A great deal of recent biological research has been concerned with elucidating the structure and functions of the nucleic acids, both the DNA of the chromosomes and similar bodies and the RNA which is believed to function as an intermediary between the DNA and the extra-nuclear protein-synthetic activities of the cell. The DNA molecules are now believed to be the physical embodiment of at least the major part of the genotype of the organism. As is indicated in Chapter 9, phylogenetic classification is concerned to express genetica! relationships, and there is reason to suppose that these relationships would be most clearly seen if we could compare the genotypes directly. Just this is what we may hope to be able to do if a practical technique can be devised for unravelling the structure of the DNA molecules of numerous species. This, however, would be a forbiddingly complex and difficult task, unlikely to be achieved in the near future.