_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.
[82] Nematophyton, etc. See "Geological History of Plants."
[83] Psilophyton, Protannularia, etc.
[84] Rhizocarpeæ.
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.
[85] _Cordaites_, etc. As I have elsewhere shown, these are distinct
sub-floras in the Lower, Middle and Upper Devonian, and in the Lower,
Middle and Upper Carboniferous and Permian, sufficiently different to
allow these periods to be determined by the evidence of these fossil
plants. Reports prepared for Geological Survey of Canada.
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]
[86] Saporata, "Ancienne Vegetation Polaire"; Hooker, Presidential
Address to Royal Society, 1878; Thistleton Dyer, "Lecture on Plant
Distribution "; Mr. Starkie Gardner, Letters in _Nature_, 1878, etc.
The basis of most of these brochures is to be found in Heer's "Flora
Fossilis Arctica."
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:--
[87] Address to American Association.
[88] _American Journal of Science_, xvi., 1878.
"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:--
[89] "Fossil Plants of the Devonian and Upper Silurian Formations of
Canada," pp. 92, twenty plates. Montreal, 1871.
"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."
[90] The term Erian is used as synonymous with Devonian, and probably
should be preferred to it, as pointing to the best development of this
formation known, which is on the shores of Lake Erie.
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.
[91] Trans. Swedish Academy, 1871, _Journal London Geological Society_,
vol. xxvlii.
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]
[92] "Fossil Plants of Lower Carboniferous and Millstone Grit
Formations of Canada," pp. 47, 10 plates. Montreal, 1873.
[93] G. M. Dawson, "Report on Arctic Regions of Canada."
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]
[94] Nordenskiöld, Expedition to Greenland, _Geological Magazine_, 1872.
[95] Yet even here the Bald Cypress (_Taxodium distichum_), or a tree
nearly allied to it, is found, though this species is now limited to
the Southern States. Fielden and De Ranee, _Journal of Geological
Society_, 1878.
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.
[96] Lesquereux, Report on Cretaceous Flora. The reader not interested
in American details may pass over to the middle of page 213.
[97] This flora has since been described in Virginia and Maryland by
Fontaine, and has been recognised in Montana by Newberry.
[98] G. M. Dawson, Report on Forty-ninth Parallel.
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.
[99] _Trans. Royal Society of Canada._
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]
[100] Ward, Repts. and Bulletins Am. Geol. Survey.
[101] Lesquereux's Tertiary Flora; White and Ward on the Laramie Group;
Stevenson, Geological Relations of Lignitic Groups, _Am. Phil. Soc._,
June, 1875.
[102] G. M. Dawson, Report on the Geology of the Forty-ninth Parallel,
1875, where full details on these points may be found.
[103] _Nature_, Dec. 12th, 1878; Publications Palæontographical
Society; Reports to British Association. It seems certain that the
so-called Miocene of Bovey Tracey in Devon, and of Mull in Scotland, is
really Eocene. The Tertiary plant-bearing beds of Greenland are said by
Nathorst to rest unconformably on the Cretaceous, and are characterized
by _M'Clintockia_ and other forms known in the Eocene of Great Britain
and Ireland.
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.
[104] Reports Geological Survey of Canada.
[105] Fontaine has well described the Mesozoic flora of Virginia,
_American Journal of Science_, January, 1879.
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.
[106] Pleistocene Plants of Canada, Dawson and Penhallow, _Bull, Geol.
Socy._, America, 1890. In Europe the Arctic flora extended, relatively
to present climate, farther south.
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.
[107] _Florula Discoana_, Botanical Society of Edinburgh, 1868.
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.
Other plants equally illustrate the decadence of important types of
vegetable life. In the beautiful family of the Magnolias there exists
in America a most remarkable and elegant tree, whose trunk attains
sometimes a diameter of 7 feet and a height of 80 or 90 feet. Its
broad deep green leaves are singularly truncate at the end, as if
artificially cut off, and in spring it puts forth a wealth of large and
brilliant orange and yellow flowers, from which it obtains the name of
Tulip tree. It is the _Liriodendron tulipifera_ of botanists, and the
sole species of its genus. This Tulip tree has a history. All through
the Tertiary beds we find leaves referable to the genus, and belonging
not to one species only, but to several, and as we go back into the
Cretaceous, the species seem to become more numerous. Many of them
have smaller leaves than the modern species, others larger, and some
have forms even more quaint than that of the existing Tulip tree. The
oldest that I have seen in Canada is one from the Upper Cretaceous of
Port McNeil in the north of Vancouver Island, which is as large as that
of the modern species, and very similar in form. Thus this beautiful
vegetable type culminated long geological ages ago, and was represented
by many species, no doubt occupying a prominent place in the forests of
the northern hemisphere. To-day only a single species exists, in our
warmer regions, to keep up the memory of this almost perished genus;
but that species is one of our most beautiful trees.
