Читать книгу The modes of origin of lowest organisms including a discussion of the experiments of M. Pasteur - H. Charlton Bastian - Страница 7

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The evidence on this part of the subject is, I think, sharply defined and conclusive. Simple experiments can be had recourse to, which are not admissible in the discussion of the question as to the origin of Bacteria and Torulæ by Heterogenesis. There, we wish to establish the fact that living matter is capable of undergoing a certain metamorphosis, and consequently, we must deal with living matter. Here, however, with the view of establishing the fact that living matter can arise de novo, if we are able, shortly after beginning our experiment, to arrive at a reasonable and well-based assurance that no living thing exists in the hermetically sealed experimental vessel—if the measures that we have adopted fully entitle us to believe that all living things which may have pre-existed therein have been killed—we may feel pretty sure that any living organisms which are subsequently found, when the vessel is broken, must have originated from some re-arrangements which had taken place amongst the not-living constituents of the experimental solutions, whereby life-initiating combinations had been formed.

The possibility of the de novo origination of Bacteria, Torulæ, and other such organisms, is one which is intimately associated with the doctrine as to the cause of fermentation and putrefaction. With regard to the almost invariable association of such organisms with some of these processes, almost all are agreed. There is, moreover, a very frequent association of particular kinds of organisms with particular kinds of fermentation. Hence the assumption is an easy and a natural one to many persons, that the organisms which are invariably met with in some cases are the causes of these fermentations,9 although it is quite obvious that the facts on which this view is based, are equally explicable on the supposition that the organisms are concomitant results or products (due to new chemical combinations) of the fermentative changes. In the one case the fermentative changes are believed to be initiated by the influence of living organisms; and those who regard living things as the only true ferments, for the most part also believe that living things are incapable of arising de novo. They think that those organisms which serve to initiate the changes in question, have been derived from a multitudinous army of omnipresent atmospheric germs, which are always ready, in number and kind suitable for every emergency. This is the doctrine of M. Pasteur and others. On the other hand, fermentations and putrefactions may be regarded as sets of chemical changes, which are apt to occur in organic and other complex substances—these changes being due either to the intrinsic instability of the body which manifests them, or to molecular movements communicated to it by a still more unstable body. Baron Liebig says:—“Many organic compounds are known, which undergo, in presence of water, alteration and metamorphosis, having a certain duration, and ultimately terminating in putrefaction; while other organic substances that are not liable to such alteration by themselves, nevertheless, suffer a similar displacement or separation of their molecules, when brought into contact with the ferments.”

Each substance belonging to the first class, would be at the same time, therefore, both ferment and fermentable substance; whilst a small portion of such substance, when brought into contact with a less unstable substance, might induce such molecular movements as to make it undergo a process of fermentation. With regard to the cause of such induced fermentative changes, Gerhardt10 says, in explaining Liebig’s views:—“Every substance which decomposes or enters into combination is in a state of movement, its molecules being agitated; but since friction, shock, mechanical agitation, suffice to provoke the decomposition of many substances (chlorous acid, chloride of nitrogen, fulminating silver), there is all the more reason why a chemical decomposition in which the molecular agitation is more complete, should produce similar effects upon certain substances. In addition, bodies are known which when alone are not decomposed by certain agents, but which are attacked, when they exist in contact with other bodies incapable of resisting the influence of these agents. Thus platinum alone does not dissolve in nitric acid, but when allied with silver, it is easily dissolved; pure copper is not dissolved by sulphuric acid, but it does dissolve in this when it is allied with zinc, &c. According to M. Liebig it is the same with ferments and fermentable substances; sugar, which does not change when it is quite alone, changes—that is to say ferments—when it is in contact with a nitrogenous substance undergoing change, that is, with a ferment.”

