method, which is that of common and ordinary occurrence, the living
parent gives rise to offspring which passes through the same cycle of
changes as itself--like gives rise to like; and this has been termed
_Homogenesis_. By the other mode, the living parent was supposed to give
rise to offspring which passed through a totally different series of
states from those exhibited by the parent, and did not return into the
cycle of the parent; this is what ought to be called _Heterogenesis_, the
offspring being altogether, and permanently, unlike the parent. The term
Heterogenesis, however, has unfortunately been used in a different sense,
and M. Milne-Edwards has therefore substituted for it _Xenogenesis_,
which means the generation of something foreign. After discussing Redi's
hypothesis of universal Biogenesis, then, I shall go on to ask how far
the growth of science justifies his other hypothesis of Xenogenesis.
The progress of the hypothesis of Biogenesis was triumphant and unchecked
for nearly a century. The application of the microscope to anatomy in the
hands of Grew, Leeuwenhoek, Swammerdam, Lyonnet, Vallisnieri, Réaurnur,
and other illustrious investigators of nature of that day, displayed such
a complexity of organisation in the lowest and minutest forms, and
everywhere revealed such a prodigality of provision for their
multiplication by germs of one sort or another, that the hypothesis of
Abiogenesis began to appear not only untrue, but absurd; and, in the
middle of the eighteenth century, when Needham and Buffon took up the
question, it was almost universally discredited.[7]
[Footnote 7: Needham, writing in 1750, says:--
"Les naturalistes modernes s'accordent unaninement à établir, comme une
vérité certaine, que toute plante vient do sa sémence spécifique, tout
animal d'un oeuf ou de quelque chose d'analogue préexistant dans la
plante, ou dans l'animal de même espèce qui l'a produit."--_Nouvelles
Observations_, p. 169.
"Les naturalistes out généralemente cru que les animaux microscopiques
étaient engendrés par des oeufs transportés dans l'air, ou déposés dans
des eaux dormantes par des insectes volans."--_Ibid._ p. 176.]
But the skill of the microscope makers of the eighteenth century soon
reached its limit. A microscope magnifying 400 diameters was a _chef
d'oeuvre_ of the opticians of that day; and, at the same time, by no
means trustworthy. But a magnifying power of 400 diameters, even when
definition reaches the exquisite perfection of our modern achromatic
lenses, hardly suffices for the mere discernment of the smallest forms of
life. A speck, only 1/25th of an inch in diameter, has, at ten inches
from the eye, the same apparent size as an object 1/10000th of an inch in
diameter, when magnified 400 times; but forms of living matter abound,
the diameter of which is not more than 1/40000th of an inch. A filtered
infusion of hay, allowed to stand for two days, will swarm with living
things among which, any which reaches the diameter of a human red blood-
corpuscle, or about 1/3200th of an inch, is a giant. It is only by
bearing these facts in mind, that we can deal fairly with the remarkable
statements and speculations put forward by Buffon and Needham in the
middle of the eighteenth century.
When a portion of any animal or vegetable body is infused in water, it
gradually softens and disintegrates; and, as it does so, the water is
found to swarm with minute active creatures, the so-called Infusorial
Animalcules, none of which can be seen, except by the aid of the
microscope; while a large proportion belong to the category of smallest
things of which I have spoken, and which must have looked like mere dots
and lines under the ordinary microscopes of the eighteenth century.
Led by various theoretical considerations which I cannot now discuss, but
which looked promising enough in the lights of their time, Buffon and
Needham doubted the applicability of Redi's hypothesis to the infusorial
animalcules, and Needham very properly endeavoured to put the question to
an experimental test. He said to himself, If these infusorial animalcules
come from germs, their germs must exist either in the substance infused,
or in the water with which the infusion is made, or in the superjacent
air. Now the vitality of all germs is destroyed by heat. Therefore, if I
boil the infusion, cork it up carefully, cementing the cork over with
mastic, and then heat the whole vessel by heaping hot ashes over it, I
must needs kill whatever germs are present. Consequently, if Redi's
hypothesis hold good, when the infusion is taken away and allowed to
cool, no animalcules ought to be developed in it; whereas, if the
animalcules are not dependent on pre-existing germs, but are generated
from the infused substance, they ought, by and by, to make their
appearance. Needham found that, under the circumstances in which he made
his experiments, animalcules always did arise in the infusions, when a
sufficient time had elapsed to allow for their development.
