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Bejan granted that no two trees -- like no two snowflakes -- are identical. Nature does not produce carbon copies. "Their similarities are far more important than their slight differences, just as cars may have different designs but they are all still cars."

On the plane ride home, he wrote down the constructal law: "For a finite size flow system to persist in time (to live) its configuration must evolve in such a way that provides easier access to the currents that flow through it."

That moment proved a turning point in his distinguished career. Bejan was already one of the world's most influential engineers -- to date he has published 460 scholarly articles and 23 books including "Shape and Structure, from Engineering to Nature" and "Thermal Design and Optimization." He has won the highest honors in his field, including the Max Jakob Memorial Award in 1999, which is considered the Nobel Prize of thermal science.

Since 1995, Bejan has devoted his energies to applying the constructal law to various phenomena. For example, one of the basic goals of any design is to accomplish the most work with the least energy. Now consider a bird, which expends energy in two ways: First, vertically, as it lifts and maintains itself in the air. Second, horizontally, as it battles air friction to move forward. Bejan's research -- which involves a great deal of math and physics -- found that birds are "designed" to achieve "an optimal balance" between these two types of energy.

He discovered the same thing studying how fish swim and animals run.

Scientists, of course, have long studied animal locomotion. But, Bejan noted, they have generally worked backward from observations in the field. They watch birds fly and fish swim, then develop explanations that fit those results. They didn't apply their conclusions about flying birds to swimming fish.

The constructal law makes that connection. It does not begin with observation but an idea -- the general tendency of things, if given freedom, to flow more easily. It then uses this single universal principle to predict a wide range of phenomena.

"This can have practical results," Bejan added, "as we see that the design goals of fish and birds can help people make better boats, airplanes and robots."

One more example: The constructal law predicts that flow systems try to balance the amount of time it takes to go slow and fast. For example, the amount of time it takes to draw the breath into your lungs -- a relatively long distance -- is the same amount of time it takes that air to fill the alveoli in the lungs -- a relatively short distance. This same dynamic is everywhere in nature. The time it takes rain to seep into the ground is equal to the time it spends flowing in the stream, which is the same as the time it spends in the river.

"This is a universal principle," Bejan says, "which can be used, for example, when designing roads. Ideally, the time we spend driving down our little streets should equal the time we spend on the access roads, which should equal the time we're driving on the highway."

Application beyond nature

While applying his law, Bejan has also worked to disseminate it. He has written books and papers on the subject and teaches a course on it at Duke. Through grants from the National Science Foundation he has organized symposiums to share his findings with scholars from around the world -- not just other engineers, but biologists, social scientists, economists and linguists.

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