Spontaneous or Equivocal generation is the term applied to the alleged production of living beings without the pre-exist-ence of germs of any kind, and therefore without the pre-exist-ence of parent organisms. The question is one which has been long and closely disputed, and is far from being settled; so that it will be sufficient to indicate the facts upon which the theory rests.
If an animal or vegetable substance be soaked in hot or cold water, so as to make an organic infusion, and if this infusion be exposed for a sufficient length of time to the air, the following series of changes is usually observed:
1. At the end of a longer or shorter time, there forms upon the surface of the infusion a thin scum, or pellicle, which, when examined microscopically, is found to consist of an incalculable number of extremely minute molecules (fig. 4, A).
Fig. 4. - A, Living particles or molecules developed in organic infusions ; B, Bacteria developed in organic infusions, highly magnified. (After Beale.)
2. In the next stage these molecules appear, many of them, to have increased in size by endogenous division, till they form short staff-shaped filaments, called "bacteria" (fig. 4, B). Others increase in length by the same process until we get long filamentous bodies produced, which are termed "vibriones." Both the bacteria and the vibrios now exhibit a vibratile or serpentine movement through the surrounding fluid.
3. After a varying period, the bacteria and vibrios become motionless, and disintegrate so as to produce again a finely molecular pellicle.
4. Little spherical bodies may now appear, each of which is provided with a vibratile cilium with which it moves actively through the infusion (Monas lens).
5. Varied forms of ciliated Infusoria - some of which possess a mouth and are otherwise highly organised - may make their appearance in the fluid.
The above is the general sequence of the phenomena which have been observed, and the following are the two theories which have been advanced to account for them:
a. By the advocates of spontaneous generation, "Abio-genesis" or "Heterogeny," it is affirmed that the Infusoria which finally appear in the infusion are produced spontaneously out of the molecular pellicle, the molecules of which are also of spontaneous origin, and are not derived from any preexisting germs.
b. By the "panspermists," or the opponents of spontaneous generation, it is alleged, on the other hand, that the production of Bacteria, Vibrios, Monads, and Infusoria, in organic infusions, is due simply to the fact that the atmosphere, and probably the fluid itself, is charged with innumerable germs - too minute, perhaps, to be always detectable by the microscope-which, obtaining access to the fluid, and finding there favourable conditions, are developed into living beings.
A large number of elaborate experiments have been carried out to prove that atmospheric air is absolutely necessary for the production of these living beings, and that if the air be properly purified by passage through destructive chemical reagents, no such organisms will be produced, provided that the infusion have been previously boiled. As the results of all these experimental trials have hitherto proved more or less contradictory, it is unnecessary to enter into the question further, and it will be sufficient to indicate the following general considerations:
a. The primary molecules which appear in the fluid are extremely minute, and if they are developed from germs, these may be so small as to elude any power of the microscope yet known to us. As they subsequently become converted into bacteria and vibrios, and as there can be little dispute as to these being truly living organisms, we are obliged to believe that they must have had some definite origin. It appears, however, to be hardly philosophical to assume that they form themselves out of the inorganic materials of the infusion; since this implies the sudden appearance, or creation, of new force, for which there seems to be no means of accounting.
b. The nature of the vibrios must be looked upon as uncertain. To say the least of it, they are quite as likely to be plants as animals; and the most probable hypothesis would place them near the filamentous Confervae, or would regard them as the mycelium of various species of Moulds (Penicil-lium). The bacteria are undoubtedly of a vegetable nature, and referable to the Algae.
c. What has been said above with regard to the origin of the bacteria and vibrios applies equally to the origin of the Monads, which appear in the infusion subsequently to the death of the vibrios.
d. These monads, as shown by recent researches, are probably to be looked upon as, in some cases at any rate, the embryonic, or larval, forms of the higher Infusoria which succeed them.
e. Many of the Infusoria which finally appear are of a comparatively high grade of organisation, being certainly the highest of the Protozoa, and being placed by some competent observers in the neighbourhood of the Trematode Worms (Scolecida). It is therefore very unlikely that these should be generated spontaneously; since, if this ever occurs, it is reasonable to suppose that the creatures thus produced will be of the lowest possible organisation (such as the Gregarinidae or the Monera, for example), and will be far below the Infusoria in point of structure.
f. The reproductive process in many of these same Infusoria is perfectly well known, and it consists either in a true sexual process, for which proper organs are provided (as in Paramoe-cium), or in a process of gemmation or fission. It is therefore improbable that they should be generated in the manner maintained by the heterogenists, since this mode of reproduction would appear to be superfluous.
g. In the absence of any direct proof to the contrary, it is safer to adopt an explanation of the observed phenomena which does not have recourse to laws with which we are as yet unacquainted. Thus, it is not at variance with any known law to suppose that the primary molecules are the result of the development of germs which find in the organic infusion a suitable nidus; that these primary molecules and the vibrios which they produce are referable to the Protophyta, and should probably be placed near the filamentous Confervae; that by the death of these vegetable organisms the fluid is prepared for the reception and development of the germs of the Protozoa, for which the former serve as pabulum ; and that many of the forms which are observed are the larval stages of the higher Infusoria.
h. Recent researches, especially those of Dr Bastian, have established some new facts as to the possibility of spontaneous generation, but they can by no means be said to have settled the question, if only upon the ground that they require confirmation by other experimentalists. The chief fact which appears to have been established upon a tolerably firm basis is, that living beings, vegetable or animal, may make their appearance in organic infusions which have been subjected to a temperature of considerably over the boiling-point, even though the said infusions have been hermetically sealed in a flask from which all atmospheric air has been previously withdrawn. The chief deduction which appears to flow from this - assuming its correctness - is, that there are low organisms which can exist, for a certain length of time at any rate, with an extremely small amount of air; for it is to be remembered that the production of a theoretically perfect vacuum is probably practically impossible. If it were conceded, in fact, that a perfect vacuum had been formed in the experiments in question, the sole result would be that we should have to alter all our beliefs as to the conditions under which life is a possibility. The only tangible result of these experiments, so far, is, that any supposed "pre-existent germs" must have been contained, if present at all, in the infinitesimal portion of air which could not be expelled from the flasks experimented on; or, they must have been able to withstand without injury a temperature of over 212o. Mr Crace-Calvert, indeed, asserts that he has experimentally shown that vibrios can survive exposure to a temperature exceeding 3000 Fahrenheit, and Messrs Dallinger and Drysdale have shown the same of the germs of bacteria. Neither of these hypotheses is wholly incredible; but the question ought to be regarded as still sub judice. Under any circumstances, the entire question is one of such complexity as to be altogether unsuited for discussion here.
i. Still more recent researches, carried out in a series of elaborate experiments by Professor Tyndall, have supplied us with a complete physical demonstration that ordinary atmospheric air is invariably charged with innumerable particles of solid matter, many of which are so immeasurably minute as to be incapable of detection by the highest known powers of the microscope. The same observer has further shown that vibrios and bacteria are never produced in organic infusions, which, subsequent to boiling, are exposed only to air from which these floating molecules have been completely removed. In the face of these observations, it is difficult to see how the doctrine of "abiogenesis" can maintain its ground.