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down the Alpine valleys, far below the snouts of present glaciers. He
reasoned that glaciers must once have extended over a much greater
range than they do today and proposed that there had once been a
great ice age. This was not so hard to imagine when looking up a
Swiss valley, but other scientists, finding glacial deposits far below
the furthest extent of present ice sheets, extended the reasoning to
conclude that huge ice sheets as much as a mile thick had advanced
over much of the Northern Hemisphere.
Lyell presented the other type of uniformity—of rate and state,
which Gould has called substantive uniformitarianism —as an a priori
description of the way the earth works: Over the long span of earth
history there has been no directional change, no progression. But
substantive uniformity was tested and falsified in Lyell's own cen-
tury, when it was learned that glaciers of vast size had advanced over
the continents, that the seas at times had risen to drown the land
and at other times had dried up, that mountain ranges had risen and
been eroded away. Clearly, processes have operated at different
rates and the earth has changed. The coup de grace to substantive
uniformitarianism was the obvious progression shown by the fossil
record, leading from one-celled bacteria in Precambrian rocks to
modern Homo sapiens. But Lyell accepted evolution only in the
1866 edition of his Principles, and only then, Gould believes, be-
cause "it permitted him to preserve all other meanings of unifor-
mity." 2 5 Since he believed the rate of biological change always to be
the same, Lyell was forced to conclude that the vast difference be-
tween the creatures that lived in the Cretaceous and those that
lived in the Tertiary implied that the missing interval between them,
which today we call the K-T boundary, represented as much time
as all that has passed since. Today we know that time to amount to
65 million years, and the K-T boundary clay to represent to only a
few thousand years at most.
To sum up, one type of uniformitarianism amounts to the state-
ment that geology is a science; the second, which requires the adop-
tion and maintenance of an a priori position regardless of the evi-
dence, amounts to the statement that geology is not a science. Both
cannot be true. But how then are we to account for the persistence
of both in geological thought for nearly two centuries?
• The two types were so inextricably entangled that few students
of geology ever realized that they were accepting "two-for-one."
Since methodological uniformitarianism worked, the substantive
variety tended to be accepted without anyone realizing that a fast
one had been pulled.
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