Charles Darwin read Charles Lyell's Principles of Geology on the voyage of the Beagle, and the experience was transformative in a way that had nothing to do with fossils or finches. What Lyell gave Darwin was time — not more time, but a different kind of time: geological time, time measured in the slow accumulation of sediment, in the patient erosion of cliffs by tides that never stopped. Lyell's uniformitarianism held that the processes shaping the Earth today are the same processes that shaped it in the past, which implied that the Earth must be incomprehensibly old. Darwin needed that age. Natural selection only works if there is enough time for tiny advantages to compound across thousands of generations. The mechanism is plausible only on a canvas of millions of years.
Before deep time was established as a geological fact, the idea of gradual species transformation was almost incoherent. Archbishop Ussher's 1650 calculation that the Earth was created in 4004 BC was the dominant framework for most of Christian Europe; even those who doubted it had nothing to replace it with. William Paley, whose Natural Theology Darwin knew almost by heart as a student, argued that the complexity of a watch demands a watchmaker — and that living organisms, being vastly more complex than watches, demand a designer even more urgently. This argument is defeated not primarily by mechanism but by time. Given enough time, the blind watchmaker can work. Deny the time, and the whole edifice collapses.
The 19th century's geological revolution was itself an act of radical recalibration. James Hutton, writing in 1788, looked at the unconformity at Siccar Point on the Scottish coast — where near-horizontal rock layers rest atop near-vertical ones, requiring two complete cycles of deposition, tilting, erosion, and redeposition — and concluded that he could see "no vestige of a beginning, no prospect of an end." This was not mysticism but geology. The rock record simply required an Earth far older than any chronology based on biblical genealogy could accommodate. Hutton and Lyell didn't just add years to the clock; they dissolved the clock and replaced it with a different metaphysics of change: slow, continuous, cumulative, immense.
Darwin internalized this so thoroughly that the struggle of the Origin of Species was partly the struggle to make his readers feel the time, not just accept it intellectually. He estimated, in an early edition, that the denudation of the Weald — the valley between the North and South Downs in England — had taken about three hundred million years. He used this calculation not to establish geology but to establish a sense of scale in his readers' minds. The estimate was wrong (the actual time is much less), and he later removed it under pressure from physicists who thought the Earth couldn't be that old. But the instinct was right: natural selection is unintuitive until you can actually feel the expanse it operates across.
The crisis came from Lord Kelvin, who in 1862 calculated from thermodynamic principles that the Sun could only have been burning for between twenty and forty million years, and that the Earth could only be between twenty and one hundred million years old. Both estimates were far too short for Darwin's theory. Darwin was troubled. He couldn't refute the physics. He wrote privately that the age of the Sun was "an odious spectre." What saved deep time — and natural selection — was the discovery of radioactivity in 1896 and nuclear fusion in the 1930s, which revealed an energy source for the Sun that Kelvin couldn't have known about. The Earth is 4.5 billion years old. The Sun will burn for another five billion. The time is there after all.
What I find profound about this episode is how the acceptability of a correct idea depended on a fact about the external world that had nothing obviously to do with the idea itself. Darwin's mechanism was right. But whether it could be taken seriously depended entirely on whether geologists and then physicists could establish an adequate temporal canvas. Truth came in installments, each dependent on truths in other domains. The history of science is full of this: ideas that were correct but unacceptable until a supporting scaffold was built, often by people working on entirely different problems, who had no idea they were providing it. I think about this when I encounter questions I can't settle — not because the reasoning is unclear but because something crucial is missing from the canvas, waiting to be established somewhere else entirely.