is only a continuation of the mother-individual; it contains not only half of the substance, but also the organization, and life is continued directly from mother to daughter. The daughter is simply half of the mother, which is subsequently regenerated; and the other half of the mother lives on in the other daughter, so that nothing dies in this multiplication. It may be said that the daughter has to develop the other half of its body anew, and that therefore it is a new individuality, and not merely a continuation of the old, and that therefore the unicellular animals are not immortal. The 'immortality' of the Protozoa may be scoffed at; the idea may seem absurd that the 'immortal' Protozoa are still the same individuals which lived upon the earth millions of years ago, but all such objections mean no more than doctrinaire quibbling with the concepts of 'individual' and 'immortality,' which do not exist in nature at all, but are mere human abstractions, and therefore only of relative value. My thesis as to the potential immortality of the Unicellulars aims at nothing more than impressing on Science the fact that the occurrence of physiological, that is, natural, death is causally associated with the transition from single-celled to many-celled organisms; and this is a truth which will not be overthrown by any sophisms. It is the Volvocineæ which show us, so to speak, the exact point at which natural death set in, at which it was introduced into the world of life. In _Pandorina_ the state of things is still the same as in single-celled organisms, for each cell is still all in all, each can bring forth the whole, none dies from physiological causes involved in the course of development, and they are therefore 'immortal' in the sense stated. But in _Volvox_ the 'individual' dies when it has given off its reproductive cells, because here the contrast between germ-cells and body has developed. Only the body is mortal in the sense of being subject to natural death; the germ-cells possess the potential immortality of the single-celled animals, and it is necessary that they should possess it if the species is to continue to exist.