Lectures delivered at the Institute of Statistics of the University of Bologna on 24th, 26th and 27th March 1962, and published in Italian in Statistica, April-June 1962 (N. 2, Anno XXI I). They are reproduced here by kind permission of Professor P. Fortunati, Director of that journal and of the Institute. English versions of the lectures have appeared in Our History, Winter-Spring 1963, and in Science and Society, Winter and Spring 1964 (Vol. XXVIII, Nos. 1 and 2) respectively, and are used with the consent of their editors.