Rise of Modern Science - Redi's Experiments with Meat, Maggots, and Fly Eggs

Redi's Experiments with Meat, Maggots, and Fly Eggs

Redi, court physician to the Florentine Duke, Ferdinando II, performed a series of experiments in the 1660s to pursue and demonstrate Harvey’s proposition that life derived from eggs. He also intended his experiments to be a demonstration of an error of Arisotle’s — the idea that maggots spontaneously emerge in rotting meat, as a property of such putrefaction.

Redi’s case shows us the power of experimentation in the rhetoric of the 1600s — trusting the senses, in a way even more pure and true than the knowledge of the Scholastics. Redi and colleagues argued that Reason was informed by the Senses, agreeing with the Scholastics that knowledge starts with common sense observation. The new philosophers thought the Scholastics had been misled, however, by excessive dependence on philosophical reasoning without constraint by observation; they are led “astray” into “hasty and faulty verdict.” What is needed is more than merely “a superficial report of the senses.” Redi also is taking up Galileo’s challenge, to extend the senses by instruments and measurement and experiment. The microscope is valuable in this regard for examining insects, but Redi’s even more powerful observing is made by controlling circumstances in order to be able to see what is true and what is myth or mere tradition.

Redi was a member of the academy that included many followers of Galileo, fond of using experiments to debunk their opponents’ traditional ideas. The experiments culminated in this text, Experiments on the Generation of Insects, published in 1668. Historians have focused on the few pages near the beginning where he describes the precisely reasoned and carried out experiments that demonstrate conclusively that maggots come from fly eggs. These are brilliant, and early, experiments that incorporate in design both controls and the process of successive experiments honing in on the variables or potentially confounding factors.

The experiments did have great impact in discussions of generation. But Redi’s concern was not only (or not even specifically) spontaneous generation theory. He should be seen in a broader context of correction, via observation and experiment, and new explanations. The text has another 100 pages, much of it devoted to point by point demolishing of claims made in ancient texts about insect reproduction, physiology, behavior, and lives. It reminds us to pay serious attention to the need in the 17th century to clear away a great deal of erroneous, supposedly observed knowledge. As Redi noted with the book’s epigram, an Arabic proverb:

Experiment add to knowledge,

Credulity leads to error.

Here in the first few pages [19-21] of his treatise, Redi makes his pitch to his patron for the use of the senses and for critically testing received wisdom:

" SIR:

There is no doubt that the senses were given to Reason by the Supreme Architect as aids to the better comprehension of natural things. They are like windows or doors through which she may look out on those things, or through which they may come in and make themselves known. Still better said: the senses are scouts, or spies, that seek to discover the nature of things, and report these observations to Reason within, who passes judgment on everything, describing with more or less clearness and precision, according to the validity, alertness, and accuracy of her informers. Hence it is that in order to verify observations, we frequently approach or recede from the object that we wish to examine, change its position or its light, and perform many other actions relating not only to the sense of sight, but also to those of hearing, smell and touch. In fact, no one of the slightest intelligence would attempt to exact judgment from Reason in any other way than this. Therefore, I believe Nature could not possibly choose any more useful gift for man than his five perfect senses. It is evident that a man searching for the truths of Natural History would go far astray if he did not keep his senses clear, for Reason, if set to work on a superficial report of the senses, would render a hasty and faulty verdict. Thus it happens that even young men new to the schools hold this opinion, which is but common sense, and has been stated by wise men of early days, who in philosophical matters were singularly in advance of their time. Among these, that great genius who knew everything and could write wonderfully well on all subjects, said in the second canto of Paradiso :

Somewhat she smiled ; and then, " If the opinion of mortals be erroneous," she said,

"Where'er the key of sense doth not unlock,

Certes the shaft of wonder should not pierce thee

Now, forasmuch as following the senses.

Thou seest that the reason has short wings."

