Devonian and Carboniferous Trilobites.
Fig. 78.—Devonian and Carboniferous Trilobites.
a,Phaceps latifrons(Bronn).b,Philipsia Howi(Billings) (tail).
Palaeozoic Ostracod Crustaceans.
Fig. 79.—Palæozoic Ostracod Crustaceans. Magnified.
a,Bairdia.b,Cytherella inflata(Jones).c,Cythere. Carboniferous.d,Beyrichia Jonesii(Dn.). Carboniferous.e,Beyrichia pustulosa(Hall). Silurian.
The higher, or decapod Crustaceans, now familiar to us in the modern crabs and lobsters, are first found in a few small species in the Devonian22and Carboniferous, and they are accompanied in the Devonian by at least one species of the allied group of the Stomapods (Figs. 81, 82).
Pterygotus anglicus.
Fig. 80.—Pterygotus anglicus. Reduced.—After Page and Woodward.
Amphipeltis paradoxus.
Fig. 81.—Amphipeltis paradoxus(Salter). A Devonian Stomapod from New Brunswick.
Anthropalaemon Hilliana
Fig. 82.—Anthropalæmon Hilliana(Dn). A Carboniferous Decapod from Nova Scotia. The carapace only.
The Palæozoic age of geology is thus emphatically an age of invertebrates of the sea. In this period they were dominant in the waters, and until toward its close almost without rivals. We shall find, however, that in the Upper Silurian, fishes made their appearance, and in the Carboniferous amphibian reptiles, and that, before the close of the Palæozoic, vertebrate life inthese forms had become predominant. We shall also see that just as the leading groups of Mollusks and Crustaceans seem to have had no ancestors, so it is with the groups of Vertebrates which take their places. It is also interesting to observe that already in the Palæozoic all the types of invertebrate marine life were as fully represented as at present, and that this swarming marine life breaks upon us in successive waves as we proceed upward from the Cambrian. Thus the progress of life is not gradual, but intermittent, and consists in the sudden and rapid influx of new forms destined to increase and multiply in the place of those which are becoming effete and ready to vanish away or to sink to a lower place. Farther, since the great waves of aquatic life roll in with each great subsidence of the land, a fact which coincides with their appearance in the limestones of the successive periods, it follows that it is not struggle for existence, but expansion under favourable circumstances and the opening up of new fields of migration that is favourable to the introduction of new species. The testimony of palæontology on this point, which I haveelsewhere adduced at length,23in my judgment altogether subverts the prevalent theory of “survival of the fittest,” and shows that the struggle for existence, so far from being a cause of development and improvement, has led only to decay and extinction, whereas the advent of new and favourable conditions, and the removal of severe competition, are the circumstances favourable to introduction of new and advanced species. This testimony of the invertebrates of the sea we shall find is confirmed by other groups of living beings, to be noticed in the sequel.24
Note.—The term “Siluro-Cambrian,” as used in this and the next chapter, is synonymous with “Ordovician” of Lapworth, which is now coming into somewhat general use.
Cordaites, of the Group of Dory-Cordaites.
Cordaites, of the Group of Dory-Cordaites. Branch Restored.—After Grand’ Eury.
Ifthe graphite of the Laurentian rocks was derived from vegetable matter, the further question arises, Was this vegetation of the land, or of the sea? and something may be said on both sides of this question. If there were land plants in the Laurentian period, they must have grown either on rocks older than the Laurentian itself, or on such portions of the beds of the latter as had been raised out of the sea, forming perhaps swampy flats of newly-made soil. But we know no rocks older than the Laurentian, and there is no positive evidence that any of the beds of that formation were other than marine. Still it is not impossible that some of the beds which are now graphitic gneisses may originally have been similar to the bituminous shales, coals, or underclays of the coal formation. The graphite occurring in veins, if of vegetable origin, must have been derived from liquid bitumen oozing into fissures; and veins of this kind occur in later formations, both in marine and fresh-water beds. The only other positive argument which has been adduced in favour of the existence of abundant land plants in the Laurentian is that of Dr. Sterry Hunt, derived from the great beds of iron ore, which it is difficult to account for chemically except on the hypothesis of the decay in the air of great quantities of vegetable matter. The question must remain in doubt till some one is fortunate enough to findportions of the Laurentian carbon retaining traces of organic structure. My own observations, though somewhat numerous, allow me only to say that the graphite sometimes presents fibrous forms, that it occasionally appears as vermicular threads—which, however, I suppose to be fillings of canals ofEozoon—and that in the graphitic beds there are occasionally slender root-like bodies of a lighter colour than the mass; but none of these indications are sufficient to determine anything as to its vegetable origin, or the nature of the plants from which it may have been derived.
