Discourses - Thomas H. Huxley
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To sum up the effect of this long chain of evidence:--
It is demonstrable that a fluid eminently fit for the development of the
lowest forms of life, but which contains neither germs, nor any protein
compound, gives rise to living things in great abundance if it is exposed
to ordinary air; while no such development takes place, if the air with
which it is in contact is mechanically freed from the solid particles
which ordinarily float in it, and which may be made visible by
appropriate means.
It is demonstrable that the great majority of these particles are
destructible by heat, and that some of them are germs, or living
particles, capable of giving rise to the same forms of life as those
which appear when the fluid is exposed to unpurified air.
It is demonstrable that inoculation of the experimental fluid with a drop
of liquid known to contain living particles gives rise to the same
phenomena as exposure to unpurified air.
And it is further certain that these living particles are so minute that
the assumption of their suspension in ordinary air presents not the
slightest difficulty. On the contrary, considering their lightness and
the wide diffusion of the organisms which produce them, it is impossible
to conceive that they should not be suspended in the atmosphere in
myriads.
Thus the evidence, direct and indirect, in favour of _Biogenesis_ for all
known forms of life must, I think, be admitted to be of great weight.
On the other side, the sole assertions worthy of attention are that
hermetically sealed fluids, which have been exposed to great and long-
continued heat, have sometimes exhibited living forms of low organisation
when they have been opened.
The first reply that suggests itself is the probability that there must
be some error about these experiments, because they are performed on an
enormous scale every day with quite contrary results. Meat, fruits,
vegetables, the very materials of the most fermentable and putrescible
infusions, are preserved to the extent, I suppose I may say, of thousands
of tons every year, by a method which is a mere application of
Spallanzani's experiment. The matters to be preserved are well boiled in
a tin case provided with a small hole, and this hole is soldered up when
all the air in the case has been replaced by steam. By this method they
may be kept for years without putrefying, fermenting, or getting mouldy.
Now this is not because oxygen is excluded, inasmuch as it is now proved
that free oxygen is not necessary for either fermentation or
putrefaction. It is not because the tins are exhausted of air, for
_Vibriones_ and _Bacteria_ live, as Pasteur has shown, without air or
free oxygen. It is not because the boiled meats or vegetables are not
putrescible or fermentable, as those who have had the misfortune to be in
a ship supplied with unskilfully closed tins well know. What is it,
therefore, but the exclusion of germs? I think that Abiogenists are bound
to answer this question before they ask us to consider new experiments of
precisely the same order.
And in the next place, if the results of the experiments I refer to are
really trustworthy, it by no means follows that Abiogenesis has taken
place. The resistance of living matter to heat is known to vary within
considerable limits, and to depend, to some extent, upon the chemical and
physical qualities of the surrounding medium. But if, in the present
state of science, the alternative is offered us,--either germs can stand
a greater heat than has been supposed, or the molecules of dead matter,
for no valid or intelligible reason that is assigned, are able to re-
arrange themselves into living bodies, exactly such as can be
demonstrated to be frequently produced in another way,--I cannot
understand how choice can be, even for a moment, doubtful.
But though I cannot express this conviction of mine too strongly, I must
carefully guard myself against the supposition that I intend to suggest
that no such thing as Abiogenesis ever has taken place in the past, or
ever will take place in the future. With organic chemistry, molecular
physics, and physiology yet in their infancy, and every day making
prodigious strides, I think it would be the height of presumption for any
man to say that the conditions under which matter assumes the properties
we call "vital" may not, some day, be artificially brought together. All
I feel justified in affirming is, that I see no reason for believing that
the feat has been performed yet.
