Chapter IX: Part II: Longevity in the Animal Kingdom (1)

I

THEORIES OF LONGEVITY

Relation between longevity and size—Longevity and the
period of growth—Longevity and the doubling in weight
after birth—Longevity and rate of reproduction—Probable
relation between longevity and the nature of the food

The duration of the life of animals varies within very wide limits. Some, as for instance, the males of certain wheel animalculæ (Rotifera) complete their cycle of life from birth to death in 50 or 60 hours, whilst others, like some reptiles, live more than 100 years, and quite possibly may live for two or three centuries.

Enquiry has been made for many years as to whether there are laws governing these different durations of life. Even the most casual observation of domesticated animals has shown that, as a general rule, small animals do not live so long as large ones; mice, guinea pigs, and rabbits for instance, have shorter lives than geese, ducks, and sheep, whilst these again are survived by horses, deer, and camels. Of all the mammals which have lived under the protection of man, the elephant is at once the largest, and the most long-lived.

However, it is not difficult to show that there is no absolute relation between size and longevity, since parrots, ravens, and geese live much longer than many mammals, and than some much larger birds.

As a general rule it may be said that a large animal takes more time than a small one to reach maturity, and it has been inferred from this that the length of the periods of gestation and of growth were in proportion to the longevity. Buffon[28] long ago stated his opinion that the “total duration of life bore some definite relation to the length of the period of growth.” Therefore, as the period of growth is, so to say, inherent in the species, longevity would have to be regarded as a very stable phenomenon. Just as any species has acquired a fixed and practically invariable size, so it would have acquired a definite longevity. Buffon, therefore, thought that the duration of life did not depend on habits or mode of life, or on the nature of food, that, in fact, nothing could change its rigid laws, except an excess of nourishment.

Taking as his standard the total period of development of the body, Buffon came to the conclusion that the duration of life is six or seven times that of the period of growth. Man, for instance, he said, who takes 14 years to grow, can live 6 or 7 times that period, that is to say, 90 or 100 years. The horse, which reaches its full size in 4 years, can live 6 or 7 times that length of time, that is to say from 25 to 30 years. The stag takes 5 or 6 years to grow, and reckoned in the same way, its longevity should be 35 to 40 years.

Flourens[29] although supporting his principle, thought that Buffon had been inexact in calculating the period of growth. In his opinion a better result can be obtained by taking the limit of growth as that age at which the epiphyses of the long bones unite with the bones themselves. Using such a mode of computation, Flourens laid down that an animal lived 5 times the length of its period of growth. Man, for instance, takes 20 years to grow, and he can live for 5 times that space, that is to say, 100 years; the camel takes 8 to grow, and lives 5 times as long, _i.e._, 40 years; the horse, 5 to grow, and lives 25 years.

However, even if we consider only the mammalia, it is impossible to accept Flourens’ law, without considerable reserve. Weismann[30] has referred to the case of the horse, which is completely adult at 4, but lives not merely 5 times that period, but 10 or even 12 times. Mice grow extremely quickly, so that they are able to reproduce at the age of 4 months. Even if we take 6 months as their period of growth, their longevity of 5 years is twice as long as it would be according to the rule of Flourens. Amongst domesticated animals, the sheep is slow in reaching maturity; it does not acquire its adult set of teeth until it is 5 years old, and cannot be regarded as adult until then. None the less, at the age of 8 or 10 years, it loses its teeth and begins to grow old, whilst by 14 it is quite senile.[31] The longevity of the sheep, therefore, is not quite three times its period of growth.

If we turn to other vertebrates, the variations in the relation of growth and the duration of life are still greater. Parrots, for instance, the longevity of which is extremely great, grow very quickly. At the age of 2 years, they have acquired the adult plumage and are able to reproduce, whilst the smaller species are in the same condition at the age of one. Incubation, moreover, is very short, not more than 25 days, and in some species not three weeks. None the less, parrots are birds which enjoy a quite remarkable longevity. The incubation period of domestic geese is 30 days, and their period of growth is also short. However, they may reach a great age, cases of 80 years and of 100 years being on record. In contrast with these, ostriches, the incubation period of which is 42 to 49 days, and which take 3 years to become adult, have a relatively short life.

