Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker


Week 4.  Sensation

Experiments on the Generation of Insects (1668; 1688)
by Francesco Redi of Arezzo (1626-1697)
trans. (1909) Mab Bigelow


Experiment adds to knowledge,
Credulity leads to error.

-- Arab proverb, Erpenius [Dutch Orientalist and Arabic scholar, Thomas van Erpen (1584-1624)], 57.

Nature is nowhere to be seen to greater perfection than in the very smallest of her works.  For this reason then, I must beg of my readers, notwithstanding the contempt they may feel for many of these objects, not to feel a similar disdain for the information I am about to give relative thereto, seeing that in the study of nature, there is none of her works that is unworthy of our consideration.

--Pliny [Roman natural historian, Pliny the Elder (c. 23-79)].


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....

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 [disciple of Galileo and Torricelli, Carlo Dati (1619-1675)], 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....  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....

After a long period of fertility ... the Earth Mother became at last exhausted and sterile and lost her power of producing men and the larger animals [leaving them on their own to reproduce themselves], still she retained enough vigor to bring forth (besides plants that are presumed to be generated spontaneously) certain small creatures such as flies, wasps, spiders, ants, scorpions, and all the other terrestrial and aerial insects.... 

The schools, both ancient and modern, all agree in this, and constantly teach that the Earth has continued to produce these creatures and will produce them so long as she exists.  They do not, however, agree as to the manner in which these insects are generated, nor how life is communicated to them; for they say that not only does the Earth possess this occult power, but that it is possessed by all animals living and dead, also by all things produced from the Earth, and finally by those which are about to decay and return to dust. 

Hence others have claimed putrefaction itself to be the all-potent cause of generation, and still others, natural heat.  Many additional causes have been adduced, conforming to the divers modes of thought of different sects, who speak of active and efficient forces, the world soul, so to say -- the spirit of the elements, ideas, the heavens, their light and  motion, and higher influences.  Nor was there lacking the assertion that the generation of all insects is caused by the generative principle residing in the original and sentient vegetative souls, of which particles remain alive in the dead bodies of animals and plants, in a quiescent state, which is changed into activity by contact with surrounding heat and communicates new life to corrupt matter. 

There is still another class of wise persons who hold it to be true that generation proceeds from certain minute agglomerations of atoms, which contain the seed of all things.  These persons say further that the seed was created by God at the beginning of the world and scattered in all directions for the fertilization of the elements, bestowing upon them, not a transitory, but a permanent fecundity as stable as the elements themselves; in this way, they say, are to be interpreted the words of the Sacred Book, "God created all things together." 

But that great philosopher of our time, the immortal William Harvey [English physician (1578-1657)], also held that all living things are born from seed as from an egg, be it the seed of animals of the same species or elsewhere derived; thus he says, "Because this is common to all living creatures, viz:  that they derive their origin either from semen or eggs, whether this semen have proceeded from others of the same kind, or have come by chance or something else.  For what sometimes happens in art occasionally occurs in nature also; those things, namely, take place by chance or accident which otherwise are brought about by art, of this health (according to Aristotle) is an illustration.  And the thing is not different as respects generation (in so far as it is from seed) in certain animals; their semina are either present by accident, or they proceed from an univocal agent of the same kind.  For even in fortuitous semina there is an inherent motive principle of generation, which procreates from itself and of itself; and this is the same as that which is found in the semina of congenerative animals -- a power, to wit, of forming a living creature." [Anatomical Exercises on the Generation of Animals (1651), translated by Robert Willis.  Works of William Harvey.  London:  1847, p. 427.]

But at first he had said that those invisible seeds, like atoms floating in the air, were scattered hither and thither by the winds; although he never explains whence or from whom they take origin; only it may be gathered from the above quoted words that he believes that those fortuitous seeds, flying in the air and carried by winds, proceed from an agent not univocal, to express myself in the language of the schools, but equivocal.  Perhaps, however, he would have stated his opinion with greater clearness and precision if the notes which he had collected on this subject had not been dispersed during the tumult of civil war, to the deplorable loss of the republic of philosophy.

Many persons would have difficulty in believing that Harvey could have hit upon the truth, in as much as they obstinately assert that it is impossible to indicate the efficient cause of the procreation of insects.  The subtlest philosopher of past centuries, after vainly seeking it in our world, declared that the immediate cause of the generation of insects was none other than the Omnipotent Hand of Him whose knowledge transcends all, that is, the great and good God, from whom all flying animals received their spirit directly....

Hereupon others add that it is no wonder Galen should confess so modestly in his book his inability to find this origin, and therefore prays all philosophers who may happen to fall in with it to let him know of it....