The history of the Sequoias or giant Cypresses, of which two species
now exist in limited areas in California, is still more striking.
These giant trees, monsters of the vegetable kingdom, are, strange to
say, very limited in their geographical range. The greater of the two,
_Sequoia gigantea_, the giant tree _par excellence_, seems limited to a
few groves in California. At first sight this strikes us as anomalous,
especially as we find that the tree will grow somewhat widely both
in Europe and America when its seeds are sown in suitable soil. The
mystery is solved when we learn that the two existing species are but
survivors of a genus once diffused over the whole northern hemisphere,
and represented by many species, constituting, in the Later Cretaceous
and Eocene ages, vast and dark forests extending over enormous areas
of our continents, and forming much of the material of the thick and
widely distributed Lignite beds of North-western America. Thus the
genus has had its time of expansion and prevalence, and is now probably
verging on extinction, not because there are not suitable habitats, but
either because it is now old and moribund, or because other and newer
forms have now a preference in the existing conditions of existence.
The Plane trees, the Sassafras, the curious Ginkgo tree or fern-leaved
yew of Japan, are cases of similar decadence of genera once represented
by many species, while other trees, like the Willows and Poplars, the
Maples, the Birches, the Oaks and the Pines, though of old date, are
still as abundant as they ever were, and some genera would seem even
to have increased in number of species, though on the whole the flora
of our modern woods is much less rich than those of the Miocene and
Eocene, or even than that of the Later Cretaceous. The early Tertiary
periods were, as we know, times of exuberant and gigantic animal life
on the land, and it is in connection with this that the vegetable
world seems to have attained its greatest variety and luxuriance. Even
that early post-glacial age in which primitive man seems first to have
spread himself over our continents was one richer both in animal and
plant life than the present. The geographical changes which closed
this period and inaugurated the modern era seem to have reduced not
only the area of the continents but the variety of land life in a very
remarkable manner. Thus our last lesson from the genesis and migrations
of plants is the humbling one that the present world is by no means the
best possible in so far as richness of vegetable and animal life is
concerned.
Reference has been made to the utility of fossil plants as evidence
of climate; but the subject deserves more detailed notice. I have
often pondered on the nature of the climate evidenced by the floras of
the Devonian and Carboniferous; but the problem is a difficult one,
not only because of the peculiar character of the plants themselves,
so unlike those of our time, but because of the probably different
meteorological conditions of the period. It is easy to see that a
flora of tree-ferns, great lycopods and pines is more akin to that of
oceanic islands in warm latitudes than anything else that we know.
But the Devonian and Carboniferous plants did not flourish in oceanic
islands, but for the most part on continental areas of considerable
dimensions, though probably more flat and less elevated than those
of the present day. They also grew, from Arctic latitudes, almost,
if not altogether, to the equator; and though there are generic
differences in the plants of these periods in the southern hemisphere,
yet these do not affect the general facies. There are, for example,
characteristic Lepidodendroids in the Devonian and Carboniferous of
Brazil, Australia, and South Africa. If now we consider the plants a
little more in detail, coniferous and taxine trees grow now in very
different latitudes and climates. There is therefore nothing so very
remarkable in their occurrence. The great group of Cordaites may have
been equally hardy; but it is noteworthy that their geographical
distribution is more limited. In Europe, for example, they are more
characteristic in France than in Great Britain. Ferns and Lycopods and
Mare's-tails are also cosmopolitan, but the larger species belong to
the warmer climates, and nowhere at present do they become so woody and
so complex in structure as they were in the older geological periods.
At the present day, however, they love moisture rather than aridity,
and uniformity of temperature rather than extreme light and heat. The
natural inference would be that in these older periods geographical
and other conditions must have conspired to produce a uniform and
moist climate over a large portion of the continents. The geographical
conditions of the Carboniferous age, and the distribution of animal
life on the sea and land, confirm the conclusion based on the flora.
