Many Junes ago, in the early summer of 1740, a thirty-year-old tutor from Geneva—his name was Abraham Trembley—walked out into Holland’s countryside to collect bits and pieces of Nature that might tease the minds of his young wards, the two sons of Count William Bentinck. Stopping by a ditch on the count’s estate that connected to a stream that passed not far from the town of The Hague, the tutor selected a horsetail, a handful of duckweed, and a plump water lily. Not until he returned to the count’s manor did he notice, with the help of his hand-held magnifier, the countless little green nubs clinging to the stems of these plants.

If the samplings fascinated his young pupils for more than a minute, Trembley was to be congratulated. More to the point of this story, those little green nubs would very much fascinate Trembley. In fact, the very same oddities, together with other creatures that possessed similar properties, in time would motivate an army of naturalists to probe and paw at Nature so thoroughly that entire populations of plants and animals would be at risk of being trampled. So riveting did those little green nubs prove to be that some historians would go so far as to say that they inspired the beginnings of modern biology.

Whether the tiny specimen was plant or animal, the young naturalist couldn’t at once say. Under his magnifying glass the mystery object looked fairly inconsequential—not unlike a thin scrap of tubing—and offered no clues. Trained as a mathematician and an ace at problem solving, Trembley took it upon himself to try and solve the identity of the little nub, whose firm clutch on freshwater plants made him fairly certain that it was, at the very least, alive.

It was green and stationary, so it probably was a plant, he initially guessed. But the more he stared through his magnifier, the more he changed his mind. The object appeared to have something like a head, as well as skinny arms emerging from its head. Trembley would later describe these appendages in his Mémoires as “arms shaped like horns.” And sometimes the arms—or were they tentacles?—did, in fact, move about. Or did that happen only when he inadvertently sloughed the water the object sat in? Maybe his first guess—plant—was correct after all, and the waving appendages were branches. A third possibility was that he had stumbled on an example of a “plant-animal,” a life form that some people imagined must exist but no one had found. It was theorized (this representing but one theory among a chaotic panoply of theories that characterized Trembley’s era) that a Chain of Being linked plants to animals, and it seemed a reasonable supposition that plant-animals, or zoophytes, occurred as intermediaries somewhere in between.

Plant? animal? or zoophyte? As much as Trembley kept changing his mind, he slowly became more and more convinced that his found species was an animal, largely because it moved forward by contracting and expanding “much in the same way as do inchworms,” he later jotted down. Before long he would also observe that it used its spindly arms to catch water fleas, which it stuck into its extremely small mouth and appeared to eat! He would hold to this animal view, even though the more of these tiny individuals he studied, the more he saw that their arms varied in number. One individual might have five arms while another had eight, a variability seen in plants but never in animals.

One simple experiment, Trembley decided, could solve the plant-or-animal conundrum. He would cut his “insecte” (a word used for all manner of beast in those days) in two, and if it were animal, as he believed it must be, one or both halves would surely expire. But if he was wrong and it was plant, both halves would presumably stay alive and grow bigger again, just like plant cuttings.

So that’s what he did. Using sharp scissors, he cut his “insecte” straight across its middle, creating an anterior piece identifiable by its head and arms, as well as a posterior piece. After putting both parts in a glass of water, he watched and waited. Because he was thinking animal, he imagined that while the anterior piece just might hang on to life, soon he’d see the posterior piece curl up and die. As he elaborated in his Mémoires, “I assumed that the second part was only a kind of tail without the organs vital to the life of an animal. I did not think that it could survive for long separated from the rest of the body.”

The days passed, but as Trembley stood witness with his magnifying glass, the expected was not happening. Neither piece of his green creature was giving up the ghost, and, in fact, both portions kept moving about. Even more amazing, little by little both pieces began getting longer. On day 9, Trembley thought he even saw arms starting to form on the posterior piece. “I continued to see these protuberances throughout the day, and I became extremely excited and impatient for the moment when I would know clearly what they were,” he wrote. By day 10 there was no question that they were arms—first five of them, then eight. Within two weeks, each of the two pieces had regrown into a whole individual!

Although it had happened right before his eyes, Trembley was incredulous, especially over the posterior piece’s resiliency. “Who would have imagined that it would grow back a head!” he exclaimed. The behavior of the organism—its moving and its eating—made him continue to think animal, but what an unusual and remarkable animal. “I had not the least expectation of being a spectator to this marvelous kind of reproduction,” he marveled.