H. Milne-Edwards[32] many years ago contended that there was no importance in the supposed law of relation between gestation and longevity. He sums up his criticism as follows: “Although the period of uterine life is longer in the horse, that animal does not live so long as a human being; and some birds, the incubation of which only lasts a few weeks, can live more than a century.”

Bunge[33] has recently taken up the study of the relations between the duration of growth and longevity, and has suggested a new means of investigation. He has observed that the period in which the new-born mammal doubles its weight is a good index of the rapidity of its growth. He has shown that whilst a human child requires 180 days to reach double its weight at birth, the horse, the longevity of which is very much less, doubles its weight in 60 days; a calf takes only 47 days for this; a kid 15 days; a pig 14 days; a cat 9-1/2; and a dog only 9 days. Although these facts are very interesting, the exceptions are too great to make it possible to base a law of longevity upon them. The period of weight-doubling in the horse is nearly 7 times longer than that in the dog, and yet the longevity of the horse is not more than 3 times that of the dog. The goat, which takes much longer than the dog to double its weight, has a shorter total life.

I observed myself that new-born mice quadruple their weight in the first 24 hours. The doubling of weight in their case requires a time 36 times less long than that of the cat, and yet the cat lives only 5 times as long as the mouse.

It is fair to say, however, that Bunge himself does not draw a definite conclusion from these figures and has published them only to stimulate interest in the subject. He is against the view of Flourens, and points out that although the multiple 5 is valid for man, it is not so in the case of the horse which finishes its growth in 4 years and yet reaches the age of 40 much less often than human beings attain that of 100 years.

Although it is impossible to admit the existence of exact relations between size and the period of growth on the one side, and longevity on the other, in the mode which Buffon and Flourens have followed, it is none the less true that there is something intrinsic in each kind of animal which sets a definite limit to the length of years it can attain. The purely physiological conditions which determine this limit leave room for a considerable amount of variation in longevity. Duration of life therefore, is a character which can be influenced by the environment. Weismann in his well-known essay on the duration of life, has laid stress on this side of the problem. Longevity, according to him, although in the last resort depending on the physiological properties of the cells of which the organism is composed, can be adapted to the conditions of existence and influenced by natural selection, like other characters useful for the existence of the species.

If a species is to remain in existence, its members must be able to reproduce and the progeny must be able to reach adult life so that they in their turn may reproduce. Now, it happens that there are some animals the fecundity of which is extremely limited. Most birds which are adapted to aerial life, and the weight of which is therefore to be kept down, lay very few eggs. This happens in the case of birds of prey, such as eagles and vultures. These birds nest only once a year, and generally rear two or frequently only a single nestling. In such circumstances the duration of life becomes a factor in the preservation of the species, more important since eggs and chicks are subject to many dangers. Eggs are devoured by many kinds of animals, whilst unseasonable cold may kill the chicks. If the members of such a species were incapable of living long, the unfavourable conditions of life would soon lead to extinction. Those animals which reproduce rapidly generally have a relatively brief duration of life. Mice, rats, rabbits, and many other rodents seldom live more than 5 or 10 years, but reproduce with enormous rapidity. It is almost possible to imagine that there is some sort of intimate link, possibly physiological, between longevity and low fertility. It is a current opinion that reproduction wastes the maternal organism and that mothers of many children grow old prematurely and seldom reach an advanced age. This would seem to mean that fecundity was the cause of the short duration of life. However, we must guard ourselves against such a theory. Longevity, at least in the case of vertebrate animals, differs extremely little in the two sexes, although the cost of the new generation to the adult organism is very much greater in the case of the female than of the male parent. None the less, females frequently reach a great age, especially in the human race where women reach 100 years, or live beyond that time, much more often than men.

