Critiques and Addresses by Thomas Henry Huxley

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These discoveries prepared the way for the illustrious Frenchman,
Lavoisier, who first approached the problem of fermentation with a
complete conception of the nature of the work to be done. The words
in which he expresses this conception, in the treatise on elementary
chemistry to which reference has already been made, mark the year 1789
as the commencement of a revolution of not less moment in the world of
science than that which simultaneously burst over the political world,
and soon engulfed Lavoisier himself in one of its mad eddies.

"We may lay it down as an incontestable axiom that, in all the
operations of art and nature, nothing is created; an equal quantity
of matter exists both before and after the experiment: the quality and
quantity of the elements remain precisely the same, and nothing takes
place beyond changes and modifications in the combinations of these
elements. Upon this principle, the whole art of performing chemical
experiments depends; we must always suppose an exact equality between
the elements of the body examined and those of the products of its
analysis.

"Hence, since from must of grapes we procure alcohol and carbonic
acid, I have an undoubted right to suppose that must consists of
carbonic acid and alcohol. From these premisses we have two modes
of ascertaining what passes during vinous fermentation: either
by determining the nature of, and the elements which compose, the
fermentable substances; or by accurately examining the products
resulting from fermentation; and it is evident that the knowledge
of either of these must lead to accurate conclusions concerning the
nature and composition of the other. From these considerations it
became necessary accurately to determine the constituent elements of
the fermentable substances; and for this purpose I did not make use
of the compound juices of fruits, the rigorous analysis of which
is perhaps impossible, but made choice of sugar, which is easily
analysed, and the nature of which I have already explained. This
substance is a true vegetable oxyd, with two bases, composed of
hydrogen and carbon, brought to the state of an oxyd by means of a
certain proportion of oxygen; and these three elements are combined
in such a way that a very slight force is sufficient to destroy the
equilibrium of their connection."

After giving the details of his analysis of sugar and of the products
of fermentation, Lavoisier continues:--

"The effect of the vinous fermentation upon sugar is thus reduced to
the mere separation of its elements into two portions; one part is
oxygenated at the expense of the other, so as to form carbonic acid;
while the other part, being disoxygenated in favour of the latter, is
converted into the combustible substance called alkohol; therefore,
if it were possible to re-unite alkohol and carbonic acid together, we
ought to form sugar."[1]

[Footnote 1: "Elements of Chemistry." By M. Lavoisier. Translated by
Robert Kerr. Second Edition, 1793 (pp. 186--196).]

Thus Lavoisier thought he had demonstrated that the carbonic acid and
the alcohol which are produced by the process of fermentation, are
equal in weight to the sugar which disappears; but the application of
the more refined methods of modern chemistry to the investigation of
the products of fermentation by Pasteur, in 1860, proved that this is
not exactly true, and that there is a deficit of from 5 to 7 per cent.
of the sugar which is not covered by the alcohol and carbonic acid
evolved. The greater part of this deficit is accounted for by the
discovery of two substances, glycerine and succinic acid, of the
existence of which Lavoisier was unaware, in the fermented liquid.
But about 1-1/2 per cent. still remains to be made good. According to
Pasteur, it has been appropriated by the yeast, but the fact that such
appropriation takes place cannot be said to be actually proved.

However this may be, there can be no doubt that the constituent
elements of fully 98 per cent. of the sugar which has vanished during
fermentation have simply undergone rearrangement; like the soldiers
of a brigade, who at the word of command divide themselves into the
independent regiments to which they belong. The brigade is sugar, the
regiments are carbonic acid, succinic acid, alcohol, and glycerine.

From the time of Fabroni, onwards, it has been admitted that the agent
by which this surprising rearrangement of the particles of the sugar
is effected is the yeast. But the first thoroughly conclusive evidence
of the necessity of yeast for the fermentation of sugar was furnished
by Appert, whose method of preserving perishable articles of food
excited so much attention in France at the beginning of this century.
Gay-Lussac, in his "Memoire sur la Fermentation,"[1] alludes to
Appert's method of preserving beer-wort unfermented for an indefinite
time, by simply boiling the wort and closing the vessel in which the
boiling fluid is contained, in such a way as thoroughly to exclude
air; and he shows that, if a little yeast be introduced into such
wort, after it has cooled, the wort at once begins to ferment, even
though every precaution be taken to exclude air. And this statement
has since received full confirmation from Pasteur.

[Footnote 1: "Annales de Chimie," 1810.]

