The problems, however, with which he deals are so extremely complicated that his work may really be said to have propounded more problems than he solved. He believed he had found and even named specific phagocytes attacking most, but not all, of the main tissues and organs of the body. Cohorts of them encamp about cells, very slowly absorbing their substance and depleting their energies--some attacking muscles; others, heart and arteries, etc.; others consuming the pigment cells of the hair which, however, as Pohl showed, continues to grow as rapidly in old age as in youth, as do the finger nails; others making the bones porous and brittle by removing the lime from them and transferring some of it to the walls of the arteries; some even specializing to attack brain and nerve cells. We must fight fire with fire, and to do this we must not only introduce the sugar-making bacteria but provide them with food _in situ_ in order that they may do their great work of purifying the cradle or breeding ground of noxious bacilli. Some of his disciples are still enthusiastic enough to believe that just as we purified the Panama Zone; as vaccination has almost annihilated smallpox, which once caused about one-tenth of all deaths; as Behring’s antitoxin has greatly abated the scourge of diphtheria; as Wright’s vaccine has lessened death from typhoid; as Ehrlich’s salvarsan treatment has done so much for syphilis, and as he and Wassermann hope may be done for cancer--so we may yet find and learn how to use a specific that, although it will not realize the dreams of those who once sought an elixir of life, will nevertheless contribute to its perhaps indefinitely great prolongation. This Metchnikoff does not hesitate to call “the most important problem of humanity.” His ideal is what he calls orthobiosis, which is “the development of human life so that it passes through a long period of old age in active and vigorous health leading to the final period in which there shall be present a sense of satiety of life and a wish for death.” Mere prolongation of life in the sense of Herbert Spencer is not in itself desirable. When the wish for death comes, he thinks that under certain circumstances suicide would be quite justifiable. Old age, he believes, will not only be greatly prolonged but will become optimistic. Pessimism he finds commonest among young men, while many avowed pessimists have become optimistic in their old age. Young men will not so precipitately attempt to displace the old, as he finds to be too much the case now, but the latter will attain greater power and influence.