1. Burnet, F. M., Intrinsic Mutagenesis: a Genetic Approach to Ageing (Medical and Technical, Lancaster, 1974).
2. Higginson, J., Can. Cancer Conf., 40–75 (Pergamon, Oxford, 1968).
3. Higginson, J., and Muir, C. S., in Cancer Prevention and Detection, 1 (in the press).
4. Cairns, J., Sci. Amer., 233, 64–78 (1975).
5. Doll, R., Prevention of Cancer—Pointers for Epidemiology (Whitefriars Press, London, 1967).