On the other hand, Schwann, Schroeder and Dusch, and Pasteur, have
amply proved that air may be allowed to have free access to beer-wort,
without exciting fermentation, if only efficient precautions are taken
to prevent the entry of particles of yeast along with the air.

Thus, the truth that the fermentation of a simple solution of sugar in
water depends upon the presence of yeast, rests upon an unassailable
foundation; and the inquiry into the exact nature of the substance
which possesses such a wonderful chemical influence becomes profoundly
interesting.

The first step towards the solution of this problem was made two
centuries ago by the patient and painstaking Dutch naturalist,
Leeuwenhoek, who in the year 1680 wrote thus:--

"Saepissimo examinavi fermentum cerevisiae, semperque hoc ex
globulis per materiam pellucidam fluitantibus, quam cerevisiam
esse censui, constare observavi: vidi etiam evidentissime,
unumquemque hujus fermenti globulum denuo ex sex distinctis
globullis constare, accurate eidem quantitate et formae, cui
globulis sanguinis nostri, respondentibus.

"Verum talis mini de horum origine et formatione conceptus
formabam; globulis nempe ex quibus farina Tritici, Hordei,
Avenae, Fagotritici, se constat aquae calore dissolvi et aquae
commisceri; hac, vero aqua, quam cerevisiam vocare licet,
refrigescente, multos ex minimis particulis in cerevisia
coadunari, et hoc pacto efficere particulam sive globulum,
quae sexta pars est globuli faecis, et iterum sex ex hisce
globulis conjungi."[1]

[Footnote 1: Leeuwenhoek, "Arcana Naturae Detecta." Ed. Nov., 1721.]

Thus Leeuwenhoek discovered that yeast consists of globules floating
in a fluid; but he thought that they were merely the starchy particles
of the grain from which the wort was made, re-arranged. He discovered
the fact that yeast had a definite structure, but not the meaning of
the fact. A century and a half elapsed, and the investigation of
yeast was recommenced almost simultaneously by Cagniard de la Tour in
France, and by Schwann and Kuetzing in Germany. The French observer
was the first to publish his results; and the subject received at his
hands and at those of his colleague, the botanist Turpin, full and
satisfactory investigation.

The main conclusions at which they arrived are these. The globular,
or oval, corpuscles which float so thickly in the yeast as to make it
muddy, though the largest are not more than one two-thousandth of
an inch in diameter, and the smallest may measure less than one
seven-thousandth of an inch, are living organisms. They multiply with
great rapidity, by giving off minute buds, which soon attain the size
of their parent, and then either become detached or remain united,
forming the compound globules of which Leeuwenhoek speaks, though the
constancy of their arrangement in sixes existed only in the worthy
Dutchman's imagination.

It was very soon made out that these yeast organisms, to which Turpin
gave the name of _Torula cerevisiae_, were more nearly allied to the
lower Fungi than to anything else. Indeed Turpin, and subsequently
Berkeley and Hoffmann, believed that they had traced the development
of the _Torula_ into the well-known and very common mould--the
_Penicillium glaucum_. Other observers have not succeeded in verifying
these statements; and my own observations lead me to believe, that
while the connection between _Torula_ and the moulds is a very close
one, it is of a different nature from that which has been supposed. I
have never been able to trace the development of _Torula_ into a true
mould; but it is quite easy to prove that species of true mould,
such as _Penicillium_, when sown in an appropriate nidus, such as
a solution of tartrate of ammonia and yeast-ash, in water, with or
without sugar, give rise to _Torulae_, similar in all respects to _T.
cerevisiae_, except that they are, on the average, smaller. Moreover,
Bail has observed the development of a _Torula_ larger than _T.
cerevisiae_, from a _Mucor_, a mould allied to _Penicillium_.

It follows, therefore, that the _Torulae_, or organisms of yeast,
are veritable plants; and conclusive experiments have proved that the
power which causes the rearrangement of the molecules of the sugar is
intimately connected with the life and growth of the plant. In fact,
whatever arrests the vital activity of the plant also prevents it from
exciting fermentation.

