Graduate Thesis Or Dissertation
 

Bridging the gap : the problem of vertebrate ancestry, 1859-1875

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  • Determination of the Vertebrate pedigree was a particularly enigmatic problem for evolutionary morphologists of the early post-Darwinian period. At that time, practically no characteristics were known by which the Vertebrates could be linked to any of the other animal groups. Up to the 1850's, most research in embryology and anatomy had reinforced the idea that wide differences existed in the basic structural and developmental patterns of Vertebrates and animals belonging to other branches of the animal kingdom. As a result, morphological practice was affected by a strong tradition against comparison of Vertebrates and Invertebrates. With the acceptance of the evolutionary viewpoint, it became theoretically valid to compare animals of different types, based on the possibility of their remote common ancestry. However, the practical matter of demonstrating that relationship with concrete evidence proved to be one of the most difficult challenges confronting early evolutionary morphology. Three different theories were proposed between 1864 and 1870, each attempting to link the Vertebrates and one of the Invertebrate groups to a hypothetical common ancestor. In 1864, Franz Leydig proposed an evolutionary relationship between Arthropods and Vertebrates; in 1866, Ernst Haeckel attempted to connect Vertebrates with Roundworms; and in 1868, on the basis of new research by Aleksandr Kovalevskii, Haeckel proposed another theory which linked Vertebrates with Tunicates. The proposals which formed the basis of the Tunicate theory became the center of a heated debate in the late 1860's and early 1870's. As a result of new research and criticism related to this debate, Haeckel made significant modifications in a revised version of the Tunicate theory which was published in 1874. In this dissertation, the problem of Vertebrate ancestry is presented as a testing ground for the newlyemerging principles of evolutionary morphology. The content and reception of the first three theories of Vertebrate ancestry are examined and compared and the development of the Tunicate theory is traced from its original form as a provisional hypothesis to its later form as a definitive theory. Particular emphasis has been placed on describing and analyzing the scientific work on which support and criticism of the various theories of Vertebrate ancestry was based, and in determining the working principles that various morphologists applied in approaching the problem of Vertebrate ancestry. Such an emphasis demonstrates that a wide variety of philosophical, methodological and empirical concerns existed within the world community of evolutionary morphologists during the early post-Darwinian period. By testing the applicability of their various viewpoints to the solution of the problem of Vertebrate ancestry, these morphologists made significant contributions to the conceptual progress of evolutionary morphology, particularly to the development of a new, genetic concept of homology.
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