In my introduction to session 1, I made the point that most of the material presented in this session could be seen almost as a continuation of the corresponding part of the Oxford symposium in 1961. This continuity, which was very noticeable in other sessions also, is one of the assets and attractions of these symposia. There are people (and I have met several) who think that such a continuity signifies lack of ideas, a state of stagnation. They argue that we must be ‘with it’, that there must be something new, even topical, every time. I do not agree with this at all. On the contrary, if we let ourselves be dominated by the notion that there must be a frantic search for a new ‘in’ subject every time we plan a symposium, we are in danger of becoming organisers of yet another set of technical gatherings, useful, most likely, but not necessarily bearing the stamp of fundamental research.