And looking back through the prodigious vista of the past, I find no
record of the commencement of life, and therefore I am devoid of any
means of forming a definite conclusion as to the conditions of its
appearance. Belief, in the scientific sense of the word, is a serious
matter, and needs strong foundations. To say, therefore, in the admitted
absence of evidence, that I have any belief as to the mode in which the
existing forms of life have originated, would be using words in a wrong
sense. But expectation is permissible where belief is not; and if it were
given me to look beyond the abyss of geologically recorded time to the
still more remote period when the earth was passing through physical and
chemical conditions, which it can no more see again than a man can recall
his infancy, I should expect to be a witness of the evolution of living
protoplasm from not living matter. I should expect to see it appear under
forms of great simplicity, endowed, like existing fungi, with the power
of determining the formation of new protoplasm from such matters as
ammonium carbonates, oxalates and tartrates, alkaline and earthy
phosphates, and water, without the aid of light. That is the expectation
to which analogical reasoning leads me; but I beg you once more to
recollect that I have no right to call my opinion anything but an act of
philosophical faith.
So much for the history of the progress of Redi's great doctrine of
Biogenesis, which appears to me, with the limitations I have expressed,
to be victorious along the whole line at the present day.
As regards the second problem offered to us by Redi, whether Xenogenesis
obtains, side by side with Homogenesis,--whether, that is, there exist
not only the ordinary living things, giving rise to offspring which run
through the same cycle as themselves, but also others, producing
offspring which are of a totally different character from themselves,--
the researches of two centuries have led to a different result. That the
grubs found in galls are no product of the plants on which the galls
grow, but are the result of the introduction of the eggs of insects into
the substance of these plants, was made out by Vallisnieri, Reaumur, and
others, before the end of the first half of the eighteenth century. The
tapeworms, bladderworms, and flukes continued to be a stronghold of the
advocates of Xenogenesis for a much longer period. Indeed, it is only
within the last thirty years that the splendid patience of Von Siebold,
Van Beneden, Leuckart, Kuechenmeister, and other helminthologists, has
succeeded in tracing every such parasite, often through the strangest
wanderings and metamorphoses, to an egg derived from a parent, actually
or potentially like itself; and the tendency of inquiries elsewhere has
all been in the same direction. A plant may throw off bulbs, but these,
sooner or later, give rise to seeds or spores, which develop into the
original form. A polype may give rise to Medusae, or a pluteus to an
Echinoderm, but the Medusa and the Echinoderm give rise to eggs which
produce polypes or glutei, and they are therefore only stages in the
cycle of life of the species.
But if we turn to pathology, it offers us some remarkable approximations
to true Xenogenesis.
As I have already mentioned, it has been known since the time of
Vallisnieri and of Reaumur, that galls in plants, and tumours in cattle,
are caused by insects, which lay their eggs in those parts of the animal
or vegetable frame of which these morbid structures are outgrowths.
Again, it is a matter of familiar experience to everybody that mere
pressure on the skin will give rise to a corn. Now the gall, the tumour,
and the corn are parts of the living body, which have become, to a
certain degree, independent and distinct organisms. Under the influence
of certain external conditions, elements of the body, which should have
developed in due subordination to its general plan, set up for themselves
and apply the nourishment which they receive to their own purposes.
From such innocent productions as corns and warts, there are all
gradations to the serious tumours which, by their mere size and the
mechanical obstruction they cause, destroy the organism out of which they
are developed; while, finally, in those terrible structures known as
cancers, the abnormal growth has acquired powers of reproduction and
multiplication, and is only morphologically distinguishable from the
parasitic worm, the life of which is neither more nor less closely bound
up with that of the infested organism.
If there were a kind of diseased structure, the histological elements of
which were capable of maintaining a separate and independent existence
out of the body, it seems to me that the shadowy boundary between morbid
growth and Xenogenesis would be effaced. And I am inclined to think that
the progress of discovery has almost brought us to this point already. I
have been favoured by Mr. Simon with an early copy of the last published
of the valuable "Reports on the Public Health," which, in his capacity of
their medical officer, he annually presents to the Lords of the Privy
Council. The appendix to this report contains an introductory essay "On
the Intimate Pathology of Contagion," by Dr. Burdon-Sanderson, which is
one of the clearest, most comprehensive, and well-reasoned discussions of
a great question which has come under my notice for a long time. I refer
you to it for details and for the authorities for the statements I am
about to make.