Low fertility, however, cannot itself be regarded as a cause of longevity, as there are some very fertile animals which none the less attain great ages. There are parrots which lay two or three times a year, producing six to nine eggs in each clutch. The ducks (Anatidæ) are distinguished for considerable longevity and very high fertility, each nest containing rarely less than six and sometimes as many as sixteen eggs. The common Sheldrake lays from twenty to thirty eggs. Tame ducks, in some parts of the tropics, lay an egg daily throughout the season. Wild ducks lay from seven to fourteen eggs in one nest. Ducks and geese, none the less, frequently attain considerable ages, ducks having been known to live for 29 years. Even the common fowl, which is a notoriously prolific bird, may reach an age of twenty to thirty years.

It will be said, however, that these birds are exposed to many enemies during youth. Chickens, ducklings, and goslings are ready prey for hawks, foxes and small carnivora. The longevity is possibly to be explained as an adaptation for the preservation of the species by compensating for the great destruction of the young. Weismann explains in this way the longevity of many aquatic birds and other creatures that are much preyed on. It must be noted, however, that the longevity cannot depend on the risks run by the young birds, but must have arisen independently. If this had not occurred, creatures, the young of which are destroyed in great numbers, would have ceased to exist, as many species have disappeared in geological time. The longevity of prolific animals, the young of which are destroyed in numbers, must be due to some cause which is neither fertility nor the destruction of their offspring. This cause must be sought in the physiological processes of the organism and can be attributed neither to the length of the period of growth nor to the size attained by the adults.

After having discussed various theories of the cause of the duration of life, M. Oustalet,[34] in a most interesting essay on the longevity of vertebrates, came to the conclusion that diet was the chief factor. He thinks that there is a “definite relation between diet and longevity. For the most part herbivorous animals live longer than carnivorous forms, probably because the former find their food with ease and regularity, whilst the latter alternate between semi-starvation and repletion.” There are certainly many instances which give support to the view. Elephants and parrots, for instance, are vegetarian and reach very great ages. On the other hand, there exist long-living carnivorous animals. Many observations have made it certain that owls and eagles reach great ages, and these birds live on animal food. Ravens, which live on carrion, are also notorious for the duration of their lives. There is no exact knowledge as to the ages reached by crocodiles, but although these live on flesh, it is certain that their longevity is great.

We must seek elsewhere for the real factors that control duration of life. Before stating my conclusion, I will review what is known as to the duration of life of different animals.

II.

LONGEVITY IN THE ANIMAL KINGDOM

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.

Mr. Gurney found that parrots were not the only birds capable of reaching a great age. One raven reached 69 years and another 50, an Eagle-owl (_Bubo maximus_) 68 years, another 53, a condor 52, an imperial eagle 56, a common heron 60, a wild goose 80, and a common swan 70 years. None of these examples approaches the legendary three centuries attributed to the swan, but it is evident that many different kinds of birds may attain great age. I can add some cases to those of Mr. Gurney. In the Royal Park at Schönbrunn, near Vienna, a white-headed vulture (_Neophron percnopterus_) died aged 118 years, a golden eagle (_Aquila chrysaëtus_) aged 104, and another aged 80 (according to Oustalet). Mr. Pycraft (_Country Life_, June 25th, 1904) reported that a female eagle, captured in Norway in 1829, had been brought to England and had lived for 75 years. In the last thirty years of its life, it had produced ninety eggs. The same writer mentions the case of a falcon having lived to 162 years.

The collection of facts that I have passed in review make it manifest that birds may have a great duration of life, but that reptiles surpass them in this respect. Birds certainly do not reach the very great ages of crocodiles and tortoises.

Longevity, therefore, is reduced as we ascend in the scale of vertebrate life. We find a still greater reduction when we turn from birds to mammals. Some mammals, it is true, may live as long as birds. Elephants are a good instance. It used to be thought that these giant mammals could live three or four centuries, but I can find no confirmation of the legend, which seems as mythical as that relating to the life of swans. There are no exact data as to the ages reached by wild elephants, but it has been stated that in captivity an elephant rarely but occasionally has completed its century. In zoological gardens and in good menageries, where elephants are well cared for, they seldom live more than 20 to 25 years. Chevrette, an African elephant presented to the Jardin des Plantes by Mehemet Ali, in 1825, lived for only 30 years. In the official list of the Indian Government, which gives the deaths of elephants, it appears that of 138 examples, only one lived more than 20 years after it had been purchased (Brehm’s _Mammals_).

