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LONGEVITY
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LONGEVITY , a See also:term applied to See also:express either the length or the duration of See also:life in any organism, but, as cases of See also:long duration excite most See also:interest, frequently used to denote a relatively unusual prolongation of life. There is no See also:reason to suppose that See also:protoplasm, the living material of organisms, has a necessarily limited duration of life, provided that the conditions proper to it are maintained, and it has been argued that since every living organism comes into existence as a piece of the protoplasm of a pre-existing living organism, protoplasm is potentially immortal. Living organisms exist, however, as particles or communities of particles of protoplasm (see LIFE), and as such have a limited duration of life. Longevity, as E. See also:Ray Lankester pointed out in 1869, for See also:practical purposes must be understood to mean the " length of See also:time during which life is exhibited in an individual." The word " individual " must be taken in its See also:ordinary sense as a wholly or partially See also:independent, organized See also:mass produced from a pre-existing organized mass, as otherwise the problem will be confused by arguments as to the meaning of biological individuality. Empirical Data.—A multitude of observations show that only a very brief life, ranging from a few See also:hours to a few days, is the normal See also:fate of the vast See also:majority of single-celled organisms, whether these be See also:animal or See also:vegetable or on the border-See also:line between the two kingdoms. See also:Death comes to them rapidly from See also:internal or See also:external causes, or the individual life ends in conjugation or See also:division or spore-formation. Under See also:special conditions, natural or artificial, the individual life may be prolonged by See also:desiccation, or freezing, or by some similar See also:arrest of functional activity. The duration of life among See also:plants is varied. The popular division into annuals, biennials and perennials is not See also:absolute, for natural and artificial conditions readily prolong the lives of annuals and biennials for several seasons, whereas the See also:case of perennials is much complicated by the mode of growth, and the problem of individuality, however we See also:desire to exclude it, obtrudes itself. In the vast majority of cases where a plant is obviously a See also:simple individual, its life is See also:short, ranging from a few days in the case of See also:fungi, to two seasons in the case of biennial herbs. Most of the simple See also:algae are See also:annual, their life enduring only for See also:part of the See also:year; the branching algae are more often perennial, but in their cases not only are observations as to duration lacking, but however simply we may use the term individual, its application is difficult. The larger terrestrial plants with woody tissues which we denote roughly as shrubs and trees have an individuality which, although different from that of a See also:hyacinth or See also:carrot, is usually obvious. Shrubs live from four to ten or more years, and it apparently is the case that odoriferous shrubs such as See also:sage and See also:lavender display the longer duration. Trees with soft See also:wood, such as poplars and willows, last for about fifty years, See also:fruit-trees rather longer. Estimates of the See also:age which large trees can attain, based partly on attempts to See also:count the annual rings, have been given by many writers, and range from about three See also:hundred years in the case of the See also:elm to three to five thousand years in the case of See also:Sequoia gigantea of See also:California, and over five thousand years in that of the See also:baobab (Adansonia digitata) of Cape Verde. It is impossible to See also:place exact reliance on these estimates, but it is at least certain that very many trees have a duration of life exceedingly See also:great in comparison with the longest-lived animals.
