In molecular dating, branch lengths are considered parameters that may be estimated by counting similarities and differences in DNA sequences. Long and short branches imply long and short time spans, which might appear informative additions when imposed on a tree that is otherwise cladistic. Recent attempts to apply molecular dating to southern hemisphere biogeography (Nothofagus, Adansonia) seem only another “excursion into futility,” as was the fate of phenetic systematics [H.H. Ross, 1964]. Some conceptual matters are clarified with reference to the experimental approach of Claude Bernard (1813–1878).

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