There is no need for me to say how great an honour it is to be invited to give the Silvanus Thompson Memorial Lecture of the British Institute of Radiology and how inadequate I feel when I recall the names of the previous Memorial lecturers. It is also a very great pleasure to be asked to give this lecture in Southampton, the city of my birth, and in the University where I studied, though now very different in appearance from the University College I knew.Silvanus Thompson and my student days here are connected because, like so many others, it was his book that introduced me to the Calculus and also like many others I was greatly comforted by what appeared on the first page …“What one fool can do, another can”(Ancient Simian Proverb)though it was false comfort, since Silvanus Thompson was indeed no fool, but a man who illuminated any subject to which he turned his attention, be it science, biography or art.However, this philosophy has perhaps had more influence on me than I suspect, and without the conceit it inspired I could not possibly have taken as my topic the subject of the comparative cell population kinetics of normal and malignant tissues and its application to cancer therapy. I am also grateful for one other circumstance, that Professor Tubiana should have taken for the subject of last year's Mackenzie Davidson Lecture “The kinetics of tumour cell proliferation and radiotherapy” (Tubiana, 1971).