Francesco Redi (1626 – 1698)





“…Belief would be vain without the confirmation of experiment” Francesco Redi, 1668


Francesco Redi, who lived in the seventeenth century, did not believe in spontaneous generation.  He explained the presence of worms in decayed meat and plants in this way:

“… I feel … inclined to believe that these worms are all generated by insemination (= breeding) and that the putrefied matter in which they are found has no other office than that of serving as a place, or suitable nest, where animals deposit their eggs at the breeding season, and in which they also find nourishment: otherwise, I assert that nothing is ever generated therein …”


Apparently Redi’s ideas had come from reading of the death of Patroclus in the Iliad, by Homer.  In his poem Homer describes how Achilles was reluctant to leave his friend Patroclus’ body.

“… I am terribly afraid … that flies may defile the corpse of my lord Patroclus by settling on the open wounds and breeding worms in them.  Life has gone out of him and all his flesh would rot.”

Patroclus’ body being carried from the field of battle.       

Redi carefully observed the worms – we would call them maggots – on decaying meat exposed to the air.  About nineteen days after they had first appeared he saw that some of them had turned into pupae, which he collected and placed separately in glass vessels.  Eight days later he saw flies hatch out of the pupae.  Here, then, was proof. 


By this painstaking work Redi had recorded things which the believers in spontaneous generation had either not seen or ignored.  Their understanding had been limited by the extent of their observations.  Redi extended his observations and thus provided support for his new idea.  This comparison bears out the truth of the saying that we can only see as far as we look.  Possibly Redi’s persistent observations were encouraged by his beliefs just as those of his opponents were discouraged.

Up to this point Redi’s work was comparable with that of van Helmont (see Background).  It consisted of observation and interpretation.  The supporters of spontaneous generation concluded their work at this stage.  Once they had produced an explanation, they thought that nothing further was needed.  Redi, however, turned his explanation into a hypothesis and set up an experiment to test it.  Belief, he said, would be vain without the confirmation of experiment.

Put briefly, Redi’s hypothesis was that the worms did not originate by spontaneous generation but from eggs laid by the flies.  This was how he tested it:

“I put a snake, some fish, some eels of the Arno , and a slice of milk-fed veal in four large, wide mouthed flasks: having well closed and sealed them   (The veal was not just a touch of renaissance magnificence, but reflected the belief that veal engendered flies.)  I then filled the same number of flasks in the same way, only leaving these open.  It was not long before the meat and the fish, in these second vessels became wormy and flies were seen entering and leaving at will; but in the closed flasks I did not see a worm, though many days had passed since the dead flesh had been put in them.”


You can see that what Redi had devised was a ‘controlled’ experiment. Half the flasks were closed, half were open.  This was the single apparent difference between them.  Therefore his result could only be related to this difference.  Only in the open flasks were flies able to enter, and only in them were the maggots produced.


Redi realised that factors which he had not taken into account might have influenced his results, so he continued his work.

“Not content with these experiments I tried many others at different seasons, using different vessels.  In order to leave nothing undone, I even had pieces of meat put underground, but though remaining buried for weeks, they never bred worms, as was always the case when flies had been allowed to light on the meat.  One day a large number of worms, which had bred on some buffalo meat, were killed by my order; having placed part in a closed first dish and part in an open one, nothing appeared in the first dish but in the second worms had hatched, which changing as usual into egg shaped balls (= pupć), finally became flies of the common kind.  In the same experiment tried with dead flies, I never saw anything breed in the closed vessel.”   

Redi published his findings in 1668.        


You will see that Redi carries out a full scientific investigation. He worked out a hypothesis, tested it by experiment, and carefully drew his conclusions from the results he obtained.  In contrast the work of the supporters of spontaneous generation was only half finished.  We may well wonder how much further advanced our knowledge of nature would be today if the exponents of spontaneous generation had adopted Redi’s method.


Even so it is interesting, and a timely reminder of how careful you have to be in experimental work, to note that there was a flaw in Redi’s experiment.  Can you work out what it is?




Grace Monger and Richard Gliddon eds, The Perpetuation of Life: revised Nuffield biology text 4, Longman 1975

Roy Porter The Greatest Benefit to Mankind, Harper Collins 1997

G. Rattray Taylor, The Science of Life, Thames and Hudson 1963