Theories of evolution before Darwin, such as those of his grandfather Erasmus Darwin, did not explain how species could evolve without help from an outside guiding force, such as a botanist who killed specific kinds of flowers or a dog breeder who bred for a specific trait. Darwin proposed natural selection. Natural selection is the pressure for species with advantageous traits to survive while those with less advantageous traits died. Darwin said that natural selection explained evolution. The Origin of Species was also important because it was the first time that a strong body of evidence, taken from a wide range of species and environments, had been assembled in support of evolution.

The Galapagos Islands were important to the formation of Darwin's theory of evolution because they showed that species in isolation tended to evolve in ways that suited their environments. Although Darwin did not immediately comprehend the importance of the different varieties of tortoises and finches that he observed in the island, he later saw that the differences in finch's beaks demonstrated the ability of evolution to fine tune a species to fit the particular demands of an environment.

The cause of Darwin's health problems has remained a mystery. Some argue that he contracted a disease while on the Beagle; others think that his physical symptoms were the result of high levels of stress and emotional repression. In any case, the effect of his illness was to isolate him from society at Down House. He continued to see people, and his correspondence was enormous, but he spent most of his time alone or in the company of Emma. This solitude meant that he was distant from the controversies and politics of science, and was able to focus on his own theories and observations without the pressures of academic fighting or the responsibilities of teaching or mentoring. This isolation helped him develop his theory of evolution, but it may also have helped delay the time at which he finally announced it to the scientific community and the public.