Boundaries - Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time - page 198

by common ancestry along time's arrow of genealogy; analogy for active
evolution of similar forms in separate lineages because immanent principles
of function specify a limited range of solutions to common problems faced
by organisms throughout time. The wings of birds, bats, and pterodactyls are
analogous, though strikingly similar in aerodynamic design, because no
common ancestor of any pair had wings—and flight evolved independently
in three separate lineages. The detailed similarity in number and arrangement
of armbones in humans, chimps, and baboons does not record a law of nature
imposed upon separate productions, but simple inheritance from a common
ancestor.
Thus, all taxonomists will tell you that they must, above all, separate
analogous from homologous similarity, discard the analogies, and base
classifications upon homologies alone—for taxonomies record pathways of
descent. But any functional morphologist will pass over homologies as
simple repetitions of the same experiment, and seek analogies that teach us
about the limits of variety when separate lineages evolve structures for
similar function.
The arrow of homology and the cycle of analogy are not warring concepts,
fighting for hegemony within an organism. They interact in tension to build
the distinctions and likenesses of each creature. They interweave and hold
one another, as the laws of time's cycle mold the changing substances of
history. The relentless arrow of history assures us that even the strongest
analogy will betray signs of uniqueness, and permit a proper placement in
taxonomy and time. Ichthyosaurus (Figure 5.8), descendant of terrestrial
reptiles that returned to the sea, evolved the most uncanny resemblances to
fishes—even developing, from no known precursor, a dorsal fin in the proper
hydrodynamic position, and a tail fin with two symmetrical lobes, as
principles of optimal swimming dictate. But ichthyosaurs retain signs of their
reptilian heritage. The dorsal fin contains no bony supports as in fishes; the
vertebral column bends into the lower lobe of the tail, not into the upper, or
ceasing in the midline, as in fishes; the supports of the flippers are finger
bones, not fin rays. In other words, the immanent and predictable features