The possibility must be borne in mind that the use of thyroid and other gland extracts that have a definite effect on metabolism is not devoid of risk to cells that are gradually undergoing involution; for, by stimulating metabolism unduly, the existing vitality may be prematurely exhausted and, though for a time there is a gratifying show of energy, it is but a flash in the pan that precedes a more rapid loss of function. We are irresistibly reminded that “no man putteth new wine into old bottles: else the new wine doth burst the bottles.” At the present time it would be unwise to express a dogmatic opinion about the relation of the endocrine glands to normal old age. That old age is due to endocrine insufficiency or loss of balance cannot be regarded as proved, and it is probably safer to regard changes, such as atrophy, in the various glands of internal secretion as concomitant with, rather than causal of, those in the senescent body. METCHNIKOFF’S PATHOLOGICAL EXPLANATION As an alternative to the belief that there is an inherent quantum of vitality which may be harmfully or beneficially influenced by environment and the wear and tear of life, it has been suggested that the process of ageing depends solely on external factors and that either there is no intrinsic limitation to the life of the organism or that the possibility is entirely cast into the shade by predominance of extrinsic influences. This doctrine is free from any tinge of fatalism and has the advantage that it is a direct stimulus to efforts in the direction of hygiene in all its forms; for as things are now old age owes its discomforts to superadded and in most instances avoidable complications. Élie Metchnikoff, as is well known, attributed the senile accompaniments of advanced years to pathological and preventible causes, namely toxaemia induced by alcohol, syphilis, and especially by bacterial activity in the colon which in common with Barclay-Smith and Arbuthnot Lane, he regarded as a harmful phylogenetic relic, this point of view being expressed by the epigram “the longer the colon the shorter is life.” He considered that the putrefactive bacteria in the colon produce phenol, indol, skatol, and aromatic bodies which cause degenerative changes in the cells of the body; and that the more resistant macrophages, which do not attack healthy tissues, absorb the damaged cells. He therefore employed means to prevent excessive bacterial activity in the large intestine, and in addition to care in diet so as to diminish the risk of introducing bacteria into the alimentary canal, he advocated the destruction of putrefactive bacteria in the colon by means of sour milk and cultures of _Bacillus bulgaricus_, which form lactic acid and thus render the contents of the bowel acid and unsuitable for the growth of the harmful micro-organisms. Metchnikoff thoroughly practised his doctrine, but he did not begin his regime until he was well over fifty, and, in spite of