Finally, he progressed towards the last phenomenon, the most mysterious in nature, Death.
Researches on the silk-worm moth—a rare example of an animal the life of which ends in natural death—allowed him to conclude that the latter is due to an auto-intoxication of the organism.
But he only just raised the veil of the great mystery; it was his last work....
Metchnikoff’s philosophical evolution ran on parallel lines with his scientific researches.
When studying the laws and the unity of vital phenomena he found that their harmony was occasionally broken by the collision of internal conditions with the environment and that regrettable consequences ensued. He saw an example of that in human nature, full of disharmonies due to its animal origin.
These considerations caused the pessimism of his youth. But his energetic, pugnacious temperament could not remain content with a passive acceptance of facts.
He started to study the lack of harmony in human nature and its causes, and sought for means to combat these causes. Gradually he reached the conclusion that the greatest human disharmonies are provoked by the rupture of the normal cycle of our life, by the precocity of senility and of death, chiefly arising from a chronic poisoning by the toxins of intestinal microbes.
But having acquired the conviction that it is possible to struggle against that intoxication, he concluded that science, which has already done so much to fight diseases, would also find means of struggling against premature old age and precocious death, thus leading us to the normal vital cycle, orthobiosis.