PART III

INVESTIGATIONS ON NATURAL DEATH

I

NATURAL DEATH AMONG PLANTS

Theory of the immortality of unicellular organisms—Examples of very old trees—Examples of short-lived plants—Prolongation of the life of some plants—Theory of the natural death of plants by exhaustion—Death of plants from auto-intoxication

It must surprise my readers to find how little science really knows about death. Although death has a preponderating place in religions, systems of philosophy, literature and folk-lore, scientific works pay little attention to it. This unfortunate fact explains, although it may not justify, the bitter attack made on science on the grounds that it is occupied with minutiæ and neglects the great problems of human life, such as death. When Tolstoi was absorbed by the problem and searched for some solution in the writings of scientific men, he found that the explanations were trivial or inexact. In consequence he was extremely indignant with the men who devoted themselves to the investigation of what seemed to him useless problems (such as the insect world, or the structure of cells and tissues) and who were yet unable to say what the destiny of man or death might be.

I am far from claiming to solve these problems; I can do little more than describe the actual state of the question of natural death. I hope in this way at least to prepare for scientific investigation, and to call attention to it as the most important problem of humanity.

By the use of the phrase “natural death” I mean to denote a phenomenon that is intrinsic in the nature of an organism and that is not the mere result of an external accident. Popular phraseology includes under natural death all cases due to diseases. But as such deaths can be avoided and are not due to qualities inherent in the organism, it is erroneous to include them in the category “natural death.”

In nature, death comes so frequently by accident that there is justification for asking if natural death really occurs. It used to be thought that death was the inevitable end of life and that the living principle contained within itself the germ of death. Accordingly, it was a surprising discovery that many low organisms die only by accident, and that if such accident be avoided, death does not fall on them. Unicellular organisms (such as infusoria, many other protozoa and low plants) multiply by simple division, the organism thus giving rise to two new organisms; the parent so to speak loses itself in its offspring without undergoing death. To criticisms of this mode of presentment of the facts, Weismann, who has attracted most attention to the view, replied as follows:—“In cultures of Infusoria, these little animals continually multiply by division and no dead bodies are found. The individual life is short, but it ends not in death but in transformation to two new individuals.”

Max Verworn,[71] a physiologist of repute, objected that Weismann had overlooked the occurrence within the organism of a process of partial destruction, and that under certain conditions a complete organ of the infusorian body (the nucleus) dies and is absorbed. Such death of a part, however, is not followed by death of the whole, and as the continuous destruction of some of the cells in our own bodies is not regarded as our death, the criticism of the German physiologist cannot be accepted.

It is not only the extremely short-lived microscopic organisms that escape death. Some of the higher plants, which may attain to gigantic size, encounter death only by accidents. There is nothing to be found in the nature of their organisation which would seem to indicate that death is the inevitable or even probable result of their constitutions.

The longevity of some trees has long been notorious, as these appear to live for many centuries and to die only when they are overwhelmed by the ravages of a storm or killed by human agency.

When the Canary Islands were discovered, in the beginning of the fifteenth century, the early explorers were struck with the gigantic size of a dragon tree which was venerated by the natives as their tutelary deity. The tree stood in a Garden at Orotava in Teneriffe, and even in these early days, its huge trunk contained a gigantic hollow. The tree did not reward the worship of the natives, who were annihilated by the Spaniards, and it survived them for nearly four centuries. At the end of the eighteenth century it was seen by Humboldt,[72] who found that the trunk was forty-five feet in circumference, and who attributed to it a great age because dragon trees grow extremely slowly. Early in the nineteenth century (1819) a furious tempest swept over Orotava and with a gigantic crash nearly a third of the crown of leaves and branches fell on the ground. Notwithstanding this shock, the monster survived for fifty years. Berthelot,[73] who visited it in 1839, described it as follows:—“A dragon tree stood in front of my dwelling, grotesque in form, gigantic in size, which a storm had smitten without overwhelming. Ten men would have much ado to girdle its vast trunk, fifty feet in circumference at the ground. The huge column had a deep cave within it, hollowed by the ages; a rustic porch gave access to the interior, and the lofty dome, although half had been destroyed by a storm, still bore an enormous crown of branches.”

The famous dragon-tree got more and more damaged, and was finally overthrown by a storm in 1868. A few years after the catastrophe (in 1871) I myself saw the remains of the colossus, lying on the ground as a huge grey mass like some antediluvian monster. No accurate estimate of its age can be formed, but it must have lived several thousand years.

Trees have been known which were still older than the dragon-tree of Teneriffe. One of the best known is the baobab of Cape Verd, described by Adanson. “This remarkable tree was thirty feet in diameter when the famous French naturalist measured and described it. Three centuries earlier, some English sailors had cut an inscription on it, and Adanson laid this bare by removing three hundred layers of wood. On his observations Adanson based an estimate of 5,150 years as the age of the tree.[74] The old cypresses of Mexico are thought to be still older. A. de Candolle[75] concluded that the cypress of Montezuma was 2,000 years old when he saw it, and that the cypress at Oazaca was much older than the tree described by Adanson. In California, trees of the species _Sequoia gigantea_ are three thousand years old, and Sargent, an American botanist, attributes to some of them an age of at least five thousand years.

The question of the nature of individuality in the vegetable world has been raised in connection with the longevity of trees. It has been asked if a tree is to be regarded as a single individual or as a colony of many plants like a branching polyp. It is a difficult question, but only of secondary importance from the point of view of this discussion. A. de Candolle,[76] having paid special attention to the subject, came to the conclusion that trees do not die of old age, that, in the real sense of the phrase, there is no natural end of their existence. Many botanists agree with him. Naegeli[77] holds that a tree several thousand years old dies only from external accidents.

