Discourses
by
Thomas H. Huxley
Part 3 out of 5
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.
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.
The results of recent inquiries into the structure of the nervous system
of animals converge towards the conclusion that the nerve fibres, which
we have hitherto regarded as ultimate elements of nervous tissue, are not
such, but are simply the visible aggregations of vastly more attenuated
filaments, the diameter of which dwindles down to the limits of our
present microscopic vision, greatly as these have been extended by modern
improvements of the microscope; and that a nerve is, in its essence,
nothing but a linear tract of specially modified protoplasm between two
points of an organism--one of which is able to affect the other by means
of the communication so established. Hence, it is conceivable that even
the simplest living being may possess a nervous system. And the question
whether plants are provided with a nervous system or not, thus acquires a
new aspect, and presents the histologist and physiologist with a problem
of extreme difficulty, which must be attacked from a new point of view
and by the aid of methods which have yet to be invented.
Thus it must be admitted that plants may be contractile and locomotive;
that, while locomotive, their movements may have as much appearance of
spontaneity as those of the lowest animals; and that many exhibit
actions, comparable to those which are brought about by the agency of a
nervous system in animals. And it must be allowed to be possible that
further research may reveal the existence of something comparable to a
nervous system in plants. So that I know not where we can hope to find
any absolute distinction between animals and plants, unless we return to
their mode of nutrition, and inquire whether certain differences of a
more occult character than those imagined to exist by Cuvier, and which
certainly hold good for the vast majority of animals and plants, are of
universal application.
A bean may be supplied with water in which salts of ammonia and certain
other mineral salts are dissolved in due proportion; with atmospheric air
containing its ordinary minute dose of carbonic acid; and with nothing
else but sunlight and heat. Under these circumstances, unnatural as they
are, with proper management, the bean will thrust forth its radicle and
its plumule; the former will grow down into roots, the latter grow up
into the stem and leaves of a vigorous bean-plant; and this plant will,
in due time, flower and produce its crop of beans, just as if it were
grown in the garden or in the field.
The weight of the nitrogenous protein compounds, of the oily, starchy,
saccharine and woody substances contained in the full-grown plant and its
seeds, will be vastly greater than the weight of the same substances
contained in the bean from which it sprang. But nothing has been supplied
to the bean save water, carbonic acid, ammonia, potash, lime, iron, and
the like, in combination with phosphoric, sulphuric, and other acids.
Neither protein, nor fat, nor starch, nor sugar, nor any substance in the
slightest degree resembling them, has formed part of the food of the
bean. But the weights of the carbon, hydrogen, oxygen, nitrogen,
phosphorus, sulphur, and other elementary bodies contained in the bean-
plant, and in the seeds which it produces, are exactly equivalent to the
weights of the same elements which have disappeared from the materials
supplied to the bean during its growth. Whence it follows that the bean
has taken in only the raw materials of its fabric, and has manufactured
them into bean-stuffs.
The bean has been able to perform this great chemical feat by the help of
its green colouring matter, or chlorophyll; for it is only the green
parts of the plant which, under the influence of sunlight, have the
marvellous power of decomposing carbonic acid, setting free the oxygen
and laying hold of the carbon which it contains. In fact, the bean
obtains two of the absolutely indispensable elements of its substance
from two distinct sources; the watery solution, in which its roots are
plunged, contains nitrogen but no carbon; the air, to which the leaves
are exposed, contains carbon, but its nitrogen is in the state of a free
gas, in which condition the bean can make no use of it;[3] and the
chlorophyll[4] is the apparatus by which the carbon is extracted from the
atmospheric carbonic acid--the leaves being the chief laboratories in
which this operation is effected.
[Footnote 3: I purposely assume that the air with which the bean is
supplied in the case stated contains no ammoniacal salts.]
[Footnote 4: The recent researches of Pringsheim have raised a host of
questions as to the exact share taken by chlorophyll in the chemical
operations which are effected by the green parts of plants. It may be
that the chlorophyll is only a constant concomitant of the actual
deoxidising apparatus.]
The great majority of conspicuous plants are, as everybody knows, green;
and this arises from the abundance of their chlorophyll. The few which
contain no chlorophyll and are colourless, are unable to extract the
carbon which they require from atmospheric carbonic acid, and lead a
parasitic existence upon other plants; but it by no means follows, often
as the statement has been repeated, that the manufacturing power of
plants depends on their chlorophyll, and its interaction with the rays of
the sun. On the contrary, it is easily demonstrated, as Pasteur first
proved, that the lowest fungi, devoid of chlorophyll, or of any
substitute for it, as they are, nevertheless possess the characteristic
manufacturing powers of plants in a very high degree. Only it is
necessary that they should be supplied with a different kind of raw
material; as they cannot extract carbon from carbonic acid, they must be
furnished with something else that contains carbon. Tartaric acid is such
a substance; and if a single spore of the commonest and most troublesome
of moulds--_Penicillium_--be sown in a saucerful of water, in which
tartrate of ammonia, with a small percentage of phosphates and sulphates
is contained, and kept warm, whether in the dark or exposed to light, it
will, in a short time, give rise to a thick crust of mould, which
contains many million times the weight of the original spore, in protein
compounds and cellulose. Thus we have a very wide basis of fact for the
generalisation that plants are essentially characterised by their
manufacturing capacity--by their power of working up mere mineral matters
into complex organic compounds.
Contrariwise, there is a no less wide foundation for the generalisation
that animals, as Cuvier puts it, depend directly or indirectly upon
plants for the materials of their bodies; that is, either they are
herbivorous, or they eat other animals which are herbivorous.
But for what constituents of their bodies are animals thus dependent upon
plants? Certainly not for their horny matter; nor for chondrin, the
proximate chemical element of cartilage; nor for gelatine; nor for
syntonin, the constituent of muscle; nor for their nervous or biliary
substances; nor for their amyloid matters; nor, necessarily, for their
fats.
It can be experimentally demonstrated that animals can make these for
themselves. But that which they cannot make, but must, in all known
cases, obtain directly or indirectly from plants, is the peculiar
nitrogenous matter, protein. Thus the plant is the ideal _proletaire_ of
the living world, the worker who produces; the animal, the ideal
aristocrat, who mostly occupies himself in consuming, after the manner of
that noble representative of the line of Zaehdarm, whose epitaph is
written in "Sartor Resartus."
Here is our last hope of finding a sharp line of demarcation between
plants and animals; for, as I have already hinted, there is a border
territory between the two kingdoms, a sort of no-man's-land, the
inhabitants of which certainly cannot be discriminated and brought to
their proper allegiance in any other way.
Some months ago, Professor Tyndall asked me to examine a drop of infusion
of hay, placed under an excellent and powerful microscope, and to tell
him what I thought some organisms visible in it were. I looked and
observed, in the first place, multitudes of _Bacteria_ moving about with
their ordinary intermittent spasmodic wriggles. As to the vegetable
nature of these there is now no doubt. Not only does the close
resemblance of the _Bacteria_ to unquestionable plants, such as the
_Oscillatorioe_ and the lower forms of _Fungi_, justify this conclusion,
but the manufacturing test settles the question at once. It is only
needful to add a minute drop of fluid containing _Bacteria_, to water in
which tartrate, phosphate, and sulphate of ammonia are dissolved; and, in
a very short space of time, the clear fluid becomes milky by reason of
their prodigious multiplication, which, of course, implies the
manufacture of living Bacterium-stuff out of these merely saline matters.
But other active organisms, very much larger than the _Bacteria_,
attaining in fact the comparatively gigantic dimensions of 1/3000 of an
inch or more, incessantly crossed the field of view. Each of these had a
body shaped like a pear, the small end being slightly incurved and
produced into a long curved filament, or _cilium_, of extreme tenuity.
Behind this, from the concave side of the incurvation, proceeded another
long cilium, so delicate as to be discernible only by the use of the
highest powers and careful management of the light. In the centre of the
pear-shaped body a clear round space could occasionally be discerned, but
not always; and careful watching showed that this clear vacuity appeared
gradually, and then shut up and disappeared suddenly, at regular
intervals. Such a structure is of common occurrence among the lowest
plants and animals, and is known as a _contractile vacuole_.
The little creature thus described sometimes propelled itself with great
activity, with a curious rolling motion, by the lashing of the front
cilium, while the second cilium trailed behind; sometimes it anchored
itself by the hinder cilium and was spun round by the working of the
other, its motions resembling those of an anchor buoy in a heavy sea.
Sometimes, when two were in full career towards one another, each would
appear dexterously to get out of the other's way; sometimes a crowd would
assemble and jostle one another, with as much semblance of individual
effort as a spectator on the Grands Mulets might observe with a telescope
among the specks representing men in the valley of Chamounix.
The spectacle, though always surprising, was not new to me. So my reply
to the question put to me was, that these organisms were what biologists
call _Monads_, and though they might be animals, it was also possible
that they might, like the _Bacteria_, be plants. My friend received my
verdict with an expression which showed a sad want of respect for
authority. He would as soon believe that a sheep was a plant. Naturally
piqued by this want of faith, I have thought a good deal over the matter;
and, as I still rest in the lame conclusion I originally expressed, and
must even now confess that I cannot certainly say whether this creature
is an animal or a plant, I think it may be well to state the grounds of
my hesitation at length. But, in the first place, in order that I may
conveniently distinguish this "Monad" from the multitude of other things
which go by the same designation, I must give it a name of its own. I
think (though, for reasons which need not be stated at present, I am not
quite sure) that it is identical with the species _Monas lens_ as defined
by the eminent French microscopist Dujardin, though his magnifying power
was probably insufficient to enable him to see that it is curiously like
a much larger form of monad which he has named _Heteromita_. I shall,
therefore, call it not _Monas_, but _Heteromita lens_.
