Metchnikoff on his part never remained indifferent to his pupils. His solicitude towards them was warm, sometimes paternal, always ready and active. Many of his pupils remained his friends and collaborators for years afterwards. His fiery and exclusive temperament, however, made him take up a very different attitude in exceptional cases, when he found himself in front of one who persisted in a path which Metchnikoff himself considered the wrong path, or before an action which he thought disloyal or work done without conscience. Then he became beside himself, and positively dangerous to those who had exposed themselves to the paroxysm of his indignation.
Fortunately such cases were rare; as a general rule, the atmosphere of his laboratory was impregnated with scientific spirit and ardour; all forces in it converged towards the same goal, being bound together by a community of aspirations and activity of which he was the soul.
The first period of his life in France was taken up by the strengthening and development of the phagocyte theory and by an eager struggle in its defence. He displayed in it his full energy as a scientist and a fighter, and this was perhaps the most agitated, the most tense period of his life.
When at last his theory was securely established and began to be accepted, he continued his researches with the same passionate ardour but in an atmosphere of peace. It was joy and bliss to him to be able to work apart from other preoccupations, and the years of his life between fifty and sixty were the happiest he ever had.
The state of his soul and his ideas had considerably evolved in the course of years; the great moral and physical sensitiveness which had so often made him miserable in his youth had decreased and he had become much less impulsive. Unpleasant sensations no longer caused him so much suffering; he could bear the mewing of a cat or the barking of a dog; personal vexations no longer made him take such a horror of life as to wish to be rid of it: he now merely tried to conquer them.
At first this change operated less upon his ideas than upon his sensations and sentiments. Accustomed as he was to analyse his emotions, he realised the development within himself of a new sense of appreciation; less sensitive now to extreme impressions, he had become more so to ordinary ones. For instance, though less enchanted by music, and less irritated by discordant noises, he enjoyed absolute calm more fully. Now indifferent to rich food, which he formerly used to enjoy, he appreciated simple fare, bread and pure water. He did not seek for picturesque sites but took infinite pleasure in watching the growth of grass or the bursting of a bud. The first halting steps or the smile of an infant charmed and delighted him.
Demanding less from life, he now appreciated it as it was, and experienced the joy of mere living. The instinct, the sense of life had been born in him. He now saw Life and Nature under a different aspect from that which they had borne for him in his youth, for he had gradually acquired more balance; he had become adapted.
In their turn, his ideas evolved towards a more optimistic conception of life. His reflections, freed from the yoke of his juvenile sensitiveness, tended towards the possibility of a correction of the disharmonies of human nature through knowledge and will. This evolution had taken years. “In order to understand the meaning of life,” he said, “it is necessary to live a long time, without which one finds oneself in the position of a congenitally blind man before whom the beauties of colour are spread out.”