While studying therapeutics and pathology under Professor Giessler, of Zurich, shortly after my return to Europe, I took up the subject of longevity, as to which Giessler had collected much curious information, and formed certain theories, one being that people of sound constitution and strong vitality, with no hereditary predisposition to disease may, by observing a correct regimen, easily live to be a hundred, preserving until that age their faculties virtually intact--in other words, only begin to be old at a hundred. So far I agree with him, but as to what constituted a "correct regimen" we differed. He held that the life most conducive to length of years was that of the scholar--his own, in fact--regular, uneventful, reflective, and sedentary. I, on the other hand, thought that the man who passed much of his time in the open air, moving about and using his limbs, would live the longer--other things being equal, and assuming that both observed the accepted rules of health. The result of our discussion was a friendly wager. "You try your way; I will try mine," said Giessler, "and we will see who lives the longer--at any rate, the survivor will. The survivor must also publish an account of his system, _pour encourageur les autres_." As we were of the same age, equally sound in constitution and strong in physique, and not greatly dissimilar in temperament, I accepted the challenge. The competition is still going on. Every New Year's day we write each other a letter, always in the same words, which both answers and asks the same questions: "Still alive?" If either fails to receive his letter at the specified time, he will presume that the other is _hors de combat_, if not dead, and make further inquiry. But I think I shall win. Three years ago I met Giessler at the meeting of the British Association, and, though he denied it, he was palpably aging. His shoulders were bent, his hearing and eye-sight failing, and the _area senilis_ was very strongly marked, while I--am what you see.