This is the Linacre lecture delivered at St. John's College,
Cambridge, in 1922. Experiments in rejuvenescence have lately attracted much attention, and it is possibly for this aspect of the subject that Sir Humphry Rolleston 's treatise will most interest the general reader. In many protozoa and in some primitive organisms it has been observed that death through senility need not occur. In an organism which propagates by division it is possible to continue life indefinitely without conjugation. In such cases, however, endomixis or internal reorganization of the cell must always take place. In man' with the heightening of individuation, the proto- plasmic cells have increased in stability, have become, so to say, rigid experts. It is thus improbable that any measure for the prolongation of life can come from an artificially induced reorganization of the cell. But few deaths occur from senility. Sir Hurnphry Rolleston regards a hundred years as the natural term, and deaths from senility should occur only at such an age. No rules for longevity can be given, but, apart from the influence of heredity (and there is undeniable evidence that some families are longer-lived than others), it is obvious that length of life depends mainly upon healthy environment, avoidance of disease, and a cultivation of good habits of life. Alcohol is a cell-poison, and over- indulgence in alcohol certainly shortens life ; but there is no evidence that smoking has any evil effects. It is interesting to observe from statistics that daring the past century the "expectation of life" has much improved.
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