432 Brian K. Hall
Homology and homoplasy do not represent a dichotomy of homologyvs. ho-
moplasy, homologyvs. analogy, or homoplasy as non-homology. The position
taken here — and developed more fully in Hall [2003a; 2007a] — is based upon
a combined analysis of homology and homoplasy, embryos and adults, and near-
ness or distance of phylogenetic relationships, to demonstrate a continuum from
homology through reversals, rudiments, vestiges, atavisms and parallelism — all
of which constitute an expanded category of homology — to convergence as the
single class of homoplasy. This reconceptualization takes an expanded homology
as the foundation of comparative studies in biology at all levels from genes to phe-
notypes and as reflecting the descent with modification that has characterized and
continues to characterize the evolution of the animal kingdom.^9 It turns out that
the position is not new; Hall [2003a], and independently Gould [2002] traced it
back to E. Ray Lankester’s subdivision of homology into homology and homogeny
in 1870.
5 HOMOLOGY AND ANALOGY DELINEATEDThe roots of homologous thinking and the use of homologous features to order and
classify organisms go back to Aristotle [Moore, 1986]. ‘Modern’ thinking on the
topic is traced to Richard Owen, the first Superintendent (Director) of the British
Museum (Natural History), who provided what remains our working definitions of
a homologue (homology) and analogue (analogy): “Homologue... The same organ
in different animals under every variety of form and function... Analogue... A
part or organ in one animal which has the same function as another part or organ
in a different animal” [Owen, 1843, 379, 374].^10
Despite Darwin having changed fundamentally how we view homology, Owen’s
definition works surprisingly well today. Although after Darwin it could be stated
that “a feature is homologous in two or more taxa if it can be traced back to
the same feature in the presumptive common ancestor” [Mayr, 1982, 45, 232] the
criteriafor determining homologous structures remain today what they were in
pre-Darwinian times, — similarity, position, connections. As a consequence, we
can take Owen’s definitions as current working/operational definitions and use
them in the everyday comparison of features.
Despite this constancy, homology has generated more words on definitional
matters than any other topic in biology with the possible exception of natural
and Wilkins [1998], Hall [1999a; 2003b], McGhee [2000], Newman and M ̈uller [2000], Wilkins
[2002] and Weiss and Buchanan [2004] for shared genes and gene networks.
(^9) I do not consider plants but the same reconceptualization applies, as exemplified by Sattler
[1984; 1992; 1994], Sattler and Jeune [1992] and Endresset al.[2000].
(^10) Analyses of Owen’s role may be found in Huxley [1894], Clark [1900], Spemann [1915],
Boyden [1943; 1947], Ghiselin [1976], Richards [1987a], Rupke [1993; 1994], in the chapters in
Hall ([1994a], especially that by Alec Panchen), Padian [1997], Panchen [1999], Matthen [2000],
Camardi [2001] and Amundson [2007]. See Rieppel [1988], Hall [1994b] and Panchen [1999] for
discussions of homology and analogy, and see Cohen [1994] for the same use of homology and
analogy in the social sciences.