434 Brian K. Hall
& Co) of each great class, I cannot doubt is one of the very highest ends of
Natural History... I shd[should] have thought that the archetype in imagination
was always in some degree embryonic, & therefore capable ofgenerally undergoing
further development.”^12 In seeing the archetype as embryonic and as capable of
change (adaptable), Darwin also was influenced by the new laws of embryology
proposed by von Baer (Box 2). Consequently, although not extensively used in
The Origin, embryological evidence provided Darwin with powerful evidence for
descent with modification.
Although Darwin embraced von Baer and the findings that were emerging on
the stability of germ layers from which structures arose — and which provided a
powerful basis for homology assessment [Hall, 1998] — Darwin wasnotprepared
to cede primacy to community in embryonic structure as the ultimate arbiter
of homologous relationships. “Thus, community in embryonic structure reveals
community of descent; but dissimilarity in embryonic development does not prove
discommunity of descent, for in one of two groups the developmental stages may
have been suppressed, or may have been so greatly modified through adaptation
to new habits of life, as to be no longer recognizable” [Darwin, 1910, 371–372].
Darwin recognized that develop evolves, something that many later workers forgot,
ignored, or simply did not know.
Owen had taken the same position in 1846 when he stated that homology “is
mainly, if not wholly, determined by the relative position and connection of the
parts, and may exist independently of... similarity of development,” and con-
tinued (admittedly, 162 pages later) that “there exists doubtless a close general
resemblance in the mode of development of homologous parts; but this is subject
to modification, like the forms, proportions, functions and very substance of such
parts, without their essential homological relationships being thereby obliterated”
[Owen, 1846, 6, 174]. Thus, it is important to recognize that embryonic devel-
opment was not a criterion for homology from the very outset, taking Owen’s
definitions of 1843 as the outset.
Not all stood with Owen and Darwin, however. Haeckel [1866] concluded that
homologous features reflect common descent. Many ever since expect homologues
to arise using similar developmental processes, for example, “homology has come
to signify an agreement in evolutionary derivation and in embryonic development”
(cited in [Hubbs, 1944, 305]).^13
(^12) Letter from Charles Darwin to Thomas Henry Huxley, written on 23 April 1853, cited from
Darwin and Seward [1903, Vol. 1, 73]. See Ospovat [1995], Hall [1992; 1994a; 1999a], Bowler
[1996] and Richards [1987b] for Darwin’s view of archetypes.
(^13) See Moment [1945] and Hall [1994a; 1995a; 1999a] for homology and developmental processes,
and Nyhart [1995], Bowler [1996], Hall [1999a; 2000a; 2007b] and Guralnick [2002] for homology
and evolutionary embryology in the late 19thC.