In any case, the quantity of carbon which has been accumulated in the Laurentian rocks is very great. I have measured one bed at Buckingham, on the Ottawa, estimated to contain 20 per cent. of carbon, and which is at least eight feet in thickness. Sir William Logan has described another similar bed from ten to twelve feet thick, and more recent reports of the Geological Survey of Canada mention a bed supposed to be twenty-five feet thick, in which Mr. Hoffman finds 30 per cent. of carbon. On the whole the quantity of carbon in the graphitic zone of the Laurentian is comparable with that in certain productive coal-fields, and we certainly have in the subsequent geological history no examples of such accumulations except from remains of the luxuriant vegetation of swampy flats.
The Upper Laurentian and Huronian have as yet afforded no evidence of land vegetation. The Cambrian, as already stated, abounds in remains of sea-weeds; but though the forms which have been namedEophytonhave been regarded as land plants, this claim is, to say the least, very doubtful; and I have as yet seen nothing of this kind which did not appear to me to be merely markings made by objects drifted over the bottom or remains of marine plants. Yet in the Upper Cambrian there are wide surfaces of littoral sandstone often containing minute carbonised fragments, and which might be expected to afford indications of land vegetation, had such existed. I have myselfdevoted many days of fruitless labour to the examination of the large areas of Potsdam sandstone exposed in some parts of Canada. But as these rocks were evidently formed along the borders of a Laurentian continent capable of supporting vegetation, we may still hope for some discovery of this kind, more especially if we could find the point where some fresh-water stream ran into the Cambrian sea.
Protannularia Harknessii.
Fig. 83.—Protannularia Harknessii(Nicholson). A Siluro-Cambrian Plant, from the Skiddaw series.
The oldest plants, probably higher than Algæ, known to me by their external forms, are those described by Nicholson25from the Siluro-Cambrian Skiddaw slates of the north of England(Fig. 83). Their discoverer has named themButhotrephis HarknessiiandB. radiata,26stating, however, that these two species are not improbably portions of the same plant, and that its form is rather that of a land plant than of an Alga. The specimens of these plants which I have seen appear to me to support the conclusion that they represent one species, and this allied to theAnnulariæof the Devonian and Carboniferous periods, which probably grew in shallow water with only their upper parts in the air, and bore whorls or verticles of narrow leaves. They were either relatives of the Mare’s-tails, or of the Rhosocarps, of our modern swamps and ponds.
American Lower Silurian Plants.
Fig. 84.—American Lower Silurian Plants.—After Lesquereux.
a,Sphenophyllum primævum.b,Protostigma sigillarioides.
Somewhat higher up in the Lower Silurian, in the Cincinnati group of America, Lesquereux finds objects which herefers to the genusSphenophyllum, which is closely allied toAnnularia(Fig. 84,a), and also a plant which he termsProtostigma(Fig. 84,b), and believes to be the stem of a tree allied to the club-mosses.27He also finds minute branching stems, which he refers to the genusPsilophyton, to be mentioned in the sequel; but as to these I have some doubts whether they may not be Zoophytes allied to the Graptolites, rather than plants of that genus. These discoveries tend to show the probable existence in the Siluro-Cambrian of plants representing two of the three leading families of the higher cryptogams or flowerless plants, namely, the Club-mosses and the Mare’s-tails. Thus land vegetation begins with the highest members of the lower of the two great series into which botanists divide the vegetable kingdom.