Further, if, as seems probable, there was a larger proportion of carbon
dioxide in the atmosphere than at present, this would not only directly
affect the growth of plants, but would impede radiation, and so prevent
escape of heat by that means, while the moisture exhaled from inland
seas and lagoons and vastly extended swamps, would tend in the same
direction.
It would, however, be a mistake to infer that there were not local
differences of climate. I have elsewhere[109] advocated the theory
that the great ridge of boulders, the New Glasgow conglomerate, which
forms one margin of the coal field of Picton, in Nova Scotia, is an
ice-formed ridge separating the area of accumulation of the great
thirty-six feet seam from an outer area in which aqueous conditions
prevailed, and little coal was formed. In this case, an ice-laden
sea, carrying boulders on its floes and fields of ice, must have been
a few miles distant from forests of Lepidodendra, Cordaites, and
Sigillariæ, and the climate must have been anything but warm, at least
at certain seasons. Nor have we a right to infer that the growth of
the coal-plants was rapid. Stems, with woody axes and a thick bark,
containing much fibrous and thick-walled cellular tissue, are not to
be compared with modern succulent plants, especially when we consider
the sparse and rigid foliage of many of them. Our conclusion should,
therefore, be that geographical conditions and the abundance of
carbon dioxide in the atmosphere favoured a moist climate and uniform
temperature, and that the flora was suited to these conditions.
[109] "Acadian Geology," Carboniferous of Picton.
As to the early Mesozoic flora, I have already suggested that it must
have been an invader from the south, for which the intervening Permian
age had made way by destroying the Palæozoic flora. This was probably
effected by great earth-movements changing geographical conditions.
But in the Mesozoic the old conditions to some extent returned, and
the Carboniferous plants being extinct, their places were taken by
pines, lycopods, and ferns, whose previous home had been in the insular
regions of the tropics, and which, as climatal conditions improved,
pushed their way to the Arctic circle. But, being derivatives of warm
regions, their vitality and capacity for variation were not great,
and they only locally and in favourable conditions became great coal
producers. The new flora of the Later Cretaceous and the Tertiary, as
previously stated, originated in the Arctic, and marched southward.
These newer Cretaceous plants presented from the first the generic
aspects of modern vegetation, and so enable us much better to gauge
their climatal conditions. In general, they do not indicate tropical
heat in the far north, but only that of the warm temperate zone; but
this in some portions of the period certainly extends to the middle of
Greenland, unless, without any evidence, we suppose that the Cretaceous
and lower Tertiary plants differed in hardiness of constitution from
their modern representatives. They prove, however, considerable
oscillations of climate. Gardner, Nathorst and Reid have shown this in
Europe, and that it extends from the almost tropical flora of the lower
Eocene to the Arctic flora of the Pleistocene. In America, owing, as
Grey has suggested, to its great north and south extension, the changes
were more regular and gradual. In the warmer periods of the Cretaceous,
the flora as far north as 55° was similar to that of Georgia and
Northern Florida at the present day, while in the cooler period of the
Laramie (Lower Eocene, or more probably Paleocene) it was not unlike
that of the Middle States. In the Pleistocene, the flora indicates a
boreal temperature in the Glacial age. Thus there are no very extreme
contrasts, but the evident fact of a warm temperate or subtropical
climate extending very far north at the same times when Greenland had
a temperate climate. As I have elsewhere shown,[110] discoveries in
various parts of North America are beginning to indicate the precise
geographical conditions accompanying the warmer and colder climates.
[110] Trans. Royal Society of Canada, 1890-1.
It would be wrong to leave this subject without noticing that
remarkable feature in the southward movement of the later floras,
to which I believe Prof. Gray was the first to direct attention. In
those periods when a warm climate prevailed in the Arctic regions,
the temperate flora must have been, like the modern Arctic flora,
circumpolar. When obliged to migrate to the south, it had to follow the
lines of the continents, and so to divide into separate belts. Three of
these at present are the floras of Western Europe, Eastern Asia, and
Eastern America, all of which have many representative species. They
are separated by oceans and by belts of land occupied by plants which
have not been obliged to migrate. Thus, while the flora of the Eastern
United States resembles that of China and Japan, that of California
and Oregon is distinct from both, and represents a belt of old species
retained in place by the continued warmth of the Pacific shore, and the
continuous extension of the American continent to the south affording
them means of retreat in the Glacial age. Were the plants of China
and Eastern America enabled to return to the Arctic, they would then
reunite into one flora. Gray compares the process of their separation
to the kind of selection which might be made by a botanical distributor
who had the whole collection placed in his hands, with instructions
to give one species of each genus to Europe, to Eastern Asia, and to
Eastern America; and if there was only one species in a genus, or if
one remained over, this was to be thrown into one of the regions, with
a certain preference in favour of America and Asia. This remarkable
kind of geographical selection opens a wide field not only for thought,
but for experiment on the actual relationship of the representative
species. There is a similar field for comparison between the trees
of Georgia in latitude 30° to 35°, and the same species or their
representatives as they existed in Cretaceous times in the latitudes of
50° and 60°. The two floras, as I know from actual comparison, are very
similar.