The duration of life amongst multicellular invertebrate animals is little known, except in the frequent instances where it is normally brief. Many See also:sponges and polyps See also:die at the end of the See also:season, leaving See also:winter eggs or buds. The much-branched masses of the larger sponges and See also:compound See also:hydrozoa certainly may be perennial. A See also:sea-See also:anemone (Actinia mesembryanthemum), captured in 1828 by See also:Sir See also: The variation in the length of life of molluscs appears to be great. Many See also:species of gastropods live only a few years; others, such as Natica laeros, have reached See also:thirty years, whilst the large Tridacna gigas is stated to live from sixty to a hundred years. Among See also:insects, the adult See also:stage has usually only a very short duration of life, extending from a few hours to a few months, but the larval stages may last much longer. Including these latter, the range of duration among insects, taking the whole life from hatching to death, appears to See also:lie between the limits of a few See also:weeks in the case of plant-lice to seventeenyears in the case of the See also:American See also:Cicada septemdecim, the larva of which lives seventeen years, the adult only a See also:month. Most butterflies are annuals, but those which fail to copulate may hibernate and live through a second season, whilst the lives of some have been preserved artificially for seven years. Worker bees and drones do not survive the season, but queens may live from two to five years. In the case of vertebrates, the duration of life appears to be greater among See also:fish and See also:reptiles than among birds and mammals. The See also:ancient See also:Romans have noted that eels, kept in aquaria, could reach the age of sixty years. Estimates based on size and See also:rate of growth have led to the inference that See also:salmon may live to the age of a hundred years, whilst G. L. L. See also:Buffon set down the See also:period of life of See also:carp in ponds as one hundred and fifty years, and there is See also:evidence for a See also:pike having reached the age of over two centuries. More recently it has been claimed that the age of fish can be ascertained exactly by counting the annual rings of the otoliths. No great ages have as yet been recorded by this method, whilst, on the other See also:hand, by revealing great See also:variations of See also:weight and size in fishes with the same number of annual rings, it has thrown doubt on the validity of estimates of age based on size and rate of growth. The evidence as a whole is unsatisfactory, but it is highly probable that in the See also:absence of accidents most fish can attain very great ages. The duration of life among See also:batrachia is little known, but small frogs have been recorded as living over twelve years, and toads up to thirty-six years. Almost nothing is known as to the longevity of See also:snakes and lizards, but it is probable that no great ages are reached. Crocodiles, alligators and caymans grow slowly and are believed to live very long. There is exact evidence as to alligators in caps tivity in See also:Europe reaching See also:forty years without signs of senescence, and some of the sacred crocodiles of See also:India are believed to be more than a hundred years old. Chelonians live still longer. A See also:tortoise has lived for eighty years in the See also:garden of the See also:governor of Cape See also:Town, and is believed to be at least two hundred years old. There are records of small See also:land-tortoises that have been kept in captivity for over a See also:century, whilst the very large tortoises of the Galapagos Islands certainly attain ages of at least two centuries and possibly much more. A considerable See also:body of See also:information exists regarding the longevity of birds, and much of this has been brought together by J. H. See also:Gurney. From his lists, which include more than fifty species, it appears that the duration is least in the case of small passerine and picarian birds, where it ranges from eight or nine years (See also:goat-suckers and swifts) to a maximum of twenty-five years, the latter age having been approached by larks, canaries and See also:goldfinch. Gulls have been recorded as living over forty years, ducks and geese over fifty years (the duchess of See also:Bedford has recorded the case of a See also:Chinese See also:goose having been in See also:possession of the same See also:family for fifty-seven years). Parrots frequently live over eighty years, swans nearly as long, ravens and owls rather less, whilst there is excellent evidence of eagles and falcons considerably exceeding a hundred years. Notwithstanding their relatively large size, struthious birds do not reach great ages. The records for cassowaries and rheas do not exceed thirty years, and the maximum for ostriches is fifty years, and that on doubtful evidence. Exact records regarding the longevity of mammals are surprisingly few. There is no evidence as to Monotremes. The life of Marsupials in captivity is seldom long; a See also:phalanger has lived in the See also:London Zoological Gardens and showed no signs of age at more than ten years old; it may be inferred that the larger forms are capable of living longer. Reliable records as to Edentates do not exist; those in captivity have short lives, but the size and structure of some of the See also:extinct forms suggests that they may have reached a great age. Nothing is known regarding the longevity of Sirenians, except that they do not live long in captivity. In the case of Cetaceans, estimates based on the growth of See also:whale-See also:bone assign an age of several centuries to whale-bone whales; exact records do not exist. More is known regarding Ungulates, as many of these are domesticated, semi-domesticated or are frequently kept in captivity. Great length of life has been assigned to the See also:rhinoceros, but the longest actual LONGEVITY See also:average duration of life and that centenarians occur more frequently amongst men than amongst most of the See also:lower animals. Theories of Longevity.