He had arranged to take advantage of these holidays to execute work of which he had been thinking for a long time. As it has been said above, he thought that the life instinct was only developed gradually and produced at the same time an optimistic conception of life; he wished to verify this personal impression by the psychological evolution of divers other thinkers. He turned to Maeterlinck, as a representative of modern ideas. This author, mystical and pessimistic in his youth, had acquired in his maturity a far more optimistic conception of life. He himself explained this change by the influence of circumstances, but Metchnikoff saw in it a deeper cause, connected with the progressive evolution of the vital instinct which, by bringing equilibrium with it, suggests optimism. The study of Maeterlinck's works confirmed his opinion. Time flowed peacefully between rest and these occupations; at the end of the holidays, we congratulated ourselves on their result on my husband's health; on our return, his friends thought him looking well. Yet on the 19th October, about seven in the morning, he had a terrible cardiac attack without any apparent cause. I found him seated at his desk, and was terrified by his appearance; his lips were blue, and he was breathing with difficulty. And yet he was writing, and this is what he was writing: SÈVRES, _19th October 1913_, 7.45 A.M. This morning, after a good night, my heart was working well; I had from 58 to 59 regular pulsations. But, as I rose, I suddenly felt acute pain along the sternum; at the same time began a strong crisis of tachycardia. I had never in my life felt anything like it.... Here he had to stop as the crisis was becoming intolerable, but a few hours later he took up his pen again: _19th October_, 3 P.M. The crisis lasted till one o'clock (six hours' duration). There were times when the pain in the chest was unendurable. I was thirsty and drank hot, weak tea; I vomited; I felt wind in the stomach and the intestine. About noon the pain decreased, but the heart-beats were frequent and extremely irregular. I lunched in order not to alarm my wife, though I feared to aggravate the attack by filling my stomach.