In much of his work Needham was associated with Buffon, and the results
of their experiments fitted in admirably with the great French
naturalist's hypothesis of "organic molecules," according to which, life
is the indefeasible property of certain indestructible molecules of
matter, which exist in all living things, and have inherent activities by
which they are distinguished from not living matter. Each individual
living organism is formed by their temporary combination. They stand to
it in the relation of the particles of water to a cascade, or a
whirlpool; or to a mould, into which the water is poured. The form of the
organism is thus determined by the reaction between external conditions
and the inherent activities of the organic molecules of which it is
composed; and, as the stoppage of a whirlpool destroys nothing but a
form, and leaves the molecules of the water, with all their inherent
activities intact, so what we call the death and putrefaction of an
animal, or of a plant, is merely the breaking up of the form, or manner
of association, of its constituent organic molecules, which are then set
free as infusorial animalcules.
It will be perceived that this doctrine is by no means identical with
_Abiogenesis_, with which it is often confounded. On this hypothesis, a
piece of beef, or a handful of hay, is dead only in a limited sense. The
beef is dead ox, and the hay is dead grass; but the "organic molecules"
of the beef or the hay are not dead, but are ready to manifest their
vitality as soon as the bovine or herbaceous shrouds in which they are
imprisoned are rent by the macerating action of water. The hypothesis
therefore must be classified under Xenogenesis, rather than under
Abiogenesis. Such as it was, I think it will appear, to those who will be
just enough to remember that it was propounded before the birth of modern
chemistry, and of the modern optical arts, to be a most ingenious and
suggestive speculation.
But the great tragedy of Science--the slaying of a beautiful hypothesis
by an ugly fact--which is so constantly being enacted under the eyes of
philosophers, was played, almost immediately, for the benefit of Buffon
and Needham.
Once more, an Italian, the Abbé Spallanzani, a worthy successor and
representative of Redi in his acuteness, his ingenuity, and his learning,
subjected the experiments and the conclusions of Needham to a searching
criticism. It might be true that Needham's experiments yielded results
such as he had described, but did they bear out his arguments? Was it not
possible, in the first place, he had not completely excluded the air by
his corks and mastic? And was it not possible, in the second place, that
he had not sufficiently heated his infusions and the superjacent air?
Spallanzani joined issue with the English naturalist on both these pleas,
and he showed that if, in the first place, the glass vessels in which the
infusions were contained were hermetically sealed by fusing their necks,
and if, in the second place, they were exposed to the temperature of
boiling water for three-quarters of an hour,[8] no animalcules ever made
their appearance within them. It must be admitted that the experiments
and arguments of Spallanzani furnish a complete and a crushing reply to
those of Needham. But we all too often forget that it is one thing to
refute a proposition, and another to prove the truth of a doctrine which,
implicitly or explicitly, contradicts that proposition; and the advance
of science soon showed that though Needham might be quite wrong, it did
not follow that Spallanzani was quite right.
[Footnote 8: See Spallanzani, _Opere_, vi. pp. 42 and 51.]
Modern Chemistry, the birth of the latter half of the eighteenth century,
grew apace, and soon found herself face to face with the great problems
which biology had vainly tried to attack without her help. The discovery
of oxygen led to the laying of the foundations of a scientific theory of
respiration, and to an examination of the marvellous interactions of
organic substances with oxygen. The presence of free oxygen appeared to
be one of the conditions of the existence of life, and of those singular
changes in organic matters which are known as fermentation and
putrefaction. The question of the generation of the infusory animalcules
thus passed into a new phase. For what might not have happened to the
organic matter of the infusions, or to the oxygen of the air, in
Spallanzani's experiments? What security was there that the development
of life which ought to have taken place had not been checked or prevented
by these changes?
The battle had to be fought again. It was needful to repeat the
experiments under conditions which would make sure that neither the
oxygen of the air, nor the composition of the organic matter, was altered
in such a manner as to interfere with the existence of life.
