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separately, by prevention or treatment, and it is well known that Metchnikoff has made great advances in that direction. The most striking practical side of The Nature of Man, however, was the discussion of the cause and prevention of intestinal putrefaction. Metchnikoff believes that the inherited structure of the human large intestine and the customary diet of civilised man are specially favourable to the multiplication of a large number of microbes that cause putrefaction. The avoidance of alcohol and the rigid exclusion from diet of foods that favour putrefaction, such as rich meats, and of raw or badly cooked substances containing microbes, do much to remedy the evils. But the special introduction of the microbes which cause lactic fermentation has the effect of inhibiting putrefaction. By such measures Metchnikoff believes that life will be greatly prolonged and that the chief evils of senility will be avoided. It may take many generations before the final result is attained, but, in the meantime, great amelioration is possible. There remains the last enemy, death. Metchnikoff shows that in the vast majority of cases death is not natural," but comes from accidental and preventable causes. When diseases have been suppressed and the course of life regulated by scientific hygiene, it is probable that death would come only at an extreme old age. Metchnikoff thinks that there is evidence enough at least to suggest that when death comes in its natural place at the end of the normal cycle of life, it would be robbed of its terrors and be accepted as gratefully as any other part of the cycle of life. He thinks, in fact, that the instinct of life would be replaced by an instinct of death.

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Metchnikoff's suggestion, then, was that science should be encouraged and helped in every possible way in its task of removing the diseases and habits that now prevent

human life from running its normal course, and his belief is that were the task accomplished, the great causes of pessimism would disappear.

In this new volume, The Prolongation of Life, the main thesis is carried further, and a number of criticisms and objections are met. The latter, so far as they relate to technical details, I need say nothing of here, as Metchnikoff and his staff at the Pasteur Institute are the most skilled existing technical experts on these matters, but I cannot refrain from a word of comment on the brilliant treatment of the objection to the suggested amelioration of human life that it considered only the individual and neglected the just subordination of the individual to society. In the sixth Part of this volume, Metchnikoff discusses the relation of the individual to the species, society or colony, from the general point of view of comparative biology, and shows that as organisation progresses, the integrity of the individual becomes increasingly important. Were orthobiosis, the normal cycle of life, attained by human beings, there still would be room for specialisation of individuals and for differentiation of the functions of individuals in society, but instead of the specialisation and differentiation making individuals incomplete throughout their whole lives, they would be distributed over the different periods of the life of each individual.

As these lines are intended to be an introduction, not a commentary, I will now leave the reader to follow the argument in the book itself.

P. CHALMERS MITCHELL.

LONDON, August, 1907.

PREFACE

It is now four years since I wrote a volume, the English translation of which was called The Nature of Man, and which was an attempt to frame an optimistic conception of life. Human nature contains many very complex elements, due to its animal ancestry, and amongst these there are some disharmonies to which our misfortunes are due, but also elements which afford the promise of a happier human life.

My views have encountered many objections, and I wish to reply to some of these by developing my arguments. This was my first task in this book, but I have also brought together a series of studies on problems which closely affect my theory.

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 SudOuest 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.

Finally, my special thanks are due to Drs. Em. Roux and Burnet, and M. Mesnil, who have been so good as to correct my manuscript and the proofs of this volume.

PARIS, Feb. 7, 1907.

É. M.

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