106 THE STRUCTURE OF EVOLUTIONARY THEORY
world, beginning with individual variants as the source of subspecies, then moving
to subspecies as incipient species, and finally to species as potential ancestors for
branches of life's tree—a full range of scales from variation within a population to
the entire pageant of life: "I look at individual differences, though of small interest
to the systematist, as of high importance for us, as being the first step towards such
slight varieties as are barely thought worth recording in works of natural history.
And as I look at varieties which are in any degree more distinct and permanent, as
steps leading to more strongly marked and more permanent varieties; and at these
latter, as leading to subspecies, and to species" (p. 51).
Darwin invoked this first method, a strong argument based on maximal
information at smallest scale, as his favored choice when available. To cite just
three instances as a sampler: (1) the paleontological panorama may be read as a
story of gradual evolution because species in adjacent strata show minimal
differences, but these differences increase gradually as stratigraphic distance
expands (p. 335). (2) When we find hints of the feather patterns of rock pigeon in
highly modified breeds, we do not hesitate to interpret these designs as vestiges of
an ancestral stock; therefore, the faint stripes that we sometimes observe in coats of
young horses point to a common origin for all species in the clade of horses, asses
and zebras (pp. 166-167). (3) Marine molluscs often exhibit brighter colors in
warmer waters. We note this pattern both among varieties of a single species living
in cold and warm waters, and among related species. A creationist explanation
requires uncomfortable special pleading: God sometimes makes a species with
bright shells in warm climates, but he allows other species to vary naturally, in the
same geographic pattern, within a single created kind. An evolutionist, using
method one, will recognize these phenomena as two stages in a single sequence of
extrapolation from smaller to larger scale (p. 133).
SEQUENCING. We can use a second style of inference about temporal order
when we cannot obtain adequate data about the nature of immediate changes at
smallest scale. Since historical processes begin at different times and proceed at
varying rates, all stages of a sequence may exist simultaneously (for example,
stage one in case A, which began very recently; stage two in case B, which began
at the same time, but has proceeded at an uncommonly rapid rate; and stage three
in case C, which began long ago). Thus, fringing reefs, barrier reefs and atolls all
exist now. When we recognize these forms as sequential stages of a single process,
we may infer the pathway of history.
Darwin epitomizes method two in writing (p. 51): "A series impresses the
mind with the idea of an actual passage." Invoking his usual starting point, Darwin
presents a first example from breeds of domesticated pigeons. The more adequate
data of method one—observed steps of passage, accumulating to greater and
greater difference in time—no longer exist, for the transitional populations have
died, and only a set of morphological "islands," representing a set of established
breeds, remains. But these islands can be ordered as a plausible sequence of change
between ancestral rock pigeons and the most aberrant of artificially produced
breeds: "Although an English carrier or