Discourses - Thomas H. Huxley
Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23
When I had the pleasure of seeing Principal Dawson in London last summer,
I showed him my sections of coal, and begged him to re-examine some of
the American coals on his return to Canada, with an eye to the presence
of spores and sporangia, such as I was able to show him in our English
and Scotch coals. He has been good enough to do so; and in a letter dated
September 26th, 1870, he informs me that--
"Indications of spore-cases are rare, except in certain coarse shaly
coals and portions of coals, and in the roofs of the seams. The most
marked case I have yet met with is the shaly coal referred to as
containing _Sporangites_ in my paper on the conditions of accumulation of
coal ("Journal of the Geological Society," vol. xxii. pp. 115, 139, and
165). The purer coals certainly consist principally of cubical tissues
with some true woody matter, and the spore-cases, &c., are chiefly in the
coarse and shaly layers. This is my old doctrine in my two papers in the
"Journal of the Geological Society," and I see nothing to modify it. Your
observations, however, make it probable that the frequent _clear spots_
in the cannels are spore-cases."
Dr. Dawson's results are the more remarkable, as the numerous specimens
of British coal, from various localities, which I have examined, tell one
tale as to the predominance of the spore and sporangium element in their
composition; and as it is exactly in the finest and purest coals, such as
the "Better-Bed" coal of Lowmoor, that the spores and sporangia obviously
constitute almost the entire mass of the deposit.
Coal, such as that which has been described, is always found in sheets,
or "seams," varying from a fraction of an inch to many feet in thickness,
enclosed in the substance of the earth at very various depths, between
beds of rock of different kinds. As a rule, every seam of coal rests upon
a thicker, or thinner, bed of clay, which is known as "under-clay." These
alternations of beds of coal, clay, and rock may be repeated many times,
and are known as the "coal-measures"; and in some regions, as in South
Wales and in Nova Scotia, the coal-measures attain a thickness of twelve
or fourteen thousand feet, and enclose eighty or a hundred seams of coal,
each with its under-clay, and separated from those above and below by
beds of sandstone and shale.
The position of the beds which constitute the coal-measures is infinitely
diverse. Sometimes they are tilted up vertically, sometimes they are
horizontal, sometimes curved into great basins; sometimes they come to
the surface, sometimes they are covered up by thousands of feet of rock.
But, whatever their present position, there is abundant and conclusive
evidence that every under-clay was once a surface soil. Not only do
carbonized root-fibres frequently abound in these under-clays; but the
stools of trees, the trunks of which are broken off and confounded with
the bed of coal, have been repeatedly found passing into radiating roots,
still embedded in the under-clay. On many parts of the coast of England,
what are commonly known as "submarine forests" are to be seen at low
water. They consist, for the most part, of short stools of oak, beech,
and fir-trees, still fixed by their long roots in the bed of blue clay in
which they originally grew. If one of these submarine forest beds should
be gradually depressed and covered up by new deposits, it would present
just the same characters as an under-clay of the coal, if the
_Sigillaria_ and _Lepidodendron_ of the ancient world were substituted
for the oak, or the beech, of our own times.
In a tropical forest, at the present day, the trunks of fallen trees, and
the stools of such trees as may have been broken by the violence of
storms, remain entire for but a short time. Contrary to what might be
expected, the dense wood of the tree decays, and suffers from the ravages
of insects, more swiftly than the bark. And the traveller, setting his
foot on a prostrate trunk, finds that it is a mere shell, which breaks
under his weight, and lands his foot amidst the insects, or the reptiles,
which have sought food or refuge within.
The trees of the coal forests present parallel conditions. When the
fallen trunks which have entered into the composition of the bed of coal
are identifiable, they are mere double shells of bark, flattened together
in consequence of the destruction of the woody core; and Sir Charles
Lyell and Principal Dawson discovered, in the hollow stools of coal trees
of Nova Scotia, the remains of snails, millipedes, and salamander-like
creatures, embedded in a deposit of a different character from that which
surrounded the exterior of the trees. Thus, in endeavouring to comprehend
the formation of a seam of coal, we must try to picture to ourselves a
thick forest, formed for the most part of trees like gigantic club-
mosses, mares'-tails, and tree-ferns, with here and there some that had
more resemblance to our existing yews and fir-trees. We must suppose
that, as the seasons rolled by, the plants grew and developed their
spores and seeds; that they shed these in enormous quantities, which
accumulated on the ground beneath; and that, every now and then, they
added a dead frond or leaf; or, at longer intervals, a rotten branch, or
a dead trunk, to the mass.