Flourens, using his own formula, assigned the age of 150 years to elephants as their epiphyses do not fuse with the long bones until the age of 30. So far, I know of no fact to support the conclusion, although it seems fairly well established that occasionally an elephant may reach a century. It is stated that one elephant was in service throughout the whole period of more than 140 years in which Ceylon was occupied by the Dutch. This elephant was found in the stables in 1656. Natives with special knowledge of elephants set down their duration of life as from 80 to 150 years, but say that they begin to grow old at from 50 to 60 years of age. My general conclusion from the facts is that the life of these very large mammals is about the same as that of man who is very much smaller.

Centenarians, extremely rare amongst elephants, do not appear to exist in any other kind of mammals except man. The rhinoceros, another large mammal which is a native of the same countries as the elephant, does not reach a great age. According to Oustalet an Indian rhinoceros died in the menagerie of the Paris Museum at about the age of 25 years, and showed all the signs of senility. Another Indian rhinoceros lived for 37 years in the London Zoological Gardens. Grindon has stated his opinion that the rhinoceros may live for 70 or 80 years, but this seems rather an inference from the slowness of growth than a statement of observed fact.

Horses and cattle are large animals, but do not enjoy very long lives. The usual duration of life in horses is from 15 to 30 years. They begin to grow old about 10 years, and in very rare cases may reach 40 or more. A Welsh pony is said to have reached the age of sixty, but such a case is excessively rare. Two other extreme cases are that of a horse belonging to the Bishop of Metz which died at the age of 50 years, and the charger of Field-Marshal Lacy which died at 46.

The duration of life of cattle is still shorter. Domestic cattle show the first sign of age, a yellow discoloration of the teeth, when five years old. In the sixteenth to eighteenth year the teeth fall out, or break, and the cow ceases to give milk, whilst the bull has lost reproductive power. According to Brehm, cattle live for 25 to 30 years or more. Although the duration of life is short, cattle are not prolific. The gestation period of a cow approaches that of the human race (242-287 days), and there is only one birth a year. The total period of reproductivity lasts only a few years.

The sheep, another domesticated Ruminant, has a life even shorter. According to Grindon, sheep do not live longer than 12 years as a rule, but may reach 14 years, which in their case would be extreme age, as they generally lose their teeth at from 8 to 10 years.

Some Ruminants, such as camels and deer, apparently live longer than sheep or cattle, but I do not know exact facts about them.

The short life of domesticated carnivorous animals is well known. Dogs seldom live more than 16 or 18 years, and even before that, at an age of from 10 to 12, they usually show plain signs of senility. Jonatt has mentioned as an extreme rarity a dog of 22 years of age, and Sir E. Ray Lankester (_Comparative Longevity_, p. 60) cites another instance, in this case the age being 34 years. The oldest dog that I have been able to procure died at the age of 22.

It is generally believed that cats do not live so long as dogs. The average age which they may attain is usually thought to be 10 or 12 years, but certainly a cat of that age has not the decrepid appearance of an old dog. Thanks to the kindness of M. Barrier, the Director of the Ecole d’Alfort, I have had in my possession a cat 23 years old. It appeared to be quite vigorous, and died from cancer in the liver.

Most rodents, particularly the domesticated kinds, are extremely prolific and very short lived. It is extremely rare for a rabbit to reach the age of 10 years, whilst 7 years is the utmost limit for a guinea-pig. Mice, so far as I can ascertain, do not live more than 5 or 6 years.

It is plain from the facts that I have brought together, that mammals, whether they are large or small, as a rule, have shorter lives than birds. It is probable, therefore, that there is something in the structure of mammals which has brought about a shortening in the duration of their lives.