I have been unable to devote to my _Heteromita_ the prolonged study
needful to work out its whole history, which would involve weeks, or it
may be months, of unremitting attention. But I the less regret this
circumstance, as some remarkable observations recently published by
Messrs. Dallinger and Drysdale[5] on certain Monads, relate, in part, to
a form so similar to my _Heteromita lens_, that the history of the one
may be used to illustrate that of the other. These most patient and
painstaking observers, who employed the highest attainable powers of the
microscope and, relieving one another, kept watch day and night over the
same individual monads, have been enabled to trace out the whole history
of their _Heteromita_; which they found in infusions of the heads of
fishes of the Cod tribe.
[Footnote 5: "Researches in the Life-history of a Cercomonad: a Lesson in
Biogenesis"; and "Further Researches in the Life-history of the Monads,"
--_Monthly Microscopical Journal_, 1873.]
Of the four monads described and figured by these investigators, one, as
I have said, very closely resembles _Heteromita lens_ in every
particular, except that it has a separately distinguishable central
particle or "nucleus," which is not certainly to be made out in
_Heteromita lens_; and that nothing is said by Messrs. Dallinger and
Drysdale of the existence of a contractile vacuole in this monad, though
they describe it in another.
Their _Heteromita_, however, multiplied rapidly by fission. Sometimes a
transverse constriction appeared; the hinder half developed a new cilium,
and the hinder cilium gradually split from its base to its free end,
until it was divided into two; a process which, considering the fact that
this fine filament cannot be much more than 1/100000 of an inch in
diameter, is wonderful enough. The constriction of the body extended
inwards until the two portions were united by a narrow isthmus; finally,
they separated and each swam away by itself, a complete _Heteromita_,
provided with its two cilia. Sometimes the constriction took a
longitudinal direction, with the same ultimate result. In each case the
process occupied not more than six or seven minutes. At this rate, a
single _Heteromita_ would give rise to a thousand like itself in the
course of an hour, to about a million in two hours, and to a number
greater than the generally assumed number of human beings now living in
the world in three hours; or, if we give each _Heteromita_ an hour's
enjoyment of individual existence, the same result will be obtained in
about a day. The apparent suddenness of the appearance of multitudes of
such organisms as these in any nutritive fluid to which one obtains
access is thus easily explained.
During these processes of multiplication by fission, the _Heteromita_
remains active; but sometimes another mode of fission occurs. The body
becomes rounded and quiescent, or nearly so; and, while in this resting
state, divides into two portions, each of which is rapidly converted into
an active _Heteromita_.
A still more remarkable phenomenon is that kind of multiplication which
is preceded by the union of two monads, by a process which is termed
_conjugation_. Two active _Heteromitoe_ become applied to one another,
and then slowly and gradually coalesce into one body. The two nuclei run
into one; and the mass resulting from the conjugation of the two
_Heteromitoe_, thus fused together, has a triangular form. The two pairs
of cilia are to be seen, for some time, at two of the angles, which
answer to the small ends of the conjoined monads; but they ultimately
vanish, and the twin organism, in which all visible traces of
organisation have disappeared, falls into a state of rest. Sudden wave-
like movements of its substance next occur; and, in a short time, the
apices of the triangular mass burst, and give exit to a dense yellowish,
glairy fluid, filled with minute granules. This process, which, it will
be observed, involves the actual confluence and mixture of the substance
of two distinct organisms, is effected in the space of about two hours.
The authors whom I quote say that they "cannot express" the excessive
minuteness of the granules in question, and they estimate their diameter
at less than 1/200000 of an inch. Under the highest powers of the
microscope, at present applicable, such specks are hardly discernible.
Nevertheless, particles of this size are massive when compared to
physical molecules; whence there is no reason to doubt that each, small
as it is, may have a molecular structure sufficiently complex to give
rise to the phenomena of life. And, as a matter of fact, by patient
watching of the place at which these infinitesimal living particles were
discharged, our observers assured themselves of their growth and
development into new monads. In about four hours from their being set
free, they had attained a sixth of the length of the parent, with the
characteristic cilia, though at first they were quite motionless; and, in
four hours more, they had attained the dimensions and exhibited all the
activity of the adult. These inconceivably minute particles are therefore
the germs of the _Heteromita_; and from the dimensions of these germs it
is easily shown that the body formed by conjugation may, at a low
estimate, have given exit to thirty thousand of them; a result of a
matrimonial process whereby the contracting parties, without a metaphor,
"become one flesh," enough to make a Malthusian despair of the future of
the Universe.
I am not aware that the investigators from whom I have borrowed this
history have endeavoured to ascertain whether their monads take solid
nutriment or not; so that though they help us very much to fill up the
blanks in the history of my _Heteromita_, their observations throw no
light on the problem we are trying to solve--Is it an animal or is it a
plant?
Undoubtedly it is possible to bring forward very strong arguments in
favour of regarding _Heteromita_ as a plant.
For example, there is a Fungus, an obscure and almost microscopic mould,
termed _Peronospora infestans_. Like many other Fungi, the _Peronosporoe_
are parasitic upon other plants; and this particular _Peronospora_
happens to have attained much notoriety and political importance, in a
way not without a parallel in the career of notorious politicians,
namely, by reason of the frightful mischief it has done to mankind. For
it is this _Fungus_ which is the cause of the potato disease; and,
therefore, _Peronospora infestans_ (doubtless of exclusively Saxon
origin, though not accurately known to be so) brought about the Irish
famine. The plants afflicted with the malady are found to be infested by
a mould, consisting of fine tubular filaments, termed _hyphoe_, which
burrow through the substance of the potato plant, and appropriate to
themselves the substance of their host; while, at the same time, directly
or indirectly, they set up chemical changes by which even its woody
framework becomes blackened, sodden, and withered.
In structure, however, the _Peronospora_ is as much a mould as the common
_Penicillium_; and just as the _Penicillium_ multiplies by the breaking
up of its hyphoe into separate rounded bodies, the spores; so, in the
_Peronospora_, certain of the hyphoe grow out into the air through the
interstices of the superficial cells of the potato plant, and develop
spores. Each of these hyphoe usually gives off several branches. The ends
of the branches dilate and become closed sacs, which eventually drop off
as spores. The spores falling on some part of the same potato plant, or
carried by the wind to another, may at once germinate, throwing out
tubular prolongations which become hyphoe, and burrow into the substance
of the plant attacked. But, more commonly, the contents of the spore
divide into six or eight separate portions. The coat of the spore gives
way, and each portion then emerges as an independent organism, which has
the shape of a bean, rather narrower at one end than the other, convex on
one side, and depressed or concave on the opposite. From the depression,
two long and delicate cilia proceed, one shorter than the other, and
directed forwards. Close to the origin of these cilia, in the substance
of the body, is a regularly pulsating, contractile vacuole. The shorter
cilium vibrates actively, and effects the locomotion of the organism,
while the other trails behind; the whole body rolling on its axis with
its pointed end forwards.
The eminent botanist, De Bary, who was not thinking of our problem, tells
us, in describing the movements of these "Zoospores," that, as they swim
about, "Foreign bodies are carefully avoided, and the whole movement has
a deceptive likeness to the voluntary changes of place which are observed
in microscopic animals."
After swarming about in this way in the moisture on the surface of a leaf
or stem (which, film though it may be, is an ocean to such a fish) for
half an hour, more or less, the movement of the zoospore becomes slower,
and is limited to a slow turning upon its axis, without change of place.
It then becomes quite quiet, the cilia disappear, it assumes a spherical
form, and surrounds itself with a distinct, though delicate, membranous
coat. A protuberance then grows out from one side of the sphere, and
rapidly increasing in length, assumes the character of a hypha. The
latter penetrates into the substance of the potato plant, either by
entering a stomate, or by boring through the wall of an epidermic cell,
and ramifies, as a mycelium, in the substance of the plant, destroying
the tissues with which it comes in contact. As these processes of
multiplication take place very rapidly, millions of spores are soon set
free from a single infested plant; and, from their minuteness, they are
readily transported by the gentlest breeze. Since, again, the zoospores
set free from each spore, in virtue of their powers of locomotion,
swiftly disperse themselves over the surface, it is no wonder that the
infection, once started, soon spreads from field to field, and extends
its ravages over a whole country.
However, it does not enter into my present plan to treat of the potato
disease, instructively as its history bears upon that of other epidemics;
and I have selected the case of the _Peroganspora_ simply because it
affords an example of an organism, which, in one stage of its existence,
is truly a "Monad," indistinguishable by any important character from our
_Heteromita_, and extraordinarily like it in some respects. And yet this
"Monad" can be traced, step by step, through the series of metamorphoses
which I have described, until it assumes the features of an organism,
which is as much a plant as is an oak or an elm.
Moreover, it would be possible to pursue the analogy farther. Under
certain circumstances, a process of conjugation takes place in the
_Peronospora_. Two separate portions of its protoplasm become fused
together, surround themselves with a thick coat and give rise to a sort
of vegetable egg called an _oospore_. After a period of rest, the
contents of the oospore break up into a number of zoospores like those
already described, each of which, after a period of activity, germinates
in the ordinary way. This process obviously corresponds with the
conjugation and subsequent setting free of germs in the _Heteromita_.
