© Sheila Terry/Science Photo Library
Charles Darwin (1809–1882).
Charles Darwin (1809–1882).
Charles Darwin is best known for his theory of natural selection, which explains how evolution works. But before he turned his attention to evolution he was a geologist, and he made his name in scientific circles with his accounts of the geological observations he made in South America during the famous voyage of the Beagle. The most dramatic of these concerned his first-hand experience of a major earthquake, the observations of which led him to the correct explanation of the origin of the Andes and other mountain ranges.
Darwin was ashore in Chile, near the town of Valdivia, when he experienced the earthquake, on 20 February 1835. He wrote: ‘[I] was lying down in the wood to rest myself. It came on suddenly, and lasted two minutes, but the time appeared much longer. The rocking of the ground was very sensible … It was something like the movement … felt by a person skating over thin ice, which bends under the weight of his body. A bad earthquake at once destroys our oldest associations: the earth, the very emblem of solidity, has moved beneath our feet like a thin crust over a fluid.’14
Robert FitzRoy, the Captain of the Beagle, put his ship out to sea as soon as possible, and headed north along the coast to the city of Concepcion to offer what help he could. They arrived at the devastated city on 4 March. It was near here that Darwin saw along the coastline rock layers raised above the level of high tide, but covered with the remains of dead and dying shell fish, including mussels and limpets, and seaweed anchored to the rocks. It was clear that the earthquake had lifted the land. FitzRoy thought that this must be a temporary phenomenon, and that the land would soon settle back down. But Darwin realized the long-term implications. He had read the work of the pioneering geologist Charles Lyell, who, in his great book, Principles of Geology, made the case that instead of the kind of catastrophes described in the Bible, all that was needed to explain the appearance of the Earth was the accumulation of gradual changes, exactly the same as the processes we see going on around us now, over immense periods of time. He developed these ideas following the work of the Scot James Hutton in the 1780s, according to whom the origins of the Earth were lost in the mists of time, and as far as his observations of the geological record were concerned he could find ‘no vestige of a beginning, no prospect of an end’. This flew in the face of the orthodox view at the time, that the Earth was only some six thousand years old. This relatively new idea, known as uniformitarianism, contrasted with the older-established ‘catastrophist’ view, which held that sudden, dramatic changes unlike anything seen on Earth today were responsible for the formation of mountain ranges, ocean beds, and other features of the Earth’s surface. Lyell was the leading light of uniformitarianism.
Compared with the Biblical Flood, even the earthquake Darwin had just experienced was a gradual change. And it was normal for Chile. If one earthquake could raise the land noticeably, then over the kind of timescale Lyell talked about the whole Andes mountain range could have been lifted out of the sea. The clinching evidence came when Darwin noticed old shell beds higher up the rocks rising from the sea – proof that there had indeed been similar earthquakes producing similar amounts of uplift in the past. On an expedition inland, he even found fossilized sea shells high up in the mountains. All of this would in due course provide Darwin with the time frame needed for evolution to do its work by a similar accumulation of tiny changes over immense intervals. He concluded that ‘it is hardly possible to doubt that this great elevation has been effected by successive small uprisings … by an insensibly slow rise’. And when he came to develop his theory of natural selection, Darwin thanked Lyell in print for giving him ‘the gift of time’ – time enough for natural selection to operate as the mechanism of evolution.
The ‘skating on thin ice’ analogy also led Darwin to think about what lay beneath the seemingly solid surface of the Earth, and to come to a remarkably modern conclusion: ‘In all probability, a subterranean lake of lava is here stretched out … We may confidently come to the conclusion that the forces which slowly and by little starts uplift continents, and those which at successive periods pour forth volcanic matter from open orifices, are identical. From many reasons, I believe that the frequent quakings of the earth on this line of coast are caused by the rending of the strata, necessarily consequent on the tension of the land when upraised, and their injection by fluidified rock.’15