>
>>At our current point in our search for Cambrian ancestors, however, the
lack
>>of fossils is due more to a lack of holes (i.e., a lack of regions) rather
>>than a lack of marbles. A lack of holes would constitute a true absence
of
>>evidence -- i.e., a lack of positive evidence that verifies or refutes the
>>hypothesis; in other words, negative evidence -- but we cannot say that
this
>>constitutes evidence of absence because we are lacking the very data we
>>would need to make that determination.
>
Art Chadwick responded:
>
>On what basis do you figure there is a lack of regions with Precambrian
>outcrops?
>
That's not what Mark meant by "regions". He was using it as a way of
equating a search of Precambrian strata to his marble-in-hole example. He
suggested that Precambrian strata could be divided up into "regions" that
would be the equivolent of the holes in his marble analogy. Searching each
region for Precambrian fossils would then be like searching holes for
marbles. As such, his conclusion is that if we search enough regions and
find no Precambrian fossils we can make certain statistical statements
regarding the probability that such fossils actually exist.
>
>What in your mind constitutes a lack?
>
There is certainly no lack of Precambrian sedimentary strata, though I
question the optimism that claims that the vast majority of these strata are
capable of preserving delicate soft-bodied creatures. When I refer to a
lack of regions, I mean that there is a lack of places to search. Currently
the only way we can search an individual stratum is to find one that has
been exposed by erosion. Yet the exposed portions of this stratum are only
a thin veneer compared to the mass of the stratum that is still buried back
into the hillside (or wherever) and therefore inaccessible to us. We trust
that any fossils that are located in the interior of the stratum will have a
few representatives in the exposed portion, but that is by no means certain.
And if the stratum covers a huge area, say much of a continent, then some
(if not all) of the fossils we are looking for may not have lived in the
area represented by the exposed portion, but instead in areas that are now
located deeper inside the stratum, and so inaccessble to us.
So when I say there is a lack of regions, I mean that the vast majority of
places we should be searching for Cambrian ancestors are hidden from us and
thus inaccessible. This constitutes negative evidence, not positive
evidence that refutes the hypothesis, so it cannot tell us whether Cambrian
ancestors actually exist.
>
>And exactly what data
>would we need to make that determination? A few Precambrian metazoan
>fossils?
>
That would help, but I was thinking of those inaccessible regions of
Precambrian strata. We need to be able to access them before we can say we
have the kind of data we need to determine if Cambrian ancestors exist.
Otherwise, to claim or imply that Cambrian ancestors do not exist based
solely on this missing evidence is to engage in a fallacious argument.
Kevin L. O'Brien