Today, a biologist won't be tricked by mother nature. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

We knew how to tell plants from animals in 1740. Animals reproduced sexually. Plants stayed in one place. Animals hunted. Plants took nutrients from water and soil. The lines were clear enough. Then a young Swiss tutor named Abraham Trembley went to work for a wealthy patron in Holland. Trembley spent all his free time studying the plant and animal life on the estate.

He soon ran across a half-inch aquatic life-form. It had a slender torso with a mouth on top and tentacles reaching out from the mouth. He called it a polyp. Today we call it a hydra. It's kin to jellyfish and corals. At first the hydra seemed to be immobile, and Trembley took it to be a plant. Then it surprised him. He saw it stretch out, put its mouth to the ground, lift its base overhead, and bring it down on the other side. It traveled by somersaulting. Animals do things like that, but not plants.

The hydra also sought light the way plants do. Trembley kept up pursuit. He cut one in two to see if a shoot would live. It did. Maybe it was a plant, but one that could walk. So he kept watch. Then one of these plants seized and ate prey like an animal. But if it was an animal, it reproduced asexually -- by budding. Some plants did that, but no known animals.

The importance of this plant/animal link was enormous. It took great tenacity for Trembley to see what was there, instead of what he expected to see. In the end, his sure observations opened the door to a far more sophisticated level of thinking in biology. As a part of this work, Trembley was first to isolate protoplasm and to realize it was a common building block of life.

Trembley published his findings in 1744. It was a beautifully illustrated volume whose impact was powerful. It's a testament to its revolutionary character that it troubled some thinkers. He blurred the plant/animal boundary, and he'd raised theological questions. What, for example, happened to an animal's soul if you could split it in two without killing it?

Oddly enough, we've nearly forgotten Trembley today. Two factors have consigned him to oblivion. One is that he offered no body of theory -- only dazzlingly clear observations. The other is that he wrote in French. The mainstream of biological study passed into an English-speaking world in the 19th century. Most biologists simply stopped reading his work.

But Trembley had shown 18th-century biologists how to gaze clearly at a world ready to tell us things. He showed them how to suspend prejudice and see what was right before their eyes.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Lenhoff, H.M. and Lenhoff, S.G., Trembley's Polyps. Scientific American, April, 1988, pp. 108-113.