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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