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.
Fig. 4.—Henry Jenkins, reputed to have lived 169 years (1501–1670). Present at Battle of Flodden Field on September 9, 1513.
The authenticity of persons alleged to be of great age rightly excites critical examination, and some of the most famous examples, such as Henry Jenkins (169), Thomas Parr (152¾), Katherine, Countess of Desmond (145), as Sir George Cornewall Lewis and Mr. W. Thom showed, as well as many of the 1712 centenarians in James Easton’s[30] list covering the years A.D. 66 to 1799, cannot be regarded as established.[31] In the same category we must place Petrasch Zorten (1537–1724), whose portrait at the reputed age of 185 was reproduced by Sir John Sinclair.[32] Old people take a natural pride in their age and tend to exaggerate it; according to Mr. G. King[33] the excess in the census returns of persons over 91 and the deficit in those between 85 and 90 years of age could only be explained by the conclusion that the temptation to overstate the age so as to appear among the nonagenarians had not been resisted. The vast majority of centenarians are naturally among the bulk of the population, namely, the poor, among whom natural selection is provided with a correspondingly greater opportunity of finding those “vigorous frames which promise a long life,” and in these circumstances the means of checking the exact age may be less easy than in the case of the well-to-do.
Sex.—Females are more long-lived than males; the 1911 census for England and Wales shows that after 10 years of age there are more females than males living in all the quinquennial periods, the proportion of females progressively increasing until at the age of 85 and upwards there are 645 females to 355 males in 1000 living at that age group. Among centenarians also the ratio of the sexes is much in favour of that popularly said to be the weaker. Out of the 691 reputed centenarian deaths registered in England and Wales during the 10 years 1910–19 inclusive, 504, or 73 per cent, were females, and 187, or 27 per cent, males. But during the same period in Ireland the disproportion was much less: among the 945 reputed centenarian deaths 545, or 58 per cent, were females, and 400, or 42 per cent, males. It has been said, though without sufficient statistical evidence, that although the female sex is favourable to longevity extreme length of days is attained only by males. The superior longevity of the female sex, in spite of the risks attaching to child-birth, depends on several factors, such as an existence less exposed to accident and infection, a more temperate life, and in addition, so Sir George Humphry argued, on a stronger inherent vitality, for during the first year of life when there is no difference as regards the first two circumstances more boys than girls succumb. The higher male death rate in the first year of life, however, may be partly due to a mechanical cause, namely, the large size of the male infant’s head, as a result of which the numerical superiority of male infants born is reduced by the effects of trauma. Another factor is, perhaps, that old women do not feel so old as old men do, and that therefore auto-suggestion plays a less powerful part with them.
Fig. 5.—Thomas Parr. Reputed to have lived 152 years (1483–1635).
From a Print by Payne.
The question must arise as to what is the object of the prolongation of life after the reproductive function has waned as it does in women—the more long-lived of the sexes—about half-way through the physiological term of years. In some lower animals life terminates very shortly after propagation of the species has been effected; but this is not the rule, and the biological meaning of the continuation of the life of individuals that but cumber the ground and may lead a parasitic existence requires explanation, though educational advantages and moral lessons may undeniably be provided by healthy old age with its store of what a Cambridge man, now justly famous, described in his rather rebellious youth as “that greatly over-rated property experience.” It has been suggested by Sir Ray Lankester that the inherent property resulting in longevity may be bound up as a “correlated variation” with some other characteristic useful in the struggle for existence and the perpetuation of the species. An obvious possibility is that the long-lived have a higher fertility rate than average mortals. Hufeland[34] argued that as very old people are nearly always married more than once and generally at a very late period of life “a certain abundance in the power of generation is favourable to longevity.” There are few satisfactory data to determine the converse proposition with which we are now concerned, namely, is longevity accompanied by unusual fertility? As already mentioned, the great majority of centenarians are found among the bulk of the population, namely the poor, who are far more prolific than the well-to-do; but this does not prove much, for the inverse correlation of fertility and good social status is largely artificial and due to voluntary birth control. Among the 824 persons between 80 and 100 years of age analysed by Sir George Humphry[35] in 1889 there were 335 married men and 292 married women, and in each group the average number of children was six, which is somewhat in excess of what is popularly supposed to be the average size of a family; but the question of average fertility is one on which Dr. Major Greenwood warns me that it is difficult to give an unambiguous answer, for it all depends on whether completed or incompleted fertilities are meant; thus in the 1911 census the completed fertilities average 5·8 and the uncompleted 4·2. No value could, he considers, be attached to the general figure obtained by dividing the total number of children by the total number of couples enumerated. Hence Sir George Humphry’s and other statistics are not really comparable. While finding that longevity was equally shared by the single and married women and by those who had children and by those who were barren, Sir George Humphry stated that among those who had children fertility was associated with longevity.