mopped up much of the variation in metabolic rate that size alone could not explain. Correcting for temperature also brought different groups onto the same line: Reptiles’ relatively slower metabolic rates are a consequence of their lower body temperatures, showing that cold-and warm-blooded animals share fundamental metabolic processes. The same goes for plants and animals.

Like the network model, this is another simplification. The chemistry of metabolism involves many reactions, each of which will require a certain amount of energy to get going. Applied to metabolism, the Boltzmann factor is a black box. It could be a kind of average for all the hundreds of chemical reactions in metabolism, or it might be the energy needed to get over one crucial hump in the path. But including it reduces the size-adjusted variation in metabolic rates from a factor of 200 to a factor of 20.

Perhaps this remaining 20-fold variability is some indication of the wiggle room that organisms have that enables them to tinker with their metabolic rate to match their circumstances. Animals living in cold environments might crank their metabolic rates above the grand average, whereas plants in impoverished soils might depress theirs below it. Or perhaps it is a measure of how far organisms can deviate from the optimal network before they become so inefficient that natural selection weeds them out. This leftover variation, if you like, is what really needs to be explained—the place where biology will come into its own.

“I’d like for biology to have a sense of average idealized organisms that share similar principles, which can be understood mathematically—and that what you should be studying are the deviations from that,” says West. “At the end of this century there’ll be a fantastic theory that’ll predict lots of things—everything from ecology to how genes are turned on and off. And I’d like it if there were a footnote in the textbooks saying: ‘At the end of the twentieth century people started taking the problem seriously, and there were these guys who pointed the way to getting rid of the uninteresting part of the problem, the bit that depends on mass and temperature, that determines 90 percent of what we see. Now this real theory of biology is devoted to the other 10 percent.’”

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