Proceedings of the National Academy of Sciences of the United States of America. 09/16/97, Vol. 94 Issue 19, p10341. 5p. 1 Black and White Photograph, 7 Graphs.
Proceedings of the National Academy of Sciences of the United States of America. 8/20/96, Vol. 93 Issue 17, p9120. 6p. 12 Color Photographs, 15 Black and White Photographs, 4 Graphs.