The aim of the paper is to analyse the judgments involved in making phyletic classifications. It is shown that in addition to the phyletic information obtainable solely from comparative data, other evidence is often available which makes one conclusion about the evolutionary relationships of different forms more probable than others. This evidence principally bears on recognising and allowing for convergence and determining likely evolutionary changes. It comes from a knowledge of the geological age and geographical distribution of forms and from analyses of function, ecology, genetics and intra-superspecific variation. An understanding of the functional inter-relationships between structures and the ecological demands made on organisms is shown to be particularly important, since it makes possible the recognition of necessary correlations between characters, evolutionary trends, different adaptations to the same environment, and general improvements. A method is offered for quantifying the judgments where possible, and the procedure for constructing common ancestors is discussed.