Proceedings of the National Academy of Sciences of the United States of America. 06/05/2001, Vol. 98 Issue 12, p6680. 6p. 6 Diagrams, 3 Charts, 7 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 06/05/2001, Vol. 98 Issue 12, p6674. 6p. 8 Diagrams, 1 Chart, 1 Graph.