Morphology and Phylogeny.

The concept that renders morphology a tool for phylogeny reconstruction is homology. The concept of homology is rooted in pre-evolutionary idealistic morphology. The claim that the goal of idealistic morphology was the seriability of form may sound paradoxical given that this discipline proceeded within a framework of strictly delimited types. But the types only demarcate where seriability starts and where it comes to an end. Carl Gegenbaur's (Grundzüge der vergleichenden Anatomie, Wilhelm Engelmann, Leipzig, 1859) was recognized as a milestone in idealistic morphology. A comparison with the second edition of 1870 illustrates Gegenbaur's turn to evolutionary morphology. The methodology remained the same-seriability of form-but the series was no longer merely descriptive or conceptual but now a historical, evolutionary one. Gegenbaur emphasized that seriability of form was possible not only between species of the same type, but also between parts (organs) of organisms of the same type. Pursuing this project, he found that different parts of organisms evolve at different rates, resulting in an incongruence between the series of parts (organs) relative to the series of species under comparison. This incongrence was called chevauchement des spécialisations by Louis Dollo, Spezialisationskreuzungen by Othenio Abel, and heterobathmy of characters by Armen Takhtajan. Willi Hennig, the founder of modern methods in phylogenetic systematics, discovered that the heterobathmy of characters was a precondition for the establishment of the phylogenetic relationships based on shared derived characters. The result was a replacement of the search for ancestors by a search for relative degrees of phylogenetic relationships (sister-group relationships).