The pretheoretical use of the concept of homology reaches far back into the preevolutionary history of comparative biology (Panchen 1994). Belon (1555) is usually taken as locus classicus for the earliest illustration of the relation of homology (O Figure 6.1), although the concept must have been implicit in Aristotle's recognition of dolphins as mammals (Russell 1982 p 6).
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In a comparison of the skeleton of man and a bird, Belon (1555) recognized the topological equivalence of constitutive elements (courtesy of the Field Museum Library, Mary W. Runnells Rare Book Room)
For Etienne Geoffroy St. Hilaire, the unity of type was based on the relation of "analogie," a concept that captured the correspondence of relative topological relations (connectivité) of constituent elements of the organism tied together throughout ontogeny by the blood-vascular system (Rieppel 1988, 2001). Geoffroy's "analogie" became Richard Owen's "homology", meaning "the same organ in different animals under every variety of form and function'' (Panchen 1994 p 40; Williams 2004 p 192). Owen contrasted this concept with "analogy," meaning "a part or organ in one animal which has the same function as another part or organ in a different animal'' (Panchen 1994 p 40; Williams 2004 p 192). Panchen (1994 p 44) drew attention to the fact that, as used by Owen, homology and analogy were not necessarily mutually exclusive concepts, so a third term was required to characterize structures analogous between two or more organisms but not homologous. This was Lankester's (1870) "homoplasy." In fact, Lankester (1870) restricted the use of the term homology to its preevolutionary connotations; he called the evolutionary use of the concept that implies common ancestry "homogeny."
At the hands of Geoffroy St. Hilaire or Owen, homology was meant to capture structural correspondence, rooted in "laws of structure'' as expressed during ontogeny. The relation of homology (and analogy) was reconceptua-lized (Kitcher 1993 p 32) by Darwin (1859), who found that, on his theory, "unity of type is explained by unity of descent'' (Darwin 1859 p 206). The term homology continued to apply to the same constituent parts of organisms that were recognized as homologs by Owen, but the meaning of the term changed to something roughly like "shared similarity due to common ancestry.'' The history of the concept of homology has been variously sketched, sometimes in great detail (Panchen 1994; Williams 2004), and needs not to be retraced here. Instead, the focus of this chapter will be on issues that are central to the contemporary discussion of the concept of homology in systematics and developmental biology. How does "similarity'' relate to the concept of homology, or, in other words, how can the relation of homology be rendered empirically accessible? Is there such a thing as "partial homology''? What does it mean to say, in Owen's words, that homologs are "namesakes," i.e., parts that "merit the same name'' (Patterson 1988 p 605); or that "the same names can be given to the homologous bones in widely different animals'': (Darwin 1859 p 434). What does "sameness," "structural identity'' (Wagner 1994), or "historical identity'' (Grant and Kluge 2004) mean in reference to homologs, or, in other words, what kind of names refer to homologs? Are homologs particulars (individuals), denoted by proper names, or tokens of natural kinds, denoted by general names?
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