Proceedings of the National Academy of Sciences of the United States of America. 11/07/2000, Vol. 97 Issue 23, p12513. 6p. 14 Diagrams, 1 Chart, 3 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 09/26/2000, Vol. 97 Issue 20, p10978. 6p. 9 Color Photographs, 8 Black and White Photographs, 7 Graphs.