Proceedings of the National Academy of Sciences of the United States of America. 03/14/2000, Vol. 97 Issue 6, p2579. 6p. 19 Black and White Photographs, 2 Diagrams, 20 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 06/23/98, Vol. 95 Issue 13, p7445. 6p. 1 Black and White Photograph, 12 Graphs.