Boundaries - Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time - page 197

of mineralogists visited the Alhambra of Granada, the architectural primer for
Islam's keen understanding of geometrical regularity in ornamentation. One
of my colleagues noted with pleasure that patterns of symmetry in tile
designs of the Alhambra included every two-dimensional arrangement
recognized by mineralogists in earthly rocks.
This similarity of time's cycle teaches us something deep about nature's
structure because the congruence of ions and tiles is not a product of "just
history." The complex likenesses of organic genealogy are passive retentions
from common ancestry—contingencies of historical pathways, not records of
immanent regularities. (I type with the same bones used by a bat to fly, a cat
to run, and a seal to swim because we all inherited our fingers from a
common ancestor, not because laws of nature fashioned these bones
independently, and in necessary arrangement.) The complex similarity of tile
and mineral patterns records an active, separate development to the same
result under immanent rules of natural order.
These two kinds of similarity—by genealogical connection, or time's arrow,
and by separate reflection of the same immanent laws, or time's cycle—join
forces when we try to unravel nature's complexity. The vision of time's cycle
enabled Hutton and Lyell to grasp deep time, but we couldn't mark units
within this immensity until time's arrow of the fossil record established a
criterion of uniqueness for each moment. The neptunists failed to unlock
stratigraphy because they falsely assumed that rocks carried signatures of
temporal uniqueness. But rocks are simple objects, and their similarities
designate formation under recurring conditions, not time's arrow of
genealogy. The proper paleontological criterion, based we now know on
contingent pathways of evolutionary change, allowed us to mark by time's
arrow what the matrix of time's cycle had established.
Evolutionary biologists have long recognized, as the fundamental operation
of our profession, the proper distinction between similarities of time's arrow
and time's cycle. We designate them by different names— homology for
passive retention of features shared