Such being the facts with regard to the nature of yeast, and the
changes which it effects in sugar, how are they to be accounted for?
Before modern chemistry had come into existence, Stahl, stumbling,
with the stride of genius, upon the conception which lies at the
bottom of all modern views of the process, put forward the notion that
the ferment, being in a state of internal motion, communicated
that motion to the sugar, and thus caused its resolution into new
substances. And Lavoisier, as we have seen, adopts substantially the
same view, (But Fabroni, full of the then novel conception of acids
and bases and double decompositions, propounded the hypothesis that
sugar is an oxide with two bases, and the ferment a carbonate with two
bases; that the carbon of the ferment unites with the oxygen of the
sugar, and gives rise to carbonic acid; while the sugar, uniting with
the nitrogen of the ferment, produces a new substance analogous to
opium. This is decomposed by distillation, and gives rise to alcohol.)
Next, in 1803, Thenard propounded a hypothesis which partakes somewhat
of the nature of both Stahl's and Fabroni's views. "I do not believe
with Lavoisier," he says, "that all the carbonic acid formed proceeds
from the sugar. How, in that case, could we conceive the action of the
ferment on it? I think that the first portions of the acid are due
to a combination of the carbon of the ferment with the oxygen of the
sugar, and that it is by carrying off a portion of oxygen from
the last that the ferment causes the fermentation to commence--the
equilibrium between the principles of the sugar being disturbed, they
combine afresh to form carbonic acid and alcohol."

The three views here before us may be familiarly exemplified by
supposing the sugar to be a card-house. According to Stahl, the
ferment is somebody who knocks the table, and shakes the card-house
down; according to Fabroni, the ferment takes out some cards, but puts
others in their places; according to Thenard, the ferment simply takes
a card out of the bottom story, the result of which is that all the
others fall.

As chemistry advanced, facts came to light which put a new face upon
Stahl's hypothesis, and gave it a safer foundation than it previously
possessed. The general nature of these phenomena may be thus
stated:--A body, A, without giving to, or taking from, another
body, B, any material particles, causes B to decompose into other
substances, C, D, E, the sum of the weights of which is equal to the
weight of B, which decomposes.

Thus, bitter almonds contain two substances, amygdalin and synaptase,
which can be extracted, in a separate state, from the bitter almonds.
The amygdalin thus obtained, if dissolved in water, undergoes no
change; but if a little synaptase be added to the solution, the
amygdalin splits up into bitter almond oil, prussic acid, and a kind
of sugar.

A short time after Cagniard de la Tour discovered the yeast plant,
Liebig, struck with the similarity between this and other such
processes and the fermentation of sugar, put forward the hypothesis
that yeast contains a substance which acts upon sugar, as synaptase
acts upon amygdalin. And as the synaptase is certainly neither
organized nor alive, but a mere chemical substance, Liebig treated
Cagniard de la Tour's discovery with no small contempt, and, from
that time to the present, has steadily repudiated the notion that the
decomposition of the sugar is, in any sense, the result of the vital
activity of the _Torula_. But, though the notion that the _Torula_ is
a creature which eats sugar and excretes carbonic acid and alcohol,
which is not unjustly ridiculed in the most surprising paper that
ever made its appearance in a grave scientific journal[1], may be
untenable, the fact that the _Torulae_ are alive, and that yeast does
not excite fermentation unless it contains living _Torulae_, stands
fast. Moreover, of late years, the essential participation of living
organisms in fermentation other than the alcoholic, has been clearly
made out by Pasteur and other chemists.

[Footnote 1: "Das entraethselte Geheimniss der geistigen Gaehrung
(Vorlaeufige briefliche Mittheilung)" is the title of an anonymous
contribution, to Woehler and Liebig's "Annalen der Pharmacie" for
1839, in which a somewhat Rabelaisian imaginary description of the
organization of the "yeast animals" and of the manner in which their
functions are performed, is given with a circumstantiality worthy
of the author of Gulliver's Travels. As a specimen of the writer's
humour, his account of what happens when fermentation comes to an end
may suffice. "Sobald naemlich die Thiere keinen Zucker mehr vorfinden,
so fressen sie sich gegenseitig selbst auf, was durch eine eigene
Manipulation geschicht; alles wird verdaut bis auf die Eier, welche
unveraendert durch den Darmkanal hineingehen; man hat zuletzt wieder
gaehrungsfaehige Hefe, naemlich den Saamen der Thiere, der uebrig
bleibt."]

However, it may be asked, is there any necessary opposition between
the so-called "vital" and the strictly physico-chemical views of
fermentation? It is quite possible that the living _Torula_ may excite
fermentation in sugar, because it constantly produces, as an essential
part of its vital manifestations, some substance which acts upon the
sugar, just as the synaptase acts upon the amygdalin. Or it may
be, that, without the formation of any such special substance,
the physical condition of the living tissue of the yeast plant is
sufficient to effect that small disturbance of the equilibrium of the
particles of the sugar, which Lavoisier thought sufficient to effect
its decomposition.

