I feel greatly honored, doubly so, in having this opportunity to deliver a Presidential Address. Twenty-one years ago (1947), I had the privilege of delivering the twelfth Charles Franklin Craig Lecture. Coincidentally, that also was here in Atlanta, and my theme was a somewhat philosophical arbovirus problem. The press described the agent I discussed as a possible “grandfather” virus. For a few of you who heard that lecture or read it, I will now briefly relate the then unknown final act. This concerned an isolate from mites of a bird's nest in California, with immunologic properties of A and B group arboviruses. It turned out to be a simple mixture of Western equine and St. Louis encephalitis viruses. Although artificial mixtures of laboratory-established viruses of these types separated spontaneously in early serial passage in several species of laboratory animal, this mixture of wild viruses did not, which suggested the interesting possibility that it might be a primitive stem virus, or that at some previous time one such might have existed in nature, which by mutation and selection could lose some antigens and become one of the many arboviruses now found.