Fragment of outer surface of Glyptodendron of Claypole.
Fig. 86.—Fragment of outer surface ofGlyptodendronof Claypole. A Silurian Tree.
If we now turn to the Silurian, further evidence of land vegetation presents itself. Near the base of this great series, the club-moss family is represented by a plant discovered by Claypole in the Clinton group, and referred to a new genus (Glyptodendron,Fig. 86). Plants of this family have also been noticed by Barrande in Bohemia, and by page in Scotland; and a humble but interesting member of the family, connectingit with the pillworts,Psilophyton(Fig. 87), though more characteristic of the Devonian, has been found in the Upper Silurian both in Canada and the United States. No Ferns or Equiseta have as yet been found in the Silurian; but in 1870 I recognised in some fragments of wood from the Ludlow bone-bed, in the Museum of the Geological Survey of Great Britain, the structure of that curious prototypal tree, to which I have given the nameNematophyton, and which was first recognised in the Devonian of Gaspé. Since that time I have found in the Upper Silurian beds of Cape Bon Ami, in New Brunswick, similar fragments of fossil wood, associated with round seed-like bodies, having a central nucleus and a thick wall or test of radiating fibres. These bodies show a structure similar to that of those found in the Upper Ludlow of England, and described by Hooker under the namePachytheca. In my judgment they are certainly true seeds.28Seeds of this kind have also been found by Hicks in the still older Denbighshire grits of North Wales, along with fragments of the wood ofNematophyton, and with remains of branching stems which have been described under the nameBerwynia, though it is not unlikely that they represent the branches ofNematophyton. It is proper to add that these ancient vegetable fossils are regarded by some English botanists as gigantic algæ or sea-weeds, but I confess I am unable to adopt this view of their nature. The supposed fern of the Upper Silurian, figured in the first edition of this work, has proved on further examination to be merely an imitative form produced by crystallisation. On the other hand, the recent discovery of a cockroach and two species of Scorpion in the Silurian, proves the existence of land animals as well as plants at this period.
Psilophyton princeps.
Fig. 87.—Psilophyton princeps(Dn.) Silurian and Devonian. Restored.
a, Fruit, natural size.b, Stem, natural size.c, Scalariform tissue of the axis, highly magnified. In the restoration one side is represented in vernation, and the other in fruit.
It is probable that these discoveries represent merely a small proportion of the plants actually existing in the Silurian period. All the deposits of this age at present knownto us are marine; and most of them were probably formed at a distance from land, so that it is little likely that land plantscould find their way into them. At any time the discovery of an estuarine or lacustrine deposit of Silurian age might wonderfully extend our knowledge of this ancient flora.
The Devonian or Erian age, that of the classic Old Red Sandstone of Scotland, is that in which we find the first great and complete land flora; and though this is inferior in number of species to that of the succeeding Carboniferous, and greatly less important with reference to its practical bearing on our welfare, it is in some respects superior in that variety which depends on diversity of soil and of station. To appreciate this, it will be necessary to glance at the range and subdivisions of the modern flora.
In the modern world we divide all vegetation into two great series, that of the Flowering Plants (Phænogams), which also produce true fruits and seeds, and that of the Flowerless Plants (Cryptogams), which produce minute spores instead of seeds. The latter is in every respect the lower group. This lower series is again divisible into three classes—first and lowest, that of the Seaweeds, Moulds, and Lichens (Thallophytes). Secondly, that of the Mosses and their allies (Anophytes). Thirdly, that of the Ferns, Equisetums and Club-mosses (Acrogens). In like manner the second, or higher series is divisible into three classes: that of the Pines and Cycads (Gymnosperms), having naked seeds not covered by true fruits, and woody tissue of simple structure; that of the Palms and Grasses and their allies (Endogens); and last and highest, that of the ordinary timber trees and other plants allied to them, with exogenous stems, netted-veined leaves, and a two-leaved embryo (Exogens). These last are in every respect the dominant plants on our present continents. Carrying with us this twofold division of the vegetable kingdom and its subdivisions, we shall be prepared to understand the relation of the more ancient floras to that now living.
