1. C. Singer, A Short History of Scientific Ideas (Oxford, 1959).
2. Jean Heidmann, this volume, Chapter V.
3. G. Burbidge, Nature 233, 36–40 (1971).
4. J.-P. Vigier, this volume, Chapter XI.
5. H. Alfvén, Worlds-Antiworlds (Freeman, San Francisco, 1968); Sci Am. 1967 (April), 106.