Although content to be corrected by any one wiser than myself, if I should make erroneous statements, I shall express my belief that the Earth, after having brought forth the first plants and animals at the beginning by order of the Supreme and Omnipotent Creator, has never since produced any kinds of plants or animals, either perfect or imperfect; and everything which we know in past or present times that she has produced, came solely from the true seeds of the plants and animals themselves, which thus, through means of their own, preserve their species.  And, although it be a matter of daily observation that infinite numbers of worms are produced in dead bodies and decayed plants, I feel, I say, inclined to believe that these worms are all generated by insemination 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.  And, in order, Signor Carlo, to demonstrate to you the truth of what I say, I will describe to you some of those insects, which, being most common, are best known to us.

It being thus, as I have said, the dictum of ancients and moderns, and the popular belief, that the putrescence of a dead body, or the filth of any sort of decayed matter engenders worms; and being desirous of tracing the truth in the case, I made the following experiment:

At the beginning of June I ordered to be killed three snakes, the kind called eels of Aesculapius.  As soon as they were dead, I 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 worms were intent on devouring the meat, increasing meanwhile in size, and from day to day I observed that they likewise increased in number; but, although of the same shape, they differed in size, having been born on different days.  But all, little and big, after having consumed the meat, leaving only the bones intact, escaped from a small aperture in the closed box, and I was unable to discover their hiding place. 

Being curious therefore, to know their fate, I again prepared three of the same snakes, which in three days were covered with small worms.  These increased daily in number and size, remaining alike in form, though not in color.  Of these the largest were white outside, and the smallest ones, pink.  When the meat was all consumed, the worms eagerly sought an exit, but I had closed every aperture.  On the nineteenth day of the same month some of the worms ceased all movements, as if they were asleep, and appeared to shrink and gradually to assume a shape like an egg.  On the twentieth day all the worms had assumed the egg shape, and had taken on a golden white color, turning to red, which in some darkened, becoming almost black.  At this point the red, as well as the black ones, changed from soft to hard, resembling somewhat those chrysalides formed by caterpillars, silkworms, and similar insects. 

My curiosity being thus aroused, I noticed that there was some difference in shape between the red and the black eggs, though it was clear that all were formed alike of many rings joined together; nevertheless, these rings were sharply outlined, and more apparent in the black than in the red, which last were almost smooth and without a slight depression at one end, like that in a lemon picked from its stack, which further distinguished the black egg-like balls.  I placed these balls separately in glass vessels, well covered with paper, and at the end of eight days, every shell of the red balls was broken, and from each came forth a fly of gray color, torpid and dull, misshapen as if half finished, with closed wings; but after a few minutes they commenced to unfold and to expand in exact proportion to the tiny body, which also in the meantime had acquired symmetry in all its parts.  Then the whole creature, as if made anew, having lost its gray color, took on a most brilliant and vivid green; and the whole body had expanded and grown so that it seemed incredible that it could ever have been contained in the small shell. 

Though the red eggs brought forth green flies at the end of eight days, the black ones labored fourteen days to produce certain large black flies striped with white, having a hairy abdomen, of the kind that we see daily buzzing about butchers' stalls.  These at birth were misshapen and inactive, with closed wings, like the green ones mentioned above. 

Not all the black eggs hatched after fourteen days; on the contrary, a large part of them delayed until the twenty-first day, at which time there came out some curious flies, quite distinct from the other two broods in size and form, and never before described, to my knowledge, by any historian, for they are much smaller than the ordinary house-flies.  They have two silvery wings, not longer than the body, which is entirely black.  The lower abdomen is shiny, with an occasional hair, as shown by the microscope, and resembles in shape that of the winged ants.  The two long horns, or antennae (a term used by writers of natural history) protrude from the head; the first four legs do not differ from those of the ordinary fly, but the two posterior ones are much larger and longer than would appear to be suitable for such a small body; and they are scaly, like the legs of the locusta marina; they are of the same color, but brighter, so red, in fact, that they would put cinnabar to shame; being all covered with white spots, they resemble fine enamel work.

That different generations of flies issued from the same dead body 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. 

The worms, five or six days after birth, changed as usual to eggs.  From those in the snakes there hatched, after two days, large flies, some blue and some purple.  The eggs in the second box, some of which were red and others black, hatched out flies; green flies being produced from the red eggs after eight days, and after fourteen days the black eggs broke in the place where there was no depression, and there escaped from the shell the same number of black flies striped with white.  Similar flies were seen issuing from al the other eggs in the boxes containing the veal, the capon, the horse-flesh, and the sheep's heart; but with this difference, that in the sheep's heart, blue and violet flies were produced, as well as the black flies striped with white.

