376 THE STRUCTURE OF EVOLUTIONARY THEORY
bumpy and blocky equals advanced. By contrast, regressive characters include
decrease in size, loss of ornamentation, thinning of the shell and, above all, a
tendency for irregular growth by uncoiling (loss of order as a sign of both
ontogenetic and phyletic senility).
Hyatt identifies three of his four lineages as progressive, distinguishing them
by different combinations of the key characters. The most purely progressive
steinheimensis-trochiformis lineage advances by all criteria to greater size,
thickness, and quadrate shape with keels and carinae (rather than a smooth whorl
profile) and, as the formal name states, a domed outline. Shells of the oxystomus-
supremus lineage become larger and more ornamented; the spire does not increase
in height, but the shell still grows taller because the underside of the whorl profile
becomes more inflated. The parvus-crescens lineage shows less advance, as the
shell remains flat and smooth, but increase in size establishes the primarily
progressive character.
Retrogressive tendencies appear in the three sublineages of the remaining
branch, all derived from P. minutus. The turbinatus sublineage shows a mixture of
progressive and retrogressive characters (Hyatt, 1880, p. 17, refers to this melding
as "the battle of the tendencies"), with modest size increase and some
strengthening of ornament offsetting a basic decline. The middle, or denudatus,
sublineage is purely retrogressive, as shells become smaller, smoother and
irregular in growth by increasingly erratic coiling. Finally, the distortus lineage
also mixes phyletic strength and weakness. Ornament remains strong and size
increases in portions of the lineage; but, as the name implies, coiling becomes
irregular as the stock declines.
- Although later convention (and emerging practice in his own time) would
lead us to read Hyatt's chart as a stratigraphic sequence, his phylogeny employs an
unconventional iconography. Vertical position does not represent time or
stratigraphy, but rather stage in an orthogenetic sequence. Snails drawn at the same
level did not necessarily live at the same time, but show common "attainment" in a
phyletic series. Thus, for example, P. trochifortnis, the ultimate stage of the most
progressive series, lived near the bottom of the stratigraphic sequence—implying
that this lineage ran its full course with geological rapidity at the base of the
section. Conversely, P. oxystomus, the initial form of the second progressive
lineage, makes an initial appearance high in the sequence.
This unconventional iconography illustrates the power of theory to channel
perception—orthogenesis as an organizing principle, in this specific case. Consider
the immense confidence that a scientist must be willing to invest in the validity of a
chosen surrogate to substitute any other criterion for the eminently available (and
obviously meaningful) stratigraphic order of time as the measuring rod for vertical
position in phyletic charts. (Cladists have created quite a fuss in our day by using
inferred branching order in preference to time of observed paleontological
appearance, if they include fossils in their phylogenies at all—see Schaeffer,
Hecht, and Eldredge, 1972. In the heyday of overweening confidence in
recapitulation, several paleontologists reversed the conventional geological
procedure and inferred stratigraphic order from