The last-mentioned experiments prove[89] that fermentation can take place in vacuo, when the conditions are more favourable than those which present themselves within the almost full tins containing provisions. The change in these latter cases cannot (in the great majority of instances) proceed far[90], because there is no adequate space into which residual gases may be emitted. When this emission (which is almost always one of the accompaniments of a fermentative change) has taken place to a slight extent, the meats are in the very best condition for preservation. There is an utter absence of light, an absence of free oxygen, and also an absence of that diminished pressure which my experiments seem to show is favourable to the promotion of many kinds of fermentative change. So that if fermentation does not take place in a closed flask which is full of a boiled infusion of hay,[91] it may be owing to the fact that there is no space for the residual gases, and that undue pressure retards many fermentative changes. This is also perfectly compatible with the other fact that the same kind of fluid will undergo change when a small quantity of it is contained in a comparatively large flask—owing to there being, in such a case, plenty of room for residual gases to be effused, before that undue amount of pressure is brought about, in the presence of which such a fluid will no longer ferment or putrefy. Fluids, therefore, whose putrefaction is hindered by increased pressure and favoured by diminution of pressure, may be placed under conditions which are successively more favourable than the last, by putting a gradually smaller and smaller quantity of fluid into a flask, to which calcined air is admitted, and, better still—if the stimulus of oxygen is not absolutely needed in order to incite fermentation in the fluid employed—by only half filling the flask, and procuring a more and more perfect vacuum.
In accordance with the doctrines of Baron Liebig, therefore, my experiments, as well as those of many other investigators, tend to show that fermentative and putrefactive changes are merely processes of chemical re-arrangement, which frequently take place—as it were “spontaneously”—owing to the inherent instability of certain nitrogenous compounds in the presence of free oxygen. My experiments have, however, also revealed the additional fact that, under the combined influence of heat and diminished pressure, some fluids will undergo fermentation even in closed vessels, from which all air has been expelled. They lend no support to the idea that the air is so thickly laden with living germs as some would have us suppose; and in view of the mass of positive information now in our possession concerning the degree of heat which suffices to kill the lowest organisms, they also, as I think, entitle us to come to the conclusion that such organisms are (as the microscopical evidence might lead us to believe) really capable of being evolved de novo. These lowest organisms are, in fact, to be regarded as occasional concomitant products, rather than as invariable or necessary causes of all fermentative changes.
It would thus appear that specks of living matter may be born in suitable fluids, just as specks of crystalline matter may arise in other fluids. Both processes are really alike inexplicable—both products are similarly the results of the operation of inscrutable natural laws, and what seem to be inherent molecular affinities. The properties of living matter, just as much as the properties of crystalline matter, are dependent upon the number, kind, and mode of collocation of the atoms and molecules entering into its composition. There is no more reason for a belief in the existence of a special “vital force,” than there is for a similar belief in the existence of a special “crystalline force.” The ultimate elements of living matter are in all probability highly complex, whilst those of crystalline matter are comparatively simple. Living matter develops into Organisms of different kinds, whilst crystalline matter grows into Crystals of diverse shapes. The greater modifiability of living matter, and the reproductive property by which it is essentially distinguished from crystalline matter, seem both alike referable to the great molecular complexity and mobility of the former. Crystals are statical, whilst organisms are dynamical aggregates, though the evolution of both, marked by their peculiar characteristics, may be regarded as visible expressions testifying to the existence of one all-pervading Power
“Whose dwelling is the light of setting suns,
And the round ocean, and the living air,
And the blue sky, and in the mind of man:
A motion and a spirit that impels
All thinking things, all objects of all thought,
And rolls through all things.”
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MR. A. R. WALLACE’S CONTRIBUTIONS TO THE THEORY OF NATURAL SELECTION. A Series of Essays.
“He has combined an abundance of fresh and original facts, with a liveliness and sagacity of reasoning which are not often displayed so effectively on so small a scale.”—Saturday Review.