An Ever-Changing Earth
Throughout his trip, Darwin saw ample evidence that Earth had undergone vast changes. Mountains had risen from the sea. Climates had changed. Many species had gone extinct, leaving fossils as evidence. But how, Darwin wondered, had these changes occurred?
Darwin's geological studies left him increasingly convinced that most of these changes had happened over a very long time. He climbed high into the Andes and found his proof: fossil tree trunks, still upright. Long ago, a forest must have been buried and slowly petrified. Then, in a long series of earthquakes, the land had been uplifted to form the Andes Mountains. As a result of the Beagle voyage Darwin would wonder: if mountains had taken shape over millions of years, was it possible that the same was true of species?
The Moving Earth
A turning point in Darwin's thinking came when he experienced an earthquake that leveled the Chilean town of Concepción. To his amazement, the earthquake had elevated a bank of mussels about eight feet above the sea. If that could happen in one day, Darwin wondered, why couldn't the same process, repeated over millions of years, raise entire mountains?
Darwin was fascinated by fossils, such as these pieces of petrified wood collected on his Beagle voyage. Petrified wood forms when trees are buried and over many years minerals seep in, slowly turning them to stone while preserving the structure of the ancient wood.
Darwin was especially interested in where he found each fossil. Studying the geology of the rocks, shells and fossils around it could reveal when the fossilized plant or animal lived. Fossils could also reveal how an environment had changed over time. For example, petrified wood had to start as living trees in a forest habitat, then spend time buried and later rise to its present elevation. Thus a single fossil could tell the story of a slowly changing landscape.
Even after the original material has been replaced by stony minerals, fine details of petrified wood are sometimes minutely preserved, down to the structure of individual cells.