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Periodical. Levisohn, Reuben, and Sol Spiegelman. "Further Extracellular Darwinian Experiments with Replicating RNA Molecules: Diverse Variants Isolated under Different Selective Conditions." Proceedings of the National Academy of Sciences of the United States of America 63, 3 (July 1969): 805-811. Article. 7 Images.. Proceedings of the National Academy of Sciences of the United States of America