But it may be said that the _Peronospora_ is, after all, a questionable
sort of plant; that it seems to be wanting in the manufacturing power,
selected as the main distinctive character of vegetable life; or, at any
rate, that there is no proof that it does not get its protein matter
ready made from the potato plant.
Let us, therefore, take a case which is not open to these objections.
There are some small plants known to botanists as members of the genus
_Colcochaete_, which, without being truly parasitic, grow upon certain
water-weeds, as lichens grow upon trees. The little plant has the form of
an elegant green star, the branching arms of which are divided into
cells. Its greenness is due to its chlorophyll, and it undoubtedly has
the manufacturing power in full degree, decomposing carbonic acid and
setting oxygen free, under the influence of sunlight. But the
protoplasmic contents of some of the cells of which the plant is made up
occasionally divide, by a method similar to that which effects the
division of the contents of the _Peronospora_ spore; and the severed
portions are then set free as active monad-like zoospores. Each is oval
and is provided at one extremity with two long active cilia. Propelled by
these, it swims about for a longer or shorter time, but at length comes
to a state of rest and gradually grows into a _Coleochaete_. Moreover, as
in the _Peronospora_, conjugation may take place and result in an
oospore; the contents of which divide and are set free as monadiform
germs.
If the whole history of the zoospores of _Peronospora_ and of
_Coleochaete_ were unknown, they would undoubtedly be classed among
"Monads" with the same right as _Heteromita_; why then may not
_Heteromita_ be a plant, even though the cycle of forms through which it
passes shows no terms quite so complex as those which occur in
_Peronospora_ and _Coleochaete_? And, in fact, there are some green
organisms, in every respect characteristically plants, such as
_Chlamydomonas_, and the common _Volvox_, or so-called "Globe
animalcule," which run through a cycle of forms of just the same simple
character as those of _Heteromita_.
The name of _Chlamydomonas_ is applied to certain microscopic green
bodies, each of which consists of a protoplasmic central substance
invested by a structureless sac. The latter contains cellulose, as in
ordinary plants; and the chlorophyll which gives the green colour enables
the _Chlamydomonas_ to decompose carbonic acid and fix carbon as they do.
Two long cilia protrude through the cell-wall, and effect the rapid
locomotion of this "monad," which, in all respects except its mobility,
is characteristically a plant. Under ordinary circumstances, the
_Chlamydomonas_ multiplies by simple fission, each splitting into two or
into four parts, which separate and become independent organisms.
Sometimes, however, the _Chlamydomonas_ divides into eight parts, each of
which is provided with four instead of two cilia. These "zoospores"
conjugate in pairs, and give rise to quiescent bodies, which multiply by
division, find eventually pass into the active state.
Thus, so far as outward form and the general character of the cycle of
modifications, through which the organism passes in the course of its
life, are concerned, the resemblance between _Chlamydomonas_ and
_Heteromita_ is of the closest description. And on the face of the matter
there is no ground for refusing to admit that _Heteromita_ may be related
to _Chlamydomonas_, as the colourless fungus is to the green alga.
_Volvox_ may be compared to a hollow sphere, the wall of which is made up
of coherent Chlamydomonads; and which progresses with a rotating motion
effected by the paddling of the multitudinous pairs of cilia which
project from its surface. Each _Volvox_-monad, moreover, possesses a red
pigment spot, like the simplest form of eye known among animals. The
methods of fissive multiplication and of conjugation observed in the
monads of this locomotive globe are essentially similar to those observed
in _Chlamydomonas_; and, though a hard battle has been fought over it,
_Volvox_ is now finally surrendered to the Botanists.
Thus there is really no reason why _Heteromita_ may not be a plant; and
this conclusion would be very satisfactory, if it were not equally easy
to show that there is really no reason why it should not be an animal.
For there are numerous organisms presenting the closest resemblance to
_Heteromita_, and, like it, grouped under the general name of "Monads,"
which, nevertheless, can be observed to take in solid nutriment, and
which, therefore, have a virtual, if not an actual, mouth and digestive
cavity, and thus come under Cuvier's definition of an animal. Numerous
forms of such animals have been described by Ehrenberg, Dujardin, H.
James Clark, and other writers on the _Infusoria_. Indeed, in another
infusion of hay in which my _Heteromita lens_ occurred, there were
innumerable such infusorial animalcules belonging to the well-known
species _Colpoda cucullus_.[6]
[Footnote 6: Excellently described by Stein, almost all of whose
statements I have verified.]
Full-sized specimens of this animalcule attain a length of between 1/300
or 1/400 of an inch, so that it may have ten times the length and a
thousand times the mass of a _Heteromita_. In shape, it is not altogether
unlike _Heteromita_. The small end, however, is not produced into one
long cilium, but the general surface of the body is covered with small
actively vibrating ciliary organs, which are only longest at the small
end. At the point which answers to that from which the two cilia arise in
_Heteromita_, there is a conical depression, the mouth; and, in young
specimens, a tapering filament, which reminds one of the posterior cilium
of _Heteromita_, projects from this region.
The body consists of a soft granular protoplasmic substance, the middle
of which is occupied by a large oval mass called the "nucleus"; while, at
its hinder end, is a "contractile vacuole," conspicuous by its regular
rhythmic appearances and disappearances. Obviously, although the
_Colpoda_ is not a monad, it differs from one only in subordinate
details. Moreover, under certain conditions, it becomes quiescent,
incloses itself in a delicate case or _cyst_, and then divides into two,
four, or more portions, which are eventually set free and swim about as
active _Colpodoe_.
But this creature is an unmistakable animal, and full-sized _Colpodoe_
may be fed as easily as one feeds chickens. It is only needful to diffuse
very finely ground carmine through the water in which they live, and, in
a very short time, the bodies of the _Colpodoe_ are stuffed with the
deeply-coloured granules of the pigment.
And if this were not sufficient evidence of the animality of _Colpoda_,
there comes the fact that it is even more similar to another well-known
animalcule, _Paramoecium_, than it is to a monad. But _Paramoecium_ is so
huge a creature compared with those hitherto discussed--it reaches 1/120
of an inch or more in length--that there is no difficulty in making out
its organisation in detail; and in proving that it is not only an animal,
but that it is an animal which possesses a somewhat complicated
organisation. For example, the surface layer of its body is different in
structure from the deeper parts. There are two contractile vacuoles, from
each of which radiates a system of vessel-like canals; and not only is
there a conical depression continuous with a tube, which serve as mouth
and gullet, but the food ingested takes a definite course, and refuse is
rejected from a definite region. Nothing is easier than to feed these
animals, and to watch the particles of indigo or carmine accumulate at
the lower end of the gullet. From this they gradually project, surrounded
by a ball of water, which at length passes with a jerk, oddly simulating
a gulp, into the pulpy central substance of the body, there to circulate
up one side and down the other, until its contents are digested and
assimilated. Nevertheless, this complex animal multiplies by division, as
the monad does, and, like the monad, undergoes conjugation. It stands in
the same relation to _Heteromita_ on the animal side, as _Coleochaete_
does on the plant side. Start from either, and such an insensible series
of gradations leads to the monad that it is impossible to say at any
stage of the progress where the line between the animal and the plant
must be drawn.
There is reason to think that certain organisms which pass through a
monad stage of existence, such as the _Myxomycetes_, are, at one time of
their lives, dependent upon external sources for their protein matter, or
are animals; and, at another period, manufacture it, or are plants. And
seeing that the whole progress of modern investigation is in favour of
the doctrine of continuity, it is a fair and probable speculation--though
only a speculation--that, as there are some plants which can manufacture
protein out of such apparently intractable mineral matters as carbonic
acid, water, nitrate of ammonia, metallic and earthy salts; while others
need to be supplied with their carbon and nitrogen in the somewhat less
raw form of tartrate of ammonia and allied compounds; so there may be yet
others, as is possibly the case with the true parasitic plants, which can
only manage to put together materials still better prepared--still more
nearly approximated to protein--until we arrive at such organisms as the
_Psorospermioe_ and the _Panhistophyton_, which are as much animal as
vegetable in structure, but are animal in their dependence on other
organisms for their food.
The singular circumstance observed by Meyer, that the _Torula_ of yeast,
though an indubitable plant, still flourishes most vigorously when
supplied with the complex nitrogenous substance, pepsin; the probability
that the _Peronospora_ is nourished directly by the protoplasm of the
potato-plant; and the wonderful facts which have recently been brought to
light respecting insectivorous plants, all favour this view; and tend to
the conclusion that the difference between animal and plant is one of
degree rather than of kind, and that the problem whether, in a given
case, an organism is an animal or a plant, may be essentially insoluble.
VII
A LOBSTER; OR, THE STUDY OF ZOOLOGY
[1861]
Natural history is the name familiarly applied to the study of the
properties of such natural bodies as minerals, plants, and animals; the
sciences which embody the knowledge man has acquired upon these subjects
are commonly termed Natural Sciences, in contradistinction to other so-
called "physical" sciences; and those who devote themselves especially to
the pursuit of such sciences have been and are commonly termed
"Naturalists."
Linnaeus was a naturalist in this wide sense, and his "Systema Naturae" was
a work upon natural history, in the broadest acceptation of the term; in
it, that great methodising spirit embodied all that was known in his time
of the distinctive characters of minerals, animals, and plants. But the
enormous stimulus which Linnaeus gave to the investigation of nature soon
rendered it impossible that any one man should write another "Systema
Naturae," and extremely difficult for any one to become even a naturalist
such as Linnaeus was.
