Once a Week (magazine)/Series 1/Volume 1/Spontaneous generation
SPONTANEOUS GENERATION.
It was as easy for the ancients to conceive that animals could be produced from putrefying matters, as it is difficult for the instructed physiologist of our day to conceive any generation whatever except that by direct parentage. Aristotle found no difficulty in believing that worms and insects were generated by dead bodies, and that mice could become impregnated by licking salt. The successors of Aristotle were even less sceptical than he. They were constantly observing animals and plants suddenly springing into existence where no animals or plants had been before. Every dead dog, or decaying tree, was quickly beset with numerous forms of life; how could it be doubted that the putrefaction, which was observed as an invariable accompaniment, was the necessary cause of these sudden appearances of life?
To the mind imperfectly acquainted with the results of modern science, Spontaneous Generation is as easy of belief as it was to Aristotle. Do we not constantly see vegetable mould covering our cheese, our jam, our ink, our bread? Do we not, even in air-tight vessels, see plants and microscopic animals develop where no plants and animals could be seen before, and where, as we think, it was impossible that their seeds should have penetrated? And when we hear that Mr. Crosse produced an insect by means of electricity, startled as we may be, do we really find any better argument than our prejudice for disbelieving such a statement? Where do parasitic animals come from, if not spontaneously generated in the body? These parasites are found in the blood, in the liver, in the brain, in the eye, nay, even in the excessively minute egg itself. “How gat they there?” is our natural question. This question, which is so easily answered on the supposition that generation can take place spontaneously, presents the most serious difficulties to science, because the massive weight of scientific evidence has been year after year accumulating against such a supposition; until the majority of physiologists have come to regard it as an axiom, that no generation whatever can occur except by direct parentage. This axiom, which a small minority has always rejected, has quite recently met with a formidable questioner in M. Pouchet, the well-known physiologist of Rouen; and his experiments and arguments having agitated the Academy of Sciences, our readers may be interested if a review of the whole subject be laid before them.
The first person who assailed the notion of Spontaneous Generation was Redi, the excellent Italian naturalist, to whom we owe so many valuable observations. I have at this moment on my table the brief but pregnant treatise, “Experimenta circa Generationem Insectorum,” in which he reviewed the facts, and proved that the worms and insects which appear in decaying substances, are really developed from eggs deposited in those substances by the parents. So masterly was the treatise, that no one since then has had the courage to maintain the production of worms and insects spontaneously. It has been held as preposterous to suppose that putrefaction could generate an insect as that it could generate a mouse — which Cardan believed. Driven from the insect world, the hypothesis has sought refuge in the world of animalcules and parasites; and there the hypothesis is not so easily defeated. Who ever turns over the pages of old Leeuwenhoek, the first who extensively applied himself to microscopic observations, will see that the Dutchman steadily set his face against Spontaneous Generation, because the microscope showed him that many even of these minute animals had their eggs, and were generated like the larger animals. Since that time thousands of observers have brought their contributions to the general stock, and each extension of our knowledge has had the effect of narrowing the ground on which the “spontaneous” hypothesis could possibly find footing; the modes of generation of plants and animals are becoming more and more clearly traced; and the necessity in each case of a parent-stock is becoming more and more absolute. It is true that there are organic beings of which, as yet, we can only say that there is the strongest presumption against their being exceptions to the otherwise universal rule of generation. We do not know, for example, how the Amoeba arises; no one has ever seen its eggs; no one has ever seen its reproduction— and, what is more, it is perfectly easy to make them in any quantities. I have done so repeatedly. Nevertheless, they can only be “made” under the conditions which would be indispensable for their birth and development if they were really generated from eggs; and that they are so generated is a presumption which has every argument in its favour, except the direct evidence of the eggs themselves. The question then comes to this: Is it more probable that a law of generation which is found to reach over the whole organic world should have an exception, or that our researches have not yet been able to detect the evidence which would bring this seeming exception also under the law? One after the other, cases which seemed exceptions have turned out to be none at all ; one after the other, the various obscurities have been cleared away, showing one law to be general ; and it is therefore the dictate of philosophic caution which suggests that, so long as we remain in positive ignorance of the actual process, we must assume that in this case also the general law prevails.
Positive evidence would of course settle the dispute ; but every one who has made any experiments, or has attentively followed the experiments of others, will admit that it is excessively difficult to devise any experiment which shall be conclusive. The facts elicited admit of such different interpretations; the avenues by which error may enter are so numerous. I will not narrate here the experiments of Fray, Gruithuisen, Burdach, Baer, and others, since they cannot withstand serious discussion; nor -will I adduce my own, for the same reason. But those recently made by M. Pouchet have a more imposing character, and demand the strictest examination.
