Spontaneous generation or Equivocal generation is an obsolete theory regarding the origin of life from inanimate matter, which held that this process was a commonplace and everyday occurrence, as distinguished from univocal generation, or reproduction from parent(s). The theory was synthesized by Aristotle[1], who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it held sway for two millennia. It is generally accepted to have been ultimately disproven in the 19th Century by the experiments of Louis Pasteur, expanding upon the experiments of other scientists before him (such as Francesco Redi who had performed similar experiments in the 17th century). Ultimately, it was succeeded by germ theory and cell theory.
The disproof of ongoing spontaneous generation is no longer controversial, now that the life cycles of various life forms have been well documented. However, the question of abiogenesis, how living things originally arose from non-living material, remains relevant today.
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Spontaneous generation refers to both the supposed process by which life would systematically emerge from sources other than seeds, eggs or parents and to the theories which explained the apparent phenomenon. The first form is abiogenesis, in which life emerges from non-living matter. This should not be confused for the modern hypothesis of abiogenesis, in which life emerged once and diversified. The second version is heterogenesis (sometimes called xenogenesis), in which one form of life emerges from a different form.[2]
As part of his overall attempt to give natural explanations of things that had previously been ascribed to the agency of the gods, Anaximander believed that everything arose out of the elemental nature of the universe, which he called the "apeiron" or "unbounded". According to Hippolytus in the third century CE, Anaximander claimed that living creatures were first formed in the "wet" when acted on by the Sun, and that they were different then than they are now. For example, he claimed humans, in a different form, must have earlier been born mature like other animals, or they would not have survived. Anaximander also claimed that spontaneous generation continued to this day, with aquatic forms being produced directly from lifeless matter.[3]
Anaximenes, a pupil of Anaximander, thought that air was the element that imparted life, motion and thought, and speculated that there was a primordial terrestrial slime, a mixture of earth and water, which when combined with the sun's heat formed plants, animals and human beings directly.[3]
Xenophanes traced the origin of man back to the transitional period between the fluid stage of the earth and the formation of land. He too held to a spontaneous generation of fully formed plants and animals under the influence of the sun.[3]
Empedocles accepted the spontaneous generation of life, but held that there had to be trials of combinations of parts of animals that spontaneously arose. Successful combinations formed the species we now see, unsuccessful forms failed to reproduce.[3]
Anaxagoras also adopted a terrestrial slime account, although he thought that the seeds of plants existed in the air from the beginning, and of animals in the ether.[3]
Aristotle lay the foundations of Western natural philosophy. In his book, The History of Animals, he stated in no uncertain terms:
Now there is one property that animals are found to have in common with plants. For some plants are generated from the seed of plants, whilst other plants are self-generated through the formation of some elemental principle similar to a seed; and of these latter plants some derive their nutriment from the ground, whilst others grow inside other plants, as is mentioned, by the way, in my treatise on Botany. So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs.[4]—Aristotle , History of Animals, Book V, Part 1
According to this theory, living things came forth from nonliving things because the nonliving material contained pneuma, or "vital heat". The creature generated was dependent on the proportions of this pneuma and the five elements he believed comprised all matter.[3] While Aristotle recognized that many living things emerged from putrefying matter, he pointed out that the putrefaction was not the source of life, but the byproduct of the action of the "sweet" element of water.[5]
Animals and plants come into being in earth and in liquid because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full of soul. Therefore living things form quickly whenever this air and vital heat are enclosed in anything. When they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble.—Aristotle , On the Generation of Animals, Book III, Part 11
Numerous forms were attributed to various sources. The testaceans (shelled molluscs) are characterized by forming by spontaneous generation in mud, but differ based upon the material they grow in — for example, clams and scallops in sand, oysters in slime, and the barnacle and the limpet in the hollows of rocks. Some reddish worms form from long-standing snow which has turned reddish. Another grub was said to grow out of fire.[4]
