THE ORIGIN OF PLANT LIFE ON THE LAND.
If the 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 under-clays 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 find portions 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 threadsaEuro"which, however, I suppose
to be fillings of canals of _Eozoon_aEuro"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 named _Eophyton_ have
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 myself devoted 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.
[Illustration: FIG. 83.aEuro"_Protannularia Harknessii_ (Nicholson). A
Siluro-Cambrian Plant, from the Skiddaw series.]
The oldest plants, probably higher than AlgA|, known to me by their
external forms, are those described by Nicholson[25] from the
Siluro-Cambrian Skiddaw slates of the north of England (Fig. 83). Their
discoverer has named them _Buthotrephis Harknessii_ and _B.
radiata_,[26] stating, 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 the _AnnulariA|_ of 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 MareaEuro(TM)s-tails, or of the
Rhosocarps, of our modern swamps and ponds.
[Illustration: FIG. 84.aEuro"American Lower Silurian Plants.aEuro"After
Lesquereux.
_a_, _Sphenophyllum primA|vum_. _b_, _Protostigma sigillarioides_.]
Somewhat higher up in the Lower Silurian, in the Cincinnati group of
America, Lesquereux finds objects which he refers to the genus
_Sphenophyllum_, which is closely allied to _Annularia_ (Fig. 84, _a_),
and also a plant which he terms _Protostigma_ (Fig. 84, _b_), and
believes to be the stem of a tree allied to the club-mosses.[27] He also
finds minute branching stems, which he refers to the genus
_Psilophyton_, 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 MareaEuro(TM)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.
[Illustration: FIG. 86.aEuro"Fragment of outer surface of _Glyptodendron_ of
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,
connecting it 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 name _Nematophyton_, and which was first
recognised in the Devonian of GaspA(C). 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 name _Pachytheca_. In my
judgment they are certainly true seeds.[28] Seeds of this kind have also
been found by Hicks in the still older Denbighshire grits of North
Wales, along with fragments of the wood of _Nematophyton_, and with
remains of branching stems which have been described under the name
_Berwynia_, though it is not unlikely that they represent the branches
of _Nematophyton_. It is proper to add that these ancient vegetable
fossils are regarded by some English botanists as gigantic algA| 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.
[Illustration: FIG. 87.aEuro"_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 known to us are marine; and most of
them were probably formed at a distance from land, so that it is little
likely that land plants could 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 (_PhA|nogams_), 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
classesaEuro"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.
[Illustration: FIG. 88.aEuro"Trunk of a Devonian Tree-fern (_Caulopteris
Lockwoodi_, Dn.). Gilboa, New York. One-third natural size.]
[Illustration: FIG. 89.aEuro"Frond of _ArchA|opteris Jacksoni_ (Dn.).
Devonian, of Maine.]
[Illustration: FIG. 90.aEuro"Portion of a branch of _Leptophleum rhombicum_
(Dn.). A Lycopodiaceous tree of the Devonian of Maine. Natural size.]
[Illustration: FIG. 91.aEuro"_Calamites radiatus_ (Brongniart). Middle
Devonian of N. Brunswick.]
In the Devonian age we meet with no land plants of the two lower
classes of the Cryptogams, and with scarcely any that can be referred to
the two higher classes of PhA|nogams, 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 gigantic _Calamites_, allied to the MaresaEuro(TM)-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 leaves[29] which 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 true
exogens. 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 in Fig. 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 type which 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 Devonian rocks
has been that of the immense abundance of spores of those humble plants
the Rhizocarps, represented in modern times by the Pillworts and
SalviniA|, &c. To these it is believed that _Sphenophyllum_ and
_Psilophyton_ were 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
genus _Protosalvinia_.[30] In Ohio there are beds of this kind 350 feet
thick, and extending across the State. They occur also in Canada, where
these forms were first recognised by the writer in the bituminous shale
of Kettle Point, Lake Huron.
[Illustration: FIG. 92.aEuro"A Devonian Taxine Conifer (_Dadoxylon
ouangondianum_, Dn.). St. John, New Brunswick.
