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It has been claimed that quantified features are inappropriate for phylogenetic anal- ysis. We consider that claim to be true under most conditions for characters discovered by com- monly used morphometric methods, including outline-based and conventional multivariate meth- ods. The most important reason these characters are unsuitable is that one of the tests of homol- ogy, the test of similarity, may be difficult to apply to them. This test is not even possible if the methods for comparing forms, such as outline-based techniques, do not ensure that the characters are located in the same part of the anatomy. Conventional methods, including principal compo- nents analysis, have no explicit basis for localizing characters. In addition, unless the transfor- mation between forms is homogeneous, conventional methods cannot dissect transformations re- gion by region to discover characters. However, one morphometric method, the thin-plate spline decomposed by its partial warps (TPS), finds characters that can be subjected to the same tests of homology (conjunction, similarity, and congruence) that we would apply to all other characters. Among available methods, TPS is unique in being able to locate the center and spatial extent of regional differences in shape and ensures that the same regions are compared among forms. We provide an example using the teleost fishes piranhas, in which tests of homology are applied to a found by the method. (Morphometrics; homology; synapomorphy; thin-plate spline; character analysis; piranha; Pygocentrus; Pygopristis; Serrasalmus.) Shape similarities among organisms Because much of our argument depends have long been recognized as having sys- on our meaning of homology, we begin tematic value. Whether shave features in with a brief foray into semantics. Follow- quantitative form can be a source of infor- ing a discussion of the definition of ho- mation about phylogenetic relationships is mology, we examine several morphometric more controversial. This doubt is reflected methods and judge their potential for in the literature, particularly in the claims yielding characters appropriate for phylo- that quantitative shape features cannot be genetic analyses. subjected to the same tests of homology that other characters must pass (Pimentel HOMOLOGY and Riggins, 1987). This position strikes di- Patterson (1982), like Rieppel (1980), dis- rectly at the use of morphometrics as a tinguished between taxic homology, a fea- class of tools for phylogenetic analysis. ture of a monophyletic group, and trans- Herein, we argue that ,at least one morpho- formational homology. The evidence of metric method is both suitable and useful taxic homologies is synapomorphies that in phylogenetic analysis because charac- have passed three tests: conjunction, simi- ters found by it can be treated in the same larity, and congruence. Systematists who way as can characters from any other use the term for shared source. However, we also contend that derived states, whether or not shown to be many methods currently in use are indeed congruent, qualify this definition by re- inappropriate because they cannot gener- stricting it to corroborated synapomor- ate results meeting the criteria applied to phies. Systematists who use terms such as phylogenetic characters. putative synapomorphy to refer to the coded characters prior to testing their con-