It is plain that amongst the lower plants and the higher plants there are cases where natural death does not exist. Theoretically, life would have an unlimited duration, subject to the continuous replacement of the substance of the organism in the normal metabolism. It must not be inferred, however, that there is no such occurrence as natural death amongst plants. There are numerous cases where death comes quite apart from the agency of external forces. Even amongst closely related plants there are some cases where natural death does not occur, and others where it is normal. The lower fungi offer a good instance. Some of these pass through a longer or shorter vegetative stage and then the living mass breaks up into spores (_Myxomycetes_). The whole bulk of matter is not transformed, but the remnant consists only of cuticular secretions, not living cells. In other fungi, only some of the cells transform to spores, the others dying naturally.

One stage of the life history of some lower plants is of short duration. The prothalli of some cryptogams (_Marsiliaceæ_) live only a few hours, just long enough for the appearance of the sexual organs. When these are ripe the body of the prothallus and all its constituent cells fall a prey to natural death. In such cases there is a “corpse,” composed of dead cells and protoplasm. Even amongst the higher plants there are instances of an extremely short duration of life. _Amaryllis lutea_ passes through all the stages of its life-history in ten days, the minimum time necessary for the sprouting of the leaves and flowers and the production of the seeds, after which it dies naturally.[78] It is interesting to find that in the same family there are other plants notable for long duration of life. The Agave requires a century to produce its flowers before death comes naturally.

Everyone is familiar with the so-called “annual” plants which live only a few months, from the time when they sprout, until, after the production of seed, death comes to them naturally. The life of annuals, however, can be preserved for two or for several years. Rye is normally an annual, but some varieties are able to live for two years and produce two crops. The Cossacks of the Don have established this fact, and have cultivated a biennial variety of rye for many years.[79] Beetroot[80] is normally biennial, but has been changed to a plant which lives for from three to five years. Such instances are by no means unique.

Natural death can be postponed if the plant be prevented from seeding. Professor Hugo de Vries has prolonged the life of the Oenotheras he cultivates, by cutting the flowers before fertilisation. Under ordinary conditions the stem dies after producing from forty to fifty flowers, but, if cutting be practised, new flowers are produced until the winter cold intervenes. By cutting the stem sufficiently early, the plants are induced to develop new buds at the base, and these buds survive winter, and resume growth in the following spring.” (Extract from a letter of Prof. H. de Vries.)

The grass of lawns is usually mowed before it begins to flower, so as to prevent the ripening of the seeds and the death of the plant. When this is done, the grass remains continually green, and its life lasts for several years.

The connection between the seeding of plants and their natural death has been recognised for long, and is usually explained as being due to the exhaustion of the plant.

As I am not a botanist, and was anxious to know the views of botanists on natural death, I wrote to Prof. de Vries, as a universally accepted authority. The distinguished botanist replied to me as follows. “Your question is extremely difficult. I do not think that much is known as to the exact cause of the death of annual plants, but it is customary to attribute it to exhaustion.” All the botanists who have expressed opinions on this matter appear to hold a similar view. Hildebrand,[81] the author of a memoir on the duration of life in plants, stated this view again and again. According to him “the life of annuals is usually short because they are exhausted by their extensive production of seeds (p. 116).” “Even amongst plants which produce seeds for several years, there are some which are prematurely exhausted by fructification and which die spontaneously” (p. 67). In the prothallus of many of the higher cryptogams, the formation of a single embryo is followed by natural death; as Goebel[82] points out, the embryo completely absorbs the prothallus.

As plants generally obtain their food with ease, it is natural to ask what is the cause of the exhaustion after seeding. When a plant which cannot resist cold dies after it has produced its seeds in the end of the summer, the event is natural enough. But how can we explain the death of an annual plant which is growing in a rich soil, and which seeds in the beginning of the summer, as being due to exhaustion long before the winter cold. It frequently happens that after harvest new shoots spring up from grains which have fallen. The soil which can support this new vegetation cannot have been exhausted by the cereal in question; and there has been enough warmth for the new crop. It cannot be the external conditions which have caused the death of the parent plant. The explanation of this apparent contradiction has been sought in the constitution of the plant itself. Hildebrand remarks that “certain species have a constitution which tends to early fructification. As soon as the seeds have been set, the strength of the plant is exhausted in the swelling of the grains, so that the plant dies.” “Other species, on the contrary, are so constituted that they vegetate for a long time, before fruiting, after which, however, they also die. A third set of plants have such a constitution that “they do not die after seeding, that they can seed often and live for many years” (p. 113).

Being unable to indicate exactly the intrinsic mechanism of these different “constitutions,” several botanists explain them by a kind of teleological predestination. According to Hildebrand “the nutritive processes of a plant have no other purpose than to make it capable of reproduction; this final end, however, can be reached in different modes and after different periods of time” (p. 132). Goebel sets down similar views. “In heterosporous plants the whole course of the development of prothalli is predetermined. The prothalli, so far as we actually know, to use the phrase of theologians, are predestined; their fate is determined once for all” (p. 403). M. Massart[83] expresses the same kind of view, when he says that “sometimes cells die because their work is finished, and they have no longer any reason for existing.”

Such an interpretation of the facts is quite opposed to determinism, and makes the problem of natural death in the plant world more difficult but more interesting.

The modern scientific conception of the universe excludes the idea of predestination. The relations between fructification and natural death must be regulated by the law of selection, according to which no organism survives if its reproduction is impossible. It occasionally happens that children are born without organs which are indispensable to life. Such monsters of different kinds being non-viable, cannot be said to be predestined to death, as they die because of defects in their structure. Others are born with all that is necessary for life, and survive for that reason, not because they are predestined to life. So also species of plants which develop incompletely and which die before they have produced spores or seeds, cannot survive; whilst those which die after having given birth to the next generation survive in their descendants. However quickly death follow the production of seed, the species will survive equally well. The cause of the natural death of plants must be sought, therefore, not in predestination, but in the mechanism of the organic processes.

