The principal causes which bring about _premature_ senility are: alcoholism, chronic poisoning by intestinal microbes, and infectious diseases, headed by syphilis. Surely Science will discover efficacious means against all these. The strengthening of the beneficent cells in our organism; the transformation of the wild intestinal flora into a cultivated flora, by the introduction of useful microbes; the struggle against infectious diseases and alcoholism--all these are workable means of fighting pathological and premature senility. When old age becomes physiological and no longer painful it will become proportionate with the other epochs of our lives and cease to alarm us. But how is the fear of death to be explained, since it is a general and inevitable phenomenon? How is it that we have no _natural instinct_ for death? Metchnikoff supposes that this lack of harmony in our nature comes from the fact that death is as _premature_ as senility and arrives before the _natural instinct for it_ has had time to develop. This supposition is confirmed by the fact that old people who have reached an exceptionally advanced age are often satiated with life and feel the _need_ of death as we feel a need of sleep after a long day's work. That is why we have a right to suppose that, when the limit of life has been extended, owing to scientific progress, the instinct of death will have time to develop normally and will take the place of the fear which death provokes at the present day. Both death and old age will become physiological and the greatest discord in our nature will be conquered. Our manner of life will have to be modified and directed according to rational and scientific data if we are to run through the normal cycle of life--_orthobiosis_. The pursuit of that goal will even influence the basis of morals. Orthobiosis cannot be accessible to all until knowledge, rectitude, and solidarity increase among men, and until social conditions are kinder. Man will then no longer be content with his natural inheritance; he will have to intervene actively in order to correct his disharmonies. "Even as he has modified the nature of plants and animals Man will have to modify his own nature in order to make it more harmonious." In order to obtain a new race, one forms an ideal in relation to the organism to be modified. "In order to modify human nature, it is necessary to realise what is the ideal in view, after which every resource of which Science disposes must be taxed in order to obtain that result. If an ideal is possible, capable of uniting men in a sort of religion of the future, it can only be based on scientific principles. And if it is true, as is so often affirmed, that it is impossible to live without faith, that faith must be faith in the power of Science." In those words, Metchnikoff ends his book on Human Nature. * * * * *