Pattern and Progress on the Geological Stage 497
probably far longer than, the whole interval from the Silurian age to the present
day" (1859, p. 307).
Thus, Kelvin's 100 million years since the initial consolidation of the earth's
crust implied substantially less than 50 million years for the entire fossil record
since the Cambrian explosion. The situation only worsened as Darwin pondered
Kelvin's views. If early changes had been so rapid and intense, then the earth must
have spent most of its history just "calming down" enough to enter a realm where
natural selection might work. Even more of Kelvin's limited time must therefore be
allotted to a pre-Darwinian earth, and even less to a severely restricted later world
amenable to natural selection as the cause of life's pattern. In January 1869,
Darwin wrote to James Croll: "Notwithstanding your excellent remarks on the
work which can be effected within a million years, I am greatly troubled at the
short duration of the world according to Sir W. Thomson, for I require for my
theoretical views a very long period before the Cambrian formation" (in F. Darwin
and Seward, 1903, volume 2, p. 163). And, more graphically, Darwin wrote to A.
R. Wallace in 1871: "I can say nothing more about missing links than I have said. I
should rely much on pre-Silurian times; but then comes Sir W. Thomson like an
odious spectre" (in Marchant, 1916, Letters of Wallace, volume 1, p. 268).
Lest we construe Darwin's anxiety as exaggerated, consider the corroboration
that he received for his distress right from the horse's mouth. Kelvin himself
addressed Darwin's greatest fear in his own direct and succinct way. The great
physicist did not dispute evolution per se, but Kelvin argued that his own
limitations upon time had effectively debarred natural selection as an important
mechanism. Kelvin, following a cultural tradition that Darwin had transcended,
believed that some spiritual force must be guiding the progress of evolution, if only
because geology provided insufficient time for producing the observed order by
mechanical processes. As Burchfield observes (1975, p. 73): "Kelvin was
convinced that the complexity of life bore witness to the work of a Creative
Intelligence. He was equally convinced that whereas natural selection would
require almost endless time, divine guidance would enable evolution to produce
the diversity of life in a relatively short period. Thus as far as evolution was
concerned, his arguments for limiting the earth's age were also proofs of design in
nature." Kelvin wrote: "A correction of this kind [on the duration of geological
time] cannot be said to be unimportant in reference to biological speculation. The
limitation of geological periods, imposed by physical science, cannot, of course,
disprove the hypothesis of transmutation of species; but it does seem sufficient to
disprove the doctrine that transmutation has taken place through 'descent with
modification by natural selection'" (Thompson, 1868, p. 222).
A question of direction (too much geology)
Thus, the vector imposed by Kelvin's Second Law—intense energy of early causes,
diminishing continuously through time—troubled Darwin by compressing the
fossil record into a restricted realm of adequate calmness. But the