Phylogeny, which is a representation of the evolutionary history of taxa, requires critical interpretation of biological diversity and of the relationship of this with the unity of organisation which underlies living systems. Some degree of personal judgement may be involved in particular cases, and so, because of this subjective element, phylogenetic propositions are always open to discussion in the light of new information. On the larger issues, however, and on many smaller ones as well, there is substantial agreement, founded on the rich classical resources of descriptive anatomy, which take account of large numbers of species, both living and extinct, and with embryological studies as a valuable supplement. The strength of these views derives not only from the wide range of material studied, but also from the possibility of evaluating the anatomical interrelationships of organs and systems which may be unduly influenced by convergence10. Further evidence is, of course, provided by physiological, biochemical and molecular studies, but in the euphoria engendered by recent advances in some of these areas it is easy to overlook their limitations: the paucity of species studied, ignorance of extinct forms, and the flexibility of functional organisation and its consequent susceptibility to opportunistic and convergent evolution. Caution is therefore demanded.