The reader will observe that the cardinal point in the investigation is to be certain that no organic germs could by any possibility be present in the liquid which is to produce the animalcules. On the hypothesis that the animalcules, like other animals and plants, are produced from germs, or eggs, these germs must be excessively minute, and easily overlooked. If they exist, it is in the water and the air, awaiting the proper conditions for their development. Supposing them to be floating about in the air, under the form of dust-like particles, they would fall into, or enter, any vessel containing organic matter in a state of decomposition, and there develope; as the deposited eggs of the insect developed in the decaying body of the dog. Now, inasmuch as the presence of atmospheric air is one of the indispensable conditions of vitality, and without it the animalcules could not develope and live, the initial difficulty is how to secure the presence of this air, and yet be sure that the air itself does not bring with it the germs of the animalcules which we find in the liquid. Schultze of Berlin devised an experiment which was thought to have finally settled this point, and to have refuted the hypothesis of Spon- taneous Generation. An account of this experi- ment, to be found in the “Edinburgh New Philosophical Journal” for October, 1837, shows that an infusion of organic substances, supplied with atmospheric air, but not with an air containing living germs, was suffered to remain thus from the end of May till the beginning of August; but, during the whole of that time, no plant or animal was developed in the infusion. The apparatus was now removed from the flask, atmospheric air was allowed to enter freely — without first passing through the acid or potass — and, in three days, the infusion was swarming with animalcules.
This really looked like a conclusive experiment. No sooner were measures taken which would destroy the germs, supposed to be suspended in the atmosphere, than the infusion was kept free from animalcules ; no sooner was the air allowed to enter the flask in the ordinary manner, than animalcules abounded. The proof did not, however, seem to me quite rigorous. It was by no means clear that the air in its passage through sulphuric acid would not suffer some alteration, perhaps electrical, affecting its vital properties; and this doubt seemed confirmed by the experiments of M. Morren, communicated to the French Academy, May 22, 1854; from which it appeared that air having passed through sulphuric acid was incompetent to sustain life, since the animalcules subject to it died in a few days. But M. Pouchet announces experiments which, if correct, not only scatter this doubt, and M. Mori on’s confirmation, but point-blank contradict the experiment of Schultze. He declares that in following Schultze’s experiment in every particular, and also in repeating it with fresh precautions, he can constantly exhibit animalcules and plants developed in an infusion in which every organic germ has been previously destroyed, and to which the air has only access after passing through concentrated sulphuric acid, or through a labyrinth of porcelain fragments at red-heat. Nay, M. Pouchet goes further. Feeling the difficulty of satisfying his opponents that the atmospheric air really contained no germs, he determined on substituting artificial air. This he did in conjunction with a chemist, M. Hongeau. Artificial air, as the reader knows, is simply a mixture of twenty-one parte of oxygen gas with seventy-nine parts of nitrogen gas. This air was introduced into a flask containing an infusion of hay, the hay having previously been subjected for twenty minutes to a heat of 100 degrees Centigrade (212 degrees Fahrenheit), a temperature which would destroy every germ. He thus guarded against the presence of any germs, or animalcules, in the infusion, or in the air. The whole was then hermetically sealed, so that no other air could gain access. In spite of these precautions cryptogamic plants and animalcules appeared in the infusion. M. Pouchet repeated the experiment with pure oxygen gas, instead of air; and with similar results.
In presence of such statements as these, only two courses were open to the antagonists of Spontaneous Generation. They could deny or disprove the facts; or they could argue that the precautions taken were not sufficiently rigorous to exclude the presence of germs. I have already said how difficult it is for the modem physiologist to admit Spontaneous Generation, and the reader will be therefore prepared to hear that M. Pouchet has roused immense opposition; but the opponents have not disputed his facts; one and all they accept the statements as he makes them, and, by criticism and counter-statement, endeavour to show that Spontaneous Generation is just as inadmissible as ever. These criticisms, and M. Pouchet’s replies, may here be grouped in order, and with all possible brevity.
Milne-Edwards objected to the conclusions of M. Pouchet, saying: — There is no proof that the hay itself had been subjected to the temperature of 100 degrees Cent, (or the boiling point of water), it being very probable that although the furnace was at that heat, the hay, which was in a glass vessel and surrounded with air at rest, was not at anything like that temperature.
To this M. Pouchet replied, that he and M. Hougeau ascertained that the hay was at the temperature of 100 degrees, before they proceeded in their experiments.