Whilst most of the lower vertebrates, and all birds, reproduce by laying eggs, the vast majority of mammals are viviparous. As the tax on the parent organism is greater when the young are produced alive than when eggs are laid, it might be thought that in this difference lay the cause of the shorter life of mammals. It is well known that an animal may be made feeble by too great fecundity, and it is conceivable that the kind of parasitic life of the embryos within the body of the mother may weaken her system.

There are many facts, however, which make it impossible to accept such a view. The longevity of mammals is nearly equal in the two sexes, although the tax on the organism caused by reproduction is much greater in the case of females than in males. Longevity, however, cannot be regarded as a character stable in each species and necessarily identical in the two sexes. The animal kingdom presents many cases of disparity in this respect, the difference in longevity in the two sexes being specially striking in species of insects. Generally, the females live longer than the males, as, for instance, amongst the Strepsiptera, where the females have 64 times the duration of life of the males. On the other hand, amongst butterflies, there are cases (_e.g._, _Aglia tau_) where the males live longer than the females. In the human race, there is a difference in the longevity of the sexes, the females having the advantage.

As in most cases of disparity in the duration of life the female lives longer than the male, it is plain that the difference cannot be assigned to the drain on the organism caused by reproduction, which, of course, is much greater in females.

Moreover, a closer scrutiny of the facts shows that although mammals do not live so long as birds, the reproductive drain is greater in the case of birds.

It is well known that the productivity of an animal is not necessarily identical with its fecundity. Fish or frogs which lay thousands of eggs at a time (a pike, for example, produces 130,000) are obviously more prolific than, for instance, a sparrow which lays only 18 eggs in a year, or than a rabbit, which in the same time gives birth to from 25 to 50. However, to produce this much smaller quantity of eggs or of young, the sparrow and the rabbit (I have chosen the most prolific bird and mammal) expend a much larger quantity of material than the frog or the fish. The sparrow and the rabbit employ in producing their progeny a bulk of material greater than the weight of their body, whilst the enormous quantity of eggs laid by the frog does not weigh more than one-seventh part of the body of the frog. It may be laid down, as a general rule, that although fecundity, that is to say the number of eggs or of young which are produced, diminishes as the organism becomes more complex, the productivity on the other hand increases, expressed in percentage of weight. The productivity, which is not more than 18 per cent. in batrachia, reaches 50 per cent. in reptiles, 74 per cent. in mammals, and 82 per cent. in birds.

It is plain that if reproduction shortens the life of mammals by weakening the organism, it must be the productivity, not the fecundity, which is the important factor. I have just shown that productivity is greater in birds than in mammals, and in consequence it cannot be on account of any greater burden of reproduction that mammals have a shorter life than birds. The shortness of mammalian life, again, cannot be attributed to the fact that mammals give birth to young, whilst the long-lived reptiles and birds produce eggs, because the longevity of the males, which produce neither young nor eggs, is none the less practically equal to that of the females of the same species. The reason of the short life of mammals must be sought for elsewhere.

III

THE DIGESTIVE SYSTEM AND SENILITY

Relations between longevity and the structure
of the digestive system—The Cæca in birds—The
large intestine of mammals—Function of the large
intestine—The intestinal microbes and their agency in
producing auto-intoxication and auto-infection in the
organism—Passage of microbes through the intestinal wall

We have seen that the duration of life in mammals is relatively shorter than that in birds, and in the so-called “cold-blooded” vertebrates. No indication as to the cause of this difference can be found in the structure of the organs of circulation, respiration, or urinary secretion, or in the nervous or sexual apparatus. The key to the problem is to be found in the organs of digestion.

In reviewing the anatomical structure of the digestive apparatus in the vertebrate series, one soon comes to the striking fact that mammals are the only group in which the large intestine is much developed. In fish, the large intestine is the least important part of the digestive tube, being little wider in calibre than the small intestine. Amongst batrachia, where it is a relatively wide sack, it has begun to assume some importance. In several reptiles it is still larger, and may be provided with a lateral out-growth, which is to be regarded as a cæcum. In birds, the large intestine still remains relatively badly developed; it is short and straight. In most birds, at the point where the large intestine passes into the small intestine, there is a pair of cæca, more or less developed. These cæca are absent in climbing birds, such as the wood-pecker, the oriole, and many others. They are reduced to a pair of tiny outgrowths in the eagles, sparrow-hawks, and other diurnal birds of prey, and in pigeons, and perching birds. These organs are larger in the nocturnal birds of prey, in gallinaceous birds, and in ducks, etc.[40]

