390 THE STRUCTURE OF EVOLUTIONARY THEORY
of evidence for viewing his orthogenetic sequence as an extension of the
ontogenetic pathway. First, in comparison with their own juvenile plumages, adult
birds generally develop patterns of coloration that may be designated as "further
along" the orthogenetic pathway. In a world of recapitulation, this palpable change
in the course of weeks becomes a surrogate for the invisible alterations of past
millennia. Whitman uses this ontogenetic evidence to assert orthogenesis against
his two chief rivals, Darwin and de Vries:
Moreover—and this is as close as we can hope to get to actual seeing— we
find that progress of just the kind we are looking for is certainly made in
passing from the juvenal [alternate spelling of juvenile, generally archaic in
English, but still occasionally used in ornithology] to the adult plumage.
This is an ontogenetic change of a few weeks, which we can easily
demonstrate by experiment to be progressive and continuous (p. 33). ...
Even in the widest departures, when every spot has vanished in the adult
plumage, the young bird frequently exhibits more or less perfect traces of
the old marking and sometimes requires several molts to reach its mature
condition (p. 58). ... Juvenile phases of color patterns become luminous as
recapitulations in the sense of the biogenetic law and do not stand as
isolated prodigies of natural selection or as meaningless exhibitions of
mutations (p. 65).Second, as Eimer also maintained (and for complex reasons rooted both in
cultural biases about sexual differences, and in the facts of embryology), Whitman
viewed sexual distinctions as products of the same ontogenetic trajectory, with
males "further along" than females. He then argued that plumages of adult male
pigeons display more advanced stages of the orthogenetic series than females of
the same population. Third, Whitman argued that, within the adult plumage of
individual birds, last-formed feathers developed more advanced characters—as the
biogenetic law required, based on its key principle of terminal addition for
evolutionary novelty. Thus, three criteria, all interpreted as manifestations of
ontogeny—differences among molts of a single bird, between sexes of adults, and
within the adult plumage by order of formation—indicated a pervasive channel of
variation, virtually compelling evolution into an extended ontogenetic pathway
directed towards reduced and concentrated coloration.
To these categories of ontogenetic evidence, Whitman then added two
additional sources of data to buttress his orthogenetic series: (1) From comparative
anatomy, he asserted that phyletic series, established by criteria independent of
coloration, illustrated the orthogenetic sequence in many parallel lineages. (I doubt
the claimed independence in many cases, and Whitman may therefore have
advanced a largely circular argument by basing phyletic inferences on the
supposed trends in color themselves.) (2) From breeding, Whitman found that
selection along the orthogenetic trajectory could only move "forward" from
checkers to bars, and never in Darwin's proposed order from bars to checkers.
Selection must push, but the phyletic sequence can only proceed in one direction—
down the channel of orthogenetic variation: