One of the first manifestations of senility being the whitening of hair, he began to study the mechanism of that. He had previously observed the dominant part played by phagocytosis in all phenomena of atrophy, and it occurred to him that it may be phagocytes which destroy the colouring matter of hair, a substance which, in the form of tiny granules, is enclosed within the hair cells. In fact, he found that the whitening process is accompanied by a stimulation of the amoeboid cells which introduce their protoplasmic prolongations into the periphery of the hair. They absorb the coloured granules, or pigment, and digest it, partly on the spot, partly after carrying it into the root of the hair, often even in the connective tissue which supports the hairy scalp. As the pigment becomes destroyed, the hair loses its colour and whitens. The cells which devour the pigment--pigmentophages--belong to the category of macrophages which, in general, absorb all the enfeebled cells in the organism. Metchnikoff was able to note similar phenomena in divers other senile atrophies either by his own ulterior researches or by collaboration with his pupils (MM. Salimbeni and Weinberg). In the same way that the whitening of the hair depends on the destruction of pigment by pigmentophages, the wrinkles of the skin, weakness of the muscles, friability of the bones, and senile degenerescence of divers organs are caused by the destruction of weakened cells which do not defend themselves and thus become the prey of the stronger and more resisting macrophages. Senility is thus no other than a generalised atrophy. What is it that provokes it? The answer is: The swarming microbes in our large intestine. They form the permanent source of a slow poisoning of our organism. This fact alone suffices to explain one of the principal causes of the enfeebling of our tissues. It is not simultaneous in all the cells because of their different powers of resistance. The struggle and destruction of the weak by the strong is the cruel law of nature; therefore the macrophages, more resisting to poisons, take advantage of the weakening of other cells in order to devour them, and this is one of the causes of senility. These reflections and the biological researches which confirmed them allowed Metchnikoff gradually to build up a philosophical doctrine, which he expounded in 1903 in his work, _Études sur la nature humaine_.