Apr 202018

On this date in 1862, Louis Pasteur (and colleagues) concluded and published a series of experiments that definitively refuted the theory of spontaneous generation: the notion that living organisms can be generated by inanimate substances. Spontaneous generation was the dominant theory for thousands of years, and it’s not hard to understand why. When I tried to germinate avocado seeds in water in Myanmar for a school project, I had to dump the water constantly because every few days you could see mosquito larvae swimming in it. Where did they come from? Rotting meat frequently breeds maggots; old fruit seems to generate fruit flies. You need a good microscope, and controlled experiments, to figure out that living things are generated only by living things that are alike. Pasteur settled the matter, although there were holdouts for a while.

In the 6th and 5th centuries BCE, Greek philosophers, called physiologoi (φυσιολόγοι) that is, investigators of “nature” (φυσις – from which we get “physics”), attempted to give natural explanations of phenomena that had previously been ascribed to the agency of the gods. The physiologoi sought the material principle or arche (ἀρχή) of things, emphasizing the rational unity of the external world and rejecting theological or supernatural explanations. Anaximander, who believed that all things arose from the elemental nature of the universe, the apeiron (ἄπειρον) or the “unbounded” or “infinite,” was likely the first Western thinker to propose that life developed spontaneously from nonliving matter. The primal chaos of the apeiron, eternally in motion, served as a substratum in which elemental opposites (e.g., wet and dry, hot and cold) generated and shaped the many and varied things in the world. According to Hippolytus of Rome in the 3rd century CE, Anaximander claimed that fish or fish-like creatures were first formed in the “wet” when acted on by the heat of the sun and that these aquatic creatures gave rise to human beings. Censorinus, writing in the 3rd century, reports:

Anaximander of Miletus considered that from warmed up water and earth emerged either fish or entirely fishlike animals. Inside these animals, men took form and embryos were held prisoners until puberty; only then, after these animals burst open, could men and women come out, now able to feed themselves.

Anaximenes, a pupil of Anaximander, thought that air was the element that imparted life and endowed creatures with motion and thought. He proposed that plants and animals, including human beings, arose from a primordial terrestrial slime, a mixture of earth and water, combined with the sun’s heat. Anaxagoras, too, believed that life emerged from a terrestrial slime. However, he held that the seeds of plants existed in the air from the beginning, and those of animals in the aether. Xenophanes traced the origin of man back to the transitional period between the fluid stage of the earth and the formation of land, under the influence of the sun.

In what has occasionally been seen as a prefiguration of a concept of natural selection, Empedocles accepted the spontaneous generation of life but held that different forms, made up of differing combinations of parts, spontaneously arose as though by trial and error: successful combinations formed the species we now see, whereas unsuccessful forms failed to reproduce.

Aristotle proposed that in sexual reproduction, the child inherits form (eidos) from the father and matter from the mother, as well as πνεῦμα (pneuma) – breath, life, or spirit – either from the father or from the environment. In spontaneous generation, the environment could effectively replace the parents’ contributions of form, matter, and pneuma:

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 … 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.

(History of Animals, Book V, Part 1)

I first came across this notion when I studied Virgil’s Georgics, Book IV, on bee keeping. Virgil advises the following, if a bee keeper loses his stock:

First they choose a narrow place, small enough for this purpose:
they enclose it with a confined roof of tiles, walls close together,
and add four slanting window lights facing the four winds.

Then they search out a bullock, just jutting his horns out
of a two-year-old’s forehead: the breath from both its nostrils
and its mouth is stifled despite its struggles: it’s beaten to death,
and its flesh pounded to a pulp through the intact hide.

They leave it lying like this in prison, and strew broken branches
under its flanks, thyme and fresh rosemary.
This is done when the Westerlies begin to stir the waves
before the meadows brighten with their new colours,
before the twittering swallow hangs her nest from the eaves.

Meanwhile the moisture, warming in the softened bone, ferments,
and creatures, of a type marvelous to see, swarm together,
without feet at first, but soon with whirring wings as well,
and more and more try the clear air, until they burst out,
like rain pouring from summer clouds,
or arrows from the twanging bows,
whenever the lightly-armed Parthians first join battle.

Spontaneous generation is discussed as a fact in literature well into the Renaissance. Shakespeare says snakes and crocodiles form from the mud of the Nile:

Your serpent of Egypt is bred now of your mud by the operation of your sun. So is your crocodile.  

(Anthony and Cleopatra Act 2 scene 7)

Izaak Walton agrees when he says that 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.”

