These are only striking examples which show what the young man has accomplished. The same thing was true in other countries. Corrigan wrote his famous essay on the "Permanent Patency of the Aortic Valve" when he was only twenty-nine years of age, and the work for it had been done during the preceding three years, at a hospital in which there were beds for only six medical patients. Trousseau declared this the greatest medical work, from a clinical standpoint, that had ever been accomplished, and hailed young Corrigan as one of the masters of clinical medicine. He maintained that disease of the aortic valve should receive the name Corrigan's disease. Stokes, Corrigan's contemporary and friend in Dublin, wrote his little book on the stethoscope when he was not yet twenty-one, and at a time when the {16} distinguished clinicians of the day were all asking if these young men expected the old physicians to carry this toy about with them and use it for any serious purpose. Graves, also of the Irish school of medicine, made some of the clinical observations on which his reputation is founded, including a short description of characteristic cases of the affection that still bears his name, when he was well under thirty-five. Further examples might well be cited, but they will be met with in the course of this book. The history of most of the sciences is like medicine in this respect, and it is to young men that the great ground-breaking ideas come. How true this is in biology can be noted even from the lives of the physician-biologists that are included in this volume. Theodore Schwann, the father of the cell doctrine, did all the work for which he deserves the name of founder of modern biology when he was scarcely more than thirty. Part of the best of it was accomplished before he was twenty-five. Claude Bernard had shown the precious metal of his originality before he was far on in his twenties. Pasteur, the most original genius of them all, began his work when he was scarcely more than a boy, and though every five years of a long life was filled with original observations of the most precious kind, his genius had received the bent which it was to follow from the successful accomplishment of observations during his third and fourth decades.