Great as have been the advances made by all the three branches of
science, of old included under the title of natural history, there can be
no doubt that zoology and botany have grown in an enormously greater
ratio than mineralogy; and hence, as I suppose, the name of "natural
history" has gradually become more and more definitely attached to these
prominent divisions of the subject, and by "naturalist" people have meant
more and more distinctly to imply a student of the structure and function
of living beings.
However this may be, it is certain that the advance of knowledge has
gradually widened the distance between mineralogy and its old associates,
while it has drawn zoology and botany closer together; so that of late
years it has been found convenient (and indeed necessary) to associate
the sciences which deal with vitality and all its phenomena under the
common head of "biology"; and the biologists have come to repudiate any
blood-relationship with their foster-brothers, the mineralogists.
Certain broad laws have a general application throughout both the animal
and the vegetable worlds, but the ground common to these kingdoms of
nature is not of very wide extent, and the multiplicity of details is so
great, that the student of living beings finds himself obliged to devote
his attention exclusively either to the one or the other. If he elects to
study plants, under any aspect, we know at once what to call him. He is a
botanist, and his science is botany. But if the investigation of animal
life be his choice, the name generally applied to him will vary according
to the kind of animals he studies, or the particular phenomena of animal
life to which he confines his attention. If the study of man is his
object, he is called an anatomist, or a physiologist, or an ethnologist;
but if he dissects animals, or examines into the mode in which their
functions are performed, he is a comparative anatomist or comparative
physiologist. If he turns his attention to fossil animals, he is a
palaeontologist. If his mind is more particularly directed to the specific
description, discrimination, classification, and distribution of animals,
he is termed a zoologist.
For the purpose of the present discourse, however, I shall recognise none
of these titles save the last, which I shall employ as the equivalent of
botanist, and I shall use the term zoology is denoting the whole doctrine
of animal life, in contradistinction to botany, which signifies the whole
doctrine of vegetable life.
Employed in this sense, zoology, like botany, is divisible into three
great but subordinate sciences, morphology, physiology, and distribution,
each of which may, to a very great extent, be studied independently of
the other.
Zoological morphology is the doctrine of animal form or structure.
Anatomy is one of its branches; development is another; while
classification is the expression of the relations which different animals
bear to one another, in respect of their anatomy and their development.
Zoological distribution is the study of animals in relation to the
terrestrial conditions which obtain now, or have obtained at any previous
epoch of the earth's history.
Zoological physiology, lastly, is the doctrine of the functions or
actions of animals. It regards animal bodies as machines impelled by
certain forces, and performing an amount of work which can be expressed
in terms of the ordinary forces of nature. The final object of physiology
is to deduce the facts of morphology, on the one hand, and those of
distribution on the other, from the laws of the molecular forces of
matter.
Such is the scope of zoology. But if I were to content myself with the
enunciation of these dry definitions, I should ill exemplify that method
of teaching this branch of physical science, which it is my chief
business to-night to recommend. Let us turn away then from abstract
definitions. Let us take some concrete living thing, some animal, the
commoner the better, and let us see how the application of common sense
and common logic to the obvious facts it presents, inevitably leads us
into all these branches of zoological science.
I have before me a lobster. When I examine it, what appears to be the
most striking character it presents? Why, I observe that this part which
we call the tail of the lobster, is made up of six distinct hard rings
and a seventh terminal piece. If I separate one of the middle rings, say
the third, I find it carries upon its under surface a pair of limbs or
appendages, each of which consists of a stalk and two terminal pieces. So
that I can represent a transverse section of the ring and its appendages
upon the diagram board in this way.
If I now take the fourth ring, I find it has the same structure, and so
have the fifth and the second; so that, in each of these divisions of the
tail, I find parts which correspond with one another, a ring and two
appendages; and in each appendage a stalk and two end pieces. These
corresponding parts are called, in the technical language of anatomy,
"homologous parts." The ring of the third division is the "homologue" of
the ring of the fifth, the appendage of the former is the homologue of
the appendage of the latter. And, as each division exhibits corresponding
parts in corresponding places, we say that all the divisions are
constructed upon the same plan. But now let us consider the sixth
division. It is similar to, and yet different from, the others. The ring
is essentially the same as in the other divisions; but the appendages
look at first as if they were very different; and yet when we regard them
closely, what do we find? A stalk and two terminal divisions, exactly as
in the others, but the stalk is very short and very thick, the terminal
divisions are very broad and flat, and one of them is divided into two
pieces.
I may say, therefore, that the sixth segment is like the others in plan,
but that it is modified in its details.
The first segment is like the others, so far as its ring is concerned,
and though its appendages differ from any of those yet examined in the
simplicity of their structure, parts corresponding with the stem and one
of the divisions of the appendages of the other segments can be readily
discerned in them.
Thus it appears that the lobster's tail is composed of a series of
segments which are fundamentally similar, though each presents peculiar
modifications of the plan common to all. But when I turn to the forepart
of the body I see, at first, nothing but a great shield-like shell,
called technically the "carapace," ending in front in a sharp spine, on
either side of which are the curious compound eyes, set upon the ends of
stout movable stalks. Behind these, on the under side of the body, are
two pairs of long feelers, or antennae, followed by six pairs of jaws
folded against one another over the mouth, and five pairs of legs, the
foremost of these being the great pinchers, or claws, of the lobster.
It looks, at first, a little hopeless to attempt to find in this complex
mass a series of rings, each with its pair of appendages, such as I have
shown you in the abdomen, and yet it is not difficult to demonstrate
their existence. Strip off the legs, and you will find that each pair is
attached to a very definite segment of the under wall of the body; but
these segments, instead of being the lower parts of free rings, as in the
tail, are such parts of rings which are all solidly united and bound
together; and the like is true of the jaws, the feelers, and the eye-
stalks, every pair of which is borne upon its own special segment. Thus
the conclusion is gradually forced upon us, that the body of the lobster
is composed of as many rings as there are pairs of appendages, namely,
twenty in all, but that the six hindmost rings remain free and movable,
while the fourteen front rings become firmly soldered together, their
backs forming one continuous shield--the carapace.
Unity of plan, diversity in execution, is the lesson taught by the study
of the rings of the body, and the same instruction is given still more
emphatically by the appendages. If I examine the outermost jaw I find it
consists of three distinct portions, an inner, a middle, and an outer,
mounted upon a common stem; and if I compare this jaw with the legs
behind it, or the jaws in front of it, I find it quite easy to see, that,
in the legs, it is the part of the appendage which corresponds with the
inner division, which becomes modified into what we know familiarly as
the "leg," while the middle division disappears, and the outer division
is hidden under the carapace. Nor is it more difficult to discern that,
in the appendages of the tail, the middle division appears again and the
outer vanishes; while, on the other hand, in the foremost jaw, the so-
called mandible, the inner division only is left; and, in the same way,
the parts of the feelers and of the eye-stalks can be identified with
those of the legs and jaws.
But whither does all this tend? To the very remarkable conclusion that a
unity of plan, of the same kind as that discoverable in the tail or
abdomen of the lobster, pervades the whole organisation of its skeleton,
so that I can return to the diagram representing any one of the rings of
the tail, which I drew upon the board, and by adding a third division to
each appendage, I can use it as a sort of scheme or plan of any ring of
the body. I can give names to all the parts of that figure, and then if I
take any segment of the body of the lobster, I can point out to you
exactly, what modification the general plan has undergone in that
particular segment; what part has remained movable, and what has become
fixed to another; what has been excessively developed and metamorphosed
and what has been suppressed.
But I imagine I hear the question, How is all this to be tested? No doubt
it is a pretty and ingenious way of looking at the structure of any
animal; but is it anything more? Does Nature acknowledge, in any deeper
way, this unity of plan we seem to trace?
The objection suggested by these questions is a very valid and important
one, and morphology was in an unsound state so long as it rested upon the
mere perception of the analogies which obtain between fully formed parts.
The unchecked ingenuity of speculative anatomists proved itself fully
competent to spin any number of contradictory hypotheses out of the same
facts, and endless morphological dreams threatened to supplant scientific
theory.
Happily, however, there is a criterion of morphological truth, and a sure
test of all homologies. Our lobster has not always been what we see it;
it was once an egg, a semifluid mass of yolk, not so big as a pin's head,
contained in a transparent membrane, and exhibiting not the least trace
of any one of those organs, the multiplicity and complexity of which, in
the adult, are so surprising. After a time, a delicate patch of cellular
membrane appeared upon one face of this yolk, and that patch was the
foundation of the whole creature, the clay out of which it would be
moulded. Gradually investing the yolk, it became subdivided by transverse
constrictions into segments, the forerunners of the rings of the body.
Upon the ventral surface of each of the rings thus sketched out, a pair
of bud-like prominences made their appearance--the rudiments of the
appendages of the ring. At first, all the appendages were alike, but, as
they grew, most of them became distinguished into a stem and two terminal
divisions, to which, in the middle part of the body, was added a third
outer division; and it was only at a later period, that by the
modification, or absorption, of certain of these primitive constituents,
the limbs acquired their perfect form.
Thus the study of development proves that the doctrine of unity of plan
is not merely a fancy, that it is not merely one way of looking at the
matter, but that it is the expression of deep-seated natural facts. The
legs and jaws of the lobster may not merely be regarded as modifications
of a common type,--in fact and in nature they are so,--the leg and the
jaw of the young animal being, at first, indistinguishable.
