In September 1833 Darwin set out to cross the Argentine pampas, heading for Buenos Aires. Immediately he found himself in the midst of a bitter struggle between the local Indians and the forces of General Rosas, who were engaged in a policy of extermination.

In 1835 an earthquake destroyed the town of Concepcion in Southern Chile. The effect of it was felt on board the Beagle, and a few days later Darwin witnessed the devastation for himself. It confirmed his view that the whole range of the Andes had been pushed up from the bed of the ocean.

On the Galapagos Islands, Darwin not only encountered creatures found nowhere else on earth, but also observed subtle differences between the inhabitants of one island and those of another. But he nearly missed the significance of the little brown finches that could be seen everywhere. Some of these had beaks thin enough to pick up seeds from the hard ground; others had beaks thick enough to crack a nut. Here was the clue which led him directly to the theory of natural selection.