To test the animal’s rejuvenative capabilities further, Trembley began a series of experiments. Maintaining a fine balancing act, whereby he cradled an “insecte” encased in a water drop in the palm of his left hand while wielding either scissors or a boar’s bristle—also used for cutting—with his right hand, all the while constantly swapping these implements for his magnifying glass, he went about cutting one green “insecte” after another either transversely or lengthwise, and in varying ways. For instance, he cut one polyp transversely into four pieces, then another longitudinally into four pieces. Low and behold, every piece became a full individual, each with a head. In another astonishing outcome, he discovered that when he sliced the head end down to nearly the animal’s midpoint, each of the two longitudinal segments acquired a head. The result was a most disconcerting two-headed, Y-shaped critter. Through an elaborate process of slicing the head lengthwise, then waiting for two heads to bud back, slicing again, then waiting some more, he found he could even design a seven-headed animal!

It was at this point that he called the organism a hydra, after the difficult-to-kill mythical monster with nine heads. (By Edith Hamilton’s account: “one of the heads was immortal and the others almost as bad, inasmuch as when Hercules chopped off one, two grew up instead.”) Hydra is what this spectacularly regenerative little creature—which belongs to the phylum Cnidaria, making it a cousin to the sea anemone and the jellyfish—has been called ever since. As it happens, freshwater lakes, ponds, and streams all over the world carry dozens of species of hydra. Its saltwater equivalent, colonial hydroids, similarly festoon the oceans.

Historical records tell us that Trembley wasn’t the first to describe the hydra. Among those before him was Anton van Leeuwenhoek, the Dutch naturalist renowned for beholding a wide variety of microscopic living things—including bacteria, single-celled slimed molds, tiny roundworms, and even blood cells and sperm cells—with his crude two-inch-long, hand-held microscope. When, in 1702, Leeuwenhoek plucked hydra from a stream, that stream

very likely ran by his home town of Delft, Delft being situated only a few kilometers down the road from the stream that coughed up Trembley’s hydra a few decades later. There is a faint chance, then, that the two men might have retrieved their hydra from the very same rambling waters.

Nor was Trembley the first naturalist to describe the process of regeneration in animals. As the ancient myth of a monster with returning heads suggests, since Aristotle’s day there had been reports, both fictional and real, of animals regaining lost appendages. The tail of a lizard, the claw of a crab, the leg of a cockroach: missing parts of a not inconsequential number of critters had been observed to grow back like new. A salamander, it would be noted, could even replace the same leg many times over. Just before Trembley did his experiments, René-Antoine Ferchault de Réaumur—the esteemed leader of the Paris Academy of Sciences and himself a student of regeneration—had in fact caused quite a stir with his drawings of crustaceans and their regenerating limbs.

But although Trembley was hardly the first naturalist to be awed by the spectacle of regeneration, he was the first to conduct a systematic study of the regenerative ability of an entire animal, as opposed to just one of its parts, and the first to report some substantial findings, according to biologist Howard Lenhoff, a Trembley authority who himself spent years probing hydra at the University of California, Irvine. It astonished Trembley to discover how closely the process of regeneration followed the small-to-big growth pattern seen during an animal’s development. A little piece from an older hydra could grow into the entire animal just the way an embryo did. But this was incredible! How was it possible that the head of a hydra—or the middle section of a worm, for that matter—had the power to act like an embryo?

Trembley’s experiments impinged on sacred ground. To many minds, his demonstration that an animal severed in two could yield two whole animals wasn’t amazing—it was unthinkable. At the time of Trembley’s hydra trials, which lasted from 1740 to 1744, many

natural historians and educators were hooked on certain rigid Rules of Nature derived from Calvinist or Cartesian doctrine, and one such Rule declared that the only way animals could reproduce was sexually, through a male and a female. Yet here Trembley was waving evidence of a mode of procreation that bypassed this union. Look, his experiments demanded, you can cut one of these creatures into several parts, and all parts turn into full individuals! Such was the incredulity produced by his experiments that Trembley, in a letter to Réaumur, observed of his skeptics, “Apparently these gentlemen have some cherished system they are frightened of disturbing.”