Concerning sexual reproduction, Aristotle argued that the male parent provided the "form," or soul, that guided development through semen, and the female parent contributed unorganized matter, allowing the embryo to grow.[6]
Vitruvius, a Roman architect and writer of the 1st century BCE, advised that libraries be placed facing eastwards to benefits from morning light, but not towards the south or the west as those winds generate bookworms.[7]
Aristotle claimed that eels were lacking in sex and lacking milt, spawn and the passages for either.[8] Rather, he asserted eels emerged from earthworms.[9] Later philosophers dissented. Pliny the Elder did not argue against the anatomic limits of eels, but stated that eels reproduce by budding, scraping themselves against rocks, liberating particles that become eels.[10] Athenaeus described eels as entwining and discharging a fluid which would settle on mud and generate life. On the other hand, Athenaeus also dissented towards spontaneous generation, claiming that a variety of anchovy did not generate from roe, as Aristotle stated, but rather, from sea foam. [11]
On its surface, creation according to Genesis would seem to be incompatible with spontaneous generation. Despite this, the Christian Churches adopted Aristotelianism. Of the major writers, one the most influential, Augustine of Hippo discussed the subject in The City of God and The Literal Meaning of Genesis. He reconciled spontaneous generation by citing passages such as "Let the waters bring forth abundantly the moving creature that hath life" (Genesis 1:20) as decrees that would enable ongoing creation.[12] Augustine emphasized that appearing ignorant of the natural world would not cast Christians in a good light.[13]
From the fall of the Roman Empire in 5th century to the East-West Schism in 1054, the influence of Greek science declined, although spontaneous generation generally went unchallenged. New descriptions were made. Of the numerous beliefs, some had doctrinal implications outside of the Book of Genesis. For example, the idea that a variety of bird known as the Barnacle Goose emerged from a crustacean known as the Goose Barnacle, had implications on the practice of fasting during Lent. In 1188, Gerald of Wales, after having traveled in Ireland, argued that the "unnatural" generation of barnacle geese was evidence for the Immaculate Conception.[14]. Where the practice of fasting during Lent allowed fish, but prohibited fowl, the idea that the goose was in fact a fish suggested that its consumption be permitted during Lent. The practice was eventually prohibited by decree of Pope Innocent III in 1215.[15]
Aristotle, in Arabic translation, was reintroduced to Western Europe. During the 13th century, Aristotle reached his greatest acceptance. With the availability of Latin translations Saint Albertus Magnus and his student, Saint Thomas Aquinas, raised Aristotelianism to its greatest prominence. Albert wrote a paraphrase of Aristotle, De causis et processu universitatis, in which he removed some and incorporated other commentaries by Arabic scholars.[16] The influential writings of Aquinas, on both the physical and metaphysical, are predominantly Aristotelian, but show numerous other influences.[17]
Spontaneous generation is discussed as a fact in literature well into the Renaissance. Where, in passing, Shakespeare discusses snakes and crocodiles forming from the mud of the Nile (Ant 2.7 F1), Izaak Walton again raises the question of the origin of eels "as rats and mice, and many other living creatures, are bred in Egypt, by the sun's heat when it shines upon the overflowing of the river…". While the ancient question of the origin of eels remained unanswered and the additional idea that eels reproduced from corruption of age was mentioned, the spontaneous generation of rats and mice engendered no debate.[18]
The last great proponent, as experimentation began to transform science, was Jan Baptist van Helmont (1580–1644). He used experimental techniques, most famously growing a willow for five years and showing it increased mass while the soil showed a trivial decrease in comparison. As the process of photosynthesis was not understood, he attributed the increase of mass to the absorption of water.[19] His notes also describe a recipe for mice (a piece of soiled cloth plus wheat for 21 days) and scorpions (basil, placed between two bricks and left in sunlight). His notes suggest he may even have done these things.[20]
Where Aristotle held that the embryo was formed by a coagulation in the uterus, William Harvey's dissection of deer showed that there was no visible embryo during the first month.[6] Although his work predated the microscope, this led him to suggest that life came from invisible eggs. In the frontispiece of his book Exercitationes de Generatione Animalium (Essays on the Generation of Animals), he made an expression of biogenesis: "omnia ex ovo" (everything from eggs).[12]
The ancient beliefs were subjected to testing. Francesco Redi challenged the idea that maggots arose spontaneously from rotting meat. In the first major experiment to challenge spontaneous generation, he placed meat in a variety of sealed, open, and partially covered containers. [21] Realizing that the sealed containers were deprived of air, he used "fine Naples veil", and observed no worm on the meat, but they appeared on the cloth.[22]
