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.