In the large running birds, such as ostriches, rheas, and tinamous, the cæca are relatively largest. Thus, for instance, in a rhea (_Rhea americana_) which I dissected, the cæca were nearly two-thirds as long as the small intestine. The latter was 1·65 m. in length, whereas one of the cæca was 1·01 m., and the other 0·95 m. The weight of the two cæca with their contents was more than 10 per cent. of the total weight of the bird.

Notwithstanding the exceptions, which are relatively rare, the large intestine is badly developed in the case of birds. On the other hand, it reaches its largest size amongst mammals. In these animals, “only the posterior portion of the latter, or rectum, which passes into the pelvic cavity, corresponds to the large intestine of lower Vertebrates; the remaining, and far larger part, must be looked upon as a neomorph, and is called the colon.”[41]

Gegenbaur,[42] another well-known authority on comparative anatomy, writes as follows on this subject:—“The hind-gut is longest in the Mammalia, where it forms the large intestine, and is distinguished as such, from the mid-gut, or small intestine. Owing to its greater length, it is arranged in coils, so that the terminal portion only has the straight course taken by the hind-gut of other Vertebrata.”

The two series of facts are not to be disputed. On the one hand mammals are shorter lived than birds and lower vertebrates, on the other hand the large intestine is much longer in them than in any other vertebrates. Is there here any link of causality, binding the two characters, or is it a mere coincidence?

To answer the question we must turn to the function of the large intestine in vertebrates. In the lower members of the group (fish, batrachia, reptiles, birds, etc.), the large intestine is not more than a mere reservoir for the waste matter in the food. It takes no share in digestion, as that is the function of the stomach and the small intestine. Only the cæcum can be thought to have some digestive property. In reptiles, the lowest vertebrates in which the cæcum is present, it is so little differentiated from the large intestine itself, that it is difficult to assign to it any specialised function. In very many birds, however, the cæca are well separated from the main digestive tube. The food material passes into them in considerable quantities, and is retained there sufficiently long for some digestive process to take place. M. Maumus has found, in the cæca of birds, secretions which can dissolve albumen and invert sugar cane, but he has been unable to make out that the cæcal juice has any action upon fatty matter. Such digestive power, however, is slight, and when M. Maumus removed the cæca in fowls and ducks, no evil consequences followed. As in many birds the cæca are rudimentary and in others absent, it may be inferred that these organs are useless, and are in process of degeneration in the class. The cæca can be regarded as playing an important part in the organism only in the case of large running birds, where they are very highly developed, but we have not precise information as to their digestive function.

The variations in the structure in the large intestine are greater in mammals than in birds. In some mammals, the large intestine is a simple prolongation of the small intestine, similar in calibre and in structure. In these conditions it may fulfil a definite digestive function. Th. Eimer[43] has determined that in insectivorous bats the large intestine digests insects like the small intestine. Such cases, however, are rare. In most mammals the large intestine is sharply separated from the small intestine by a valve, and opens directly into the cæcum which may be very large. In the horse, the cæcum is an enormous bag, cylindrical and tapering, generally well filled, and holding on an average 35 litres. It is equally large in many other herbivorous animals, such as the tapir, the elephant, and most rodents. In such cases, the food remains for a considerable time in the organ and without doubt undergoes some digestive changes. In many other mammals, particularly carnivorous forms, the cæcum may be quite absent, whilst in some, as for instance, the cat and dog, it is very small; in the latter cases its digestive function must be non-existent or insignificant.[44]

As for the large intestine itself, apart from the special cases, such as bats, it cannot fulfil any notable digestive function. Th. Eimer was unable to find a proof of any such action in rats and mice, and the very many investigations that have been made in the case of man seem to have established the absence of digestive power in the colon.