A certain proportion of the spores and seeds no doubt fulfilled their
obvious function, and, carried by the wind to unoccupied regions,
extended the limits of the forest; many might be washed away by rain into
streams, and be lost; but a large portion must have remained, to
accumulate like beech-mast, or acorns, beneath the trees of a modern
forest.
But, in this case it may be asked, why does not our English coal consist
of stems and leaves to a much greater extent than it does? What is the
reason of the predominance of the spores and spore-cases in it?
A ready answer to this question is afforded by the study of a living
full-grown club-moss. Shake it upon a piece of paper, and it emits a
cloud of fine dust, which falls over the paper, and is the well-known
Lycopodium powder. Now this powder used to be, and I believe still is,
employed for two objects which seem, at first sight, to have no
particular connection with one another. It is, or was, employed in making
lightning, and in making pills. The coats of the spores contain so much
resinous matter, that a pinch of Lycopodium powder, thrown through the
flame of a candle, burns with an instantaneous flash, which has long done
duty for lightning on the stage. And the same character makes it a
capital coating for pills; for the resinous powder prevents the drug from
being wetted by the saliva, and thus bars the nauseous flavour from the
sensitive papilla; of the tongue.
But this resinous matter, which lies in the walls of the spores and
sporangia, is a substance not easily altered by air and water, and hence
tends to preserve these bodies, just as the bituminized cerecloth
preserves an Egyptian mummy; while, on the other hand, the merely woody
stem and leaves tend to rot, as fast as the wood of the mummy's coffin
has rotted. Thus the mixed heap of spores, leaves, and stems in the coal-
forest would be persistently searched by the long-continued action of air
and rain; the leaves and stems would gradually be reduced to little but
their carbon, or, in other words, to the condition of mineral charcoal in
which we find them; while the spores and sporangia remained as a
comparatively unaltered and compact residuum.
There is, indeed, tolerably clear evidence that the coal must, under some
circumstances, have been converted into a substance hard enough to be
rolled into pebbles, while it yet lay at the surface of the earth; for in
some seams of coal, the courses of rivulets, which must have been living
water, while the stratum in which their remains are found was still at
the surface, have been observed to contain rolled pebbles of the very
coal through which the stream has cut its way.
The structural facts are such as to leave no alternative but to adopt the
view of the origin of such coal as I have described, which has just been
stated; but, happily, the process is not without analogy at the present
day. I possess a specimen of what is called "white coal" from Australia.
It is an inflammable material, burning with a bright flame and having
much the consistence and appearance of oat-cake, which, I am informed
covers a considerable area. It consists, almost entirely, of a compacted
mass of spores and spore-cases. But the fine particles of blown sand
which are scattered through it, show that it must have accumulated,
subaerially, upon the surface of a soil covered by a forest of
cryptogamous plants, probably tree-ferns.
