Popular perception notwithstanding, the theory of natural selection was accepted by every serious evolutionist long before Darwin. Earlier scientists interpreted it as the clearest possible evidence for intelligent design of the universe. William Paley's Natural Theology (1802), for example, employs the famous image of the "great watchmaker" to account for the perfect adaptation of creatures to harmonious ecosystems. Darwin's innovation, which may appear small but is in fact immense, lay in his claim that natural selection is the only cause of evolution.

In one sense, this was merely a change of emphasis: The impulse of pre-Darwinian evolutionists, faced with incontrovertible evidence of natural selection, had been to ask why it occurred. They sought after the "final cause" of evolution, and they found it in the proposal of an intelligent designer. But one of the essential principles of modern science is that such final causes are unknowable. Science must limit itself to "efficient" or "material" causes; it must not ask why things happen, but how. Darwin applied this principle to evolution. Whereas his predecessors had seen the adaptation of organisms to their environment as the effects of design, Darwin saw the physical development of creatures as the sole cause of evolution. The great watchmaker had been overthrown.

As Stephen J. Gould (who died as this issue was going to press) shows in The Structure of Evolutionary Theory, Darwin's breakthrough was essentially methodological. Darwinism is what you get when you focus on the micrological details, resolutely refusing to lift your eyes to the level of the whole. Over the course of the nineteenth century, this methodological sine qua non for scientific investigation was imposed on every discipline, but it originated in the "dismal science" of economics. The "political economy" of Adam Smith began from the material actions of individuals in pursuit of their own selfish ends, and extrapolated from this micrological level to abstract generalizations about the economy as a whole.

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