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        Tetrapod Limbs

        As this illustration from Evolution by Monroe W. Strickberger shows, the limbs of tetrapods all have the same pattern of bones. Darwin was one of the first to comment that it seems unlikely that this single skeletal structure could be the best one possible for each of the activities it is required to perform in different animals. The explanation for this common structure lies in a common heritage--a pattern of development laid down in the ancestor of all modern tetrapods and adapted over time, by different environmental pressures, to perform different functions.

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