As regards this important point of the subaerial region of coal, I am
glad to find myself in entire accordance with Principal Dawson, who bases
his conclusions upon other, but no less forcible, considerations. In a
passage, which is the continuation of that already cited, he writes:--
"(3) The microscopical structure and chemical composition of the beds of
cannel coal and earthy bitumen, and of the more highly bituminous and
carbonaceous shale, show them to have been of the nature of the fine
vegetable mud which accumulates in the ponds and shallow lakes of modern
swamps. When such tine vegetable sediment is mixed, as is often the case,
with clay, it becomes similar to the bituminous limestone and calcareo-
bituminous shales of the coal-measures. (4) A few of the under-clays,
which support beds of coal, are of the nature of the vegetable mud above
referred to; but the greater part are argillo-arenaceous in composition,
with little vegetable matter, and bleached by the drainage from them of
water containing the products of vegetable decay. They are, in short,
loamy or clay soils, and must have been sufficiently above water to admit
of drainage. The absence of sulphurets, and the occurrence of carbonate
of iron in connection with them, prove that, when they existed as soils,
rain-water, and not sea-water, percolated them. (5) The coal and the
fossil forests present many evidences of subaerial conditions. Most of
the erect and prostrate trees had become hollow shells of bark before
they were finally embedded, and their wood had broken into cubical pieces
of mineral charcoal. Land-snails and galley-worms (_Xylobius_) crept into
them, and they became dens, or traps, for reptiles. Large quantities of
mineral charcoal occur on the surface of all the large beds of coal. None
of these appearances could have been produced by subaqueous action. (6)
Though the roots of the _Sigillaria_ bear more resemblance to the
rhizomes of certain aquatic plants; yet, structurally, they are
absolutely identical with the roots of Cycads, which the stems also
resemble. Further, the _Sigillarioe_ grew on the same soils which
supported Conifers, _Lepidodendra_, _Cordaites_, and Ferns-plants which
could not have grown in water. Again, with the exception perhaps of some
_Pinnularioe_, and _Asterophyllites_, there is a remarkable absence from
the coal measures of any form of properly aquatic vegetation. (7) The
occurrence of marine, or brackish-water animals, in the roofs of coal-
beds, or even in the coal itself, affords no evidence of subaqueous
accumulation, since the same thing occurs in the case of modern submarine
forests. For these and other reasons, some of which are more fully stated
in the papers already referred to, while I admit that the areas of coal
accumulation were frequently submerged, I must maintain that the true
coal is a subaerial accumulation by vegetable growth on soils, wet and
swampy it is true, but not submerged."
I am almost disposed to doubt whether it is necessary to make the
concession of "wet and swampy"; otherwise, there is nothing that I know
of to be said against this excellent conspectus of the reasons for
believing in the subaerial origin of coal.
But the coal accumulated upon the area covered by one of the great
forests of the carboniferous epoch would in course of time, have been
wasted away by the small, but constant, wear and tear of rain and streams
had the land which supported it remained at the same level, or been
gradually raised to a greater elevation. And, no doubt, as much coal as
now exists has been destroyed, after its formation, in this way. What are
now known as coal districts owe their importance to the fact that they
were areas of slow depression, during a greater or less portion of the
carboniferous epoch; and that, in virtue of this circumstance, Mother
Earth was enabled to cover up her vegetable treasures, and preserve them
from destruction.
Wherever a coal-field now exists, there must formerly have been free
access for a great river, or for a shallow sea, bearing sediment in the
shape of sand and mud. When the coal-forest area became slowly depressed,
the waters must have spread over it, and have deposited their burden upon
the surface of the bed of coal, in the form of layers, which are now
converted into shale, or sandstone. Then followed a period of rest, in
which the superincumbent shallow waters became completely filled up, and
finally replaced, by fine mud, which settled down into a new under-clay,
and furnished the soil for a fresh forest growth. This flourished, and
heaped up its spores and wood into coal, until the stage of slow
depression recommenced. And, in some localities, as I have mentioned, the
process was repeated until the first of the alternating beds had sunk to
near three miles below its original level at the surface of the earth.
In reflecting on the statement, thus briefly made, of the main facts
connected with the origin of the coal formed during the carboniferous
epoch, two or three considerations suggest themselves.
In the first place, the great phantom of geological time rises before the
student of this, as of all other, fragments of the history of our earth--
springing irrepressibly out of the facts, like the Djin from the jar
which the fishermen so incautiously opened; and like the Djin again,
being vaporous, shifting, and indefinable, but unmistakably gigantic.
However modest the bases of one's calculation may be, the minimum of time
assignable to the coal period remains something stupendous.
