This is an edited record of the dialogue of eminent scientists attending the fourth conference in the series of the origins of life, supported by a grant from the Biosciences Program of the National Aeronautics and Space Admini­ stration. The first conference at Princeton, 1967, was held under the direc­ tion of Dr. Frank Fremont-Smith at the time when the Interdisciplinary Co m­ munications Program (lCP) was associated with the New York Academy of Sciences. In 1968, ICP was integrated into the Office of the Assistant Secretary for Science of the Smithsonian Institution, and the entire operation was set up in Washington, D. C. The second conference, also in Princeton, was held in 1968 and the third was in Santa Ynez, California in 1970. (See Margulis, ed. 1970, 1971 and 1972 for previously published proceedings.) The fourth conference was held at the Belmont Conference Center in Elkridge, Maryland, April 13-16, 1971. The proceedings are recorded and edited by the Interdisciplinary Com­ munications Associates, Inc. (lCA, a nonprofit foundation), for ICP. Dr. Lynn Margulis, who served as Scientific Editor for the entire series, has succeeded admirably in carrying out a difficult and usually inadequately appreciated task. She was ably assisted in her offices by Barbara Miranda and Harriet Eklund, the ICP Staff Editor for this series. I am grateful to all.