1302 THE STRUCTURE OF EVOLUTIONARY THEORY
longer perhaps in some cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication of species from
some one or some few parent-forms; and in the succeeding formation such species
will appear as if suddenly created" (pp. 302-303).
The second, and more sophisticated, argument follows from this principle of
non-deposition during most intervals. We fall, Darwin argues, into circular reasoning
in claiming that similar events in widely separated regions must have occurred
simultaneously—for we make our judgment of temporal coincidence from the
geological similarity alone, and not from any independent measure of time. For
example, if we note the disappearance of several brachiopods in one stratum and the
first appearance of several clams in the stratum just above, and we find the same
pattern in a distant region on the other side of the earth, we might be tempted to
proclaim a truly momentary wipeout followed by effectively simultaneous origin of
functionally similar creatures. But this transition might actually occur very slowly in
any single place, and leave no record of its true pace because a long interval of
nondeposition followed the last preserved stratum of brachiopods. Moreover, this
truly slow transition, prompted by ordinary biological competition of superior clams
against inferior brachiopods, one species at a time, might have unfolded at quite
different times in separated regions of the globe—for the process can only begin
when the clam fauna migrates to a new area, and these migrations may span a
considerable range of time (falsely compressed to simultaneity by our error in
viewing the first stratum with clams as coeval throughout the world). Darwin
summarizes this complex argument (pp. 327-329):
Therefore as new and improved groups spread throughout the world, old
groups will disappear from the world; and the succession of forms in both
ways will everywhere tend to correspond ... If the several formations in these
regions have not been deposited during the same exact period, —a formation
in one region often corresponding with a blank interval in the other ... in this
case, the several formations in the two regions could be arranged in the same
order, in accordance with the general succession of the form of life, and the
order would appear to be strictly parallel.In a striking example, summarizing both the biological argument for gradual
replacement by competition and the geological claim for false appearance of
simultaneity by imperfection of preserved records, Darwin makes his plausible case
for extrapolation and uniformitarian explanation, even for the two most famous cases
of mass extinction for formerly prominent groups (see pp. 1314-1316 for a modern
perspective on the demise of these taxa): trilobites at the Permo-Triassic event, and
ammonites at the Cretaceous-Tertiary mass dying (pp. 321-322):
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the Paleozoic period and of Ammonites
at the close of the secondary period, we must remember