Spontaneous Generation is a term applied to the real or imaginary development of lowly organisms from non-living matter. The facts are that when organic substance is exposed to the air it putrefies, and at the same time living animals—maggots, infusoria, and others, according to the nature and conditions of the substance—appear in it. The question is, whence do the living animals come; from the organic stuff or from germs present in it or in the air? There is a further question—what is the cause of the putrefaction; is it an ordinary process of slow oxidation or is it caused by the living organisms? If the dead organic matter can give rise to life, then we know something of the mode of origin of life upon this planet, for we can make solutions of inorganic salts that will support life, and might therefore also give origin to it; if not, then we are entirely ignorant as to that origin. For many years no one has doubted but that, if one is careful to exclude all germs from the organic stuff, no life can proceed from non-living matter, even if it has once formed part of some living organism and is in the most complex state in which such matter is known to us. The matter is therefore one of historic interest chiefly. As long ago as 1870 Huxley, in his presidential address to the British Association, was able to say that it appeared to him, within certain limitations, that the doctrine of Biogenesis, that life proceeds from life only, was victorious along the whole line. These limitations were, he said, that if he could have been a witness of the early stages of the earth's history, when the physical and chemical conditions were different from those that now hold, he would have expected to see the evolution of living protoplasm from non-living matter. As these limitations are still often thought inconsistent, and as by other people the denial of spontaneous generation seems inconsistent with a belief in evolution, it will be well to quote Herbert Spencer's reply to a critic who urged objections on this ground of inconsistency. In his Principles of Biology (vol. i., Appendix, 1868) he says, 'I do not believe in the "spontaneous generation" commonly alleged, and so little have I associated in thought this alleged "spontaneous generation," which I disbelieve, with the generation by evolution, which I do believe, that the repudiation of the one never occurred to me as liable to be mistaken for repudiation of the other. That creatures having quite specific structures are evolved in the course of a few hours, without antecedents calculated to determine their specific forms, is to me incredible. Not only the established truths of Biology, but the established truths of science in general, negative the supposition that organisms having structures definite enough to identify them as belonging to known genera and species can be produced in the absence of germs derived from antecedent organisms of the same genera and species. If there can suddenly be imposed upon simple protoplasm the organisation which constitutes it a Paramecium I see no reason why animals of greater complexity, or indeed of any complexity, may not be constituted in the same manner. In brief, I do not accept these alleged facts as exemplifying evolution, because they imply something immensely beyond that which evolution, as I understand it, can achieve. In the second place, my disbelief extends not only to the alleged cases of "spontaneous generation," but to every case akin to them. The very conception of spontaneity is wholly incongruous with the conception of evolution.' On the other hand he says, 'Granting that the formation of organic matter and the evolution of life in its lowest forms may go on under existing cosmical conditions, but believing it more likely that the formation of such matter and such forms took place at a time when the heat of the earth's surface was falling through ranges of temperature at which the higher organic compounds are unstable, I conceive that the moulding of such organic matter into the simplest types must have commenced with portions of protoplasm more minute, more indefinite, and more inconstant in their characters than the lowest Rhizopods—less distinguishable from a mere fragment of albumen than even the Protogenes of Professor Haeckel. The evolution of specific shapes must, like all other organic evolution, have resulted from the actions and reactions between such incipient types and the environment, and the continual survival of those which happened to have specialities best fitted to the specialities of their environments. To reach by this process the comparatively well-specialised forms of ordinary infusoria must, I conceive, have taken an enormous period of time.' Again, 'That organic matter was not produced all at once, but was reached through steps, we are well warranted in believing by the experience of chemists. Organic matters are produced in the laboratory by what we may literally call artificial evolution. Chemists find themselves unable to form these complex combinations directly from their elements, but they succeed in forming them indirectly by successive modifications of simpler combinations.' We may say then that it is certain that living organisms, large enough to be visible with the help of a microscope and definite enough in form and structure to be classified with other known genera, do not grow at present from non-living matter. But it is not therefore certain that protoplasm of living matter may not be so formed in extremely small quantities, too small to be visible and of simple or no structure, but yet sufficiently complex in composition to serve as food for other and more highly developed animals. Whether this be so or not, Huxley and Spencer and nearly all biologists agree in believing that in past time molecules of simple matter by some series of reactions became aggregated until a matter, sufficiently complex and sufficiently unstable to be called living matter, was formed, whilst there is no evidence that any such generation is taking place at present.
We will now give a summary of the history of this inquiry, based upon Huxley's presidential address of 1870, and to a less extent upon Tyndall's article in the Nineteenth Century, January 1878. Expressions and phrases will be freely quoted from these authors.
