This volume, together with a few others that are being published in this new series at about the same time, inaugurates a publishing program entitled Classics of Science. Each volume is to be a collection of the fundamental essays and other basic original articles in a certain field of science, presented in the sequence of its development, together with an introduction, commentary, and clarifying notes by the scholar responsible for the selection of the papers. All areas of science are to be included as the series grows. The titles of volumes commissioned for early completion range from The Discovery of Radio-activity and Transmutations to The Theory of Evolution (1830 to Modern Genetic Theory). Thus, in time, we shall have here a convenient network of roads to take us to the usually inaccessible sources of the great rivers of science, through its widely visible mountain ranges, its renowned battlefields, and its little-known but nevertheless choice vineyards.