Consequently, as soon as sexual methods of propagation superseded the more primitive a-sexual methods, it became desirable in the interests of the sexually-propagating species that their constituent individuals should cease to be immortal, so that the species should always be recuperated by fresh, young, and well-formed representatives. Consequently, also, natural selection would speedily see to it that all sexually-propagating species should become deprived of the aboriginal endowment of immortality, with the result that death is now universal among all the individuals of such species—that is to say, among all the metazoa and metaphyta. Nevertheless, it is to be remembered that this destiny extends only to the parts of the individual other than the contents of those specialized cells which constitute the reproductive elements. For although in each individual metazoon or metaphyton an innumerable number of these specialized cells are destined to perish during the life, or with the death, of the organism to which they belong, this is only due to the accident, so to speak, of their contents not having met with their complements in the opposite sex: it does not belong to their essential nature that they should perish, seeing that those which do happen to meet with their complements in the opposite sex help to form a new living individual, and so on through successive generations _ad infinitum_. Therefore the reproductive elements of the metazoa and metaphyta are in this respect precisely analogous to the protozoa: potentially, or in their own nature, they are immortal; and, like the protozoa, if they die, their death is an accident due to unfavourable circumstances. But the case is quite different with all the other parts of a multicellular organism. Here, no matter how favourable the circumstances may be, every cell contains within itself, or in its very nature, the eventual doom of death. Thus, of the metazoa and metaphyta it is the “germ-plasms” alone that retain their primitive endowment of everlasting life, passed on continuously through generation after generation of successively perishing organisms.