Thus, in accordance with this latter view, living ferments are not needed—mere dead, organic or nitrogenous matter suffices to initiate the processes in question.11 Those who hold this opinion may or may not believe that organisms are capable of arising de novo;12 though there can be little doubt that a belief in the truth of such a doctrine does, almost inevitably, entail a belief in the de novo origination of living things. No one who has looked into the evidence, doubts the fact of the association between some of these processes and the presence of organisms; the only question is, as to the relation in which they stand to one another. If organisms are not the causes of those fermentative changes with which they are invariably associated, then they are, in all probability, the results of such changes; and they must certainly have been produced de novo if it can be shown that fermentation or putrefaction may take place under the influence of conditions which make it certain that pre-existing living organisms could have had nothing to do with the process.

Now, in order to lend some air of probability to the former hypothesis, concerning the necessity for the existence of living ferments, it was incumbent upon its supporters to endeavour to show that the air did contain such a multitude of “germs,” or living things, as were demanded by the requirements of their theory. Spallanzani and Bonnet had, as far as the imagination was concerned, done all that was necessary. They had proclaimed the universal diffusion of “germs” of all kinds of organisms throughout the atmosphere—which were ready to develop, whenever suitable conditions presented themselves. So far, however, this was but another hypothesis. To establish the doctrine that fermentation cannot take place without the agency of living ferments, we cannot receive hypotheses in evidence: facts are needed. These, no one attempted to supply in an adequate manner13 anterior to the investigations of M. Pasteur. Speaking of his researches, even M. Milne-Edwards says,14 “Previous to this time, the existence of reproductive particles, or infusorial germs in the atmosphere was nothing more than a plausible hypothesis, put forward in order to explain the origin of such creatures in a manner conformable with the general laws of reproduction; but it was only a mere supposition, and no one had been able actually to see or to handle these reproductive corpuscles.”

We have to look, therefore, to M. Pasteur’s investigations, and to others which may have been since conducted, for all the scientific evidence in support of what has been called the “Panspermic hypothesis.”

By an ingenious method of filtration, which is fully described in his memoir,15 M. Pasteur separated from the air that passed through his apparatus the solid particles which it contained. This search convinced him that there were, as he says, “constantly in ordinary air a variable number of corpuscles whose form and structure declare them to be organized.” Some of these, he thinks, resemble the spores of fungi, and others the ova of ciliated infusoria, though he adds:—“But as to affirming that this is a spore, much less the spore of any definite species, and that one is an egg, and belonging to such an infusorium, I believe that this is not possible.” He limits himself, in fact, to the statements, that the corpuscles which he found, were (in his opinion) evidently organized; that they resembled in form and appearance the germs of the lower kinds of organisms; and that, from their variety in size, they probably belonged to many different sorts of living things. Even here, therefore, we have to do with the impressions of M. Pasteur, rather than with verified statements. All that has been established by his direct investigation as to the nature of the solid bodies contained in the atmosphere is this: that the air contains a number of round or ovoidal corpuscles, often quite structureless, which he could not distinguish from the spores of fungi16—some of which, being about the right size, were round or ovoidal, and structureless. In addition, however, it has been shown that the air contains other rounded corpuscles which are similarly structureless, though composed of silica or starch. It may therefore be asked, in the first place, whether the conclusion is a sufficiently safe one that many of the corpuscles found by M. Pasteur were spores of fungi; and in the next place, supposing this to have been established, whether such spores were living or dead. These questions would have been answered satisfactorily if M. Pasteur could state that he had actually watched the development of such corpuscles, in some suitable apparatus, into distinct organisms. But any such development, he distinctly states, he never witnessed. He says17:—“What would have been the better and more direct course would have been to follow the development of these germs with the microscope. Such was my intention; but the apparatus which I had devised for this purpose not having been delivered to me at a convenient time, I was diverted from this investigation by other work.” The evidence which he does adduce, in subsequent portions of his memoir, in order to prove that some of these corpuscles were really “fertile germs,” is almost valueless, because all the facts are open to another interpretation, which is just as much, nay, even more, in accordance with Baron Liebig’s than with his own doctrine of fermentation.