You are familiar with what happens in vaccination. A minute cut is made
in the skin, and an infinitesimal quantity of vaccine matter is inserted
into the wound. Within a certain time a vesicle appears in the place of
the wound, and the fluid which distends this vesicle is vaccine matter,
in quantity a hundred or a thousandfold that which was originally
inserted. Now what has taken place in the course of this operation? Has
the vaccine matter, by its irritative property, produced a mere blister,
the fluid of which has the same irritative property? Or does the vaccine
matter contain living particles, which have grown and multiplied where
they have been planted? The observations of M. Chauveau, extended and
confirmed by Dr. Sanderson himself, appear to leave no doubt upon this
head. Experiments, similar in principle to those of Helmholtz on
fermentation and putrefaction, have proved that the active element in the
vaccine lymph is non-diffusible, and consists of minute particles not
exceeding 1/20000th of an inch in diameter, which are made visible in the
lymph by the microscope. Similar experiments have proved that two of the
most destructive of epizootic diseases, sheep-pox and glanders, are also
dependent for their existence and their propagation upon extremely small
living solid particles, to which the title of _microzymes_ is applied. An
animal suffering under either of these terrible diseases is a source of
infection and contagion to others, for precisely the same reason as a tub
of fermenting beer is capable of propagating its fermentation by
"infection," or "contagion," to fresh wort. In both cases it is the solid
living particles which are efficient; the liquid in which they float, and
at the expense of which they live, being altogether passive.
Now arises the question, are these microzymes the results of
_Homogenesis_, or of _Xenogenesis?_ are they capable, like the
_Toruloe_ of yeast, of arising only by the development of pre-existing
germs? or may they be, like the constituents of a nut-gall, the results
of a modification and individualisation of the tissues of the body in
which they are found, resulting from the operation of certain conditions?
Are they parasites in the zoological sense, or are they merely what
Virchow has called "heterologous growths"? It is obvious that this
question has the most profound importance, whether we look at it from a
practical or from a theoretical point of view. A parasite may be stamped
out by destroying its germs, but a pathological product can only be
annihilated by removing the conditions which give rise to it.
It appears to me that this great problem will have to be solved for each
zymotic disease separately, for analogy cuts two ways. I have dwelt upon
the analogy of pathological modification, which is in favour of the
xenogenetic origin of microzymes; but I must now speak of the equally
strong analogies in favour of the origin of such pestiferous particles by
the ordinary process of the generation of like from like.
It is, at present, a well-established fact that certain diseases, both of
plants and of animals, which have all the characters of contagious and
infectious epidemics, are caused by minute organisms. The smut of wheat
is a well-known instance of such a disease, and it cannot be doubted that
the grape-disease and the potato-disease fall under the same category.
Among animals, insects are wonderfully liable to the ravages of
contagious and infectious diseases caused by microscopic _Fungi_.
In autumn, it is not uncommon to see flies motionless upon a window-pane,
with a sort of magic circle, in white, drawn round them. On microscopic
examination, the magic circle is found to consist of innumerable spores,
which have been thrown off in all directions by a minute fungus called
_Empusa muscoe_, the spore-forming filaments of which stand out like a
pile of velvet from the body of the fly. These spore-forming filaments
are connected with others which fill the interior of the fly's body like
so much fine wool, having eaten away and destroyed the creature's
viscera. This is the full-grown condition of the _Empusa_. If traced back
to its earliest stages, in flies which are still active, and to all
appearance healthy, it is found to exist in the form of minute corpuscles
which float in the blood of the fly. These multiply and lengthen into
filaments, at the expense of the fly's substance; and when they have at
last killed the patient, they grow out of its body and give off spores.
