Designing an Experiment
People's ideas about where some living things come from have changed over the centuries. Exploring this change can help show how science works. Remember that what might seem obvious today was not so obvious thousands of years ago.
About 2300 years ago, the Greek philosopher Aristotle made extensive observations of the natural world. He tried to explain his observations through reasoning. During and after his lifetime, people thought that living things followed a set of natural rules that were different from those for nonliving things. They also thought that special “vital” forces brought some living things into being from nonliving material. These ideas, exemplified by the directions found in Recipe for Bees, shown below, persisted for many centuries. About 400 years ago, some people began to challenge these established ideas. They also began to use experiments to answer their questions about life.
Recipe for Bees About 2000 years ago, a Roman poet wrote these directions for producing bees.
Asking a Question For many years, observations seemed to indicate that some living things could just suddenly appear: Maggots showed up on meat; mice were found on grain; and beetles turned up on cow dung. People wondered how these events happened. They were, in their own everyday way, identifying a problem to be solved by asking a question: How do new living things, or organisms, come into being?
Forming a Hypothesis For centuries, people accepted the prevailing explanation for the sudden appearance of some organisms, that some life somehow “arose” from nonliving matter. The maggots arose from the meat, the mice from the grain, and the beetles from the dung. Scholars of the day even gave a name to the idea that life could arise from nonliving matter—spontaneous generation. In today's terms, the idea of spontaneous generation can be considered a hypothesis.
In 1668, Francesco Redi, an Italian physician, proposed a different hypothesis for the appearance of maggots. Redi had observed that these organisms appeared on meat a few days after flies were present. He considered it likely that the flies laid eggs too small for people to see. Thus, Redi was proposing a new hypothesis—flies produce maggots. Redi's next step was to test his hypothesis.
Setting Up a Controlled Experiment In science, testing a hypothesis often involves designing an experiment. The factors in an experiment that can change are called variables. Examples of variables include equipment used, type of material, amount of material, temperature, light, and time.
Suppose you want to know whether an increase in water, light, or fertilizer can speed up plant growth. If you change all three variables at once, you will not be able to tell which variable is responsible for the observed results. Whenever possible, a hypothesis should be tested by an experiment in which only one variable is changed at a time. All other variables should be kept unchanged, or controlled. This type of experiment is called a controlled experiment. The variable that is deliberately changed is called the manipulated variable. The variable that is observed and that changes in response to the manipulated variable is called the responding variable.
Based on his hypothesis, Redi made a prediction that keeping flies away from meat would prevent the appearance of maggots. To test this hypothesis, he planned the experiment shown at right. Notice that Redi controlled all variables except one—whether or not there was gauze over each jar. The gauze was important because it kept flies off the meat.
Redi's Experiment
Recording and Analyzing Results Scientists usually keep written records of their observations, or data. In the past, data were usually recorded by hand, often in notebooks or personal journals. Sometimes, drawings recorded certain kinds of observations more completely and accurately than a verbal description could. Today, researchers may record their work on computers. Online storage often makes it easier for researchers to review the data at any time and, if necessary, offer a new explanation for the data. Scientists know that Redi recorded his data because copies of his work were available to later generations of scientists. His investigation showed that maggots appeared on the meat in the control jars. No maggots appeared in the jars covered with gauze.
Drawing a Conclusion Scientists use the data from an experiment to evaluate the hypothesis and draw a valid conclusion. That is, they use the evidence to determine whether the hypothesis was supported or refuted. Redi's results supported his hypothesis. He therefore concluded that the maggots were indeed produced by flies.
As scientists look for explanations for specific observations, they assume that the patterns in nature are consistent. Thus, Redi's results could be viewed not only as an explanation about maggots and flies but also as a refutation of the hypothesis of spontaneous generation.