LONGEVITY, or length of days, is the prolongation of life to or beyond the standard duration. Generally speaking there is a rough correspondence between the bodily bulk and the span of life; thus the imagines of Ephemerids or dayflies after an hour or two of "aerial life devoted to love" die, whereas the tortoise may survive for one or even two centuries ; but this relation is not absolute, parrots, ravens, and geese live longer than many larger birds and than most mammals (Mitchell) ; some fish, such as salmon ( oo years), carp ( 15o) and pike (20o), have a longev ity contrasting with the 3o years of horses. Trees are constructed on an entirely different plan from that underlying the complex higher animals, and are thus endowed with a kind of potential im mortality. The section of the trunk of a mammoth tree (Sequoia gigantea) in the Natural History Museum, South Kensington, showing 1,335 rings, might have been as many years old.
Death, the termination of life, is avoided in the protozoa by the division of the individual into two, which thus start afresh, su that, as Weismann long ago argued, the organism is immortal. But in the higher grades of the animal kingdom such rejuvena tion is impossible, and the processes of senescence and death must be regarded as the penalty to be paid for their higher differen tiation; they are dependent for their term of life on an inherited physicochemical constitution, and are thus, like a clock, set for a definite period. Since the time of Aristotle, the vital cycle has been thought to be a multiple of the period of growth ; Francis Bacon considered that as a rule animals should live eight times, Flourens five times, as long as they take to reach maturity; in the case of man both Buffon and Flourens, though on rather different grounds, estimated that a hundred years is the physi ological duration of life, and this has been widely accepted, in spite of the psalmist's "three score years and ten," with perhaps a sorrowful extension to four score.
In more modern times the number of reputed centenarians has been shown by critical enquiry to be much in excess of the real figure, and that the ages popularly ascribed to Henry Jen kins (169), Thomas Parr (152), Katherine, Countess of Desmond (140), and many of the 1,712 centenarians in James Easton's list covering the years A.D. 66 to 2799 cannot be accepted as authen
tic. Fallacies easily creep in and memories and records fail; thus in the census of 1911 the excess of persons alive over 91 and the deficit of those between 85 and 90 could only be explained by exaggeration of their age by old people. With the great ad vance in public health and the resulting fall in infant mortality the expectation of life at birth has risen in a remarkable manner; in 1854 the expectation of life, as given by the Registrar-General for England and Wales, was for males 39.9 years and for females years, whereas in 1922 the corresponding figures were 55.6 and 59.58 years. The expectation of life at later ages also shows an improvement, though much less; thus in 1854, males at the age of 6o had an expectation of 13.53 years and in 1922 of 14.36 years; for females the corresponding figures were 14.34 and 16.2 years. In America the expectation of life at birth for both sexes combined in the white population in 1910 was 51.49 years, and in 1920 had risen, being for males to 55.3 and for females to 57.52 years. The saving of life in infancy and early childhood, which to some extent means a survival of the most unfit, does not ensure prolongation of life at the other end of life; but this is not sur prising as death naturally becomes less preventable with the passage of years. Karl Pearson's investigations, made in 1902, into the ages recorded on Egyptian mummies showed that, though an individual aged 25 at the beginning of this century lived on an average 15 years more than an Egyptian 2,000 years before, the expectation of life after the age of 68 for an Egyptian at about the time of the Christian era was greater than that of an English man or woman of the same age at the beginning of this century. Thus in the case of these ancient people the influence of the survival of the fittest was obviously the factor concerned.