1. Camp, W. H., Brittonia, 7, 113 (1951).
2. Gilmour, J. S. L., Nature, 168, 400 (1951).
3. Gilmour, J. S. L., and Heslop Harrison, J., Genetica, 27, 147 (1954).
4. “Species Studies in the British Flora”, 160, 178 (Bot. Soc. Brit. Isles. London, 1955).
5. Crawshay-Williams, R., “Methods and Criteria of Reasoning” (Routledge and Kegan Paul, 1957).