Principal Dawson is the last person likely to be guilty of exaggeration
in this matter, and it will be well to consider what he has to say about
it:--
"The rate of accumulation of coal was very slow. The climate of the
period, in the northern temperate zone, was of such a character that the
true conifers show rings of growth, not larger, nor much less distinct,
than those of many of their modern congeners. The _Sigillarioe_ and
_Calamites_ were not, as often supposed, composed wholly, or even
principally, of lax and soft tissues, or necessarily short-lived. The
former had, it is true, a very thick inner bark; but their dense woody
axis, their thick and nearly imperishable outer bark, and their scanty
and rigid foliage, would indicate no very rapid growth or decay. In the
case of the _Sigillarioe_, the variations in the leaf-scars in different
parts of the trunk, the intercalation of new ridges at the surface
representing that of new woody wedges in the axis, the transverse marks
left by the stages of upward growth, all indicate that several years must
have been required for the growth of stems of moderate size. The enormous
roots of these trees, and the condition of the coal-swamps, must have
exempted them from the danger of being overthrown by violence. They
probably fell in successive generations from natural decay; and making
every allowance for other materials, we may safely assert that every foot
of thickness of pure bituminous coal implies the quiet growth and fall of
at least fifty generations of _Sigillarioe_, and therefore an undisturbed
condition of forest growth enduring through many centuries. Further,
there is evidence that an immense amount of loose parenchymatous tissue,
and even of wood, perished by decay, and we do not know to what extent
even the most durable tissues may have disappeared in this way; so that,
in many coal-seams, we may have only a very small part of the vegetable
matter produced."
Undoubtedly the force of these reflections is not diminished when the
bituminous coal, as in Britain, consists of accumulated spores and spore-
cases, rather than of stems. But, suppose we adopt Principal Dawson's
assumption, that one foot of coal represents fifty generations of coal
plants; and, further, make the moderate supposition that each generation
of coal plants took ten years to come to maturity--then, each foot-
thickness of coal represents five hundred years. The superimposed beds of
coal in one coal-field may amount to a thickness of fifty or sixty feet,
and therefore the coal alone, in that field, represents 500 x 50 = 25,000
years. But the actual coal is but an insignificant portion of the total
deposit, which, as has been seen, may amount to between two and three
miles of vertical thickness. Suppose it be 12,000 feet--which is 240
times the thickness of the actual coal--is there any reason why we should
believe it may not have taken 240 times as long to form? I know of none.
But, in this case, the time which the coal-field represents would be
25,000 x 240 = 6,000,000 years. As affording a definite chronology, of
course such calculations as these are of no value; but they have much use
in fixing one's attention upon a possible minimum. A man may be puzzled
if he is asked how long Rome took a-building; but he is proverbially safe
if he affirms it not to have been built in a day; and our geological
calculations are all, at present, pretty much on that footing.
A second consideration which the study of the coal brings prominently
before the mind of any one who is familiar with palaeontology is, that the
coal Flora, viewed in relation to the enormous period of time which it
lasted, and to the still vaster period which has elapsed since it
flourished, underwent little change while it endured, and in its peculiar
characters, differs strangely little from that which at present exist.
The same species of plants are to be met with throughout the whole
thickness of a coal-field, and the youngest are not sensibly different
from the oldest. But more than this. Notwithstanding that the
carboniferous period is separated from us by more than the whole time
represented by the secondary and tertiary formations, the great types of
vegetation were as distinct then as now. The structure of the modern
club-moss furnishes a complete explanation of the fossil remains of the
_Lepidodendra_, and the fronds of some of the ancient ferns are hard to
distinguish from existing ones. At the same time, it must be remembered,
that there is nowhere in the world, at present, any _forest_ which bears
more than a rough analogy with a coal-forest. The types may remain, but
the details of their form, their relative proportions, their associates,
are all altered. And the tree-fern forest of Tasmania, or New Zealand,
gives one only a faint and remote image of the vegetation of the ancient
world.
Once more, an invariably-recurring lesson of geological history, at
whatever point its study is taken up: the lesson of the almost infinite
slowness of the modification of living forms. The lines of the pedigrees
of living things break off almost before they begin to converge.
Finally, yet another curious consideration. Let us suppose that one of
the stupid, salamander-like Labyrinthodonts, which pottered, with much
belly and little leg, like Falstaff in his old age, among the coal-
forests, could have had thinking power enough in his small brain to
reflect upon the showers of spores which kept on falling through years
and centuries, while perhaps not one in ten million fulfilled its
apparent purpose, and reproduced the organism which gave it birth: surely
he might have been excused for moralizing upon the thoughtless and wanton
extravagance which Nature displayed in her operations.