But another most important consideration presents itself. M. Pasteur’s researches as to the nature of the dust contained in the atmosphere enable him to say nothing concerning the presence of Bacteria, although he himself admits that these are generally the first organisms which display themselves in fermentations or putrefactions, and that in a very large majority of the cases in which fermentation occurs in closed vessels they are the only organisms which make their appearance.18 And yet, notwithstanding these facts, M. Pasteur says, in reference to the common form of Bacterium:—“This infusorial animal is so small that one cannot distinguish its germ, and still less fix upon the presence of this germ, if it were known, amongst the organized corpuscles of the dust which is suspended in the air.”

Here, then, we have a confession from M. Pasteur himself, that all evidence fails, where it is most wanted, in support of his hypothesis.

If a large number of fermentations begin with the presence of Bacteria as the only living things, and if in a number of cases no other organisms ever occur, it is useless to adduce as evidence, in proof of the view that fermentations are always initiated by air-derived organisms, the fact that certain corpuscles (supposed to be spores of fungi) are recognizable in the atmosphere—capped by the distinct statement19 that Bacteria or their germs are not recognizable. If Bacteria are not recognizable in the atmosphere, what scientific evidence is there that the fermentations in which these alone occur are initiated by Bacteria derived from the atmosphere, or from certain imaginary Bacteria germs,20 which we are supposed to be unable to distinguish? M. Pasteur may, moreover, be reminded that when he resorts to the supposition of Bacteria possessing “germs” which are indistinguishable, he is again resorting to hypothesis rather than to fact, in order to prove the truth of the particular doctrine of fermentation which he advocates. Bacteria are known to reproduce and multiply only by a process of fission; each of the parts into which they divide being nothing more than a part of the original Bacterium, and therefore endowed with similar properties of resisting heat, desiccation, and other agencies. Any resort to invisible germs to account for the multiplication of Bacteria, which are known to reproduce freely in other ways, is obviously not permissible, unless such postulation be more or less necessitated by the occurrence of facts otherwise inexplicable.

Although, therefore, no direct evidence has been adduced tending to show that Bacteria are present in the atmosphere, even if this evidence had been forthcoming, it would have been necessary, in reference to M. Pasteur’s hypothesis, for it to be supplemented by further evidence to the effect that Bacteria were well capable of resisting such an amount of desiccation as must have been involved by their presence for an indefinite time in the atmosphere even of the hottest and driest regions of the earth. For, organic substances in solution do not only putrefy in moist weather or moist climates; they putrefy most rapidly and surely when the temperature is high, and quite irrespectively of the amount of moisture contained in the atmosphere. A property of resisting the effects of desiccation—the possession of which, by Bacteria, is so necessary for the truth of M. Pasteur’s argument—ought to have been shown by scientific evidence to be a real attribute of such organisms; though it seems, on the contrary, to have been assumed to exist, with almost equal readiness by both parties, in the controversies concerning the possibility of “spontaneous generation.” This error may be ascribed to the misguiding influence of a treacherous analogy. Whilst it may be true that certain seeds and spores, and also that Rotifers, “Sloths,” and some Nematoids are capable of resisting the influence of a prolonged exposure to desiccating influences, it may well be asked, whether the same fact necessarily holds good for organisms such as Bacteria, which have no chitinous or other envelopes to protect them, and which are merely minute fragments of naked protoplasm. Having elsewhere21 shown how far presumptions had stolen a march upon established facts, in reference to the supposed possession of a similar property by the Free Nematoids, my eyes were opened to the reality of this uncertainty with regard to Bacteria. It is, however, no easy matter definitely to prove or to disprove the possession of this property by organisms so minute as Bacteria, and therefore so difficult to identify. If dried Bacteria are added to a drop of a suitable solution—similar to that in which they had been bred—it soon becomes quite impossible to distinguish those which have been added from those which arise in the fluid. Taking into consideration the fate of other simple organisms, however, it is by no means improbable that they should be killed even by a short desiccation. I have found, for instance, that desiccation for half-an-hour in a room at a temperature of 65° F. suffices to kill all the larger, naked, lower organisms with which I have ex­peri­men­ted—in­clud­ing long Vibrios, Amœbæ, Monads, Chlamydomonads, Euglenæ, Desmids, Vorticellæ and all other Ciliated Infusoria.

