We analyze the relationship between evolutionary theory and classification of higher taxa in the work of three ichthyologists: Albert C.L.G. Günther (1830–1914), Edward Drinker Cope (1840–1897), and Theodore Gill (1837–1914). The progress of ichthyology in the early years following the Origin has received little attention from historians, and offers an opportunity to further evaluate the extent to which evolutionary theorizing influenced published views on systematic methodology. These three ichthyologists held radically different theoretical views. The apparent commensurability of claims about relationships (...) among groups of fishes belies differences in what the relationships actually were supposed to be. As well, interpreting classification as genealogical did not lead to agreement about taxonomic methodology; instead, applying evolutionary theory raised new axes of disagreement. (shrink)