Darwin-L Message Log 7:6 (March 1994)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.

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<7:6>From DARWIN@iris.uncg.edu  Sat Mar  5 04:21:44 1994

Date: Sat, 05 Mar 1994 00:57:57 -0500 (EST)
From: DARWIN@iris.uncg.edu
Subject: Re: Superposition
To: darwin-l@ukanaix.cc.ukans.edu
Organization: University of NC at Greensboro

Just catching up on some messages that I have let accumulate in my mailbox
for the last few days.

John Sutton asked an interesting question about the concept of "superposition"
which he says is a hot topic now in connectionism and related fields.  The
most common sense of the term in the historical sciences is of course the
geological one: the "principle of superposition" says that in a series of
geological strata, the oldest layers are on the bottom and the youngest layers
are on the top, because sediments are deposited horizontally, one on top of
another.  This is a very important principle of historical reconstruction
which goes back to Steno in the 1600s at least; perhaps some of our historians
of geology could provide more information about its history.

The sense I have from what John was saying, though, is that although the same
word may happen to be used by people in some other fields, they are not really
talking about the same concept.  (Just as an historian might refer to
"character" as something in the the personality of an individual, whereas
"character" to a systematist means a difference among taxa from which we infer
and evolutionary event.  Same word, different meaning.)  The concept John was
describing sounds like what I would call "superimposition" rather than
"superposition" (which I have never really heard outside of geology, but that
means nothing).  "Superimposition" to me describes the placement of an image
or abstract object of some kind on top of another image or abstract object,
such that the two form a single image.  George Gale mentioned the case of two
waves being superimposed on one another.  This strikes me as a different thing
from "superposition", which I take to be the placement of one space-filling
object _above_ another, just as one layer of rock is deposited above another.

The notion of stratigraphic superposition certainly exists in some fields
outside geology in the strict sense.  Dendrochronology is based on the
superposition of tree rings.  Someone reconstructing the sequence of brush
strokes used by a painter to produce a particular painting would similarly
assume that if a particular paint later covers another then the uppermost is
the younger and the lowermost is the older.  All of these applications of the
principle of superpostion (as a tool of historical reconstruction) presume
that there process of "deposition" going on, and that that process is
understood.  I can imagine situations where such an assumption might not hold.
I seem to recall reading a long time ago (maybe in some popular work like
Sagan's _Dragons of Eden_?) a reference to the human brain being structured in
layers: an ancient reptilian later, a middle mammalian layer, and a later
human layer.  This strikes me as a rather naive "superpostion" argument, since
I rather doubt that our brains have been deposited like sediments over the
course of evolution.  ;-)

Bob O'Hara, Darwin-L list owner

Robert J. O'Hara (darwin@iris.uncg.edu)
Center for Critical Inquiry and Department of Biology
100 Foust Building, University of North Carolina at Greensboro
Greensboro, North Carolina 27412 U.S.A.

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