from open university uk

Living proof
It was during Darwin's journeys on the British survey ship HMS Beagle that he saw the variations in different species that led him to develop the idea of natural selection. Darwin's experiences on The Galápagos Islands were a catalyst for his thinking about evolution.

The species on these islands - birds, plants, insects and reptiles - resembled those on the South American mainland, but they were also different in many subtle ways. After his journey back to England, Darwin began to develop the idea that the species from the mainland had reached the Galápagos, and then changed, adapting to their new environment. Until this point it was universally accepted that species did not change yet Darwin's emerging theory was about to challenge this in a radical way.
What did finches teach Darwin about evolution?
An evolving idea
Unlike today's scientists with their sophisticated lab techniques and computer equipment, Darwin was heavily reliant on observation and deduction; yet one of the amazing things about Darwin's theory is that it has remained at the heart of scientific thinking about the origins and development of life. Through our understanding of DNA we now have a much more sophisticated view of genetically inherited traits, which provides an even stronger scientific basis for Darwin's 150 year old theory.