These are wonderful truths, the more so because the zoologist finds them
to be of universal application. The investigation of a polype, of a
snail, of a fish, of a horse, or of a man, would have led us, though by a
less easy path, perhaps, to exactly the same point. Unity of plan
everywhere lies hidden under the mask of diversity of structure--the
complex is everywhere evolved out of the simple. Every animal has at
first the form of an egg, and every animal and every organic part, in
reaching its adult state, passes through conditions common to other
animals and other adult parts; and this leads me to another point. I have
hitherto spoken as if the lobster were alone in the world, but, as I need
hardly remind you, there are myriads of other animal organisms. Of these,
some, such as men, horses, birds, fishes, snails, slugs, oysters, corals,
and sponges, are not in the least like the lobster. But other animals,
though they may differ a good deal from the lobster, are yet either very
like it, or are like something that is like it. The cray fish, the rock
lobster, and the prawn, and the shrimp, for example, however different,
are yet so like lobsters, that a child would group them as of the lobster
kind, in contradistinction to snails and slugs; and these last again
would form a kind by themselves, in contradistinction to cows, horses,
and sheep, the cattle kind.
But this spontaneous grouping into "kinds" is the first essay of the
human mind at classification, or the calling by a common name of those
things that are alike, and the arranging them in such a manner as best to
suggest the sum of their likenesses and unlikenesses to other things.
Those kinds which include no other subdivisions than the sexes, or
various breeds, are called, in technical language, species. The English
lobster is a species, our cray fish is another, our prawn is another. In
other countries, however, there are lobsters, cray fish, and prawns, very
like ours, and yet presenting sufficient differences to deserve
distinction. Naturalists, therefore, express this resemblance and this
diversity by grouping them as distinct species of the same "genus." But
the lobster and the cray fish, though belonging to distinct genera, have
many features in common, and hence are grouped together in an assemblage
which is called a family. More distant resemblances connect the lobster
with the prawn and the crab, which are expressed by putting all these
into the same order. Again, more remote, but still very definite,
resemblances unite the lobster with the woodlouse, the king crab, the
water flea, and the barnacle, and separate them from all other animals;
whence they collectively constitute the larger group, or class,
_Crustacea_. But the _Crustacea_ exhibit many peculiar features in common
with insects, spiders, and centipedes, so that these are grouped into the
still larger assemblage or "province" _Articulata_; and, finally, the
relations which these have to worms and other lower animals, are
expressed by combining the whole vast aggregate into the sub-kingdom of
_Annulosa_.
If I had worked my way from a sponge instead of a lobster, I should have
found it associated, by like ties, with a great number of other animals
into the sub-kingdom _Protozoa_; if I had selected a fresh-water polype
or a coral, the members of what naturalists term the sub-kingdom
_Coelenterata_, would have grouped themselves around my type; had a snail
been chosen, the inhabitants of all univalve and bivalve, land and water,
shells, the lamp shells, the squids, and the sea-mat would have gradually
linked themselves on to it as members of the same sub-kingdom of
_Mollusca_; and finally, starting from man, I should have been compelled
to admit first, the ape, the rat, the horse, the dog, into the same
class; and then the bird, the crocodile, the turtle, the frog, and the
fish, into the same sub-kingdom of _Vertebrata_.
And if I had followed out all these various lines of classification
fully, I should discover in the end that there was no animal, either
recent or fossil, which did not at once fall into one or other of these
sub-kingdoms. In other words, every animal is organised upon one or other
of the five, or more, plans, the existence of which renders our
classification possible. And so definitely and precisely marked is the
structure of each animal, that, in the present state of our knowledge,
there is not the least evidence to prove that a form, in the slightest
degree transitional between any of the two groups _Vertebrata, Annulosa,
Mollusca_, and _Coelenterata_, either exists, or has existed, during that
period of the earth's history which is recorded by the geologist.[1]
Nevertheless, you must not for a moment suppose, because no such
transitional forms are known, that the members of the sub-kingdoms are
disconnected from, or independent of, one another. On the contrary, in
their earliest condition they are all similar, and the primordial germs
of a man, a dog, a bird, a fish, a beetle, a snail, and a polype are, in
no essential structural respects, distinguishable.
[Footnote 1: The different grouping necessitated by later knowledge does
not affect the principle of the argument.--1894.]
In this broad sense, it may with truth be said, that all living animals,
and all those dead faunae which geology reveals, are bound together by an
all-pervading unity of organisation, of the same character, though not
equal in degree, to that which enables us to discern one and the same
plan amidst the twenty different segments of a lobster's body. Truly it
has been said, that to a clear eye the smallest fact is a window through
which the Infinite may be seen.
Turning from these purely morphological considerations, let us now
examine into the manner in which the attentive study of the lobster
impels us into other lines of research.
Lobsters are found in all the European seas; but on the opposite shores
of the Atlantic and in the seas of the southern hemisphere they do not
exist. They are, however, represented in these regions by very closely
allied, but distinct forms--the _Homarus Americanus_ and the _Homarus
Capensis:_ so that we may say that the European has one species of
_Homuarus_; the American, another; the African, another; and thus the
remarkable facts of geographical distribution begin to dawn upon us.
Again, if we examine the contents of the earth's crust, we shall find in
the latter of those deposits, which have served as the great burying
grounds of past ages, numberless lobster-like animals, but none so
similar to our living lobster as to make zoologists sure that they
belonged even to the same genus. If we go still further back in time, we
discover, in the oldest rocks of all, the remains of animals, constructed
on the same general plan as the lobster, and belonging to the same great
group of _Crustacea_; but for the most part totally different from the
lobster, and indeed from any other living form of crustacean; and thus we
gain a notion of that successive change of the animal population of the
globe, in past ages, which is the most striking fact revealed by geology.
Consider, now, where our inquiries have led us. We studied our type
morphologically, when we determined its anatomy and its development, and
when comparing it, in these respects, with other animals, we made out its
place in a system of classification. If we were to examine every animal
in a similar manner, we should establish a complete body of zoological
morphology.
Again, we investigated the distribution of our type in space and in time,
and, if the like had been done with every animal, the sciences of
geographical and geological distribution would have attained their limit.
But you will observe one remarkable circumstance, that, up to this point,
the question of the life of these organisms has not come under
consideration. Morphology and distribution might be studied almost as
well, if animals and plants were a peculiar kind of crystals, and
possessed none of those functions which distinguish living beings so
remarkably. But the facts of morphology and distribution have to be
accounted for, and the science, the aim of which it is to account for
them, is Physiology.
Let us return to our lobster once more. If we watched the creature in its
native element, we should see it climbing actively the submerged rocks,
among which it delights to live, by means of its strong legs; or swimming
by powerful strokes of its great tail, the appendages of the sixth joint
of which are spread out into a broad fan-like Propeller: seize it, and it
will show you that its great claws are no mean weapons of offence;
suspend a piece of carrion among its haunts, and it will greedily devour
it, tearing and crushing the flesh by means of its multitudinous jaws.
Suppose that we had known nothing of the lobster but as an inert mass, an
organic crystal, if I may use the phrase, and that we could suddenly see
it exerting all these powers, what wonderful new ideas and new questions
would arise in our minds! The great new question would be, "How does all
this take place?" the chief new idea would be, the idea of adaptation to
purpose,--the notion, that the constituents of animal bodies are not mere
unconnected parts, but organs working together to an end. Let us consider
the tail of the lobster again from this point of view. Morphology has
taught us that it is a series of segments composed of homologous parts,
which undergo various modifications--beneath and through which a common
plan of formation is discernible. But if I look at the same part
physiologically, I see that it is a most beautifully constructed organ of
locomotion, by means of which the animal can swiftly propel itself either
backwards or forwards.
But how is this remarkable propulsive machine made to perform its
functions? If I were suddenly to kill one of these animals and to take
out all the soft parts, I should find the shell to be perfectly inert, to
have no more power of moving itself than is possessed by the machinery of
a mill when disconnected from its steam-engine or water-wheel. But if I
were to open it, and take out the viscera only, leaving the white flesh,
I should perceive that the lobster could bend and extend its tail as well
as before. If I were to cut off the tail, I should cease to find any
spontaneous motion in it; but on pinching any portion of the flesh, I
should observe that it underwent a very curious change--each fibre
becoming shorter and thicker. By this act of contraction, as it is
termed, the parts to which the ends of the fibre are attached are, of
course, approximated; and according to the relations of their points of
attachment to the centres of motions of the different rings, the bending
or the extension of the tail results. Close observation of the newly-
opened lobster would soon show that all its movements are due to the same
cause--the shortening and thickening of these fleshy fibres, which are
technically called muscles.
Here, then, is a capital fact. The movements of the lobster are due to
muscular contractility. But why does a muscle contract at one time and
not at another? Why does one whole group of muscles contract when the
lobster wishes to extend his tail, and another group when he desires to
bend it? What is it originates, directs, and controls the motive power?
Experiment, the great instrument for the ascertainment of truth in
physical science, answers this question for us. In the head of the
lobster there lies a small mass of that peculiar tissue which is known as
nervous substance. Cords of similar matter connect his brain of the
lobster, directly or indirectly, with the muscles. Now, if these
communicating cords are cut, the brain remaining entire, the power of
exerting what we call voluntary motion in the parts below the section is
destroyed; and, on the other hand, if, the cords remaining entire, the
brain mass be destroyed, the same voluntary mobility is equally lost.
