Life at the Edge of Chaos?

Darwinism Evolving

by David J. Depew, by Bruce H. Weber
MIT Press, 588 pp., $49.95

Darwinism Evolving is a history of ideas about biological diversity and evolution, from Aristotle to the present day. The last part of the book is an account of some recent developments, and an attempt to forecast the future. Most of this review will be concerned with the final section, which seems to me mistaken. I must therefore start by saying that I found the historical part well informed, and full of valuable insights. The ideas discussed are fundamental, not only to biology, but also to our view of our relationship to the rest of the natural world. The last twenty years have seen an explosion of scholarship centered on Darwin, by historians and philosophers. The book is an admirable summary of, and addition to, that scholarship.

The historical thesis can be summarized by saying that Darwinism has a central core, the idea of natural selection, but that it has undergone three stages of evolution, according to the dynamic theories used to formulate it: first, the deterministic dynamics of Newton, then the probabilistic dynamics of Clerk Maxwell and Ludwig Boltzmann, and finally the dynamics of complex systems now being developed. Darwin himself, the authors suggest, formulated his theory in Newtonian terms. When I first met this claim, I was doubtful. Of course Darwin, like many of his contemporaries, wished to be seen as following the Newtonian method: Who would not claim to be following such a successful example. But how can one have “dynamics” without any mathematical equations? After reading their account, however, I was persuaded that the claim is reasonable, provided that one interprets dynamics in what Darwin himself might have called a loose and metaphorical sense.

The essential difference between Aristotle and Newton is that Aristotle thought that bodies move as they do because it is natural for them to do so, whereas Newton explained the elliptical orbits of planets as caused by an external force, gravity. A similar contrast exists between Lamarck and Darwin. Lamarck held that organisms evolve because they have an inherent tendency to become more complex. It was this idea that Darwin was rejecting when he said his theory had nothing in common with Lamarck’s. Instead of explaining evolution by an inherent tendency, Darwin thought that change was directed by an external force, natural selection.

Thus I think the authors make a good case for the claim that Newton is to Aristotle as Darwin is to Lamarck. Later in the nineteenth century, Newton’s deterministic dynamics was replaced in some fields of physics by the “stochastic” (i.e., probabilistic) dynamics of Maxwell and Boltzmann. They showed that the behavior of large aggregates of things (initially gases, which are aggregates of molecules) could be predicted by a dynamics which ignored the precise behavior of individuals, and took into account only the average behavior. Boltzmann once wrote that the nineteenth century should be seen as the century of Darwin, because Darwin explained evolution according to the chances of death or survival of millions of individuals. Boltzmann …

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