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Springer-Verlag has set up a progromme to publish books in fast-moving fields in biomedical science, going from manuscript to finished article in as little as 90 days. The Mammalian Molecular Clock by Simon Easteal, Chris Collet and David Betty (R G. Landes, Texas, $59, ISBN 157 059 177 6) is timely – and the topic is fast-moving. Molecular evolution is based on two related and controversial theories, the neutral theory and the molecular clock theory. They strongly support a molecular clock, at least in mammals (their field of study). This is a very useful introduction to a subject that has had an important impact on traditional biology.