A. Lambert. (2010)cite arxiv:1009.0118
Comment: 16 pages, 4 figures. To appear in Journal of Mathematical Biology.
The final publication is available at http://www.springerlink.com.
W. Feller. Proceedings of the Second Berkeley Symposium on Mathematical Statistics and Probability, 1950, page 227--246. Berkeley and Los Angeles, University of California Press, (1951)