How did Darwin come up with the theory of natural selection?

Charles Darwin, one of the most popular scientists in history, was well known for his theory on natural selection. Natural selection is when nature chooses or " selects" which organisms survive.  In other words, if their adaptations help them to survive, then they pass on their genes to their offspring. If their adaptations don't allow this, then they starve or are killed off.  In 1861, Darwin ventured out to the coastal South America with aristocrat Robert Fitzroy.Darwin ended up spending most of the  trip exploring the wildlife of South America, mainly the Galapagos islands.While on the islands, Darwin saw many animal species that he had never seen before in England. Darwin began to pay attention to the various species on the island, and he noticed that finches, a common bird species, were slightly different on every island. The main difference was their beak sizes. Some had small beaks, while others had large beaks. This affected their dietary habits, and the size of food they ate. For example, small beaked birds would survive in an environment with only small seeds, where as large beaked birds wouldn't. ( more difficult for birds with large beaks). This would lead to their starvation, and at one point, they would die out.