—Ray Lankester has pointed out that several meanings are attached to the word longevity. It may be used of an individual, and in this sense has little importance, partly because of the inevitable variability of the individual, and partly because there may be individuals that are abnormal in duration of life, just as there are abnormalities in weight or height. It may be used for the average duration of life of all the individuals of a species and so be another way of expressing the average mortality that affects the species, and that varies not only with structure and constitution but with the See also:kind of enemies, accidents and conditions to which the members of the species are subject. If we reflect on the large incidence of mortality from external causes affecting a species and particularly the See also:young of a species, we shall see that we must conclude that See also:intrinsic, physiological causes can have relatively little weight in deter-See also:mining the average mortality rate. Finally, longevity may be used, and is most conveniently used, to denote the specific potential longevity, that is to say the duration of life that would be attained by normal individuals of a species if the conditions were most favourable. It is necessary to keep in mind these various applications of the term when considering the theoretical explanations that have been associated with the empirical facts. There is a certain relation between size and longevity. As a See also:general See also:rule small animals do not live so long as larger creatures. Whales survive elephants, elephants live longer than camels, horses and See also:deer, and these again than rabbits and mice. But the relation is not absolute; parrots, ravens and geese live longer than most mammals and than many larger birds. G. L. L. Buffon tried to find a more definite measure of longevity, and believed that it was given by the ratio between the whole period of life and the period of growth. He believed that the possible duration of life was six or seven times that of the period of growth. See also:Man, he said, takes fourteen years to grow, and his duration of life is ninety to one hundred years; the See also:horse has reached its full size at four years of age and may live for a See also:total period of twenty-five to thirty years. M. J. P. See also:Flourens attempted to make Buffon's See also:suggestion more exact; he took the end of the period of growth as the time at which the epiphyses of the long bones See also:united with the bones themselves, and on this basis held that the duration of life was five times the length of the period of growth. The theories of Buffon and Flourens, however, do not apply to all vertebrates and have no meaning in the case of invertebrates. Y. Bunge has suggested that in the case of mammals the period taken by the new-See also:born young to See also:double in weight is an See also:index of the rapidity of growth and is in a definite relation to the possible duration of life. M. Oustalet has discussed the existence of definite relations between duration of life and size, rate of growth, period of gestation and so forth, and found so many exceptions that no general conclusion could be See also:drawn. He finally suggested that See also:diet was the See also:chief See also:factor in determining the span of life. E. Metchnikoff has provided the most See also:recent and fullest See also:criticism and theory of the physiological causes of longevity. He admits that many factors must be involved, as the results vary so much in different kinds of animals. He thinks that too little is known of the physiological processes of invertebrates to draw any valid conclusions in their case. With regard to vertebrates, he calls See also:attention to the See also:gradual reduction of longevity as the See also:scale of life is ascended. On the whole, reptiles live much longer than birds, and birds than mammals, the contrast being specially notable when birds and mammals are compared. He dismisses the effect of the reproductive tax from possible causes of short duration of life, for the obvious reason that longevity is nearly equal in the two sexes, although See also:females have a much greater reproductive drain. He points out that the See also:hind-gut or large See also:intestine is least See also:developed in fishes, relatively small in reptiles, still small but relatively larger in birds and largest in mammals, relatively and absolutely, the caecum or caeca being reckoned as part of the hind-gut. The See also:area of the intestinal See also:tract in question is of relatively little importance in digestion, although a considerable record is that of an See also:Indian rhinoceros which lived for thirty-seven years in the London Zoological Gardens. The usual duration of life in the case of horses, asses and zebras is from fifteen to thirty years, but instances of individuals reaching fifty years are fairly well authenticated. Domestic See also:cattle may live from twenty-five to thirty years, See also:sheep and goats from twelve to fourteen years, antelopes rather longer, especially in the case of the larger forms. A See also:giraffe has lived for nineteen years in the London Zoological Gardens. Deer are reputed to live longer than sheep, and records of individuals