But, certain indirect evidence seems to speak most authoritatively against the supposition that the air contains any notable quantity of living Bacteria, or Bacteria germs, whether visible or invisible. I have always found that a simple solution of ammonic tartrate, which has been placed—without previous boiling—in a corked bottle of greater capacity, will become turbid in two or three days, owing to the presence of myriads of Bacteria; whilst a similar solution, previously boiled, may remain for ten days, three weeks, or more, without showing the least trace of turbidity, although the open neck of the bottle or flask in which it is contained, may be covered only by a loose cap of paper. And yet, at any time, in order to make this fluid become turbid in from 24 to 48 hours, all that one has to do is to bring it into contact with a small glass rod which has just been dipped into a solution containing living Bacteria.22

If we find that an eminently inoculable fluid will remain for two or three weeks, or perhaps more, in contact with the air without becoming turbid, though it will always become turbid in two or three days if brought into contact with living Bacteria, what can we conclude, but that living Bacteria are not very common in the atmosphere? These most striking facts can be easily verified by other observers.

Thus we find ourselves, at present, in this position. After all that has been said and done to prove the wonderful prevalence of “germs” in the atmosphere, we are really still in the region of hypothesis—no further advanced than we were in the time of Bonnet and of Spallanzani, so far as it concerns the organisms which are all important—Bacteria. Neither these nor their germs have been shown to exist in any recognizable abundance in the atmosphere, and yet in most fermentations they are the first organisms which display themselves; whilst in many such fermentations Bacteria alone occur. Nay more, even were they present in any great abundance, there is some reason to believe that the majority of them would exist as mere dead, organic particles—because Bacteria are more than likely to be unable to resist anything like an extreme or prolonged exposure to desiccating influences.

The first and essential data in support of M. Pasteur’s hypothesis must, therefore, be regarded as entirely unproved in respect to Bacteria—which are the most important of all organisms, in relation to the cause of fermentation and putrefaction.

Without the aid of elaborate experiments, however, the evidence which the microscope can supply is capable of leading us to the conclusion that such search for atmospheric Bacteria germs, was comparatively useless. If it can be shown that Bacteria can arise in a fluid independently of visible germs, then, obviously, any inquiries as to the nature of the visible contents of the atmosphere, can have only a very indirect bearing upon the question as to the mode of origin of these organisms. And yet by the aid of the microscope, as I have elsewhere stated, one can watch the appearance of almost motionless specks, more or less uniformly diffused through a motionless film of fluid, and can see them gradually develop into moving Bacteria or into Torulæ. So that, where no visible germs previously existed, visible particles of living matter develop, and more or less rapidly grow into distinct Bacteria. This may be best seen in a drop of a fresh and very strong turnip infusion, which has been filtered several times through the finest paper. The drop, placed in a live-box, should be flattened into a thin film by the application of the cover.

Thus protected, evaporation takes place very slowly, and with the live-box resting on one of Stricker’s hot-water plates, at a temperature of 85° to 90° F., and the latter upon the stage of the microscope, one can easily select a portion of the field in which either no particles or only a countable number exist. If, therefore, around and between any mere granules which may pre-exist, or in a clear space, one gradually sees in the course of two or perhaps three hours, a multitude of almost motionless specks (at first about 1/100000″ in diameter) in positions where no such specks previously existed; and if these specks may be seen gradually to increase in size and develop into Bacteria and Torulæ, then, at all events, we are able to say that these organisms can be developed without pre-existing visible germs, and we have just the same amount of actual evidence for believing that they have been formed de novo, as we should have for believing that crystals had been formed de novo, if we had seen them appearing under our eyes in the same manner. Whether they really arise after the fashion of crystals, without the aid of pre-existing though invisible germs, is a matter which can only be settled inferentially, by a subsequent resort to strict methods of experimentation.

