Charles Lyell, Historian of Time's Cycle - Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time - page 161

of all signatures in the bag as now constituted. We can then study the
beanmaster's four photos and tabulate the 1000 signatures in each. The longer
any bean is in the bag, the greater its chance of removal (since beans are
pulled out at random as new ones enter). Thus, the more recently any bean
entered, the greater the chance that it still resides in the bag. Lyell exclaims
triumphantly that we need only tabulate, for each photo of the bag at a
previous time, the percentage of beans that remain in the bag at day's end.
The higher the proportion of current beans, the younger the photo.
What other criterion could we use? No bean betrays its age, but we do have
the present bag, and can tell time by a gradual and continuous approach to
current composition. The present bag is not better or more distinctive than
any other; it only possesses the virtue of precise location in time—and we
may therefore compare the similarity of other bags with it.
Lyell dated the Tertiary (last "day" of just a few) in this manner precisely. He
proposed a statistical measure based on the relative percentage of living
species of mollusks. (He used mollusks because they are numerous and
distinct in Tertiary strata, and because he could pay his French colleague
Deshayes to compile faunal lists for all major sections of the European
Tertiary, based on the consistent criterion of Deshayes's personal taxonomic
expertise.) Such a statistical criterion cannot yield overly fine distinctions,
since several random factors operate (not just in removal, as in our bean
experiment, but also, for the complexities of reality, because species don't
arise at equally spaced intervals, and because total numbers of species are not
truly constant). Thus, Lyell split the Tertiary into four subdivisions (as the
beanmaster took four photos)—named, in order, Eocene. Miocene, older
Pliocene, and newer Pliocene (Figures 4.8 and 4.9), and defined as bearing
about 3 percent of living species (Eocene), about 20 percent (Miocene), more
than a third and often more than a half (older Pliocene), and about 90 percent
(newer Pliocene). (Readers who have been forced to memorize the geological
time scale will recognize that our modern system retains LyelTs names, with
a few additions in the same mold—Paleocene,