Today, we wonder why we don't have wheels. 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.

So! Wheels are so great: why don't I have wheels? Why do I have legs? Animals move by creeping, leaping, walking, running, slithering, swimming, and flying. Some animals, and plants too, even move by rolling. But never on wheels. Why no wheels?

Once we found out how to make wheels, we went rolling off into the sunset with them. We took their use further and further into our lives. We made carts and chariots -- then potter's wheels, spinning wheels, gears, and water wheels. We made clockwork. We made machinery. Finally we made the automobile.

Our traveling machines roll, but they hardly ever walk. We've experimented with walking machines, but we haven't managed to make one that works. Wheels are the means of choice for moving about. Yet, if they're so fine, why has natural selection never fitted any living thing with wheels?

Stephen Jay Gould raises these questions. He points out that at one time wheels entered human history and then disappeared. Imperial chariots ranged North Africa and the Middle East for 2000 years. But when the Romans finally left, so did chariots. Wheels were almost forgotten until modern times. Camels took up the burden of transportation. They needed less manpower. One boy could handle six camels; but it took a drayer and several thirsty oxen to pull a cart. The camel was better suited to everyday needs in the arid lands. Wheels didn't fully return to the desert until there were engines to turn them.

When we look closer we find that the mechanics of legs and wings are so superbly complex that our technology simply hasn't mastered them. Maybe wheels are just a simple stopgap we've had to use because we can't design legs.

Maybe! But if we can't design legs, neither has nature ever made a wheel. A wheel poses what we call a topological problem. It cannot be attached to the axle about which it turns. It cannot share the blood supply or be controlled by its owner's nervous system. The best nature has done is to equip certain stomach bacteria with a kind of propeller. That propeller, or flagellum, as it's called, is so small it can take nourishment, and neural control, through its cell walls. That's out of the question for larger organisms.

Of course, nature has bred the wheel. Nature has bred the one animal clever enough to invent the wheel. Someday we'll master the wing and the leg as well. Only when we've learned to make both legs and wheels will we be able to talk about the superiority of one over the other.

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

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Gould, S.J., Hen's Teeth and Horse's Toes. New York: W.W. Norton & Co., 1984.