Foreword. In summarizing this symposium, I expressed to our hosts, the Office of NavalResearch and the University of Pennsylvania, our deep appreciation for their hospitalityand for what all agreed was the great success and stimulation of this meeting. It wasrather good to see Government, and particularly military science, tied up with civilianscience. It was also good to see the applied so deeply woven with the theoretical. We wereconcerned, of course, on the one hand, with the enormously important specific problemsof the antibiotics, herbicides, and insecticides, with drug resistance and addiction, withthe development of resistance to epidemics due to bacteria and other organisms. Yet, onthe other hand, these very practical matters do reduce, I think, to the general theory ofthe interaction of systems with their environment over the course of time; and this is aproblem of microevoluition. So we were dealing, for those 2½ days, with modern experiments in the general field of evolution.