Whence the inevitable conclusion is, that the power of originating these
motions resides in the brain and is propagated along the nervous cords.
In the higher animals the phenomena which attend this transmission have
been investigated, and the exertion of the peculiar energy which resides
in the nerves has been found to be accompanied by a disturbance of the
electrical state of their molecules.
If we could exactly estimate the signification of this disturbance; if we
could obtain the value of a given exertion of nerve force by determining
the quantity of electricity, or of heat, of which it is the equivalent;
if we could ascertain upon what arrangement, or other condition of the
molecules of matter, the manifestation of the nervous and muscular
energies depends (and doubtless science will some day or other ascertain
these points), physiologists would have attained their ultimate goal in
this direction; they would have determined the relation of the motive
force of animals to the other forms of force found in nature; and if the
same process had been successfully performed for all the operations which
are carried on in, and by, the animal frame, physiology would be perfect,
and the facts of morphology and distribution would be deducible from the
laws which physiologists had established, combined with those determining
the condition of the surrounding universe.
There is not a fragment of the organism of this humble animal whose study
would not lead us into regions of thought as large as those which I have
briefly opened up to you; but what I have been saying, I trust, has not
only enabled you to form a conception of the scope and purport of
zoology, but has given you an imperfect example of the manner in which,
in my opinion, that science, or indeed any physical science, may be best
taught. The great matter is, to make teaching real and practical, by
fixing the attention of the student on particular facts; but at the same
time it should be rendered broad and comprehensive, by constant reference
to the generalisations of which all particular facts are illustrations.
The lobster has served as a type of the whole animal kingdom, and its
anatomy and physiology have illustrated for us some of the greatest
truths of biology. The student who has once seen for himself the facts
which I have described, has had their relations explained to him, and has
clearly comprehended them, has, so far, a knowledge of zoology, which is
real and genuine, however limited it may be, and which is worth more than
all the mere reading knowledge of the science he could ever acquire. His
zoological information is, so far, knowledge and not mere hearsay.
And if it were nay business to fit you for the certificate in zoological
science granted by this department, I should pursue a course precisely
similar in principle to that which I have taken to-night. I should select
a fresh-water sponge, a fresh-water polype or a _Cyanoea_, a fresh-water
mussel, a lobster, a fowl, as types of the five primary divisions of the
animal kingdom. I should explain their structure very fully, and show how
each illustrated the great principles of zoology. Having gone very
carefully and fully over this ground, I should feel that you had a safe
foundation, and I should then take you in the same way, but less
minutely, over similarly selected illustrative types of the classes; and
then I should direct your attention to the special forms enumerated under
the head of types, in this syllabus, and to the other facts there
mentioned.
That would, speaking generally, be my plan. But I have undertaken to
explain to you the best mode of acquiring and communicating a knowledge
of zoology, and you may therefore fairly ask me for a more detailed and
precise account of the manner in which I should propose to furnish you
with the information I refer to.
My own impression is, that the best model for all kinds of training in
physical science is that afforded by the method of teaching anatomy, in
use in the medical schools. This method consists of three elements--
lectures, demonstrations, and examinations.
The object of lectures is, in the first place, to awaken the attention
and excite the enthusiasm of the student; and this, I am sure, may be
effected to a far greater extent by the oral discourse and by the
personal influence of a respected teacher than in any other way.
Secondly, lectures have the double use of guiding the student to the
salient points of a subject, and at the same time forcing him to attend
to the whole of it, and not merely to that part which takes his fancy.
And lastly, lectures afford the student the opportunity of seeking
explanations of those difficulties which will, and indeed ought to, arise
in the course of his studies.
What books shall I read? is a question constantly put by the student to
the teacher. My reply usually is, "None: write your notes out carefully
and fully; strive to understand them thoroughly; come to me for the
explanation of anything you cannot understand; and I would rather you did
not distract your mind by reading." A properly composed course of
lectures ought to contain fully as much matter as a student can
assimilate in the time occupied by its delivery; and the teacher should
always recollect that his business is to feed, and not to cram the
intellect. Indeed, I believe that a student who gains from a course of
lectures the simple habit of concentrating his attention upon a
definitely limited series of facts, until they are thoroughly mastered,
has made a step of immeasurable importance.
But, however good lectures may be, and however extensive the course of
reading by which they are followed up, they are but accessories to the
great instrument of scientific teaching--demonstration. If I insist
unweariedly, nay fanatically, upon the importance of physical science as
an educational agent, it is because the study of any branch of science,
if properly conducted, appears to me to fill up a void left by all other
means of education. I have the greatest respect and love for literature;
nothing would grieve me more than to see literary training other than a
very prominent branch of education: indeed, I wish that real literary
discipline were far more attended to than it is; but I cannot shut my
eyes to the fact, that there is a vast difference between men who have
had a purely literary, and those who have had a sound scientific,
training.
Seeking for the cause of this difference, I imagine I can find it in the
fact that, in the world of letters, learning and knowledge are one, and
books are the source of both; whereas in science, as in life, learning
and knowledge are distinct, and the study of things, and not of books, is
the source of the latter.
All that literature has to bestow may be obtained by reading and by
practical exercise in writing and in speaking; but I do not exaggerate
when I say, that none of the best gifts of science are to be won by these
means. On the contrary, the great benefit which a scientific education
bestows, whether is training or as knowledge, is dependent upon the
extent to which the mind of the student is brought into immediate contact
with facts--upon the degree to which he learns the habit of appealing
directly to Nature, and of acquiring through his senses concrete images
of those properties of things, which are, and always will be, but
approximatively expressed in human language. Our way of looking at
Nature, and of speaking about her, varies from year to year; but a fact
once seen, a relation of cause and effect, once demonstratively
apprehended, are possessions which neither change nor pass away, but, on
the contrary, form fixed centres, about which other truths aggregate by
natural affinity.
Therefore, the great business of the scientific teacher is, to imprint
the fundamental, irrefragable facts of his science, not only by words
upon the mind, but by sensible impressions upon the eye, and ear, and
touch of the student, in so complete a manner, that every term used, or
law enunciated, should afterwards call up vivid images of the particular
structural, or other, facts which furnished the demonstration of the law,
or the illustration of the term.
Now this important operation can only be achieved by constant
demonstration, which may take place to a certain imperfect extent during
a lecture, but which ought also to be carried on independently, and which
should be addressed to each individual student, the teacher endeavouring,
not so much to show a thing to the learner, as to make him see it for
himself.
I am well aware that there are great practical difficulties in the way of
effectual zoological demonstrations. The dissection of animals is not
altogether pleasant, and requires much time; nor is it easy to secure an
adequate supply of the needful specimens. The botanist has here a great
advantage; his specimens are easily obtained, are clean and wholesome,
and can be dissected in a private house as well as anywhere else; and
hence, I believe, the fact, that botany is so much more readily and
better taught than its sister science. But, be it difficult or be it
easy, if zoological science is to be properly studied, demonstration,
and, consequently, dissection, must be had. Without it, no man can have a
really sound knowledge of animal organisation.
A good deal may be done, however, without actual dissection on the
student's part, by demonstration upon specimens and preparations; and in
all probability it would not be very difficult, were the demand
sufficient, to organise collections of such objects, sufficient for all
the purposes of elementary teaching, at a comparatively cheap rate. Even
without these, much might be effected, if the zoological collections,
which are open to the public, were arranged according to what has been
termed the "typical principle"; that is to say, if the specimens exposed
to public view were so selected that the public could learn something
from them, instead of being, as at present, merely confused by their
multiplicity. For example, the grand ornithological gallery at the
British Museum contains between two and three thousand species of birds,
and sometimes five or six specimens of a species. They are very pretty to
look at, and some of the cases are, indeed, splendid; but I will
undertake to say, that no man but a professed ornithologist has ever
gathered much information from the collection. Certainly, no one of the
tens of thousands of the general public who have walked through that
gallery ever knew more about the essential peculiarities of birds when he
left the gallery than when he entered it. But if, somewhere in that vast
hall, there were a few preparations, exemplifying the leading structural
peculiarities and the mode of development of a common fowl; if the types
of the genera, the leading modifications in the skeleton, in the plumage
at various ages, in the mode of nidification, and the like, among birds,
were displayed; and if the other specimens were put away in a place where
the men of science, to whom they are alone useful, could have free access
to them, I can conceive that this collection might become a great
instrument of scientific education.
The last implement of the teacher to which I have adverted is
examination--a means of education now so thoroughly understood that I
need hardly enlarge upon it. I hold that both written and oral
examinations are indispensable, and, by requiring the description of
specimens, they may be made to supplement demonstration.
Such is the fullest reply the time at my disposal will allow me to give
to the question--how may a knowledge of zoology be best acquired and
communicated?
But there is a previous question which may be moved, and which, in fact,
I know many are inclined to move. It is the question, why should teachers
be encouraged to acquire a knowledge of this, or any other branch of
physical science? What is the use, it is said, of attempting to make
physical science a branch of primary education? Is it not probable that
teachers, in pursuing such studies, will be led astray from the
acquirement of more important but less attractive knowledge? And, even if
they can learn something of science without prejudice to their
usefulness, what is the good of their attempting to instil that knowledge
into boys whose real business is the acquisition of reading, writing, and
arithmetic?
