Francesco Redi was a poet and a physician, a learned man in 17th century Florence, Italy. He is considered by some to be the author of the “experimental control” and the author of parasitology. He had a particular interest in flies and studied the life cycle of the blow fly. Redi conducted experiments that showed that maggots were produced not from rotting meat, but by flies that laid eggs on the meat. This observation was a refutation of spontaneous generation and part of the emerging paradigm of learning through observation and experimentation rather than sole reliance on authorities such as Aristotle.
Based on his observations of blow flies, Redi hypothesized that plant galls and the gall flies that emerged from galls, originated from fly eggs. However, Redi was not able to observe gall flies laying eggs or adequately exclude them in his study of galls. This led him to conclude (erroneously) that galls were produced by plants by the same process that caused plants to produce flowers and fruits. Redi’s error was soon corrected by Vallisneri, who observed the eggs laid in the plants.
Today, Redi’s error is obvious, but in the 17th century, genes, DNA and plant hormones were all unknown and no good model of flower production had been proposed. Even today, the mechanisms behind gall creation (secretions of the fly induce gall formation) are vaguely understood and are the subject of intense investigation.