at the London Gardens confirm this, but it is doubtful if they live as long as cattle. Camels are long-lived, according to repute, but actual records show no great age; a See also:llama which died in the London Gardens at the age of seventeen years showed unmistakable signs of senility. The See also:hippopotamus is another large ungulate to which great longevity has been assigned, but the longest actual record is the case of a See also:female born in the London Gardens which died in its thirty-fifth year. The duration of life assigned to domestic See also:swine is about twenty years; an Indian See also:wild See also:boar, alive in the London Zoological Gardens in 1910, and apparently in full vigour, was fifteen years old. Elephants are usually supposed capable of reaching great ages, but the actual records of See also:menagerie and military animals show that thirty to forty years is a normal limit. Facts as to rodents are not numerous; the larger forms such as See also:hares and rabbits may live for ten years, smaller forms such as rats and mice, for five or six years. Bats have a reputation for long duration of life, and tropical fruit-bats are known to have lived for seventeen years. No great ages have been recorded for See also:Carnivora, but the average is fairly high. Twenty-five years appears to be a limit very rarely exceeded by lions, tigers or bears; domestic See also:cats may live for from twelve to twenty-three years, and See also:dogs from sixteen to eighteen years, though cases of as many as thirty-four years have been noted. Less is known of the smaller forms, but menagerie records show that ages between twelve and twenty are frequently reached. There were in r9ro in the London Zoological Gardens, apparently in See also:good See also:health, a meerkat at least twelve years old, a See also:sand-See also:badger fourteen years and a See also:ratel nineteen years of age. Records regarding monkeys are unsatisfactory, for these creatures are notoriously delicate in captivity, and it is practically certain that under such circumstances they rarely die of old age. A See also:grey See also:lemur eleven years old and a See also:chimpanzee eleven and a See also:half, both in good health in the London Zoological Gardens, appear to be the See also:oldest See also:primates definitely recorded. Estimates based on size, See also:condition of the See also:skull and so forth obtained by examination of wild specimens that have been killed would seem to establish a rough See also:correspondence between the size of monkeys and their duration of life, and to set the limits as between seven or eight and thirty years.
With regard to the human See also:race, there seems to be almost no doubt but that the average duration of life has increased with See also:civilization; the generally improved conditions of life, the greater care of the young and of the aged and the advance in medical and surgical See also:science far more than outweigh any depressing effect caused by the more strenuous and See also:nervous activity required by See also:modern social organization. The expectation of life of those who attain the age of sixty varies with race, See also:sex and occupation, but is certainly increasing, and an increasing number of persons have a See also:chance of reaching and do reach ages between ninety and one hundred. Careful investigation has thrown doubt almost amounting to disproof on the much-quoted cases of great longevity, such as that of See also: The products of putrefaction are absorbed by the See also:blood and there results a See also:constant auto-See also:intoxication of the body which Metchnikoff believes to be the See also:principal See also:agent in senile degeneration. Mammals, if they See also:escape from enemies, diseases and accidents, fall victims to premature senility as the result of the putrefactive changes in their intestines, and the average mortality of the species is much too high, the normal specific longevity being rarely if ever attained. Metchnikoff urges, and so far probably is followed by all competent authorities, that improvements in the conditions of life, greater knowledge of disease and of See also:hygiene and simplification of habits are tending to reduce the average mortality of man and the domestic animals, and to bring the average longevity nearer the specific longevity. He adds to this, however, a more special theory, which, although it appears rapidly to be gaining ground, is yet far from being accepted. The theory is that duration of life may be prolonged by See also:measures directed against intestinal putrefaction. The See also:process of putrefaction takes place in masses of badly-digested See also:food, and may be combated by careful dieting, avoidance of See also:rich foods of all kinds and particularly of flesh and See also:alcohol. Putrefaction, however, cannot take place except in the presence of a particular See also:group of bacteria, the entrance of which to the body can be prevented to a certain extent. But it would be impossible or impracticable to secure a sterilized diet, and Metchnikoff urges that the bacteria of putrefaction can be replaced or suppressed by another set of microbes. He found that there was a widely spread popular belief in the See also:advantage of diet consisting largely of products of soured See also:milk and that there was a See also:fair parallel between unusual longevity and such a diet. Experimentally he showed that the presence of the bacilli which produce lactic See also:acid inhibited the process of putrefaction. Accordingly he recommends that the diet of human beings should include preparations of milk soured by cultures of selected lactic acid bacilli, or that the spores of such bacilli should