In the meanwhile I had placed in a glass dish some skinned river frogs, and having left the dish open, I found the next day, on examination, that some small worms were occupied in devouring them, while some others swam about, at the bottom of the dish, in a watery matter that had run out of the frogs.  The next day the worms had all increased in size and many others had appeared that also swam below and on top of the water, where they devoured the floating fragments of flesh; and after two days, having consumed all that was left of the frogs, they swam and sported about in the fetid liquid, now creeping up, all soft and slimy, on the side of the glass, now wriggling back to the water until at last on the following day, without my knowledge, they all disappeared, having reached the top of the dish.

At the same time I enclosed some fish, called Barbi, in a box full of holes, with a lid perforated in the same way.  When I opened it after four hours, I found a large number of very minute maggots on the fish, and I saw a great many tiny eggs adhering in bunches to the joints and around all the holes in the interior of the b ox:  some of these were white and others, yellow.  I crushed them between my nails and the cracked shell emitted a kind of whitish liquid, thinner and less viscuous than the white of a fowl's egg.

Having rearranged the box as it was before, and having opened it, on the following day, I observed that all the eggs had hatched into the same number of maggots, and that the empty shells were still attached in the places where the hatching occurred; I also noted that the first maggots hatched had increased to double their size; but what surprised me most was that on the following day they had grown so large that every one of them weighed about seven grains, while only the day before there would have been twenty-four or thirty to a grain.  All the later ones hatched were very small.  The whole lot, almost in the twinkling of an eye, finished devouring the flesh of the fish, leaving all the bones so clean and white that they looked like skeletons polished by the hand of the most skilful anatomist.

All these maggots, having been placed where they could not escape in spite of all their endeavors, five or six days after birth turned as usual into as many eggs, some of red and some of black color, and not of the same size; subsequently, at the proper time, different kinds of flies came out, green flies, big blue flies, black flies striped with white, and others resembling the marine locust and winged ants, which I have described.  Besides these four kinds I also saw eight or ten common flies, such as daily hover and buzz about our dinner tables.

Having on the twentieth day noticed that among the larger eggs, there were some still unhatched, I separated them from the others in a different vessel, and two days after there gradually came out of them some very small gnats, the number of which after two days had greatly exceeded the number of eggs.  I opened the vessel and having broken five or six of the eggs I found them so packed with gnats that each shell held at least twenty-five or thirty, and at most forty.

I continued similar experiments with the raw and cooked flesh of the 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.  Besides these, there were to be seen many broods of small black flies, some of which were so minute as to be scarcely visible, and almost always I saw that the decaying flesh and the fissures in the boxes where it lay were covered not alone with worms, but with the eggs from which, as I have said, the worms were hatched.  These eggs made me think of those deposits dropped by flies on meats, that eventually become worms, a fact noted by the compilers of the dictionary of our Academy, and also well known to hunters and to butchers, who protect their meats in Summer from filth by covering them with white cloths.  Hence great Homer, in the nineteenth book of the Iliad, has good reason to say that Achilles feared lest the flies would breed worms in the wounds of dead Patrocles, whilst he was preparing to take vengeance on Hector.

Having considered these things, 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.  Meanwhile the different things placed in the flasks had become putrid and stinking; the fish, their bones excepted, had all been dissolved into a thick, turbid fluid, which on settling became clear, with a drop or so of liquid grease floating on the surface; but the snake kept its form intact, with the same color, as if it had been put in but yesterday; the eels, on the contrary, produced little liquid, though they had become very much swollen, and losing all shape, looked like a viscous mass of glue; the veal, after many weeks, became hard and dry.

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.  One day a large number of worms, which had bred in some buffalo-meat, were killed by my order; having placed part in a closed 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-shape balls, finally became flies of the common kind.  In the same experiment tried with dead flies, I never saw anything breed in the closed vessel....

...[I]t is now necessary to tell you that 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.  For further protection against flies, I placed the vessel in a frame covered with the same  net.  I never saw any worms in the meat, though many were to be seen moving about on the  net-covered frame.  These, attracted by the odor of the meat, succeeded at last in penetrating the fine meshes and would have entered the vase had I not speedily removed them.  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.  I noted that some left six or seven at a time there, and others dropped them in the air before reaching the net....

Hence as I have shown, no dead animal can breed worms....

Go to:
  • Philosophical Letters between Mr. [John] Ray (1628-1705) and several of his Ingenious Correspondents.... (1718)
Readings for Week
Lecture Notes for