But, if the senses do not do their duty, if they do not obtain correct information of what is happening in Nature and thus do not aid Reason, is it strange that she should make but uncertain progress, now hastening forward impetuously, now retarded by fallacy and caught in the net of error? Hence, though my philosophical studies have been pursued with more zeal than profundity, I have nevertheless given myself all possible trouble and have taken the greatest care to convince myself of facts with my own eyes by means of accurate and continued experiments before submitting them to my mind as matter for reflection. In this manner, though I may not have arrived at a perfect knowledge of anything, I have gone far enough to perceive that I am still entirely ignorant of many things the nature of which I supposed was known to me, and when I discover a palpable falsehood in ancient writings or in modern belief, I feel so irresolute and doubtful of my own knowledge that I scarcely dare attack it without first consulting some learned and prudent friends. Thus having recently made many experiments especially in regard to the origin of those living creatures considered, to the present day, by all schools to have been generated by chance, that is spontaneously, without paternal seed ; and being distrustful of myself, but still desirous of submitting the results of my labors to other minds, it occurred to me that I might have recourse to you, Signor Carlo, as you have graciously given me a place among your closest friends.

Your great knowledge fortified by philosophy and nobly adorned with varied erudition is admired by all men of learning, and is the pride of Tuscany, who envies neither Latium her Varros, nor Greece her Plutarchs. Therefore I beg you to take the trouble to read this letter in your leisure moments and to give me your sincere opinion of it, together with your friendly advice and wise counsel, by the aid of which I shall be enabled to remove all superfluous and trivial matter and to add whatever may be necessary.

" Perchance I may with greater diligence

And patient study yet perfect this work."

An example of his diligent and patient study is his series of experiments on the idea "that the putrescence of a dead body, or the filth of any sort of decayed matter engenders worms." His series of experiments, summed up as he described them (pp. 27-37, where there is a lot more detail of observation in the original):

He "ordered to be killed" three snakes and "placed them in an open box to decay. Not long afterwards I saw that they were covered with worms of a conical shape and apparently without legs." These grew but then escaped.
"Being curious, therefore, to know their fate," he placed three dead snakes in a closed box. They grew, and after 19 days ceased moving and became smaller, harder, and egg-shaped, "resembling somewhat those chrysalides formed by caterpillars."
He placed these "separately in glass vessels, well covered with paper," and after 8 days a fly emerged from each. There had been two colors of the "balls" (pupae) and two types of flies emerged.
This was "perplexing, and I sought further knowledge from experiment. To this end, having made ready six boxes without covers, I placed in the first, two of the snakes described above, in the second, a large pigeon, in the third two pounds of veal, in the fourth a large piece of horse-flesh, in the fifth, a capon, in the sixth, a sheep's heart; and all became wormy in little more than twenty-four hours." A variety of flies ultimately emerged, mostly not associated with a particular kind of meat.
He did similar experiments with dead frogs and "ox, the deer, the buffalo, the lion, the tiger, the dog, the lamb, the kid, the rabbit; and sometimes with the flesh of ducks, geese, hens, swallows, etc., and finally I experimented with different kinds of fish, such as sword-fish, tun, eel, sole, etc."
"In every case, one or other of the above-mentioned kinds of flies were hatched, and sometimes all were found in a single animal."
"I began to believe that all worms found in meat were derived directly from the droppings of flies, and not from the putrefaction of the meat, and I was still more confirmed in this belief by having observed that, before the meat grew wormy, flies had hovered over it, of the same kind as those that later bred in it. Belief would be vain without the confirmation of experiment, hence in the middle of July 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, 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. Outside on the paper cover there was now and then a deposit, or a maggot that eagerly sought some crevice by which to enter and obtain nourishment."
"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 under ground, 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."
He killed some maggots, and nothing emerged from those flasks. Nor did anything ever emerge from an initially closed vessel.
In the text, he then criticizes a number of alchemists and ancient authors for promoting an idea of creating life -- he's trying to prove that the new flies came from flies, not generation out of other matter such as dead flesh.
"Although I thought I had proved that the flesh of dead animals could not engender worms unless the semina of live ones were deposited therein, still, to remove all doubt, as the trial had been made with closed vessels into which the air could not penetrate or circulate, I wished to attempt a new experiment by putting meat and fish in a large vase closed only with a fine Naples veil, that allowed the air to enter." [This countered the claim that the bad air inside his closed flasks stopped the spontaneous generation.] The result? "I never saw any worms in the meat, though many were to be seen moving about on the net-covered frame. . . . It was interesting, in the meanwhile, to notice the number of flies buzzing about which, every now and then, would light on the outside net and deposit worms there."

Quotations from the 1909 translation: Francesco Redi, Experiments on the Generation of Insects [1668, translated from the 1688 edition by Mab Bigelow], Chicago: The Open Court Publishing Company (1909), pp. 19-21, 27-37.