be taken along with food favourable to their development. In a short time the bacilli establish themselves in the large intestine and rapidly stop putrefactive change. The treatment has not yet been persisted in sufficiently long by a sufficient number of different persons to be accepted as universally satisfactory, and there is even more difference of See also:opinion as to Metchnikoff's theory that the chief agent in senile degeneration is the stimulation of phagocytes by the products of putrefaction with the resulting destruction of the specific cells of the tissues. Metchnikoff, however, gave it to the See also:world, not as a proved and completed See also:doctrine, but as the line of inquiry that he himself had found most promising. He has suggested further that if the normal specific longevity were attained by human beings, old and not degenerate individuals would lose the See also:instinct for life and acquire an instinct for death, and that as they had fulfilled the normal See also:cycle of life, they would accept death with the same relieved acquiescence that they now accept See also:sleep. The various writers whose opinions have been briefly discussed agree in supposing that there is a normal specific longevity, although Metchnikoff alone has urged that this differs markedly from the average longevity, and has propounded a theory of the causes of the divergence. It is See also:common ground that they believe the organism to be See also:wound up, so to say, for a definite period, but have no very definite theory as to how this period is determined. A. See also:Weismann, on the other hand, in a well-known See also:essay on the duration of life, has developed a theory to explain the various fashions in which the See also:gift of life is measured out to different kinds of creatures. He accepts the position that purely physiological conditions set a limit to the number of years that can be attained by each kind of multi-cellular organism, but holds that these conditions leave See also:room for a considerable amount of variation. Duration of life, in fact, according to Weismann, is a See also:character that can be influenced by the environment and that by a process of natural selection can be adapted to the conditions of existence of different species. If a species is to maintain its existence or to increase, it is obvious that its members must be able to replace the losses caused by death. It is necessary, moreover, for the success of the species, that an average See also:population of full vigour should be maintained. Weismann argues that death itself is an See also:adaptation to secure the removal of useless and worn-out individuals and that it comes as soon as may be after the period of reproductive activity. It is understood that the term reproductive activity covers not merely the See also:production of new individuals but the care of these by the parents until they are self-sufficient. The average longevity, according to Weismann, is adapted to the needs of the species; it is sufficiently long to secure that the requisite number of new individuals is produced and protected. He has brought together a large number of instances which show that there is a relation between duration of life and fertility. Birds of See also:prey, which breed slowly, usually producing an annual brood of no more than one or two, live to great ages, whilst rabbits which produce large litters at frequent invervals have relatively short lives. See also:Allowance has to be made in cases where the young are largely preyed upon by enemies, for this See also:counter-acts the effect of high fecundity. In short, the duration of life is so adapted that a pair of individuals on the average succeed in rearing a pair of offspring. Metchnikoff, however, has pointed out that the longevity of such fecund creatures must have arisen independently, as otherwise species subject to high risks of this nature would have ceased to exist and would have disappeared, as many species have vanished in the past of the world's See also:history. The normal specific longevity, the age to which all normal individuals of a species would survive under the most favourable conditions, must depend on constitution and structure. No doubt selection is involved, as it is obvious that creatures would perish if their constitution and structure were not such that they could live long enough to reproduce their kind. The See also:direct explanation, however, must be sought for in size, complexity of structure, length of period of growth, capacity to withstand the See also:wear and See also:tear of life and such other intrinsic qualities. The average specific longevity, on the other hand, depends on a multitude of extrinsic conditions operating on the intrinsic constitution; these extrinsic conditions are given by the environment of the species as it affects the young and the adults, enemies, diseases, abundance of food, See also:climatic conditions and so forth. It would seem most natural to suppose that in all cases, except perhaps those of intelligent man and the domestic animals or plants he harbours, the average longevity must vary enormously with changing conditions, and must be a factor of greater importance in the survival of the species than the ideal normal specific longevity. It also seems more probable that the reproductive capacity, which is extremely variable, has been adapted to the average longevity of the species, than that, as Weismann supposed, it should itself be the determining cause of the duration of life. A. Weismann, Essays upon See also:Heredity (See also:Oxford, 1889). (P. C. Additional information and CommentsThere are no comments yet for this article.
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