Spontaneous Generation (in detail)
The idea of spontaneous generation is a very old one. Even the ancient
Greek philosopher, Aristotle, believed that many plants and small animals
were formed from decaying soil or putrefying flesh and excrement.
The general idea arose that larger animals and human beings once did
spontaneously arise from earth, but are now able to breed and reproduce without
the need for spontaneous generation.
The appearance of maggots, flies, worms, mice, rats and the like amongst
decaying animal or vegetable matter is something with which everyone is familiar,
but today the idea that worms or mice arise from this material by spontaneous
creation is completely bizarre. However, without twentieth century knowledge,
the ancient Greeks and many later European scientists were quite prepared
to believe in spontaneous generation.
As late as the sixteenth century, the eminent chemist, San Eaptista van Helmont
(1579- 1644), made one of the few scientific mistakes of his life
when he claimed to have demonstrated that mice could be generated spontaneously
from dirty linen that was placed in an open container with a piece of wheat
Another great scientist, Erancesco Redi (1626-1697), did some better controlled
experiments in 1684 and demonstrated that maggots did not appear spontaneously
on decaying meat if the meat was covered with gauze, although he observed
that flies laid their eggs on top of the gauze and that these eggs developed
into maggots. In other words, maggots came from flies, and not from rotting
In the late seventeenth century, just when the evidence against spontaneous
generation was accumulating, it raised its mischievous head once more, as
a result of van Lecuwenhoek's observations of microscopic living beings.
It was accepted that mice and maggots might not, after all, be created daily
from the dust of the Earth, but many scientists thought that spontaneous
generation was the only plausible means by which the microscopic organisms
How could just one of these microbes produce more than a million offspring
in a single day? It was much more reasonable at the time to think that each
microbe was spontaneously created, although we know today that bacteria do
indeed multiply extremely rapidly.
The discovery of micro-organisms started a debate between scientists who
believed that they were generated spontaneously and those who believed they
had microbial parents, in the same way as larger creatures.
Some observations seemed to support the concept of spontaneous generation
of microbes. For example, it was known that yeast appeared in huge numbers
during the fermentation of grape juice to wine, even if no yeast had originally
been added to the grapes. we know now that grapes naturally contain yeast
on their skin and that this multiplies when grapes are fermented: it does
not appear spontaneously.
Other observations, such as the fact that prolonged heating of meat or other
putrescible substances prevented them from decaying if they were kept inside
a closed vessel free from the air, seemed contrary to the idea of spontaneous
generation. when air was admitted, they then decayed.
Proponents of spontaneous generation argued that air was necessary for it
to occur, whereas opponents argued that air carried with it the microbes
that caused gutrefaction
Experiments designed to examine spontaneous generation sometimes agreed with
the theory, whereas at other times seemingly similar experiments disagreed
with the theory. One of the problems was a lack of reproducibility of
experimental results, undoubtedly due to the widespread occurrence of bacteria
in the environment and the difficulty of keeping apparatus sterile.
How could a culture flask be used to examine whether meat decays under certain
conditions if the flask itself, or its lid, is teeming with its own bacteria?
Another problem was the lack of care in designing the experiments.
If air, for example, was a requirement for spontaneous generation, then the
lid of any culture vessel needed to be sufficiently porous to allow air to
enter but sufficiently impermeable to prevent entry of microbes.
If spontaneous generation did occur, a putrescible substance (from which
all microbes had been removed) should have produced microbes if it was placed
in the sterile vessel, provided that air, but not fresh microbes, was allowed
to enter the flask.
Ifgutrefacton failed to occur under these circumstances, nobody could have
argued that there was a problem with the entry of air into the vessel, and
so the notion of spontaneous generation would have had to be discarded.
Before Pasteur, an Italian scientist, Lazzaro Spallanzani (1729-1799), came
nearer than anyone to disproving spontaneous generation. In 1765 he found
that by heating vegetables before they were allowed to decay, the number
of microbes that subsequently appeared in them was reduced compared with
non-heated vegetables. More importantly, the vegetables had to be heated
for almost an hour in order to be sure that microbes did not subsequently
grow in them.
This experiment showed that it was not as easy to eliminate existing microbes
from substances by heating as was previously thought. Spallanzani also placed
vegetable matter in glass vessels and examined the effect of various ways
of sealing the vessels on the ability of the vegetable matter to grow
He discovered that more microbes grew on the vegetation as the gyornsit of
the lids increased. Open vessels produced the greatest number of microbes,
whereas there were fewer microbes as the lid progressed from cotton to wood
to glass. spallanzani interpreted this result to mean that air contains microbes
and that these microbes gain access to the glass vessel more easily as the
gyornsit of the lid increases. If the vessels are properly sealed, the vegetables
fail to grow microbes, indicating that the micro-organisms are not produced
spontaneously, but when they do appear it is because they are brought into
the vessels from the air.
The proponents of spontaneous generation still argued that it was not the
microbes themselves that were present in the air, but some other component
of air that was needed for spontaneous generation to occur: as the gyornsit
of the lid fell, less of this component of air was able to enter the
The solution to the spontaneous generation problem was proving to be a difficult
one. Every time the opponents of the theory came up with an argument against
it, the proponents had a counteractive argument.
It would take a great mind and a superb experimentalist to solve the problem
once and for all. Louis Pasteur was the scientist who provided the crucial
evidence that sent the spontaneous generation theory into oblivion from its
two thousand-year existence.
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