496 THE STRUCTURE OF EVOLUTIONARY THEORY
have been more active in ancient times than at present, because of more
rapid augmentation of temperature downwards below the earth's surface;
and it cannot be reasonably urged that a hotter sun is not a probable
explanation of the supposed warmer climate of the Paleozoic ages (1868, p.
230).Kelvin's argument brings us to the nub of Darwin's objections. Darwin's
strong opposition to Kelvin, a reaction that could almost be described as fear and
loathing, has often been recorded, but rarely understood. To appreciate Darwin's
intense reaction, we must focus upon the geological prerequisite for his account of
evolution—a steady and intermediate rate of change, enough to prod, but not too
much to overwhelm, natural selection. Kelvin's directional geology did not invoke
the paroxysmal specter of traditional catastrophism. But he had raised, in many
ways, an even greater threat—for his challenge operated as a double-edged sword
to attack natural selection from both sides of Darwin's geological needs.
A question of time (too little geology)
Kelvin's estimate of time seemed generous to most naturalists, even to supporters
of natural selection like Wallace, and to Darwin's self-appointed spokesman, T. H.
Huxley. But Darwin envisioned natural selection as working so slowly, especially
in its progressive mode of extended biotic competition, that any talk about
limitation made him intensely nervous—for too little time could be equated with
insufficient geological impetus for evolutionary change. Darwin had shown his
hand in calculating a greatly exaggerated 300 million years for the denudation of
the Weald (see p. 153), a figure that he expunged with embarrassment from later
editions of the Origin (see Burchfield, 1975, pp. 70-72). Darwin had also urged
readers to close his volume if they could not accept Lyell's views on the
"incomprehensibly vast" time available for natural selection.
Kelvin's 100 million years sounded sufficiently long, but Darwin harbored
deep doubts, especially in the light of Kelvin's related argument for a vector of
diminishing rates of physical change through time. Kelvin's directional vector
clashed with the most important item in Darwin's apologetics for the imperfection
of the geological record. Darwin had been troubled, for example, by the abrupt
appearance of so many complicated anatomies in the Cambrian explosion: "The
case at present must remain inexplicable; and may be truly urged as a valid
argument against the views here entertained" (1859, p. 308). Darwin concluded
that Precambrian seas had "swarmed with living creatures" not yet found as fossils.
But the anatomical distance from the first living molecule to a trilobite certainly
exceeded all later change from Cambrian forms to modern organisms. Following
the gradualistic premise that amounts of change provide a rough measure of time,
Darwin concluded that most of the earth's history had passed before the Cambrian
explosion: "If my theory be true, it is indisputable that before the lowest Silurian
stratum was deposited [Cambrian of modern terminology], long periods elapsed, as
long as, or