Here was the realm of secular traditions, deeply rooted, and of theories generally admitted but resting on no biological basis. Attacks and objections against his theories came following upon each other with a rush, only to be compared with the racing clouds of a stormy sky or the hurrying waves of a tempestuous sea. An epic struggle began for Metchnikoff which was to last for twenty-five years, until the moment when the phagocyte theory, his child now grown up, emerged victoriously. To each attack, to each objection, he answered by fresh experiments, fresh observations annihilating objections; his theory was assuming a wider and wider scope, becoming more solid, more convincing.... But only his intimates knew how much the struggle cost him in vital force, what sleepless nights, due to continuous cerebral tension and to the effort to conceive some new and irrefragable experiment, what alternations of hope and depression.... In an ardent, stormy life such as this, each year counted for many. As soon as he arrived at the Pasteur Institute he undertook active researches with the object of developing and defending the phagocyte theory. By experiments on the _rouget_ of pigs he refuted the objections of Emmerich, who affirmed that, in that disease, the destruction of the microbes was not due to phagocytes. By experiments on the anthrax of pigeons he answered the attacks of Baumgarten and his pupils. To Behring, who affirmed that immunity was due to the bactericidal power of the serum, he replied by a series of experiments on the anthrax of rats. By all these researches Metchnikoff proved that recovery and immunity depended on the absorption and digestion of _living, virulent_ microbes by phagocytes. Natural or artificial vaccination by attenuated microbes allows the phagocytes to become gradually accustomed to digest more virulent ones, and this confers immunity upon the organism. That phenomenon is comparable to that by which we can accustom ourselves gradually to doses of poison which would be very harmful if taken at the start (arsenic, opium, nicotine, etc.). Little by little, the accuracy of Metchnikoff's observations began to be realised, and, moreover, other scientists supported him by their personal investigations. The part played by phagocytosis was becoming more and more evident and the question was ripening in France and in England, but in Germany it still met with great opposition. At the Berlin Congress in 1890 the theory was received very favourably by Lister, whilst Koch attacked it, trying to prove that phagocytes played no part in immunity, which, according to him, depended upon the chemical properties of the blood. Soon after that, Behring discovered antitoxins, and this seemed to favour the chemical or humoral theory of immunity. According to the latter, microbes and their poisons were rendered harmless by the chemical properties of the blood serum, properties similar to those of disinfecting substances.