1. Seal, H., Multivariate Statistical Analysis for Biologists (Methuen, London, 1964).
2. Sokal, R. R., and Sneath, P. H. A., Principles of Numerical Taxonomy (Freeman, San Francisco and London, 1963).
3. Macnaughton-Smith, P., Biometrics, 19, 364 (1963).
4. Williams, W. T., and Dale, M. B., Adv. Bot. Res., 2 (in the press).
5. Goodall, D. W., Nature, 203, 1098 (1964).