In 1891, he wrote _The Law of Life_, in which we find the dawning idea that the lack of harmony in human structure does not make a happy existence and a rational code of morals impossible. Morals must consist "not in rules of conduct adapted to our present defective human nature, but on conduct based upon human nature modified, according to the ideal of human happiness." _The Flora of the Human Body_, published in 1901, is a study in which Metchnikoff's optimism assumes a definite form, for he speaks of the efficacy of certain means of struggling with our lack of harmony. The last chapter in the book, "A Conception of Life and of Medical Science," introducing the word Orthobiosis, strikes the optimistic chord, winged and conclusive, which must result from victory over the disharmonies of human nature. This is Metchnikoff's ultimate formula, summing up the problems of life and of morals: The ethical problem reduces itself to this: to allow the majority of human beings to reach life's goal, that is, to accomplish the whole cycle of a rational existence to its natural end. We are still very far from that. We can but sketch the rules to follow in order to attain this ideal. Its final realisation will demand more scientific researches, which must be allowed the widest and freest scope. It is to be foreseen that existence will have to be modified in many ways. Orthobiosis demands an active, healthy, and sober life, devoid of luxury and excess. We must therefore modify present customs and eliminate those extremes of wealth and poverty which now bring us so many evils. As time goes on, when Science has caused present evils to disappear, when men no longer tremble for the life and welfare of their dear ones, when individual life follows a normal course--then Man can attain a higher level and more easily devote himself to exalted goals. Then Art and pure Science will occupy the place which is due to them and which they lack at the present moment in consequence of our many cares. Let us hope that men will understand their true interests and contribute to the progress of orthobiosis. Many efforts are necessary, much self-sacrifice, but they will be attenuated by the consciousness of an activity directed towards the real goal of human existence. CHAPTER XXXIII First our pleasures die, and then Our hopes, and then--our fears, and when These are dead--the debt is due. Dust claims dust--and we die too. SHELLEY. Unpleasant incidents--The fabrication of lacto-bacilli--St. Léger-en-Yvelines--Return to Paris--First cardiac attack--Evolution of the death-instinct--Notes on his symptoms. The end of 1912 had some unexpected emotions in store for us.