In 1745, John Needham performed a series of experiments on boiled broths. Believing that boiling would kill all living things, he showed that when sealed right after boiling, the broths would cloud, allowing the belief in spontaneous generation to persist.[21]
Lazzaro Spallanzani modified the Needham experiment in 1768, attempting to exclude the possibility of introducing a contaminating factor between boiling and sealing. His technique involved boiling the broth in a sealed container with the air partially evacuated to prevent explosions. Although he did not see growth, the exclusion of air left the question of whether air was an essential factor in spontaneous generation.[21]
In 1837, Charles Cagniard de la Tour, a physicist, and Theodor Schwann, one of the founders of cell theory, published their independent discovery of yeast in alcoholic fermentation. They used the microscope to examine foam left over from the process of brewing beer. Where Leeuwenhoek described "small spheroid globules", they observed yeast cells undergo cell division. Fermentation would not occur when sterile air or pure oxygen was introduced if yeast were not present. This suggested that airborne microorganisms, not spontaneous generation, was responsible.[23]
Louis Pasteur's 1859 experiment put the question to rest. He boiled a meat broth in a flask that had a long neck which curved downward, like a goose. The idea being that the bend in the neck prevented any particles from reaching the broth, while still allowing the free flow of air. The flask remained free of growth for an extended period. When the flask was turned so that particles could fall down the bends, the broth became quickly clouded.[21]
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The theory known as spontaneous generation held that complex, living organisms are generated by decaying organic substances. Mice might spontaneously appear in stored grain or maggots could spontaneously appear in meat.
According to Aristotle it was a readily observable truth that aphids arise from the dew which falls on plants, fleas from putrid matter, mice from dirty hay, crocodiles from logs rotting at the bottom of bodies of water, and se same timo forth. In the 17th century these things started to be questioned. Sir Thomas Browne wrote a book called Pseudodoxia Epidemica. He subtitled it Enquiries into Very many Received Tenets, and Commonly Presumed Truths. He published it in 1646. He wrote it as an attack on false beliefs and "vulgar errors". Scientists failed to accept the conclusions he reached. That way, Alexander Ross, a scientist who lived at the same time as Browne, wrote about it: "To question this he meant: spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants."[1]
In 1546 the physician Girolamo Fracastoro theorized that epidemic diseases were caused by tiny, invisible particles or "spores", which might not be living creatures, but this was not widely accepted. Next, Robert Hooke published the first drawings of a microorganism in 1665. He is also credited for naming the cell which he discovered while observing cork samples.
In 1676, Anthony van Leeuwenhoek discovered microorganisms that were probably protozoa and bacteria.[2] This sparked new interest in the microscopic world.[3]
The first step was taken by the Italian Francesco Redi. In 1668, Redi proved that no maggots appeared in meat when flies were prevented from laying eggs. From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be omne vivum ex ovo: that every living thing came from a pre-existing living thing (literally, everything from an egg).
In 1768 Lazzaro Spallanzani proved that microbes came from the air, and could be killed by boiling. Yet it was not until 1861 that Louis Pasteur performed a series of careful experiments. With these experiments, Pasteur proved that organisms such as bacteria and fungi do not appear in nutrient-rich media of their own accord in non-living material. This experiment supported cell theory.
   | The Slow Death of Spontaneous Generation (1668-1859) - The Slow Death of Spontaneous Generation (1668-1859) |
| | Topographia Hiberniae - Medieval Sourcebook: Gerald of Wales: Barnacle Geese should convince the Jew of the Immaculate Conception, 1188 |
| | Natural History - Pliny the Elder, The Natural History (eds. John Bostock, M.D., F.R.S., H.T. Riley, Esq., B.A.) |
| | The Compleat Angler or the Contemplative Man's Recreation - The Complete Angler |
| | Embryology - History Of Embryology As A Science - Embryology - History Of Embryology As A Science |
| | Stanford Encyclopedia of Philosophy - Stanford Encyclopedia of Philosophy |
| | The History of Animals - The History of Animals, by Aristotle |
| | On the Generation of Animals - On the Generation of Animals, by Aristotle |
| | The deipnosophists, or, Banquet of the learned of Athenæus - The Literature Collection: Search Results |
| | Dictionary of the History of Ideas - Dictionary of the History of Ideas |
| | Spontaneous Generation and the Origin of Life - Pasteur, fermentation, contagion, and proving a negative |
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