7. Metchnikoff[309] has furnished the scientific facts for our understanding of senescence. He has demonstrated that the changes in tissue which give rise to phenomena of senility are due to the action of phagocytes. Thus the ganglion cells are altered (digested?) and destroyed by “neuronophags” and this is the main cause of mental senility. Definite phagocytic cells, the osteoclasts, slowly dissolve the bones (by the excretion of an acid?) and this leads to the known fragility of the bones in old age. The whiteness of the hair is due to the action of phagocytes; in the muscles in old age the contractile elements are destroyed by the sarcoplasm, and so on. It agrees with these facts that where organs are absorbed in the embryonic development of an animal, as _e. g._, the tail of the tadpole in metamorphosis, the phenomenon is due to a process of phagocytosis (and autolysis). We have mentioned the fact that in the larva of the _Amblystoma_ the absorption of the gills and of the tail occurs simultaneously and that both must be caused by a constituent of the blood. Such a constituent may be responsible for phagocytosis and autolysis in the organs undergoing absorption. Metchnikoff calls attention to the fact that certain infectious diseases, _e. g._, syphilis, may bring about precocious senility; and he mentions also the senile appearance of young cretins which is due to the diseased thyroid. “It is no mere analogy to suppose that human senescence is the result of a slow but chronic poisoning of the organism.” He assumes that in man this poisoning is caused by the products of fermentation in the large intestine and that the micro-organisms responsible for these fermentations may therefore be regarded as the real cause of senility in man. Parrots which are long-lived birds have a limited flora of microbes in their intestine, while cows and horses which are short-lived in comparison with man possess an extraordinary richness of the intestinal flora. But, needless to say, it is not the quantity of microbes alone which is to be considered, the nature of the microbes is of much greater importance. [309] Metchnikoff, E., _The Prolongation of Life_. New York, 1907. Certain plants like the Californian _Sequoia gigantea_ may be considered as practically immortal since they live several thousands of years; other plants, the annuals, die after fructification. Metchnikoff quotes from a letter by de Vries that this author prolonged the life of _Œnotheras_ by cutting the flowers before fertilization. Under ordinary conditions the stem dies after producing from forty to fifty flowers, but if cutting be practised new flowers are produced until the winter cold intervenes. By cutting the stem sufficiently early the plants are induced to develop new buds at the base and these buds survive winter and resume growth in the following spring.