The Genevan naturalist came to notice that if left to their own devices, his teensy green creatures did reproduce sexually, just like other animals. However, they also had the ability to make more copies of themselves by budding, an activity that was curiously similar to a plant’s extending side shoots. Oblong buds formed roughly two-thirds of the way down a hydra’s body column, and each bud eventually detached and inched off into the watery beyond to become its own little hydra. It would be shown one day that if a parent hydra budded continuously, in a month’s time it could give rise to as many as twenty chips off the old block—or clones, individuals that are genetically identical to the parent.

As was to be expected, Trembley’s evidence of asexual procreation in the animal kingdom was too iconoclastic to be believed by everyone overnight. Some of his contemporaries, unable to reconcile a hydra’s incredible regenerativeness when cut in two—or four, or eight—remained convinced that his green oddity was a plant. Voltaire, for one, with an eye toward the hydra’s tubular shape considered it a type of carrot or asparagus.

Many other Europeans, however, were mightily impressed with the completeness of Trembley’s experiments and accepted them as valid. It helped that the Enlightenment was underway; novel observations and ideas that once might have been branded as ludicrous were now discussed with fervor. Réaumur, one of Europe’s most distinguished scientists, was satisfied that Trembley’s small regenerative

life forms indeed were animals and spread the word that the young Genevan’s investigations might bring important new knowledge of hidden life forces. Because the green nub’s cylindrical body and head-circling tentacles reminded him of a miniature octopus, Réaumur referred to the animal as a “polyp.”

Someone else who was fascinated by Trembley’s investigations was his cousin Charles Bonnet. Ten years younger and also from Geneva, Bonnet coincidentally had encountered another form of nonsexual reproduction in Nature—parthenogenesis, which con-notes “birth without father”—shortly before Trembley had launched his hydra experiments. When scrutinizing plant parasites called aphids, Bonnet noticed that the eggs of female aphids could develop and produce offspring without fertilization by male sperm.

Eager to explore the boundaries of regeneration as his cousin was doing, Bonnet chose worms to work on, and he and other naturalists would provide abundant proof that a mud worm was a master of regrowth. An inquiry he pursued much later in his life, beginning in 1779, deserves greater description, however, because by that time Bonnet was no longer focused on a small invertebrate but on a larger animal—a salamander. Like humans, salamanders are vertebrates with backbones, yet salamanders and other tailed amphibians have the distinction of being the only vertebrates that can proficiently regenerate a lost appendage during adulthood. A remarkable assertion by Lazzaro Spallanzani, another leading naturalist of the day, was that a salamander’s eye, if injured, could bounce back as completely as its tail did, and Bonnet, perhaps motivated by his own increasingly poor vision, decided to look into the Italian’s claim by dislodging the eye of a salamander. Here is biologist-historian Charles Dinsmore’s account of Bonnet’s dismal yet revealing attempt:

With his eyesight steadily waning, Bonnet must have viewed the salamander’s restored eye with fabulous wonder, if not downright envy. Biologists would later declare that they would give their right arms to understand regeneration, and Bonnet must have felt the same way. Tantalizing questions likely brushed across his mind, such as—Just how far do regenerative capabilities extend in more complex animals? Is there a glimmer of any such capability in humans? If he was as astute as historians have portrayed him, the regenerative prowess seen in many lower species must have caused him to speculate that although a human could not spontaneously replace an arm, leg, or eye with a new arm, leg, or eye, perhaps scientists of the future would be able to figure out how to recreate parts of the body. At least one eighteenth-century visionary made his musings known on this subject. Voltaire was said to be so confident that humans would master the skills to regenerate themselves, he expected they’d be able to acquire whole new heads. Certainly many people could benefit from such a strategy, he intimated.

All the singular observations made by Trembley and Bonnet—as well as Réaumur, Spallanzani, Mazolleni, Müller, Guettard, Thévenot, Lyonet, De Jussieu, and other early students of regeneration—created an infectious interest in this enviable phenomenon, encouraging people to press their noses closer to Nature than ever before. One short-term consequence was that snails, particularly those in the French countryside, fell victim to an intense manhunt. Hoards of naturalists went tromping over hill and dale decapitating these gastropods in order to see if, as reported by Spallanzani, their heads really would regrow. Indeed they could, as could portions of a snail’s antenna, mantle, and foot. Poor pestered snails!