Trunk of a Devonian Tree-fern.
Fig. 88.—Trunk of a Devonian Tree-fern (Caulopteris Lockwoodi, Dn.). Gilboa, New York. One-third natural size.
Archaeopteris Jacksoni
Fig. 89.—Frond ofArchæopteris Jacksoni(Dn.). Devonian, of Maine.
Leptophleum rhombicum.
Fig. 90.—Portion of a branch ofLeptophleum rhombicum(Dn.). A Lycopodiaceous tree of the Devonian of Maine. Natural size.
Calamites radiatus.
Fig. 91.—Calamites radiatus(Brongniart). Middle Devonian of N. Brunswick.
In the Devonian age we meet with no land plants of the twolower classes of the Cryptogams, and with scarcely any that can be referred to the two higher classes of Phænogams, so that the vegetation of this period presents a remarkable character of mediocrity, being composed almost entirely of the highest class of the flowerless plants and the lowest class of those that flower. Of the former there are Tree-ferns and vast numbers of herbaceous forms (Figs. 88, 89), great Lycopodiaceous plants, immensely better developed than those now existing (Fig. 90), and giganticCalamites, allied to the Mares’-tails (Fig. 91), along with humbler members of the same group (Fig. 95). Of the latter there were Pines of great stature, known to us at present only by their wood (Fig. 92); and that other allied trees existed we have evidence in numerous seeds which must have belonged to this class (Fig. 93), and in long flag-like leaves29which modern discoveries would refer to the same group. As yet we know no Devonian Palms or Grasses; and only a single specimen has been found indicating the existence of a plant of the highest vegetable class, that of the trueexogens. This unique specimen, found by Hall in the Devonian of the shores of Lake Erie, is a fragment of mineralised wood, the structures of which are represented inFig. 94. The large ducts seen in cross section in Nos. 1, 2, and 3, and in longitudinal section in Nos. 4 and 5, and the medullary rays, seen in Nos. 1, 4, and 6, testify to the fact that this chip of wood must have belonged to a tree of the same type which contains our oaks, maples, and poplars; a typewhich does not appear to have become dominant till near the close of the Mesozoic, but which already existed, though perhaps only in few species, and only in upland and inland positions, as far back as the Middle Devonian. Perhaps one of the most interesting discoveries in the Erian or Devonianrocks has been that of the immense abundance of spores of those humble plants the Rhizocarps, represented in modern times by the Pillworts and Salviniæ, &c. To these it is believed thatSphenophyllumandPsilophytonwere allied; but in addition to this there are thick and vastly extended beds of bituminous shale which owe their inflammable properties to countless multitudes of Macrospores (Sporangites) of the genusProtosalvinia.30In Ohio there are beds of this kind 350 feet thick, and extending across the State. They occur also in Canada, wherethese forms were first recognised by the writer in the bituminous shale of Kettle Point, Lake Huron.
A Devonian Taxine Conifer.
Fig. 92.—A Devonian Taxine Conifer (Dadoxylon ouangondianum, Dn.). St. John, New Brunswick.
A, Fragment showingSternbergiapith and wood;a, Medullary sheath;b, Pith;c, Wood;d, Section of pith.
B, Wood cella, and hexagonal areole and poreb.
C, Longitudinal section of wood, showinga, Areolation, andb, Medullary rays.
D, Transverse section showinga, Wood-cells, andb, Limit of layer of growth.
Group of Devonian Fruits.
Fig. 93.—Group of Devonian Fruits, &c. Middle Devonian, New Brunswick.
A,Cardiocarpum cornutum.
B,Cardiocarpum acutum.
C,Cardiocarpum Crampii.
D,Cardiocarpum Baileyi.
E,Trigonocarpum racemosum.
E1, E2, Fruits enlarged.
F,Antholithes Devonicus.
F1, Fruit of the same.
G,Annularia acuminata.
H,Asterophyllites acicularisH1, Leaf.
K,Cardiocarpum. (? young of A.)
L,Pinnularia dispalans.
FromAcadian Geology.
FromAcadian Geology.
