206 THE STRUCTURE OF EVOLUTIONARY THEORY
for Allmacht, and against Lamarckian inheritance, rested upon a logical structure of
inferences from premises, not upon observation—for an empirical approach,
Weismann held, could not achieve resolution, given the impossibility of "seeing,"
at their minute sizes, the material bearers of heredity. Panmixia did compromise
Allmacht in a sense, for this process yielded evolutionary change without selection.
But following Kellogg's key distinction of auxiliary from contradictory hypotheses
(see pp. 163-169), panmixia worked as an adjunct and aid—a mopping-up
operation for organs fallen below the purview of selection, and, more importantly,
a moat to prevent the incursion of a true enemy, the antiselectionist forces of Neo-
Lamarckism. (Lamarck battled against Darwin for the common ground of universal
adaptation, while panmixia only worked to finish what selection had started, and
only in the limited domain of degeneration.)
But Weismann's panmixia, having no support beyond the internal logic of the
argument itself, could not survive the detection and exposure of crucial flaws.
Spencer was not the first writer to illustrate the weaknesses of panmixia, but the
debate of 1893 does mark Weismann's last attempt to explain degeneration by
panmixia alone, and therefore contains the seeds for his next and final attempt—
the theory of germinal selection.
Spencer, referring to "the vexed question of panmixia" (1893b, p. 22), offered
three major rebuttals. "When from the abstract statement of it we pass to a concrete
test, in the case of the whale, we find that it necessitates an unproved and
improbable assumption respecting plus and minus variation; that it ignores the
unceasing tendency to reversion; and that it implies an effect out of all proportion
to the cause" (1893b, pp. 28-29). The second point, based on Galton's principle of
regression to the mean, denies that "minus" variations can continue to accumulate
differentially; the third brands panmixia as too weak a force to secure the total
elimination of a useless organ. The first argument, however, proved to be not only
decisive in itself, but unusual in scientific discourse by accusing Weismann
(correctly) of conflating linguistic usage with biological reality.
Weismann continually argued that selection maintained an organ "at its
highest level." Relaxation of selection might then impel an accumulation of
previously eliminated variation in the minus direction only. But, as Spencer and
others protested, why should selective optimization hold an organ at the summit of
its potential size and complexity. Shouldn't optimality lie somewhere in the middle
of a possible range, with selective elimination of both plus and minus variations?
"Take the case of the tongue," Spencer argued (1893b, pages 23-24). "Certainly
there are tongues inconveniently large, and probably tongues inconveniently small.
What reason have we for assuming that the inconveniently small tongues occur
more frequently than the inconveniently large ones?" Without the invalid metaphor
of selective summits, panmixia cannot reduce an organ to oblivion, for release
from selection does not impart an inexorably downward trend to preserved
variation.
All these objections can be combined into a single claim, which Weismann
found so compelling that he eventually surrendered panmixia as a fully adequate