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 "Règne 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.
Animals further needed muscles for locomotion and nerves for sensibility.
Hence, says Cuvier, it was necessary that the chemical composition of the
animal body should be more complicated than that of the plant; and it is
so, inasmuch as an additional substance, nitrogen, enters into it as an
essential element; while, in plants, nitrogen is only accidentally joined
with he three other fundamental constituents of organic beings--carbon,
hydrogen, and oxygen. Indeed, he afterwards affirms that nitrogen is
peculiar to animals; and herein he places the third distinction between
the animal and the plant. The soil and the atmosphere supply plants with
water, composed of hydrogen and oxygen; air, consisting of nitrogen and
oxygen; and carbonic acid, containing carbon and oxygen. They retain the
hydrogen and the carbon, exhale the superfluous oxygen, and absorb little
or no nitrogen. The essential character of vegetable life is the
exhalation of oxygen, which is effected through the agency of light.
Animals, on the contrary, derive their nourishment either directly or
indirectly from plants. They get rid of the superfluous hydrogen and
carbon, and accumulate nitrogen. The relations of plants and animals to
the atmosphere are therefore inverse. The plant withdraws water and
carbonic acid from the atmosphere, the animal contributes both to it.
Respiration--that is, the absorption of oxygen and the exhalation of
carbonic acid--is the specially animal function of animals, and
constitutes their fourth distinctive character.
Thus wrote Cuvier in 1828. But, in the fourth and fifth decades of this
century, the greatest and most rapid revolution which biological science
has ever undergone was effected by the application of the modern
microscope to the investigation of organic structure; by the introduction
of exact and easily manageable methods of conducting the chemical
analysis of organic compounds; and finally, by the employment of
instruments of precision for the measurement of the physical forces which
are at work in the living economy.
That the semi-fluid contents (which we now term protoplasm) of the cells
of certain plants, such as the _Charoe_ are in constant and regular
motion, was made out by Bonaventura Corti a century ago; but the fact,
important as it was, fell into oblivion, and had to be rediscovered by
Treviranus in 1807. Robert Brown noted the more complex motions of the
protoplasm in the cells of _Tradescantia_ in 1831; and now such movements
of the living substance of plants are well known to be some of the most
widely-prevalent phenomena of vegetable life.
Agardh, and other of the botanists of Cuvier's generation, who occupied
themselves with the lower plants, had observed that, under particular
circumstances, the contents of the cells of certain water-weeds were set
free, and moved about with considerable velocity, and with all the
appearances of spontaneity, as locomotive bodies, which, from their
similarity to animals of simple organisation, were called "zoospores."
Even as late as 1845, however, a botanist of Schleiden's eminence dealt
very sceptically with these statements; and his scepticism was the more
justified, since Ehrenberg, in his elaborate and comprehensive work on
the _Infusoria_, had declared the greater number of what are now
recognised as locomotive plants to be animals.
At the present day, innumerable plants and free plant cells are known to
pass the whole or part of their lives in an actively locomotive
condition, in no wise distinguishable from that of one of the simpler
animals; and, while in this condition, their movements are, to all
appearance, as spontaneous--as much the product of volition--as those of
such animals.
Hence the teleological argument for Cuvier's first diagnostic character--
the presence in animals of an alimentary cavity, or internal pocket, in
which they can carry about their nutriment--has broken down, so far, at
least, as his mode of stating it goes. And, with the advance of
microscopic anatomy, the universality of the fact itself among animals
has ceased to be predicable. Many animals of even complex structure,
which live parasitically within others, are wholly devoid of an
alimentary cavity. Their food is provided for them, not only ready
cooked, but ready digested, and the alimentary canal, become superfluous,
has disappeared. Again, the males of most Rotifers have no digestive
apparatus; as a German naturalist has remarked, they devote themselves
entirely to the "Minnedienst," and are to be reckoned among the few
realisations of the Byronic ideal of a lover. Finally, amidst the lowest
forms of animal life, the speck of gelatinous protoplasm, which
constitutes the whole body, has no permanent digestive cavity or mouth,
but takes in its food anywhere; and digests, so to speak, all over its
body. But although Cuvier's leading diagnosis of the animal from the
plant will not stand a strict test, it remains one of the most constant
of the distinctive characters of animals. And, if we substitute for the
possession of an alimentary cavity, the power of taking solid nutriment
into the body and there digesting it, the definition so changed will
cover all animals except certain parasites, and the few and exceptional
cases of non-parasitic animals which do not feed at all. On the other
hand, the definition thus amended will exclude all ordinary vegetable
organisms.
