A key assumption in science is that experimental results can be reproduced because nature behaves in a consistent manner. When one particular variable is manipulated in a given set of variables, the result should always be the same. In keeping with this assumption, scientists expect to test one another's investigations. Thus, communicating a description of an experiment is an essential part of science. Today's researchers often publish a report of their work in a scientific journal. Other scientists review the experimental procedures to make sure that the design was without flaws. They often repeat experiments to be sure that the results match those already obtained. In Redi's day, scientific journals were not common, but he communicated his conclusion in a book that included a description of his investigation and its results.

Needham's Test of Redi's Findings Some later tests of Redi's work were influenced by an unexpected discovery. About the time Redi was carrying out his experiment, Anton van Leeuwenhoek (LAY-vun-hook) of the Netherlands discovered a world of tiny moving objects in rainwater, pond water, and dust. Inferring that these objects were alive, he called them “animalcules,” or tiny animals. He made drawings of his observations and shared them with other scientists. For the next 200 years or so, scientists could not agree on whether the animalcules were alive or how they came to exist.

In the mid-1700s, John Needham, an English scientist, used an experiment involving animalcules to attack Redi's work. Needham claimed that spontaneous generation could occur under the right conditions. To prove his claim, he sealed a bottle of gravy and heated it. He claimed that the heat had killed any living things that might be in the gravy. After several days, he examined the contents of the bottle and found it swarming with activity. “These little animals,” he inferred, “can only have come from juice of the gravy.”

Spallanzani's Test of Redi's Findings An Italian scholar, Lazzaro Spallanzani, read about Redi's and Needham's work. Spallanzani thought that Needham had not heated his samples enough and decided to improve upon Needham's experiment. The figure shown at right illustrates that Spallanzani boiled two containers of gravy, assuming that the boiling would kill any tiny living things, or microorganisms, that were present. He sealed one jar immediately and left the other jar open. After a few days, the gravy in the open jar was teeming with microorganisms. The sealed jar remained free of microorganisms.

Spallanzani's Experiment

Spallanzani concluded that nonliving gravy did not produce living things. The microorganisms in the unsealed jar were off-spring of microorganisms that had entered the jar through the air. This experiment and Redi's work supported the hypothesis that new organisms are produced only by existing organisms.

Pasteur's Test of Spontaneous Generation Well into the 1800s, some scientists continued to support the spontaneous generation hypothesis. Some of them argued that air was a necessary factor in the process of generating life because air contained the “life force” needed to produce new life. They pointed out that Spallanzani's experiment was not a fair test because air had been excluded from the sealed jar.

In 1864, an ingenious French scientist, Louis Pasteur, found a way to settle the argument. He designed a flask that had a long curved neck, as shown in the figure at right. The flask remained open to the air, but microorganisms from the air did not make their way through the neck into the flask. Pasteur showed that as long as the broth was protected from microorganisms, it remained free of living things. About a year after the experiment began, Pasteur broke the neck of the flask, and the broth quickly became filled with microorganisms. His work convinced other scientists that the hypothesis of spontaneous generation was not correct. In other words, Pasteur showed that all living things come from other living things. This change in thinking represented a major shift in the way scientists viewed living things.

Pasteur's Experiment

The Impact of Pasteur's Work During his lifetime, Pasteur made many discoveries related to microorganisms. His research had an impact on society as well as on scientific thought. He saved the French wine industry, which was troubled by unexplained souring of wine, and the silk industry, which was endangered by a silkworm disease. Moreover, he began to uncover the very nature of infectious diseases, showing that they were the result of microorganisms entering the bodies of the victims. Pasteur is considered one of biology's most remarkable problem solvers.

Major Discoveries