Читать книгу The Prolongation of Life: Optimistic Studies - Élie Metchnikoff - Страница 11

Table of Contents

Longevity in the lower animals—Instances of long life in sea-anemones and other invertebrates—Duration of life of insects—Duration of life of “cold-blooded” vertebrates—Duration of life of birds—Duration of life of mammals—Inequality of the duration of life in males and females—Relations between longevity and fertility of the organism

It is wonderful to what an extent the duration of life varies amongst animals, the slightest examination of the facts showing that very many factors must be involved.

As the higher animals are nearly always larger than invertebrates, if there be a definite relation between longevity and size, one would expect to find that vertebrates live longer than invertebrates. However, this is not the case. Amongst animals of extremely simple organisation, there are some which reach a great age. A striking example of this is found in sea-anemones. These animals have a very simple structure, without a separate digestive canal, and with a badly developed, diffused nervous system, and yet have lived very long in captivity. More than forty years ago, I remember having seen in the possession of M. Lloyd, the Director of the Aquarium at Hamburg, an anemone that he had kept alive for several dozen years in a glass bowl. Another sea-anemone, belonging to the species Actinia mesembryanthemum, is known to have lived 66 years. It was captured in 1828 by Dalyell, a Scottish zoologist, and was then quite adult, and probably about 7 years old. It survived its owner for 36 years, and died in Edinburgh in 1887, the cause of death being unknown. Although they are thus capable of living so long, the rate of growth of members of this species is rapid, and their fertility is very high. According to Dalyell, these anemones reach the adult condition in 15 months. The specimen in his possession, in the 20 years from 1828 to 1848 produced 334 larvæ, then after a period of sterility it gave birth, in one night (1857) to 230 young anemones. This extraordinary prolificness decreased with age, but even when it was 58 years old it used to produce from 5 to 20 at a time. In the seven years from 1872 onwards, it gave birth to 150 young anemones.35 This animal, which certainly was not more than the fortieth or the fiftieth of the weight of an adult rabbit, lived six or seven times as long.

Ashworth and Nelson Annandale have published their observations on another sea-anemone, of the species Sagartia troglodytes, which was 50 years old. It differed from younger examples only in being less prolific.

There are other polyps, such as Flabellum, which do not live more than 24 years, although we have no knowledge as to the cause of the different duration of life.

The variation in the length of the life of molluscs and insects is extremely great. Some species of gasteropods (Vitrina, Succinea) live only a very few years, whilst others (Natica heros) can reach thirty years. Some of the marine bivalves, as for instance, Tridacna gigas, can live to sixty or a hundred years.36

Insects are animals as variable in their duration of life as they are in other respects. Some live only a few weeks; some of the plant-lice, for instance, die in a month. In the same order of Insects, however, (Hemiptera) there are species of cicada which live thirteen to seventeen years, that is to say, much longer than such little Rodents as rats, mice, and guinea-pigs. The larva of an American species spends seventeen years buried in the ground in orchards, where it feeds on the roots of apple trees, and the species is known as Cicada septemdecim, because of this duration of life. In the adult stage the insect lives little more than a month, just time enough to lay the eggs, and bring into the world the new generation, which in its turn will not appear above ground until after another period of seventeen years.

Between these extremes of long and short life, there is to be found amongst insects almost every gradation of longevity. Science, in its present state, has failed to find any law governing these facts. Rules which hold good up to a certain point in the case of the higher animals break down in their application to insects. The large grasshoppers and locusts, for instance, live a much shorter time than many minute beetles. Queen bees, the fertility of which is very great, live two or three years and may reach a fifth year, whilst worker bees, which are infertile, die in the first year of their existence. Female ants, although these are small and extremely prolific, reach the age of seven years.37

We know so little about the physiological processes of insects, that we cannot as yet make even a guess at the cause of this great variation in their longevity. It is more probable that we shall find some explanation in the case of vertebrates concerning which we know much more.

Analysis of the facts shows that whilst in the evolution from fish to mammal there has been a great increase in complexity of organisation, there has at the same time been a reduction in the duration of life. As a general rule, it may be laid down that the lower vertebrates live longer than mammals.