But we have the advantage over our shovel-headed predecessor--or possibly
ancestor--and can perceive that a certain vein of thrift runs through
this apparent prodigality. Nature is never in a hurry, and seems to have
had always before her eyes the adage, "Keep a thing long enough, and you
will find a use for it." She has kept her beds of coal many millions of
years without being able to find much use for them; she has sent them
down beneath the sea, and the sea-beasts could make nothing of them; she
has raised them up into dry land, and laid the black veins bare, and
still, for ages and ages, there was no living thing on the face of the
earth that could see any sort of value in them; and it was only the other
day, so to speak, that she turned a new creature out of her workshop, who
by degrees acquired sufficient wits to make a fire, and then to discover
that the black rock would burn.
I suppose that nineteen hundred years ago, when Julius Caesar was good
enough to deal with Britain as we have dealt with New Zealand, the
primaeval Briton, blue with cold and woad, may have known that the strange
black stone, of which he found lumps here and there in his wanderings,
would burn, and so help to warm his body and cook his food. Saxon, Dane,
and Norman swarmed into the land. The English people grew into a powerful
nation, and Nature still waited for a full return of the capital she had
invested in the ancient club-mosses. The eighteenth century arrived, and
with it James Watt. The brain of that man was the spore out of which was
developed the modern steam-engine, and all the prodigious trees and
branches of modern industry which have grown out of this. But coal is as
much an essential condition of this growth and development as carbonic
acid is for that of a club-moss. Wanting coal, we could not have smelted
the iron needed to make our engines, nor have worked our engines when we
had got them. But take away the engines, and the great towns of Yorkshire
and Lancashire vanish like a dream. Manufactures give place to
agriculture and pasture, and not ten men can live where now ten thousand
are amply supported.
Thus, all this abundant wealth of money and of vivid life is Nature's
interest upon her investment in club-mosses, and the like, so long ago.
But what becomes of the coal which is burnt in yielding this interest?
Heat comes out of it, light comes out of it; and if we could gather
together all that goes up the chimney, and all that remains in the grate
of a thoroughly-burnt coal-fire, we should find ourselves in possession
of a quantity of carbonic acid, water, ammonia, and mineral matters,
exactly equal in weight to the coal. But these are the very matters with
which Nature supplied the club-mosses which made the coal She is paid
back principal and interest at the same time; and she straightway invests
the carbonic acid, the water, and the ammonia in new forms of life,
feeding with them the plants that now live. Thrifty Nature! Surely no
prodigal, but most notable of housekeepers!
VI
ON THE BORDER TERRITORY BETWEEN THE ANIMAL AND THE VEGETABLE KINGDOMS
[1876]
In the whole history of science there is nothing more remarkable than the
rapidity of the growth of biological knowledge within the last half-
century, and the extent of the modification which has thereby been
effected in some of the fundamental conceptions of the naturalist.
In the second edition of the "Regne Animal," published in 1828, Cuvier
devotes a special section to the "Division of Organised Beings into
Animals and Vegetables," in which the question is treated with that
comprehensiveness of knowledge and clear critical judgment which
characterise his writings, and justify us in regarding them as
representative expressions of the most extensive, if not the profoundest,
knowledge of his time. He tells us that living beings have been
subdivided from the earliest times into _animated beings_, which possess
sense and motion, and _inanimated beings_, which are devoid of these
functions and simply vegetate.
Although the roots of plants direct themselves towards moisture, and
their leaves towards air and light,--although the parts of some plants
exhibit oscillating movements without any perceptible cause, and the
leaves of others retract when touched,--yet none of these movements
justify the ascription to plants of perception or of will. From the
mobility of animals, Cuvier, with his characteristic partiality for
teleological reasoning, deduces the necessity of the existence in them of
an alimentary cavity, or reservoir of food, whence their nutrition may be
drawn by the vessels, which are a sort of internal roots; and, in the
presence of this alimentary cavity, he naturally sees the primary and the
most important distinction between animals and plants.
Following out his teleological argument, Cuvier remarks that the
organisation of this cavity and its appurtenances must needs vary
according to the nature of the aliment, and the operations which it has
to undergo, before it can be converted into substances fitted for
absorption; while the atmosphere and the earth supply plants with juices
ready prepared, and which can be absorbed immediately. As the animal body
required to be independent of heat and of the atmosphere, there were no
means by which the motion of its fluids could be produced by internal
causes. Hence arose the second great distinctive character of animals, or
the circulatory system, which is less important than the digestive, since
it was unnecessary, and therefore is absent, in the more simple animals.