Platinum in a very fine state of division--known as platinum black, or
_noir de platine_--has the very singular property of causing alcohol
to change into acetic acid with great rapidity. The vinegar plant,
which is closely allied to the yeast plant, has a similar effect upon
dilute alcohol, causing it to absorb the oxygen of the air, and become
converted into vinegar; and Liebig's eminent opponent, Pasteur, who
has done so much for the theory and the practice of vinegar-making,
himself suggests that in this case--

"La cause du phenomene physique qui accompagne la vie de la
plante reside dans un etat physique propre, analogue a celui
du noir de platine. Mais il est essentiel de remarquer que cet
etat physique de la plante est etroitement lie avec la vie de
cette plante."[1]

[Footnote 1: "Etudes sur les Mycodermes," Comptes-Rendus, liv., 1862.]

Now, if the vinegar plant gives rise to the oxidation of alcohol,
on account of its merely physical constitution, it is at any rate
possible that the physical constitution of the yeast plant may exert a
decomposing influence on sugar.

But, without presuming to discuss a question which leads us into the
very arcana of chemistry, the present state of speculation upon the
_modus operandi_ of the yeast plant in producing fermentation is
represented, on the one hand, by the Stahlian doctrine, supported by
Liebig, according to which the atoms of the sugar are shaken into new
combinations, either directly by the _Torulae_, or indirectly, by some
substance formed by them; and, on the other hand, by the Thenardian
doctrine, supported by Pasteur, according to which the yeast plant
assimilates part of the sugar, and, in so doing, disturbs the rest,
and determines its resolution into the products of fermentation.
Perhaps the two views are not so much opposed as they seem at first
sight to be.

But the interest which attaches to the influence of the yeast plants
upon the medium in which they live and grow does not arise solely
from its bearing upon the theory of fermentation. So long ago as 1838,
Turpin compared the _Torulae_ to the ultimate elements of the tissues
of animals and plants--"Les organes elementaires de leurs tissus,
comparables aux petits vegetaux des levures ordinaires, sont aussi les
decompositeurs des substances qui les environnent."

Almost at the same time, and, probably, equally guided by his study of
yeast, Schwann was engaged in those remarkable investigations into
the form and development of the ultimate structural elements of the
tissues of animals, which led him to recognize their fundamental
identity with the ultimate structural elements of vegetable organisms.

The yeast plant is a mere sac, or "cell," containing a semi-fluid
matter, and Schwann's microscopic analysis resolved all living
organisms, in the long run, into an aggregation of such sacs or cells,
variously modified; and tended to show, that all, whatever their
ultimate complication, begin their existence in the condition of such
simple cells.

In his famous "Mikroskopische Untersuchungen," Schwann speaks of
_Torula_ as a "cell;" and, in a remarkable note to the passage in
which he refers to the yeast plant, Schwann says:--

"I have been unable to avoid mentioning fermentation, because
it is the most fully and exactly known operation of cells,
and represents, in the simplest fashion, the process which is
repeated by every cell of the living body."

In other words, Schwann conceives that every cell of the living body
exerts an influence on the matter which surrounds and permeates it,
analogous to that which a _Torula_ exerts on the saccharine solution
by which it is bathed. A wonderfully suggestive thought, opening up
views of the nature of the chemical processes of the living body,
which have hardly yet received all the development of which they are
capable.

Kant defined the special peculiarity of the living body to be that the
parts exist for the sake of the whole and the whole for the sake of
the parts. But when Turpin and Schwann resolved the living body into
an aggregation of quasi-independent cells, each, like a _Torula_,
leading its own life and having its own laws of growth and
development, the aggregation being dominated and kept working towards
a definite end only by a certain harmony among these units, or by the
superaddition of a controlling apparatus, such as a nervous system,
this conception ceased to be tenable. The cell lives for its own sake,
as well as for the sake of the whole organism; and the cells, which
float in the blood, live at its expense, and profoundly modify it, are
almost as much independent organisms as the _Torulae_ which float in
beer-wort.

Schwann burdened his enunciation of the "cell theory" with two false
suppositions; the one, that the structures he called "nucleus" and
"cell-wall" are essential to a cell; the other, that cells are usually
formed independently of other cells; but, in 1839, it was a vast and
clear gain to arrive at the conception, that the vital functions of
all the higher animals and plants are the resultant of the forces
inherent in the innumerable minute cells of which they are composed,
and that each of them is, itself, an equivalent of one of the lowest
and simplest of independent living beings--the _Torula._

From purely morphological investigations, Turpin and Schwann, as we
have seen, arrived at the notion of the fundamental unity of structure
of living beings. And, before long, the researches of chemists
gradually led up to the conception of the fundamental unity of their
composition.

