Spontaneous generation
Spontaneous generation is the idea that complex forms of life arise anew from non-living matter. This concept goes back to at least 330BC when the philosopher Aristotle observed that when meat is left to decay, maggots would appear on the meat within a few days. From this, he asserted that non-living matter, such as the meat, could give rise to a life form, such as the maggots.
For many centuries, this theory was generally accepted, as most common observations seemed to support it. Many experiments were performed on the subject; most notably that of the biologist Jean Baptist van Helmont in the mid-1600s. In attempt to prove this concept, he placed a moist cloth and wheat grain inside a closed, wooden box. To his astonishment, he found a mouse nibbling at the box within a few days.
Even in the 1600s there were skeptics about spontaneous generation in the scholarly community, including Francesco Redi, who had had doubts of its truth. He protested that Helmont's experiment was flawed because he had no way of determining whether the mouse was nibbling in or out of the box. Thus, Redi performed his own experiment in a controlled environment, placing meat inside a sealed jar. Several days passed, and no maggots appeared on the meat. While this may seem to be proof that spontaneous generation is false, his colleagues insisted that the no life forms could have been created because he had cut off the meat from the air supply.
Still obstinately refusing to believe the law of spontaneous generation, Redi performed his experiment once more, now with a very fine netting. Thus, air would still be able to move freely in and out of the jar, but flies could not. Again, no maggots formed inside the jar, thus providing solid evidence that this "law" which had been accepted for nearly 1900 years, was false.