444 Brian K. Hall
bSee Hall [1995a; 2002c; 2003a; 2003b; 2005c] and Stone and Hall [2004, and
references therein] for coding and latent homology.Box 2
von Baer’s Lawsvon Baer’s laws had their origin in a parallelism between human embryonic
development and the history of life proposed independently by the German, J.
F. Meckel [1811] and the Frenchman, E. R. A. Serres [1824; 1830]. Sometimes
referred to as the Meckel’s-Serres’ law of parallelism, human embryonic devel-
opment — and, as extended by Serres, to the development of non-mammalian
vertebrates — progresses through a hierarchical series of animal types, each
embryonic stage in turn representing fish, reptilian, mammalian, and finally
human stages of human evolution. The progress of life on earth, which was
being revealed during the 19thC through discoveries of fossilized remains, par-
alleled this progression of embryonic development, not in any trivial way, but
as a reflection of underlying common natural laws.a
von Baer [1828; 1835] went well beyond either Meckel or Serres to extend the
concept of parallelism to the entire animal kingdom. Because some features
in animals thought to be ‘higher’ up (more deeply nested in) the evolutionary
tree were not present in animals considered ‘lower’ down the tree (more basal)
— the avian yolk sac is not found in frog embryos, and birds were considered
‘higher’ up the evolutionary tree than frogs — von Baer concluded that higher
forms donotrecapitulate lower forms. Organs not organisms are recapitulated
during embryonic development: “The embryo successively adds the organs
that characterize the animal classes in the ascending scale. When the human
embryo, for instance, is but a simple vesicle, it is an infusorian; when it has
gained a liver, it is a mussel; with the appearance of the osseous system,
it enters the class of fishes; and so forth, until it becomes a mammal and
then a human being” ([Ospovat, 1976, 4–5], citing Oken’s summary of von
Baer’s theory of recapitulation). Meckel and Serres independently proposed
a law of comparison and recapitulation between levels of the natural world
and proposed a universal mechanism of parallel change. von Baer’s law was
a statement about parallelism — development paralleled the classification of
organisms — von Baer’s theory, a theory of development (ontogeny) with
evolutionary (phylogenetic) implications, not a law of recapitulation.
von Baer’s law had several elements: (1) Embryonic stages are highly con-
served. (2) Because development proceeds from the general to the specific,