In revising this translation of Metchnikoff’s book for the English-speaking public I have had to content myself with seeing that the plain meaning of the French was transformed to plain English, and that references to French editions were changed, so far as was possible, to corresponding references to English editions. Some of the phrases that recur were difficult to express. “Human nature” for instance is not an exact equivalent of _la nature humaine_, for the latter phrase has a complete significance, and very definitely implies not only the mental qualities of man, but his bodily framework, with its inherited and acquired anatomical structure and physiological functions. The phrase “human constitution,” especially in the common medical sense, carries more of the meaning, and I have used it occasionally. The word “harmony” means harmony with the environment, and disharmony is want of harmony or imperfect adaptation to the existing environment. In the case of the human organism, which has passed through profound changes at a rate prodigious in the history of evolution, many parts of the constitution are no longer in gear with the existing environment, and it is in such disharmonies that Metchnikoff finds the source of the troubles that have perplexed mankind. In several parts of this volume, and particularly in the chapter dealing with disharmonies in the reproductive functions, there is much plain speaking on matters that modern civilisation attempts to conceal. I have not had the impertinence to suppress or to alter a line or a word of these pages. They are written in high seriousness on fundamental facts of the constitution of man; they relate to problems and difficulties that every age in the history of man has had to face, and that are dealt with in the plainest language in the books of all the religions. For the first time proper knowledge has been brought to the task, and it is to be remembered that this volume is an attempt to explain mysteries of the flesh and of the spirit of which all existing explanations have failed to satisfy humanity. The volume is avowedly no more than a preliminary statement, a rallying-point for the work of future generations. But it awakens a new hope for humanity now that the old are fallen dumb; as Metchnikoff himself says, “If it be true that man cannot live without faith, this volume, when the age of faith seemed gone by, has provided a new faith, that in the all-powerfulness of science.” In every country, the new Order of priests of science, in the vigils of the laboratory, is working for the future of humanity. P. CHALMERS MITCHELL. PREFACE