374 THE STRUCTURE OF EVOLUTIONARY THEORY
verbal mode (Rudwick, 1992; Gould, 1989c, 1993d, 1996a). Hyatt's phylogeny of
the Steinheim planorbids (Fig. 5-5) epitomizes hardline orthogenesis under the
guidance of phyletic life cycles.
Hyatt depicts four lineages within the lakebeds, each beginning from an
ancestral Planorbis levis stock (not shown). This putative phylogeny rests upon
two principles derived from his orthogenetic "old age" theory (and showing that
Hyatt's convictions directed his observations, rather than the conventionally touted
vice versa).
- Hyatt distinguished the four lineages on the basis of supposedly
progressive and retrogressive characters. The rationale for these designations
probably owed more to vague and general cultural conventions (largely the
folklore of more and less as better and worse) than to any explicitly biological
argument. Progressive characters include increase in size, shell thickness, strength
of ornamentation, and change of shape from planispiral (flat like an ammonite) to
trochiform (domed like a conventional snail). In other words, small, thin, smooth,
and flat specifies a primitive state; whereas large, thick,