This chapter discusses the role of viruses in nature. Viral transduction of structural and regulatory genes provides a means for information to leave the body of an organism other than through the germ cells. Natural selection acts upon the cell–virus nucleic acid coupling and the rate and direction of the evolution of any species depends upon the number of associated viruses and the extent to and speed with which they allow information to be cycled through the total gene pool of that population. There are three mechanisms by which gene material can be transferred from cell to cell: (1) transformation, (2) transduction, and (3) sexual conjugation. Transformation is the most random and inefficient process; it requires the laws of diffusion and the existing chemistry of the cell membrane, modified in contemporary cells by the development of transport systems, which facilitate membrane penetration. Transduction requires the development of genes for capsomere proteins to encapsidate nucleic acid and a sophistication of the process of membrane evagination to package nucleic acid into free particles, These are relatively modest genetic adaptations. However, true sexual union as it occurs in modern eukaryotes, requires such a high degree of cytological organization that it is inconceivable that it could have operated efficiently during the first billion or so years of cell evolution.