Although it has been possible to support my conception by new facts, some of which have been established by my fellow-workers, others by myself, there still remain many sides of the subject where it is necessary to fall back on hypotheses. I have accepted such imperfections instead of delaying the publication of my book. Even at present there are critics who regard me as incapable of sane and logical reasoning. The longer I postpone publication, the longer would I leave the field open to such persons. What I have been saying may serve also as a reply to the remark of one of my critics, that my ideas have been “suggested by self-preoccupation.” It is, of course, quite natural that a biologist whose attention had been aroused by noticing in his own case the phenomena of precocious old age should turn to study the causes of it. But it is equally plain that such a study could give no hope of resisting the decay of an organism which had already for many years been growing old. If the ideas which have come out of my work bring about some modification in the onset of old age, the advantage can be gained only by those who are still young, and who will be at the pains to follow the new knowledge. This volume, in fact, like my earlier one on the “Nature of Man,” is directed much more to the new generation than to that which has already been subjected to the influence of the factors which produce precocious old age. I think that thus the experience of those who have lived and worked for long can be made of service to others. As this volume is a sequel to _The Nature of Man_, I have tried as much as possible to avoid repetition of what was fully explained in the earlier volume. Here I bring together the results of work that has been done since the publication of _The Nature of Man_. Some of the chapters relate to subjects upon which I have lectured, or which, in a different form, have been printed before. For instance, the section on the psychic rudiments of man appeared in the _Bulletin de l’Institut général psychologique_ of 1904, the essay on Animal Societies was published in the _Revue Philomatique de Bordeaux et du Sud-Ouest_ of 1904, and in the _Revue_ of J. Finot of the same year, whilst a German translation of it appeared in Prof. Ostwald’s _Annalen der Naturphilosophie_. The chapter on soured milk first appeared as a pamphlet, published in 1905. The substance of my views on natural death was published in June last in “Harper’s Monthly Magazine” of New York, while the chapter on natural death in animals appeared in the first number of the _Revue du Mois_ for 1906. I have to thank most sincerely the friends and pupils who have helped me by bringing before me new facts, or other materials; the names of these will appear in their proper places in the volume. I have not mentioned by name, however, Dr. J. Goldschmidt, whose continual encouragement and practical sympathy have made my work much easier.