The shorter the intestine, the fewer microbes it contains and the longer the relative duration of life. As an example, he quoted the relatively great longevity of birds and bats. Those animals, adapted to aerial life, have to weigh as little as possible. To that end, they empty their intestine very frequently and this in consequence is not used as a reservoir for alimentary refuse; as it is but little developed, it contains a much smaller number of microbes. The longevity of flying animals is relatively much greater than that of mammals with a large intestine full of microbes, a constant source of slow poisoning.
After treating the question of longevity, Metchnikoff dealt with that of death.
Living beings die, in the great majority of cases, in consequence of diseases or accidents with an external cause; one involuntarily wonders whether there is such a thing as “natural death,” i.e. arising exclusively from causes due to the organism itself. A review of known facts allowed Metchnikoff to draw the following conclusions: unicellular inferior beings have no natural death; they merely die by accident. Their individual life is very short and comes to an end by multiplication or division of a unit into two; there is no trace of a corpse in this loss of previous individuality.
Among superior plants, certain trees attain considerable dimensions (dragon-tree, baobab, oak, cypress), live for centuries, and die from external causes. Their organism presents no internal necessity for a natural death. On the other hand, a multitude of other plants have but a short life and their natural death coincides usually with the ripening of the seed. It has even been observed that it is possible to retard the death of a plant by preventing it from fructifying. For instance, lawns made up of grass mown before it runs to seed remain green and living whilst grass allowed to flower and bear seed becomes yellow and dries up. It is a well-known fact that fruits and seeds are frequently poisonous. Therefore Metchnikoff supposed that the death of the plant may be due to an auto-intoxication by poisons manufactured by it in order to defend its seeds and ensure the next generation; in Nature, the individual does not count, but the species. Once the survival of this is ensured the individual may disappear.
A similar phenomenon of auto-intoxication is manifested by lower vegetables, yeasts, and microbes. Pasteur, who discovered the microbe of lactic fermentation, found that this micro-organism, which itself produces lactic acid, perishes because of the over-production of this substance. Yeasts, again, cannot bear an excess of alcohol, their own product. Thus the vegetable kingdom offers us examples of the absence of natural death as well as examples of a natural death due to an auto-intoxication of the organism.
In the animal kingdom examples of natural death are also to be found, but only very exceptionally. Those examples are provided by Rotifera (inferior worms) and by Ephemeridæ. Their adult life is reduced to the sexual act, almost immediately followed by death without an external cause. Their life is so short that they do not even feed and lack developed buccal organs. That in itself constitutes an organic cause of inevitable, i.e. natural, death.
Among human beings natural death is extremely rare. It sometimes occurs in very old people, under the shape of a peaceful last sleep. The likeness it bears to sleep is so striking that Metchnikoff thought himself authorised to form the following hypothesis concerning the analogy in their mechanism.
According to a theory of Preyer’s, fatigue and sleep are due to a periodical auto-intoxication set up by the products of the vital activity of our organism. These products are destroyed by oxidation during sleep, after which fatigue disappears and awakening comes. According to Metchnikoff it may be that the mechanism of natural death also consists in an auto-intoxication by the progressive accumulation of toxic products during the whole of life. The analogy between sleep and natural death allows the supposition that, as before going to sleep an instinctive desire for rest is felt, in the same way natural death must be preceded by an instinctive desire to die. Moreover, this is confirmed by concrete examples. Thus that of an old woman of ninety-three who expressed that desire in the following terms to her great-nephew: “If ever you reach my age, you will see that death becomes desired just like sleep.” The same thought had been expressed by the biblical patriarchs who fell asleep satiated with life.
When, owing to the progress of Science, men reach the development of the instinct of death, they will look upon Death with the same calm as do very old people, and it will cease to be one of the principal causes of pessimism. It is for that reason that we must learn to prolong life and to allow all men to realise their complete and natural vital cycle, thus ensuring their moral balance.