904 THE STRUCTURE OF EVOLUTIONARY THEORY
clade must drift to the right, even though the mode may not move from initial
smallness, just because the space of possible change includes substantial room in
the domain of larger size, and little or no space between the founding lineage and
the left wall. Thus, as Stanley (1973) stated so incisively, Cope's Law receives a
reversed interpretation as the structurally constrained and passive evolution (of an
abstracted central tendency, I might add) from small size, rather than as active
evolution towards large size based on the organismic advantages of greater bodily
bulk under natural selection.
But we must then carry the revision one step further and ask an even more
iconoclastic question: does Cope's Law hold at all? Could our impressions about its
validity arise as a psychological artifact of our preferential focus upon lineages that
grow larger, while we ignore those that remain in stasis or get smaller—just as we
focus on fishes, then dinosaurs, then mammoths, then humans, all the while
ignoring the bacteria that have always dominated the diversity of life from the
pinnacle of their unchanging mode throughout geological time?
Again, we cannot even ask this question until we reformulate the entire issue
in speciational terms. If we view a temporal vector of a single number as adequate
support for Cope's Law, we will not be tempted to study all species in a
monophyletic clade that includes signature lineages showing the documented
increase in size. But when we know, via Stanley's argument, that Cope's Law can
be generated as a summary statement about passively drifting central tendencies in
random systems with constraining boundaries, then we must formulate our tests in
terms of the fates of all species in monophyletic groups. Jablonski (1997) has
published such a study for late Cretaceous mollusks of the Gulf and Atlantic coasts
(a rich and well-studied fauna of 1086 species in Jablonski's tabulation) and has,
indeed, determined that, for this prominent group at least, prior assertions of
Cope's Law only represent an artifact of biased attention (see commentary of
Gould, 1997b). Jablonski found that 27-30 percent of genera do increase in mean
size through the sequence of strata. But the same percentage of genera (26- 27
percent) also decrease in mean size—although no one, heretofore, had sought them
out for equal examination and tabulation.
Moreover, and more notably for its capacity to lead us astray when we operate
within a conceptual box defined by anagenetic flux rather than variation in
numbers of taxa, an additional 25-28 percent of genera fall into a third category of
generally and symmetrically increasing variation through the sequence—that is,
the final range for all species within the genus includes species both smaller and
larger than the extremes of the ancestral spread. I strongly suspect that a previous
inclusion of these genera as affirmations of Cope's Law engendered the false result
of dominant relative frequency for phyletic size increase. Older treatments of the
topic usually considered extreme values only, and affirmed Cope's Law if any later
species exceeded the common ancestor in size—thus repeating, in miniature, the
same error generally committed for life's totality by ignoring the continuing
domination of bacteria, and using the motley sequence of trilobite to dinosaur to
human as evidence for a central and defining thrust. Obviously, from a variational
and