This method makes use of comparative studies of related living taxa in order to determine the function of a structure. The observed relationship between structure and function in extant(living) species can be applied to extinct taxa of interest. The assumption is that homologous structures in living and extinct taxa have homologous functions. Problems arise when there is no extant homolog, or when there are more than one extant taxa with the structure and it serves a different function in each taxon.
a)E(which is extinct) shares structure S3 with living taxa A and B. S3 is related to function F3 in A and B, and so F3 is inferred for E.
b)E(which is extinct) has unique structures S7 and S8, so the inference of function must come from analogous structures in other clades(arrow). As well, it shares S2 with living taxa A and B, But S2 is related to F3 in A, and F4 in B, which makes it difficult, if not impossible to infer the function of S2.
c)The phylogenetic method makes it easy to infer function F1 for S1 of taxon E(which is extinct). A,B,C,D,F are living.