This appearance of an important correspondence vanishes as soon as we examine Weismann’s pronouncement on the problem of death. For Weismann admits the differentiation between the mortal soma and the immortal germ-plasm only in relation to multicellular organisms; with the unicellular beings the individual and the reproductive cell are still one and the same.[21] The unicellular he thus affirms to be potentially immortal; death appears only among the metazoa, the multicellular. This death of the higher organisms is, it is true, a natural one, a death from inner causes, but it does not depend on an inherent quality of the living substance,[22] is not to be conceived as an absolute necessity based on the nature of life.[23] Death is rather a purposive contrivance, a phenomenon of adaptation to the external conditions of life, because after the differentiation of the corporeal cells into soma and germ-plasm the indefinite prolongation of the life of the individual would have become a quite inexpedient luxury. With the appearance of this differentiation among multicellular organisms death became possible and expedient. Since then the soma of the higher organisms dies after a certain time from internal causes; the protozoa, however, remain immortal. Propagation, on the other hand, was not first introduced with death; it is on the contrary a primordial property of living matter like growth, in which it originated, and life has gone on uninterruptedly from its inception on the earth.[24] It is easy to see that to concede natural death to the higher organisms does not greatly help our case. If death is a late acquisition of life, then death instincts traceable to the beginning of life on this planet no longer come into question. Multicellular organisms may continue to die from internal causes, whether defect of differentiation or imperfections of their metabolism; it possesses no interest for the inquiry on which we are engaged. Such a conception and derivation of death certainly more nearly approaches the ordinary human view of it than the unwonted assumption of ‘death-instincts’. The discussion which has centred round Weismann’s assertations has in my opinion had no decisive result in any direction.[25] Many writers have reverted to the standpoint of Goette (1883) who saw in death the direct consequence of propagation. Hartmann does not regard as the characteristic of death the appearance of a ‘corpse’, a piece of living substance which has ‘died off’, but defines it as the ‘definitive end of individual development’. In this sense protozoa are also subject to death; with them death invariably coincides with propagation, but it is, so to speak, disguised by the latter, for the whole substance of the parent organism may be absorbed directly into the new individuals.[26]