These questions are, and will be, very commonly asked, for they arise
from that profound ignorance of the value and true position of physical
science, which infests the minds of the most highly educated and
intelligent classes of the community. But if I did not feel well assured
that they are capable of being easily and satisfactorily answered; that
they have been answered over and over again; and that the time will come
when men of liberal education will blush to raise such questions--I
should be ashamed of my position here to-night. Without doubt, it is your
great and very important function to carry out elementary education;
without question, anything that should interfere with the faithful
fulfilment of that duty on your part would be a great evil; and if I
thought that your acquirement of the elements of physical science, and
your communication of those elements to your pupils, involved any sort of
interference with your proper duties, I should be the first person to
protest against your being encouraged to do anything of the kind.
But is it true that the acquisition of such a knowledge of science as is
proposed, and the communication of that knowledge, are calculated to
weaken your usefulness? Or may I not rather ask, is it possible for you
to discharge your functions properly without these aids?
What is the purpose of primary intellectual education? I apprehend that
its first object is to train the young in the use of those tools
wherewith men extract knowledge from the ever-shifting succession of
phenomena which pass before their eyes; and that its second object is to
inform them of the fundamental laws which have been found by experience
to govern the course of things, so that they may not be turned out into
the world naked, defenceless, and a prey to the events they might
control.
A boy is taught to read his own and other languages, in order that he may
have access to infinitely wider stores of knowledge than could ever be
opened to him by oral intercourse with his fellow men; he learns to
write, that his means of communication with the rest of mankind may be
indefinitely enlarged, and that he may record and store up the knowledge
he acquires. He is taught elementary mathematics, that he may understand
all those relations of number and form, upon which the transactions of
men, associated in complicated societies, are built, and that he may have
some practice in deductive reasoning.
All these operations of reading, writing, and ciphering, are intellectual
tools, whose use should, before all things, be learned, and learned
thoroughly; so that the youth may be enabled to make his life that which
it ought to be, a continual progress in learning and in wisdom.
But, in addition, primary education endeavours to fit a boy out with a
certain equipment of positive knowledge. He is taught the great laws of
morality; the religion of his sect; so much history and geography as will
tell him where the great countries of the world are, what they are, and
how they have become what they are.
Without doubt all these are most fitting and excellent things to teach a
boy; I should be very sorry to omit any of them from any scheme of
primary intellectual education. The system is excellent, so far as it
goes.
But if I regard it closely, a curious reflection arises. I suppose that,
fifteen hundred years ago, the child of any well-to-do Roman citizen was
taught just these same things; reading and writing in his own, and,
perhaps, the Greek tongue; the elements of mathematics; and the religion,
morality, history, and geography current in his time. Furthermore, I do
not think I err in affirming, that, if such a Christian Roman boy, who
had finished his education, could be transplanted into one of our public
schools, and pass through its course of instruction, he would not meet
with a single unfamiliar line of thought; amidst all the new facts he
would have to learn, not one would suggest a different mode of regarding
the universe from that current in his own time.
And yet surely there is some great difference between the civilisation of
the fourth century and that of the nineteenth, and still more between the
intellectual habits and tone of thought of that day and this?
And what has made this difference? I answer fearlessly--The prodigious
development of physical science within the last two centuries.
Modern civilisation rests upon physical science; take away her gifts to
our own country, and our position among the leading nations of the world
is gone to-morrow; for it is physical science only that makes
intelligence and moral energy stronger than brute force.
The whole of modern thought is steeped in science; it has made its way
into the works of our best poets, and even the mere man of letters, who
affects to ignore and despise science, is unconsciously impregnated with
her spirit, and indebted for his best products to her methods. I believe
that the greatest intellectual revolution mankind has yet seen is now
slowly taking place by her agency. She is teaching the world that the
ultimate court of appeal is observation and experiment, and not
authority; she is teaching it to estimate the value of evidence; she is
creating a firm and living faith in the existence of immutable moral and
physical laws, perfect obedience to which is the highest possible aim of
an intelligent being.
But of all this your old stereotyped system of education takes no note.
Physical science, its methods, its problems, and its difficulties, will
meet the poorest boy at every turn, and yet we educate him in such a
manner that he shall enter the world as ignorant of the existence of the
methods and facts of science as the day he was born. The modern world is
full of artillery; and we turn out our children to do battle in it,
equipped with the shield and sword of an ancient gladiator.
Posterity will cry shame on us if we do not remedy this deplorable state
of things. Nay, if we live twenty years longer, our own consciences will
cry shame on us.
It is my firm conviction that the only way to remedy it is to make the
elements of physical science an integral part of primary education. I
have endeavoured to show you how that may be done for that branch of
science which it is my business to pursue; and I can but add, that I
should look upon the day when every schoolmaster throughout this land was
a centre of genuine, however rudimentary, scientific knowledge, as an
epoch in the history of the country.
But let me entreat you to remember my last words. Addressing myself to
you, as teachers, I would say, mere book learning in physical science is
a sham and a delusion--what you teach, unless you wish to be impostors,
that you must first know; and real knowledge in science means personal
acquaintance with the facts, be they few or many.[2]
[Footnote 2: It has been suggested to me that these words may be taken to
imply a discouragement on my part of any sort of scientific instruction
which does not give an acquaintance with the facts at first hand. But
this is not my meaning. The ideal of scientific teaching is, no doubt, a
system by which the scholar sees every fact for himself, and the teacher
supplies only the explanations. Circumstances, however, do not often
allow of the attainment of that ideal, and we must put up with the next
best system--one in which the scholar takes a good deal on trust from a
teacher, who, knowing the facts by his own knowledge, can describe them
with so much vividness as to enable his audience to form competent ideas
concerning them. The system which I repudiate is that which allows
teachers who have not come into direct contact with the leading facts of
a science to pass their second-hand information on. The scientific virus,
like vaccine lymph, if passed through too long a succession of organisms,
will lose all its effect in protecting the young against the intellectual
epidemics to which they are exposed.
[The remarks on p. 222 applied to the Natural History Collection of the
British Museum in 1861. The visitor to the Natural History Museum in 1894
need go no further than the Great Hall to see the realisation of my hopes
by the present Director.]]
VIII
BIOGENESIS AND ABIOGENESIS
(THE PRESIDENTIAL ADDRESS TO THE BRITISH ASSOCIATION FOR THE ADVANCEMENT
OF SCIENCE FOR 1870)
It has long been the custom for the newly installed President of the
British Association for the Advancement of Science to take advantage of
the elevation of the position in which the suffrages of his colleagues
had, for the time, placed him, and, casting his eyes around the horizon
of the scientific world, to report to them what could be seen from his
watch-tower; in what directions the multitudinous divisions of the noble
army of the improvers of natural knowledge were marching; what important
strongholds of the great enemy of us all, ignorance, had been recently
captured; and, also, with due impartiality, to mark where the advanced
posts of science had been driven in, or a long-continued siege had made
no progress.
I propose to endeavour to follow this ancient precedent, in a manner
suited to the limitations of my knowledge and of my capacity. I shall not
presume to attempt a panoramic survey of the world of science, nor even
to give a sketch of what is doing in the one great province of biology,
with some portions of which my ordinary occupations render me familiar.
But I shall endeavour to put before you the history of the rise and
progress of a single biological doctrine; and I shall try to give some
notion of the fruits, both intellectual and practical, which we owe,
directly or indirectly, to the working out, by seven generations of
patient and laborious investigators, of the thought which arose, more
than two centuries ago, in the mind of a sagacious and observant Italian
naturalist.
It is a matter of everyday experience that it is difficult to prevent
many articles of food from becoming covered with mould; that fruit, sound
enough to all appearance, often contains grubs at the core; that meat,
left to itself in the air, is apt to putrefy and swarm with maggots. Even
ordinary water, if allowed to stand in an open vessel, sooner or later
becomes turbid and full of living matter.
The philosophers of antiquity, interrogated as to the cause of these
phenomena, were provided with a ready and a plausible answer. It did not
enter their minds even to doubt that these low forms of life were
generated in the matters in which they made their appearance. Lucretius,
who had drunk deeper of the scientific spirit than any poet of ancient or
modern times except Goethe, intends to speak as a philosopher, rather
than as a poet, when he writes that "with good reason the earth has
gotten the name of mother, since all things are produced out of the
earth. And many living creatures, even now, spring out of the earth,
taking form by the rains and the heat of the sun."[1] The axiom of
ancient science, "that the corruption of one thing is the birth of
another," had its popular embodiment in the notion that a seed dies
before the young plant springs from it; a belief so widespread and so
fixed, that Saint Paul appeals to it in one of the most splendid
outbursts of his fervid eloquence:--
"Thou fool, that which thou sowest is not quickened, except it die."[2]
[Footnote 1: It is thus that Mr. Munro renders
"Linquitur, ut merito maternum nomen adepta
Terra sit, e terra quoniam sunt cuncta creata.
Multaque nunc etiam exsistant animalia terris
Imbribus et calido solis concreta vapore."
_De Rerum Natura_, lib. v. 793-796.
But would not the meaning of the last line be better rendered "Developed
in rain-water and in the warm vapours raised by the sun"?]
[Footnote 2: 1 Corinthians xv. 36.]
The proposition that life may, and does, proceed from that which has no
life, then, was held alike by the philosophers, the poets, and the
people, of the most enlightened nations, eighteen hundred years ago; and
it remained the accepted doctrine of learned and unlearned Europe,
through the Middle Ages, down even to the seventeenth century.