Milne-Edwards is ready to grant that the tem- perature may have been reached, but argues that even that would not suffice for the destruction of all the germs, if they were perfectly dry. He refers to the observations of M. Doyère, which prove that the Tardigrada (“water bears,” microscopic animals common in stagnant water), when thoroughly desiccated, preserve their power of reviving even after having been subjected to a temperature of 140 degrees Cent. (316 degrees Fahr.). If, therefore, animals of so complex a structure as these water-spiders can resist the action of so high a temperature, there is no reason for supposing that the germs of the simpler ani- malcules would be destroyed by it. Not content with this argument, which is sufficiently forcible, Milne-Edwards narrates an experiment of his own, which is very similar, both in method and results, to one I have performed. Unhappily, it is an experiment the value of which is either destroyed by the argument just adduced, or else it destroys the argument. It is this: In two tubes a little water containing organic matter is placed, one of them hermetically sealed, the other left open to the air. They are then placed in a bath of boiling water and kept there till their temperature has reached that point. After this they are left undisturbed for a few days. In the tube which was exposed to the air there were animalcules; in the tube which was excluded from the air, before the action of heat had destroyed all the germs, not an animalcule could be seen.
Is not this something like a proof? “Why, no, sir,” as Johnson would have said. At least, not if the argument previously urged is worth anything. Because every one will see that if it be true, as Milne-Edwards maintains, that the temperature of boiling water is not by any means high enough to destroy the organic germs of animalcules, then it could not have destroyed those germs in the closed tube, and animalcules ought to have made their appearance there. If I could lay any particular stress on my own experiments (which I do not), they would lead to the conclusion that the organic germs do not resist the action of boiling water; for I found that a piece of fish divided into three, and placed in boiling water in three different tubes, one closed and excluded from the light, the second closed but exposed to the light, and the third open and exposed to the light, gave me no animalcules at all: had there been any germs in the water or meat, these must have been destroyed. But all such observations go for nothing in the presence of M. Pouchet’s assertion that he had found animalcules in the infusion after subjecting the organic matters to a temperature of 250 degrees Cent. (546 degrees Fahr.), and this, too, with artificial water. Unless the germs are supposed to be incombustible, it is difficult, he says, to maintain, after this, that the animalcules were developed from germs.
Milne-Edwards being thus disposed of by M. Pouchet, let us see how M. Quatrefages will come off. He says, that having examined the dust remaining on the filter after some observations on rain water, he found that the organic elements presented a confused assemblage of particles; and this continued to be the case for a few minutes after their immersion in water. But a few hours afterwards, he detected a great number of vegetable spores, infusoria, and those minute spherical and ovoid bodies familiar to microscopists, which inevitably suggest the idea of eggs of extremely small dimensions. He also declares that he has frequently seen monads revive and move about after a few hours of immersion. The conclusion drawn is, that the air transports myriads of dust-like particles, which have only to fall into the water to appear in their true form as animalcules.
The reply of M. Pouchet is crushing. If the air is filled with animalcules and their eggs, they will of course fall into any vessel of water, and as water is their natural element, will there exhibit their vitality. But if half a dozen vessels of distilled water, perfectly free from animalcules, be left exposed to the air, beside one vessel of distilled water containing organic substances in decay, the half dozen will be free from animalcules and eggs, but the one will abound with them. Now, it is perfectly intelligible that inasmuch as organic matter is said to form the indispensable condition for the development of the eggs, it is only in the vessel containing such matter that the eggs will develops; but why are they not also visible as eggs in the other vessels? why are not the animalcules themselves visible there, as they were in the water examined by M. Quatrefages? If both eggs and animalcules are blown about like dust in the air, it is an immense stretch of credulity to believe they will only be blown into the vessel containing organic matter; but the opponents of Spontaneous Generation go further even than this, for they declare these dust-like animalcules will be blown into a closed vessel, if it contain organic matter, but not into several open vessels, if they only contain distilled water.
M. Quatrefages is on better ground when he rejects the evidence, long supposed to be so weighty, of parasitic animals. He refers to the modern investigations which have not only made the generation of these parasites intelligible, but in many cases have demonstrated it. M. Pouchet’a reply is feeble, and unworthy of a physiologist of his eminence. He doubts the truth of the results obtained in Germany, Italy, and Belgium: “the monopoly of which,” he adds, “has, by a strange anomaly, belonged to foreigners.” Because France has not the honour of this splendid discovery, the Frenchman begs to doubt its value! Every physiologist, however — not French — will be ready to admit that whereas the parasitic animals formerly furnished the advocates of Spontaneous Generation with their most striking illustrations, the investigations of Von Siebold, Van Beneden, Küchenmeister, Philippi, and others, have entirely changed the whole aspect of the question, and given the opponents of Spontaneous Generation new grounds for believing that in time all obscurities will be cleared away, all contradictions explained.
