The Slow Death of Spontaneous Generation (1668-1859)
Russell Levine and Chris Evers
From the time of the ancient Romans, through the Middle Ages, and
until the late nineteenth century, it was generally accepted that some
life forms arose spontaneously from non-living matter. Such
"spontaneous generation" appeared to occur primarily in decaying
matter. For example, a seventeenth century recipe for the spontaneous
production of mice required placing sweaty underwear and husks of
wheat in an open-mouthed jar, then waiting for about 21 days, during
which time it was alleged that the sweat from the underwear would
penetrate the husks of wheat, changing them into mice. Although such
a concept may seem laughable today, it is consistent with the other
widely held cultural and religious beliefs of the time.
The first serious attack on the idea of spontaneous generation was
made in 1668 by Francesco Redi, an Italian physician and poet. At
that time, it was widely held that maggots arose spontaneously in
rotting meat. Redi believed that maggots developed from eggs laid by
flies. To test his hypothesis, he set out meat in a variety of
flasks, some open to the air, some sealed completely, and others
covered with gauze. As he had expected, maggots appeared only in the
open flasks in which the flies could reach the meat and lay their
This was one of the first examples of an experiment in the modern
sense, in which controls are used. In spite of his well-executed
experiment, the belief in spontaneous generation remained strong, and
even Redi continued to believe it occurred under some circumstances.
The invention of the microscope only served to enhance this belief.
Microscopy revealed a whole new world of organisms that appeared to
arise spontaneously. It was quickly learned that to create
"animalcules," as the organisms were called, you needed only to place
hay in water and wait a few days before examining your new creations
under the microscope.
The debate over spontaneous generation continued for centuries. In
1745, John Needham, an English clergyman, proposed what he considered
the definitive experiment. Everyone knew that boiling killed
microorganisms, so he proposed to test whether or not microorganisms
appeared spontaneously after boiling. He boiled chicken broth, put it
into a flask, sealed it, and waited - sure enough, microorganisms
grew. Needham claimed victory for spontaneous generation.
An Italian priest, Lazzaro Spallanzani, was not convinced, and he
suggested that perhaps the microorganisms had entered the broth from
the air after the broth was boiled, but before it was sealed. To test
his theory, he modified Needham's experiment - he placed the chicken
broth in a flask, sealed the flask, drew off the air to create a
partial vacuum, then boiled the broth. No microorganisms grew. Proponents of
spontaneous generation argued that Spallanzani had only proven that
spontaneous generation could not occur without air.
The theory of spontaneous generation was finally laid to rest in 1859
by the young French chemist,
Louis Pasteur. The
French Academy of Sciences sponsored a contest for the best experiment
either proving or disproving spontaneous generation.
experiment was a variation of the methods of Needham and Spallanzani.
He boiled meat broth in a flask, heated the neck of the flask in a
flame until it became pliable, and bent it into the shape of an S.
Air could enter the flask, but airborne microorganisms could not -
they would settle by gravity in the neck. As
Pasteur had expected,
no microorganisms grew. When
Pasteur tilted the
flask so that the broth reached the lowest point in the neck, where
any airborne particles would have settled, the broth rapidly became
cloudy with life.
Pasteur had both
refuted the theory of spontaneous generation and convincingly
demonstrated that microorganisms are everywhere - even in the air.
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