Proceedings of the National Academy of Sciences of the United States of America. 05/08/2001, Vol. 98 Issue 10, p5649. 6p. 1 Diagram, 1 Chart, 23 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 08/29/2000, Vol. 97 Issue 18, p10038. 6p. 17 Color Photographs, 13 Black and White Photographs, 6 Graphs.