Nothing seems more probable than that a plant should die when all its organic forces have been exhausted. It would be interesting, however, to ascertain the mechanism of that exhaustion, and this especially because it is often very difficult to imagine a cause for it. Many plants exist which produce several generations each season, in the same soil, without exhausting it. In perennial plants, some parts, such as the flowers, die periodically, although the plant itself is not exhausted. Everyone has seen that in geraniums some of the flowers wither whilst others are blooming, the process going on throughout the season. We can scarcely attribute such a natural death of the flowers to any exhaustion of the plant which continues to produce new flowers.

The fairly frequent prolongation of the life of plants is also out of harmony with the theory of natural death as the result of exhaustion. It sometimes happens that male plants produce female flowers abnormally; cases of this kind have been observed in willows, stinging-nettles, hops, and especially in maize.[84] Here we have to deal with a kind of monstrosity, differing, however, from the non-viable monsters of the human race, in the respect that the production of female flowers on the male branches results in the prolongation of their lives. Generally the male branches die a natural death as soon as the pollen has been shed, and therefore some time before the death of the female flowers. If, however, a male branch bears a female flower which becomes fertilised, then the life of the branch is prolonged until the seeds ripen. If the natural death of the male flowers is the result of exhaustion due to the development of the pollen, how can we reconcile this with the prolongation of life in a case where the male branch has also female flowers to nourish and seeds to mature?

It is quite clear that natural death, in such cases, is the result of a mechanism more complex than simple exhaustion.

Prof. de Vries has already noted that the duration of life in plants depends on their vital processes. That view implies that there are some qualities inherent in its organisation which can prolong or shorten the life of a plant, and it is here that we ought to find the key to the problem of natural death in the vegetable world. However, to gain exact knowledge of such factors, it would be necessary to have information on many points in plant physiology which unfortunately are very imperfectly known. In this respect, the vital conditions of the simplest plants, such as yeasts and bacteria, have been investigated much more fully. It is true that such low organisms reproduce freely either by division or by budding, so that they are amongst the organisms in which natural death is not inevitable. None the less, in their lives phenomena occasionally present themselves which can be interpreted as cases of natural death.

At a time when it was still unknown that all fermentation was due to the action of microscopic plants, it had been observed that, in certain conditions, fermentation ceased much more quickly than in other conditions. For instance, when sugar is being transformed to lactic acid, it is useful to add chalk, as otherwise the fermentation stops before the greater part of the sugar has been acted upon. When, in 1857, Pasteur made his great discovery of the lactic acid microbe, he showed that that little organism, although it could produce lactic acid, was interfered with by an excess of the acid. To secure complete fermentation, it was necessary to neutralise the acid by the addition of chalk.

When the action of lactic acid is continued too long, it not only arrests the process of fermentation but definitely kills the microbe. It is for that reason that it has been found difficult to preserve the lactic acid ferment for a long time in a living condition. Amongst the ferments which have been isolated from Egyptian ‘leben’ by MM. Rist and Khoury[85] there is one which is extremely delicate.

When it is inoculated deep in a nutritive medium, it dies in a few days, death, without doubt, being due to the lactic acid produced by the microbe from the sugar and not neutralised. As this transformation of sugar into lactic acid is a fundamental property of the microbe, depending on its constitution, the arrest of the fermentation and the death of the ferment in these definite conditions can be interpreted only as natural death due to auto-intoxication, that is to say to poisoning by a product of the physiological activity of the microbe itself. As death takes place at a time when the medium still contains enough sugar for the nutrition of the microbe, it is certain that it cannot be the result of exhaustion. This case of the lactic acid ferment is not unique. The microbe which produces butyric acid is also interfered with by the acid it secretes. M. G. Bertrand, who has examined carefully the microbe which produces fermentation in sorbose (sugar extracted from fruit of the service-tree) (_Sorbus domestica_) has informed me that this fermentation, too, ceases under the influence of the secretions of the microbes, and that the microbes undergo natural death at a time when the medium is far from exhausted of the nutritive material. The yeast which produces alcohol is also interfered with by an excess of alcohol, and as soon as a certain limit of alcoholic strength has been reached, fermentation stops. When the yeast is grown in media rich in nitrogen and poor in sugar, the plant takes the nitrogenous material and produces salts of ammonia. These alkalies damage the yeast and cause its death by auto-intoxication.[86]

In the examples that I have given, natural death was a result of the activity of the microbes, and was in correlation with their organisation. Such death can be avoided by changing the external conditions, and, if the acids or alkalies produced by these bacteria are neutralised, the bacteria survive. The facts are in harmony with those that I described in the case of the higher plants. By preventing the ripening of seed, the life of many annual plants may be preserved and the plants changed to biennials or perennials. In such cases death, although the result of the constitution of the plant, may be postponed.

We may ask then if the natural death of higher plants, usually attributed to exhaustion, cannot be explained more simply as the result of poisons produced in their metabolism. Many plants produce poisons which are fatal to animals and man. May they not also produce substances fatal to themselves? There is nothing improbable in the supposition that some of the poisons may develop when the seeds are ripening. By preventing the latter process, the ripening of the whole organism may also be prevented. Such a theory would explain the many cases of natural death which occur whilst the cell is far from having reached exhaustion. The equally numerous cases of partial death, such as that of flowers, whilst the same stem is still producing other flowers (_e.g._ geraniums) would be explained by a local action of the poisons not strong enough to kill the whole plant.

I must insist that this theory, that natural death of the higher plants, is the result of auto-intoxication, is a mere hypothesis which future investigations may disprove. If, however, it comes to be confirmed, it would explain the coincidence of death and fructification more simply than the hypothesis of predestination.

