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Rereading Darwin

Science now takes for granted the importance of forces and time spans we can’t perceive directly

Robert L. Dorit

The Dangers of Extrapolation

By Darwin’s time, 19th-century geologists and paleontologists were well aware of the fossil record—and they knew that fossils were old. Naturalists recognized the characteristic appearance and extinction of faunas in the geologic record; by the mid-1800s, they had named the periods and even established their order of succession. But the implications of the fossil record for the age of the Earth remained controversial. One group of scholars constructed increasingly elaborate theories of successive catastrophes that reconciled the fossil record with a young Earth. Lyell and Darwin belonged to another camp, pioneered by James Hutton, which argued that the fossil history was evidence of a far more ancient Earth. As Hutton put it in a 1785 paper presented to the Royal Society of Edinburgh, Earth’s geology revealed “no vestige of a beginning, no prospect of an end.”

The matter was far from settled. Indeed, the assignment of accurate absolute dates to the geologic timescale would have to wait for the discovery of radioactive decay at the turn of the 20th century. That’s not to say no one tried; clever calculations were certainly attempted during Darwin’s lifetime. Most notably, the influential physicist William Thomson—better known as Lord Kelvin—argued for the power of physical approaches to the question of time spans. He used the rate of cooling of an originally molten Earth to conclude that our planet was likely 100 million years old.

Here again, as with Bishop Ussher, the flaw was not in the method, but in its underlying assumptions. Kelvin knew nothing about radioactivity and the constant heat it supplied. His model of a cooling Earth failed to account for new heat contributed by radioactive decay. Nor did he consider the transfer of heat from Earth’s deep interior to its crust by convection. These omissions led to a serious underestimate of the planet’s age.

At the same time that Kelvin was making the case for a 100-million-year-old Earth, geologists championed a completely different approach. Their argument depended on careful observation of the speed of deposition and erosion in contemporary landscapes. Geologists in the 19th century understood that these processes, writ large, could carve canyons, build up or erase thick sedimentary layers and reshape topography. By looking at these large-scale phenomena in terms of slow—but measurable—current rates of deposition and erosion, they could estimate how long those processes had been at work to generate contemporary geologic features. This approach, sometimes called uniformitarianism, would lead to significantly greater estimates of the age of the Earth.

In the first edition of the Origin, Darwin employed this method to calculate the age of the Weald, a sandstone formation in Southern England. This feature, once covered by a layer of chalk (still visible in nearby formations), had been completely denuded. Based on estimates of the rate of erosion, Darwin calculated that it would have taken at least 300 million years to remove all of the sediments covering the Weald. This estimate was three times greater than even the most generous prevailing notions of the age of the Earth. Kelvin was unimpressed with both the method and the result that Darwin obtained. And, as it turns out, Darwin was wrong, too. The Weald, in an unusual turn, was younger than he thought. Shortly after its publication in 1859, Darwin acknowledged that his estimate was likely incorrect, and the episode continued to disturb him. In an 1860 letter to his friend Asa Gray, he complained that “in fact geologists have no means of gauging the infinitude of past time.” A few months later, he wrote to his friend Lyell about the dangers of extrapolation:

Having burnt my own fingers so consumedly with the Wealden, I am fearful for you.… for Heaven-sake take care of your fingers; to burn them severely, as I have done, is very unpleasant.

By the third edition of the Origin, published in 1861, the discussion of the age of the Weald was entirely gone. Telling time was proving trickier than Darwin had thought.

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