Some of the basic tenets of cladism (phylogenetic systematics) and the techniques that have been used to apply them to palaeontological data are examined. Using as an example the studies by Eldredge of phacopids, the most complete and rigorous work yet published on Palaeozoic invertebrates using cladistic methods, it is concluded that the results are ambiguous, and that the geological inferences involving speciation and migration in northeastern U.S.A. are independent of the cladistic models.