In order to appreciate Darwin’s reflections on the only gradual differences between the mental abilities
of humans and animals, I must expound a bit more.
When people of the Enlightenment recognised that the world was not always as it is today, the first explanation was that the old world, the one that had been discovered here and there in the form of the bones of extinct animals, had been the first creation that had perished in the deluge narrated in the Bible.
This gave rise to catastrophe theory, the idea that there had been several acts of creation, and that the world had been destroyed in between them by floods or enormous volcanic eruptions and earthquakes. However, the geology of Darwin’s time had shown that many processes in nature were not catastrophic but rather gradual and that they took place in small and even tiny steps.
The Scottish geologist James Hutton had gained experience from known remains out of the Roman era with just how slowly geological processes take place. He also had recognised that the geological faults that could be observed on the coasts of his Scottish homeland had to have taken an incredible amount of time to form. This gave rise to the theory of gradualism which attributed the changes in the world to a continuous, gradual sort of change.
A supporter of this theory was the geologist Charles Lyell, Darwin’s friend and teacher. Darwin adopted gradualism from Lyell in his theory of evolution. Darwin was able to demonstrate one of the centrepieces of Darwinism: evolution does not happen in large leaps, rather evolutionary changes in populations of individuals occur in the smallest of steps. When they occur they require very long periods of time.