Pattern and Progress on the Geological Stage 493
in Geology Briefly Refuted." This manifesto became the stalking horse in Kelvin's
40 - year campaign to refute the substantive uniformities of rate and state (see pp.
479 - 484) by arguing that the earth's limited age did not provide enough time for
explanations based solely on the accumulation of small effects produced by causes
acting at current rates. Over the years, Kelvin developed a set of arguments,
yielding broad ranges rather than precise ages, and based on limits to the age of
both the sun (derived from estimates of meteoric influx), and of the earth (derived
from outflow of heat and rotational slowing by tidal friction). Kelvin originally
favored a date of some 100 million years (with an upper bound at 400 million), but
he refined his estimates downward as the years passed, and finally settled upon a
limited span of only 10-30 million years (at least for the duration of a solidified
outer crust).
In Kelvin's most famous argument (and sole subject of the 1866 note),
measurements of interior heat restrict the earth's age on the assumption that
outflow represents continued cooling from an initially molten state. By measuring
rates of outflow, we should be able to set an outer limit of maximal age for the
origin of life by specifying the initial time of formation for a solid planetary
surface. (In practice, such a calculation must remain highly uncertain due to in
homogeneities of planetary composition and our own ignorance about the earth's
interior.) More importantly, the entire argument rests upon an assumption that no
sources of novel heat exist, and that all current efflux must therefore represent a
residual flow from the original fireball of an initially molten planet. When the
discovery of radioactivity revealed an engine of new heat, Kelvin's argument
collapsed. In the delicious irony mentioned above, the same force that dethroned
Kelvin's limited duration soon provided a clock to measure the earth's actual age—
and the billions favored by many geologists triumphed over Kelvin's long
campaign for restriction. (Kelvin lived into the age of radioactivity, but never
publicly acknowledged his defeat. Lord Rutherford tells an interesting story of an
early lecture that he delivered in 1904 on determining the age of the earth by
radioactive decay—see Gould, 1985c. Rutherford spotted the aged Kelvin in his
audience and realized that he was "in for trouble." "To my relief," Rutherford
writes, "Kelvin fell fast asleep, but as I came to the important point, I saw the old
bird sit up, open an eye and cock a baleful glance at me. Then a sudden inspiration
came, and I said Lord Kelvin had limited the age of the earth, provided no new
source of heat was discovered. That prophetic utterance refers to what we are now
considering tonight, radium!")
At this point in the conventional morality play, the story becomes a homily in
the Manichean mode: The elegant mathematics of an arrogant physicist expires on
the Achilles' heel of a false assumption. The humble and patient observers of
nature, who always knew, in the bones of their rich empirical experience, that
Kelvin must be wrong, but who dared not oppose such a powerful foe, triumph in
the end. Empiricism wins the day: "speak to the earth, and it shall teach thee" (Job
12:8). In this canonical version, Darwin stands with his fellow geologists and
biologists, forging a common front of natural historians against an intruder with no
feel for the empirics of history. But this