Structures of the oldest-known Angiospermous Exogen.
Fig. 94.—Structures of the oldest-known Angiospermous Exogen (Syringoxylon mirabile, Dn.). From Eighteen-mile Creek, Lake Erie.
1, Transverse section x 100. 2 and 3, Portions of the same x 300. 4, Longitudinal section x 300. 5, Fragment of duct from the same x 600. 6, Wood cells and medullary ray x 600.
The Devonian flora seems to have been introduced in the northern parts of the American continent at a time of warm and equable climate, and of elevation of new land out of the Silurian sea. It spread itself to the southward, and was finally destroyed in the great subsidences and disturbances which closed the Devonian age, and which were probably accompanied with refrigeration of climate. It was succeeded by the more massive and richer, but more monotonous flora of the Carboniferous, a period in which large areas of ourcontinents were in the state of swampy and often submerged flats, and in which the climate was again warm and uniform.
Asterophyllites parvula.
Fig. 95.—Asterophyllites parvula(Dn.), andSphenophyllum antiquum(Dn.). Middle Devonian, New Brunswick.
The Carboniferous age was, even more emphatically than the Devonian, an age of Acrogens and Conifers. A few Carboniferous Fungi have recently been discovered, but there are no known Lichens or Mosses. There seem to be a few Endogens, but no true Exogens. The great bulk of the plants consists of Acrogens and Gymnosperms, as in the previous period. As this flora is so very important and so much better known than any other of those belonging to the infancy of the vegetable kingdom, we may notice a little in detail some of its leading forms.
Calamites. Carboniferous.
Fig. 96.—Calamites.Carboniferous.
A,C. Suckovii.B,C. Cistii(Bt.).C, Base ofCalamites.D,E, Structures.
FromAcadian Geology.
FromAcadian Geology.
Carboniferous Ferns.
Fig. 97.—Carboniferous Ferns.
A,Odontopteris subcuneata(Bunbury).B,Neuropteris cordata(Brongniart).C,Alethopteris tonchitica(Brongniart).
Beginning with the Mares’-tails, we find these represented in the Carboniferous by many gigantic species, attaining to almosttree-like dimensions (Fig. 96). These are theCalamites, which formed dense brakes and jungles on the margins of the great swampy flats of this period. Their tall stems, ribbed and jointed, bore whorls of leaves or branchlets. Sending out horizontal root-stocks and budding out from the base, they grew in great clumps, and had the capacity to resist the effects of accumulating sediment by constantly sending out new stems at higher and higher levels. The larger species assumed a complexity in the structure of their stems unknown in their modern congeners, and enabling them to grow to a great height;31but their foliage and fructification were not correspondingly advanced. Thus the family of the Equisetaceæ culminatedin the Carboniferous, and thenceforth descended gradually in the succeeding ages, leaving the comparatively humble Mares’-tails and Scouring Rushes as its present representatives.
The Ferns of the Carboniferous, like those of the Devonian, presented both gigantic forms like those of the tree-ferns of the modern tropics, and delicate herbaceous species, and these in great profusion. On the whole, they do not strike the observer as very dissimilar from those of modern times. A more critical examination, however, shows that the bulk of the tree-ferns of the Devonian and Carboniferous are allied not to the Polypod type, which is the most common at present, but to certain comparatively rare southern ferns, theMarattiasand their allies, characterised by a peculiar style of fructification, perhaps adapting them to a moist and warm atmosphere (Fig. 97).32Thus the ferns, while a wonderfully persistent type, were in their grander forms far more widely distributed in the Carboniferous than at present; and genera now comparatively rare, and limited to warm and moist climates, were then abundant, and ranged over those temperate and boreal regions of the Northern Hemisphere where only a few humble and hardy species can now subsist. There were also some remarkable and anomalous tree-ferns, of which that represented inFig. 98is an example.
Carboniferous Tree-ferns.
Fig. 98.—Carboniferous Tree-ferns.
A,Megaphyton magnificum(Dn.).C,Palæopteris Hartii(Dn.).D,P. Acadica(Dn.).
