In order to spare his family the sorrow of an obvious suicide, he inoculated himself with relapsing fever, choosing this disease in order to ascertain at the same time whether it could be inoculated through the blood. The answer was in the affirmative: he became very seriously ill. His condition was aggravated by anxiety concerning the University; for he was sufficiently conscious to be aware of the events which were taking place in Russia. The murder of Alexander II. caused him to foresee a political reaction of the most terrible type; already, a reactionary Rector had been appointed. Metchnikoff developed intense jaundice and had a serious relapse with alarming cardiac weakness; during the crisis he had a very distinct prevision of approaching death. This semi-conscious state was accompanied by a feeling of great happiness; he imagined that he had solved all human ethical questions. Much later, this fact led him to suppose that death could actually be attended by agreeable sensations. His robust nature, however, triumphed over all these grave complications, and, during his convalescence, he was filled with a joy of living such as he had never experienced before; from that moment his moral and physical balance was completely restored. There was one unpleasant sequel to his illness, an acute affection of the sight (choroiditis), but it fortunately disappeared without leaving any traces, and, in fact, he never suffered again from his eyes, in spite of his constant use of the microscope. After his recovery he had a renascence of vital intensity; the life instinct developed in him in a high degree; his health became flourishing, his energy and power for work greater than ever, and the pessimism of his youth began to pale before the optimistic dawn of his maturity. However, the relapsing fever had very probably increased, if not started, the cardiac trouble which eventually caused his death. During the time when Metchnikoff was forbidden the use of the microscope on account of his eye weakness, he studied Ephemeridæ from the point of view of natural selection. He wished to elucidate the manner in which this selection operates during the very short life of those insects: the rudimentary structure of their buccal organs does not allow them to feed themselves, and they have no time to adapt themselves to external conditions.