Intra-individual competition and the consequent intra-individual cell line selection are mostly, but not necessarily always (see XII.126.96.36.199) undesirable from the standpoint of maintenance of the integrity of a multicellular organism. If the cells could compete together, those that were capable of the fastest reproduction and the most effective spreading in the body of the organism would, in time, predominate in the individual. Then these cells would multiply at the expense of those that invest their energy into performing their physiological functions in the body of the multicellular organism rather than into their own reproduction. Those cells that would be capable of preferentially occupying the position and function of precursors of germinal cells would then predominate at the population level. The most important consequence of the existence of a single-cell phase in the life cycle of organisms is that all the cells in the body of a multicellular organism are mutually genealogically related and thus genetically more similar than the cells occurring in the bodies of two multicellular individuals. Genetic similarity and, if we ignore newly formed somatic mutations, even genetic identity of cells in an organism fundamentally limit the occurrence of mutual competition of cells and cell lines within the multicellular organism – intra-individual competition of cell lines.