The higher plants may be subjects of auto-intoxication in the same fashion as bacteria and yeasts. If these poisons were produced before the ripening of the seeds, the plants would remain sterile, leaving no descendants, so that the race would become extinct. The production of poisons at the time of fructification would not interfere with the succession of generations, and the race would be preserved. As the poisoning is not necessary, it is easy to understand why many plants survive seeding and escape natural death. The Dragon-tree, baobab, and the cedars, which I spoke of earlier, would be examples of such escape.

Although the existence of auto-intoxication in the higher plants is still only a hypothesis, the natural death of bacteria and yeasts by poisons which they themselves produce is an ascertained fact.

In the plant world, therefore, there are examples of natural death (bacteria and yeasts) due to auto-intoxication, and there are other cases where high or low plants escape natural death.

II

NATURAL DEATH IN THE ANIMAL WORLD

Different origins of natural death in animals—Examples of natural death associated with violent acts—Examples of natural death in animals without digestive organs—Natural death in the two sexes—Hypothesis as to the cause of natural death in animals

The cases of natural death amongst animals differ from those found in the vegetable world by their greater variety and complexity. As M. Massart has shown for plants, so also natural death must have become established independently in different groups of animals. In some cases, the characters presented are strange and almost paradoxical.

It is usual to contrast natural death with violent death on account of the difference between the two. None the less, natural death may occur in the animal kingdom, that is to say death resulting directly from the constitution, and yet in intimate association with violent acts. I will give some examples.

Small, helmet-shaped organisms, transparent and graceful, are common on the surface of the sea. These have been described by zoologists under the name _Pilidium_. The organisation is simple. The body wall is a delicate pellicle, through which, on the lower surface, a mouth leads into a capacious stomach. Continual movements of waving cilia direct small particles of food through the mouth to the digestive stomach. As there are no organs of reproduction, it was assumed that these creatures were not adults, but floating larvæ of some marine animal, and, after a good deal of trouble, it was found that the Pilidia were the young stages of ribbon-shaped worms of the group of Nemertines. At a definite stage in the life-history, a fœtus begins to develop round about the stomach of the Pilidium, and eventually completely encloses it and detaches it by violent muscular contractions. The end of the story is that the fœtus abandons the body of the Pilidium carrying off with it the stomach, an organ necessary to the maintenance of life. The remnant of the Pilidium swims about in the sea-water, but soon dies as the result of the mortal wound caused by the removal of the digestive organs.

The act by which the Nemertine separates from its mother is violent, and yet the death of the Pilidium must be regarded as natural. It is the result of agencies within the body and not, as in most cases of accidental death, of violence from without.

The group of Nematode worms contains many common intestinal parasites of man, such as _Ascaris_, _Trichina_, _Trichocephalus_, _Oxyuris_, &c., but also others that live free in soil or water or in such fluids as vinegar. They are protected by a strong cuticle, and some of them are viviparous, that is to say, instead of laying eggs they give birth to young worms already well grown and capable of independent activity. Amongst the human Nematode parasites, the _Trichinæ_ give birth to swarms of small larvæ which easily escape from the body of the mother by the female generative aperture. In the case of some free-living Nematodes, however, the female aperture is too small to give passage to the rather stout larvæ. More than forty years ago, when I was investigating the life-history[87] of one of these Nematodes (_Diplogaster tridentatus_) I was struck by the fact that the larvæ could leave the body of the mother only by violence and after they had devoured most of its substance. These larvæ develop from eggs produced within the maternal body. As the external reproductive aperture of the female is minute, the larvæ cannot escape through it, but wander amongst the tissues tearing and absorbing them. The mother soon dies, and although her death is violent, it must be included in the category of natural death.

From the teleological point of view it might be said that Pilidium and Diplogaster cease to live because they have fulfilled their function of giving rise to a Nemertine or young Nematodes. Their natural death would thus be predestined. There is no ground for such an interpretation. On the other hand, it is certain that this death, coming after the birth of the new generation, is in no way against the preservation of the species in which the extraordinary natural death by violence occurs. If the female orifice of Diplogaster were slightly larger, the larvæ would emerge without difficulty and without causing the death of the mother which none the less would have fulfilled her purpose.

All the cases of natural death amongst animals are not so brutal as those of the Pilidium and the Nemertine worms. In many instances the death is peaceful. As very frequently it is difficult to establish definitely that the death is natural, I shall select clear cases.

Animals are occasionally found which are devoid of some organ necessary for prolonged life. The absence of a digestive tract in an animal that lives in an environment rich in dissolved nutritive material (as for instance tapeworms living in the intestinal tract) is not surprising. But when creatures of the sea or of fresh water have no digestive tract, their life can be maintained only at the expense of nutritive material stored within them during embryonic life. The death which comes eventually is truly natural. The best cases, that is to say those which can be studied most completely, of such natural death occur amongst the Rotifera. These are minute creatures of fresh or sea water, at one time confused with the Infusoria, but possessed of a much more complex organisation. They have a well-developed digestive tube, organs of excretion, nervous system, and organs of sense. The animals are diœcious; in each species both males and females exist. Whilst the females have the complete structure of the species, the males are much reduced, and are devoid of a digestive canal. The cuticle is fairly stout, and they are unable to absorb dissolved nutriment through it; as they have no organs of digestion, their life must be short.

To study in detail the life and death of these creatures, I selected a species sent to me by M. Haffkine. So far as I can judge, the species in question is a hitherto unknown member of the genus _Pleurotrocha_, and I propose for it the name _Pleurotrocha haffkini_. This rotifer is convenient to study as it thrives in vessels containing fresh-water to which some bread-crumb has been added (in the proportion of a gram of bread to 500 grams of water).