Cuvier himself practically gives up his second distinctive mark when he
admits that it is wanting in the simpler animals.
The third distinction is based on a completely erroneous conception of
the chemical differences and resemblances between the constituents of
animal and vegetable organisms, for which Cuvier is not responsible, as
it was current among contemporary chemists. It is now established that
nitrogen is as essential a constituent of vegetable as of animal living
matter; and that the latter is, chemically speaking, just as complicated
as the former. Starchy substances, cellulose and sugar, once supposed to
be exclusively confined to plants, are now known to be regular and normal
products of animals. Amylaceous and saccharine substances are largely
manufactured, even by the highest animals; cellulose is widespread as a
constituent of the skeletons of the lower animals; and it is probable
that amyloid substances are universally present in the animal organism,
though not in the precise form of starch.
Moreover, although it remains true that there is an inverse relation
between the green plant in sunshine and the animal, in so far as, under
these circumstances, the green plant decomposes carbonic acid and exhales
oxygen, while the animal absorbs oxygen and exhales carbonic acid; yet,
the exact researches of the modern chemical investigators of the
physiological processes of plants have clearly demonstrated the fallacy
of attempting to draw any general distinction between animals and
vegetables on this ground. In fact, the difference vanishes with the
sunshine, even in the case of the green plant; which, in the dark,
absorbs oxygen and gives out carbonic acid like any animal.[1] On the
other hand, those plants, such as the fungi, which contain no chlorophyll
and are not green, are always, so far as respiration is concerned, in the
exact position of animals. They absorb oxygen and give out carbonic acid.
[Footnote 1: There is every reason to believe that living plants, like
living animals, always respire, and, in respiring, absorb oxygen and give
off carbonic acid; but, that in green plants exposed to daylight or to
the electric light, the quantity of oxygen evolved in consequence of the
decomposition of carbonic acid by a special apparatus which green plants
possess exceeds that absorbed in the concurrent respiratory process.]
Thus, by the progress of knowledge, Cuvier's fourth distinction between
the animal and the plant has been as completely invalidated as the third
and second; and even the first can be retained only in a modified form
and subject to exceptions.
But has the advance of biology simply tended to break down old
distinctions, without establishing new ones?
With a qualification, to be considered presently, the answer to this
question is undoubtedly in the affirmative. The famous researches of
Schwann and Schleiden in 1837 and the following years, founded the modern
science of histology, or that branch of anatomy which deals with the
ultimate visible structure of organisms, as revealed by the microscope;
and, from that day to this, the rapid improvement of methods of
investigation, and the energy of a host of accurate observers, have given
greater and greater breadth and firmness to Schwann's great
generalisation, that a fundamental unity of structure obtains in animals
and plants; and that, however diverse may be the fabrics, or _tissues_,
of which their bodies are composed, all these varied structures result
from the metamorphosis of morphological units (termed _cells_, in a more
general sense than that in which the word "cells" was at first employed),
which are not only similar in animals and in plants respectively, but
present a close resemblance, when those of animals and those of plants
are compared together.
The contractility which is the fundamental condition of locomotion, has
not only been discovered to exist far more widely among plants than was
formerly imagined; but, in plants, the act of contraction has been found
to be accompanied, as Dr. Burdon Sanderson's interesting investigations
have shown, by a disturbance of the electrical state of the contractile
substance, comparable to that which was found by Du Bois Reymond to be a
concomitant of the activity of ordinary muscle in animals.
Again, I know of no test by which the reaction of the leaves of the
Sundew and of other plants to stimuli, so fully and carefully studied by
Mr. Darwin, can be distinguished from those acts of contraction following
upon stimuli, which are called "reflex" in animals.
On each lobe of the bilobed leaf of Venus's fly-trap (_Dionoea
muscipula_) are three delicate filaments which stand out at right angle
from the surface of the leaf. Touch one of them with the end of a fine
human hair and the lobes of the leaf instantly close together[2] in
virtue of an act of contraction of part of their substance, just as the
body of a snail contracts into its shell when one of its "horns" is
irritated.
[Footnote 2: Darwin, _Insectivorous Plants_, p. 289.]
The reflex action of the snail is the result of the presence of a nervous
system in the animal. A molecular change takes place in the nerve of the
tentacle, is propagated to the muscles by which the body is retracted,
and causing them to contract, the act of retraction is brought about. Of
course the similarity of the acts does not necessarily involve the
conclusion that the mechanism by which they are effected is the same; but
it suggests a suspicion of their identity which needs careful testing.