388 THE STRUCTURE OF EVOLUTIONARY THEORY
by two criteria: general reduction in coloration, and concentration of remaining
color into regionally differentiated bands or swaths. Thus, in the first stages, spots
disappear from much of the plumage, while local areas may develop strong
concentrations. Subsequently, these local intensities diminish in number and area
(bars, for example, may become both narrower and fewer in number). Finally, the
regional concentrations become effaced as well, and the bird turns light and
monochromatic.
In a bold move towards complete generality, Whitman then tried to extend
this idea of an archetype, followed by an orthogenetic trend, beyond pigeons and
doves to the entire field of avian plumage. He postulated that the uniform
turtledove pattern should be regarded as ancestral for all coloration in birds. With
differing degrees of heterochrony in the inexorable process of reduction, and
varying places and styles of regional concentrations, all observed plumages might
then be rendered as extensive variations upon a single orthogenetic trend. Even the
ocelli of peacocks, for example, can be interpreted as altered spots of the turtle-
dove pattern, while their restriction to limited areas of the plumage (however
showy and conspicuous the result) denotes one form of participation in the
universal trend: "With this [turtle-dove] pattern as an archetype it is possible to get
an orientation of the whole field of avian patterns and to thread our way through
what before seemed an impenetrable maze of multifarious variations, with no
discoverable beginning or end of order" (1919, p. 58).
The mere claim for a trend, even such a pervasive and inevitable series, does
not of itself complete an argument for orthogenesis, or internally directed variation.
After all, both the archetypal turtle-dove pattern and all subsequent stages of
reduction might be adaptations, externally selected from isotropic Darwinian
variation. But Whitman well understood the ingredients required to distinguish true
orthogenesis from orthoselection or some other functionalist explanation of
trends—namely, (1) evidence that the trend proceeds independently from (or even
despite) adaptive pressures from local environments (selection may alter rates or
add details, but cannot derail the basic route); and (2) data supporting an internally
based directionality of variation available for shaping into evolutionary change
(ontogenetic channeling in Whitman's view, as we shall see).
On the first criterion, Whitman upheld the inexorable character of a trend towards
local differentiation and final effacement, independent of what environment might
favor in functional terms.
The process of evolution in color patterns has been a sweeping one,
involving the whole surface and taking the same general direction. The
stages reached are various, ranging all the way from the full chequered to
the wholely unchequered state; from chequers and bars combined in
different proportions to bars alone; from many bars to three, two, one, a
remnant, or none; and in all shades of brown, black, gray, red, to pure
white. Nowhere in this field of variations do we find any indications that