Discourses - Thomas H. Huxley
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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.