Spontaneous Generation, the direct production of living beings from inanimate material, in contradistinction to the ordinary mode of generation, in which young animals or plants appear only as the progeny of other living organisms. The views held by physiologists on the question of spontaneous generation have varied greatly at different times. In the earlier periods of scientific culture, the Grecian naturalists, as represented by Aristotle, recognized among animals three different modes of generation: 1, viviparous generation, as in man and the quadrupeds, where the young were known to be produced alive from the bodies of their parents; 2, oviparous generation, as in birds, reptiles, and fish, where the young were hatched from eggs produced by the female; 3, spontaneous generation, where no connection could be traced between the young animals and any previously existing parents, and where they were consequently thought to be formed by the spontaneous organization of earthy deposits or decaying organic material. Spontaneous generation was therefore regarded as one of the regular and natural methods for the production of living forms; but as a physiological doctrine it rested entirely upon negative grounds, and was due to the incomplete knowledge then possessed by naturalists as to the real origin of many animal species.
Maggots, for instance, were thought to be formed by spontaneous generation from putrefying meat, because they always appeared at a certain stage of its decomposition, although no similar creatures existed in it beforehand, and because there was no other apparent cause for their production. A great change in opinion on this subject was introduced by the discoveries of Francesco Redi in 1668. He exposed fresh meat, during summer weather, in wide-mouthed bottles, protected by pieces of paper fastened over their necks. In the bottles thus secured no maggots were developed, notwithstanding that the putrefaction of the meat went on as usual; while in other similar vessels, unprotected by paper covers, maggots swarmed in abundance at the customary time. It was evident therefore that their origin was due to something introduced from without, and it soon appeared that .they were really the progeny of flesh flies, which, attracted by the odor of the meat, hovered over it until they gained access to it, and deposited their eggs upon its surface. The eggs then hatched into maggots, which, after a certain period of growth, became transformed into perfect insects similar to their parents.
The idea thus suggested was rapidly carried out by means of further observations on the reproduction and metamorphosis of insects in general. The investigations showed that in what had been supposed to be cases of spontaneous generation the animals were really produced from parents like themselves. The microscope soon brought into view many minute forms of life not previously known. The multiplicity of these forms, their endless variation, their small size, and their different conditions of life made it impossible at first to ascertain their complete physiological history or their mode of origin; and in regard to many of them the idea of spontaneous generation was again adopted. This was especially the case with the class known as infusoria; that is, microscopic animals living in water or in watery infusions of organic material, some of the smallest of which received the name of monads. Investigations upon this point were consequently taken up afresh, with a view of determining whether the infusoria in a watery liquid were produced by the ingredients of the solution itself, or by germs derived from without.
Experimenters boiled the watery infusions, to destroy the vitality of any animalcules or germs which they might already contain, and afterward kept them> with a due supply of air, in hermetically sealed flasks. If, under these circumstances, living forms still made their appearance in the infusions, they must be attributed to spontaneous generation; if not, they must be regarded as dependent on the preex-istence or introduction of germs. These experiments were tried by different observers, with results which varied according to the nature and extent of the precautions adopted; but the general conclusion, derived especially from the investigations of Spallanzani in 1775, was that a preliminary boiling in closed flasks, for a few minutes, effectually prevented the appearance of all the larger and more highly organized infusoria; while, if the boiling were prolonged from half an hour to an hour, the infusion afterward remained absolutely destitute of all forms of life, even the smallest and simplest.
Although at that time the real mode of generation of the infusoria had never been ascertained, nor their eggs detected by the microscope, it was considered certain that these animalcules must require for their production the existence of living germs, and consequently that they did not originate by spontaneous generation. During the early and middle part of the present century the common opinion of naturalists became gradually more decided in opposition to the doctrine of spontaneous generation, owing to the occasional repetition of experiments like Spallan-zani's, and also to important discoveries in regard to the sexless internal parasites, such as cysticercus and trichina. These creatures were found inhabiting the solid tissues of other animals, and furthermore were seen to be incapable of exercising the function of generation. It was difficult therefore to account for their presence in the animal tissues unless by a growth upon the spot, and also to understand how the species could be reproduced by ordinary modes of generation. But continued investigation removed both of these difficulties.
It was shown by the researches of Siebold, Kuchenmeister, Leuckart, Pagenstecher, and others, that the sexless parasites were in reality the embryonic or youthful progeny of perfectly developed parents; their mode of introduction into the internal cavities and tissues of the body was ascertained; and they were found to acquire after a time sexual organs, and to produce a new progeny by sexual generation. Thus, one by one, a great variety of obscure animal species became more perfectly known; and a complete study of their physiological history revealed in every instance the regular mode of their origin and reproduction. But the class of infusoria still remained somewhat refractory in this respect, and notwithstanding that the question had been for some years regarded as settled, it was reopened in 1858. M. Pouch et, an eminent naturalist and physiologist of Rouen, took the ground that the former experiments in regard to boiled infusions were incorrect, and that in point of fact a preliminary boiling did not prevent the appearance of infusorial life.
