1. Dept of Mathematics and Statistics, Hamilton Hall, Room 218; McMaster University; 1280 Main Street West Hamilton ON L8S 4K1 Canada
2. Dept of Biosciences; Univ. of Helsinki; PO Box 65 (Viikinkaari 1) FIN-00014 Helsinki Finland
3. Dept of Renewable resources; Univ. of Alberta; 751 General Services Building Edmonton AB T6G 2H1 Canada
4. Dépt de sciences biologiques; Univ. de Montréal; C.P. 6128, succursale Centre-ville Montréal QC, H3C 3J7 Canada