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Re: Ancestors and descendants

<it is when we try to remove the birds that we run into trouble, since I
believe putting birds in a category with the same rank as all of the rest of
Dinosauria implies that all members of the bird category are related equally
to all members of the dinosaur category, and that we know not to be true.>

That implication does not appear very logical.  Any group you choose to
carve out will have a direct  ancestor which is a single member of another
group.  We assign categories based on descendants, and decide which animals
deserve to be considered a category.  Our reasons can vary, and some schemes
can be considered better than others based on criteria we establish, but no
scheme can be considered an exclusive logical necessity.
Means the argument can go on for a while, I guess.