The sexes of the little rotifer can be distinguished from the earliest age, for eggs that are to become females are much larger than those from which males develop. It is easy to isolate the male eggs and to follow the life-history up to the moment of natural death. The whole course of life from the laying of the egg until death lasts only about three days, and is probably the shortest duration of life in the animal kingdom. Although some Ephemeridæ live only a few hours in the adult state, their total life-cycle is much longer than that of the rotifers, as the larval stages last for months or even for years.

The little males (Fig. 16) begin to swim soon after hatching, the wheel-apparatus and the musculature being vigorous. They seek out the females, as their reproductive organs are mature almost at the moment of hatching. The transparent body, which is devoid of digestive apparatus, swarms with mobile spermatozoa. As soon as the male has seized a female, he discharges the contents of his body. It might be supposed that such an evacuation would cause a violent perturbation of the system leading to the death of the organism. There is no question of this however. The males are able to live for twenty-four hours after having accomplished their function, and the period represents a third of their total duration of life. Moreover, I have isolated males from females without any prolongation of their lives. In one experiment, I isolated two males and placed a third in company with two females. It was the third specimen that lived longest.

The natural death of the males is foreshadowed by a weakening of the movements; although the muscles and cilia remain mobile, the whole animal moves only spasmodically; sometimes the muscles of the head contract, sometimes those of the tail, but no locomotion occurs. Occasionally there is a violent effort of ciliary motion as if the attempt were being made to overcome the immobility of the body. Such a condition lasts for several hours and is followed by death. The spermatozoa inside the body retain activity last of all.

Towards the crisis, bacteria, which abound in the medium occupied by the rotifers, begin to attack the males. Some cluster round the head, others round the tail, although none of them can effect entrance to the body. The death of the males cannot be attributed to microbial infection, but comes from some intrinsic cause.

Is it inanition that is the cause of death? I do not think so, because up to the time of death the tissues appear to be unmodified. In the case of the females I have sometimes seen phenomena of inanition. In old and exhausted cultures the starved females become thin, flattened and quite transparent, and the tissues lose their granular appearance. No such changes are visible in the dying males, the tissues of which, on the contrary, retain a normal aspect.

The most probable explanation is that death comes from poisoning by the secretions of the tissues themselves. The large size of the organs of excretion indicates that in the course of metabolism waste matter is produced some of which is got rid of. If, after a time, the secretions are insufficiently eliminated, the tissues must be poisoned. As death is preceded by a spasm of uncoordinated movement, it appears as if the fatal intoxication of the males affected the nervous system first. The vibrating cilia and the muscles are attacked later.

There can be no doubt but that the death of these male rotifers is natural in the fullest sense. The females, however, although they are provided with complete digestive organs, do not escape a similar fate. Their life is longer and more complex than that of the males, and so is subject to many more chances. The females therefore may come to die from starvation or from other external, accidental causes. But, if they are kept in favourable conditions, they may live for about fifteen days, towards the end of which they die naturally, exhibiting the symptoms that I have described in the case of the males (Fig. 17).

Rotifers are not the only animals which undergo natural death in a fashion quite unlike the violent end of Pilidium and Diplogaster. There are other cases amongst invertebrates, but I shall limit myself to describing one that is well ascertained.

More than fifty years ago, Dana, the American naturalist, discovered a pelagic marine creature with characters so curious that he gave to it the name _Monstrilla_. It is a little crustacean akin to the _Cyclops_ of lakes. But although the latter is endowed with the organs necessary to capture and digest food, _Monstrilla_ has neither organs of prehension nor a digestive canal. It is a highly muscular animal with organs of sense and reproduction and a nervous system; but it is devoid of apparatus for prolonging life by nutrition. _Monstrilla_ therefore is a creature doomed to natural death.

The detailed observations of M. Malaquin[88] have supplied full information regarding this strange life-history. _Monstrilla_ passes a portion of its life as a parasite on Annelid worms. In that stage it accumulates the necessary material for the growth of the sexual products (ova and spermatozoa) and for free life in the sea whilst the young are developing. It is not only the males which have no digestive apparatus. The females also lack it, which is the more surprising as they carry about the eggs attached to the body (as is done by many other Crustacea, such as crayfish and lobsters) until the young are ready to hatch (Fig. 18). M. Malaquin thinks that the Monstrillas die of starvation.

“As they are without a digestive tube or organs of prehension or mastication,” M. Malaquin says (p. 192), “the Monstrillas, which have no means of nutrition, are doomed to death from inanition after a short pelagic life. This is a logical inference from their structure.”

In support of his view, M. Malaquin states that before death the tissues and organs show plain signs of degeneration.

“The eyes first show traces of degeneration. The pigment spreads and disappears little by little and then the visual elements fade out.”

“Finally, individuals, usually females, show complete degeneration. A female taken in a fine-meshed net showed no trace of organs in the head; the eyes, the brain and the intestinal tract had disappeared almost completely. The antennæ were reduced to stumps consisting of the lowest joint and a portion of the second. These were clear indications of the senility that precedes death” (p. 194).

Such evidence not only supports the hypothesis that the natural death of Monstrilla is due to inanition, but is opposed to a similar interpretation being applied to the case of male rotifers, in which death is not preceded by wasting of the organs. The death of some insects, which comes rapidly after the adult stage has been reached, cannot readily be attributed to starvation. In the strange butterflies known as psychids (_Solenobia_) some of the females lay eggs without having been fertilised,[89] and their life in the adult condition lasts only a day. On the other hand, other females of the same butterfly are fertilised before laying their eggs and in this case survive for more than a week although they take no food. The rapid death of the first-mentioned set cannot be attributed to inanition.

