The Fruitful Facets of Galton's Polyhedron 391
The conclusion supported by comparative study admits of experimental
confirmation. We may take pigeons of the two-barred type, and try to
advance from this condition to that of the chequered type, by selecting in
each generation birds with the widest bars, and especially any that may
have a trace of a third bar. This I have tried continuously for 6 years and
with several different stocks. I have not been able to establish a third bar, or
to extend chequers in front of the vestigial third bar, which is often found.
With purebred birds, not allowed to mingle with chequered birds, I believe
it is impossible to advance from bars to the chequered bird state. With
chequered pigeons, on the other hand, it is fairly easy to advance in the
opposite direction, gradually clearing the field and leaving two bars. The
process has been carried to the point of completely eliminating the bars
(1919, p. 60).These accumulated sources of evidence led Whitman to strong assertions
about the primacy of orthogenesis among rival theories: "The orthogenetic process
is the primary and fundamental one" (1919, p. 35). In his boldest statement (1919,
p. 191), Whitman advocated a model of inevitable evolutionary flow, and
explicitly limited the role of natural selection to tinkering with the style and rate of
a determined sequence:
The steps are seriated in a causal, genetic order—an order that admits of no
transpositions, no reversals, no mutation-skips, and no unpredictable
chance intrusions. This series may conceivably be lengthened or shortened,
strengthened or weakened; indeed, we may multiply the number of steps at
will; that is, we may provoke one or more steps to arise between any two
normal steps; but in that case the new steps will be measured true to the
time and place of introduction, and their direction will invariably coincide
with that of the series as a whole, so that if the time and place of origin are
noted, the nature and extent of the strides may be approximately predicted.Whitman wrote most of his work on pigeons and orthogenesis between 1900
and 1910, the period of greatest agnosticism and debate about evolutionary
mechanisms (see Kellogg, 1907). He therefore upheld orthogenesis as an explicit
preference among competing theories. Rejecting Lamarckism, Whitman faced the
macromutationism of de Vries and the selectionism of Darwin as chief rivals. And
with Darwin's own pigeons reinterpreted as the bulwark of orthogenesis, we can
hardly be surprised that Whitman singled out natural selection for special criticism:
"To attempt to explain all this as the work of natural selection would lead into an
endless tangle of conjecture that would leave even the simplest facts as
unapproachable mysteries. Natural selection has probably had most to do with the
end stages in the evolution of characters, but little or no direct influence in
originating them. The two-barred condition has been reached in the simplest
possible way, not by accidental variation or chance mutation, but by progressive
modification of a chequered condition previously established" (1919, p. 61).