Seeing however, that we are able, with the aid of the microscope alone, to demonstrate that Bacteria and Torulæ can develop in situations where no visible germs had previously existed, it is useless, as I have said before—so far as the question of their mode of origin is concerned—to search the atmosphere to ascertain what visible germs it may contain. If some Bacteria and Torulæ arise from germs at all, it must be from germs which are invisible to us. The finding of visible germs in the atmosphere can, therefore, only have an indirect bearing upon the solution of the problem. Since it can be shown that some visible spores and ova exist in the atmosphere, this affords a certain amount of warrant for the supposition that invisible, living, reproductive particles may also exist—more especially if the existence of an amount of organic matter, which is ordinarily invisible, can be revealed in the air, by the agency of the electric beam, or by any other means.

Nothing can be more illegitimate, however, in the way of inference, than the assumption at once indulged in by Prof. Tyndall and others (who might have been expected, by their previous scientific work, to have learned more caution) that this impalpable organic dust was largely composed of impalpable germs. Yet, without a shadow of proof, without even an attempt to prove it, the air was for a time represented to be a mere stirabout, thick with invisible germs. The briefest reflection, however, upon the probabilities of the case, should have sufficed to suggest a totally different interpretation. The surface of the earth is clothed with living things of all kinds, animal and vegetal, which are not only continually throwing off organic particles and fragments during their life, but are constantly undergoing processes of decay and molecular disintegration after their death. The actual reproductive elements of these living things are extremely small in bulk, when compared with the other parts which are not reproductive, and although Bacteria and Torulæ do exist abundantly, and do materially help to bring about some of the decay in question, yet their bulk, also, is extremely small in comparison with the amount of organic matter itself that is continually undergoing disintegration of a dry kind, in which Bacteria and Torulæ take no part. When, moreover, it is considered that in the neighbourhood of populous cities (the air of which alone exhibits this very large quantity of impalpable, mixed with palpable, organic dust), there is constantly going on a wear and tear of the textile fabrics and of the organic products of various kinds which are daily subservient to the wants of man; and that the chimneys of manufactories and dwelling-houses are also continually emitting clouds of smoke thick with imperfectly consumed organic particles, some idea may be gained of the manifold sources whence the organic particles and fragments found in the atmosphere may emanate, and also as to what proportion of them is likely to be composed of living or dead reproductive elements, or “germs.”

Thus, then, so far as the two rival doctrines of fermentation are concerned, the investigation of the nature of the solid particles contained in the atmosphere has revealed facts which are thoroughly in harmony with all the requirements of Liebig’s physical theory, though it has almost utterly failed to give anything like a scientific basis to the vital theory of Pasteur. So far from being able to show that living Bacteria (which are the first and oftentimes the only organisms concerned in many processes of fermentation and putrefaction) are universally diffused through the air, Pasteur admits that these cannot be detected, and that their “germs” are not recognizable.

If, therefore, M. Pasteur still maintains the truth of his theory, it should be distinctly understood that it rests originally, not upon established facts, but upon a mere hypothesis—the hypothesis that the air teems with multitudes of invisible Bacteria germs. He is driven to such a doctrine, not only by his own confessions concerning Bacteria, but also by the microscopical evidence to which I have referred.

So that in explaining the results of any experiments made with the view of throwing light upon the cause of fermentation or putrefaction, it is especially necessary to bear in mind two considerations:—

I. That dust filtered from the atmosphere cannot be proved to include living Bacteria; though it is known to contain a multitude of organic particles which may be capable in the presence of water, in accordance with Liebig’s hypothesis, of acting as ferments.

II. It must also be recollected that, in the opinion of many, Life represents a higher function which is displayed by certain kinds of organic matter; and that this higher function may be deteriorated or rendered non-existent by an amount of heat which might not be adequate to decompose the organic matter itself.