History.—It must always have been a matter of common experience that many articles of food are apt to become mouldy and to putrefy if kept too long. Associated with mould and with putrefaction are various sorts of low forms of life. The ancient philosophers never doubted that these were generated in the matters in which they made their appearance. Indeed, all men believed this until past the middle of the 17th century. But in 1660 in Italy, in those days the home of learning, Francesco Redi published his Esperienza intorno alla Generazione degli Insetti. He was no theorist, but a careful experimenter. Here, said he, is meat; if I expose it to the air in hot weather, in a few days it putrefies and swarms with maggots; but if I protect similar pieces of meat by covering them with fine gauze, then, though they still putrefy, not a maggot makes its appearance. From this experiment it becomes obvious that the maggots are not generated in the meat, but that the cause of their formation is something that is kept away by fine gauze. This something can be easily shown to be blowflies, for these, attracted by the meat, swarm near it and lay their eggs on the protecting gauze, eggs from which maggots are shortly hatched. Now this is the principle of the whole matter; keep away all living things which might come to the meat and the meat will not create any living things, will not even putrefy if one kills any living animals or germs of animals that may be in the meat. The protecting gauze must be fine enough, that is all. Redi of course was accused of controverting Scripture, because of the story of the bees which were said to be generated in the carcass of the lion. But his doctrine of Biogenesis flourished for a century. Indeed when, through the development of the microscope, the numerous provisions for the production of germs were made known, the hypothesis of Abiogenesis, that life could come from what was not living, appeared absurd. Leenwenhoek (q.v.), 1632–1723, is remarkable as being the first man to demonstrate existence of unicellular organisms. During the 18th century the microscope was greatly improved. The animalcules (infusoria) which in a few days will swarm in any infusion of organic matter became visible, and Needham, on theoretical grounds, doubted whether Redi's generalisation, 'no life without antecedent life,' held true for these lowly forms of life. He put his doubts to the test. He argued that if the infusorial animalcules came from germs, then the germs must exist either in the substance infused or in the water used to make the infusion or in the air that touches both of them. The life of all germs is destroyed by heat. If, therefore, the infusion be boiled, any germs present will be killed; and then if the infusion be shut off from the air no more germs can get into it. Now, if after this treatment animalcules still appear in the infusion they will have been generated from the infused substance or from the water; but if they do not appear, then Redi's dogma will be true for infusoria. He therefore boiled and corked infusions of various substances, and in every case after a longer or shorter time animalcules appeared and flourished. Needham was associated in much of his work with Buffon. The French naturalist had a theory of life to which Needham's experiments lent support. Life, he thought, was the indestructible property of certain molecules, which he described as 'organic molecules.' All living organisms, he said, are built up of such molecules; death is nothing more than their dissociation. When they are thus set free they take the form of infusorial animalcules. It is necessary to distinguish this theory, which is so ingenious, from the theory of Abiogenesis, the antithesis to that of Biogenesis, which supposes that life may and does arise from non-living matter.
But the theory of Buffon, and especially the experiments of Needham, which lent it support, did not seem satisfactory to Spallanzani. He saw two sources of error—first, the germs present in the infusion might have escaped death through not having been boiled long enough; and secondly, the corks, perhaps, were not perfectly effective, and air containing germs might have got into the flask and infected the infusion. So he too prepared infusions; but he boiled them for three-quarters of an hour, and then fused the necks of the flasks. He found after this treatment that the infusions remained perfectly free from living organisms for as long as he chose to keep them. It might seem that this must be the end of the whole matter; but the event proved otherwise. This time it was the chemists who reopened the discussion. Oxygen was discovered, the theory of respiration was begun, and it was proved that a supply of free oxygen is one of the conditions of life and also of putrefaction. So it was possible that Spallanzani's infusions did not produce life either because the 'organic molecules' were altered in some way by being boiled, or because they were unable, owing to the absence of oxygen, to live. So the experiments had to be repeated in such a manner that the organic matter was not altered, and so that there was sufficient oxygen. Schulze and Schwann in 1836 took up the matter. They carefully boiled their infusions, and then supplied air; but they made it pass through red-hot tubes first, so that any germs present in it would be burned. In these conditions no animalculæ appeared in the infusions; but if they were exposed to air which had not been heated then animalculæ appeared in abundance. Therefore boiling does not injure the vitality of the 'organic molecules,' if there are such, and there is only one possible objection to the conclusions drawn from such experiments, if they be properly conducted, and that is that what the red-hot tubes destroy is not germs, but something else that may be non-living and yet essential to life. Now about this time Cagniard de la Tour discovered that fermentation, like putrefaction, is always accompanied by the presence of minute living creatures. Common yeast, for instance, is a mass of minute plants. When it was suggested that the living creatures not only accompanied but actually caused the processes of fer- mentation and putrefaction, the chemists, led by Liebig and Berzelius, laughed the idea to scorn. But in 1843 Helmholtz ingeniously separated a putrescent from a putrescible fluid by a membrane, so that the products of putrefaction could mix with the putrescible matter; but that did not in consequence putrefy. Therefore it followed that the cause of putrefaction must be either a colloid—indiffusible stuff—or a solid. In 1854–59 Schroeder and Dusch cleared up this point by experiments which were simply refinements upon the original ones of Redi; instead of using a screen of gauze to keep off blowflies they used a screen of cotton-wool, a screen with meshes so fine that not even the tiny germs can pass through them. They boiled infusions, and while the steam was coming off freely they plugged the neck of the flask with cotton-wool. Now this plug did not keep away oxygen, nor did it in any way heat or alter the air that passed to the fluid, as the red-hot tubes of Schulze and Schwann had done, and yet no animalcules appeared in the boiled infusion screened by cotton-wool. It is therefore proved that the cause of putrefaction and fermentation and the origin of the living forms that accompany these processes must be small particles that exist in the air.