Schulze and Schwann took up the question from this point of view in 1836
and 1837. The passage of air through red-hot glass tubes, or through
strong sulphuric acid, does not alter the proportion of its oxygen, while
it must needs arrest, or destroy, any organic matter which may be
contained in the air. These experimenters, therefore, contrived
arrangements by which the only air which should come into contact with a
boiled infusion should be such as had either passed through red-hot tubes
or through strong sulphuric acid. The result which they obtained was that
an infusion so treated developed no living things, while, if the same
infusion was afterwards exposed to the air, such things appeared rapidly
and abundantly. The accuracy of these experiments has been alternately
denied and affirmed. Supposing then, to be accepted, however, all that
they really proved was that the treatment to which the air was subjected
destroyed _something_ that was essential to the development of life in
the infusion. This "something" might be gaseous, fluid, or solid; that it
consisted of germs remained only an hypothesis of greater or less
probability.
Contemporaneously with these investigations a remarkable discovery was
made by Cagniard de la Tour. He found that common yeast is composed of a
vast accumulation of minute plants. The fermentation of must, or of wort,
in the fabrication of wine and of beer, is always accompanied by the
rapid growth and multiplication of these _Toruloe_. Thus, fermentation,
in so far as it was accompanied by the development of microscopical
organisms in enormous numbers, became assimilated to the decomposition of
an infusion of ordinary animal or vegetable matter; and it was an obvious
suggestion that the organisms were, in some way or other, the causes both
of fermentation and of putrefaction. The chemists, with Berzelius and
Liebig at their head, at first laughed this idea to scorn; but in 1843, a
man then very young, who has since performed the unexampled feat of
attaining to high eminence alike in Mathematics, Physics, and Physiology--
I speak of the illustrious Helmholtz--reduced the matter to the test of
experiment by a method alike elegant and conclusive. Helmholtz separated
a putrefying or a fermenting liquid from one which was simply putrescible
or fermentable by a membrane which allowed the fluids to pass through and
become intermixed, but stopped the passage of solids. The result was,
that while the putrescible or the fermentable liquids became impregnated
with the results of the putrescence or fermentation which was going on on
the other side of the membrane, they neither putrefied (in the ordinary
way) nor fermented; nor were any of the organisms which abounded in the
fermenting or putrefying liquid generated in them. Therefore the cause of
the development of these organisms must lie in something which cannot
pass through membranes; and as Helmholtz's investigations were long
antecedent to Graham's researches upon colloids, his natural conclusion
was that the agent thus intercepted must be a solid material. In point of
fact, Helmholtz's experiments narrowed the issue to this: that which
excites fermentation and putrefaction, and at the same time gives rise to
living forms in a fermentable or putrescible fluid, is not a gas and is
not a diffusible fluid; therefore it is either a colloid, or it is matter
divided into very minute solid particles.
The researches of Schroeder and Dusch in 1854, and of Schroeder alone, in
1859, cleared up this point by experiments which are simply refinements
upon those of Redi. A lump of cotton-wool is, physically speaking, a pile
of many thicknesses of a very fine gauze, the fineness of the meshes of
which depends upon the closeness of the compression of the wool. Now,
Schroeder and Dusch found, that, in the case of all the putrefiable
materials which they used (except milk and yolk of egg), an infusion
boiled, and then allowed to come into contact with no air but such as had
been filtered through cotton-wool, neither putrefied, nor fermented, nor
developed living forms. It is hard to imagine what the fine sieve formed
by the cotton-wool could have stopped except minute solid particles.
Still the evidence was incomplete until it had been positively shown,
first, that ordinary air does contain such particles; and, secondly, that
filtration through cotton-wool arrests these particles and allows only
physically pure air to pass. This demonstration has been furnished within
the last year by the remarkable experiments of Professor Tyndall. It has
been a common objection of Abiogenists that, if the doctrine of Biogeny
is true, the air must be thick with germs; and they regard this as the
height of absurdity. But nature occasionally is exceedingly unreasonable,
and Professor Tyndall has proved that this particular absurdity may
nevertheless be a reality. He has demonstrated that ordinary air is no
better than a sort of stirabout of excessively minute solid particles;
that these particles are almost wholly destructible by heat; and that
they are strained off, and the air rendered optically pure, by its being
passed through cotton-wool.
