Nature, May 6, 1999

Acknowledgements. We are grateful to H. Bertsch, A. Brush, A. Gaunt, P. MacCready, K. Padian, J. Ostrom, G. Spedding, and M. Warner for discussions, to P. Rollings for the illustrations and to S. Orell for editorial assistance. J. M. V. Rayner provided insightful comments on the earlier versions of the manuscript. Support provided by the Frank M. Chapman Memorial Fund of the American Museum of Natural History.

Correspondence and requests for materials should be addressed to P.B.

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