Structural similarity is helpful in recognizing homologous structures, but it does not define them. Such structures must also have phylogenetic continuity, a criterion that is ignored by Aboitiz et al. and by proponents of “field homology.” “Similar” structures, as well as “field homologues” from “the same” embryonic field, are not necessarily homologous, and an outgroup analysis of developmental stages should be performed to establish homologies.