Some historians maintain that it was this utter captivation with

regeneration in the eighteenth century that would sweeten the way for experimental zoology and, over time, the entire modern sweep of biological sciences. To whatever extent this may be true, it stands to reason that in Trembley and Bonnet’s era, when so little was known about how and why living things lived and grew and breathed and moved, an amphibian’s physical capacity for regeneration must have seemed so foreign to humans as to be totally bewildering. How, for starters, could a full-grown creature regrow parts of itself when it had already undergone the process of development, from a zygote on forward? And by what means did the newly regrown material—say a crab’s new claw—so seamlessly operate with the rest? Or how on earth could one section of an organism give rise to a wholly unrelated section, as in the case of the hydra’s head end re-forming its foot end? Or strangest of all, how could a piece of an animal that had no mouth, hence took in no food—as was true of a hydra’s foot end—re-form the creature’s head end? How was this possible?

But it was too early in the history of science to begin to say. The cell—biology’s coming empire—was still cloaked in inky obscurity. The answers to such questions would have to wait for tools that would take the eye much deeper into biological minutiae. Only in the previous century had Robert Hooke, with the help of his magnifying glass, glimpsed the intricate design of cells in a piece of cork, hardly guessing their central role in Life. After that, it may in fact have been Abraham Trembley who first witnessed the activity of cells dividing and multiplying. With the aid of his magnifier, he observed single-celled diatoms and stentors—aquatic organisms—dividing in two, yielding more diatoms and stentors. But he does not appear to have realized the worth of these observations, either that he was observing cells or that Life largely reaches fruition through the division and multiplication of these tiny universes.

Not until 1838 and 1839 would two Germans, Matthias Schleiden and Theodor Schwann, make perfectly clear that the cell represents the organized fabric of both plant and animal tissue, the vital component of all living things. It would be a few decades more

before the notion that Life is made up of a continuing wave of cells begetting cells really took hold. Still more decades would pass before scientists gained any substantial knowledge of how cells divide, how they stack into tissues, and how they are sustained. After Schleiden and Schwann, as many as 150 years would go by before biologists determined that a hydra has ten different types of cells that are organized into two layers, and that the creature’s remarkable gift of regeneration hinges on embryonic-like cells that are as ubiquitously present in hydra as oxygen molecules are in air.

In the eighteenth century, whatever lengths Trembley, Bonnet, and other naturalists went to in their endeavors to fathom how regeneration happened, they couldn’t have had a clue. They didn’t have the resources to get to the bottom of their questions. Not that that stopped them from coming up with some interesting theories. Bonnet was under the impression that an animal’s lost part was replaced by pre-formed material already inside its body. This thinking grew out of the popular supposition that held that human embryos arose from minute pre-formed organs in a parent’s egg or sperm. Another explanation for regeneration, espoused by a German biologist, was that an organism inherited Bildungstrieb, a strong inner force that was capable of reorganizing an animal’s structures.

Greatly influenced by Réaumur, who avoided speculating about why things in Nature happened, Trembley offered no theories about regeneration. Unlike many of his contemporaries, including his cousin, he didn’t try to fit his findings to a fashionable explanation. “Trembley didn’t like theories and stayed away from them,” notes Howard Lenhoff. “He felt that theories kept people from knowing what fishermen probably had observed and known about regeneration since way back in time. He felt that all that counts is your data, since interpretations will change as more is learned.” Patient observation combined with smart experiments, believed Trembley, would in time reveal the workings of Nature. This “empirical” approach,

which emphasizes observation over theory, would prevail as one of the Enlightenment’s foremost contributions to modern science.

Trembley may have seldom wondered aloud about how a hydra performed its magic, yet he must have sometimes silently brooded over what inner properties allowed it to bounce back the way a pasture did after pruning. Perhaps in the end he simply realized that these inner things, which were undoubtedly too small to see, were meant for another generation’s discovery. He and his contemporaries’ dearth of knowledge about the mechanisms underlying living systems set up a roadblock beyond which they could not travel. Due to all his empirical pokings, however, Trembley would be crowned by future biologists as “the father of experimental zoology,” when, scores of years later, the considerable interest in regeneration that he, Réaumur, Bonnet, and a swarm of other naturalists had stirred up would explosively reignite. Just as Trembley predicted in his Mémoires, in time new observations would bring long-awaited explanations:

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