In some Ephemeridæ, which supply good cases of natural death, the end comes after a few hours of adult life without any sign of degeneration of the organs. As in others (_Chloë_), life lasts for several days without food having been taken, it is clear that inanition is not the cause of the swift arrival of death in the first set. It is much more probable that the natural death is due to an auto-intoxication which takes effect at different intervals of time in different circumstances.[90]

In the higher animals such as vertebrates the conditions are less favourable than in the case of insects for the investigation of the causes of natural death. Vertebrates have always well-developed organs of digestion and so live a relatively longer time and encounter a greater number of chances of accident, with the result that in most cases death comes from external accidental causes. Vertebrates usually perish from hunger or cold, or are devoured by their enemies or killed by the attacks of parasites or diseases. There remains only the human race amongst the more highly developed animals, in which to study the onset of natural death. And in the human race cases which may be designated as natural are extremely rare.

III

NATURAL DEATH AMONGST HUMAN BEINGS

Natural death in the aged—Analogy of natural death and sleep—Theories of sleep—_Ponogenes_—The instinct of sleep—The instinct of natural death—Replies to critics—Agreeable sensation at the approach of death

The death of old people, which has often been described as natural death, is in most cases due to infectious diseases, particularly pneumonia (which is extremely dangerous) or to attacks of apoplexy. True natural death must be very rare in the human race. Demange[91] has described it as follows:—“Arrived at extreme old age, and still preserving the last flickers of an expiring intelligence, the old man feels weakness gaining on him from day to day. His limbs refuse to obey his will, the skin becomes insensitive, dry, and cold; the extremities lose their warmth; the face is thin; the eyes hollow and the sight weak; speech dies out on his lips which remain open; life quits the old man from the circumference towards the centre; breathing grows laboured, and at last the heart stops beating. The old man passes away quietly, seeming to fall asleep for the last time.” Such is the course of what properly speaking is natural death.

The natural death of human beings cannot be regarded as due to exhaustion from reproduction or from inanition, as in the case of _Monstrilla_. It is much more likely that it is due to an auto-intoxication of the organism. The close analogy between natural death and sleep supports this view, as it is very probable that sleep is due to poisoning by the products of organic activity.

It is more than fifty years since sleep was explained as the result of auto-intoxication. Obersteiner, Binz, Preyer, and Errera are among the competent men of science who have taken this view. The first two attributed sleep to an accumulation in the brain of the products of exhaustion which are carried away by the blood during repose. The attempt has been made even to discover the nature of these narcotic substances. Some investigators think that an acid, produced during the activity of the organs, is stored up in quantities that cannot be tolerated. During sleep, the organism gets rid of this excess of acid.

Preyer[92] tried to put the problem upon a more exact basis by the theory that the activity of all the organs gives rise to substances which he called _ponogenes_ and which he regarded as producing the sensation of fatigue. According to him these substances accumulate during the waking hours, and are destroyed by oxidation during sleep. Preyer thinks that lactic acid is the most important of the ponogenes, and lays stress on its narcotic effect. If his theory were correct, there would be a remarkable analogy between the auto-intoxication by lactic acid in the cases of man and animals, and the case of bacteria which produce the same acid and the fermenting activity of which is arrested as the acid accumulates. Just as sleep may be transformed to natural death, so also the arrest of lactic fermentation may be followed by the death of the bacteria which form the acid.

So far, however, there has been no confirmation of Preyer’s theory. Errera[93] has brought forward against it another theory according to which the cause of sleep is not acid products, but certain alkaline substances described by M. Armand Gautier under the name of _leucomaines_. Gautier laid down that these substances act on the nervous centres and produce fatigue and sleepiness. According to Errera they might very well be the cause of sleep, as that comes on at a time when there is the greatest accumulation of these leucomaines in the body. He thinks that their action in producing sleep is a direct intoxication of the nerve centres. During sleep they are removed, and the disturbance which was produced in the organism is arrested.

If it were possible to accept Errera’s theory, a kind of analogy could be established between sleep and natural death on the one hand, and the arrest of development and death of yeast grown in nitrogenous media on the other hand, because in the latter case the poisoning is produced by an alkaline salt of ammonia. It must be confessed, however, that the actual state of our knowledge does not allow of a definite view of the real mechanism of the sleep-producing intoxication. Our ideas regarding leucomaines in general are still incomplete, and, recently, one of them, _adrenaline_, the product of the supra-renal capsules, has been investigated. Adrenaline is an alkaloid[94] which is produced in the supra-renal bodies and is discharged into the blood. It has the power of contracting arteries strongly, and has been used to control blood-pressure. When it is given in large quantities or in frequent doses, it acts as a true poison, whilst, in small doses, it produces anæmia of the organs and has a special influence on the nervous centres. Dr. Zeigan[95] has shown that a milligramme of adrenaline, mixed with five grammes of normal salt solution injected into the brain of cats, produces a soporific action. “About a minute after the injection, the animal appears to be plunged into deep sleep which lasts from 30 to 50 minutes. During this time, the sensitiveness of the animal has completely ceased throughout the body, and for some time after that it is much decreased. When they awake the animals seem to have been drunk with sleep for some time.” Sleep is generally associated with anæmia of the brain, and as adrenaline can actually produce such anæmia, it might be supposed that this narcotic substance is the most important of the organic products which give rise to sleep. Against this hypothesis, however, some weight must be given to recent investigations on fatigue and its causes.

Each stage in the advance of knowledge has had its influence on the study of the interesting and complex problem of sleep. When it was thought that alkaloids (ptomaines) were of great importance in infectious diseases, it was attempted to explain sleep as due to the action of similar bodies. Now, when we believe that in such diseases the chief part is played by poisons of extremely complex chemical composition, the attempt is made to explain fatigue and sleep by similar bodies.

Weichardt[96] has recently made the best known investigations in this direction. This young man maintains with ardour the view that during the activity of organs there is an accumulation of special materials which are neither organic acids nor leucomaines, but which are much more like the toxic products of pathogenic bacteria.

