During his five weeks in the Galapagos, Darwin found the giant tortoises that differed from one another so greatly that anybody with half an eye could immediately say which island they came from. Two forms of iguanas lived in the islands. Each type had affinities with the common South American green iguana, yet they had adapted so profoundly to different ecologic niches in the islands that they had evolved into separate genera. Conolophus, adept at living on the arid islands and feeding on the sharp-spined Opuntia cactus became the land iguana, while Amblyrhynchus, with its flattened tail for swimming, its strong claws for hauling itself out on the water, and its blunt, shortened snout for scraping algae off of rocks, became the marine iguana. Moreover, many islands developed their own races of these unusual lizards. Many of the birds that Darwin found, especially the land birds, were endemic species found nowhere else on earth. Here were thirteen different types of finches whose beeks were modified to different sub-environments on the islands.
The Galapagos islands were volcanic in nature and relatively recently formed, Darwin reasoned, and the animals that dwelt there had to have come from someplace else. Those most closely resembling the Galapagos community were the animals that lived closest to the islands on the mainland. But they were not the same animals. Why?
Why? That was the question that plagued Darwin. Why? Eight years after his return, Darwin wrote to his close friend and colleague, Sir Joseph Hooker: “At last gleams of light have come, and I am almost convinced (quite to the contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable.” According to Genesis, God created all plants and animals, and they have not changed significantly since that time. Yet the only way that Darwin could explain all of his observations was that they had indeed changed. The pivotal role of the Galapagos islands in shaping Darwin’s new world view is clear from a passage in his ornithological notebooks:
“If there is the slightest foundation for evolution, the zoology of the Galapagos will be well worth examining…”