One: Problems and Methods.- 1 Systems and classifications.- 1.1 Systematics and taxonomy.- 1.2 Classification versus system.- 1.3 Biological classifications from Andrea Cesalpino to the New Systematics.- 1.4 Evolutionary systematics.- 1.5 Numerical taxonomy.- 1.6 Hennig's phylogenetic systematics.- 1.7 Contrasting systematic schools.- 1.8 Towards a natural system of living organisms.- 2 Some steps in comparative biology.- 2.1 Characters as 'symptoms' for recognizing taxa.- 2.2 Characters for choice.- 2.3 Homology.- 2.4 Homoplasy.- 2.5 Character coding.- 2.6 Monophyly, paraphyly, polyphyly.- 2.7 Determining character polarity.- 2.8 Cladograms and trees.- 2.9 Numerical methods for the reconstruction of phylogeny.- 2.10 Ancestors.- 2.11 Fossils and cladistic analysis.- 2.12 Grouping and ranking.- 2.13 Phylogeny versus adaptation.- 3 Biochemical and molecular systematics.- 3.1 Micromolecules.- 3.2 Macromolecules.- 4 The species.- 4.1 Species concepts.- 4.2 Taxonomic diversity within the species.- 4.3 Hybrids.- 4.4 Speciation.- 5 Resources and media.- 5.1 Human resources.- 5.2 Institutions.- 5.3 Literature.- 5.4 Nomenclature.- Two: The State of the Art.- 6 The inventory of natural diversity.- 6.1 How many species do we know?.- 6.2 Continuing discovery.- 6.3 How many species are still to be discovered?.- 7 Towards the system.- 7.1 Kingdoms and phyla.- 7.2 'Prokaryotes'.- 7.3 The major groups of eukaryotes.- 7.4 Fungi.- 7.5 'Protists'.- 7.6 Metazoans.- 7.7 Placozoans.- 7.8 Sponges.- 7.9 Cnidarians.- 7.10 Ctenophorans.- 7.11 Platyhelminths.- 7.12 Gnathostomulids.- 7.13 Mesozoans.- 7.14 Aschelminths.- 7.15 Pogonophorans.- 7.16 Annelids.- 7.17 Molluscs.- 7.18 Arthropods, excluding insects.- 7.19 Insects.- 7.20 Onychophorans, tardigrades and pentastomids.- 7.21 Bryozoans, brachiopods and phoronids.- 7.22 Deuterostomes, excluding chordates.- 7.23 Chordates, excluding vertebrates.- 7.24 Vertebrates.- 7.25 Green plants, excluding angiosperms.- 7.26 Angiosperms.- 8 Interviews on the daily work of systematists: problems and trends.- 8.1 Specialist groups as natural groups.- 8.2 Genera.- 8.3 Species.- 8.4 Infraspecific taxa.- 8.5 Characters.- 8.6 From field work to monograph.- 9 The unequal distribution of taxonomic diversity.- 9.1 The very large genera.- 9.2 Size distributions of higher taxa.- 10 Domesticated animals and cultivated plants.- 10.1 Taxonomy and nomenclature of domesticated animals.- 10.2 Taxonomy and nomenclature of cultivated plants.- Three: Epilogue.- 11 Some dangerous trends, and a hope for the future.- Appendices.- 1 Zoological checklists and catalogues.- 2 Moehn's (1984) general classification of living organisms.- 3 'Provisional classification' of the Protista, according to Corliss.- 4 Phyla and classes of the Protoctista (Corliss' Protista) according to Margulis et al (1990).- 5 Moehn's (1984) classification of animals.- 6 Nielsen's (1985) classification of the Animalia.- 7 Ehlers's (1985) system of Plathelminthes.- 8 Jamieson's (1988) system of the Oligochaeta.- 9 Salvini-Plawen's (1980) classification of the Phylum Mollusca.- 10 Haszprunar's (1986) classification of gastropods.- 11 Weygoldt and Paulus's (1979) system of the Chelicerata.- 12 Shultz's (1990) system of the Chelicerata.- 13 Schram's (1986) classification of the Crustacea.- 14 Starobogatov's (1988) classification of the Crustacea.- 15 Hennig's (1969) system of the Insecta.- 16 Hennig's (1985) system of the Chordata.- 17 The major groups of Chordata according to Nelson (1969).- 18 Rosen et al.'s (1981) classification of gnathostome vertebrates.- 19 Carroll's (1987) classification of vertebrates, including both.- extinct and living forms.- 20 Lauder and Liem's (1983) classification of living bony fishes.- 21 Sibley and Ahlquist's (1990) classification of birds.- 22 Bremer's (1985) cladistic classification of green plants.- 23 Dahlgren's (1989a,b) classification of the flowering plants.- References.- Author index.- Taxonomic index.