In conclusion, I must say that as far as regards the particular discussion, M. Pouchet seems to me to have the best of it. Their objections to his experiments are all set aside. If the facts are as he states them — and his antagonists at present do not dispute the facts — their criticisms go for very little. They have not shown it probable that any germs could have been present, under the conditions stated by him. Are we, then, to accept Spontaneous Generation as proven? By no means. It is very far from proven. The massive preponderance of fact and argument against such an hypothesis forces us to pause long before we accept it. What M. Pouchet has done is to destroy many of the arguments against Spontaneous Generation, and to have devised experiments which may finally lead to a conclusion. It is still on the cards that some source of error as yet overlooked vitiates his experiments; but until that error has been detected, he must be considered to have on his side the evidence of experiment, whereas we have on our side the massive evidence of extensive inductions. His experiment may be conclusive, and an exception to the general law will thereby be established But it may also, on further investigation, turn out to be illusory; some little oversight may be detected which will rob the experiment of all its force.
Perhaps you will ask why this suspicion should be entertained? Why ought we not to accept M. Pouchet’s statement with confidence, although it does contradict our inductions? The reason can only be, that the massive weight of these inductions naturally predisposes the mind to believe that it is more probable the experiment which contradicts them should be misconceived, than that they should be contradicted. Two years ago I became acquainted with an observation made by Cienkowski, the botanist, which seemed finally to settle this question of Spontaneous Generation, to place the fact beyond doubt, because it caught Nature in the act, so to speak, of spontaneously generating. Cienkowski’a statement is as follows: If a slice of raw potato be allowed to decompose in a little water, it will be found, after some days, that the starch grains have a peculiar border, bearing a strong resemblance to a cell-membrane. This shortly turns out to be a real cell-membrane, and is gradually raised above the starch-grain, which grain then occupies the position of a cell- nucleus. Thus, out of a grain of starch, a cell has been formed under the observer's eye. Inside this cell, little granular masses are developed, which begin to contract. Finally, minute eel- like animalcules are developed there, which bore their way through the cell-wall into the water.
Funke in his report of this observation, which he says, he has verified, asks, how is it possible to deny Spontaneous Generation here? Before our eyes a grain of starch becomes a cell, in that cell are developed living forms, which bore their way out.
The reader will imagine the sensation which such an observation created. He will agree with Funke, as I did, that if the fact were as he stated it, all discussion was at an end. But was the fact as stated? I tried in vain to verify it. Not less than twenty separate potatoes were employed, always in conjunction with ordinary starch, as a point of comparison; but although the animalcules were abundant enough, I never could satisfy myself of the first and all-important step, namely, the formation of a cell wall round the starch-grain. This was the more distressing, because it is at all times unpleasant to be unable to verify an observation, especially one made by a careful and competent observer, and described in precise terms.
I could not reject what Cienkowski had positively affirmed, and Funke positively confirmed, and was willing to suppose that there was some necessary condition in the observation which I had not fulfilled. On the other hand, I could not reject a doctrine on the strength of a fact about which any doubt was permissible. In this state of suspense I had the satisfaction of hearing from Professor Naegeli, the celebrated microscopist, that he too had been baffled at first in the attempt to verify this observation, but that, after nearly a hundred trials, he had succeeded. He positively confirmed all the statements Cienkowski had made. But, from that moment, my suspense vanished. If the phenomenon was of such rare occurrence, there were reasons for suspecting some other explanation than that of Spontaneous Generation. What the source of the error was might not be easily divined; but it seemed very probable that error had crept in somewhere.
In the last number of the Annales des Sciences Naturelles (x. 140), there is a note which clears up the whole mystery. Cienkowski has himself discovered the source of his own error. The membrane which seemed to form itself round the starch-grain has had quite another origin. He has observed the little monads swimming about, and has noticed one of them adhere to a starch-grain, spread its elastic body round it, and finally envelope it, as the Amœba wraps itself round its food. This explains how the starch-grain comes to be inside a cell; and as this process was never suspected, and the starch-grain was seen with a cell-wall, the idea of natural formation was inevitable, the more so, as the wall seemed to grow larger and larger.
Thus has even this, the most striking case in favour of Spontaneous Generation ever adduced, been finally cleared up; and the reader will probably agree in the conclusion to which the whole of the facts advanced in this paper lead, namely, that the Law of Generation is universal; the exceptions which have been hitherto urged have, one by one, been found to be no exceptions; and the presumption is that even M. Pouchet’s cases will be likewise explained. It is quite possible that the generation of animalcules may take place spontaneously; but although possible, it is not probable, and certainly is not proven.