Bock, W. J. (Department of Biological Sciences, Columbia University, New York, New York, 10027) 1974. Philosophical foundations of classical evolutionary classification. Syst. Zool. 22:375–392.—The primary objective of biological classification is to provide the foundation of all comparative studies in biology. These studies require consideration of two factors, namely: (a) the degree of genetic similarity between organisms; and (b) the phylogenetic sequence of events in their history. Classical evolutionary classification provides the best approach to classification, based on Popper's criterion of content, because it attempts to maximize simultaneously both of these semi-independent variables. Evolutionary classification is based on the evolution of organisms, not just their phylogeny. Application of the Popperian philosophy of the demarcation and methodology of science to classification suggests that a major task is the development of severe tests of falsification by which classifications can be tried. Although phenetic methods appear to be the best means of recognizing taxa and deducing classifications, cladistic methods appear to provide the best tests in attempts to disprove these statements of relationships. The best approach to definitions in this historical science is a theoretical one in which the defining criteria and the recognizing criteria may differ. Comparisons, which are closely akin to definitions, must be based on evolutionary theory. Homology is the primary principle in comparative biology with the possibility that all other principles of comparison are reducible to homology. The defining criterion of homology is phylogeny and the only recognizing criterion of homology is similarities of all sorts between features. Any approach to classification that excludes homology and its recognizing criterion of similarity as a primary step in deducing relationships is invalid. Use of falsifying tests suggests that relative weighting of characters may have greatly reduced importance in taxonomic methods. Lastly, the formal classification and the phylogenetic diagram of a group of organisms are not different, but redundant images of each other. Both are essential parts of the conclusions of any study of biological relationships.