1. T. Fagerström is in the Department of Theoretical Ecology, S-223 62 Lund, Sweden.
2. P. Jagers is in the Department of Mathematics, Chalmers and Gothenburg Universities, S-412 96 Gothenburg, Sweden.
3. P. Schuster is at the Institut fur Theoretische Chemie und Strahlenchemie, Universität Wien, A-1090 Wien, Austria.
4. E. Szathmary is at the Collegium Budapest, 1014 Budapest, Hungary.