The family of the Club-mosses, already, even in the Devonian, in advance of its modern development, experiences in the Carboniferous a remarkable and portentous extension into great trees of several genera and many species, constituting apparently extensive forests, and having the woody tissues of their stems developed to a degree unheard of in their present representatives (Fig. 99). Further, they become closely linked, in external form at least, with another and more advanced type, that of theSigillariæ. These remarkable trees were the most abundant of all in the swamps of the coal-formation, and probably thosewhich most contributed to the accumulation of coal. They presented tall pillar-like trunks, often ribbed longitudinally, and with perpendicular rows of scars of fallen leaves. Dividing at top into a few thick branches, they were covered with long rigid grass-like foliage. Their fruit was borne in rings or whorls of spikes surrounding the branches at intervals (Fig. 100). Their roots were strangely symmetrical, spreading out like underground branches into the soft soil by a regular process of bifurcation, and were covered with rootlets diverging in every direction, and so jointed to the main root that when broken off they left round marks regularly arranged. Theseroots are the so-calledStigmariæ, so abundant in every coal-field, and especially filling the “under-clays” of the coal-beds, which are the soils on which the plants forming these beds were supported. The true botanical position of theSigillariæhas been a matter of much controversy. Some of them undoubtedly have structures akin to those of the tree-like Club-mosses, as Williamson has well shown, and may have been cryptogamous. Others have structures of higher character, akin to those of the modern Cycads, and seem to have borne nutlets allied to those of these plants. Yet the external forms of these diverse sorts are so similar that no definite separation of them has yet been made. Either these anomalous trees constitute a link connecting the two great series of the vegetable kingdom, or we have been confounding two distinct groups, owing to imperfect information.
Lepidodendron corrugatum.
Fig. 99.—Lepidodendron corrugatum(Dn.). A characteristic Lycopod of the Lower Carboniferous of America.
A, Restoration.B, Leaf, natural size.C, Cone.D, Leafy branch.E, Forms of leaf-bases.F, Sporangium.I, L, M, N, O, Markings on stem and branches, in various states.
Sigillariae of the Carboniferous.
Fig. 100.—Sigillariæof the Carboniferous.
A,Sigillaria Brownii(Dn.).B,S. elegans(Brongniart).B1, &c. Leaf and Leaf-scars.
Another curious, and till recently little understood, group of Carboniferous trees is that known asCordaites, which existed already in some of its species in the Devonian. Their leaves are long, and often broad as well, and with numerous delicate parallel veins, resembling in this the leaves of grasses. Corda long ago showed that one species at least has a stem allied to the Club-mosses. More recently Grand’ Eury has found in the South of France admirably preserved specimens, which show that others more resembled in their structure the Pines and Yews, and were probably Gymnosperms, approaching to the Pines, but with very peculiar and exceptional foliage, of which the only modern examples are the broad-leaved Pines of the genusDammara(Frontispiece to Chapter). Here again we have either two very distinct groups, combined through our ignorance, or a connecting link between the Lycopods and the Pines.
Trigonocarpum Hookeri.
Fig. 101.—Trigonocarpum Hookeri(Dn.). A Gymnospermous seed.
a, Testa.b, Tegmen.c, Nucleus.d, Embryo.
The Yews and their allies among modern trees, while members of the great Cone-bearing order, bear nut-like seeds in fleshy envelopes, sometimes, as in the Gínkgo of Japan, constituting edible fruits. Seeds of this type seem to have beenextremely abundant in the Carboniferous age in all parts of the world, and were probably produced by trees of several genera (Dadoxylon,Sigillaria,Cordaites, etc.) (Fig. 101). Charles Brongniart has recently described no less than seventeen genera of these seeds from the coal-field of St. Étienne alone, and it would be a low estimate to say that we probably know as many as sixty or seventy species in all, while the trunks of great coniferous trees allied to Taxineæ, and showing well-preserved structure, are by no means uncommon in the Devonian and Carboniferous. Had these great Yews appeared for the first time in the Coal-formation, we might have supposed that they had been developed from such Lycopods as Lepidodendra, and that theCordaitesare the intermediateforms; but unfortunately the Pines go almost as far back in geological time as the Lycopods, and it does not help us, when in search of evidence of evolution, to find the link which is missing or imperfect in the Early Devonian supplied in the Coal-formation, where, for this purpose at least, it is no longer needed.