Healthy flies shut up with diseased ones catch this mortal disease, and
perish like the others. A most competent observer, M. Cohn, who studied
the development of the _Empusa_ very carefully, was utterly unable to
discover in what manner the smallest germs of the _Empusa_ got into the
fly. The spores could not be made to give rise to such germs by
cultivation; nor were such germs discoverable in the air, or in the food
of the fly. It looked exceedingly like a case of Abiogenesis, or, at any
rate, of Xenogenesis; and it is only quite recently that the real course
of events has been made out. It has been ascertained, that when one of
the spores falls upon the body of a fly, it begins to germinate, and
sends out a process which bores its way through the fly's skin; this,
having reached the interior cavities of its body, gives off the minute
floating corpuscles which are the earliest stage of the _Empusa_. The
disease is "contagious," because a healthy fly coming in contact with a
diseased one, from which the spore-bearing filaments protrude, is pretty
sure to carry off a spore or two. It is "infectious" because the spores
become scattered about all sorts of matter in the neighbourhood of the
slain flies.
The silkworm has long been known to be subject to a very fatal and
infectious disease called the _Muscardine_. Audouin transmitted it by
inoculation. This disease is entirely due to the development of a fungus,
_Botrytis Bassiana_, in the body of the caterpillar; and its
contagiousness and infectiousness are accounted for in the same way as
those of the fly-disease. But, of late years, a still more serious
epizootic has appeared among the silkworms; and I may mention a few facts
which will give you some conception of the gravity of the injury which it
has inflicted on France alone.
The production of silk has been for centuries an important branch of
industry in Southern France, and in the year 1853 it had attained such a
magnitude that the annual produce of the French sericulture was estimated
to amount to a tenth of that of the whole world, and represented a money-
value of 117,000,000 francs, or nearly five millions sterling. What may
be the sum which would represent the money-value of all the industries
connected with the working up of the raw silk thus produced, is more than
I can pretend to estimate. Suffice it to say, that the city of Lyons is
built upon French silk as much as Manchester was upon American cotton
before the civil war.
Silkworms are liable to many diseases; and, even before 1853, a peculiar
epizootic, frequently accompanied by the appearance of dark spots upon
the skin (whence the name of "Pebrine" which it has received), had been
noted for its mortality. But in the years following 1853 this malady
broke out with such extreme violence, that, in 1858, the silk-crop was
reduced to a third of the amount which it had reached in 1853; and, up
till within the last year or two, it has never attained half the yield of
1853. This means not only that the great number of people engaged in silk
growing are some thirty millions sterling poorer than they might have
been; it means not only that high prices have had to be paid for imported
silkworm eggs, and that, after investing his money in them, in paying for
mulberry-leaves and for attendance, the cultivator has constantly seen
his silkworms perish and himself plunged in ruin; but it means that the
looms of Lyons have lacked employment, and that, for years, enforced
idleness and misery have been the portion of a vast population which, in
former days, was industrious and well-to-do.
In 1858 the gravity of the situation caused the French Academy of
Sciences to appoint Commissioners, of whom a distinguished naturalist, M.
de Quatrefages, was one, to inquire into the nature of this disease, and,
if possible, to devise some means of staying the plague. In reading the
Report[11] made by M. de Quatrefages in 1859, it is exceedingly
interesting to observe that his elaborate study of the Pebrine forced the
conviction upon his mind that, in its mode of occurrence and propagation,
the disease of the silkworm is, in every respect, comparable to the
cholera among mankind. But it differs from the cholera, and so far is a
more formidable malady, in being hereditary, and in being, under some
circumstances, contagious as well as infectious.
[Footnote 11: _Etudes sur les Maladies actuelles des Vers a Soie_, p.
53.]
The Italian naturalist, Filippi, discovered in the blood of the silkworms
affected by this strange disorder a multitude of cylindrical corpuscles,
each about 1/6000th of an inch long. These have been carefully studied by
Lebert, and named by him _Panhistophyton_; for the reason that in
subjects in which the disease is strongly developed, the corpuscles swarm
in every tissue and organ of the body, and even pass into the undeveloped
eggs of the female moth. But are these corpuscles causes, or mere
concomitants, of the disease? Some naturalists took one view and some
another; and it was not until the French Government, alarmed by the
continued ravages of the malady, and the inefficiency of the remedies
which had been suggested, despatched M. Pasteur to study it, that the
question received its final settlement; at a great sacrifice, not only of
the time and peace of mind of that eminent philosopher, but, I regret to
have to add, of his health.
