One of the most obvious properties of life on Earth is that its organisms form a hierarchically ordered system of mutually immersed groups. The lowest level represents species, groups of mutually similar individuals. A separate chapter, Chapter XX, was concerned with the aspect of species and the existence of species. Species can be ordered into higher groups on the basis of similarity of their members and these groups of species can again be classified in groups at higher and higher levels. We are currently aware that the reason for hierarchies in taxa lies in the mechanism through which the individual species emerged during evolution. Species gradually branched off from a common ancestor and the individually established phylogenetic lines subsequently further branched or some of them disappeared as a consequence of extinction of all their species. The lines that branched off only recently contain a number of mutual relative, and thus more similar species than the lines that branched off at an earlier time. The subject of study of phylogenetics consists in phylogenesis, i.e. the formation and evolution of the individual phylogenetic lines. Phylogenetics attempts particularly to reconstruct the course of cladogenesis, i.e. the order and manner of branching of all the phylogenetic lines during evolution. Simultaneously, it must necessarily be based on the study of anagenesis, i.e. on the study of the evolution of the individual properties of organisms within relevant phylogenetic lines. Thus, phylogenetics studies both the mutually interconnected aspects of phylogenesis - the specific history of the evolution of life on the Earth.