One may wonder what it was that motivated Kelvin to examine this problem in the first place. The answer is actually quite simple. Even a cursory examination leaves little doubt that the publication of Darwin’s The Origin in 1859 provided the main impetus for Kelvin’s direct attack on the estimates of the ages of both the Sun and the Earth. To be clear, Kelvin did not object to the theory of evolution per se. In his 1871 presidential address to the British Association for the Advancement of Science, for instance, he expressed moderate support for some of Darwin’s conclusions in The Origin. However, he completely rejected natural selection because he had “always felt that this hypothesis does not contain the true theory of evolution, if evolution there has been, in biology.” Why not? Because, he explained, he was “profoundly convinced that the argument of design has been greatly too much lost sight of in recent zoological speculations.” In other words, even this committed mathematical physicist, who passionately declared that the “essence of science . . . consists in inferring antecedent conditions, and anticipating future evolutions, from phenomena which have actually come under observation,” still believed that “overpoweringly strong proofs of intelligent and benevolent design lie all around us.” In fact, Kelvin held that the laws of thermodynamics themselves were a part of that universal design. Nevertheless, we should remember that even if Kelvin felt a certain emotional attachment to the concept of “design,” there is no doubt that he anchored his scathing criticism of the geologists’ practices in genuine physics, not in his religious beliefs.

What was Kelvin’s impact on geology? Until the 1860s, geologists were much more preoccupied with discussions on whether the Earth’s interior was solid or fluid than with the Earth’s chronology. By the mid-1860s, however, quite a few of the influential geologists started to pay serious attention to Kelvin’s claims. Foremost among these were John Phillips, Archibald Geikie, and James Croll. Based on studies of sediments, Phillips himself had suggested in 1860 an age of about ninety-six million years for the Earth. By 1865, he was publicly supporting Kelvin. Geikie, the new director of the Geological Survey for Scotland, more or less assumed the role of a conduit and mediator between physics and geology. On one hand, he criticized Kelvin’s assertion that the Earth’s geological past had been more active, citing evidence that seemed to show that if anything “the intensity . . . has, on the whole, been augmenting.” On the other, in a paper published in 1871, he essentially abandoned uniformitarianism and stated that based on research in physics “about 100 millions of years is the time assigned within which all geological history must be comprised.” Croll, an impressive self-taught physicist and geologist, was entirely convinced by Kelvin’s calculation of the cooling Earth, and even though he was extremely skeptical about Kelvin’s estimate of the age of the Sun, accepted one hundred million years for the Earth’s age.

Often you can judge whether a certain scientific theory has had an impact by the vehemence with which the heavyweights with something at stake announce their objections to it. In Kelvin’s case, the sure sign that the opposition had taken notice came when biologist Thomas Huxley attacked Kelvin’s calculation in February 1869.

Huxley had earned the title “Darwin’s Bulldog” because of his aggressive support of the theory of evolution and his eagerness to debate in its defense. Huxley loved controversy as much as Darwin hated it. He is perhaps best known for his legendary, brief verbal encounter with Samuel Wilberforce, bishop of Oxford, on June 30, 1860. The event took place at Oxford University’s New Museum library as part of the annual conference of the British Association for the Advancement of Science. The story was told in colorful, although probably partly imaginary, detail in the October 1898 issue of Macmillan’s Magazine. The writer reminisced:

I was happy enough to be present on the memorable occasion at Oxford when Mr. Huxley bearded Bishop Wilberforce . . . Then the Bishop rose, and in a light scoffing tone, florid and fluent, he assured us there was nothing in the idea of evolution; rock-pigeons were what rock-pigeons had always been. Then, turning to his antagonist with a smiling insolence, he begged to know, was it through his grandfather or his grandmother that he claimed his descent from a monkey? On this Mr. Huxley slowly and deliberately arose. A slight tall figure stern and pale, very quiet and very grave, he stood before us, and spoke those tremendous words—words which no one seems sure of now, nor I think, could remember just after they were spoken, for their meaning took away our breath, though it left us in no doubt as to what it was. He was not ashamed to have a monkey for his ancestor; but he would be ashamed to be connected with a man who used great gifts to obscure the truth. No one doubted his meaning and the effect was tremendous. One lady fainted and had to be carried out.