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 embryo-