No doubt a certain number of deaths are recorded in statistics as being due to old age, without visible malady. Sometimes decrepit old men feel no pain and seem to fall quietly into their eternal sleep; but autopsy reveals serious lesions of the internal organs. There is reason to believe that even such deaths are in reality violent and are usually caused by infectious microbes. The general effect on the mind produced by examination of the collected facts is not an acceptance of the view that natural death is essentially inherent in living organisms, but the production of a wish to discover if there be any real proof of its existence. For some time natural death has been ascribed only to the parts of the body that are of use in the individual life. Those cells, the function of which is to secure reproduction of the species, are, like unicellular organisms, potentially immortal. The egg-cell of the female is transformed into a fœtus, and so is the starting-point of the new generation, while the sexual cells of the new generation give rise to the third generation, and so on, in an endless chain of life. The greater number, by far, of the eggs and spermatozoa perish; but their death is not natural but violent, being due to harmful external agencies. An infinitesimal minority of the sexual cells survive indefinitely in the successions of generations. Scientific proof exists, therefore, that our bodies contain immortal elements, eggs or spermatozoa. As these cells not only are truly alive but exhibit properties that are within the category of psychical phenomena, it would be possible to build up a serious thesis on the immortality of the soul. Observations on protozoa, and especially on the infusorian group of protozoa, show that these simple beings, each of which is composed of no more than a single cell, possess a high degree of sensibility. They select their food, distinguish living from dead animalculæ,[324] seek out their mates for conjugation, avoid danger, and hunt their prey; in fact, they are in possession of a set of qualities that must be included in psychical phenomena. Although such phenomena are very much lower in the case of the infusorians than in the case of higher animals, it is possible to speak of the soul of protozoa. Moreover, as the body is immortal by reason of its indefinite power of reproduction by division, the soul also of these creatures is immortal. However, the soul is so primitive that it is impossible to speak in definite terms about it.