Philosophy of Biology

Homology and Homoplasy 445

earlier stages of embryos from different groups of organisms are more alike than

are later stages. (3) As a consequence of 1 and 2, development parallels the

classification of the same organisms into ‘natural’ groups. The first features

to appear in development are those of the phylum, followed by features of the

class, order, family, genus, and species; Matt Vickaryous, my cladistic conscience,

would prefer that I write: “The first features to appear in development are

those of the largest, most inclusive clade, followed by features of successively less

inclusive clades”.

The consequences of von Baer linking homology, development and classification

were enormous, especially for the primacy of embryonic criteria in determining

homology. E. S. Russell captured this beautifully: “And the surest way to de-

termine the true homologies of parts will be to study their early development...

Parts therefore, which develop from the same ‘fundamental organ’, and in the

last resort from the same germ-layer, have a certain kinship, which may even

reach the degree of exact homology” [Russell, 1916, 126]. The rapid dissemina-

tion of von Baer’s laws into English by Martin Barry and William Carpenter,

their adoption into the conceptualization of the archetype and of homology by

Richard Owen, and into descent with modification by Charles Darwin, brought

embryos and embryonic development to the forefront. Archetypes were to be

sought in embryos, homology was to be sought in embryonic development, and

the embryological criterion of homology was born.b

aSee Russell [1916], Oppenheimer [1967], Gould [1977] and Bowler [1996] more

for Meckel and Serres.

bSee Desmond [1989], Hall [1992; 1999a], Bowler [1996], Richards [1987a,b] and

Richmond [2000] for evaluations of how von Baer’s laws were received by English

morphologists.

BIBLIOGRAPHY

[Abouheif, 1997] E. Abouheif. Developmental genetics and homology: a hierarchical approach.
Trends Ecol. and Evol., 12: 405–408, 1997.
[Abouheif, 1999] E. Abouheif. Establishing homology criteria for regulatory gene networks:
prospects and challenges. In G. R. Bock and G. Cardew (eds.),Homology, Novartis Foun-
dation Symposium 222, pages 207–221. Wiley, Chichester, 1999.
[Abzhanov and Kaufman, 2000]A. Abzhanov and T. C. Kaufman. Homologs ofDrosophilaap-
pendage genes in the patterning of arthropod limbs.Devel. Biol., 227: 673–689, 2000.
[Amundson, 2007]R. Amundson (ed.). Reissue ofOn the nature of Limbsby R. Owen, with
introductory essays and a Foreword by B. K. Hall. The University of Chicago Press, Chicago
(in press), 2007.
[von Baer, 1828]K. E. von Baer.Uber Entwickelungsgeschichte der Thiere: Beobachtung und ̈
Reflexion, Gebr ̈uder, Borntr ̈ager, K ̈onisberg. Reprinted 1967 by Culture et Civilisation, Brux-
elles, 1828.
[von Baer, 1835]K. E. von Baer.Untersuchungen ̈uber die Entwickelungsgeschichte der Fische.
Leipzig, 1835.
[Barry, 1836-1837a] M. Barry. On the unity of structure in the animal kingdom.Edinburgh New
Philos. J., 22: 116–141, 1836-1837.
[Barry, 1836-1837b]M. Barry. Further observations on the unity of structures in the animal
kingdom, and on congenital anomalies, including ‘Hermaphrodites’; with some remarks on