So stealthy is the onset of senescence that commonly it is not recognized by its victim and, though he may seldom mention it, every man is firmly convinced that he not only looks but is at least ten years less than the register would tell him. Though unconscious of the change in himself he notes it with perhaps some self-congratulation in his contemporaries. It is only if he tests himself, for example by timing himself for a mile’s walk, that a healthy man of say 70 years has it borne in on him that he is not what he was. The information that he is old may be suddenly conveyed by overhearing the chance remarks of others, by catching a reflection of his bent back in a mirror, by tardy recovery from illness, or the advent of some disability, such as hypertrophy of the prostate or dyspnoea on holiday exertion. Perhaps the commonest warning is a feeling of fatigue. Sir Andrew Clark regarded the onset of old age as the period when a man ceased to adjust himself to his environment; but some persons never are able to do this, and misfits are not necessarily old. Laurentius divided old age beginning at 50 into the three stages of “the green because it is accompanied with prudence, full of experience, and fit to gouerne common weales,” the second at 70 years is “very cold and drie,” and the last, without a specified age incidence, is that of decrepitude corresponding to the famous description in the 12th chapter of Ecclesiastes. A more modern and comforting division is that of Lacassagne[41] of pre-senility, old age beginning at 60 or sometimes 70 in men, and in women 10 to 15 years earlier, advanced old age, and lastly decrepitude. Nascher[42] speaks of a senile climacteric about the latter part of the 7th or 8th decade--the transitional period between old age and decrepitude--corresponding to the changes at puberty and at the menopause. _Precocious old age_ due to the effects of disease, especially syphilis and acute infections, and to metabolic defects, which permanently damage the cells of the body, has a special interest as it supports Metchnikoff’s pathological view that old age as ordinarily seen is a result of toxic injury (_vide_ p. 82). Hastings Gilford[43] has specially investigated premature senility, and under the title progeria has described a remarkable condition of premature senility combined with, or secondary to, infantilism. This condition was independently described in 1910 by Variot and Pironneau[44] as the senile type of nanism. In such cases it would appear reasonable to ascribe these two opposite conditions to the same toxic or other factor, the infantilism depending more on damage to the ductless glands before development was complete, and the senile changes to direct injury of the cells in the body in general. IV FACTORS INFLUENCING LONGEVITY