So far back as 1803, Thenard pointed out, in most distinct terms, the
important fact that yeast contains a nitrogenous "animal" substance;
and that such a substance is contained in all ferments. Before him,
Fabroni and Fourcroy speak of the "vegeto-animal" matter of yeast.
In 1844 Mulder endeavoured to demonstrate that a peculiar substance,
which he called "protein," was essentially characteristic of living
matter. In 1846, Payen writes:--

"Enfin, une loi sans exception me semble apparaitre dans les
faits nombreux que j'ai observes et conduire a envisager sous
un nouveau jour la vie vegetale; si je ne m'abuse, tout ce
que dans les tissus vegetaux la vue directe ou amplifiee nous
permet de discerner sous la forme de cellules et de vaisseaux,
ne represente autre chose que les enveloppes protectrices,
les reservoirs et les conduits, a l'aide desquels les corps
animes qui les secretent et les faconnent, se logent, puisent
et charriant leurs aliments, deposent et isolent les matieres
excretees."

And again:--

"A fin de completer aujourd'hui l'enonce du fait general, je
rappellerai que les corps, doue des fonctions accomplies
dans les tissus des plantes, sont formes des elements qui
constituent, en proportion peu variable, les organismes
animaux; qu'ainsi l'on est conduit a reconnaitre une immense
unite de composition elementaire dans tous les corps vivants
de la nature."[1]

[Footnote 1: "Mem. sur les Developpements des Vegetaux," &c.--"Mem.
Presentees." ix. 1846.]

In the year (1846) in which these remarkable passages were published,
the eminent German botanist, Von Mohl, invented the word "protoplasm,"
as a name for one portion of those nitrogenous contents of the cells
of living plants, the close chemical resemblance of which to the
essential constituents of living animals is so strongly indicated by
Payen. And through the twenty-five years that have passed, since the
matter of life was first called protoplasm, a host of investigators,
among whom Cohn, Max Schulze, and Kuehne must be named as leaders, have
accumulated evidence, morphological, physiological, and chemical, in
favour of that "immense unite de composition elementaire dans tous les
corps vivants de la nature," into which Payen had, so early, a clear
insight.

As far back as 1850, Cohn wrote, apparently without any knowledge of
what Payen had said before him:--

"The protoplasm of the botanist, and the contractile substance
and sarcode of the zoologist, must be, if not identical, yet
in a high degree analogous substances. Hence, from this point
of view, the difference between animals and plants consists
in this; that, in the latter, the contractile substance, as
a primordial utricle, is enclosed within an inert cellulose
membrane, which permits it only to exhibit an internal motion,
expressed by the phenomena of rotation and circulation, while,
in the former, it is not so enclosed. The protoplasm in the
form of the primordial utricle is, as it were, the animal
element in the plant, but which is imprisoned, and only
becomes free in the animal; _or_, to strip off the metaphor
which obscures simple thought, the energy of organic vitality
which is manifested in movement is especially exhibited by a
nitrogenous contractile substance, which in plants is limited
and fettered by an inert membrane, in animals not so."[1]

[Footnote 1: Cohn, "Ueber Protococcus pluvialis," in the "Nova Acta"
for 1850.]

In 1868, thinking that an untechnical statement of the views current
among the leaders of biological science might be interesting to the
general public, I gave a lecture embodying them in Edinburgh. Those
who have not made the mistake of attempting to approach biology,
either by the high _a priori_ road of mere philosophical speculation,
or by the mere low _a posteriori_ lane offered by the tube of a
microscope, but have taken the trouble to become acquainted with
well-ascertained facts and with their history, will not need to be
told that in what I had to say "as regards protoplasm" in my lecture
"On the Physical Basis of Life," there was nothing new; and, as I
hope, nothing that the present state of knowledge does not justify us
in believing to be true. Under these circumstances, my surprise may be
imagined, when I found, that the mere statement of facts and of views,
long familiar to me as part of the common scientific property of
continental workers, raised a sort of storm in this country, not only
by exciting the wrath of unscientific persons whose pet prejudices
they seemed to touch, but by giving rise to quite superfluous
explosions on the part of some who should have been better informed.

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