We are so accustomed to look upon death as something natural and inevitable, that it has long since come to be regarded as inherent in organisms. However, when biologists investigated the matter more carefully, they failed to discover any proof of the accepted doctrine. Observation of members of the lowest grade of animal life, such as infusorians and other protozoa, has shown that these reproduce by simple division, and in a very short time multiply to an astonishing extent. Generation succeeds generation, with the utmost rapidity and without the intervention of death; no single corpse appears in the swarming masses of animalculæ. From such facts, which are extremely easy to confirm, several biologists, and in specially Bütschli and Weismann,[319] have deduced an immortality of the unicellular organisms. When an infusorian has divided, each daughter organism rapidly completes itself and sets about again dividing in the fashion of its parent. The process may be more complicated, as in the cases where a single organism breaks up into several portions each of which contains an essential part of the parent organism. Many unicellular organisms reproduce in such a fashion, and as each animal divides simultaneously into a number of individuals of the new generation, the individuality is destroyed. It is possible to admit with Götte[320] that such a process is natural death, although there is no actual destruction and no corpse. In any event it cannot be disputed that lower organisms are not subject to the natural death that comes inevitably to man and the higher animals. It has been suggested that the debility of infusorians after a rapid series of divisions, and before conjugation, is to be interpreted as natural death. But the rejuvenescence that follows conjugation is incompatible with such an interpretation. Moreover, when conjugation does not occur, and the debility leads to death, the deaths must be regarded as accidental. The theory of the immortality of unicellular organisms is now generally accepted. However, there are animals, higher in the scale of life, to which natural death does not come. Among these occur certain forms of considerable complexity, composed of many organs and very many cells, such as many polyps, and some worms, especially annelid worms. Some annelids (Fig. 17) reproduce by transverse divisions very actively. “Throughout the summer,” said E. Perrier,[321] “the Naïdimorpha are devoid of genital organs, and apparently (according to unpublished observations of Maupas), they may be kept alive for several years, and perhaps indefinitely, in this sexless condition.” This certainly may be regarded as a case of immortality due to the indefinite power of regeneration possessed by a complex animal.