The Fruitful Facets of Galton's Polyhedron 371
time, acceleration has become so intense that most middle stages in ontogeny begin
to drop out entirely. The earliest embryonic stages, however, remain stubbornly
persistent. As acceleration intensifies, newly introduced senile features push back
the older progressive traits of phyletic midlife, until these middle stages encounter
the persistent juvenile features. Pushed at one end by senile features, and pressed
against the impenetrable wall of persistent embryonic traits at the other end, these
progressive middle stages finally tumble off the phyletic conveyor belt. Characters
of phyletic old age now merge with juvenile features (see Fig. 5-3). Ontogeny
becomes shorter (by excision of intermediate stages), and simpler (because the
remaining juvenile and old-age stages resemble each other in external appearance).
In this way, the simplified ontogeny of regressive lineages does not represent
retardation or truncation of development, but rather acceleration so intense that all
intermediate complexity (once intercalated between true youth and "second
childhood") disappears by compression. Hyatt writes (1889, p. x): "Acceleration
produces first, the earlier development of some of the progressive characteristics
5 - 3. Hyatt combined his concept of a predetermined phyletic life cycle with his principle of
universal acceleration to explain how even the simplified ontogenies of regressive evolution can
originate by acceleration. In Hyatt's "old age theory" (his designation) as extinction nears; senile
stages of phyletic youth and maturity become the adult stages of a waning stock in racial
senescence. Ontogeny becomes so shortened by acceleration and deletion that senile stages
merge with persistent juvenile stages to produce a greatly simplified and senile course of life.
From Gould, 1977b.