We may be sure that the insane war which broke out in consequence of the lack of knowledge or of power of those who should have watched over peace, will be followed by a long period of calm. It is to be hoped that this unexampled butchery will, for a long time, do away with the desire for fighting, and that soon the need will be felt of a more rational activity. Let those who will have preserved the combative instinct direct it towards a struggle, not against human beings, but against the innumerable microbes, visible or invisible, which threaten us on all sides and prevent us from accomplishing the normal and complete cycle of our existence. The results acquired by the progress of the new medical science allow us to hope that, in a more or less distant future, humanity will be freed from the principal diseases which oppress it. After describing the state of medical science before Pasteur, Lister, and Koch, Metchnikoff compared with it modern medicine, created by these three Founders, and showed the great horizons opened by them to the medicine of the future. On the 26th of September 1914, whilst we were still in Paris, he had, in the laboratory, an attack of tachycardia, which lasted three hours but was much less violent than that of the year before. The winter, however, passed fairly well in spite of the emotions and continuous excitement caused by the war, and he had no other attack until April 1915, when again he had a slight tachycardiac crisis of a short duration. Yet he was very much changed: his hair was much whiter, his movements were slow, and his figure bent. His infectious gaiety and vivacity had disappeared, but he remained energetic and enthusiastic in his work, and gained more and more in serenity. Little children in the street called him "Father Christmas," and came confidingly to ask him for presents. They knew him well, and were aware that his pockets were always filled with sweets for them. He used to say that his growing love for children was the revelation of the grandfatherly instinct, for which he had reached the proper age. He especially loved one of his god-daughters, little Lili; he had become attached to the child on account of her kind heart and exceptional sweetness, and also because, from the cradle, she had shown a marked preference for him. And yet his love for children was not to him a source of joy, for anxiety on their account predominated over other feelings. In spite of the physical change which had supervened, his brain continued to work untiringly as in the past, and he tackled new problems with youthful courage and boldness. He had planned a work on the sexual question, which, according to him, was treated erroneously, with the result that grave disharmonies occurred in human existence.