Coincidence or homology?

Remember that story from last month about how fruit flies have some kind of free will because they navigate their flight in nondeterministic directions?

Only after the team analyzed the fly behavior with methods developed by co-authors George Sugihara and Chih-hao Hsieh from the Scripps Institution of Oceanography at UC San Diego did they realize the origin of the fly's peculiar spontaneity. "We found that there must be an evolved function in the fly brain which leads to spontaneous variations in fly behavior" Sugihara said. "The results of our analysis indicate a mechanism which might be common to many other animals and could form the biological foundation for what we experience as free will".

Well, here's a passage I happened across this morning in Ontogeny and Phylogeny, discussing the developmental theories of Etienne Geoffroy Saint-Hilaire and his student, Etienne Serres:

Geoffroy tried to compare the exoskeleton of arthropods with the internal skeleton of vertebrates (relegating insects to a life within their own vertebrae); he sought identityin the location of parts by likening the basic design of vertebrates to a worm turned over (yielding both the happy circumstance of dorsal nerve cords and such problems as a mouth above the brain). Serres agreed, attributing the inversion to a reversed position of the embryo relative to the yolk (1860, pp. 825-826).
Yet Serres acknowledged the difficulty of comparing adults and set out to prove the unity of plan on another basis: by the fact of recapitulation. The nervous systems of vertebrates and invertebrates have a common design (though this may shock some physiologists since it implies that invertebrates have a will). This identity is not apparent in adult vertebrates, but transient stages of the vertebrate fetus repeat the permanent configurations of invertebrate systems and display thereby a unity of plan (Gould 1977:48, emphasis added).

I'm not sure why the design of the vertebrate nervous system necessarily yields a "will," but of course the new results show a functional commonality that may reflect the developmental and genetic homologies.


Gould SJ. 1977. Ontogeny and Phylogeny. Harvard University Press, Cambridge MA.