On the preceding day, one who hardly knew my husband had come to ask him for information concerning his life, with the object of writing his biography. We were saying to each other how inevitably superficial and incomplete such a biography was bound to be; how difficult such a task is for a biographer, even when fully informed; how necessary it is to be thoroughly acquainted with a man and with every phase of his existence in order to give a truthful picture of his character and of his life. The intimate side is bound to remain more or less closed to a stranger; in order to decipher it, it is indispensable for the writer of a biography to have lived in complete communion of spirit with its subject. Our long past, spent together, fulfilled all these conditions. My husband's whole life was well known to me. My mother-in-law had often told me vivid stories of his childhood; he himself willingly talked to me about his past. As to the second part of his existence, we had lived it together. In order clearly to understand his character, at once both complex and one-sided, it was necessary to possess the key to his psychology. In his life, as in his work, everything was so closely knitted that it was impossible to understand the whole without knowledge of every link of his evolution. In the soothing calm of that summer evening, I submitted my reflections to him; he warmly encouraged me, and I then and there resolved to write his biography. He advised me to relate his whole life without any reticence, considering that thus alone does a biography justify its existence. That advice was to guide me, within limits, for to dissect an individual life without touching other lives as well is not always possible. Numerous were the difficulties before me; yet, I considered the task as a mission, hoping, in spite of all, that this biography would present a true picture of the life and evolution of Elie Metchnikoff. We talked over this project for a long time. The moon now appeared above the trees, the soft light tracing silver designs through the ivy leaves. The lawn, the walnut tree in front of the house, and everything around us was bathed in peaceful radiance. Under its mysterious charm, we ceased to speak, we listened to the inward voices of nature and of our own hearts. In youth, vague reveries fill our minds; after a long life, distant memories.... He whose life I describe is no more.... Without his help my task could not have been accomplished. Often, when he was not too tired, he would sit comfortably in his armchair and recount to me with his usual spirit and animation some period or episode of his past. I read to him a sketch of the first part of this biography and a few chapters only of the second, which was hardly begun. Thus we spent many evenings, never to be forgotten.