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This is the theory of evolution applied widely, linking such things as the actions of the immune system, bird calls and even the progress of science. Darwin Machines and the Nature of Knowledge, by Henry Plotkin, (Penguin, £7.99, ISBN 0 14 123092 0) assembles the chain. Adaptation is all, Plotkin argues in admirably limpid style. This is a pleasurable combination of case and clarity.