Pouchet's views and assertions were supported by the testimony of several other experimenters, among the most distinguished of whom have been Mante-gazza and Bastian. They were opposed by many others, of equal reputation; and the weight of the discussion turned for a time upon the dissemination of germs in the atmosphere, as the supposed source of life in organic infusions. The most important experiments in this direction were those of Pasteur, from 1860 to 1865. This chemist had been especially interested in the study of fermentation, which was shown to be a change dependent on the presence and growth of microscopic vegetable cells. He boiled a suitable organic infusion in glass flasks, the necks of which were drawn out and scaled while ebullition was going on, thus excluding completely the atmospheric air. Afterward, when the flask and its contents had become cooled, the air was readmitted through the neck of the flask, by breaking off its narrow end. But this operation was done, with different sets of flasks, in different localities, in order to determine whether the difference of locality had any influence upon the subsequent appearance or non-appearance of life within the flask. The bearing of these experiments upon the question at issue was as follows.
If it were the constituent gases of the atmosphere alone which excited the spontaneous growth of living forms by the necessary supply of oxygen, then the production of these forms should follow with the same readiness in all localities, because the gaseous constitution of the atmosphere is everywhere the same. But if, in order to produce life, the atmosphere must also bring with it certain organic germs, then the locality might make a difference in the result, because these floating particles would naturally vary in abundance in different regions. Investigation showed a manifest difference, according to the place where the air was admitted to the flask. In one of the most significant of Pasteur's experiments, a flask containing an organic infusion was boiled, sealed, allowed to cool, and afterward carried to the valley of Chamouni in Sa-voy, where its neck was opened and the air admitted on the Montanvert, 6,000 ft. above the sea level. It was immediately resealed, brought back to Paris, and kept for four years perfectly unchanged. It was then reopened and again exposed to the air, and in three days afterward contained a growth of microscopic vegetation.
These experiments were considered by Pasteur and his associates as demonstrating the existence in the atmosphere of extraneous particles, the introduction of which into an infusion was the necessary condition of infusorial life. A further difficulty now began to bo appreciated in this method of investigation. It had at first been taken for granted that a boiling temperature would necessarily destroy the vitality of both the infusoria and their germs. Put this gradually became a matter of doubt, especially as the length of time during which the boiling was continued evidently had an influence on the subsequent appearance of life in the infusion. It was found necessary to determine more exactly the limits of this influence; and among the most valuable experiments in that respect were those of Jeffries Wyman in 1867. He showed that, in infusions of a certain constitution, the minute forms known as bacteria might appear in closed flasks after boiling; that the longer the boiling was continued, the fewer the instances in which bacteria were afterward developed; and that they never made their appearance in infusions which had been boiled continuously for five or six hours.
These results were variously interpreted by different observers; a certain number still maintaining that bacteria might often be developed after the application of a heat sufficient to destroy their previous vitality. - In the modern renewal of the question of spontaneous generation, dating from Pouchet in 1858, another element has had its influence upon this discussion; that is, the idea of evolution, as accounting for the present existence of organic life upon the earth. It is assumed that there was once a period in the history of the earth when, from its elevated temperature and the different combination of its chemical elements, life could not possibly exist upon it; that, as living beings subsequently made their appearance, they must necessarily have originated by the spontaneous organization of inanimate materials; and that these primitive and imperfect structures have gradually, by modification and descent, given rise to all the forms of animal and vegetable life now inhab-king the globe.
Some of those who accept the evolution doctrines believe that the conditions necessary for a spontaneous production of life have long since passed away, with the earlier stages of the world's history; others maintain that these conditions still exist, and are effective for the continued creation of bacteria and their allied forms. It is common to meet with expressions, among writers of this class, which declare that spontaneous generation is not so much a matter of question or experiment as a logical sequence of the doctrine of evolution. The stricter school of physiologists maintain, on the contrary, that it is a subject to be investigated on its own merits, by means of observa-tion and experiment, like any other question relating to the phenomena of life. - Of late years the experimental evidence bearing on this topic has received an important addition from the independent researches of naturalists in regard to the infusoria. Some of the forms originally included in this group have been found, on more extended examination, to possess a higher organization, and have been by common consent transferred to the class of worms. Like others of this class, they reproduce their species by sexual generation, and often contain living embryos in the interior of their bodies.
The infusoria proper are now known to be mostly ciliated animalcules; that is, they are provided with minute, vibrating, hair-like appendages, by which they perform rapid movements of locomotion. They have also been shown, principally by the labors of Stein, Bal-biani, Engelmann,. and Claparede and Lach-mann, to perform the act of sexual generation, and to produce their young by means of fertile eggs, from which embryos are developed. The more minute and lowly forms, on the other hand, usually included under the general term bacteria, do not belong to the animal kingdom, but are microscopic vegetables. They have a remarkable power of multiplication by division or doubling of their cells; and certain species appear to be the active agents in causing the putrefactive decomposition of albuminoid organic substances. The more modern investigations on spontaneous generation with boiled infusions have been almost exclusively confined to this class. But even in bacteria there are indications of a reproduction by germs. Cohn in 1872 observed certain bodies in connection with bacteria, which he regarded as resting spores; that is, spores which do not immediately germinate, but remain quiescent for a certain interval and afterward become developed under other conditions.
These resting spores were more fully described in 187-4 by Billroth, whose description has been confirmed by Stimson in 1875. According to Billroth, although the vitality of bacteria is destroyed by boiling, their resting spores will withstand this temperature, and are afterward capable of development into active forms. This may explain the occasional appearance of microscopic life in organic solutions which have been subjected to ebullition. - For the most complete recent defence of the doctrine of spontaneous generation, see " The Beginnings of Life," by II. Charlton Bastian, F. R. S. (2 vols., London and New York, 1872).