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Re: Cladistic taxonomy (was Dietary factors)

HP Williams observed:
Classifications are an attempt on the part of humankind to impose a system
on nature.
To which HP Knorpp responded:
Again I note, on the verge of pedantry: at least some reasonable folks
disagree with this.  This, of course, sounds a lot like nominalism, the view
that no two things are really any more similar to each other than they are
to any other arbitrarily-selected thing...   The nominalist holds that all
"similarities" just result from us calling things by the same name.

HP Williams had, I thought, considered that:
In effect, if a genus is more closely related to modern birds than
_Archaeopteryx_, then it is considered a bird *in the scientific sense*.  We
(as humans) draw the line
under _Archaeopteryx_.  But Nature makes no such clear distinctions...

He's emphasizing that the label is arbitrary for some animals, because of
the great similarities between 'birds' and 'dinosaurs' at one time.
However, his statements are predicated on the (for him proven) assumption
that the animal is in fact either a bird or a dinosaur, and not anything
else.  This is the clear distinction; bird/dino is a naming problem.
Then you go back one step further and there's a problem again:  Archie is
part of the _definition_ of  birds.  For this one animal, specific
distinctions and identification of related groups are not paramount, Archie
simply is included.  (Remember that evolutionary relationships developed
from logical analysis of physical character[istic]s are used as the basis
for naming under the system he's discussing.)
I've grumped about that a bit.

So, cladistics says (as I understand it) that there are essential
differences between reptiles/dinos/birds and others, essential because of
descent.  I'm comfortable with the view that substantial descriptive
differences are also essential, that dinos can be split from reptiles and
birds from dinos.  As you said:

Bas VanFraassen sez: you don't need clear boundaries to have a legitimate
distinction; all you need is a clear case of the one kinda thing and a clear
case of the other kinda thing.

It isn't necessary to base a system on the close decisions, right?!