms of life in favour of the introduction of others.

When we find in the glacial beds the leaves of trees still living in North America and Europe, and consider the vicissitudes of elevation and submergence of the land, and of Arctic and temperate climates which have occurred, we are struck with the persistence of the weak things of life, as compared with the changeableness of rocks and mountains. A superficial observer might think the fern or the moss of a granite hill a frail and temporary thing as compared with solid and apparently everlasting rock. But just the reverse is the case. The plant is usually older than the mountain. But the glacial age is a very recent thing. We have facts older than this. As hinted in a previous paper, in the Laramie clays associated with the Lignite beds of North-western Canada—beds of Lower Eocene or early Tertiary age—which were deposited before the Rocky Mountains or the Himalayas had reared their great peaks and ridges, and at a time when the whole geography of the northern hemisphere was different from what it is at present—are remains of very frail and delicate plants which still live. I have shown that in these clays there exist, side by side, the Sensitive Fern, Onoclea sensibilis, and one of the delicate rock ferns, Davallia tenuifolia.[108] The first is still very abundant all over North America. The second has ceased to exist in North America, but still survives in the valleys of the Himalayas. These two little plants, once probably very widely diffused over the northern hemisphere, have continued to exist through the millenniums separating the Cretaceous from the present time, and in which the greater part of our continent was again and again under the sea, in which great mountain chains have been rolled up and sculptured into their present forms, and in which giant forms, both of animal and plant life, have begun, culminated and passed away. Truly God hath chosen the weak things of the world to confound those that are strong.

[108] Report on 49th Parallel, 1875.