But the sacrifice has not been in vain. It is now certain that this
devastating, cholera-like, Pebrine, is the effect of the growth and
multiplication of the _Panhistophyton_ in the silkworm. It is contagious
and infectious, because the corpuscles of the _Panhistophyton_ pass away
from the bodies of the diseased caterpillars, directly or indirectly, to
the alimentary canal of healthy silkworms in their neighbourhood; it is
hereditary because the corpuscles enter into the eggs while they are
being formed, and consequently are carried within them when they are
laid; and for this reason, also, it presents the very singular
peculiarity of being inherited only on the mother's side. There is not a
single one of all the apparently capricious and unaccountable phenomena
presented by the Pebrine, but has received its explanation from the fact
that the disease is the result of the presence of the microscopic
organism, _Panhistophyton_.
Such being the facts with respect to the Pebrine, what are the
indications as to the method of preventing it? It is obvious that this
depends upon the way in which the _Panhistophyton_ is generated. If it
may be generated by Abiogenesis, or by Xenogenesis, within the silkworm
or its moth, the extirpation of the disease must depend upon the
prevention of the occurrence of the conditions under which this
generation takes place. But if, on the other hand, the _Panhistophyton_
is an independent organism, which is no more generated by the silkworm
than the mistletoe is generated by the apple-tree or the oak on which it
grows, though it may need the silkworm for its development in the same
way as the mistletoe needs the tree, then the indications are totally
different. The sole thing to be done is to get rid of and keep away the
germs of the _Panhistophyton_. As might be imagined, from the course of
his previous investigations, M. Pasteur was led to believe that the
latter was the right theory; and, guided by that theory, he has devised a
method of extirpating the disease, which has proved to be completely
successful wherever it has been properly carried out.
There can be no reason, then, for doubting that, among insects,
contagious and infectious diseases, of great malignity, are caused by
minute organisms which are produced from pre-existing germs, or by
homogenesis; and there is no reason, that I know of, for believing that
what happens in insects may not take place in the highest animals.
Indeed, there is already strong evidence that some diseases of an
extremely malignant and fatal character to which man is subject, are as
much the work of minute organisms as is the Pebrine. I refer for this
evidence to the very striking facts adduced by Professor Lister in his
various well-known publications on the antiseptic method of treatment. It
appears to me impossible to rise from the perusal of those publications
without a strong conviction that the lamentable mortality which so
frequently dogs the footsteps of the most skilful operator, and those
deadly consequences of wounds and injuries which seem to haunt the very
walls of great hospitals, and are, even now, destroying more men than die
of bullet or bayonet, are due to the importation of minute organisms into
wounds, and their increase and multiplication; and that the surgeon who
saves most lives will be he who best works out the practical consequences
of the hypothesis of Redi.
I commenced this Address by asking you to follow me in an attempt to
trace the path which has been followed by a scientific idea, in its long
and slow progress from the position of a probable hypothesis to that of
an established law of nature. Our survey has not taken us into very
attractive regions; it has lain, chiefly, in a land flowing with the
abominable, and peopled with mere grubs and mouldiness. And it may be
imagined with what smiles and shrugs, practical and serious
contemporaries of Redi and of Spallanzani may have commented on the waste
of their high abilities in toiling at the solution of problems which,
though curious enough in themselves, could be of no conceivable utility
to mankind.
Nevertheless, you will have observed that before we had travelled very
far upon our road, there appeared, on the right hand and on the left,
fields laden with a harvest of golden grain, immediately convertible into
those things which the most solidly practical men will admit to have
value--viz., money and life.
The direct loss to France caused by the Pebrine in seventeen years cannot
be estimated at less than fifty millions sterling; and if we add to this
what Redi's idea, in Pasteur's hands, has done for the wine-grower and
for the vinegar-maker, and try to capitalise its value, we shall find
that it will go a long way towards repairing the money losses caused by
the frightful and calamitous war of this autumn. And as to the equivalent
of Redi's thought in life, how can we over-estimate the value of that
knowledge of the nature of epidemic and epizootic diseases, and
consequently of the means of checking, or eradicating them, the dawn of
which has assuredly commenced?