Abraham Trembley

Abraham Trembley: The Tutor Who Cut Up Hydra

In 1740, a Genevan tutor fishing in a Dutch pond with his two young pupils scooped up something he could not identify. It was green, it had arms, and when he cut it in half both halves grew back into whole animals. Abraham Trembley did the experiment again. Then he did it lengthways, then in several pieces, then through a developing bud until he had produced a seven-headed creature he named Hydra, after the monster of the myth. He was not a professor. He was a private tutor with a pair of scissors, and he had just invented experimental zoology.

Geneva and the Calculus Thesis

He was born on 3 September 1710 into a prominent Geneva family — an officer's family, by one account — and studied at the Calvin Institute, the ancestor of the University of Geneva, where he completed a thesis on calculus. That mathematical training matters: it explains an experimenter who counted, varied one thing at a time, and repeated.

He was related to Charles Bonnet, who became his correspondent and scientific ally, and he would go on to correspond with René-Antoine Ferchault de Réaumur and to befriend Lazzaro Spallanzani. Then, as many educated young Genevans did, he left for the Dutch Republic to take a post as tutor — in his case to the two sons of Count Willem Bentinck van Rhoon, aged three and six, at the Count's summer estate of Sorgvliet near The Hague.

The Polyp

He was teaching the boys natural history, which in practice meant collecting things from water. The organism he found had been seen before: Antoni van Leeuwenhoek had published observations of it decades earlier, describing an "animalculum" that budded and bore tentacles, and an anonymous English gentleman had noted it in 1704. Trembley knew none of this. He proceeded as though the thing were new, which for his purposes it was.

The first question was the obvious one for 1740: plant or animal? He designed a test and stated his prediction in advance. If he cut the organism in two and both halves regenerated into complete individuals, it must be a plant — because animals were not known to do that. This is the moment the modern method appears in the story, because the prediction was falsifiable and Trembley meant to be bound by it.

The Wrong Answer, Rightly Reached

Both halves regenerated. By his own stated criterion, he had a plant.

He concluded it was an animal anyway. The polyp moved — a stepping, looping progress across the glass that he compared to an inchworm — and locomotion of that kind was not something plants did. So he abandoned his own decision rule in the face of a better observation, and in doing so overturned the premise underneath it. The finding was not that polyps are plants; it was that animals regenerate. That inversion is the whole significance of the work.

He then pushed the manipulation as far as it would go, using scissors, his own palm and very little magnification: transverse sections, longitudinal splits, division into multiple fragments, cuts through the head buds themselves. He grafted. The seven-headed polyp was a demonstration piece and a joke at once — hydra by name, and hydra in behaviour.

Réaumur, and the News

In 1741 Trembley wrote to Réaumur, reaching him through his cousin Bonnet. Réaumur announced the results to the Paris Academy of Sciences and the discovery travelled across Europe at speed. Bonnet promptly replicated the phenomenon in worms and published alongside him.

The full account came in 1744 as *Mémoires pour servir à l'histoire d'un genre de polypes d'eau douce, à bras en forme de cornes*, illustrated with engravings by Cornelis Pronk; it also appeared in volume 42 of the Royal Society's *Philosophical Transactions*. The Royal Society elected him in 1743 and awarded him the Copley Medal the same year — a remarkable pair of honours for a previously unknown private tutor. The French Académie des sciences made him a corresponding member.

What It Broke

The polyp landed in the middle of the century's central biological quarrel. Preformationists held that organisms were already present in miniature and merely unfolded; if that were true, an animal chopped into pieces should not be able to build two complete new animals from the parts. Trembley's evidence pointed instead toward epigenesis, gradual development from unorganised material. It also fed straight into the arguments about the Chain of Being — where does a creature that is unmistakably an animal but behaves like a cutting from a shrub belong? — and materialists including Julien Offray de La Mettrie seized on it for their case that animals were machines.

Trembley himself stayed out of most of this. He reported, and let others theorise.

Why Abraham Is Called a Genius

The claim rests on method rather than on the discovery, and it is stronger for it. Regeneration in the polyp is a fact he stumbled on while entertaining two small boys; what he did next is the achievement. He framed a testable question with a stated prediction, varied the intervention systematically rather than repeating one trick — transverse, longitudinal, multiple, through the bud, grafted — repeated everything many times, and refused to theorise beyond what he had seen. Historians of biology credit him with setting a standard of rigorous experimental repetition, observational acuity and cautious theorising at a moment when natural history was still largely a matter of collecting and describing. The instinct is visible in his handling of the plant-or-animal test: he had committed to a criterion, the criterion gave the wrong answer, and rather than force the data he concluded that the criterion itself was mistaken. Most people do the opposite.

The honest qualifications: he was not first to the organism — Leeuwenhoek had described it forty years earlier — and his was a discovery of enormous luck as well as skill, since few animals regenerate so spectacularly and almost any other pond creature would have yielded nothing. He built no general theory of generation, and the theoretical uses his work was put to, by Bonnet, by La Mettrie, by the epigenesists, were other people's. He was a superb experimentalist and a deliberately unambitious thinker, and he seems to have known exactly which of those he was.

Legacy

He returned to a life of tutoring and correspondence and died in Geneva on 12 May 1784. Hydra never left the laboratory: it remains a working model system for the study of regeneration, nearly three centuries after a tutor cut one in half to settle an argument with himself about whether it was a vegetable. The experiment is still repeatable by a schoolchild, which is the surest sign of how well it was designed.

Achievements

• Copley Medal — 1743
• Fields of research: zoology