Dr. Stragesco,[45] in a recent investigation carried out under the direction of the famous Russian physiologist Pawloff, established that, in normal conditions, digestion and assimilation of food are confined almost exclusively to the small intestine in mammals, and that the large intestine plays only the smallest part. It is only in certain diseases of the digestive tract, in which, on account of increased peristaltic action, the contents of the intestine with the digestive juices are passed quickly from the small intestine to the large intestine, that some digestive work is done in the latter organ.

The large intestine (excluding the cæcum), then, cannot be regarded as an organ of digestion, although absorption of the liquids which have been formed in the small intestine, may take place within its walls. It is known that in the large intestine the contents of the gut give up their water and assume the solid form of fæcal matter. However, whilst the mucous membrane of the large intestine rapidly absorbs water, it has not a similar action on other substances.

The question of the extent to which the large intestine can absorb has been closely investigated, because of its practical importance. It sometimes happens that invalids cannot take food by the mouth, so that their life would be in danger if it were not possible to supply them with food otherwise. Attempts have been made to inject nutritive substances through the skin, or, and this is a more usual procedure, by the rectum. By such means the organism can be kept alive for a certain time, but the absorbing power of the large intestine is extremely small. According to Czerny and Lautschenberger[46] the entire colon of the human being can absorb no more than 6 grammes of albumen in 24 hours, an amount which, from the point of view of nutrition, is very small. It was thought that the large intestine might more rapidly absorb albuminous material which had been previously digested and transformed to peptones, but the experiments of Ewald[47] showed that even in that case the absorption was very small. According to more recent experiments of Heile,[48] carried out upon dogs which had cæcal fistulas, and in the case of a man who had an artificial aperture in the colon, the large intestine does not absorb undigested white of egg, and absorbs water, cane sugar, and glucose only very imperfectly. The only substances which are rapidly absorbed through the wall of the colon are the alkaline fluids from fæcal matter. It is possible, however, to nourish invalids by rectal injections of certain nutritious substances, the most important of which is milk.[49]

The large intestine, which has really very slight digestive properties and cannot absorb any considerable bulk of nutriment, is an organ which secretes mucus. The latter serves to moisten the solid fæcal material, so aiding in its expulsion.

We must conclude, therefore, that the large intestine, the organ so highly developed in mammals, is an apparatus the general function of which is the preparation and elimination of the waste products of digestion. Why should such an organ be so much more developed in mammals than in the other vertebrates?

In answer to the question, I have formed the theory that the large intestine has been increased in mammals to make it possible for these animals to run long distances without having to stand still for defæcation. The organ, then, would simply have the function of a reservoir of waste matter.

Batrachia and reptiles lead a very idle life, and can move slowly, sometimes because they are protected by poison (toads, salamanders, serpents), sometimes because they have a very hard shell (turtles), sometimes because they are extremely powerful (crocodiles). Mammals, on the other hand, have to move very actively to catch their prey, or to escape from their enemies. Such activity has become possible because of the high development of the limbs, and because the capacity of the large intestine makes possible the accumulation of waste matter for a considerable time.

In order to void the contents of the intestines, mammals have to stand still and assume some particular position. Each act of this kind is a definite risk in the struggle for existence. A carnivorous mammal which, in the process of hunting its prey, had to stop from time to time, would be inferior to one which could pursue its course without pausing. So, also, a herbivorous mammal, escaping from an enemy by flight, would have the better chance of surviving the less it was necessary for it to stand still.

According to such a view, the extreme development of the large intestine would supply a real want in the struggle for existence. M. Yves Delage,[50] the well-known biologist, is unable to accept this hypothesis. He thinks that the rectal enlargement would fulfil the purpose, and adds that everyone has seen herbivorous animals pass their excretions whilst running. The rectum of mammals, however, cannot serve as a reservoir for waste matter, because as soon as such matter reaches the rectum it excites the need of excretion. The waste matter accumulates in the large intestine, from which it passes into the rectum at intervals. When it has reached that region, a sensation is caused which leads to defæcation.

