The greatest difficulty in explaining science to ordinary people is that almost every part of it is becoming mathematical. The mathematics is not always very difficult. One of the studies which are rapidly becoming mathematical is that of evolution. Charles Darwin thought in words. The teeth of horses have been getting longer for some fifty million years. Their ancestors were browsers, that is to say they ate the leaves of trees, for which they only needed short teeth. Natural selection under controlled conditions has been done with populations of flies by Dubinin in the Soviet Union, by Dobzhansky in the United States, by Kalmus in England and above all by Teissier in France. One can study human beings with various inherited abnormalities and see how long on an average they live and how many children they have. The first place where mathematics comes in is in fixing the time scale. This can be done by analysing radioactive minerals.