With the kind permission of the chair, I dedicate this lecture to the memory of my father, Leland W. Parr, who died in December of 1973. For many years Dr. Parr was Professor of Bacteriology and Public Health at George Washington University Medical School, here in Washington. He was an accomplished research scientist and a dedicated and effective teacher. And he spent hundreds of hours in the committee rooms of the National Research Council and of the National Institutes of Health, in the service of his fellow scientists.