M. Delage is not quite definite when he speaks of mammals voiding their excretions whilst they are in motion. A horse, harnessed to a vehicle, may defæcate whilst it is walking or even running slowly. But these animals cannot defæcate when in rapid motion, and competent observers state that horses never do so whilst racing. In zoological gardens, where animals have room to run about, they stand still before emptying the rectum. M. Ch. Debreuil, who keeps antelopes in a very large park at Melun, has noticed that the excreta are always to be found in masses and not scattered about as if they had been discharged by animals in motion. Antelopes, which are animals that run and leap extremely actively, have to come to a standstill before discharging their small pellets of deer-like excreta.

In the struggle for existence, when a mammal is pursuing its prey or escaping its enemy, there is no question of the leisurely movement of a horse harnessed to an omnibus or cab, but the greatest possible activity is necessary. In such circumstances the possession of an organ within which the excreta could accumulate would be of real importance. My theory of the origin of the mammalian large intestine is intrinsically probable.

Although the capacity of the large intestine may preserve a mammal in emergencies, it is attended with disadvantages that may shorten the actual duration of life.

The accumulation of waste matter, retained in the large intestine for considerable periods, becomes a nidus for microbes which produce fermentations and putrefaction harmful to the organism. Although our knowledge of the subject is far from complete, it is certain that the intestinal flora contains some microbes which damage health, either by multiplying in the organism, or by poisoning it with their secretions. Most of our knowledge on this matter has come from the study of human patients.

Persons have been known who do not defæcate except at intervals of several days, and who, none the less, do not seem to suffer in health. But the opposite result is more common. The retention of fæcal matter for several days very often brings harmful consequences. Organisms which are in a feeble state from some other cause are specially susceptible to damage of the kind referred to. Infants are frequently seriously ill as the result of constipation. Dr. du Pasquier[51] describes such cases in the following words:—“The infant is leaden in hue, with sunken eyes, dilated pupils, and pinched nostrils. The temperature may reach nearly 104° Fahr.; the pulse is rapid, feeble, and often irregular. Restlessness, insomnia, sometimes convulsions, stiffness of the neck and strabism show that the nervous system is being poisoned by toxins, and even collapse may be reached. The foul and dry tongue, the vomiting and fetid discharges show the disturbance of the digestive tract. Very often an eruption appears, as described by Hutinel, chiefly on the back and buttocks, the front of the thighs and fore-arms.” The illness may lead to death but is generally cured by simple purging.

Women in pregnancy and child-birth frequently suffer much as the result of retention of fæcal matter, and physicians are familiar with the symptoms, which have been described as follows by M. Bouchet[52]:—“After normal parturition, in the course of which the usual antiseptic precautions have been fully pursued, and where delivery has been complete and natural, occasionally the patient is seized with chill and headache. The breath is fetid and the tongue foul. The temperature, taken in the axilla, is nearly 101° Fahr. The abdomen is inflated and painful in the umbilical region. Palpation in the iliac fossæ reveals lumps or consolidations along the colon. Thirst is intense, and there is complete anorexy. On questioning, it is found that there has not been defæcation for several days. The treatment consists of purgatives, enemas, and milk diet. In the next few days the bowels are emptied freely, the abdominal pain ceases, the temperature becomes lower, appetite is restored, and the patient recovers.”

Those who suffer from affections of the heart, liver, or kidneys are specially susceptible to the evil results of retained fæcal matter. In such patients an error of diet or constipation may bring about most serious consequences.

Such facts are well known to physicians, and it has been established that complete emptying of the lower bowels leads at once to favourable symptoms. From the other side, it has been shown by experiment that artificial retention of the fæces by ligature of the rectum puts the body in a grave condition.

If we collect our knowledge of all the facts, we cannot doubt but that the cause of the evil is multiplication of microbes in the contents of the large intestine. When the fæcal matter is free from microbes, as is the case with the meconium of the fœtus or new-born infant, it is not a source of danger to the organism. The waste of cells and the secretions which are added to the undigested food cannot do any harm. Amongst the microbes of the gut, there are some that are inoffensive, but others are known to have pernicious properties.