Elie Metchnikoff (d. 1916, _a.e._ 71), a bacteriologist and the successor of Pasteur, who approached the problem of old age from a very different angle and collected many interesting data, was led by his experiments and observations to a unique theory.[169] He first sets forth the disharmonies in the life of animals and especially of man in a way that seems pessimistic; but both his volumes are subtitled “optimistic studies” because he finds hope at the bottom of the Pandora casket. Old age, he thinks, is not due to loss of the power of somatic cells to divide or reproduce themselves but is “an infectious chronic disease, whether manifested by degeneration or an enfeebling of the nobler elements and by the excessive activity of macrophags,” the latter being large wandering cells represented by the white blood corpuscles and which he holds to be true phagocytes or scavengers, which, instead of protecting as they were meant to do, are very liable to turn on and destroy the higher elements of the body. They are thus like an army raised and sent out to destroy menacing savages that may turn and attack its own city. Old age and death, then, according to Metchnikoff, are not due, as Blütschli thought, to the exhaustion of some kind of vital ferment that protozoa and germ plasm have pre-eminent power to make; nor to the mere accumulation of waste, which the more always tend to dump upon the less vital elements of the body; nor, as Delboeuf conjectured, to the precipitation of the substance of organs, which always tend to revert to their inorganic bases; nor to Roux’s hypothesis that organs are always competing with each other for the available nutritive material, and that as and when there is not enough to satisfy all, those that have to starve drag down the rest; nor to the failure of the initial momentum given at impregnation; nor to the fact that at the senium the body has passed beyond the reach of the influences of sex and its products; but it is due to a very rank and variegated flora or fauna of noxious microbes, and especially to the toxic products they make, which tend to accumulate in the large intestine, making it thus a very cesspool or latrine of the most manifold infections.