1. Comfort, A., Nature, 217, 320 (1968).
2. Comfort, A., Ageing—The Biology of Senescence (Holt, Rinehart and Winston, New York, 1964).
3. Medawar, P. B., An Unsolved Problem of Biology (H. K. Lewis, London 1952).
4. Williams, G. C., Evolution, 11, 398 (1957).