Jan Baptist van Helmont (1580–1644) used experimental techniques, such as growing a willow for five years and showing it increased mass while the soil showed a trivial decrease in comparison. He attributed the increase of mass to the absorption of water. 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 tried these things.

The ancient beliefs were subjected to testing starting in the 17th century. In 1668, 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. 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. Redi used his experiments to support the preexistence theory put forth by the Church at that time, which maintained that living things originated from parents. Pier Antonio Micheli, around 1729, observed that when fungal spores were placed on slices of melon the same type of fungi were produced that the spores came from, and from this observation he noted that fungi did not arise from spontaneous generation.

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. His studies were rigorously scrutinized by his peers and many of them agreed.

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. However, by that time there was already widespread skepticism among major scientists, to the principle of spontaneous generation. Observation was increasingly demonstrating that whenever there was sufficiently careful investigation of mechanisms of biological reproduction, it was plain that processes involved basing of new structures on existing complex structures, rather from chaotic muds or dead materials.

Louis Pasteur’s 1859 experiment is widely seen as having settled the question of spontaneous generation. He boiled a meat broth in a flask that he invented called the swan-necked flask (because ithad a long neck that curved downward, like that of a swan). The idea was that the bend in the neck prevented falling 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 quickly became clouded. A flask in which broth was boiled and immediately exposed to air, became clouded quickly. Minority objections to the conclusiveness of the experiments were persistent, however, and subsequent, more rigorous, experiments were needed to bring the question to an end for the die-hards. Hey – we still have flat earthers.

The obvious ingredient for today’s celebratory recipe is Lyle’s golden syrup. The label has the ancient slogan on it, “Out of the strong came forth sweetness,” a reference to a riddle put by Samson in Judges 14:14, the answer to which is that dead lions propagate honey bees. Here is the recipe for treacle tart taken from the Lyle’s website (unedited):


Lyle’s Treacle Tart



295g Plain flour, plus extra for dusting
165g Unsalted butter (chilled + cubed)
4½ tbsp Cold water
Pinch of salt


450g Lyle’s Golden Syrup
25g Unsalted butter
1 Large egg
3 tbsp Double cream
2 sachets Dr Oetker Lemon Ready Zest
30g breadcrumbs (increase to 80g for a denser mixture)
Crème fraîche, for serving


Pulse the flour, butter and salt in a blender until the mixture resembles large crumbs. Add the water and briefly blend until it comes together in a ball – then wrap in cling film and chill for 20 minutes.

Cut off one-third of the pastry and set aside for the lattice top. Roll the rest of pastry out on a lightly floured surface to about 4cm (1½”) bigger than a loose-bottomed tart tin, 22cm (9”) x 3.5cm (1½”) deep. Line the tin with pastry, trim the excess and lightly prick with a fork, then chill for 30 minutes. Add the excess to the pastry set aside for the lattice top.

Preheat the oven to 190°C/170° Fan, 375°F, Gas 5. Lay some baking parchment in the tin over the pastry and then put your baking beans in, over the parchment. Place in the oven and pre-bake for 15 minutes on the middle shelf. Remove the paper and beans and bake for a further 8-10 minutes to dry the pastry out. Remove the tart from the oven and put it on a baking tray. Reduce the oven temperature down to 180°C/160°Fan, 350°F, Gas 4, ready for later.

Roll the extra lattice top pastry out thinly and set aside on a tray to chill in the fridge for about 20-30 minutes – this makes it easier to handle.

Gently warm the Lyle’s Golden Syrup in a pan over a low heat, remove, then add the butter and stir until melted. Leave to cool a little. Using a fork, beat the egg and cream together in a separate bowl, then quickly beat in the syrup mixture along with the lemon zest and crumbs. Pour into the pastry case.

Remove the pastry from the fridge and cut into 10 strips of 1cm width which are long to overhang the edges of the tart tin.

Lay 5 parallel strips equally spaced over the tart. Fold back every other strip and place one strip of dough perpendicular to the parallel strips. Unfold the folded strips over the perpendicular strip. Now take the parallel strips that are running underneath the perpendicular strip and fold them back over. Lay down a second perpendicular strip (evenly spaced) and unfold the folded parallel strips.

Continue this process until all 10 strips have been placed. Trim the edges of the strips for a neat finish to fit inside the tart.

Bake on the middle shelf for 45-50 minutes until richly brown and set. (The filling will still be a bit wobbly but it will firm up on cooling.) Remove, leave to cool until warm, then remove from the tin, slide onto a plate and serve.

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