Homology is one of the most important concepts in biology (de Beer 1971;Donoghue 1992).Having been introduced in pre-Darwinian comparative biology, it continues to be fundamental to taxonomy, phylogeny, and evolutionary biology. In recent times it has come to play an important role in molecular and developmental biology. In addition to figuring prominently in biological practice, the notion of homology is the subject of extensive theoretical reflection among biologists (Bock and Cardew 1999; Hall 1994;Wagner 2001b).Curiously though, homology has been discussed only sparsely by philosophers.3 The contributions to this special issue attempt to highlight the importance of homology for philosophy as well as biology. Homology is germane to such philosophical issues as the individuation of biological natural kinds, and the patterns of inductive reasoning in the life sciences. Given the possibility of individuating cognitive kinds and mental phenomena in terms of homology (rather than their evolutionary function), the idea of homology has implications for the philosophy of mind and cognitive science, implications that are explored in the contributions of Marc Ereshefsky and Mohan Matthen.While species have been extensively discussed by philosophers and have served as the prime examples of biological natural kinds, homology is an equally important notion of natural kindhood in the biological sciences (Brigandt 2002, in press;Rieppel 2006;Wagner 1996, ( ), Brigandt (2002, Griffiths (1994Griffiths ( , 1996Griffiths ( , 2006, Matthen (1998Matthen ( , 2000, and Sober (1999).
THE IMPORTANCE OF HOMOLOGY FOR BIOLOGY AND PHILOSOPHY
2001a). Biologists partition an organism into homologues which are presumed to represent natural units of biological organization because they can be reidentified in other organisms and other species (Griffiths, this issue). A homologous character shares many biological properties in the different organisms in which it occurs, and there is a causal basis for this sharing of properties (common ancestry and shared developmental mechanisms). These two features, a cluster of properties and a causal basis for the co-occurrence of those properties, are the two hallmarks of natural kinds (Boyd 1991;Hacking 1991; Wilson et al., in press). In the course of evolution, a character typically undergoes modification, so that homologues are Over the past few decades there has been an intensification of interest in the concept of homology. Its scope of application has increased and new theoretical interpretations of homology have been proposed. In addition to traditional morphological characters, processes and functions can be homologised (Amundson and Lauder 1994;Gilbert and Bolker 2001;Gilbert et al. 1996;Lauder 1994), and behavioural patterns and cognitive features are increasingly recognized as homologous across species (see Ereshefsky, this issue; Matthen, this issue). Following the rise of molecular biology, genes and proteins came to be homologized, leading to the field of molecular evolution and the practice...