It remains yet in the order of logic, though not of history, to show that
among these solid destructible particles, there really do exist germs
capable of giving rise to the development of living forms in suitable
menstrua. This piece of work was done by M. Pasteur in those beautiful
researches which will ever render his name famous; and which, in spite of
all attacks upon them, appear to me now, as they did seven years ago,[9]
to be models of accurate experimentation and logical reasoning. He
strained air through cotton-wool, and found, as Schroeder and Dusch had
done, that it contained nothing competent to give rise to the development
of life in fluids highly fitted for that purpose. But the important
further links in the chain of evidence added by Pasteur are three. In the
first place he subjected to microscopic examination the cotton-wool which
had served as strainer, and found that sundry bodies clearly recognisable
as germs, were among the solid particles strained off. Secondly, he
proved that these germs were competent to give rise to living forms by
simply sowing them in a solution fitted for their development. And,
thirdly, he showed that the incapacity of air strained through cotton-
wool to give rise to life, was not due to any occult change effected in
the constituents of the air by the wool, by proving that the cotton-wool
might be dispensed with altogether, and perfectly free access left
between the exterior air and that in the experimental flask. If the neck
of the flask is drawn out into a tube and bent downwards; and if, after
the contained fluid has been carefully boiled, the tube is heated
sufficiently to destroy any germs which may be present in the air which
enters as the fluid cools, the apparatus may be left to itself for any
time and no life will appear in the fluid. The reason is plain. Although
there is free communication between the atmosphere laden with germs and
the germless air in the flask, contact between the two takes place only
in the tube; and as the germs cannot fall upwards, and there are no
currents, they never reach the interior of the flask. But if the tube be
broken short off where it proceeds from the flask, and free access be
thus given to germs falling vertically out of the air, the fluid, which
has remained clear and desert for months, becomes, in a few days, turbid
and full of life.
[Footnote 9: _Lectures to Working Men on the Causes of the Phenomena of
Organic Nature_, 1863. (See Vol. II. of these Essays.)]
These experiments have been repeated over and over again by independent
observers with entire success; and there is one very simple mode of
seeing the facts for one's self, which I may as well describe.
Prepare a solution (much used by M. Pasteur, and often called "Pasteur's
solution") composed of water with tartrate of ammonia, sugar, and yeast-
ash dissolved therein.[10] Divide it into three portions in as many
flasks; boil all three for a quarter of an hour; and, while the steam is
passing out, stop the neck of one with a large plug of cotton-wool, so
that this also may be thoroughly steamed. Now set the flasks aside to
cool, and, when their contents are cold, add to one of the open ones a
drop of filtered infusion of hay which has stood for twenty-four hours,
and is consequently hill of the active and excessively minute organisms
known as _Bacteria_. In a couple of days of ordinary warm weather the
contents of this flask will be milky from the enormous multiplication of
_Bacteria_. The other flask, open and exposed to the air, will, sooner or
later, become milky with _Bacteria_, and patches of mould may appear in
it; while the liquid in the flask, the neck of which is plugged with
cotton-wool, will remain clear for an indefinite time. I have sought in
vain for any explanation of these facts, except the obvious one, that the
air contains germs competent to give rise to _Bacteria_, such as those
with which the first solution has been knowingly and purposely
inoculated, and to the mould-_Fungi_. And I have not yet been able to
meet with any advocate of Abiogenesis who seriously maintains that the
atoms of sugar, tartrate of ammonia, yeast-ash, and water, under no
influence but that of free access of air and the ordinary temperature,
re-arrange themselves and give rise to the protoplasm of _Bacterium_. But
the alternative is to admit that these _Bacteria_ arise from germs in the
air; and if they are thus propagated, the burden of proof that other like
forms are generated in a different manner, must rest with the assertor of
that proposition.
[Footnote 10: Infusion of hay treated in the same way yields similar
results; but as it contains organic matter, the argument which follows
cannot be based upon it.]
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.