Weichardt made animals in his laboratory go through fatiguing movements for hours and then killed them. The extract from muscles of such animals had a powerful toxic effect when it was injected into normal animals, producing lassitude and sometimes death within 20 to 40 hours. As all attempts to determine the exact chemical nature of this fatigue-producing substance were baffled, it is impossible to get an exact account of it. Amongst its properties there is one of great interest. When it has passed into the circulation of normal animals in quantities insufficient to produce death, it excites the formation of an anti-toxin in the same way as a poison of diphtheria stimulates the production of a diphtheria anti-toxin.

When Weichardt injected into animals a mixture of the poison which produces fatigue with small doses of the serum antidote, no results followed. The neutralising effect of the antidote was apparent even when it was introduced by the mouth. Towards the end of his investigations, Weichardt supposed that it would be possible to obtain a material that would prevent fatigue.

Although it is still impossible to specify exactly the nature of the substances which accumulate during the activity of organs and which produce fatigue and sleep, it is becoming more and more probable that such substances exist, and that sleep is really an auto-intoxication of the organism. So far, such a theory has not been shaken by any argument. Recently M. E. Claparède,[97] a psychologist of Geneva, has argued against the current theory of sleep. He thinks that it is contradicted by the fact that new-born infants sleep a great deal, whilst very old people sleep very little. This fact, however, can readily be explained by the greater sensibility of the nerve centres of infants, as shown with regard to many harmful agencies. The other objections of Claparède, such as the fact that sleepiness is induced by exercise in the open air, or that excess of sleep itself produces sleepiness, are not really incompatible with the theory of auto-intoxication. They are facts of secondary importance probably depending on some complication which the present state of our knowledge makes it difficult to indicate exactly. The insomnia of neurasthenia, which Claparède brings forward as another objection, can readily be explained as due to hyperæsthesia of the nervous tissues which lose part of their sensitiveness to poisons.

On the other hand, there are many well established facts in agreement with the theory of auto-intoxication. Leaving out of the question sleep induced by narcotics, I may mention in this connection the so-called “sleeping sickness.” It has been proved that this disease is caused by a microscopic parasite, the _Trypanosoma gambiense_ of Dutton, which develops in the blood and spreads to the liquid of the membranes surrounding the central nervous system. One of the most typical symptoms of the advanced stages of this disease is continual drowsiness. “The drowsiness increases progressively, and the habitual attitude becomes characteristic; the head is bent on the breast; the eyelids are closed; in earlier stages the invalid can be aroused easily, but, after a time, incurable attacks of sleep overcome the patient in all circumstances, but especially after meals. These fits of sleepiness become longer and deeper, until they reach a comatose condition from which it is almost impossible to arouse the patient.”[98] The total result of medical knowledge of this disease is that it is impossible to doubt that the sleepiness is due to intoxication produced by the poison of the trypanosome.

Claparède has opposed what he calls an “instinctive” theory to the toxic theory of sleep. According to this theory, sleep is the manifestation of an instinct “the object of which is to arrest activity; we do not sleep because we are intoxicated or exhausted, but to prevent ourselves from falling into such a condition.” However, in order to bring this narcotic instinct into play, certain conditions are necessary, one of which certainly would be the intoxication of the nerve centres. M. Claparède supposes that sleep is an active phenomenon, induced when waste matter begins to accumulate in the organism. “To bring about sleep, the nerve centres must be influenced by waste matter, and this influence can readily be regarded as a kind of intoxication.”

Hunger is an instinctive sensation as much as sleepiness, but it does not appear until our tissues are in a condition of exhaustion, the exact nature of which cannot as yet be indicated. There is no real contradiction between the toxic and instinctive theories of sleep. The two theories represent different sides of a special condition of the organism.

The analogy between sleep and natural death is in favour of the supposition that the latter, also, is due to an intoxication much more profound and serious than that which results in sleep. Therefore, as natural death in human beings has been studied only very superficially, it is impossible to do more than frame theories regarding it.

It would be natural if, just as in sleep there is an instinctive desire for rest, so also the natural death of man were preceded by an instinctive wish for it. As I have already discussed this subject in the “Nature of Man” (chap. xi) I need not deal with it at length here. I should like, however, to add some information which I have recently obtained.

The most striking fact in favour of the existence of the instinct for natural death in man appears to me to have been related by Tokarsky in regard to an old woman. While Tokarsky was alive I asked one of his friends to obtain for me further details of this very interesting case. Unfortunately Tokarsky could add nothing to what he had already published in his article. I think that I have discovered the source of his information. In his famous book on the _Physiology of Taste_[99] Brillat-Savarin relates as follows:—“A great-aunt of mine died at the age of 93. Although she had been confined to bed for some time her faculties were still well preserved, and the only evidence of her condition was the decrease in appetite and weakening of her voice. She had always been very friendly to me, and once when I was at her bedside, ready to tend her affectionately, although that did not hinder me from seeing her with the philosophical eye that I always turned on everything about me, ‘Is it you, my nephew?’ she said in her feeble voice. ‘Yes, Aunt, I am here at your service, and I think you will do very well to take a drop of this good old wine.’ ‘Give it me, my dear; I can always take a little wine.’ I made ready at once, and gently supporting her, gave her half a glass of my best wine. She brightened up at once, and turning on me her eyes which used to be so beautiful, said: ‘Thank you very much for this last kindness; if you ever reach my age you will find that one wants to die just as one wants to sleep.’ These were her last words, and in half an hour she fell into her last sleep.” The details make it certain that this was a case of the instinct of natural death. The instinct showed itself at an age not very great in the case of a woman who had preserved her mental faculties. Generally, however, it seems not to appear till much later, for old men usually exhibit a keen wish to live.

