Griffiths, G.C.D. (Department of Entomology, University of Alberta, Edmonton 7, Alberta, Canada) 1974. Some Fundamental Problems in Biological Classification. Syst. Zool. 22:338–343.—Difficulties in the theory of each of the three main contemporary schools of systematic philosophy are discussed. If categories of taxa are identified with age classes, as proposed by phylogenetic systematists, then changes must be made in the traditional form of the Linnaean hierarchy. The central problem of evolutionary classification is that no satisfactory general measure of evolutionary differentiation has yet been found. The unit character hypothesis in numerical taxonomy is unlikely to be valid, since the logical atomist programme in philosophy proved a failure. Modifications may be expected in the theoretical positions of all schools.