A, Fragment showing _Sternbergia_ pith and wood; _a_, Medullary sheath;
_b_, Pith; _c_, Wood; _d_, Section of pith.
B, Wood cell _a_, and hexagonal areole and pore _b_.
C, Longitudinal section of wood, showing _a_, Areolation, and _b_,
Medullary rays.
D, Transverse section showing _a_, Wood-cells, and _b_, Limit of layer
of growth.]
[Illustration: FIG. 93.aEuro"Group of Devonian Fruits, &c. Middle Devonian,
New Brunswick.
A, _Cardiocarpum cornutum_.
B, _Cardiocarpum acutum_.
C, _Cardiocarpum Crampii_.
D, _Cardiocarpum Baileyi_.
E, _Trigonocarpum racemosum_.
EA╣, EA squared, Fruits enlarged.
F, _Antholithes Devonicus_.
FA╣, Fruit of the same.
G, _Annularia acuminata_.
H, _Asterophyllites acicularis_.
HA╣, Leaf.
K, _Cardiocarpum_. (? young of A.)
L, _Pinnularia dispalans_.
From _Acadian Geology_. ]
[Illustration: FIG. 94.aEuro"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 our continents were in the state of
swampy and often submerged flats, and in which the climate was again
warm and uniform.
[Illustration: FIG. 95.aEuro"_Asterophyllites parvula_ (Dn.), and
_Sphenophyllum 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.
[Illustration: FIG. 96.aEuro"_Calamites._ Carboniferous.
A, _C. Suckovii_. B, _C. Cistii_ (Bt.). C, Base of _Calamites_. D, E,
Structures.
From _Acadian Geology_.]
[Illustration: FIG. 97.aEuro"Carboniferous Ferns.
A, _Odontopteris subcuneata_ (Bunbury).
B, _Neuropteris cordata_ (Brongniart).
C, _Alethopteris tonchitica_ (Brongniart).]
Beginning with the MaresaEuro(TM)-tails, we find these represented in the
Carboniferous by many gigantic species, attaining to almost tree-like
dimensions (Fig. 96). These are the _Calamites_, 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;[31] but their foliage and fructification
were not correspondingly advanced. Thus the family of the EquisetaceA|
culminated in the Carboniferous, and thenceforth descended gradually in
the succeeding ages, leaving the comparatively humble MaresaEuro(TM)-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, the _Marattias_ and their allies,
characterised by a peculiar style of fructification, perhaps adapting
them to a moist and warm atmosphere (Fig. 97).[32] Thus 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 in Fig. 98 is an example.
[Illustration: FIG. 98.aEuro"Carboniferous Tree-ferns.
A, _Megaphyton magnificum_ (Dn.).
C, _PalA|opteris 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 the
_SigillariA|_. These remarkable trees were the most abundant of all in
the swamps of the coal-formation, and probably those which 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. These roots are the so-called
_StigmariA|_, so abundant in every coal-field, and especially filling the
aEurooeunder-claysaEuroŁ of the coal-beds, which are the soils on which the plants
forming these beds were supported. The true botanical position of the
_SigillariA|_ has 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.
[Illustration: FIG. 99.aEuro"_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.]
[Illustration: FIG. 100.aEuro"_SigillariA|_ of the Carboniferous.
A, _Sigillaria Brownii_ (Dn.).
B, _S. elegans_ (Brongniart).
BA╣, &c. Leaf and Leaf-scars.]
Another curious, and till recently little understood, group of
Carboniferous trees is that known as _Cordaites_, 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 GrandaEuro(TM) 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 genus _Dammara_ (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.
[Illustration: FIG. 101.aEuro"_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 GA-nkgo of Japan, constituting edible fruits. Seeds of this
type seem to have been extremely 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. A%tienne 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
TaxineA|, 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 the
_Cordaites_ are the intermediate forms; 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 PalA|ozoic flora; but what of
that which was not? We may answer:aEuro"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 PalA|ozoic 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 PalA|ozoic 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 PalA|ozoic 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 PalA|ozoic 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
PalA|ozoic, 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 PalA|ozoic 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 period even 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.
[Illustration: PTERASPIS. RESTORED.aEuro"AFTER LANKESTER.]