We have said something of what was in the Palæozoic flora; but what of that which was not? We may answer:—Nearly all that is characteristic of our modern forests, whether in the ordinary Exogens, which predominate so greatly in the trees and shrubs of temperate climates, or in the Palms and their allies, which figure so conspicuously within the tropics. The few rare, and to some extent doubtful, representatives of these types scarcely deserve to be noted as exceptions. Had a botanist searched the Palæozoic forests for precursors of the future, he would probably have found only a few rare species, while he would have seen all around him the giant forms and peculiar and monotonous foliage of tribes now degraded in magnitude and structure, and of small account in the system of nature.
It must not be supposed that the Palæozoic flora remained in undisturbed possession of the continents during the whole of that long period. In the successive subsidences of the continental plateaux, in which the marine limestones were deposited, it was to a great extent swept away, or was restricted to limited insular areas, and these more especially in the far north, so that on re-elevation of the land it was always peopled with northern plants. Thus there were alternate restrictions and expansions of vegetation, and the latter were always signalised by the introduction of new species, for here, as elsewhere, it was not struggle, but opportunity, that favoured improvement.
In the Lower Silurian such plants as existed must have experienced great restriction at the age of the Niagara or Wenlock limestone. Those of the Upper Silurian suffered a similar reverse at the time of the Lower Helderberg or Ludlow limestones. This recurred at the close of the Devonian and in the time of the Lower Carboniferous limestone; and finally the Palæozoic flora disappeared altogether in the Permian, to be replaced by new types in the Mesozoic. While, therefore, there is a great general similarity in the successive Palæozoic floras, there are minor differences, so that the Devonian plants are for the most part distinct specifically from those of the Lower Carboniferous, those of the Lower Carboniferous from those of the Coal-formation, and those of the latter from those of the Permian.
With all these vicissitudes it is to be observed that there is no apparent elevation of type in all the long ages from the Devonian to the Permian, that the Acrogens and Gymnosperms of these periods are in some respects superior, in all respects equal, to their modern successors, and that their history shows a decadence toward the modern period; that intermediate forms arrive too late to form connecting links in time, that several distinct types appear together at the beginning, and that all utterly and apparently simultaneously perish at the end of the Palæozoic, to make way for the entirely new vegetation of the succeeding age. Theories of evolution receive no support from facts like these, though their practical significance, as parts of the one great uniform scheme of nature, is sufficiently manifest.
Of what use then were these old floras? To the naturalist, vegetable life, with regard to its modern uses, is the great accumulator of pabulum for the sustenance of the higher forms of vital energy manifested in the animal. In the Palæozoic this consideration sinks in importance. In the Coal period we know few land animals, and these not vegetable feeders, with the exception of some insects, millipedes, and snails. But the Carboniferous forests did not live in vain, if their only use was to store up the light and heat of those old summers in the form of coal, and to remove the excess of carbonic acid from the atmosphere. In the Devonian periodeven these utilities fail, for coal does not seem to have been accumulated to any great extent, and the petroleum of the Devonian appears to have been produced from aquatic vegetation. Even with reference to theories of evolution, there seems no necessity for the long continuance and frequent changes of species of acrogenous plants without any perceptible elevation. We may have much yet to learn of the life of the Devonian; but for the present the great plan of vegetable nature goes beyond our measures of utility; and there remains only what is perhaps the most wonderful and suggestive correlation of all, namely, that our minds, made in the image of the Creator, are able to trace in these perished organisms structures similar to those of modern plants, and thus to reproduce in imagination the forms and habits of growth of living things which so long preceded us on the earth. We may indeed proceed a step further, and hold that, independently of human appreciation, these primitive plants commended themselves to the approval of their Maker, and perhaps of higher intelligences unknown to us; and that in the last resort it was for His pleasure that they were created.