The facts about the longevity of fish are not very numerous, but it seems clear that these animals reach a great age. The ancient Romans, who used to keep eels in aquaria, have noted that these fish would live for more than sixty years. There is reason to believe that salmon can live for a century, whilst pike live much longer. There is, for instance, the much quoted instance of the pike stated by Gessner to have been captured in 1230 and to have lived for 267 years afterwards. Carps are regarded as equally long lived, Buffon setting down their period of life as 150 years. There is a popular idea that the carp in the lakes at Fontainebleau and Chantilly are several centuries old, but E. Blanchard throws doubt on the accuracy of this estimate, inasmuch as during revolutionary times most of the carp were eaten when the palaces were overrun by the populace. There is no doubt, however, that the life of carp may be very long indeed. Not very much is known about the duration of life in batrachians, but it is certain at least that some small frogs may live twelve or sixteen years, and toads as many as thirty-six years.

More is known about the life of reptiles. Crocodiles and caymans, which are large and which grow very slowly, attain great ages. In the Paris Museum of Natural History there are crocodiles which have been kept for more than forty years without showing signs of senescence. Turtles, although they are smaller than crocodiles, live still longer. A tortoise has lived for eighty years in the garden of the Governor of Cape Town, and is believed to have reached the age of two hundred years. Another tortoise, a native of the Galapagos Islands, is known to be 175 years old, whilst a specimen in the London Zoological Gardens is 150 years old. A land tortoise (Testudo marginata) has been kept in Norfolk, England, for a century. I am informed that in the Archbishop’s palace at Canterbury, there is to be seen the carapace of a tortoise which was brought to the Palace in 1623 and which lived there for 107 years.38 Another tortoise, brought to Fulham by Archbishop Laud, lived in the Palace for 128 years. I have already referred to a specimen of Testudo mauritanica, the history of which is known for 86 years, but which is probably much older.

Very little is known as to the longevity of lizards and serpents, but it may be inferred from what I have said about other reptiles that reptiles as a class are able to reach great ages.

It is an easy inference that the great duration of life in cold-blooded animals is associated with the slowness of the physiological processes in these creatures. The circulation, for instance, is so slow, that the heart of a tortoise beats only 20 to 25 times in a minute. Weismann has suggested that one of the factors influencing the duration of life is the rapidity or slowness of the vital activities, the times taken by the processes of absorption and nutrition.

On the other hand, the blood is hot and the vital activities are rapid in birds, and yet birds may attain great ages. Although in the last chapter I gave a number of examples, the subject is so important that I propose to go further into details. The possibility of this is due to an admirable set of details brought together by Mr. J. H. Gurney.39 In his list, in which are included more than fifty species of birds, the lowest figures are from eight and a half to nine years (Podargus cuvieri, Chelidon urbica), and a duration of life so short is an exception, a period of from fifteen to twenty years being more common. Canaries have lived in captivity from 17 to 20 years, and goldfinches up to 23 years. Field larks have lived for 24 years, the Lesser Black-backed Gull 31 years and the Herring Gull 44 years. Birds of medium size may live for several dozens of years, whether they live on animal or on vegetable food, whether they are prolific or lay very few eggs. I will quote only a few instances. Of forty parrots the minimum and maximum ages were respectively 15 and 81 years, and the average 43 years. Without accepting the truth of the story mentioned by Humboldt according to which certain parrots survived an extinct race of Indians, at least we may be certain that great ages have sometimes been reached by these birds. Levaillant mentions a parrot (Psittacus erithaceus) which lost its memory at the age of 60 years, its sight at 90 years, and which died aged 93 years. Another individual, probably of the same species, is reported by J. Jennings to have reached the age of 77. Jones, Layard, and Butler are the authorities for instances of Sulphur-crested Cockatoos having reached respectively 30, 72 and 81 years. M. Abrahams states that an Amazon (Chrysotis amasonica) lived 102 years. I myself have observed two cases of great longevity in the same species of parrot. One of these birds died at the age of 82 years, apparently simply from old age, whilst the other, which was in my possession for several years before it died at the age of 70 to 75 years, was vigorous, showing no signs of senility, but died of pneumonia.