It is all the more necessary to call attention to these two considerations, because M. Pasteur invariably speaks as though it had been established that the air contains multitudes of living Bacteria, when, really, he had only proved that the air contains a number of corpuscles resembling spores of fungi, &c. And, as I have already intimated, the existence of spores of fungi in the atmosphere, however well established, is of little or no importance as an explanation of the cause of a very large number of fermentations. Their presence is even of still less importance, owing to the fact of the co-existence with these fungus-spores, of multitudes of organic fragments, which—in accordance with the views of Liebig, Gerhardt, and other chemists—are capable of acting as ferments. To this latter consideration M. Pasteur never even alludes when he speaks (loc. cit. p. 40) of his “ensemencements,” and of other experiments which are equally, or even more, capable of being interpreted in accordance with Liebig’s views than with his own.

Bearing these considerations in mind, we shall be in a better position to enquire into the real interpretation that may be given to many of M. Pasteur’s results, and into the question as to how far the facts which he records are favourable to his own, or to the adverse doctrine concerning the causes of fermentation.

In the memoir so often alluded to on “The Organized Corpuscles which exist in the Atmosphere,” M. Pasteur adduced various kinds of evidence, tending, as he thought, to show that the first Bacteria which make their appearance in putrefying or fermenting solutions, have been derived from living Bacteria or their “germs,” which pre-existed in the atmosphere.

Some of the experiments by which he endeavoured to substantiate this position were of a very simple nature. Their narration attracted much attention at the time, as it was supposed that by their means M. Pasteur had—as he pro­fessed—con­clu­sive­ly shown the erroneousness of the views of those who believed in what was called “spontaneous generation.” These experiments were soon repeated by other observers, who, using different fluids, obtained quite opposite results. Thus it became obvious to impartial critics, that whilst the means adopted by M. Pasteur might be adequate to check the processes of fermentation or putrefaction in certain fluids, they were quite powerless to effect this when many other fluids were employed.

These particular experiments, however, still seem to exercise a very great influence on the minds of many in this country, who are either unaware of, or disbelieve in, the possibility of obtaining opposite results.

The chapter in which M. Pasteur detailed these experiments is thus entitled:—“Another very simple method of demonstrating that all the organized products of Infusions (previously heated) owe their origin to the corpuscles which exist suspended in the Atmosphere.” Whilst claiming to have already rigorously established the validity of this conclusion by the experiments described in previous chapters, M. Pasteur adds:—“If there remained the least doubt on this subject, in the mind of the reader, it would be dissipated by the experiments of which I am now about to speak.” (p. 66.)

Sweetened yeast-water, urine, infusions of pear and of beetroot, were placed in flasks with long necks, variously drawn out and bent. The flasks were subsequently treated as follows. M. Pasteur says:—“I then raise the liquid to the boiling-point for several minutes until steam issues abundantly from the extremity of the drawn-out neck of the flask, which is permitted to remain open. I then allow the flask to cool. But, singular fact—and one well calculated to astonish every one acquainted with the delicacy of the experiments relating to what is called ‘spontaneous generation’—the liquid of this flask will remain indefinitely without alteration. The flask may be handled without any fear, it may be transported from place to place, allowed to experience all the seasonal variations of temperature, and its liquid does not undergo the slightest alteration, whilst it preserves its odour and its taste.” If, however, the neck of one of these flasks be broken off close to the flask itself, then, according to M. Pasteur, the previously unaltered fluid will, in a day or two, undergo the ordinary changes, and swarm with Bacteria and Mucedineæ.

“The great interest of this method is,” M. Pasteur adds, “that it completes, unanswerably, the proof that the origin of life in infusions which have been raised to the boiling point, is solely due to the solid particles23 which are suspended in the air.” He believes that any living things pre-existing in the fluid itself would be destroyed by the high temperature to which it had been raised; and that those contained in the air of the flask would also be destroyed, if not expelled, by the process of ebullition. Believing that the air is the source of germs from which Life is first developed in infusions, he thinks that what rapidly enters at first, on the cessation of ebullition, has its germs destroyed by contact with the almost boiling liquid; whilst the air which enters subsequently, and more slowly, is supposed to deposit its germs in the various flexures of the tubes, so that none are able to reach the fluid itself. Infusions, thus protected, do not undergo putrefaction, says M. Pasteur, because the access of pre-existing living things is necessary for the initiation of this change, and such access is prevented by the tortuous and bent neck of the flask.