Longevity. A term which in popular usage has come to mean great length of life instead of merely length of life; therefore, after a discussion of centenarianism, will follow a short account of the general theory.
The wide-spread belief that there are cases on record of persons living to the age of 150 or even 200 years, and that centenarians are numerous, is owing to a general love of the marvellous backed by superstition, and also to the fact that noted writers, such as Haller the physiologist, accepted and reasoned upon many such stories. But in 1862 Sir G. C. Lewis wrote in Notes and Queries an article in which he professed disbelief in any case of a life exceeding 100 years; he pointed out that neither the peerage and baronetage nor the books of insurance companies contained any evidence of such, and further that the current stories were nearly all of persons of humble rank, careless of registration, so that their statements could not be verified. This led to great correspondence in Notes and Queries and elsewhere: the editor, Mr Thoms, took the matter up, went into it with great care, and compiled his work on longevity which is authoritative. He examined many stories of very ancient persons, showing them to be baseless; while as to stock historical cases of Thomas Parr, Henry Jenkins, and the Countess of Desmond, reputed to be 152, 169, and 140 respectively, he found that there was no satisfactory evidence. For Jenkins there was none save his own assertion. Parr was before his death a celebrity, the poet Taylor wrote his life with numerous dates of various events, and Harvey in his post-mortem report repeats the popular hearsay—this is all the evidence to be found. As to the Countess of Desmond, Mr Thoms gives conclusive reasons for believing that the stories from which her age is deduced really relate to two, if not three, ladies of that title.
The evidence which is often said to exist in the registers has been proved in many cases to refer to two persons of the same name; and in one noted case, Carr of Shoreditch, said to be 207, the 2 was found to have been written upon the top of 1. As to tombstones, the age 309 in one case being certainly some village chiseller's manner of writing 39, will serve as an example. In fact, a review of the evidence shows that while Lewis was right in renouncing his contention that no certain instances of a greater age than 100 existed, a belief in lives of 150 years is no longer possible. It remains to add that there is no scientific evidence to support the belief that the length of human life was once much greater than it is in modern times, nor the converse opinion that the length of life has been increasing since the Psalmist cited it at three-score and ten. All that we certainly know is that in civilised countries the average length of life has been for many obvious reasons emphatically on the increase for several centuries.
There is another question of common interest: How shall we live to attain great age? There have been many teachers with many fads; but from the varied modes of life of those who have lived long it is probable that as no amount of feeding will make a man tall who is destined to be short, so no amount of care will prolong the life of one destined to die young. St Antony lives a life of excessive austerity and he dies at 105. Titian is all his life about a court and he paints a fine picture at ninety-six.
Statistics have been accumulated, and such general facts as that married people live longer than unmarried, that women live longer than men, and that the clergy have longer lives than other professional men have been established; but deductions from facts such as these are unsafe in the present state of science—the whole subject is too complex.
Turning now to the general question of the length of life of plants and animals, we may notice at the outset that the unicellular organisms cannot and do not die after the fashion of those in which death seems to be the necessary price paid for a 'body'; that increase of intelligence naturally tends to lengthen life; that perfecting of the reproductive processes has the same result; that many males live much longer than their mates, and so on. More than one popular adage makes size the criterion of longevity, but it is at most a partial factor; for while an elephant lives 200 years and a mouse only 6, carp and pike may attain the age of the former, and one of Sir John Lubbock's queen ants surpassed the mouse by almost 9 years. A horse often lives 40 years, but the donkey may exceed this, while both are outstripped by the golden eagle (60), by an almost centenarian captive raven, by a toad, or even by Sir John Dalyell's sea-anemone 'Grannie,' which, after its removal from the Firth of Forth, lived in an aquarium for 59 years—from 1828 till 4th August 1887. Flourens supposed that the length of life was five times the period of growth, but this does not hold even approximately for the majority of animals. Rapidity of life is another factor; thus the sluggish amphibian is long-lived, and trees may survive over 2000 years, in contrast to the transient life of many of our rapidly growing, brightly flowering annuals, or the yet more ephemeral existence of many intensely active insects. But on the other hand ants and bees often live for many years, and some of the most active birds attain a great age. According to Weismann, 'Duration of life is really dependent upon adaptation to external conditions; its length, whether longer or shorter, is governed by the needs of the species, and is determined by precisely the same mechanical process of regulation as that by which the structure and functions of an organism are adapted to their environment.' In other words, he maintains that the duration of life is fixed by natural selection; that, given the rate of reproduction and the average mortality, the length of life characteristic of any species is such that the numbers under fixed conditions will remain constant. His essay is suggestive, but natural selection is at present called upon to explain too much; for instance, he believes that that principle has determined that no creature shall long survive the period at which its reproductive activity ceases; but he does not seem to observe that a creature may not have sufficient energy for reproduction and yet quite enough to maintain life in an ordinary way for many years, as is notably the case with women. It may be noted that the unicellular animals in natural conditions probably never or hardly ever die a natural death; they may be eaten up, but, violence apart, they are virtually immortal; they divide, but in this there is no death. In fact, death probably began with the multicellular organisms, as the price paid for a body.
For general aspects of human longevity, see W. J. Thoms, Longevity of Man; G. M. Humphrey, Old Age; Burn Bailey, Modern Methuselahs. For general theory, see E. Ray Lankester, Comparative Longevity; and August Weismann, Essays upon Heredity (trans. by Poulton, &c.), which contain abundant references to other literature on the subject. See also INFUSORIA, PROTOZOA, REPRODUCTION, INSECTS; F. Hildebrand, Die Lebensdauer, &c. der Pflanzen, in Engler's Botan. Jahrbuch, Bd. ii. (Leipz. 1881); and, for further literature, Geddes and Thomson, Evolution of Sex (1889).