One word may be said here as to use of fossil plants in determining
geological time. In this I need only point to the fact of my
having defined in Canada three Devonian floras, a Lower, Middle,
and Upper, and that Mr. Whiteaves, in his independent study of the
fossil fishes, has vindicated my conclusions. There are also in Nova
Scotia three distinctive sub-floras of the Lower, Middle, and Upper
Carboniferous.[111] I have verified these for the Devonian and
Carboniferous of the United States, and to some extent also for those
of Europe. To the same effect is the recognition of the Kootanie or
Lower Cretaceous, the Middle Cretaceous, Upper Cretaceous, Laramie and
Miocene in Western Canada. These have in all cases corresponded with
the indications of animal fossils[112] and of stratigraphy. Fossil
plants have been less studied in this connection than fossil animals,
but I have no hesitation in affirming that, with reference to the
broader changes of the earth's surface, any competent palæobotanist is
perfectly safe in trusting to the evidence of vegetable fossils.
[111] _Transactions Royal Society of Canada_, 1883 to 1891.
[112] Reports on Fossil Plants of the Devonian and Lower Carboniferous.
It may be objected that such evidence will be affected by the
migrations of plants, so that we cannot be certain that identical
species flourished in Greenland and in temperate America at the same
time. If such species originated in Greenland and migrated southward,
the specimens found at the south may be much newer than those in the
north. This, no doubt, is locally true, but the migrations of plants,
though slow, occupy less time than that of a great geological period.
It may also be objected that the flora of swamps, plains, and mountain
tops would differ at any one period. This also is true, but the same
difficulty applies to animals of the deep sea, the shore, and the land;
and these diversities of station have always to be taken into account
by the palæontologist.
References:--Report on the Erian or Devonian Plants of Canada,
Montreal, 1871. Article in Princeton Review on Genesis and
Migrations of Plants. "The Geological History of Plants," London
and New York, 1888 and 1892. Papers on Fossil Plants of Western
Canada, 1883, and following volumes of Transactions of Royal
Society of Canada.
Note.--Since writing the above, I have obtained access to Dall and
Harris' "Neocene Correlation Papers," which throw some additional
light on the Cretaceous and Eocene Floras of Alaska, which, from
its high northern latitude, affords a good parallel to Greenland.
It would appear that plant-beds occur in that territory at two
horizons. One of these (Cape Beaufort), according to Lesquereux and
Ward, holds species of Neocomian Age, and apparently equivalent to
the Kootanie of British Columbia and the Komé of Greenland. The
other, which occurs at several localities (Elukak, Port Graham,
etc.), has a flora evidently of Laramie (Eocene) age, equivalent to
the "Miocene" of Heer and Lesquereux, and to the Lignite Tertiary
of Canada. The plants are accompanied by lignite, and evidently
_in situ_, and clearly prove harmony with Greenland and British
Columbia in two of the periods of high Arctic temperature indicated
above.
_THE GROWTH OF COAL._
DEDICATED TO THE MEMORY OF
DR. SCHIMPER,
OF STRASBURG,
The Author of "La Flore du Monde Primitif," and
many other Contributions to Fossil Botany,
and of
DR. H. R. GOEPPERT,
whose Essay on the Structure and Formation of Coal
was One of my first Guides in its Study.
Questions of Growth and Driftage--Testimony of a Block of Coal under
the Microscope--Different Kinds of Coal--Conditions Necessary to
Accumulation in Situ--Coal Beds and their Accompaniments--Underclays
and Roofs--Vegetable Remains--History of Coal Groups--Summary
of Evidence--Subsidence of Coal Areas--Stigmaria and other Coal
Plants--Later Coal Accumulations--The Story and Uses of Coal
[Illustration: Part of a Coal Group, at the South Joggins, with
underclays and erect trees and Calamites (p. 238).]