It is taken for granted today, at least by zoologists, that systematic classifications of organisms can be represented by branching diagrams (dendrograms: Mayr, Linsley, and Usinger 1953) that represent hierarchical arrangements - Darwin's (1859) “groups within groups”. The nested groups are taxa, each of which belongs to a category that represents its level in the hierarchy (Simpson 1961a). In the tenth edition of Linnaeus 's Systema Naturae (1758), he proposed the following categories: Regnum (Kingdom), Classis, Ordo, Genus, Species, to which the categories Phylum and Family were added later. All the taxa at the same level in the hierarchy occupy the same rank and are given the same category. Thus “the rank of a taxon is that of the category of which it is a member” (Simpson 1961a). Modern biological classification is therefore a process of “ordinally stratified hierarchical clustering” (Jardine and Sibson 1971, p. 127), and the result is an aggregational hierarchy (Mayr 1982, pp. 64-6) in which the units, usually species, which constitute its lowest rank, are aggregated in successively higher ranks. The hierarchy is also an inclusive one (Mayr 1982, pp. 205-8) as opposed to an exclusive one: