More modern estimates of the duration of human life under ideal conditions of protection against infection have been to the same effect, thus conforming to the estimate of Cornaro,[27] who died at that age, that a hundred years was “the time allowed to man by God and nature.” Metchnikoff,[28] while admitting that there must be variations, considered that man should live more than 100 years, and Luciani fixed the physiological duration of life at 100 years. Ebstein’s (1891) estimate of 70 years as the usual duration of life, which is the age at which the maximum number of deaths occur when the first few years of life are excluded, is different from the ideal duration under the best physiological conditions and corresponds more or less with B. W. Richardson’s rough rule that the individual’s probable age at death can be arrived at by taking the average of the ages of his parents and grandparents; in this procedure the important factor of heredity is taken into account, and the disturbing part played by accidents and infections in modifying the ideal expectation of life is to some extent neutralized by taking the average of the six lives. Although 100 years may be regarded as the physiological life of man it is rarely reached, and one of several reasons is that the physiological requirements for this apparently exaggerated term of years are very seldom provided. Sir Ray Lankester[29] suggested that just as giants in stature are exceptional variations so centenarians should be regarded in the same light, as giants in years instead of in inches. He meets the obvious objection that whereas the parents of giants are of normal height, longevity and centenarianism appear to run in families, by doubting if the actual quality of potential long life is transmitted and by ascribing family longevity to the inheritance of traditions and habits favourable to long life. In this connexion it should be remembered that many giants are pathological and due to acromegaly, whereas unusual height, over six feet, certainly runs in families; analogy therefore makes it reasonable to believe that length of days, like length in inches, may run in families, although these two approaches to giantism very seldom occur in the same individual. [Illustration: FIG. 4.--Henry Jenkins, reputed to have lived 169 years (1501–1670). Present at Battle of Flodden Field on September 9, 1513.]