But in 1859 Pouchet published his Hétérogénie. He once more raised clouds of doubt. It seemed evident to him that spontaneous generation was one of the means that nature employed for the reproduction of living beings. If, he said, all putrefaction is the result of life present, as germs, in the air, then the air in which we live would have almost the density of iron! About this time Pasteur took the matter up, although advised by his friends, in view of the difficulty of the subject, not to do so; at least, said Dumas, do not spend too much time over it. But in 1862 he published a paper On the Organised Particles existing in the Atmosphere. He had collected the floating dust of the air and examined it with a microscope. He saw that much of it actually consisted of organised particles, and on his sowing these in suitable sterilised infusions there grew from them rich crops of animalculæ. He also showed that these germs of life were not universally diffused in the air. He opened his sealed flasks in the pure air of the Mer de Glace. Only one out of twenty such flasks thus opened became filled with life; while eight out of twenty opened in the air of the plains did so, and all of them did so if opened in the air of towns. These researches of Pasteur were necessary before Lister could have brought his system of antiseptic surgery to a successful issue; while he himself, as is well known, has done great work upon the relations of these forms of life to many industries and diseases. The germs of the air were then studied by Cohn, shown by him to be bacteria, and the basis of a sound knowledge of them was laid. In 1869 Tyndall hit upon a very precise method of determining the absence or presence of dust particles in the air, a method much more searching than that furnished by the most powerful microscope. He was experimenting in another direction, and had need of air free from dust. He noticed that such matter (dust) passed easily through liquids. A beam of light shows the presence of dust particles in the air by the reflection that occurs from the surfaces of such. Tyndall showed that whenever dust was present the putrefaction occurred sooner or later, when it was absent it did not. Tyndall's apparatus was a box with glass sides, into the bottom of which the mouths of flasks penetrated, the lower parts being outside the box, so that the contained infusions could be boiled. A beam of sunlight was allowed to pass through the box; it showed the presence of dust in the air in a way with which every one is familiar. The inside of the box was then coated with glycerine, and the whole was kept as still as possible. After some time the dust particles sank to the floor of the box, and stuck to the glycerine, and a beam of sunlight passing through was quite invisible, there not being anything to reflect it. The infusions were then boiled for a long time, and then allowed to cool, after which they might be kept for months without putrefying or showing any signs of the presence of life. Here all the conditions of the infusions were natural, save that there was no dust in the air above them. In the dust of the air therefore are solid germs of life. The opponents of the doctrine of Biogenesis had long made objection to it, saying that 'if true then the air must be thick with germs; now this is absurd, therefore the doctrine is untrue.' An argument this that shows that a reductio ad absurdum is not always a proof.
The researches of Tyndall called forth the first utterances on the subject of Bastian, the latest, let us hope the last, obscurantist. Two years later, 1872, Bastian published his work, in two volumes, upon The Beginnings of Life. In the preface he states that the question of spontaneous generation turns wholly upon the probability of the de novo origin of bacteria, and further that evidence of the most convincing character shows that bacteria are killed by a temperature of 140° F., yet similar organisms will constantly appear in closed flasks containing organic fluids that have been exposed for some time to a temperature of 212° F. It is surprising that any man could at that late day have been found to urge such stale arguments after the repeated demonstration of the two sources of error in such experiments—(1) that fresh bacteria might get in after the original set were killed; (2) that though adult bacteria are killed by a temperature of 140° F., their germs are not always killed by a much higher temperature. Bastian's work has been quietly forgotten, and since then the progress of discovery as to the nature and mode of origin of bacteria and all other forms of life has progressed steadily.
See BACTERIA, BATHYBIUS, LIFE, PROTOPLASM, PUTREFACTION; and Haeckel's Natural History of Creation.