It is a well-known saying that the longer a man has lived the more he wishes to live. Charles Renouvier,[100] a French philosopher who died a few years ago, has left a definite proof of the truth of the saying. When he was eighty-eight years old, and knew that he was dying, he recorded his impressions in his last days. Let me quote from what he wrote four days before his death. “I have no illusions about my condition; I know quite well that I am going to die, perhaps in a week, perhaps in a fortnight. And I have still so much to say on my subject.” “At my age I have no longer the right to hope: my days are numbered, and perhaps my hours. I must resign myself.” “I do not die without regrets. I regret that I cannot foresee in any way the fate of my views.” “And I am leaving the world before I have said my last word. A man always dies before he has finished his work, and that is the saddest of the sorrows of life.” “But that is not the whole trouble, when a man is old, very old, and accustomed to life, it is very difficult to die. I think that young men accept the idea of dying more easily, perhaps more willingly than old men. When one is more than eighty years old, one is cowardly and shrinks from death. And when one knows and can no longer doubt that death is coming near, deep bitterness falls on the soul.” “I have faced the question from all sides in the last few days; I turn the one idea over in my mind; I _know_ that I am going to die, but I cannot _persuade_ myself that I am going to die. It is not the philosopher in me that protests. The philosopher does not fear death; it is the _old man_. The old man has not the courage to submit, and yet I have to submit to the inevitable.”

I know a lady, a hundred and two years old, who is so oppressed by the idea of death, that those about her have to conceal from her the death of any of her acquaintances. Mde. Robineau, however, when between one hundred and four and one hundred and five years old, became quite indifferent to the close approach of her own death. She often expressed a wish for it, thinking herself useless in the world.

M. Yves Delage[101] in an analysis of my “Nature of Man” doubted the existence of an instinct for death. “Animals,” said he, “cannot have the instinct for death, because they do not know of death. In their case, we must consider that what happens is an apathy tending to the abolition of the sense of self-preservation. In man, the knowledge of death implies that the indifference to its approach cannot be an instinct.” “There may be developed, at the end of life, a special state of mind which accepts death with indifference or with pleasure, but such a state cannot be designated as an instinct.” M. Delage, however, does not suggest what the state of mind in question is to be called. As the aunt of Brillat-Savarin compared her sensations just before death with the desire to sleep, and as this desire is an instinctive manifestation, I think that the cheerful acquiescence in death, exhibited by extremely old people, is also a kind of instinct. However, the important matter is that the sentiment exists, and not what we are to call it. M. Delage is far from denying its existence.

Dr. Cancalon,[102] another of my critics, cannot admit the existence of an instinct of death, “because of the theory of evolution. Of what good would it have been, as M. Metchnikoff tells us that natural death is very rare; how could it have been transmitted, as it comes into existence long after the age of reproduction, and how could it have aided the survival of the species? If its existence were proved as the result of biological evolution, it would be a contradiction of adaptation and an argument in favour of final causes.” I cannot agree in any way with these opinions. In the first place, it is well known that men and animals have many harmful instincts that do not tend to the survival of the species. I need recall only the disharmonic instincts which I described in the “Nature of Man,” such as the anomalies of the sexual instinct, the instinct which drives parents to devour their young or which attracts insects to flames. The instinct of natural death is far from being harmful, and may even have many advantages. If men were convinced that the end of life were natural death accompanied by a special instinct like that of the need for sleep, one of the greatest sources of pessimism would disappear. Now pessimism is the cause of the voluntary death of a certain number of people and of many others refraining from reproduction. The instinct of natural death would contribute to the maintenance of the life of the individual and of the species. On the other hand, there is no difficulty in admitting the existence of instincts hostile to the preservation of the species, especially in the case of man, in whom individualism has reached its highest development. As man is the only animal with a definite notion of death, there is nothing extraordinary if it is in man that the instinctive wish for death develops. M. Cancalon denies the possibility that death can be pleasant, as it is the arrest of the physiological functions; but as sleep and syncope are often preceded by very pleasant sensations, why may not this also happen in natural death? Several facts prove it beyond dispute. It is even probable that the approach of natural death is one of the most pleasant sensations that can exist.

It is indubitable that in a large number of cases of death, the cessation of life is associated with very painful sensations. One has only to see the horror shown in the faces o£ many dying people to be convinced of this, but there are diseases and serious accidents in which the approach of death does not arouse sorrowful sensations. I myself, in a crisis of intermittent fever, in which the temperature descended in a very short time from about 106° Fahr. to below normal, experienced a feeling of extraordinary weakness, certainly like that at the approach of death. This sensation was much more pleasant than painful. In two cases of serious morphia poisoning, my sensations were more agreeable; I felt a pleasant weakness, associated with a sensation of lightness of the body, as if I were floating in the air.

Those who have noted the sensations of persons rescued from death have related similar facts. Prof. Heim, of Zurich, has described a fall in the mountains which nearly killed him, as well as several similar accidents to Alpine tourists. In all these cases he states that there was a sensation of pleasure.[103] Dr. Sollier has told of a young woman addicted to morphia, who had been convinced that she was at the point of death. On recovering from a most serious attack of syncope, from which she was restored only by giving another dose of morphia, she cried: “I seem to come from far away; how happy I was!” Another of Dr. Sollier’s patients, a lady who had an attack of peritonitis from which she expected to die, felt herself “suffused with a feeling of well-being, or rather the absence of all pain.” In a third Case of Dr. Sollier, a young woman suffering from puerperal fever, feeling herself at the point of death, had a similar sensation “of physical well-being and of detachment from everything.”[104]

As a sensation of happiness occurs even in cases of pathological death, it is much more likely to occur in natural death. If natural death be preceded by the loss of the instinct of life and by the acquisition of a new instinct, it would be the best possible end compatible with the real organisation of human nature.

I do not pretend to give the reader a finished study on natural death. This chapter of Thanatology, the science of death, only opens the subject; but it is already apparent that study of the circumstances of natural death in plants, in the animal world, and in human beings, may give facts of the highest interest to science and humanity.