The similarities among animals that are on the surface very different are no coincidence. In fact, animal locomotion is no different than other flows, animate and inanimate: they all develop in space and in time such that they optimize the flow of material.

The Earth with its solar heat input, heat rejection, and wheels of atmospheric and oceanic circulation, is a heat engine without [a] shaft. Its maximized mechanical power cannot be delivered, but is instead destined to dissipate through air and water friction and other forms of heat loss. It produces maximum power, which it then dissipates at a maximum rate.

The fact that the same proportionalities rule optimal running, flying and swimming is not a coincidence; rather it is an illustration of the fact that a universal principle is involved. Running requires the least food when during each cycle a certain amount of work is destroyed by vertical impact and a certain amount to horizontal friction. The same balancing act is responsible for optimal flight and swimming.

The water can only go up because the bottom and sides of the channel are rigid. That bulge, however undetectable, is the fish's footprint.

It was swimming that stumped us. Everyone knows that, in water, fish are weightless.

Running, swimming and flying occur in vastly different physical environments and, likewise, involve quite different body mechanics. Nonetheless, there are strong convergences in certain functional characteristics of runners, swimmers and fliers.