Answer
During Darwin's journey on the Beagle, he made a stop by the Galapagos Islands. Here, he documented all of the unique characteristics found on the island animals, such as the giant tortoises and marine iguanas. He realized that the Galapagos plants and animals more closely resembled those of South America, instead of the African Cape Verde Islands he had previously been to, despite having a more similar climate and topography to the Cape Verde Islands. He also saw that each Galapagos Island contained unique variations of the same animals that resided on each of the other Islands. He realized through this that these species were products of a long history of evolutionary change, and not immutable or specifically created as they were.
When studying pigeon breeding, he was entertained by how each breed of pigeon was so unique that they could be considered different species if found in nature, but that they all came from a single wild species (the rock dove). Darwin realized that, similar to artificial selection, the "struggle for existence" in the wild could create a selection of traits in the animals that could be a powerful force of evolution of wild species.
Work Step by Step
Journey on the Beagle, study of breeding of pigeons, theory of evolution.