It is commonly counted among the many merits of our great countryman,
Harvey, that he was the first to declare the opposition of fact to
venerable authority in this, as in other matters; but I can discover no
justification for this widespread notion. After careful search through
the "Exercitationes de Generatione," the most that appears clear to me
is, that Harvey believed all animals and plants to spring from what he
terms a "_primordium vegetale_," a phrase which may nowadays be rendered
"a vegetative germ"; and this, he says, is _"oviforme_," or "egg-like";
not, he is careful to add, that it necessarily has the shape of an egg,
but because it has the constitution and nature of one. That this
"_primordium oviforme_" must needs, in all cases, proceed from a living
parent is nowhere expressly maintained by Harvey, though such an opinion
may be thought to be implied in one or two passages; while, on the other
hand, he does, more than once, use language which is consistent only with
a full belief in spontaneous or equivocal generation.[3] In fact, the
main concern of Harvey's wonderful little treatise is not with
generation, in the physiological sense, at all, but with development; and
his great object is the establishment of the doctrine of epigenesis.
[Footnote 3: See the following passage in Exercitatio I.:--"Item _sponte
nascentia_ dicuntur; non quod ex _putredine_ oriunda sint, sed quod casu,
naturae sponte, et aequivoca (ut aiunt) generatione, a parentibus sui
dissimilibus proveniant." Again, in _De Uteri Membranis:_--"In cunctorum
viventium generatione (sicut diximus) hoc solenne est, ut ortum ducunt a
_primordio_ aliquo, quod tum materiam tum elficiendi potestatem in se
habet: sitque, adeo id, ex quo et a quo quicquid nascitur, ortum suum
ducat. Tale primordium in animalibus (_sive ab aliis generantibus
proveniant, sive sponte, aut ex putredine nascentur_) est humor in
tunica, aliquaaut putami ne conclusus." Compare also what Redi has to say
respecting Harvey's opinions, _Esperienze_, p. 11.]
The first distinct enunciation of the hypothesis that all living matter
has sprung from pre-existing living matter, came from a contemporary,
though a junior, of Harvey, a native of that country, fertile in men
great in all departments of human activity, which was to intellectual
Europe, in the sixteenth and seventeenth centuries, what Germany is in
the nineteenth. It was in Italy, and from Italian teachers, that Harvey
received the most important part of his scientific education. And it was
a student trained in the same schools, Francesco Redi--a man of the
widest knowledge and most versatile abilities, distinguished alike as
scholar, poet, physician, and naturalist--who, just two hundred and two
years ago, published his "Esperienze intorno alla Generazione degl'
Insetti," and gave to the world the idea, the growth of which it is my
purpose to trace. Redi's book went through five editions in twenty years;
and the extreme simplicity of his experiments, and the clearness of his
arguments, gained for his views, and for their consequences, almost
universal acceptance.
Redi did not trouble himself much with speculative considerations, but
attacked particular cases of what was supposed to be "spontaneous
generation" experimentally. Here are dead animals, or pieces of meat,
says he; I expose them to the air in hot weather, and in a few days they
swarm with maggots. You tell me that these are generated in the dead
flesh; but if I put similar bodies, while quite fresh, into a jar, and
tie some fine gauze over the top of the jar, not a maggot makes its
appearance, while the dead substances, nevertheless, putrefy just in the
same way as before. It is obvious, therefore, that the maggots are not
generated by the corruption of the meat; and that the cause of their
formation must be a something which is kept away by gauze. But gauze will
not keep away aeriform bodies, or fluids. This something must, therefore,
exist in the form of solid particles too big to get through the gauze.
Nor is one long left in doubt what these solid particles are; for the
blowflies, attracted by the odour of the meat, swarm round the vessel,
and, urged by a powerful but in this case misleading instinct, lay eggs
out of which maggots are immediately hatched, upon the gauze. The
conclusion, therefore, is unavoidable; the maggots are not generated by
the meat, but the eggs which give rise to them are brought through the
air by the flies.
These experiments seem almost childishly simple, and one wonders how it
was that no one ever thought of them before. Simple as they are, however,
they are worthy of the most careful study, for every piece of
experimental work since done, in regard to this subject, has been shaped
upon the model furnished by the Italian philosopher. As the results of
his experiments were the same, however varied the nature of the materials
he used, it is not wonderful that there arose in Redi's mind a
presumption, that, in all such cases of the seeming production of life
from dead matter, the real explanation was the introduction of living
germs from without into that dead matter.[4] And thus the hypothesis that
living matter always arises by the agency of pre-existing living matter,
took definite shape; and had, henceforward, a right to be considered and
a claim to be refuted, in each particular case, before the production of
living matter in any other way could be admitted by careful reasoners. It
will be necessary for me to refer to this hypothesis so frequently, that,
to save circumlocution, I shall call it the hypothesis of _Biogenesis_;
and I shall term the contrary doctrine--that living matter may be
produced by not living matter--the hypothesis of _Abiogenesis_.
[Footnote 4: "Pure contentandomi sempre in questa ed in ciascuna altro
cosa, da ciascuno piu savio, la dove io difettuosamente parlassi, esser
corretto; non tacero, che per molte osservazioni molti volti da me fatte,
mi sento inclinato a credere che la terra, da quelle prime piante, e da
quei primi animali in poi, che ella nei primi giorni del mondo produsse
per comandemento del sovrano ed omnipotente Fattore, non abbia mai piu
prodotto da se medesima ne erba ne albero, ne animale alcuno perfetto o
imperfetto che ei se fosse; e che tutto quello, che ne' tempi trapassati
e nato e che ora nascere in lei, o da lei veggiamo, venga tutto dalla
semenza reale e vera delle piante, e degli animali stessi, i quali col
mezzo del proprio seme la loro spezie conservano. E se bene tutto giorno
scorghiamo da' cadaveri degli animali, e da tutte quante le maniere dell'
erbe, e de' fiori, e dei frutti imputriditi, e corrotti nascere vermi
infiniti--
'Nonne vides quaecunque mora, fluidoque calore
Corpora tabescunt in parva animalia verti'--
Io mi sento, dico, inclinato, a credere che tutti quei vermi si generino
dal seme paterno; e che le carni, e l' erbe, e l' altre cose tutte
putrefatte, o putrefattibili non facciano altra parte, ne abbiano altro
ufizio nella generazione degl' insetti, se non d'apprestare un luogo o un
nido proporzionato, in cui dagli animali nel tempo della figliatura sieno
portati, e partoriti i vermi, o l' uova o l' altre semenze dei vermi, i
quali tosto che nati sono, trovano in esso nido un sufficiente alimento
abilissimo per nutricarsi: e se in quello non son portate dalle madri
queste suddette semenze, niente mai, e replicatamente niente, vi s'
ingegneri e nasca."--REDI, _Esperienze_, pp. 14-16.]
In the seventeenth century, as I have said, the latter was the dominant
view, sanctioned alike by antiquity and by authority; and it is
interesting to observe that Redi did not escape the customary tax upon a
discoverer of having to defend himself against the charge of impugning
the authority of the Scriptures;[5] for his adversaries declared that the
generation of bees from the carcase of a dead lion is affirmed, in the
Book of Judges, to have been the origin of the famous riddle with which
Samson perplexed the Philistines:--
Out of the eater came forth meat,
And out of the strong came forth sweetness.
[Footnote 5: "Molti, e molti altri ancora vi potrei annoverare, se non
fossi chiamato a rispondere alle rampogne di alcuni, che bruscamente mi
rammentano cio, che si legge nel capitolo quattordicesimo del sacrosanto
Libro de' giudici ... "--REDI, _loc. cit._ p. 45.]
Against all odds, however, Redi, strong with the strength of demonstrable
fact, did splendid battle for Biogenesis; but it is remarkable that he
held the doctrine in a sense which, if he lead lived in these times,
would have infallibly caused him to be classed among the defenders of
"spontaneous generation." "Omne vivum ex vivo," "no life without
antecedent life," aphoristically sums up Redi's doctrine; but he went no
further. It is most remarkable evidence of the philosophic caution and
impartiality of his mind, that although he had speculatively anticipated
the manner in which grubs really are deposited in fruits and in the galls
of plants, he deliberately admits that the evidence is insufficient to
bear him out; and he therefore prefers the supposition that they are
generated by a modification of the living substance of the plants
themselves. Indeed, he regards these vegetable growths as organs, by
means of which the plant gives rise to an animal, and looks upon this
production of specific animals as the final cause of the galls and of, at
any rate, some fruits. And he proposes to explain the occurrence of
parasites within the animal body in the same way.[6]
[Footnote 6: The passage (_Esperienze_, p. 129) is worth quoting in
full:--
"Se dovessi palesarvi il mio sentimento crederei che i frutti, i legumi,
gli alberi e le foglie, in due maniere inverminassero. Una, perche
venendo i bachi per di fuora, e cercando l' alimento, col rodere ci
aprono la strada, ed arrivano alla piu interna midolla de' frutti e de'
legni. L'altra maniera si e, che io per me stimerei, che non fosse gran
fatto disdicevole il credere, che quell' anima o quella virtu, la quale
genera i fiori ed i frutti nelle piante viventi, sia quella stessa che
generi ancora i bachi di esse piante. E chi sa, forse, che molti frutti
degli alberi non sieno prodotti, non per un fine primario e principale,
ma bensi per un uffizio secondario e servile, destinato alla generazione
di que' vermi, servendo a loro in vece di matrice, in cui dimorino un
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