I read a Stephen Jay Gould article a while ago where he talked about how functions of structures evolve over generations to achieve complex or different states/functions. His example was a benthic invert. species (I think some kind of clam) that has an appendage that appeared as a small fish. His argument was something along the line that obviously it didn't just mutate into having this fish-like foot, but that it may have functioned in a different way in a simpler state. I'm actually more interested in finding the article (it was in one of his compilation books) but I figure I can find it if I know the species.

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Solved: Lampsilis Ventricosa